In the hydraulically actuated, air spring return exhaust valve design used on large two-stroke engines, the valve spindle is closed by compressed air (the "air spring") rather than a mechanical coil spring, and the opening motion is generated by hydraulic pressure acting on a piston or piston block at the top of the valve housing. Because both hydraulic oil and compressed air are involved, the valve has no rigid mechanical link to the rocker/cam; instead the hydraulic oil above the air spring is what drives the valve open and repositions it.

The term "virtual tappet" refers to the effective, controllable clearance or cushion that exists between the hydraulic actuator piston and the valve spindle extension. In a conventional mechanical tappet system the clearance must be adjusted manually. In this hydraulic system there is no physical tappet screw; instead the design creates an equivalent controlled clearance by the oil film and by the dimensional relationship between the actuator piston and the lower end of the valve spindle extension. The virtual tappet is set by machining the spindle extension to a defined length and by ensuring the piston block is positioned so that, when the valve is closed, there is a small pre-determined axial clearance (typically of the order of a few tenths of a millimetre). This setting is carried out by measuring between the piston and the spindle extension, or by using spacer/adjusting shims, and confirming the cold clearance against the manufacturer's figure. The air spring also provides a controlled cushioning effect so that the "tappet" is effectively compliant.

Furrowing and cutting of the valve seats: The seats become damaged because of burning of deposit, fuel-related corrosion and erosion. When combustion deposits or particles of uncarbonised fuel and hard sodium/vanadium compounds become trapped between the valve seat and valve insert, they act as an abrasive. The hard, brittle ash particles also soften and stick at high temperature. The high seating velocity and the excavating action of gas flow can then literally plough "furrows" round the seat and produce localized "cutting" in the valve-facing surfaces. Thermal loading and the differential expansion between spindle and seat ring further worsen it. Poor atomization and excess combustion advance promote burning on the seat land. Keeping the seats clean by proper valve rotation, correct fuel quality and adequate cooling reduces this damage.

Valve drop is the complete loss of the valve drive/retention, where the hydraulic oil pressure fails (e.g. loss of pump pressure, oil viscosity reduction, valve spindle fracturing at the neck or the spindle extension breaking) and the air spring supply fails simultaneously, so the valve head goes into the cylinder uncontrolled. The valve can then hit the piston crown at top dead centre, bending the connecting rod, breaking the crown, and leading to extensive damage to the running gear (piston, liner, crosshead and connecting rod). The mechanism usually involves failure of the hydraulic system security interlocks combined with a fractured spindle.

• The lower half of the bottom end bearing is secured using a chain block to prevent it from falling into the crankcase during disassembly.
• The hydraulic nut securing the lower half of the bottom end bearing is opened, and the bearing is carefully removed from the crankcase.
• The connecting rod is then secured using a chain block.
• The crosshead is locked in position on the crosshead guide using a dedicated locking tool.
• The crosshead bearing cap nut is opened.
• With the engine carefully turned using the turning gear, the connecting rod is slowly lowered and removed from the crankcase. This controlled movement is very important to prevent damage.

Post-Connecting Rod Removal:

Once the connecting rod is removed and piston suspended, the following steps are taken to isolate the affected cylinder and allow continued operation (following the maker's recommendation):

• The fuel pump for the affected cylinder is disabled by bypassing its cam roller, preventing fuel injection into the disabled cylinder. In the case of an Electronic engine, set the fuel index to Zero (0) from the MOP computer.
• The exhaust valve is deactivated by lifting its roller off the camshaft using a specialised lifting tool. With the Electronic engine, Disable the exhaust valve operation in the MOP computer. This prevents exhaust gases from escaping into the system from the disabled cylinder, although the cylinder will likely be vented in some other way.
• The starting air pipe to the cylinder is disconnected and blanked off at the main control air valve, preventing accidental air ingress.
• The lubricating oil supply to the crosshead of the affected cylinder is blanked off to prevent pressure drop of oil.
• The cylinder lubricator for the affected unit is set to "zero" delivery to prevent further lubrication of a seized and immobile piston.

• The damaged cylinder is isolated by suspending the piston and crosshead, removing the connecting rod, and cutting off the unit's combustion. This results in power imbalance and uneven loading on the crankshaft.
• The absence of power generation in the affected cylinder creates an imbalance in the crankshaft. Operating the engine at a reduced speed minimizes crankshaft deflection and prevents further damage to engine components.
• With one cylinder out of operation, the engine cannot develop its rated power.
• It is recommended to reduce the engine speed to 55% MCR (Maximum Continuous Rating), as this is sufficient to manoeuvre the vessel safely while reducing the risk of further damage. The engine load must remain within the manufacturer’s specified limits to avoid overloading the remaining cylinders.
• Continuous monitoring of parameters such as temperature, pressure, and vibration is essential to detect any abnormal behaviour during operation. Regular checks help ensure the engine’s condition is stable.
• The engine must be operated strictly within the conditions specified by the manufacturer for Emergency operating conditions.

Selective Catalytic Reduction (SCR) removes NOx from exhaust gas by injecting a reductant (aqueous urea, which decomposes to ammonia) into the gas stream and passing it over a catalytic reactor, where NOx is reduced to nitrogen and water:

4NO + 4NH3 + O2 => 4N2 + 6H2O

The three basic SCR system types differ mainly in where the reactor is placed relative to the turbocharger and the engine.

In an HPSCR system the reactor and urea injection are located between the engine exhaust outlet and the turbocharger inlet (upstream of the turbine), where exhaust gas pressure and temperature are high. Because the gas is hot (usually above 300 to 350 deg C), no reheating is required, and the catalyst works efficiently even at low engine loads. Disadvantages: the reactor and injection grid must withstand high pressure and vibration, the space and structure around the engine top must accommodate a large reactor, and the catalyst is exposed to soot and deposits which reduce life and require more frequent cleaning. The turbocharger operates on the cleaned gas, which reduces blade fouling.

In an LPSCR system the reactor is placed downstream of the turbocharger, in the low-pressure (near atmospheric) exhaust line. The system is lighter, cheaper and easier to retrofit, and standard marine exhaust piping can be used. The main drawback is that at low load the exhaust temperature after the turbine can be too low (below about 280 to 300 deg C) for effective reduction, so the gas must be reheated or the temperature maintained by engine management, which consumes extra energy and demands additional measures.

A static mixer is a passive device placed in the exhaust duct immediately downstream of the urea injection point. It consists of baffles, vanes or grids that create turbulence and thoroughly mix the injected urea/ammonia vapour with the exhaust gas. This ensures an even distribution of reductant across the catalyst face, avoiding both ammonia slip (excess ammonia leaving the system) and areas of high NOx leakage due to poor mixing. Static mixers improve conversion efficiency and reduce the amount of urea required. No moving parts make them robust and reliable.

Upstream installation (HPSCR) places the reactor before the turbine, utilising high gas temperature and providing efficient low-load operation and turbocharger protection. The disadvantages are high mechanical and thermal loading, more complex engine top layout and difficulty of cleaning a large high-mounted reactor.

Downstream installation (LPSCR) places the reactor after the turbine in the low-pressure exhaust. It is simpler, cheaper and easier to maintain and retrofit. Its principal drawback is the low temperature at part load, which must be managed by gas reheating or by limiting the load range in which the SCR is effective. In practice both configurations satisfy Tier III in their intended load range, and the choice is a trade-off between cost, space, temperature and maintenance.

An air distributor, also called a starting air distributor, is a device which controls the admission of starting air to each cylinder's starting air valve in the correct sequence, at the correct time relative to the crankshaft position. It consists of a rotor driven by the engine crankshaft (usually via gearing or chains) which carries a ported face that sweeps over a stationary plate containing one outlet to each cylinder. Starting air is supplied to the distributor inlet and, as the rotor rotates, compressed air is directed to each cylinder in the required firing order, only during the starting (power) stroke of each cylinder, i.e. for a piston moving down on the expansion stroke with the starting air valve pilot open. The distributor effectively acts as a rotary pilot valve that opens the pneumatically operated starting air valves (the large valves in each cylinder head) so that air enters the cylinder to turn the engine until the first firing sequence takes over. It also shuts off air to a cylinder as soon as the firing stroke would begin.

Many large slow-speed direct reversing engines use valve blocks with pilot-operated starting valves where the pilot air is timed by a camshaft or by the electronic control system (in camshaftless engines), with the whole starting sequence managed by the control system rather than a separate mechanical distributor. This avoids the mechanical complexity, drive gearing and wear associated with a distributor, and gives greater flexibility in timing, especially for improving starting air economy and for reversing. On camshaft-based engines the fuel and air valve timing can be changed by the reversing mechanism, so a distributor becomes unnecessary.

1. Wear of the rotating and stationary faces. The faces are lapped and spring-loaded; wear leads to internal air leakage and poor pilot air pressure. Solution: periodic dismantling, inspection, relapping or replacement, and correct adjustment of the axial spring load to keep faces in light contact.
2. Air leakage bypassing the faces, causing loss of start air pressure and difficulty starting. Solution: check seals and the spring pressure; ensure faces are flat and clean.
3. Misalignment of the drive gearing, so that the distributor timing no longer matches the crankshaft, giving wrong firing order or no start. Solution: check and re-set drive gear timing marks; replace worn gears and bearings.
4. Condensate and water in the start air, which causes corrosion and freezing in cold weather. Solution: proper draining of air reservoirs and filters, installation of air dryers, and periodic checks of automatic drain traps.
5. Sticking or blocked pilot passages due to oil and dirt, causing a cylinder not to receive air. Solution: filter the pilot air, keep the distributor clean, and check all ports are unobstructed.
6. Leakage of oil from the drive casing into the distributor due to poor shaft sealing. Solution: check the oil seals and vent arrangements.
7. Air not being cut off properly from a cylinder, allowing air to be admitted at the wrong time - dangerous. Solution: verify the port geometry and timing setting, inspect the rotor position and the stop/starting valves.

Difficulties During Installation:

• The ship's configuration changes when transitioning from being berthed to afloat, affecting alignment.
• Natural deflection of the shafting occurs between supports due to its length and weight.
• The propeller's weight creates a cantilevered effect on the shaft, further complicating alignment.

Difficulties During Service:

• The ship's loading conditions (cargo, ballast, fuel, and water) affect alignment.
• Movement of the ship in water causes dynamic changes in alignment.
• Off-centre thrust from the propeller can create additional forces on the shafting system.
• Wear down of bearings over time impacts alignment.
• Water forces acting on the vessel's hull cause distortion, affecting shaft alignment.

Reasons for Unreliable Results:

Results are often unreliable due to various external factors such as temperature fluctuations (high deck temperatures in tropical climates versus low sea temperatures), wind, waves, draft, and water density. The ship's hull can distort due to hogging and sagging under different loading conditions, affecting the alignment measurements. Cargo weight and distribution, ballast, fuel, and water levels are all subject to change, further impacting the accuracy of measurements. Over the ship's lifetime, extreme weather conditions can alter the hull's shape, leading to variations in shaft alignment.

Reasons for Misalignment

• Uneven wear down of bearings.
• Hull deformation caused by hogging, sagging, or prolonged stress.
• Improper or incomplete alignment during initial installation.
• Changes in loading conditions, cargo distribution, and ballast arrangements.
• Long-term effects of extreme weather and sea conditions.
• Propeller thrust misalignment due to incorrect propeller installation or damage.
• Vibration and fatigue in the shaft system.

A simple sketch to illustrate bearing load assessment using the jacking method:

Hydraulic jacks are placed on either side of the bearing and used to lift the shaft. A dial gauge measures the shaft's lift, indicating the amount of force needed to lift it. The hydraulic pressure exerted by the jacks directly corresponds to the load on the bearing. By comparing this load with design specifications, engineers can determine if the load is evenly distributed among bearings.

• Alter the height of the bearing from the tank top by loosening the foundation bolts and tightening the jacking bolts.
• Insert or remove shims between the bearing housing and foundation to achieve proper alignment.
• Compare the actual bearing load with the original load specified in the manual and make adjustments accordingly.
• If the bearing is excessively worn or clearance exceeds limits, replace the bearing to restore proper function.

Insufficient bearing clearance:

Indications:

• Increase in bearing temperature due to reduced oil flow and friction.
• Dark brown appearance of lubricating oil caused by overheating and oxidation.
• High oil mist content indicating excessive wear or overheating.
• Increased amperage of the turning gear motor, highlighting resistance during engine rotation.
• Presence of white metal particles in lubricating oil analysis, indicating bearing material damage.

Effects:

• Excessive heat can cause the bearing's white metal layer to melt or wear away.
• Metal-to-metal contact leads to surface damage (scoring) on the crankpin and bearing surfaces.
• High heat generation may cause the bearing and shaft to seize.
• Overheated oil may cause oxidation and degrade into sludge.
• Overheating and wear can lead to permanent bearing failure.

Excessive bearing clearance:

Indications:

• Noisy operation, often characterized by a knocking sound caused by the clearance between components.
• Drop in lubricating oil pressure due to increased leakage at the bearing clearance.
• Presence of white metal particles in the oil analysis, indicating wear or damage.

Effects:

• Metal-to-metal contact may occur as the hydrodynamic oil film is compromised.
• Over time, the bearing may experience accelerated wear or failure.
• Can lead to irregular engine speed.
• Excessive clearance causes imbalance and increases vibrations in shaft.

Sketch showing lubricating oil passage to crank pin bearing:

Forged Crankshaft:

• Forged crankshafts have a continuous grain flow throughout the shaft, providing greater material strength and reduced susceptibility to cracks or failures under high stress.
• Being a single-piece construction, there is no shrink fit, eliminating the risk of slippage.
• Forged crankshafts have better resistance to fatigue and reduced stress concentration due to their uniform structure and absence of assembly joints.
• These crankshafts are smaller and lighter, which is advantageous for compact designs and weight-sensitive applications.
• Primarily used in smaller engines where space and weight considerations are critical.

Built-Up Crankshaft:

Fully Built-Up Crankshaft:

• Comprised of separately forged webs, crankpins, and journals, assembled using a shrink-fit method.
• Grain flow is not continuous, leading to comparatively lower strength.
• Higher stress concentration due to assembly joints.
• The advantage is it is simpler construction and allows for easier replacement of parts.

Semi-Built-Up Crankshaft:

• Crank throws are forged as a single piece with continuous grain flow, enhancing material strength.
• Better fatigue resistance compared to fully built-up crankshafts.
• Features larger pin diameters and lighter, smaller webs, reducing overall shaft weight.

Welded Crankshaft:

• Half-journal, webs, and crankpins are forged together and welded to similar sections.
• Welding and continuous grain flow enhance material strength.
• Post-welding stress relief minimizes residual stresses.
• Absence of shrink fits eliminates slippage risks, allowing thinner webs for a compact and lightweight design.

• Stop the engine and engage the turning gear.
• Start measuring deflections from the unit farthest to flywheel
• Place the crank pin at the point of 30° (position ‘B’) past the bottom dead centre.
• Install the deflection gauge in the pop point provided for this purpose.
• Set the reading on the gauge to 0 (zero reading) at the position ‘B’ in the figure.
• Slowly conduct turning of the engine in the normal direction of rotation, and measure the reading on the scale when the crankshaft is at the angle of ‘B’, ‘C’, ‘D’, ‘E’ and ‘A’ respectively, of which data shall be recorded.

Calculating deflection (d): Calculate the deflection values as based not the measured values and in accordance with the following formula and record the calculated values.

Vertical (V) deflection: dV = D - A+B/ 2

Horizontal (H) deflection: dH = C - E

positive/ negative deflection: open downward (+), closing downward (-) A, B, C, D and E represent the measured values respective at each corresponding position shown in the figure above.

1. Draw a horizontal reference line below the crankshaft.
2. For every unit, draw a vertical line representing the vertical deflection measurement for this unit.
3. Draw a soft curve using the points obtained by the measurements.
4. Finally, draw a baseline (tangent) to this curve to see which units deviate the most from the deflection curve.

This allows us to assess whether there are additional misalignments between the webs, even though the deflection values are within the manufacturer’s limits.

A two-stroke, single-acting crosshead-type slow-speed engine is an ideal choice for large vessels that require significant propulsive power and operate primarily in open seas. 🚢 This includes ships with a huge tonnage and deep draft, such as large tankers, bulk carriers, and container ships.

A key advantage of this engine type is its power output. Since it's a two-stroke engine, it delivers a power stroke with every revolution, providing consistent and immense power to the propeller. This high power-to-weight ratio allows the vessel to carry more cargo. While its maneuverability is less than other engine types, this isn't a major issue for large ships that spend most of their time on long voyages. The engine is also known for its high efficiency and reliability. Furthermore, starting and reversing are relatively easy, making it a convenient choice for a vessel's main propulsion system.

The four-stroke, single-acting engine with a reverse/reduction gear is particularly well-suited for ships that require frequent and precise maneuvering. This makes it the preferred engine for vessels that operate often in harbors or restricted waters, such as barges, tugs, product carriers, supply vessels, research vessels, and dynamically positioned (DP) ships.

The inclusion of reverse/reduction gears allows for easy and rapid changes in direction, which is critical in confined spaces. These gears also enable the propeller to operate at various speeds while the engine maintains a constant, optimal RPM. This provides flexibility and control, especially when a wide range of speeds is needed for different operational requirements. Since these ships don't typically carry huge amounts of cargo, the smaller power output of a four-stroke engine is sufficient. Additionally, four-stroke engines generally have better scavenging, resulting in lower emissions and easier compliance with port emission standards.

In a pulse (or proportional) type cylinder lubrication system, the cylinder lubricant is delivered to the cylinder liner as individual timed pulses of oil, injected when the piston rings pass near the injection points, rather than being sprayed as a continuous fine mist. On modern systems the lubricators are driven mechanically from the engine (or electrically by servo motors in electronic systems) so that the quantity of oil injected is proportional, or directly related, to the engine speed and load (often with a feed-rate setting adjusted to PI = 1.0 g/kWh as a starting point). Each lubricator delivers a metered amount of oil once per revolution (or per few revolutions) at the moment the piston ring pack is positioned to receive and spread the oil around the liner circumference.

Working: oil is drawn from a header tank or a low-pressure supply and a variable-delivery piston pump (lubricator) discharges a fixed volume per pulse. The oil is led through a non-return valve to an injector nozzle fitted flush with the liner, opening into a small recess. The pulse is timed electronically or mechanically to coincide with piston ring passage so the oil is wiped over the ring contact band and distributed by ring motion.

Advantages: (1) feeds oil only where and when required, so oil consumption is reduced to optimal feed rates of about 0.5 to 1.5 g/kWh; (2) reduces the volume of unburt oil entering the scavenge space and the risk of scavenge fires and oil deposits; (3) improves distribution over the ring pack, giving a more uniform oil film; (4) reduces liner and ring wear; (5) control of feed rate and adjustment to load and fuel sulphur is straightforward; (6) reduces total oil carried and sludge generation.

Disadvantages: (1) relies on correct timing - if injection occurs when the piston is at the wrong position the oil may not be spread effectively; (2) more complex, with individual lubricators and timing equipment needing attention; (3) if the timing drive or electronics fails, lubrication may stop or be delivered at the wrong time, risking scuffing; (4) nozzles and non-return valves can block, giving a dry region of the liner.

Specific cylinder oil consumption is the mass of cylinder lubricating oil consumed per unit of engine power per unit time, expressed in g/kWh. It gives a comparable figure of oil used per unit of work done.

Measurement: over a defined period (typically several hours or a watch), record the engine brake power developed from the engine log/indicator card (or the ME/ML1 figure) and the quantity of cylinder oil consumed. For electronic lubrication systems the quantity can be read from the control system; for mechanical systems the oil drawn from the header tank - or the level drop in a calibrated tank - is measured, allowing for any flowmeter reading.

Fuel-oil-ratio method (workshop/lab): cylinder consumption g/kWh = (cylinder oil flow in g/h) / (engine power in kW). On the ship, power is computed from the propeller curve and all available data.

Suggested formula used by makers: SFOC of cylinder oil = (oil feed rate setting in g/kWh) adjusted by the load correction factor, but the actual ship measurement is:

Cylinder oil consumption (g/kWh) = (Oil consumed in grams over the trial period) / (Average brake power in kW x hours run).

Care must be taken that consumption is measured with the engine at steady load, that returned oil is not counted again, and that all six/eight/nine cylinders' lubricator deliveries are included. The result is compared with maker's recommended figures (typically 0.5 - 1.5 g/kWh for modern engines at part load) and with fuel sulphur content, increasing the feed rate in proportion to fuel sulphur to maintain BN reserve.

A Common Rail (CR) fuel injection system consists of a high-pressure fuel manifold (common rail) running along the length of the engine, supplying fuel at a constant high pressure to all cylinders. Unlike the conventional jerk pump system, fuel pressure generation and injection timing are completely independent.

One example is the Sulzer/Wärtsilä RT-flex two-stroke low-speed marine diesel engine, which uses electronically controlled, hydraulically actuated common rail fuel injection.

Construction and Working

• Fuel is supplied by engine-driven high-pressure fuel pumps, operated by a three-lobe cam, which deliver fuel to the common rail at approximately 1000 bar.
• A separate servo oil system, operating at about 200 bar, supplies hydraulic power for operating the injection control units.
• The common rail acts as a pressure accumulator, maintaining nearly constant fuel pressure for all cylinders irrespective of engine speed.
• Each cylinder has an independent Volumetric Injection Control (VIC) unit, which receives:
  • High-pressure fuel from the common rail.
  • Hydraulic servo oil.
  • Electronic control signals from the Fuel Control Module (FCM).
• The Fuel Control Module (FCM) determines:
  • Injection timing.
  • Quantity of fuel injected.
  • Injection pressure and duration.
  • Injection rate (shape of the injection pattern).
• The VIC unit operates quick-acting electronically controlled rail valves, which hydraulically actuate the fuel injectors.
• In RT-flex engines, three fuel injectors are fitted in each cylinder cover. Each injector is controlled independently, allowing them to inject:
  • Individually,
  • Sequentially, or
  • Simultaneously,
  • depending on engine load and operating conditions.
• Since the fuel pressure is maintained independently of engine speed, optimum injection pressure is available throughout the entire operating range, ensuring efficient combustion.

Common Rail Fuel Injection System	Jerk-Type Fuel Injection System
Injection pressure is almost constant and independent of engine speed.	Injection pressure depends directly on engine speed and pump plunger movement.
Injection timing, duration and quantity are electronically controlled.	Injection timing and quantity are mechanically controlled by the cam profile and pump helix.
Multiple or pilot injections can be provided for better combustion.	Normally only a single injection per cycle is possible.
Produces superior combustion with very low smoke and emissions.	More smoke and poorer combustion, especially at low loads.
Better fuel economy due to precise fuel metering.	Higher specific fuel consumption because of less precise control.
Stable operation at very low engine speeds due to high injection pressure.	Poor low-speed performance because injection pressure falls with engine speed.
Individual cylinder performance can be adjusted electronically.	Individual cylinder adjustment is limited and requires mechanical setting.
Easier compliance with IMO emission regulations.	Difficult to meet stringent emission limits without additional systems.

Advantages of Common Rail Fuel Injection

1. Smokeless Operation
   • High injection pressure is maintained throughout the entire operating range, resulting in superior atomization and efficient combustion with significantly reduced smoke emissions.
2. Reduced Fuel Consumption
   • Electronic control maintains optimum engine settings throughout service life, preventing deterioration in fuel economy due to wear or maladjustment.
3. Excellent Low-Speed Running
   • Constant high injection pressure, precise fuel metering and sequential operation of injectors provide smooth and stable engine operation at very low speeds without excessive smoke.
4. High Reliability and Redundancy
   • Multiple high-pressure fuel pumps and servo oil pumps provide redundancy.
   • The engine can continue to develop full power even if one fuel pump and one servo pump are out of service.
   • If additional pumps fail, engine power reduces only in proportion to the number of pumps unavailable.
5. Improved Combustion
   • Precise control of injection timing, pressure and injection pattern results in complete combustion, higher thermal efficiency and lower exhaust temperatures.
6. Lower Emissions
   • Reduced NOₓ, particulate matter and visible smoke due to optimized injection characteristics.
7. Reduced Maintenance
   • Elimination of individual jerk pumps, pump timing adjustments and mechanical linkages reduces wear and maintenance requirements.
8. Flexible Engine Control
   • Injection timing, quantity and rate can be optimized electronically for different operating conditions, improving performance over the entire load range.

Disadvantages of Common Rail Fuel Injection

1. High Initial Cost
   • More expensive than conventional jerk-type systems due to electronic control units, sensors, actuators and hydraulic components.
2. Greater System Complexity
   • Requires sophisticated electronic control systems, hydraulic servo systems and high-pressure fuel equipment.
3. Higher Maintenance Skill Requirement
   • Troubleshooting and repairs require trained personnel and specialized diagnostic equipment.
4. Sensitive to Fuel Cleanliness
   • High-pressure components and control valves are susceptible to contamination; excellent fuel filtration is essential.
5. Dependence on Electronic Systems
   • Failure of electronic sensors, control modules or wiring may affect engine operation, although redundancy minimizes this risk.

Examples in Modern Marine Diesel Engines

Common Rail Fuel Injection

• Wärtsilä (Sulzer) RT-flex low-speed two-stroke engines.
• WinGD X-DF electronically controlled dual-fuel engines (common rail variants).
• Modern medium-speed marine diesel engines equipped with electronically controlled common rail systems.

Jerk-Type Fuel Injection

• MAN B&W MC-series mechanically controlled low-speed two-stroke engines.
• Conventional medium-speed and auxiliary diesel engines using individual cam-operated jerk pumps.

Comparison: Modern Multiple-Sensor vs. Traditional Single-Sensor Oil Mist Detectors

Modern multiple-sensor crankcase oil mist detectors have significantly advanced from the traditional single-sensor type, offering major improvements in response time, detection accuracy, and overall reliability.

Traditional Single-Sensor Type

This older system used a single, centralized sensor that sequentially sampled air from each crankcase compartment.

• Sequential Sampling: The biggest drawback was the time delay caused by the sequential sampling process. A centralized suction unit drew air through a complex network of pipes and a selector valve, analyzing each compartment one by one. This meant that on large engines, a significant amount of time could pass between the development of a hot spot and its detection.
• Slow Response Time: The slow sampling cycle meant a developing oil mist could escalate into a dangerous situation before the system even got a chance to check that specific compartment.
• Complex Installation: The extensive and complex piping required for this system made it costly and difficult to install and maintain. The long pipes could also lead to condensation, reducing the system's sensitivity.

Modern Multiple-Sensor Type

Modern systems utilize multiple, dedicated sensors, with a sensor typically installed directly in or on each crankcase compartment. These sensors operate independently and in parallel.

• Simultaneous Monitoring: Each compartment is monitored continuously and simultaneously. This eliminates the delay of sequential scanning.
• Immediate Detection: A hot spot and the resulting oil mist can be detected and localized almost instantaneously, allowing for a much faster response to prevent a catastrophic crankcase explosion.
• Simpler Installation: This design eliminates the need for complex piping and a centralized suction unit. The compact sensors connect directly to a central control unit via a simple network cable, significantly reducing installation costs and complexity.

What Is Meant by Addressable Sensors?

An addressable sensor is a smart device with a unique digital identifier, or "address," that allows it to communicate its status directly to a central control unit.

In modern oil mist detectors, each sensor head is an addressable unit.

• Individual Identification: Each sensor is assigned a unique digital address (e.g., sensor #1, sensor #2) that corresponds to a specific crankcase compartment.
• Precise Localization: When an alarm is triggered, the central unit instantly knows which specific sensor (by its address) detected the oil mist. This provides the exact location of the hot spot, allowing the crew to focus their investigation immediately on the correct area, which is crucial for safety.
• Data Transmission: The sensor takes its own readings and transmits this data digitally to the central control unit. This allows for continuous, precise monitoring.
• Flexibility: Addressable systems are easily expanded or modified. If an engine has more compartments, more sensors can be added to the network without a major overhaul. This also makes troubleshooting easier, as the system can pinpoint a faulty sensor by its address.

Cylinder lubrication in a two-stroke engine is entirely separate from the crankcase lubrication system: cylinder oil must form and maintain an oil film between the piston rings and the liner to control wear, seal combustion gases and keep the liner clean. Since the piston skirt and crosshead receive oil from the crankcase, the cylinder oil is supplied fresh by lubricators at each cylinder, and it is burned in the combustion space or passes down to the scavenge space, so it is a total-loss lubricant. Because of MARPOL Annex VI, which limits the sulphur content of fuel (down to 0.50% or 0.10% sulphur in Emission Control Areas and as required by various regulators), the requirement to neutralise the acidic products of sulphur combustion (sulphuric acid) is much reduced. Excessively high cylinder oil feed rates now produce excess alkalinity, deposits, ash and increased oil costs, while feed rates must still be enough to maintain the ring/liner film and prevent corrosion wear. This tension between alkalinising and wear protection is the heart of modern cylinder lubrication practice.

"Two-level lubrication" refers to the ability of the lubrication system to deliver different feed rates of cylinder oil at different engine operating conditions, usually distinguishing between a higher feed rate for normal sea-going service and a reduced feed rate for low-load, slow-steaming or manoeuvring conditions, and increasingly with separate settings to match fuel sulphur content. It is implemented by either;

1. Two separate oil injection pumps/systems with different deliveries which are engaged by the control system according to engine speed or load, or
2. An electronic pulse lubrication system where the quantity injected per unit time (and even per injection) is programmed as a function of engine speed and load and of the selected fuel-oil sulphur content and the BN (base number) of the oil.

In normal service the feed rate is set to, say, 1.0 to 1.2 g/kWh to allow for the corrosive component of high-sulphur fuel. In slow-steaming or low-load operation the lower setting (e.g. 0.5 to 0.8 g/kWh) is selected because the corrosive load is lower but a minimum film must still be maintained. Some systems physically select a different set of plungers or a different cam to give the two rates; electronic systems simply change the injection programme. The purpose is to avoid both under- and over-lubrication over the full operating range and to minimise total oil consumption and carbon/ash deposit formation.

Under-lubrication: with too little cylinder oil, the oil film between rings and liner breaks down. This causes metal-to-metal contact, high friction, high liner and ring wear, scuffing, seizure of the ring(s), loss of compression and blow-by of combustion gas, a fall in power, and greater risk of a scavenge fire as hot blow-by gases ignite the lubricant deposits collected in the scavenge space. The liner can become polished or badly worn and the running surface can be damaged permanently. Anti-corrosion protection also fails, so acid attack (cold corrosion) increases, especially in low-sulphur/low-load conditions.

Over-lubrication: excessive oil is passed into the cylinder. Parts of the oil are burned, and the ash and carbon deposits build up on the piston crown, ring grooves and gas side, and in the exhaust valves, turbocharger (if not cleaned) and scavenge space. The pour of oil down the liner increases oil consumption, raises costs and produces large quantities of sludge and oily deposits in the scavenge space, which are a serious fire risk. Carbon in the ring grooves causes the rings to become stuck, reducing sealing and leading to blow-by. The excess alkalinity (BN) from the oil can react with fuel-ash and form hard deposits. The overall result is reduced engine reliability and higher running cost.

A magnetic pick-up unit (MPU), also known as a revolution pick-up sensor, consists of a permanent magnet and an external coil winding. It's positioned a precise distance from ferrous gear teeth or flywheel teeth. As the flywheel rotates, the proximity of the teeth to the MPU creates a constantly changing magnetic field within the sensor. This fluctuating magnetic field induces an AC voltage in the coil. The frequency of this AC voltage is directly proportional to the engine's rotational speed – higher engine speed results in a higher frequency and voltage. The formula for induced frequency is:

$$F=\frac{Number\:of\:gear\:teeth\:\times Gear\:RPM}{60}$$

This frequency signal is then used by the engine's control system to determine engine speed.

The important parameter for the MPU is the air gap between the sensor and the gear teeth. This gap should be maintained within a specified range, typically 0.25 mm to 1.02 mm at the closest point. Incorrect gap adjustment significantly impacts the sensor's output. Too small a gap risks damaging the sensor, while too large a gap leads to a significant voltage drop. Adjustment involves loosening a locking nut, adjusting the sensor's position to achieve the correct gap, and then re-tightening the nut. The sensor's condition can be verified by measuring the AC voltage at a known engine speed. A new MPU should produce at least 1.5V (AC) at the ideal air gap and operational speed. A voltage below 1.5V (AC) usually indicates a faulty sensor requiring replacement.

The pulse angle offset adjustment ensures the output waveform from the pick-up sensor aligns with the engine's flywheel position.

• Connect an oscilloscope to the sensor output.
• Monitor the output waveform and compare it to a reference waveform.
• Adjust the offset knob to align the output signal with the reference waveform.
• Ensure the signals match in both amplitude and phase.
• Once the adjustment is complete, lock the offset adjustment knob securely.
• Confirm the alignment by rechecking the waveform on the oscilloscope.

The pin diameter is made larger to distribute the load over a greater surface area, reducing the load per unit area on the bearing. This also increases the relative sliding speed between the pin and the bearing, aiding lubrication.

The bearing shells are lined with layers of materials designed for specific purposes:

• Flash Layer (2-5 µm): 100% tin to prevent oxidation and act as a dry lubricant during initial operation.
• Overlayer (20-30 µm): An alloy of 85% lead, 10% tin, and 2% copper to provide good embedability and conformity with the pin's surface geometry.
• Nickel Dam (3 µm): A pure nickel layer offering corrosion resistance to the main bearing layer.
• Main Layer (0.5 mm): Made of aluminium (60%) and tin (40%) for high strength and anti-friction properties.
• Steel Backing: Provides the structural strength needed to support the bearing shell.

The crosshead bearing features a machined wedge to assist hydrodynamic lubrication, creating an oil film that supports the load during operation. The crosshead pin is manufactured with a high surface finish to reduce metal-to-metal contact in the boundary lubrication region, further minimising edge loading.

• The 120° arc of special surface geometry on the lower shell ensures that the load from the connecting rod is spread over a larger area of the bearing surface. This prevents point or edge loading that would cause high pressures and potential failure.
• The axial and transverse oil grooves, combined with the carefully designed geometry, facilitate the establishment of a hydrodynamic oil film. This film separates the moving surfaces, significantly reducing friction and wear. The oil wedge design further helps in establishing a stable lubricating film.
• The soft overlayer in the tri-metal bearing allows the bearing surface to conform to the shape of the crosshead pin, ensuring good contact and consistent lubrication across the entire contact area.
• The use of a tri-metal bearing material ensures wear resistance, corrosion protection, and good embedability for debris.
• A larger pin diameter increases the contact area, thus reducing pressure per unit area. The smooth surface finish helps further reduce friction.

For large two-stroke marine diesel engines (MAN B&W ME-C):

• Top End Bearing Clearance (Crosshead): 0.25 mm to 0.6 mm
• Bottom end bearing clearance (Crankpin bearing): 0.4 mm to 0.8 mm.

These values depend on the engine size and design specifications

MEP (mean effective pressure) is the average pressure acting on the piston, and increasing it raises the power output of an engine of given size without increasing engine dimensions, i.e. improves the power-to-weight ratio. Power is proportional to MEP and speed, so a higher MEP (today 18 to 25 bar for large engines) produces more power per cylinder. To obtain a higher MEP, a larger mass of air must be admitted to burn a correspondingly larger mass of fuel, and the air must be supplied at a higher pressure. This is the role of the turbocharger compression ratio - the ratio of the compressor delivery pressure to the compressor inlet (atmospheric) pressure. A higher compressor pressure ratio delivers air at higher density, so more air mass is trapped per cycle and more fuel can be burned, raising MEP.

Modern engines therefore demand turbocharger pressure ratios up to about 4.5 to 5.5:1 (with charge air pressures of 4 to 5 bar), whereas older engines used ratios of about 1.5 to 3:1.

The problem with a single high-pressure-ratio compressor is that as the ratio increases, the compressor operates nearer its surge limit, its efficiency normally falls at the top of the operating range, and the air temperature rise (and hence the work needed to compress) increases markedly. It is also difficult for one stage to deliver both high pressure ratio and high efficiency across the whole engine operating speed range. It is therefore necessary to introduce two-stage turbocharging, where the air is compressed in two turbocharger stages (usually one high-pressure and one low-pressure compressor, often with intercooling between the stages and a charge-air cooler after the low-pressure stage). Two-stage compression:

• achieves a higher overall pressure ratio than a single stage at similar or better efficiency;
• lowers the compressed air temperature for a given delivery pressure (approximately isothermal-like compression), giving cooler, denser air to the engine;
• keeps each compressor operating in its most efficient region and reduces surge risk;
• reduces the work per stage and hence the power taken from the exhaust, and improves engine part-load response.

The higher boost allows engines to exceed the former limits on MEP and thus to achieve significantly higher power-to-weight ratios, with two-stage turbocharging being a key enabler for today's highest-MEP engines.

(i) Which area is varied: the effective flow area of the exhaust gas into the turbine nozzle ring (the nozzle vanes at the turbine entry) is varied. It is the throat/entry area available to the exhaust gas as it passes to the turbine blades.

(ii) Why it is varied: with a fixed area turbine, the turbocharger is matched at one design point. At low engine load the exhaust energy is small, the turbine runs slowly and the compressor delivers too little boost, giving poor air/fuel ratio, black smoke and high thermal load; at high load the fixed nozzle may choke and the turbocharger over-speed. Varying the turbine area adjusts the nozzle area to control the turbine inlet pressure and hence the boost, so that a good air/fuel ratio can be obtained over a wide load range, smoke is reduced at low load, the turbocharger over-speed is limited at high load, and the engine can be optimised for better fuel consumption and lower exhaust temperatures.

(iii) How it is varied: by a control mechanism that rotates the nozzle vanes (swirl vanes or a ring of adjustable vanes) in the turbine entry, hydraulically or pneumatically, driven by the electronic control system which usually bases the vane position (and hence area) on engine speed and load. At low load the vanes close up to reduce the area (heavy throttle effect), increasing exhaust velocity and turbine power and boost; at high load they open to increase the area, avoid choking and prevent overspeed. Alternatively a sliding sleeve or adjustable guide ring alters the nozzle area. The setting may be feedback-controlled to maintain a target charge-air pressure or scavenge pressure.

Problems in Turbocharger Nozzles, Shrouds, and Blades

Residual fuels contain significant impurities such as carbon, ash, silica, and alumina. Additives in fuel oil can also contribute to specific fuel-related issues. Incomplete combustion, often caused by

high Conradson Carbon Residue (CCR) and ignition delay, leads to carbon deposits accumulating in the gas passages of the turbocharger.

Effects on Turbocharger Operation:

• Accumulated carbon restricts gas flow through the nozzles and blades, reducing the effective working of the turbocharger.
• Speed of rotation falls, resulting in decreased air supply to the engine.
• This leads to reduced efficiency, incomplete combustion, and further carbon deposits.

Remedies to Minimize Build-up:

1. Use the correct grade of fuel as per engine specifications.
2. Proper fuel oil treatment, including heating and purification.
3. Regular maintenance of fuel equipment (fuel pump, injectors).
4. Perform dry washing of the turbocharger as per the manufacturer’s recommendations.
5. Avoid prolonged low-load operation, which promotes carbon buildup.

Residual fuels contain sodium (Na) and vanadium (V) as impurities. At high temperatures, sodium and vanadium react (in a 1:3 ratio) to form sodium vanadate, which further oxidises to Vanadium Pentoxide (V₂O₅). Vanadium Pentoxide has a low melting point and is highly corrosive. These corrosive compounds deposit on the turbocharger and exhaust trunking, causing hot corrosion.

Effects on Turbocharger Operation:

• Corrosive deposits degrade and damage the nozzles, shrouds, and blades.
• Leads to a loss of turbine efficiency due to distortion or roughening of surfaces.
• Turbine imbalance may occur, increasing vibrations and further reducing operational reliability.

Remedies to Minimize Hot Corrosion:

1. Use the proper grade of fuel with low sodium and vanadium content.
2. Carry out appropriate fuel oil treatment to remove impurities.
3. Maintain fuel-burning equipment to ensure efficient combustion and avoid after-burning or high exhaust gas temperatures.

Turbocharger turbines operate at very high rotational speeds, making them susceptible to damage from high-impact particles present in exhaust gases. Erosive particles include unburnt fuel, ash, and abrasive contaminants like silica and alumina from residual fuel. Catfines (catalytic fines) present in untreated fuel oil are particularly abrasive.

Effects on Turbocharger Operation:

• Erosion causes surface wear and damage to the turbine blades and nozzles.
• Loss of blade profile reduces turbocharger efficiency and air delivery.
• Long-term erosion may result in turbine imbalance, vibrations, and eventual mechanical failure.

Remedies to Minimize Erosion:

1. Ensure complete combustion by maintaining fuel injection equipment (fuel pumps, injectors, atomizers).
2. Implement proper fuel purification and filtration to remove abrasive contaminants such as catfines, silica, and alumina.
3. Monitor and maintain proper fuel treatment procedures to reduce unburnt fuel and residue buildup.

In large slow-speed two-stroke engines, proper cylinder liner lubrication is essential to maintain a protective oil film between the piston rings and cylinder liner. If lubrication is inadequate or improperly controlled, several operational and mechanical problems may occur.

1. Excessive Wear

• When lubrication is insufficient, metal-to-metal contact occurs between the piston rings and the cylinder liner. This results in accelerated wear of both the piston rings and liner surface, ultimately reducing the service life of the engine components.

2. Scuffing and Scoring

• Improper lubrication can cause the breakdown of the lubricating oil film. As a result, deep vertical scratches or scoring marks may develop on the liner surface. If this condition becomes severe, it may lead to piston seizure.

3. Micro-Seizure

• Micro-seizure occurs when localized welding and tearing of metal surfaces takes place between the piston rings and liner. This happens when the lubricating oil film is too thin or insufficient, causing direct metal contact.

4. Corrosive Wear

• Residual fuels contain sulphur, which during combustion forms sulphuric acid. If the cylinder oil does not have a sufficient Base Number (BN) to neutralize these acidic products, the acid can corrode the liner surface, leading to corrosive wear.

5. Piston Ring Sticking

• Poor lubrication and the formation of carbon deposits can restrict the free movement of piston rings within their grooves. This causes piston ring sticking, resulting in poor sealing and increased gas leakage.

6. Blow-by and Loss of Compression

• Worn liners or damaged piston rings allow combustion gases to leak past the piston rings, a condition known as blow-by. This reduces compression pressure, lowers engine efficiency, and increases fuel consumption.

7. Overheating

• Excessive friction due to poor lubrication increases the temperature of the piston and liner surfaces. This overheating may damage the piston crown, piston rings, and cylinder liner.

8. Increased Oil Consumption

• Incorrect cylinder oil feed rates may lead to either excessive oil consumption or insufficient lubrication, both of which negatively affect engine performance and operating costs.

1. Cloverleafing

Description

Cloverleafing refers to an uneven wear pattern on the cylinder liner. The liner develops a lobed or oval shape resembling a clover leaf rather than remaining perfectly circular. This wear pattern usually occurs at specific locations corresponding to the fuel injection points.

Causes

Cloverleafing can occur due to several factors, including:

• Uneven temperature distribution around the circumference of the liner
• Poor fuel atomization, causing localized hot spots
• Incorrect fuel injection timing
• Over-lubrication, which may lead to bore polishing
• High thermal and mechanical stresses acting on the liner

Effects

The consequences of cloverleafing include:

• Poor sealing between the piston rings and liner
• Increased blow-by of combustion gases
• Development of irregular wear patterns on the liner surface

2. Micro-Seizure

Description

Micro-seizure is a condition where localized adhesion occurs between the piston ring and the cylinder liner. Small fragments of metal may tear away from the surfaces, leaving fine scoring marks on the liner.

Causes

Micro-seizure can result from several operating conditions, such as:

• Insufficient lubrication
• Low cylinder oil feed rate
• Low oil viscosity
• Excessive engine load
• Poor distribution of lubricating oil
• Breakdown of the oil film due to high temperatures

Effects

The effects of micro-seizure include:

• Roughening of the liner surface
• Damage to piston rings
• If not corrected, it may develop into major seizure or severe liner damage

Large two-stroke marine engines operating on heavy residual fuel oil (HFO) require cylinder lubricating oil with high alkalinity, commonly expressed as a high Base Number (BN), in order to neutralize the acidic products formed during combustion.

The typical composition of such cylinder oil includes the following components:

1. Base Oil

• The main component is a high-viscosity mineral base oil.
• This base oil provides the primary lubricating film strength required to protect the piston rings and cylinder liner.

2. Alkaline Detergents (High BN Additives)

• Cylinder oils contain calcium-based alkaline detergents.
• These additives neutralize sulphuric acid formed during fuel combustion and help maintain the cleanliness of engine components.
• Typical cylinder oil Base Number (BN) ranges from 40 to 100, depending on the sulphur content of the fuel used.

3. Dispersants

• Dispersants help keep carbon particles and combustion residues suspended in the oil, preventing them from forming harmful deposits on engine components.

4. Anti-Wear Additives

• Anti-wear additives reduce direct metal-to-metal contact between moving parts, thereby minimizing wear of the piston rings and cylinder liner.

5. Antioxidants

• Antioxidants prevent oxidation of the lubricating oil at high temperatures, thereby extending the service life of the oil.

6. Corrosion Inhibitors

• These additives protect metal surfaces from acidic corrosion, particularly the cylinder liner, which is exposed to sulphurous combustion products.

7. Thermal Stability Improvers

• Thermal stability additives ensure that the lubricating oil maintains its film strength and stability at high operating temperatures, which is essential for reliable engine operation.

A single collar thrust bearing is used to absorb the axial thrust generated by the propeller shaft and transmit it safely to the ship's hull.

Construction

• A thrust collar is forged integrally with the propeller shaft.
• Thrust pads (or shoes) lined with white metal are fitted on both sides of the thrust collar.
• The pads are mounted inside a strong bearing casing (housing).
• Each thrust pad is supported by a pivot or fulcrum at its back, allowing it to tilt slightly during operation.
• Lubricating oil is supplied between the rotating thrust collar and the stationary thrust pads.
• The lower part of the bearing housing is securely bolted to the ship's structure so that the propeller thrust is transmitted to the hull.

A single collar thrust bearing operates on the hydrodynamic lubrication principle.

Working Principle

• As the shaft rotates, lubricating oil is drawn between the rotating thrust collar and the stationary thrust pads.
• Due to the tilting action of the pads, a wedge-shaped oil film is formed.
• This oil wedge develops sufficient pressure to support the axial thrust load.
• The rotating collar and stationary pads remain separated by the pressurized oil film, preventing metal-to-metal contact.
• Thus, the propeller thrust is transmitted smoothly with minimum wear.

Thrust Transmission Path

Propeller → Shaft → Thrust Collar → Thrust Pads → Bearing Housing → Ship's Hull

The essential clearances of a thrust bearing are checked mainly by the following methods:

1. Feeler Gauge Method

This is the most common method used to measure the thrust pad clearance.

Procedure

• Remove the stopper.
• Remove all lubricating oil (L.O.) pipes and connections on the forward side.
• Push the thrust pad towards the thrust collar using a small screwdriver or crowbar.
• Insert a feeler gauge into the wear groove provided on the pad (normally 1 mm deep).

Interpretation

• If a 0.1 mm feeler gauge cannot be inserted, it indicates that the pad has worn by 0.9 mm.
• This shows that the thrust pad is excessively worn and should be replaced.

2. Shaft Axial Movement Method

Procedure

• Move the shaft axially using a hydraulic jack.
• Mount a dial gauge on the shaft.
• Continue moving the shaft until the thrust pads make contact with the thrust collar.
• The total movement indicated on the dial gauge represents the thrust clearance.

Significance of Thrust Clearance

• If the clearance is too large:
  • Proper hydrodynamic lubrication cannot be achieved, resulting in poor load carrying capacity.
• If the clearance is too small:
  • Lubricating oil flow becomes restricted.
  • This causes overheating and may lead to wiping of the white metal lining of the thrust pads.

On modern four-stroke engines the topmost part of the cylinder liner, in the region of the top ring groove and at the top dead centre where rings reverse direction, is fitted with a "protection" (or chrome/flame) ring, often a separate steel or specially hardened ring pressed into a recess at the top of the liner. Its function is to protect the cylinder bore at the point of maximum thermal and mechanical loading. At TDC the rings are momentarily stationary and the gas pressure is highest, so the top ring cannot wipe away combustion products entering the clearance above it, leading to rapid localised wear, ring groove hammering and carbon build-up. The protection ring provides a hardened, corrosion-resistant wearing surface which preserves the integrity of the liner throat, reduces vibration and fretting of the liner top, prevents erosion by the flame, and prevents the top liner material from being worn away. It also gives a consistent sealing surface for the top compression ring, improving oil control and reducing the risk of bore polishing.

In a conventional four-stroke engine the fuel injection pump is driven by a cam (or, on some, by the low-speed camshaft) with a spring-return plunger and fixed injection timing set by cam profile. In modern medium-speed four-stroke engines the fuel injection drive has been modified by the introduction of electronic unit injectors and/or the replacement of the mechanical camshaft drive by electronically controlled individual pumps. There are two broad trends: (1) Common rail injection, where fuel is stored at high pressure and each cylinder has an injector opened by a solenoid or hydraulic valve, with timing, duration and (on some) pressure controlled electronically; (2) Camshaft-less, electronically controlled injection (as on some four-strokes) where each unit has a high-pressure pump and the injection timing is controlled by an electronic control unit rather than by cams. The modification removes the need for precise cam timing, allows variable injection timing (VIT) and flexible control of start/end of injection to improve combustion and reduce emissions, and reduces wear of driving gear.

In a multi-hole injection nozzle the holes are arranged so that the fuel jets from adjacent holes enter the combustion chamber at slightly different angles. Staggering refers to arranging the nozzle tip holes so that the spray from each hole does not impinge symmetrically on the piston bowl lip or collide with the spray from the neighbouring hole, and to give a uniform distribution around the bowl. The staggered (offset) layout also means that the spray axes are not all radial/equal, which, together with the swirl, ensures that the fuel is spread evenly and no two jets coincide, improving atomization and mixing, reducing wall wetting on the piston crown and liner, and giving more even heat release and lower smoke and emissions.

Swirl is a rotary motion of the air charge about the cylinder axis. It is generated primarily by a helically/tangentially vaned inlet port which imparts angular momentum to the incoming air during the suction stroke. Swirl gives high relative velocity between fuel spray and air, improving mixing, shortening the ignition delay, giving faster and more complete combustion and a more even temperature field, reducing smoke and increasing efficiency.

Squish is the radial inward movement of the air from the outer edge into the piston-bowl at the end of the compression stroke, generated by the piston crown design - when the piston approaches TDC the air in the squish band (the narrow gap between the piston crown edge and the cylinder head) is forced radially into the bowl. Squish adds turbulence close to the fuel injection point, promoting mixture formation and combustion, and helps delay knock by mixing the burning and unburned gases. Both swirl and squish together produce a turbulent flow which promotes cleaner, faster combustion.

(a) Forces due to combustion load

• Combustion forces exert alternating tension and compression loads on the engine structure.
• These forces act on the piston, crankshaft, and bedplate.
• The gas load is transmitted from the cylinder head through tie bolts to the bedplate.
• The bedplate then transfers the load to the ship's hull via holding-down bolts and resin chocks.

Structural Components to Withstand Forces:

Bedplate: Made of mild steel (MS) plates and steel castings, it is assembled and welded to ensure high longitudinal and transverse strength. It also resists twisting forces.

Cross Girders: Cast steel cross girders house the main bearings and provide additional transverse strength and resistance to twisting.

Chocks: Installed between the bedplate and the ship's double-bottom tank top, these absorb shocks and cyclic stresses, ensuring smooth load distribution.

The angular motion of the connecting rod (con-rod) during the engine cycle creates guide forces. At the top and bottom of the stroke, the con-rod is aligned with the crankshaft, but at other positions, it is inclined, generating horizontal forces. These horizontal forces are absorbed by the guides in two-stroke engines, creating a guide force moment.

• In two-stroke engines, the horizontal forces are absorbed by the guide shoes and transmitted through the engine structure.
• In four-stroke engines, the thrust on the gudgeon pin is absorbed by the piston skirt and transmitted through the cylinder liner to the engine body.

In order to counteract the possible impact from guide force moments, it is recommended to install a set of TOP BRACES between the upper gallery of the engine and hull structure. These braces increase the natural frequency of the vibration system to such an extent that resonance occurs above the running range of engine speed, and guide force moment seems harmless.

(c) Inertia Forces

The inertia forces are categorised into those acting on rotating masses and reciprocating masses:

1. Inertia Forces on Rotating Masses:
   • These forces have a constant magnitude when the engine speed is steady, but their direction changes with rotation.
2. Inertia Forces on Reciprocating Masses:
   • These forces depend on the actual position of the piston, even if the engine speed remains constant.

Unbalanced inertia forces, originating from the rotating and reciprocating masses of the engine, create external moments that are unbalanced. This requires effective countermeasures to mitigate their impact on the hull and engine operation.

Resonance can occur when these external moments coincide with the natural frequency of the system within the engine's operating speed range.

• First-Order Moment: One cycle per revolution.
• Second-Order Moment: Two cycles per revolution.

These forces are managed through flywheel design to smooth out rotational speed fluctuations and the addition of counterweights to balance the drive chain, reducing vibrations and ensuring stable operation.

(a) Reason for piston ring breakage:

• Excessive wear in the cylinder liner leads to increased piston ring movement, both radially and axially. This fluctuating motion can cause tilting and eventual breakage of the rings.
• If the piston ring does not exert sufficient pressure on the liner, gas pressure can penetrate between the ring and liner, collapsing the ring into the groove and causing breakage.
• Ridge formation near scavenge pockets can create stress concentrations at the piston ring's radial edge, promoting fracture.
• Jamming or sticking of rings caused by excessive carbon deposits, often due to improper combustion or inadequate cleaning during maintenance.
• Excessive wear in the piston ring grooves causes the rings to impact the groove walls during operation, leading to hammering and eventual breakage.
• Inadequate cylinder lubrication results in overheating and increased friction, weakening the rings and causing breakage.
• Acidic corrosion and high-temperature corrosion weaken the ring material, predisposing them to fracture.
• Excessive engine loading can cause the rings to deform beyond their elastic limit, leading to collapse and breakage.
• Using low-quality or non-manufacturer-specified rings compromises material strength and durability, increasing the risk of breakage.
• Improper installation during ring renewal can lead to misalignment, increased stress, and premature failure.

Minimizing Breakage through Maintenance and Engine Operation:

Maintenance practice:

• Perform routine inspections and overhauls of pistons, piston rings, and cylinder liners as per the PMS schedule.
• During overhauls, ensure piston rings and grooves are thoroughly cleaned, and all necessary clearances are measured to verify proper fit.
• Reuse piston rings only if measurements indicate they are within the safe operational limits until the next overhaul.
• Regularly maintain the fuel injection systems to prevent improper combustion and minimise stress on piston rings.
• Ensure that piston rings and liners are free of marks, scratches, or other signs of wear during scavenge inspections.
• During overhaul, install piston rings with proper tools and techniques, ensuring free movement of rings in their grooves.
• A proper running-in procedure after installing new pistons and rings helps to ensure correct seating and minimises initial wear.

Engine Operation:

• Maintaining adequate cylinder oil lubrication minimises friction and heat generation.
• Maintain appropriate cooling of the cylinder liner and piston to avoid thermal stresses.
• Use properly treated fuel oil and ensure correct operation of fuel pumps, injectors, and Variable Injection Timing (VIT) systems.
• Maintaining correct combustion parameters minimises improper combustion and reduces carbon deposits.
• Keep air filters clean to avoid the ingress of dust and abrasive particles into the engine.
• Avoid overloading the engine, which can stress the piston rings and cause failure.

(b) Consequences of Broken or worn-out piston rings.

• Low compression pressure, Pmax & power developed.
• Blowpast, increase in scavenge temperature and cause scavenge fire.
• Rise in exhaust temperature.
• Scuffing of liner and increase in wear rate.
• Increased SFOC.
• Fouling of turbocharger due to improper combustion.
• Fouling of EGE and can cause EGE fire.
• Damage to cylinder liner due to blowpast.
• Loss of cylinder lubrication.

The following precautions must be taken while operating an engine with broken or severely worn piston rings:

• Isolate the affected unit as excessive blowpast may cause scavenge fire.
• Monitor the scavenge temperature.
• Run the engine at low load till necessary replacement is carried out.

Static balance: the balancing of the rotating masses so that the resultant of all centrifugal forces on the crankshaft, when the shaft is not rotating, is zero, i.e. the shaft has no heavy point. Static imbalance is detected when the shaft, supported on knife edges or rollers, always rolls until the heavy side is at the bottom. It is corrected by adding or removing mass on the appropriate crank webs (counterweights) or by drilling/grinding material from the heavy side so the centre of mass lies on the axis of rotation.

Dynamic balance: even a statically balanced shaft can have a couple acting because the unbalanced masses lie in different planes along the shaft, so that when rotating a rocking couple is set up. Dynamic balancing is performed on a balancing machine where the shaft is rotated and the vibrations at the two ends are measured; correction masses are added or removed at calculated positions (usually on the crank webs) in the two end planes so that both the resultant force and the resultant couple are zero. In practice counterweights are attached to, or cast integrally with, the crank webs, and for large shafts the balance is checked during manufacture and correction machining is done on the webs.

The primary reciprocating force arises from the acceleration of the reciprocating masses (piston, rings, small-end part of connecting rod) and varies in magnitude once every revolution (at engine speed, first order). For a multi-cylinder engine, by arranging the firing order and crank positions, the primary forces of different cylinders can be made to cancel to a large extent. The methods used are:

1. Balance (counterweight) on the crank web approximately equal to the fictionally rotating part of the reciprocating mass (a rotating balance weight of about half the reciprocating mass placed on the opposite side of the crank throws) which balances part of the primary force.
2. Arrangement of crank throws so that the out-of-balance primary forces act at different phases and cancel each other in symmetrical multi-cylinder engines (e.g. six-cylinder inline engines), including using an even number of cylinders spaced evenly.
3. The use of balance shafts (counter-rotating shaft pairs) running at engine speed, carrying balance weights, which generate a downward force to cancel the primary force.

They are not completely successful because: (1) the primary force is a force (not a pure couple) that cannot be wholly eliminated for an inline engine unless balance shafts are fitted, and these themselves add weight and complexity; (2) the balance weights cancel only the rotating part of the primary force; the reciprocating part acts along the cylinder axis, so the horizontal component passes through the crank and the vertical component cannot be cancelled by webs alone; (3) remaining combined forces leave a residual imbalance which, though small, is not zero; (4) the counterweight approach only reduces the force, and the exact phase relationship varies with speed.

Secondary imbalance: the secondary reciprocating force varies at twice engine speed (second order) and arises from the finite length of the connecting rod causing the piston acceleration to have a second harmonic. Additions to control implantance are:

1. Counter-rotating balance shafts (Lanchester or reciprocating balance shafts) running at engine speed for primary and at twice engine speed for secondary, with weights arranged to generate cancelling forces. These shafts are gear- or chain-driven from the crankshaft.
2. Addition of counterweights of increased size to the crank webs.
3. For secondary forces, two shafts rotating in opposite senses at twice crankshaft speed with parallel axes are used so their horizontal components cancel and vertical components add to balance the secondary force.
4. Use of a flywheel of correct mass moment of inertia and a torsional damper/detuner to damp out the torque variations and torsional vibrations that such vibration produces.

These additions reduce frame vibration, main-bearing loading, and forces transmitted to the ship's structure, preventing excessive vibration and noise.

(a) Cast Iron Welding is a Challenging Task – Reasons

Welding cast iron is considered a difficult and highly specialized operation because its metallurgical characteristics react unfavorably to the intense heat involved in welding. The following factors explain the challenges:

1. High Carbon Content

• Cast iron contains approximately 2% to 4% carbon, which is nearly ten times higher than that of mild steel. During welding, the base metal is subjected to rapid heating and cooling cycles. This high carbon content promotes the formation of extremely hard and brittle microstructures such as martensite or white iron in the heat-affected zone (HAZ). These structures lack toughness and are highly prone to cracking.

2. Inherent Brittleness

• Unlike steel, cast iron has very low ductility. It cannot deform plastically to relieve stresses created by thermal expansion and contraction during welding. Instead of stretching or yielding under stress, it tends to crack suddenly, especially near the weld area.

3. Thermal Shock and Rapid Cooling

• The welding arc produces intense localized heat, creating steep temperature gradients between the weld zone and the surrounding metal. When cooling occurs rapidly, high internal stresses are generated. These stresses frequently result in immediate or delayed cracking in the HAZ.

4. Presence of Graphite in Gray Cast Iron

• Gray cast iron contains graphite flakes distributed within the iron matrix. These flakes act as internal stress concentrators and weaken the structure. During welding, graphite may dissolve into the weld pool, causing embrittlement and reducing the strength and integrity of the joint.

5. Porosity Due to Oil and Grease Contamination

• Engine casings made of cast iron are often porous and, over years of service, absorb oil and grease. When welding heat is applied, these trapped contaminants vaporize and form gas pockets within the molten weld metal. This leads to porosity, reducing weld strength and reliability.

6. Limited Weldability of Certain Types

• While gray cast iron can be welded with strict preheating and controlled cooling procedures, white cast iron is generally considered unweldable because of its extreme hardness and brittleness. It cannot tolerate the stresses introduced by welding.

Because of all these metallurgical and structural limitations, welding cast iron requires careful temperature control, suitable filler materials, and controlled cooling—yet it still carries a high risk of failure.

(b) Alternative Repair Methods Used by Engine Makers for Cast Iron Casings

Due to the significant risks associated with welding cast iron, engine manufacturers often prefer mechanical or “cold repair” techniques. These methods avoid excessive heat and preserve the original structure and alignment of the engine casing.

1. Metal Stitching and Locking (Metalock Method)

• This is the most widely used industrial repair method for cracked engine casings. Holes are drilled along the length of the crack, and specially designed high-tensile metal “stitching pins” and “locks” are inserted. These components mechanically pull the cracked sections together and restore structural strength. Since no heat is applied, the original metallurgy and alignment of the casing remain unaffected.

2. Braze Welding

• In this method, a filler material such as bronze or nickel alloy—having a lower melting point than cast iron—is used. The base metal is not melted; instead, the filler bonds to it. This significantly reduces thermal stress and minimizes the risk of additional cracking compared to conventional fusion welding.

3. Studding Method

• For major fractures, holes are drilled and tapped along the cracked surfaces. Steel studs are screwed into these holes to provide reinforcement. Weld metal or filler material is then deposited over the studs to secure and anchor the repair. The studs act as mechanical reinforcement, improving the strength of the repaired area.

4. Epoxy Bonding (Cold Weld Compounds)

• For non-structural cracks or minor leakages, metal-filled epoxy compounds can be applied. These adhesives provide a watertight and heat-resistant seal without introducing thermal stresses. This method is suitable for temporary repairs or low-load applications.

5. Patching with Insert Replacement

• In cases of severe damage, such as when a connecting rod breaks through the casing, the damaged section can be completely machined out. A new cast iron insert or patch piece is then fitted into the prepared opening and secured using metal stitching or pinning methods. This restores both strength and dimensional accuracy.

The bedplate's longitudinal strength and rigidity are crucial because they ensure the engine's structural integrity against bending forces. While the ship's hull provides overall support, it can flex and deform. The bedplate must resist these independent bending and torsional loads, which are particularly pronounced in longer, multi-cylinder engines. The firing sequence of each cylinder occurs at a different time, causing continuous, uneven forces along the engine's length. This dynamic loading creates significant longitudinal bending and twisting moments. A strong, rigid bedplate prevents misalignment of the crankshaft and main bearings, which could lead to bearing damage, increased wear, and ultimately, catastrophic engine failure.

The combustion loads generated in the cylinder are transferred to the bedplate through a specific load-path. The process is as follows:

1. Cylinder Head & Piston: During combustion, high-pressure gas forces act on the underside of the cylinder head and the top of the piston crown. These forces are equal and opposite.
2. Tie Rods: The upward force on the cylinder head is transferred to the bedplate through a series of tie rods that run the full height of the engine. These rods are hydraulically tightened to pre-stress the engine structure, holding the cylinder head, entablature (the main engine frame), and bedplate tightly together.
3. Piston & Connecting Rod: The downward force on the piston is transmitted through the piston rod and connecting rod to the crankshaft.
4. Bedplate: The crankshaft and its main bearings are housed within the bedplate's transverse girders. The downward combustion force is ultimately absorbed by the bedplate as the crankshaft pushes down on the main bearings.

The combined effect of the tie rods pulling up and the crankshaft pushing down means the combustion load is fully contained and absorbed by the bedplate, which is designed to withstand and distribute these massive forces.

A common large diesel arrangement uses a jerk-type fuel pump with a variable injection timing mechanism. The pump has a plunger operated by a cam, but the point at which pressure starts to build can be altered by changing the effective spill/closing point.

Main parts:

• Fuel cam and roller tappet give the plunger its upward stroke.
• Plunger and barrel pressurise fuel.
• Suction/spill ports allow filling and spilling.
• Delivery valve prevents backflow and gives sharp pressure collapse.
• Fuel rack rotates the plunger to vary effective stroke and fuel quantity.
• VIT linkage or servo changes the relative timing between plunger movement and port closure, giving earlier or later injection.

The exact design differs between engine makers. Some alter the pump barrel position, some alter the roller guide or cam follower position, and modern engines may use an electronically controlled spill valve. The principle is the same: change when high pressure begins without upsetting the required fuel quantity control.

Changing injection timing

Injection begins when the pump plunger covers the spill/suction port and fuel pressure rises enough to open the injector needle.

With VIT, the mechanism changes the point at which this happens:

• If the port is covered earlier in the plunger stroke, pressure builds earlier, so injection is advanced.
• If the port is covered later in the plunger stroke, pressure builds later, so injection is retarded.
• This may be done by moving the pump barrel, changing the roller guide position, or controlling a spill valve electronically/hydraulically.

Thus VIT varies the start of injection according to engine load to obtain correct peak pressure and fuel economy.

Changing fuel quantity

Fuel quantity is changed by rotating the pump plunger with the fuel rack.

• At low fuel setting, the plunger helix uncovers the spill port early.
• Pressure collapses early, so injection ends early and less fuel is delivered.
• At high fuel setting, the helix uncovers the spill port later.
• Injection continues for longer, so more fuel is delivered.

So, in a conventional jerk pump, VIT mainly changes the start of injection, while the fuel rack changes the end of injection and therefore the quantity delivered.

• Accumulated dirt or deposits on turbine blades or compressor impellers can cause an imbalance in the rotating assembly.
• Turbine blades or lashing wires may be damaged due to wear, fatigue, or foreign object impact.
• A loose or improperly secured blower impeller can create uneven rotation and vibrations.
• A bent or distorted shaft may result from overloading, misalignment, or bearing failure.
• Bearing wear or misalignment can lead to irregular shaft rotation and vibrations.
• Entry of foreign objects (e.g., debris, soot) into the turbine or blower side can disrupt balance.
• Loose or damaged foundation bolts may allow movement of the turbocharger during operation.

1. Perform regular dry or water washing of the compressor and turbine blades as per the manufacturer's recommendations.
2. Regularly inspect turbine blades and lashing wires for wear or damage and renew them if required.
3. Ensure foundation bolts are properly tightened and undamaged.
4. Replace bearings at intervals specified in the Planned Maintenance System (PMS), regardless of their apparent condition.
5. Maintain proper lubrication and renew the lubricating oil as per the schedule.
6. Ensure injectors and fuel pumps are maintained to provide efficient combustion and minimize deposits.
7. Follow the PMS for scheduled inspections, cleaning, and overhauling of the turbocharger system.

1. For taking the Turbocharger out of operation, the rotor must be locked to prevent rotation.

• For constant pressure turbochargers, locking the blower side is sufficient as exhaust gas pressure has minimal impact on turbine blades.
• For pulse-type turbochargers, both the turbine and blower sides must be locked.

2. If required, bypass the exhaust gas inlet by installing a specially designed bypass pipe as provided by the manufacturer.

3. If exhaust gases are allowed to flow through the locked turbine, ensure air circulates through the blower to prevent overheating of the impeller:

• If the auxiliary blower takes suction through the turbocharger, this condition is automatically satisfied.
• If not, create a small hole (as per the manufacturer’s recommendation) in the blanking plate on the air outlet to allow airflow.

4. Cooling water flow should only be stopped if significant leakage endangers engine operation.

5. Ensure the turbocharger bearing chambers are drained of lubrication if the turbocharger is out of operation.

1. The engine can only operate at reduced load as per the manufacturer’s instructions due to insufficient air supply.
2. A shortage of air leads to incomplete combustion, resulting in:
   • High Exhaust Gas Temperatures
   • Black Smoke
   • Carbon Deposits
3. Sudden speed changes during manoeuvring can result in uneven thermal expansion, leading to thermal shock in engine components.
4. Reduced air availability increases fuel consumption per unit of power (Increased SFOC).
5. Heavy carbon deposits on pistons may increase the wear rate of liners and piston rings.
6. Poor combustion produces higher levels of air pollutants such as soot and unburnt hydrocarbons.

(b) Common forms of seal failure in a stern tube

1. Loss of elasticity in seal material – Nitrile rubber seals may lose their elastic properties over time, reducing their sealing effectiveness.
2. Surface damage to chrome liner – Grooving or scoring of the chrome liner can impair sealing surfaces, leading to leakage.
3. Excessive shaft vibration – Heavy vibration of the propeller shaft can cause uneven wear and seal deformation.
4. Insufficient cooling – Inadequate cooling can cause rubber sealing elements to harden and eventually fail.
5. Exceeding running hour limits – Operating beyond the manufacturer’s recommended service life increases the risk of seal failure.
6. Deterioration of oil quality – Contaminated or degraded oil reduces lubrication and protection, accelerating seal wear.
7. Incorrect header tank level adjustment – Failure to adjust the header tank according to vessel draft can cause oil loss, leading to inadequate lubrication and seal damage.

(c) Restricting oil loss due to seal failure whilst on passage

1. Use of high-viscosity oil – Recharge the system with a thicker oil to reduce leakage rate through damaged seals.
2. Temporary oil supply arrangement –
   • Disconnect the existing oil supply line.
   • Connect a 45-gallon drum supported by a block and tackle arrangement.
   • Adjust the drum height to vary the oil head pressure, matching it to the surrounding water pressure and minimizing leakage.
3. Fresh water introduction – Supply fresh water to the gravity tank to emulsify with any leaked oil. The resulting emulsion helps coagulate around the damaged seal area while the oil is circulated to maintain lubrication and sealing.

The aft stern tube bearing is designed to distribute the propeller load uniformly and prevent excessive pressure from being concentrated at one location. The main design features are:

1. Slope Boring (Taper Boring) (Most Important Exam Point)

The aft bearing is machined with a slight taper (slope bore) to match the natural deflection of the propeller shaft caused by the weight of the propeller.

Reason:

This ensures that the load is distributed over the entire length of the bearing instead of being concentrated at the aft end, thereby reducing wear and increasing bearing life.

2. Long Bearing Length

The aft bearing is made longer than the forward bearing, providing a larger contact area between the shaft and the bearing.

Reason:

The increased bearing area reduces the unit bearing pressure and distributes the load more evenly.

3. Large Bearing Surface Area

The bearing is provided with a large diameter and a long white-metal or composite bearing surface.

Reason:

The larger bearing surface spreads the propeller load over a greater area, reducing localised stresses and wear.

4. Proper Bearing Clearance and Hydrodynamic Oil Film

The bearing is designed with the correct clearance to maintain a continuous hydrodynamic oil film between the shaft and the bearing during operation.

Reason:

The oil film prevents metal-to-metal contact and supports the shaft hydraulically, ensuring uniform load distribution and reducing friction and wear.

The axial clearance (oil clearance) in a Mitchell-type main thrust bearing is the total axial movement of the thrust shaft between the ahead and astern thrust pads. Maintaining the correct axial clearance is essential for the following reasons:

1. Formation of the Hydrodynamic Oil Wedge
   • The correct clearance allows the thrust pads to tilt freely on their pivots or ridges.
   • This tilting action draws lubricating oil between the rotating thrust collar and the stationary white-metal pads, forming a pressurized wedge-shaped hydrodynamic oil film.
   • The oil film prevents direct metal-to-metal contact and ensures smooth operation.
2. Prevention of Overheating and Seizure
   • If the clearance is too small, the oil flow between the collar and pads is restricted.
   • The resulting thin oil film produces excessive friction and heat, which can cause wiping (melting or smearing) of the white-metal lining and may eventually lead to bearing seizure.
3. Accommodation of Thermal Expansion
   • During operation, the engine and shafting expand axially due to temperature rise.
   • The axial clearance provides sufficient space to accommodate this thermal expansion without imposing excessive compressive loads on the crankshaft, thrust bearing, or engine bedplate.
4. Control of Crankshaft Axial Movement
   • Excessive clearance caused by wear allows the shafting to move too far in the axial direction.
   • This shifts the crankshaft from its designed position, which may lead to damage to the crank webs, main bearings, and misalignment of connected equipment such as the turning gear.
5. Reduction of Axial Vibrations
   • The correct clearance helps absorb and dampen axial vibrations transmitted from the propeller through the shafting, thereby protecting the main propulsion machinery.

Axial clearance can be measured by the following onboard methods:

1. Feeler Gauge Method (Static)

• Ensure the thrust collar is pressed firmly against one set of thrust pads (ahead or astern).
• Insert a long feeler gauge between the thrust collar and the opposite set of thrust pads.
• The thickness of the feeler gauge that fits snugly without forcing represents the total axial clearance.

2. Dial Gauge Method (Static)

• Mount a dial indicator securely on the thrust block casing using a magnetic base.
• Position the dial gauge tip against a machined surface of the thrust shaft and set the indicator to zero.
• Using a hydraulic jack or suitable levering arrangement, move the shaft fully forward until it contacts the ahead thrust pads and note the reading.
• Move the shaft fully aft until it contacts the astern thrust pads.
• The total movement indicated on the dial gauge represents the total axial clearance.

Removal Procedure

1. Safety and Isolation
   • Stop and isolate the main engine.
   • Engage the turning gear and lock out the starting system.
   • Isolate the lubricating oil system and display appropriate warning notices.
2. Gain Access
   • Remove the thrust bearing top cover using suitable lifting equipment such as the engine room overhead crane.
3. Shift the Shaft
   • Move the thrust shaft axially towards the opposite side of the pads to be removed (for example, move the shaft forward to remove the astern pads), creating sufficient clearance for removal.
4. Remove the Thrust Pads
   • The pads are generally mounted in a carrier ring or retaining ring.
   • Rotate the pad ring or individual pads upward using the provided eyebolts or special lifting tools.
   • Withdraw each thrust pad carefully, one at a time, from the side of the shaft.
   • Mark and keep each pad in its original position (Ahead/Astern and Port/Starboard) to ensure correct reassembly.

Inspection Points

During inspection, particular attention should be given to the following:

1. Condition of the White-Metal Lining
   • Check for scoring, scratches, pitting, erosion, overheating, wiping (melting or smearing), and signs of metal-to-metal contact.
2. Cracks and Delamination
   • Inspect for hairline cracks, fatigue cracks, crazing, or separation of the white-metal lining from the steel or bronze backing.
   • If necessary, carry out a dye penetrant test to detect fine cracks or bonding failure.
3. Pivot or Tilting Surface
   • Examine the pivot button or ridge on the back of the pad for wear or damage.
   • Excessive wear at the pivot prevents proper pad tilting and affects the formation of the hydrodynamic oil wedge.
4. Oil Grooves and Chamfers
   • Ensure that the oil grooves, leading-edge chamfers, and oil passages are clean and free from carbon deposits, sludge, or metal particles that could restrict oil flow and impair lubrication.

At the heart of the system is the exhaust gas boiler situated in the uptake. This crucial component generates superheated steam by harnessing the heat from the main engine's exhaust gas. This superheated steam serves as the primary energy source for the downstream steam turbine. The main engine exhaust gas, including outlets from turbochargers and the gas turbine, is directed into the exhaust gas boiler, facilitating efficient heat transfer.

The gas turbine, clutched and geared to a power turbine, forms an integral part of the system. This power turbine is directly linked to a generator, contributing to electrical power generation. Simultaneously, a steam turbine is connected to the gas turbine, providing an additional means of harnessing energy from the superheated steam generated in the exhaust gas boiler.

A control valve regulates the excess steam flow, directing it to a dump condenser. The system is further complemented by seawater-cooled vacuum condensers, which receive the exhaust from the steam turbine. This comprehensive waste heat recovery system ensures that the thermal energy in the main engine exhaust gas is efficiently converted into electrical power through the combined operation of gas and steam turbines.

Critical to the system's efficiency is the generation of sufficient steam by the exhaust gas boiler, a key requirement met at around 41% engine load. The generator is then synchronized to the main engine switchboard by the power management system, which not only oversees synchronization but also monitors and manages the generated power. The power management system plays a crucial role in the dynamic control of the overall electrical power distribution.

The operation also incorporates the start and stop functionality of auxiliary diesel generators, driven by the power management system to ensure a balance in the electrical power supply. Meanwhile, the main engine control system takes charge of the power (gas) turbine, initiating start and stop procedures and adjusting exhaust gas bypass when the gas turbine is not in operation. This careful design ensures that the gas turbine comes into play at around 53% engine load, contributing additional power through the steam turbine when needed. Importantly, the system is designed to cut out the power turbine when the engine load drops to 47% for a specified duration.

To maintain optimal conditions, especially as the gas turbine reduces available exhaust gas for the main engine turbochargers, the control system monitors scavenge pressure and makes necessary adjustments to the gas flow. This intricate operational scheme ensures not only the efficient generation of electrical power but also the safety and stability of the waste heat recovery system in conjunction with the main engine's operation.

The inert gas system is provided to substitute any remaining natural gas in the pipeline or system with inert gas (nitrogen). This is required when any maintenance work is carried out in the system. This is a safety process that ensures that the natural gas cannot leak into the surrounding areas with potential risks.

Gas Valve unit for your reference:

DEF is a non-hazardous solution, which is 32.5% urea and 67.5% de-ionised water.

The SCR system consists of a reactor, catalyst elements, soot blower, sensors, air reservoir, mixing devices, dosing unit urea injection nozzle, urea pump and safety control system.

When the Rail Valves are energised for injection by the Valve Driver Module, oil from the Control Oil Rail opens the Injection Control Valves. The fuel injectors are pressurised and fuel oil pressure behind a Fuel Quantity Piston in the Volumetric Control Unit maintains this pressure at the injectors. As the Piston moves to the left a feedback signal is sent to the Cylinder Control Module

When the desired amount of fuel has been injected the Valve Driver Module energies the solenoids which move the Rail valves back to the return position. The Injection Control Valves interrupt the supply to the injectors, and the increase in pressure on the LH of the fuel Quantity Piston moves it back to its starting position.

The starting air system of the RT-flex engine is similar to that of a standard RTA engine except for the control of the cylinder starting air valves which is incorporated in the WECS rather than a starting air distributor. Starting air is supplied to the engine starting air manifold from the starting air receivers via the starting air shut-off valve. Individual cylinders are supplied with starting air via branch pipes which have flame arresters (Figure below).

The cylinder starting valve is operated by pilot air and the pilot air valve is controlled electrically by the cylinder control module. The starting pilot air valve is opened and closed directly by the cylinder control module (CCM) once every revolution at defined crank angles during the starting period.

When the engine has started the starting system is shut down. The opening and closing of the starting pilot valves is controlled by the corresponding CYL-EU, depending on the crank angle. The nominal opening angle is Zero degree (0°) and the closing angle is 110 degree (110°).

The automatic main starting valve is controlled by the COM-EU. Each MCM has its own start control valve. For slow turning the automatic valve is controlled and the starting pilot valves are pulsed via the CYL-EUs to reach the desired slow turning speed

Shown below is the arrangement of a chain drive, which is used to transmit power from the crankshaft to the camshaft.

• It consists of chain sprockets mounted on the crankshaft & camshaft. There can be two or more chains.
• A chain-tightening arrangement is provided, as shown in the fig.
• The chain is guided by the guide bars, which has rubber shock-absorbing pads
• Flyweights are provided as they are the moment compensators.
• Oil spray nozzles are used to lubricate the chain & the wheels.

In the event of a chain link failure during engine operation, the following steps should be carried out:

• Turn the chain until the damaged link is positioned on the longest free end side of the chain, where it is easily accessible.
• Release tension on the chain to facilitate repair.
• Wrap a thin wire around the chain, a short distance from the damaged link, and pull the wire taut using a chain block. This ensures that the chain remains stable during repair.

Remove the Faulty Link:

• Chisel or grind off the riveted metal on the pin ends of the damaged link.
• Use a chain bursting tool:
  • Place the tool over the smallest part of the chain link.
  • Align the dismantling screws precisely over the ground pin ends.
  • Tighten the dismantling screws alternately to push the pins out of the link.
• Remove the damaged link plate and pin.

Install the Replacement Link:

• Replace the damaged plate and pin with a new spare.
• Rivet the ends of the new pin securely.
• If a second chain is present, replace the corresponding link in the other chain to ensure uniform wear and performance.

After the repair, adjust the chain tension to the correct setting.

Reasons for failure:

• Cyclic stresses resulting in fatigue failure cracks.
• Excessive wear due to improper lubrication.
• Overheating due to improper lubrication.

Setting the chain to the correct degree of tension initially:

• Turn the engine to bring the slack part of the chain on the same side as the lighter wheel.
• Place the spring & spring carrier in place. Tighten Nut 'C' till the required compression of spring is achieved (softly touching).
• Tighten nut 'B' till it touches the shaft (softly touching).
• Tighten nut 'C' further again till the shaft carrying carrier is up against the star (further compression will not affect the chain tension).
• The lock nuts A & D are then tightened & locking washers are bent in place.

Chain tightening:

This stress is cyclic, meaning the crankshaft twists and untwists along its length. In direct-drive engines, torsional vibration can be exacerbated by an unbalanced engine cylinder or propeller shaft, potentially caused by a damaged propeller.

An increase in torsional vibrations results in higher torsional stress, which adds to the existing stress levels. This increase in stress can lead to the generation and growth of cracks in high-stress areas of the crankshaft. If left unaddressed for an extended period, this condition can lead to the crankshaft breaking.

If the engine is overloaded, it exerts a high amount of stress on the crankshaft, leading to cracks and eventual failure. To minimize this risk:

1. Keep the engine cylinders balanced to ensure even loading on the crankshaft.
2. Operate the engine within the limits prescribed by the manufacturer, referencing performance results such as from sea trials.
3. Regularly check engine performance to analyze the engine's condition and ensure it remains within operational limits.

1. The engine's rotational speed might have coincidentally passed through a critical speed, where the excitation frequency matches a natural frequency of the crankshaft system. This resonance amplifies the vibrations, triggering the alarm.
2. If one or more cylinders are unbalanced (e.g., due to improper combustion or issues with the fuel system), uneven forces can generate excessive torsional vibration.
3. An imbalance in the engine's cylinders could generate irregular firing torques, leading to increased torsional vibrations and activating the alarm system. This could be due to unforeseen internal component failure or a previously undetected manufacturing defect.
4. Slight misalignment in the crankshaft's main bearings could induce high bending stresses and increase torsional vibrations
5. Maneuvering in shallow water can increase propeller load and generate additional cyclic stresses on the crankshaft, resulting in torsional vibration.
6. In rough seas, cyclic loading on the propeller shaft caused by wave action can transmit additional torsional stresses to the crankshaft, activating the alarm.

A two-stroke engine stuffing box (gland box) is fitted around the piston rod where the rod passes through the diaphragm or the bottom of the scavenge air space, to separate the crankcase/oil space from the scavenge space and to prevent:

• blow-by of scavenge air into the crankcase;
• lubricating oil from the crankcase splashing up into the scavenge space;
• gas and oil from contaminating each other.

Principal components of a stuffing box:

1. Outer and inner brass (bronze) box housings/two halves forming the gland body, bolted together on the rod.
2. Upper (scavenge-side) sealing rings: typically two to three rings (often metallic, such as hardened steel or bronze) which prevent scavenge air passing down; these may be arranged as one set above and one below.
3. Lower (crankcase-side) rings that prevent oil passing up; there are usually a series of sealing rings in the lower part (oil scraper rings).
4. The space between the upper and lower ring sets is connected by drain passages to scavenge space and crankcase space, with a drain to a spill tank/drain tray so that oil and condensate can drain away.
5. In some designs the box has a small groove fed with oil (lubricating the rod) or relies on the low-level oil splash.

The piston rod sealing rings are split and held against the rod by their own elasticity or by a spring, and are located in grooves. The rings are numbered/oriented so that the split remains tight; the ring gaps are staggered.

Procedure to overhaul a stuffing box without removing the piston:

1. Isolate and secure the engine (stop engine, turn gear engaged or the unit locked), make it impossible to start, and isolate starting air and turning gear as per permit-to-work. Place warning notices on the bridge and in the ECR. Never rely on automatic systems alone.
2. Remove the crankcase/scavenge space door(s) in the vicinity of the piston rod.
3. Purge the scavenge space of any gas which could be flammable; ensure adequate ventilation and gas monitor.
4. Drain the oil from the scavenge space drain and any oil in the gland area, collecting spill in a drip tray. Keep the work area clean, wear oil and hazard protection, and have rags/fire precautions.
5. Half-turn or lock the piston rod in a defined position so the box is accessible. Loosen and remove the two halves (bolts) of the stuffing box housing. Care: the box is heavy; support it with suitable lifting gear. Withdraw the upper half first, then the lower.
6. Mark the parts and record the order and orientation of rings so they can be returned the same way. Remove the sealing/scraper rings.
7. Clean all parts, inspect the rod for wear, scoring, taper and cracks, and measure the ring/interior clearances and the clearances in the grooves.
8. Replace worn, broken or fatigued rings with new ones of the correct size and ring gap. Check ring gap and side clearance in grooves; stagger the gaps of the fitted rings so joints do not line up.
9. Reassemble in the reverse order, tightening the housing bolts evenly to the correct torque. On reinstallation, ensure the box is correctly clocked/aligned so the ring halves match.
10. Restore drain connections and re-fit the doors, then reinstate the permit-to-work, remove the locks and turning gear interlock, and test the engine on turning gear and subsequently at low speed, checking for leaks and correct oil level.

Tools required: ring spanners/socket set with correct torque wrench, drift/punches, brass or soft-faced hammer, seal/screwdrivers, feeler gauges and micrometer, ring expander tool, appropriate lifting gear/sling and eye-bolts, cleaning rags and solvent, drip trays, and personal protective equipment (overalls, safety footwear, gloves, goggles). Always comply with the confined space entry and permit-to-work procedures.

Cast iron is widely used in auxiliary engine components because of its high strength, good wear resistance, excellent castability, and vibration damping properties. However, components operating under high temperatures, corrosive environments, and heavy mechanical loads require improved material properties. These are achieved by adding alloying elements such as silicon (Si), chromium (Cr), and manganese (Mn), which enhance corrosion resistance, hardness, thermal stability, and overall durability.

1. Silicon (Si)

Effects of Silicon

• Increases corrosion and oxidation resistance, especially at elevated temperatures.
• Improves the thermal stability of cast iron, enabling it to retain its properties under prolonged high-temperature operation.
• Promotes graphite formation, which improves machinability and helps reduce internal stresses during casting.

Applications

Silicon-alloyed cast iron is commonly used for:

• Exhaust manifolds.
• Water pump housings.
• Cooling water system components.

2. Chromium (Cr)

Effects of Chromium

• Significantly increases hardness and wear resistance.
• Improves corrosion resistance and resistance to oxidation.
• Maintains the strength of cast iron at high operating temperatures.
• Extends the service life of components subjected to continuous friction and abrasive wear.

Applications

Chromium-alloyed cast iron is commonly used for:

• Cylinder liners.
• Piston rings.
• Valve seats.
• Valve guides.

3. Manganese (Mn)

Effects of Manganese

• Increases the strength and hardness of cast iron.
• Improves toughness, enabling the material to withstand impact and repeated loading.
• Combines with sulphur to form manganese sulphide, thereby preventing hot cracking during casting.
• Enhances the wear resistance of the material.

Applications

Manganese-alloyed cast iron is commonly used for:

• Crankshafts.
• Camshafts.
• Gear housings.
• Flywheels.

Overall Benefits of Alloying Cast Iron

The addition of silicon, chromium, and manganese provides the following advantages:

• Improved resistance to corrosion caused by cooling water and combustion products.
• Increased hardness, resulting in reduced wear and longer component life.
• Enhanced thermal stability, preventing distortion, softening, and cracking at high temperatures.
• Increased mechanical strength to withstand repeated cyclic and impact loads.
• Improved reliability and reduced maintenance requirements for auxiliary engine components.

The laboratory report indicating appreciable amounts of wear metals and insoluble materials in used diesel engine oil points to possible internal wear, corrosion, or contamination. Each finding must be carefully investigated to identify the source, assess severity, and decide corrective action.

High levels of iron strongly indicate wear of ferrous engine components or corrosion.

• Possible Sources: Cylinder liners, piston rings, crankshaft, camshaft, valve train, gears, rolling element bearings, or corrosion from water contamination.
• Subsequent Investigation:
  • Compare with previous oil analysis to observe trends.
  • Correlate with other wear metals (chromium, nickel, molybdenum). If copper, lead, or tin are rising faster, bearings may be affected.
  • Check for signs of corrosion (acid number, water content in oil).
  • Inspect the air intake and filters for dust ingress.
  • Verify lubrication effectiveness, oil pressure, and surface wear patterns of components.
  Part (b)

  Copper (Cu):

  Elevated copper indicates bearing wear, cooler issues, or leaching.

  • Possible Sources: Journal/bottom-end bearings, brass/bronze bushings, thrust washers, oil coolers, radiators, or copper leaching from new coolers/oil additives.
  • Subsequent Investigation:
    • Check if other wear metals (iron, aluminum, chromium) are also elevated.
    • Inspect oil filters for copper/lead debris to confirm bearing wear.
    • If only copper is high, consider leaching from oil cooler tubes.
    • Review recent maintenance (cooler replacements, use of copper-based anti-seize).
    • Check for coolant leaks (elevated potassium in oil may confirm).
    Part (c)

    Antimony (Sb) and Tin (Sn):

    Their simultaneous presence points to wear of Babbitt/white metal bearings.

    • Possible Sources: Journal and crosshead bearings, bushings, thrust washers, or solder joints in coolers.
    • Subsequent Investigation:
      • Inspect bearings for wear, clearance, and surface damage.
      • Correlate with copper/lead levels to confirm bearing distress.
      • Check maintenance history (new/replaced bearings may initially shed metals).
      • Consider solder leaching from coolers, especially if acid number is also rising.
      Part (d)

      Silica (Si):

      High silicon usually indicates dirt/dust ingestion, but may also arise from sealants, additives, or coolant inhibitors.

      • Possible Sources: Ingress through faulty air filters/hoses, silicone-based sealants, casting sand, or coolant leakage.
      • Subsequent Investigation:
        • Inspect air filters, breather pipes, clamps, and intake hoses for leaks.
        • Compare silicon with aluminum trends (Si:Al ≈ 3.4:1 strongly indicates dust ingestion).
        • If found with sodium/potassium, suspect coolant leakage.
        • If silicon rises without aluminum, consider sealant leaching.
        • Analyze particulate matter in oil to confirm source.
        Part (e)

        n-Pentane and Toluene Insolubles:

        High levels indicate degraded oil products, soot, sludge, or external dirt leading to lubrication issues.

        • Possible Sources:
          • Pentane insolubles: Oxidation products, soot, degraded additives, fuel contamination.
          • Toluene insolubles: Carbon deposits, external dust/dirt, wear metals, varnish, and highly carbonized residues.
        • Subsequent Investigation:
          • Analyze insoluble composition (microscopy/elemental analysis).
          • Check for fuel dilution contributing to oxidation products.
          • Verify filtration efficiency (filter bypassing or clogging).
          • Assess oil life—high insolubles suggest oxidation and may require oil change.
          • Review engine operation (high thermal stress, extended drain intervals).
          • Inspect components for excessive wear producing debris.

Fully Built Crankshafts: In fully built crankshafts, various components such as crank pins, webs, and main journals are fabricated separately and then assembled using the shrink-fitting method. This method involves machining the crank pins and journals, boring matching holes in the webs, heating the webs to expand the holes, and then fitting the crank pins and journals into these holes. As the webs cool, they contract and grip the crank pins and journals firmly, ensuring a tight fit that prevents slippage during engine operation. Fully built crankshafts were commonly used in older engines and are advantageous for allowing the replacement of individual parts.

Materials: Typically made from unalloyed carbon steel (normalized).

Applications: Historically used in large, slow-speed marine engines. However, they are less common today due to lower fatigue strength and the massive size required for secure shrink fits.

Semi-Built Crankshafts: Semi-built crankshafts are constructed by forging the crank throws, which include two webs and a crank pin, as a single piece. The main journals, which are forged and machined separately, are then fitted into the webs using the shrink-fitting method. This construction method provides excellent grain flow, following the web around the crank pin and back down the other web, enhancing the strength and durability of the crankshaft. Semi-built crankshafts are often used in large two-stroke crosshead engines. This method also allows for reduced web thickness and overall weight while maintaining strength.

Materials: Often use low-alloyed Chrome-Molybdenum (Cr-Mo) steel for throws and unalloyed carbon steel for journals.

Applications: The industry standard for large two-stroke slow-speed main propulsion engines.

Solid Forged (Single-Piece) Crankshaft

Welded Crankshafts: Welded crankshafts are composed of individual forgings that include the webs, crank pins, and parts of the main journals. These forgings are welded together using processes such as submerged arc welding. This method allows for continuous grain flow and the production of thinner webs, resulting in a lighter and shorter crankshaft. Despite the high capital costs, welded crankshafts provide significant advantages in terms of strength and weight reduction, and they have been successfully used in some marine engines.

Materials: Hardened and tempered low-alloy Cr-Mo steel.

Applications: Modern high-power engines where weight reduction and high natural frequency are critical.

Solid Single-Piece Crankshafts: Solid single-piece crankshafts are forged or cast as a single unit and are typically used in small to medium-speed and high-speed engines. This construction method ensures the highest strength and durability, with continuous grain flow throughout the entire crankshaft. However, the size and weight limitations make this type less suitable for large marine engines, where the crankshaft components need to be individually forged and assembled due to the immense size and weight.

Materials: High-tensile alloy steels like Nickel-Chrome or Chrome-Molybdenum steel.

Applications: Preferred for medium and high-speed four-stroke engines, such as auxiliary generators and smaller propulsion units.

Each type of crankshaft is chosen based on the specific requirements of the engine, including size, speed, and load conditions. The materials used for these crankshafts vary, with slow-speed engines typically using plain carbon steel, while medium and high-speed engines often utilise alloy steels for enhanced performance and durability.

Slow steaming is operating the engine well below its maximum continuous rating (MCR), typically at 40 to 60 percent of MCR, to save fuel. Before and during slow steaming:

1. The engine should be operated within the manufacturer's recommended minimum load/speed band for continuous running; de-rating/load-limit the engine appropriately; ensure the turbocharger is kept off surge and the scavenge pressure is adequate.
2. Cylinder lubrication: adopt a low-load feed rate (electronic two-level lubrication) to avoid over-lubrication and deposit build-up; feed rates should be matched to load, not kept at sea-going rates.
3. Keep fuel viscosity/temperature correct: in slow steaming the fuel oil heater and viscosity control must be monitored; maintain fuel temperature to give the correct viscosity at the injector to ensure good atomization.
4. Maintain sufficient charge air pressure and scavenge pressure; monitor turbocharger speed and the air side for fouling (lambda/surge margin); keep the charge air cooler clean and water temperature high enough to give good air density.
5. Monitor exhaust temperatures, particularly the turbocharger inlet and individual cylinder exhaust temperatures, to detect over-loading of the turbocharger or its frequent operation in the low-efficiency range - no abnormal build of carbon.
6. Manage the cooling water temperatures: keep jacket water at specified high temperature to limit corrosion and reduce thermal stress; maintain scavenge air temperature above the dew point of the combustion gases to reduce acid corrosion.
7. Check sealing and lubricating of the cylinder, and keep liner wall temperature high enough (lagging and insulating of the scavenge space) to prevent cold corrosion.
8. Confirm all safety and protection systems are operative, alarms set, and that turning gear/procedures for manoeuvring remain available; ensure air system pressures are adequate.

• Cold corrosion: low liner/gas temperatures allow sulphuric acid to condense on the liner, causing corrosion wear.
• Over-lubrication at constant high feed rate leads to carbon and ash deposits in ring grooves, on the piston crown and in the scavenge space, and can cause stuck rings, liner polishing and scavenge fires.
• Turbocharger matching/tailure: single turbocharger may run in surge or at low efficiency, reducing scavenge pressure and causing poor combustion, soot and higher specific consumption; exhaust gas temperature drops.
• Reduced exhaust gas temperature may cause boiler/economiser efficiency problems and corrosion in the waste heat system.
• Deposits build up in exhaust valves and turbocharger, increasing maintenance intervals; some components (exhaust valves, fuel injection, piston rings) are prone to increased wear or fouling from running at low load for a long time.

Before returning:

• Overhaul/clean as necessary the turbocharger (air side), exhaust valves, scavenge space (if deposits), and check piston rings/crown - bring injectors and components to good condition.
• Re-adjust cylinder lubrication feed rate back to full-load setting; check oil feed and clearances.
• Gradually increase the load (in steps) so that temperatures and thermal stresses build up slowly; ensure exhaust temperatures rise evenly and turbocharger accelerates to normal speed; allow time at intermediate loads so the engine and turbocharger reach equilibrium and to burn off deposits by the higher gas temperatures.

After:

• Confirm all temperatures, pressures, ratios normal and no abnormal noise/vibration; settle the engine at MCR/appropriate service load;
• Rese-turbed procedures; carry out a round of checks (exhaust temperatures, exhaust gas against baseline) and confirm the plant runs efficiently; update logs.

(a) Need for a Moment Compensator in Large Two-Stroke Marine Engines

Large slow-speed two-stroke marine engines have very long strokes, which produce high inertia forces due to rapid acceleration and deceleration of the pistons.

The most problematic among these are 2nd-order vertical inertia forces, which vary with the square of engine speed. These forces:

• Create strong vertical shaking moments,
• Can excite the ship’s hull at its natural frequency,
• Lead to excessive hull vibration, noise, and structural fatigue.

A moment compensator is therefore installed to cancel out these secondary vertical forces. By producing equal and opposite inertial forces, it protects both the engine and the hull from harmful vibration and ensures smoother, safer operation.

Construction of a Moment Compensator

A typical moment compensator consists of:

• Two heavy rotating masses (elliptical or circular),
• Mounted inside a rigid casing,
• Driven by a gear train from the engine crankshaft,
• Rotating at twice the engine speed (2N),
• Rotating in opposite directions.

The masses and their phasing are precisely calculated so that their generated inertial forces match the magnitude and timing of the engine’s 2nd-order forces.

Working Principle

1. The reciprocating masses (pistons and rods) generate vertical unbalanced 2nd-order inertia forces.
2. The two compensator masses rotate at 2 × crankshaft speed, producing centrifugal forces of the same order.
3. Because the masses rotate in opposite directions, their horizontal components cancel, while the vertical components combine.
4. These vertical forces are timed such that:
   • When the engine’s inertia force is maximum upwards, the compensator generates a maximum downward force.
   • When the engine force is downwards, the compensator force is upwards.

Thus, the compensator dynamically balances the engine’s vertical inertia forces, preventing their transmission to the hull.

(b) Consequences of Failure or Incorrect Operation

Failure or incorrect functioning of the moment compensator can result in:

1. Severe Hull Vibration
   • Increased noise and crew discomfort.
   • Vibration of accommodation areas and decks.
2. Structural Fatigue
   • Repeated cyclic loading causing cracks in hull plating, bulkheads, and structural members.
3. Engine Damage
   • Excessive shaking loads on crankshaft, main bearings, and thrust bearings.
   • Potential misalignment of the main engine.
4. Foundation and Mounting Issues
   • Loosening of holding-down bolts,
   • Damage to engine seating and chocks.

Recommended Checks and Maintenance Practices

To ensure reliability of the moment compensator:

1. Lubrication Checks
   • Ensure reliable oil supply to gears and bearings.
   • Check for oil leaks and maintain correct oil levels.
2. Vibration Monitoring
   • Analyze vibration trends and PRU values.
   • Rising vibration levels often indicate incorrect timing or bearing wear.
3. Balance and Timing Checks
   • Ensure counterweights are correctly phased.
   • Confirm gear backlash and timing marks during overhauls.
4. Visual and Mechanical Inspection
   • Check gear teeth for wear or pitting.
   • Inspect bearings for clearance and temperature abnormalities.
   • Verify the integrity of the casing and mounting bolts.
5. Oil Analysis
   • Test for metal particles or wear debris from gears and bearings.
6. Alignment Checks
   • Ensure correct alignment between the compensator drive gears and crankshaft drives.

Definition

Specific Cylinder Lubricating Oil Consumption (SCLOC) is the quantity of cylinder lubricating oil consumed by the engine to produce a unit of power over a specified period.

It is normally expressed in:

g/kWh (grams per kilowatt-hour).

SCLOC is an important parameter for assessing the effectiveness and efficiency of the cylinder lubrication system. It helps ensure that sufficient oil is supplied for proper lubrication and acid neutralization without causing excessive cylinder oil consumption.

Formula and Derivation

Let:

• m_oil = cylinder oil consumption, kg/h
• P = engine brake power, kW

The specific cylinder oil consumption is the oil consumed per unit of engine power:

$$SCLOC=\frac{\dot{m}_{oil}}{P}$$

Since the above value is in kg/kWh, it is converted into g/kWh by multiplying by 1000:

$$SCLOC=\frac{\dot{m}_{oil}\times1000}{P}$$

Therefore:

$$\boxed{SCLOC=\frac{\dot{m}_{oil}\times1000}{P}\ g/kWh}$$

When Oil Consumption Is Measured Over a Period

If cylinder oil consumption is measured over a specific period:

• M = mass of cylinder oil consumed during the period, kg
• P_avg = average engine power during the period, kW
• t = operating time, h

The total energy produced during the period is:

$$Energy=P_{avg}\times t$$

Therefore:

$$SCLOC=\frac{M}{P_{avg}\times t}$$

Converting kg to grams:

$$SCLOC=\frac{M\times1000}{P_{avg}\times t}\ g/kWh$$

Hence:

$$\boxed{SCLOC=\frac{M\times1000}{P_{avg}\times t}\ g/kWh}$$

If Cylinder Oil Is Measured by Volume

If cylinder oil consumption is measured in litres:

• (V) = cylinder oil volume consumed, litres
• (\rho) = cylinder oil density, kg/L

The mass of oil consumed is:

$$M=V\times\rho$$

Therefore:

$$SCLOC=\frac{V\times\rho\times1000}{P_{avg}\times t}\ g/kWh$$

Hence:

$$\boxed{SCLOC=\frac{V\times\rho\times1000}{P_{avg}\times t}\ g/kWh}$$

Onboard Procedure to Optimize Engine Operation

The following procedures are normally adopted onboard to achieve and maintain the optimum SCLOC:

1. Determine the actual cylinder oil consumption by monitoring cylinder-oil tank levels, measuring oil flow, or using the flow/consumption data provided by the cylinder lubrication system.
2. Calculate the actual SCLOC using the quantity of cylinder oil consumed and the engine's average power output.
3. Compare the calculated SCLOC with the cylinder oil feed rate or consumption recommended by the engine manufacturer.
4. Check cylinder lubrication at different engine loads, because the optimum cylinder oil feed rate depends on engine load and other operating conditions.
5. Adjust the cylinder oil feed rate according to engine load and speed using the electronic or mechanical cylinder lubrication system, as fitted.
6. Ensure correct timing of cylinder oil injection. Oil should be injected at the correct crank angle and piston position as specified by the engine manufacturer.
7. Regularly inspect cylinder liners and piston rings for excessive wear, scuffing, corrosion, deposits or other abnormal conditions.
8. Carry out scrape-down oil analysis to monitor the condition of the cylinders. Important parameters include:
   • Iron content
   • Residual BN (Base Number)
   • Wear indicators
   • Signs of corrosive wear
9. Adjust the cylinder oil feed rate based on actual condition monitoring, including scrape-down oil analysis, cylinder liner and piston-ring condition, fuel sulphur content and engine load.

The objective should not simply be to reduce cylinder oil consumption. The feed rate should be optimized to maintain adequate lubrication and acid neutralization while avoiding unnecessary oil consumption.

Factors Affecting SCLOC

1. Engine Load

Engine load has a significant effect on cylinder lubrication requirements. Low-load operation generally requires different lubrication settings from high-load operation, and the cylinder oil feed rate should be adjusted accordingly.

2. Engine Speed

Engine speed affects the number and timing of cylinder lubrication events and therefore influences the quantity and distribution of cylinder oil supplied to the cylinder liner.

3. Cylinder Oil BN

The Base Number (BN) of the cylinder oil must be suitable for the fuel being used. Adequate BN is required to neutralize the acidic products formed during combustion.

4. Fuel Sulphur Content

Higher fuel sulphur content can produce more acidic combustion products. Therefore, adequate alkaline cylinder lubrication is required to neutralize these acids and prevent corrosive wear.

5. Cylinder Liner Condition

The condition of the cylinder liner, including its wear, honing condition and surface temperature, affects the quantity of oil required for effective lubrication.

6. Piston-Ring Condition

Worn, damaged or sticking piston rings can increase oil consumption and cylinder wear. Proper piston-ring condition is therefore essential for maintaining optimum SCLOC.

7. Cylinder Liner Temperature

Excessive liner temperature can promote oil degradation and deposit formation, affecting lubrication quality and cylinder oil consumption.

8. Lubricator Condition and Calibration

The condition and correct calibration of the cylinder lubricator are important. Faulty lubricator pumps, quills, non-return valves or associated components can result in incorrect or uneven cylinder oil delivery.

9. Engine Condition and Combustion Quality

Poor combustion can increase carbon deposits, corrosive wear and abnormal cylinder conditions, which may increase the cylinder oil requirement.

Methods of Monitoring and Adjustment

1. Cylinder Oil Flow and Consumption Monitoring

Regularly record cylinder-oil tank levels or use a flowmeter/flow-monitoring system to determine the actual cylinder oil consumption.

The measured consumption is then used together with engine power output and operating hours to calculate SCLOC.

2. Scrape-Down Oil Analysis

Samples of scrape-down oil are analysed for:

• Iron content
• Residual BN
• Other wear and combustion indicators

A high iron content may indicate increased cylinder liner or piston-ring wear. Excessive residual BN may indicate that more alkaline oil is being supplied than required and may therefore indicate over-lubrication.

3. Cylinder Inspection

Regularly inspect the:

• Cylinder liner
• Piston rings
• Piston crown

The inspection should look for:

• Scuffing
• Corrosion
• Excessive deposits
• Abnormal wear
• Piston-ring sticking

4. Load-Dependent Lubrication

The cylinder lubricator should be adjusted according to:

• Engine load
• Engine speed
• Manufacturer's recommended feed-rate curve
• Actual cylinder condition

Where an electronic lubrication system is fitted, the feed rate can be automatically adjusted according to the engine's operating condition.

5. Fuel-Dependent Adjustment

Cylinder oil BN and feed rate should be selected and adjusted according to the fuel sulphur content and actual operating conditions.

Adequate alkalinity must be maintained to neutralize acidic combustion products and prevent corrosive wear.

6. Avoid Over-Lubrication and Under-Lubrication

The cylinder lubrication system must be maintained within the optimum operating range.

Under-Lubrication

Insufficient cylinder oil can result in:

• Corrosive wear
• Scuffing
• Cylinder liner damage
• Piston-ring damage
• Increased wear

Over-Lubrication

Excessive cylinder oil can result in:

• Excessive deposits
• Piston-ring sticking
• Exhaust-valve deposits
• Increased maintenance requirements
• Unnecessary cylinder oil consumption and increased operating cost

The SAC volume refers to the small space within the fuel injector between the valve seat (fuel shut-off point) and the entrance to the final metering orifice.

• This volume holds fuel that vaporizes incompletely at the end of injection and enters the cylinder at low velocity during the expansion stroke.
• The unburnt fuel contributes to post-injection dripping, after-burning, and increased emissions such as unburnt hydrocarbons and NOx.

Low SAC volume injectors are introduced to address these issues:

• Minimize post-injection dripping and after-burning.
• Reduce carbon accumulation on the nozzle tip and prevent heat sinking of the nozzle, which can cause damage.
• Improve fuel combustion, lowering emissions and enhancing engine efficiency.

Additionally, nitriding treatment of the fuel valve enhances heat resistance and improves durability against corrosion, thereby prolonging service life. This directly contributes to an improved Life Cycle Value (LCV) of the engine.

Precise regulation of fuel valve opening pressure helps in optimal combustion. Higher opening pressure improves fuel atomization, leading to more complete combustion. This improved combustion helps in:

• Firstly, it reduces smoke density, particularly at low engine loads, minimizing particulate emissions and improving engine efficiency.
• Secondly, it minimizes carbon deposits within the combustion chamber, reducing the risk of pre-ignition and engine damage.

The cleaner combustion process extends the life of engine components such as pistons, exhaust valves, and the combustion chamber itself, ultimately contributing to a higher LCV through extended service intervals and reduced maintenance.

• Contaminates turbocharger which leads to premature failure of Turbocharger
• Excessive wear of liner & piston rings causing blow past.
• Burning of piston crown.
• Blockage of exhaust valves & exhaust passages.
• Emissions trouble & air pollution.

• High wear rate of liner, bearings, piston rings.
• Reduction in load carrying capacity.
• Improper lubrication & cooling.
• Bacterial attack.
• Corrosion.

All these will impact the life cycle value of the engine.

• Stop the engine and engage the turning gear.
• Start measuring deflections from the unit farthest to flywheel
• Place the crank pin at the point of 30° (position ‘B’) past the bottom dead centre.
• Install the deflection gauge in the pop point provided for this purpose.
• Set the reading on the gauge to 0 (zero reading) at the position ‘B’ in the figure.
• Slowly conduct turning of the engine in the normal direction of rotation, and measure the reading on the scale when the crankshaft is at the angle of ‘B’, ‘C’, ‘D’, ‘E’ and ‘A’ respectively, of which data shall be recorded. Before taking the measurements for each point the turning gear should be momentarily reversed to release and torsional stress that it is exerting on the crankshaft.

Calculating deflection (d): Calculate the deflection values as based not the measured values and in accordance with the following formula and record the calculated values.

Vertical (V) deflection: dV = D - A+B/ 2

Horizontal (H) deflection: dH = C - E

positive/ negative deflection: open downward (+), closing downward (-) A, B, C, D and E represent the measured values respective at each corresponding position shown in the figure above.

(c) Procedure to follow when Crankshaft deflection readings are outside the permissible limits

If the crankshaft deflection readings are found to be outside the acceptable limits, the following steps should be carried out systematically:

• Repeat the deflection measurements with careful attention to all factors that may influence the readings. These include:
  • Vessel’s loading condition
  • Trim and list
  • Weather conditions (e.g., heavy seas)
  • Cargo operations underway
  • Ambient temperature (e.g., cold sea or hot weather)
• Confirm the dial gauge being used is properly calibrated and in good working condition.
• Ensure proper tightness of holding down bolts and tie rod bolts.
• Inspect the bedplate chocking arrangement for any signs of fretting or damage.
• The most common cause of excessive crankshaft deflections is unevenly worn main bearings. This can be confirmed by measuring bearing clearances.
• If worn bearings are identified, replacing them may restore deflections within permissible limits.
• If the issue is due to worn or fretted chocks, or as a result of collision or grounding, then a full realignment of the engine may be required. This procedure must be carried out at a shore-based workshop.
• If the engine must be operated with deflections still beyond the limits, it should only be done after consulting the engine builder.
• Operation must be at a reduced load, under specific guidance.
• Immediately inform the DPA (Designated Person Ashore) and the technical superintendent.
• The engineer superintendent should initiate contact with the engine manufacturer for further instructions or support.

In large slow-speed two-stroke engines, proper cylinder liner lubrication is essential to maintain a protective oil film between the piston rings and cylinder liner. If lubrication is inadequate or improperly controlled, several operational and mechanical problems may occur.

1. Excessive Wear

• When lubrication is insufficient, metal-to-metal contact occurs between the piston rings and the cylinder liner. This results in accelerated wear of both the piston rings and liner surface, ultimately reducing the service life of the engine components.

2. Scuffing and Scoring

• Improper lubrication can cause the breakdown of the lubricating oil film. As a result, deep vertical scratches or scoring marks may develop on the liner surface. If this condition becomes severe, it may lead to piston seizure.

3. Micro-Seizure

• Micro-seizure occurs when localized welding and tearing of metal surfaces takes place between the piston rings and liner. This happens when the lubricating oil film is too thin or insufficient, causing direct metal contact.

4. Corrosive Wear

• Residual fuels contain sulphur, which during combustion forms sulphuric acid. If the cylinder oil does not have a sufficient Base Number (BN) to neutralize these acidic products, the acid can corrode the liner surface, leading to corrosive wear.

5. Piston Ring Sticking

• Poor lubrication and the formation of carbon deposits can restrict the free movement of piston rings within their grooves. This causes piston ring sticking, resulting in poor sealing and increased gas leakage.

6. Blow-by and Loss of Compression

• Worn liners or damaged piston rings allow combustion gases to leak past the piston rings, a condition known as blow-by. This reduces compression pressure, lowers engine efficiency, and increases fuel consumption.

7. Overheating

• Excessive friction due to poor lubrication increases the temperature of the piston and liner surfaces. This overheating may damage the piston crown, piston rings, and cylinder liner.

8. Increased Oil Consumption

• Incorrect cylinder oil feed rates may lead to either excessive oil consumption or insufficient lubrication, both of which negatively affect engine performance and operating costs.

1. Cloverleafing

Description

Cloverleafing refers to an uneven wear pattern on the cylinder liner. The liner develops a lobed or oval shape resembling a clover leaf rather than remaining perfectly circular. This wear pattern usually occurs at specific locations corresponding to the fuel injection points.

Causes

Cloverleafing can occur due to several factors, including:

• Uneven temperature distribution around the circumference of the liner
• Poor fuel atomization, causing localized hot spots
• Incorrect fuel injection timing
• Over-lubrication, which may lead to bore polishing
• High thermal and mechanical stresses acting on the liner

Effects

The consequences of cloverleafing include:

• Poor sealing between the piston rings and liner
• Increased blow-by of combustion gases
• Development of irregular wear patterns on the liner surface

2. Micro-Seizure

Description

Micro-seizure is a condition where localized adhesion occurs between the piston ring and the cylinder liner. Small fragments of metal may tear away from the surfaces, leaving fine scoring marks on the liner.

Causes

Micro-seizure can result from several operating conditions, such as:

• Insufficient lubrication
• Low cylinder oil feed rate
• Low oil viscosity
• Excessive engine load
• Poor distribution of lubricating oil
• Breakdown of the oil film due to high temperatures

Effects

The effects of micro-seizure include:

• Roughening of the liner surface
• Damage to piston rings
• If not corrected, it may develop into major seizure or severe liner damage

Large two-stroke marine engines operating on heavy residual fuel oil (HFO) require cylinder lubricating oil with high alkalinity, commonly expressed as a high Base Number (BN), in order to neutralize the acidic products formed during combustion.

The typical composition of such cylinder oil includes the following components:

1. Base Oil

• The main component is a high-viscosity mineral base oil.
• This base oil provides the primary lubricating film strength required to protect the piston rings and cylinder liner.

2. Alkaline Detergents (High BN Additives)

• Cylinder oils contain calcium-based alkaline detergents.
• These additives neutralize sulphuric acid formed during fuel combustion and help maintain the cleanliness of engine components.
• Typical cylinder oil Base Number (BN) ranges from 40 to 100, depending on the sulphur content of the fuel used.

3. Dispersants

• Dispersants help keep carbon particles and combustion residues suspended in the oil, preventing them from forming harmful deposits on engine components.

4. Anti-Wear Additives

• Anti-wear additives reduce direct metal-to-metal contact between moving parts, thereby minimizing wear of the piston rings and cylinder liner.

5. Antioxidants

• Antioxidants prevent oxidation of the lubricating oil at high temperatures, thereby extending the service life of the oil.

6. Corrosion Inhibitors

• These additives protect metal surfaces from acidic corrosion, particularly the cylinder liner, which is exposed to sulphurous combustion products.

7. Thermal Stability Improvers

• Thermal stability additives ensure that the lubricating oil maintains its film strength and stability at high operating temperatures, which is essential for reliable engine operation.

Starting:

• When the telegraph is moved to the desired command, e.g., Dead Slow Ahead from STOP, a solenoid valve in the control system is energized.
• This admits control air to the Ahead switch, which directs air to pneumatic cylinders fitted on each fuel pump. These cylinders shift the fuel pump roller to the “ahead firing” position.
• Control air is also supplied to the starting air distributor, preparing it for the ahead start sequence.
• After these actions, the Ahead switch supplies air to the interlock system, releasing it.
• The control air then opens the Main Automatic Valve (Auto v/v), admitting ~30 bar starting air into the engine via the starting air distributor.
• The starting air is admitted to cylinders as per the firing sequence, and the engine begins to rotate.
• Once sufficient starting RPM is achieved, starting air is cut off, and fuel admission begins, completing the starting sequence.

Stopping:

• The telegraph is moved to STOP.
• This energizes another solenoid valve, which supplies air to the puncture valves of the fuel pumps, cutting off fuel injection, and the engine stops.

Reversing:

• After the engine has completely stopped, the telegraph is moved to Dead Slow Astern.
• A solenoid valve supplies control air to the Astern switch and simultaneously vents the Ahead switch.
• The Astern switch directs control air to the fuel pump pneumatic cylinders, shifting the rollers to the astern firing position, and also supplies air to the starting air distributor.
• The air distributor now operates according to the astern firing order.
• After the interlocks are released, the engine is started in the astern direction using the same process as ahead, but with the astern firing sequence.

(i) Engine fails to turn on air

Causes:

• Low pressure in starting air receiver.
• Valve on starting air receiver closed.
• Valve to starting air distributor closed.
• No pressure in control air system.
• Main starting air valve stuck/locked.
• Turning gear interlock engaged.
• Pistons in starting air distributor sticking.

Remedies:

• Start compressors and pressurize the air bottles.
• Open the air receiver valve.
• Open the valve to the distributor.
• Check control air pressure and open supply if closed.
• Lift the locking plate to working position.
• Disengage turning gear.
• Lubricate pistons, free them, and overhaul the starting air distributor.

(ii) Engine turns on air but fails to fire on fuel

Causes:

• Puncture valves not deactivated.
• Engine shut-down system tripped.
• Sluggishness in manoeuvring gear.
• Fault in governor.
• Fault in fuel system.

Remedies:

• Identify and correct the puncture valve cause.
• Check pressures and temperatures, reset shut-down.
• Lubricate and free the manoeuvring gear.
• Attempt starting from local control, bypassing governor if required.
• Check fuel pressure and temperature.
• Drain fuel for sludge/water contamination.

(iii) Engine fails to reverse

Causes:

• Reversing solenoid valve not receiving voltage.
• Control air signal not reaching engine due to blockage or defective valve.

Remedies:

• Check electrical wiring and control circuits.
• Inspect system by removing the tappet pipe; locate and clear blockages or replace defective valves.

The protection ring is a separate hardened ring seated in a groove at the top of the cylinder liner. Its function is to protect the upper liner bore where the piston rings reverse direction at top dead centre, where combustion gas pressure is maximum and the top ring is practically stationary. At this position the ring cannot wipe away the products of combustion, so the liner throat would otherwise wear rapidly and crack face erosion by the flame. The protection ring provides a hard, corrosion- and wear-resistant surface which: preserves the liner bore diameter, gives a consistent sealing surface and prevents bore polishing, stops the flame eroding the liner rim, and protects against ring groove hammering and fretting. It is usually made of a special hardened steel, and is a replaceable part which can be renewed when worn, so the liner itself lasts longer.

In a conventional four-stroke engine the fuel pump plunger is driven by a cam on the low-speed camshaft through a follower and roller, with a fixed lift profile and a return spring; injection timing is fixed by the cam angle and the timing of the fuel delivery is adjusted by mechanical means (nozzle/rack). In a modern engine the fuel injection drive has been modified by one or both of two approaches; (1) common rail injection, where a high-pressure rail is charged by a power-cylinder-driven or dedicated pump and each injector is opened electronically (solenoid/hydraulically triggered) so timing, duration and pressure are variable, decoupling injection timing from the mechanical cam; (2) electronic unit injection (or single camshaft-less pumps) where the pump and injector are mounted in one unit and the injection is controlled by an electronic control unit (ECU) using signals of engine speed, load and temperature. The modification removes fixed cam timing, allowing variable injection timing (VIT) and flexible injection control for better combustion, lower smoke/emissions and improved fuel economy, and reduces wear on drive gear/cams.

Multi-hole nozzle tips have a series of injection holes. Staggering means the holes are arranged so that the spray axes of adjacent holes are offset by a small angle in relation to the valve centreline or to each other. This means that no two sprays issue from diametrically opposite/equal angles, so the sprays divide into the combustion chamber space evenly and are less likely to hit the piston crown/bowl lip or liner (wall wetting). The staggered arrangement, together with swirl, gives a more even spatial distribution of fuel, better mixing, avoids overlapping of adjacent sprays that would shield each other, improves atomization, and produces more uniform heat release, reducing smoke and unburnt fuel and increasing efficiency.

Swirl is the rotary motion of the air charge about the cylinder axis generated by a tangential/helical inlet port during the induction stroke: the port vanes impart angular momentum to the air. Effect: high relative velocity between the fuel spray and the air improves atomization and mixing, shortens the ignition delay, promotes fast and complete combustion, and gives more uniform gas temperature and lower smoke and emissions.

Squish is the rapid radial inward movement of the air from the outer piston-cylinder clearance (squish band) into the piston bowl as the piston approaches TDC, generated by the piston crown geometry. Effect: it induces turbulence in the combustion chamber just before and during injection, which thoroughly mixes fuel and air, accelerates the flame front, improves combustion and prevents knock, while reducing unburnt HC. Combined, swirl and squish create the turbulence that yields efficient, clean, rapid combustion.

Shown below is the arrangement of a chain drive, which is used to transmit power from the crankshaft to the camshaft.

• It consists of chain sprockets mounted on the crankshaft & camshaft. There can be two or more chains.
• A chain-tightening arrangement is provided, as shown in the fig.
• The chain is guided by the guide bars, which has rubber shock-absorbing pads
• Flyweights are provided as they are the moment compensators.
• Oil spray nozzles are used to lubricate the chain & the wheels.

In the event of a chain link failure during engine operation, the following steps should be carried out:

• Turn the chain until the damaged link is positioned on the longest free end side of the chain, where it is easily accessible.
• Release tension on the chain to facilitate repair.
• Wrap a thin wire around the chain, a short distance from the damaged link, and pull the wire taut using a chain block. This ensures that the chain remains stable during repair.

Remove the Faulty Link:

• Chisel or grind off the riveted metal on the pin ends of the damaged link.
• Use a chain bursting tool:
  • Place the tool over the smallest part of the chain link.
  • Align the dismantling screws precisely over the ground pin ends.
  • Tighten the dismantling screws alternately to push the pins out of the link.
• Remove the damaged link plate and pin.

Install the Replacement Link:

• Replace the damaged plate and pin with a new spare.
• Rivet the ends of the new pin securely.
• If a second chain is present, replace the corresponding link in the other chain to ensure uniform wear and performance.

After the repair, adjust the chain tension to the correct setting.

Reasons for failure:

• Cyclic stresses resulting in fatigue failure cracks.
• Excessive wear due to improper lubrication.
• Overheating due to improper lubrication.

Setting the chain to the correct degree of tension initially:

• Turn the engine to bring the slack part of the chain on the same side as the lighter wheel.
• Place the spring & spring carrier in place. Tighten Nut 'C' till the required compression of spring is achieved (softly touching).
• Tighten nut 'B' till it touches the shaft (softly touching).
• Tighten nut 'C' further again till the shaft carrying carrier is up against the star (further compression will not affect the chain tension).
• The lock nuts A & D are then tightened & locking washers are bent in place.

Chain tightening:

• Excessive wear in the cylinder liner leads to increased piston ring movement, both radially and axially. This fluctuating motion can cause tilting and eventual breakage of the rings.
• If the piston ring does not exert sufficient pressure on the liner, gas pressure can penetrate between the ring and liner, collapsing the ring into the groove and causing breakage.
• Ridge formation near scavenge pockets can create stress concentrations at the piston ring's radial edge, promoting fracture.
• Jamming or sticking of rings caused by excessive carbon deposits, often due to improper combustion or inadequate cleaning during maintenance.
• Excessive wear in the piston ring grooves causes the rings to impact the groove walls during operation, leading to hammering and eventual breakage.
• Inadequate cylinder lubrication results in overheating and increased friction, weakening the rings and causing breakage.
• Acidic corrosion and high-temperature corrosion weaken the ring material, predisposing them to fracture.
• Excessive engine loading can cause the rings to deform beyond their elastic limit, leading to collapse and breakage.
• Using low-quality or non-manufacturer-specified rings compromises material strength and durability, increasing the risk of breakage.
• Improper installation during ring renewal can lead to misalignment, increased stress, and premature failure.

Maintenance practice:

• Perform routine inspections and overhauls of pistons, piston rings, and cylinder liners as per the PMS schedule.
• During overhauls, ensure piston rings and grooves are thoroughly cleaned, and all necessary clearances are measured to verify proper fit.
• Reuse piston rings only if measurements indicate they are within the safe operational limits until the next overhaul.
• Regularly maintain the fuel injection systems to prevent improper combustion and minimise stress on piston rings.
• Ensure that piston rings and liners are free of marks, scratches, or other signs of wear during scavenge inspections.
• During overhaul, install piston rings with proper tools and techniques, ensuring free movement of rings in their grooves.
• A proper running-in procedure after installing new pistons and rings helps to ensure correct seating and minimises initial wear.

Engine Operation:

• Maintaining adequate cylinder oil lubrication minimises friction and heat generation.
• Maintain appropriate cooling of the cylinder liner and piston to avoid thermal stresses.
• Use properly treated fuel oil and ensure correct operation of fuel pumps, injectors, and Variable Injection Timing (VIT) systems.
• Maintaining correct combustion parameters minimises improper combustion and reduces carbon deposits.
• Keep air filters clean to avoid the ingress of dust and abrasive particles into the engine.
• Avoid overloading the engine, which can stress the piston rings and cause failure.

• Low compression pressure, Pmax & power developed.
• Blowpast, increase in scavenge temperature and cause scavenge fire.
• Rise in exhaust temperature.
• Scuffing of liner and increase in wear rate.
• Increased SFOC.
• Fouling of turbocharger due to improper combustion.
• Fouling of EGE and can cause EGE fire.
• Damage to cylinder liner due to blowpast.
• Loss of cylinder lubrication.

The following precautions must be taken while operating an engine with broken or severely worn piston rings:

• Isolate the affected unit as excessive blowpast may cause scavenge fire.
• Monitor the scavenge temperature.
• Run the engine at low load till necessary replacement is carried out.

Static balance: the balancing of the rotating masses so that the resultant of all centrifugal forces on the crankshaft, when the shaft is not rotating, is zero, i.e. the shaft has no heavy point. Static imbalance is detected when the shaft, supported on knife edges or rollers, always rolls until the heavy side is at the bottom. It is corrected by adding or removing mass on the appropriate crank webs (counterweights) or by drilling/grinding material from the heavy side so the centre of mass lies on the axis of rotation.

Dynamic balance: even a statically balanced shaft can have a couple acting because the unbalanced masses lie in different planes along the shaft, so that when rotating a rocking couple is set up. Dynamic balancing is performed on a balancing machine where the shaft is rotated and the vibrations at the two ends are measured; correction masses are added or removed at calculated positions (usually on the crank webs) in the two end planes so that both the resultant force and the resultant couple are zero. In practice counterweights are attached to, or cast integrally with, the crank webs, and for large shafts the balance is checked during manufacture and correction machining is done on the webs.

The primary reciprocating force arises from the acceleration of the reciprocating masses (piston, rings, small-end part of connecting rod) and varies in magnitude once every revolution (at engine speed, first order). For a multi-cylinder engine, by arranging the firing order and crank positions, the primary forces of different cylinders can be made to cancel to a large extent. The methods used are:

1. Balance (counterweight) on the crank web approximately equal to the fictionally rotating part of the reciprocating mass (a rotating balance weight of about half the reciprocating mass placed on the opposite side of the crank throws) which balances part of the primary force.
2. Arrangement of crank throws so that the out-of-balance primary forces act at different phases and cancel each other in symmetrical multi-cylinder engines (e.g. six-cylinder inline engines), including using an even number of cylinders spaced evenly.
3. The use of balance shafts (counter-rotating shaft pairs) running at engine speed, carrying balance weights, which generate a downward force to cancel the primary force.

They are not completely successful because: (1) the primary force is a force (not a pure couple) that cannot be wholly eliminated for an inline engine unless balance shafts are fitted, and these themselves add weight and complexity; (2) the balance weights cancel only the rotating part of the primary force; the reciprocating part acts along the cylinder axis, so the horizontal component passes through the crank and the vertical component cannot be cancelled by webs alone; (3) remaining combined forces leave a residual imbalance which, though small, is not zero; (4) the counterweight approach only reduces the force, and the exact phase relationship varies with speed.

Secondary imbalance: the secondary reciprocating force varies at twice engine speed (second order) and arises from the finite length of the connecting rod causing the piston acceleration to have a second harmonic. Additions to control implantance are:

1. Counter-rotating balance shafts (Lanchester or reciprocating balance shafts) running at engine speed for primary and at twice engine speed for secondary, with weights arranged to generate cancelling forces. These shafts are gear- or chain-driven from the crankshaft.
2. Addition of counterweights of increased size to the crank webs.
3. For secondary forces, two shafts rotating in opposite senses at twice crankshaft speed with parallel axes are used so their horizontal components cancel and vertical components add to balance the secondary force.
4. Use of a flywheel of correct mass moment of inertia and a torsional damper/detuner to damp out the torque variations and torsional vibrations that such vibration produces.

These additions reduce frame vibration, main-bearing loading, and forces transmitted to the ship's structure, preventing excessive vibration and noise.

(a) Cast Iron Welding is a Challenging Task – Reasons

Welding cast iron is considered a difficult and highly specialized operation because its metallurgical characteristics react unfavorably to the intense heat involved in welding. The following factors explain the challenges:

1. High Carbon Content

• Cast iron contains approximately 2% to 4% carbon, which is nearly ten times higher than that of mild steel. During welding, the base metal is subjected to rapid heating and cooling cycles. This high carbon content promotes the formation of extremely hard and brittle microstructures such as martensite or white iron in the heat-affected zone (HAZ). These structures lack toughness and are highly prone to cracking.

2. Inherent Brittleness

• Unlike steel, cast iron has very low ductility. It cannot deform plastically to relieve stresses created by thermal expansion and contraction during welding. Instead of stretching or yielding under stress, it tends to crack suddenly, especially near the weld area.

3. Thermal Shock and Rapid Cooling

• The welding arc produces intense localized heat, creating steep temperature gradients between the weld zone and the surrounding metal. When cooling occurs rapidly, high internal stresses are generated. These stresses frequently result in immediate or delayed cracking in the HAZ.

4. Presence of Graphite in Gray Cast Iron

• Gray cast iron contains graphite flakes distributed within the iron matrix. These flakes act as internal stress concentrators and weaken the structure. During welding, graphite may dissolve into the weld pool, causing embrittlement and reducing the strength and integrity of the joint.

5. Porosity Due to Oil and Grease Contamination

• Engine casings made of cast iron are often porous and, over years of service, absorb oil and grease. When welding heat is applied, these trapped contaminants vaporize and form gas pockets within the molten weld metal. This leads to porosity, reducing weld strength and reliability.

6. Limited Weldability of Certain Types

• While gray cast iron can be welded with strict preheating and controlled cooling procedures, white cast iron is generally considered unweldable because of its extreme hardness and brittleness. It cannot tolerate the stresses introduced by welding.

Because of all these metallurgical and structural limitations, welding cast iron requires careful temperature control, suitable filler materials, and controlled cooling—yet it still carries a high risk of failure.

(b) Alternative Repair Methods Used by Engine Makers for Cast Iron Casings

Due to the significant risks associated with welding cast iron, engine manufacturers often prefer mechanical or “cold repair” techniques. These methods avoid excessive heat and preserve the original structure and alignment of the engine casing.

1. Metal Stitching and Locking (Metalock Method)

• This is the most widely used industrial repair method for cracked engine casings. Holes are drilled along the length of the crack, and specially designed high-tensile metal “stitching pins” and “locks” are inserted. These components mechanically pull the cracked sections together and restore structural strength. Since no heat is applied, the original metallurgy and alignment of the casing remain unaffected.

2. Braze Welding

• In this method, a filler material such as bronze or nickel alloy—having a lower melting point than cast iron—is used. The base metal is not melted; instead, the filler bonds to it. This significantly reduces thermal stress and minimizes the risk of additional cracking compared to conventional fusion welding.

3. Studding Method

• For major fractures, holes are drilled and tapped along the cracked surfaces. Steel studs are screwed into these holes to provide reinforcement. Weld metal or filler material is then deposited over the studs to secure and anchor the repair. The studs act as mechanical reinforcement, improving the strength of the repaired area.

4. Epoxy Bonding (Cold Weld Compounds)

• For non-structural cracks or minor leakages, metal-filled epoxy compounds can be applied. These adhesives provide a watertight and heat-resistant seal without introducing thermal stresses. This method is suitable for temporary repairs or low-load applications.

5. Patching with Insert Replacement

• In cases of severe damage, such as when a connecting rod breaks through the casing, the damaged section can be completely machined out. A new cast iron insert or patch piece is then fitted into the prepared opening and secured using metal stitching or pinning methods. This restores both strength and dimensional accuracy.

(b) Common forms of seal failure in a stern tube

1. Loss of elasticity in seal material – Nitrile rubber seals may lose their elastic properties over time, reducing their sealing effectiveness.
2. Surface damage to chrome liner – Grooving or scoring of the chrome liner can impair sealing surfaces, leading to leakage.
3. Excessive shaft vibration – Heavy vibration of the propeller shaft can cause uneven wear and seal deformation.
4. Insufficient cooling – Inadequate cooling can cause rubber sealing elements to harden and eventually fail.
5. Exceeding running hour limits – Operating beyond the manufacturer’s recommended service life increases the risk of seal failure.
6. Deterioration of oil quality – Contaminated or degraded oil reduces lubrication and protection, accelerating seal wear.
7. Incorrect header tank level adjustment – Failure to adjust the header tank according to vessel draft can cause oil loss, leading to inadequate lubrication and seal damage.

(c) Restricting oil loss due to seal failure whilst on passage

1. Use of high-viscosity oil – Recharge the system with a thicker oil to reduce leakage rate through damaged seals.
2. Temporary oil supply arrangement –
   • Disconnect the existing oil supply line.
   • Connect a 45-gallon drum supported by a block and tackle arrangement.
   • Adjust the drum height to vary the oil head pressure, matching it to the surrounding water pressure and minimizing leakage.
3. Fresh water introduction – Supply fresh water to the gravity tank to emulsify with any leaked oil. The resulting emulsion helps coagulate around the damaged seal area while the oil is circulated to maintain lubrication and sealing.

The aft stern tube bearing is designed to distribute the propeller load uniformly and prevent excessive pressure from being concentrated at one location. The main design features are:

1. Slope Boring (Taper Boring) (Most Important Exam Point)

The aft bearing is machined with a slight taper (slope bore) to match the natural deflection of the propeller shaft caused by the weight of the propeller.

Reason:

This ensures that the load is distributed over the entire length of the bearing instead of being concentrated at the aft end, thereby reducing wear and increasing bearing life.

2. Long Bearing Length

The aft bearing is made longer than the forward bearing, providing a larger contact area between the shaft and the bearing.

Reason:

The increased bearing area reduces the unit bearing pressure and distributes the load more evenly.

3. Large Bearing Surface Area

The bearing is provided with a large diameter and a long white-metal or composite bearing surface.

Reason:

The larger bearing surface spreads the propeller load over a greater area, reducing localised stresses and wear.

4. Proper Bearing Clearance and Hydrodynamic Oil Film

The bearing is designed with the correct clearance to maintain a continuous hydrodynamic oil film between the shaft and the bearing during operation.

Reason:

The oil film prevents metal-to-metal contact and supports the shaft hydraulically, ensuring uniform load distribution and reducing friction and wear.

A common large diesel arrangement uses a jerk-type fuel pump with a variable injection timing mechanism. The pump has a plunger operated by a cam, but the point at which pressure starts to build can be altered by changing the effective spill/closing point.

Main parts:

• Fuel cam and roller tappet give the plunger its upward stroke.
• Plunger and barrel pressurise fuel.
• Suction/spill ports allow filling and spilling.
• Delivery valve prevents backflow and gives sharp pressure collapse.
• Fuel rack rotates the plunger to vary effective stroke and fuel quantity.
• VIT linkage or servo changes the relative timing between plunger movement and port closure, giving earlier or later injection.

The exact design differs between engine makers. Some alter the pump barrel position, some alter the roller guide or cam follower position, and modern engines may use an electronically controlled spill valve. The principle is the same: change when high pressure begins without upsetting the required fuel quantity control.

Changing injection timing

Injection begins when the pump plunger covers the spill/suction port and fuel pressure rises enough to open the injector needle.

With VIT, the mechanism changes the point at which this happens:

• If the port is covered earlier in the plunger stroke, pressure builds earlier, so injection is advanced.
• If the port is covered later in the plunger stroke, pressure builds later, so injection is retarded.
• This may be done by moving the pump barrel, changing the roller guide position, or controlling a spill valve electronically/hydraulically.

Thus VIT varies the start of injection according to engine load to obtain correct peak pressure and fuel economy.

Changing fuel quantity

Fuel quantity is changed by rotating the pump plunger with the fuel rack.

• At low fuel setting, the plunger helix uncovers the spill port early.
• Pressure collapses early, so injection ends early and less fuel is delivered.
• At high fuel setting, the helix uncovers the spill port later.
• Injection continues for longer, so more fuel is delivered.

So, in a conventional jerk pump, VIT mainly changes the start of injection, while the fuel rack changes the end of injection and therefore the quantity delivered.

In large slow-speed two-stroke engines, proper cylinder liner lubrication is essential to maintain a protective oil film between the piston rings and cylinder liner. If lubrication is inadequate or improperly controlled, several operational and mechanical problems may occur.

1. Excessive Wear

• When lubrication is insufficient, metal-to-metal contact occurs between the piston rings and the cylinder liner. This results in accelerated wear of both the piston rings and liner surface, ultimately reducing the service life of the engine components.

2. Scuffing and Scoring

• Improper lubrication can cause the breakdown of the lubricating oil film. As a result, deep vertical scratches or scoring marks may develop on the liner surface. If this condition becomes severe, it may lead to piston seizure.

3. Micro-Seizure

• Micro-seizure occurs when localized welding and tearing of metal surfaces takes place between the piston rings and liner. This happens when the lubricating oil film is too thin or insufficient, causing direct metal contact.

4. Corrosive Wear

• Residual fuels contain sulphur, which during combustion forms sulphuric acid. If the cylinder oil does not have a sufficient Base Number (BN) to neutralize these acidic products, the acid can corrode the liner surface, leading to corrosive wear.

5. Piston Ring Sticking

• Poor lubrication and the formation of carbon deposits can restrict the free movement of piston rings within their grooves. This causes piston ring sticking, resulting in poor sealing and increased gas leakage.

6. Blow-by and Loss of Compression

• Worn liners or damaged piston rings allow combustion gases to leak past the piston rings, a condition known as blow-by. This reduces compression pressure, lowers engine efficiency, and increases fuel consumption.

7. Overheating

• Excessive friction due to poor lubrication increases the temperature of the piston and liner surfaces. This overheating may damage the piston crown, piston rings, and cylinder liner.

8. Increased Oil Consumption

• Incorrect cylinder oil feed rates may lead to either excessive oil consumption or insufficient lubrication, both of which negatively affect engine performance and operating costs.

1. Cloverleafing

Description

Cloverleafing refers to an uneven wear pattern on the cylinder liner. The liner develops a lobed or oval shape resembling a clover leaf rather than remaining perfectly circular. This wear pattern usually occurs at specific locations corresponding to the fuel injection points.

Causes

Cloverleafing can occur due to several factors, including:

• Uneven temperature distribution around the circumference of the liner
• Poor fuel atomization, causing localized hot spots
• Incorrect fuel injection timing
• Over-lubrication, which may lead to bore polishing
• High thermal and mechanical stresses acting on the liner

Effects

The consequences of cloverleafing include:

• Poor sealing between the piston rings and liner
• Increased blow-by of combustion gases
• Development of irregular wear patterns on the liner surface

2. Micro-Seizure

Description

Micro-seizure is a condition where localized adhesion occurs between the piston ring and the cylinder liner. Small fragments of metal may tear away from the surfaces, leaving fine scoring marks on the liner.

Causes

Micro-seizure can result from several operating conditions, such as:

• Insufficient lubrication
• Low cylinder oil feed rate
• Low oil viscosity
• Excessive engine load
• Poor distribution of lubricating oil
• Breakdown of the oil film due to high temperatures

Effects

The effects of micro-seizure include:

• Roughening of the liner surface
• Damage to piston rings
• If not corrected, it may develop into major seizure or severe liner damage

Large two-stroke marine engines operating on heavy residual fuel oil (HFO) require cylinder lubricating oil with high alkalinity, commonly expressed as a high Base Number (BN), in order to neutralize the acidic products formed during combustion.

The typical composition of such cylinder oil includes the following components:

1. Base Oil

• The main component is a high-viscosity mineral base oil.
• This base oil provides the primary lubricating film strength required to protect the piston rings and cylinder liner.

2. Alkaline Detergents (High BN Additives)

• Cylinder oils contain calcium-based alkaline detergents.
• These additives neutralize sulphuric acid formed during fuel combustion and help maintain the cleanliness of engine components.
• Typical cylinder oil Base Number (BN) ranges from 40 to 100, depending on the sulphur content of the fuel used.

3. Dispersants

• Dispersants help keep carbon particles and combustion residues suspended in the oil, preventing them from forming harmful deposits on engine components.

4. Anti-Wear Additives

• Anti-wear additives reduce direct metal-to-metal contact between moving parts, thereby minimizing wear of the piston rings and cylinder liner.

5. Antioxidants

• Antioxidants prevent oxidation of the lubricating oil at high temperatures, thereby extending the service life of the oil.

6. Corrosion Inhibitors

• These additives protect metal surfaces from acidic corrosion, particularly the cylinder liner, which is exposed to sulphurous combustion products.

7. Thermal Stability Improvers

• Thermal stability additives ensure that the lubricating oil maintains its film strength and stability at high operating temperatures, which is essential for reliable engine operation.

Starting:

• When the telegraph is moved to the desired command, e.g., Dead Slow Ahead from STOP, a solenoid valve in the control system is energized.
• This admits control air to the Ahead switch, which directs air to pneumatic cylinders fitted on each fuel pump. These cylinders shift the fuel pump roller to the “ahead firing” position.
• Control air is also supplied to the starting air distributor, preparing it for the ahead start sequence.
• After these actions, the Ahead switch supplies air to the interlock system, releasing it.
• The control air then opens the Main Automatic Valve (Auto v/v), admitting ~30 bar starting air into the engine via the starting air distributor.
• The starting air is admitted to cylinders as per the firing sequence, and the engine begins to rotate.
• Once sufficient starting RPM is achieved, starting air is cut off, and fuel admission begins, completing the starting sequence.

Stopping:

• The telegraph is moved to STOP.
• This energizes another solenoid valve, which supplies air to the puncture valves of the fuel pumps, cutting off fuel injection, and the engine stops.

Reversing:

• After the engine has completely stopped, the telegraph is moved to Dead Slow Astern.
• A solenoid valve supplies control air to the Astern switch and simultaneously vents the Ahead switch.
• The Astern switch directs control air to the fuel pump pneumatic cylinders, shifting the rollers to the astern firing position, and also supplies air to the starting air distributor.
• The air distributor now operates according to the astern firing order.
• After the interlocks are released, the engine is started in the astern direction using the same process as ahead, but with the astern firing sequence.

(i) Engine fails to turn on air

Causes:

• Low pressure in starting air receiver.
• Valve on starting air receiver closed.
• Valve to starting air distributor closed.
• No pressure in control air system.
• Main starting air valve stuck/locked.
• Turning gear interlock engaged.
• Pistons in starting air distributor sticking.

Remedies:

• Start compressors and pressurize the air bottles.
• Open the air receiver valve.
• Open the valve to the distributor.
• Check control air pressure and open supply if closed.
• Lift the locking plate to working position.
• Disengage turning gear.
• Lubricate pistons, free them, and overhaul the starting air distributor.

(ii) Engine turns on air but fails to fire on fuel

Causes:

• Puncture valves not deactivated.
• Engine shut-down system tripped.
• Sluggishness in manoeuvring gear.
• Fault in governor.
• Fault in fuel system.

Remedies:

• Identify and correct the puncture valve cause.
• Check pressures and temperatures, reset shut-down.
• Lubricate and free the manoeuvring gear.
• Attempt starting from local control, bypassing governor if required.
• Check fuel pressure and temperature.
• Drain fuel for sludge/water contamination.

(iii) Engine fails to reverse

Causes:

• Reversing solenoid valve not receiving voltage.
• Control air signal not reaching engine due to blockage or defective valve.

Remedies:

• Check electrical wiring and control circuits.
• Inspect system by removing the tappet pipe; locate and clear blockages or replace defective valves.

The protection ring is a separate hardened ring seated in a groove at the top of the cylinder liner. Its function is to protect the upper liner bore where the piston rings reverse direction at top dead centre, where combustion gas pressure is maximum and the top ring is practically stationary. At this position the ring cannot wipe away the products of combustion, so the liner throat would otherwise wear rapidly and crack face erosion by the flame. The protection ring provides a hard, corrosion- and wear-resistant surface which: preserves the liner bore diameter, gives a consistent sealing surface and prevents bore polishing, stops the flame eroding the liner rim, and protects against ring groove hammering and fretting. It is usually made of a special hardened steel, and is a replaceable part which can be renewed when worn, so the liner itself lasts longer.

In a conventional four-stroke engine the fuel pump plunger is driven by a cam on the low-speed camshaft through a follower and roller, with a fixed lift profile and a return spring; injection timing is fixed by the cam angle and the timing of the fuel delivery is adjusted by mechanical means (nozzle/rack). In a modern engine the fuel injection drive has been modified by one or both of two approaches; (1) common rail injection, where a high-pressure rail is charged by a power-cylinder-driven or dedicated pump and each injector is opened electronically (solenoid/hydraulically triggered) so timing, duration and pressure are variable, decoupling injection timing from the mechanical cam; (2) electronic unit injection (or single camshaft-less pumps) where the pump and injector are mounted in one unit and the injection is controlled by an electronic control unit (ECU) using signals of engine speed, load and temperature. The modification removes fixed cam timing, allowing variable injection timing (VIT) and flexible injection control for better combustion, lower smoke/emissions and improved fuel economy, and reduces wear on drive gear/cams.

Multi-hole nozzle tips have a series of injection holes. Staggering means the holes are arranged so that the spray axes of adjacent holes are offset by a small angle in relation to the valve centreline or to each other. This means that no two sprays issue from diametrically opposite/equal angles, so the sprays divide into the combustion chamber space evenly and are less likely to hit the piston crown/bowl lip or liner (wall wetting). The staggered arrangement, together with swirl, gives a more even spatial distribution of fuel, better mixing, avoids overlapping of adjacent sprays that would shield each other, improves atomization, and produces more uniform heat release, reducing smoke and unburnt fuel and increasing efficiency.

Swirl is the rotary motion of the air charge about the cylinder axis generated by a tangential/helical inlet port during the induction stroke: the port vanes impart angular momentum to the air. Effect: high relative velocity between the fuel spray and the air improves atomization and mixing, shortens the ignition delay, promotes fast and complete combustion, and gives more uniform gas temperature and lower smoke and emissions.

Squish is the rapid radial inward movement of the air from the outer piston-cylinder clearance (squish band) into the piston bowl as the piston approaches TDC, generated by the piston crown geometry. Effect: it induces turbulence in the combustion chamber just before and during injection, which thoroughly mixes fuel and air, accelerates the flame front, improves combustion and prevents knock, while reducing unburnt HC. Combined, swirl and squish create the turbulence that yields efficient, clean, rapid combustion.

Shown below is the arrangement of a chain drive, which is used to transmit power from the crankshaft to the camshaft.

• It consists of chain sprockets mounted on the crankshaft & camshaft. There can be two or more chains.
• A chain-tightening arrangement is provided, as shown in the fig.
• The chain is guided by the guide bars, which has rubber shock-absorbing pads
• Flyweights are provided as they are the moment compensators.
• Oil spray nozzles are used to lubricate the chain & the wheels.

In the event of a chain link failure during engine operation, the following steps should be carried out:

• Turn the chain until the damaged link is positioned on the longest free end side of the chain, where it is easily accessible.
• Release tension on the chain to facilitate repair.
• Wrap a thin wire around the chain, a short distance from the damaged link, and pull the wire taut using a chain block. This ensures that the chain remains stable during repair.

Remove the Faulty Link:

• Chisel or grind off the riveted metal on the pin ends of the damaged link.
• Use a chain bursting tool:
  • Place the tool over the smallest part of the chain link.
  • Align the dismantling screws precisely over the ground pin ends.
  • Tighten the dismantling screws alternately to push the pins out of the link.
• Remove the damaged link plate and pin.

Install the Replacement Link:

• Replace the damaged plate and pin with a new spare.
• Rivet the ends of the new pin securely.
• If a second chain is present, replace the corresponding link in the other chain to ensure uniform wear and performance.

After the repair, adjust the chain tension to the correct setting.

Reasons for failure:

• Cyclic stresses resulting in fatigue failure cracks.
• Excessive wear due to improper lubrication.
• Overheating due to improper lubrication.

Setting the chain to the correct degree of tension initially:

• Turn the engine to bring the slack part of the chain on the same side as the lighter wheel.
• Place the spring & spring carrier in place. Tighten Nut 'C' till the required compression of spring is achieved (softly touching).
• Tighten nut 'B' till it touches the shaft (softly touching).
• Tighten nut 'C' further again till the shaft carrying carrier is up against the star (further compression will not affect the chain tension).
• The lock nuts A & D are then tightened & locking washers are bent in place.

Chain tightening:

• Excessive wear in the cylinder liner leads to increased piston ring movement, both radially and axially. This fluctuating motion can cause tilting and eventual breakage of the rings.
• If the piston ring does not exert sufficient pressure on the liner, gas pressure can penetrate between the ring and liner, collapsing the ring into the groove and causing breakage.
• Ridge formation near scavenge pockets can create stress concentrations at the piston ring's radial edge, promoting fracture.
• Jamming or sticking of rings caused by excessive carbon deposits, often due to improper combustion or inadequate cleaning during maintenance.
• Excessive wear in the piston ring grooves causes the rings to impact the groove walls during operation, leading to hammering and eventual breakage.
• Inadequate cylinder lubrication results in overheating and increased friction, weakening the rings and causing breakage.
• Acidic corrosion and high-temperature corrosion weaken the ring material, predisposing them to fracture.
• Excessive engine loading can cause the rings to deform beyond their elastic limit, leading to collapse and breakage.
• Using low-quality or non-manufacturer-specified rings compromises material strength and durability, increasing the risk of breakage.
• Improper installation during ring renewal can lead to misalignment, increased stress, and premature failure.

Maintenance practice:

• Perform routine inspections and overhauls of pistons, piston rings, and cylinder liners as per the PMS schedule.
• During overhauls, ensure piston rings and grooves are thoroughly cleaned, and all necessary clearances are measured to verify proper fit.
• Reuse piston rings only if measurements indicate they are within the safe operational limits until the next overhaul.
• Regularly maintain the fuel injection systems to prevent improper combustion and minimise stress on piston rings.
• Ensure that piston rings and liners are free of marks, scratches, or other signs of wear during scavenge inspections.
• During overhaul, install piston rings with proper tools and techniques, ensuring free movement of rings in their grooves.
• A proper running-in procedure after installing new pistons and rings helps to ensure correct seating and minimises initial wear.

Engine Operation:

• Maintaining adequate cylinder oil lubrication minimises friction and heat generation.
• Maintain appropriate cooling of the cylinder liner and piston to avoid thermal stresses.
• Use properly treated fuel oil and ensure correct operation of fuel pumps, injectors, and Variable Injection Timing (VIT) systems.
• Maintaining correct combustion parameters minimises improper combustion and reduces carbon deposits.
• Keep air filters clean to avoid the ingress of dust and abrasive particles into the engine.
• Avoid overloading the engine, which can stress the piston rings and cause failure.

• Low compression pressure, Pmax & power developed.
• Blowpast, increase in scavenge temperature and cause scavenge fire.
• Rise in exhaust temperature.
• Scuffing of liner and increase in wear rate.
• Increased SFOC.
• Fouling of turbocharger due to improper combustion.
• Fouling of EGE and can cause EGE fire.
• Damage to cylinder liner due to blowpast.
• Loss of cylinder lubrication.

The following precautions must be taken while operating an engine with broken or severely worn piston rings:

• Isolate the affected unit as excessive blowpast may cause scavenge fire.
• Monitor the scavenge temperature.
• Run the engine at low load till necessary replacement is carried out.

Static balance: the balancing of the rotating masses so that the resultant of all centrifugal forces on the crankshaft, when the shaft is not rotating, is zero, i.e. the shaft has no heavy point. Static imbalance is detected when the shaft, supported on knife edges or rollers, always rolls until the heavy side is at the bottom. It is corrected by adding or removing mass on the appropriate crank webs (counterweights) or by drilling/grinding material from the heavy side so the centre of mass lies on the axis of rotation.

Dynamic balance: even a statically balanced shaft can have a couple acting because the unbalanced masses lie in different planes along the shaft, so that when rotating a rocking couple is set up. Dynamic balancing is performed on a balancing machine where the shaft is rotated and the vibrations at the two ends are measured; correction masses are added or removed at calculated positions (usually on the crank webs) in the two end planes so that both the resultant force and the resultant couple are zero. In practice counterweights are attached to, or cast integrally with, the crank webs, and for large shafts the balance is checked during manufacture and correction machining is done on the webs.

The primary reciprocating force arises from the acceleration of the reciprocating masses (piston, rings, small-end part of connecting rod) and varies in magnitude once every revolution (at engine speed, first order). For a multi-cylinder engine, by arranging the firing order and crank positions, the primary forces of different cylinders can be made to cancel to a large extent. The methods used are:

1. Balance (counterweight) on the crank web approximately equal to the fictionally rotating part of the reciprocating mass (a rotating balance weight of about half the reciprocating mass placed on the opposite side of the crank throws) which balances part of the primary force.
2. Arrangement of crank throws so that the out-of-balance primary forces act at different phases and cancel each other in symmetrical multi-cylinder engines (e.g. six-cylinder inline engines), including using an even number of cylinders spaced evenly.
3. The use of balance shafts (counter-rotating shaft pairs) running at engine speed, carrying balance weights, which generate a downward force to cancel the primary force.

They are not completely successful because: (1) the primary force is a force (not a pure couple) that cannot be wholly eliminated for an inline engine unless balance shafts are fitted, and these themselves add weight and complexity; (2) the balance weights cancel only the rotating part of the primary force; the reciprocating part acts along the cylinder axis, so the horizontal component passes through the crank and the vertical component cannot be cancelled by webs alone; (3) remaining combined forces leave a residual imbalance which, though small, is not zero; (4) the counterweight approach only reduces the force, and the exact phase relationship varies with speed.

Secondary imbalance: the secondary reciprocating force varies at twice engine speed (second order) and arises from the finite length of the connecting rod causing the piston acceleration to have a second harmonic. Additions to control implantance are:

1. Counter-rotating balance shafts (Lanchester or reciprocating balance shafts) running at engine speed for primary and at twice engine speed for secondary, with weights arranged to generate cancelling forces. These shafts are gear- or chain-driven from the crankshaft.
2. Addition of counterweights of increased size to the crank webs.
3. For secondary forces, two shafts rotating in opposite senses at twice crankshaft speed with parallel axes are used so their horizontal components cancel and vertical components add to balance the secondary force.
4. Use of a flywheel of correct mass moment of inertia and a torsional damper/detuner to damp out the torque variations and torsional vibrations that such vibration produces.

These additions reduce frame vibration, main-bearing loading, and forces transmitted to the ship's structure, preventing excessive vibration and noise.

(a) Cast Iron Welding is a Challenging Task – Reasons

Welding cast iron is considered a difficult and highly specialized operation because its metallurgical characteristics react unfavorably to the intense heat involved in welding. The following factors explain the challenges:

1. High Carbon Content

• Cast iron contains approximately 2% to 4% carbon, which is nearly ten times higher than that of mild steel. During welding, the base metal is subjected to rapid heating and cooling cycles. This high carbon content promotes the formation of extremely hard and brittle microstructures such as martensite or white iron in the heat-affected zone (HAZ). These structures lack toughness and are highly prone to cracking.

2. Inherent Brittleness

• Unlike steel, cast iron has very low ductility. It cannot deform plastically to relieve stresses created by thermal expansion and contraction during welding. Instead of stretching or yielding under stress, it tends to crack suddenly, especially near the weld area.

3. Thermal Shock and Rapid Cooling

• The welding arc produces intense localized heat, creating steep temperature gradients between the weld zone and the surrounding metal. When cooling occurs rapidly, high internal stresses are generated. These stresses frequently result in immediate or delayed cracking in the HAZ.

4. Presence of Graphite in Gray Cast Iron

• Gray cast iron contains graphite flakes distributed within the iron matrix. These flakes act as internal stress concentrators and weaken the structure. During welding, graphite may dissolve into the weld pool, causing embrittlement and reducing the strength and integrity of the joint.

5. Porosity Due to Oil and Grease Contamination

• Engine casings made of cast iron are often porous and, over years of service, absorb oil and grease. When welding heat is applied, these trapped contaminants vaporize and form gas pockets within the molten weld metal. This leads to porosity, reducing weld strength and reliability.

6. Limited Weldability of Certain Types

• While gray cast iron can be welded with strict preheating and controlled cooling procedures, white cast iron is generally considered unweldable because of its extreme hardness and brittleness. It cannot tolerate the stresses introduced by welding.

Because of all these metallurgical and structural limitations, welding cast iron requires careful temperature control, suitable filler materials, and controlled cooling—yet it still carries a high risk of failure.

(b) Alternative Repair Methods Used by Engine Makers for Cast Iron Casings

Due to the significant risks associated with welding cast iron, engine manufacturers often prefer mechanical or “cold repair” techniques. These methods avoid excessive heat and preserve the original structure and alignment of the engine casing.

1. Metal Stitching and Locking (Metalock Method)

• This is the most widely used industrial repair method for cracked engine casings. Holes are drilled along the length of the crack, and specially designed high-tensile metal “stitching pins” and “locks” are inserted. These components mechanically pull the cracked sections together and restore structural strength. Since no heat is applied, the original metallurgy and alignment of the casing remain unaffected.

2. Braze Welding

• In this method, a filler material such as bronze or nickel alloy—having a lower melting point than cast iron—is used. The base metal is not melted; instead, the filler bonds to it. This significantly reduces thermal stress and minimizes the risk of additional cracking compared to conventional fusion welding.

3. Studding Method

• For major fractures, holes are drilled and tapped along the cracked surfaces. Steel studs are screwed into these holes to provide reinforcement. Weld metal or filler material is then deposited over the studs to secure and anchor the repair. The studs act as mechanical reinforcement, improving the strength of the repaired area.

4. Epoxy Bonding (Cold Weld Compounds)

• For non-structural cracks or minor leakages, metal-filled epoxy compounds can be applied. These adhesives provide a watertight and heat-resistant seal without introducing thermal stresses. This method is suitable for temporary repairs or low-load applications.

5. Patching with Insert Replacement

• In cases of severe damage, such as when a connecting rod breaks through the casing, the damaged section can be completely machined out. A new cast iron insert or patch piece is then fitted into the prepared opening and secured using metal stitching or pinning methods. This restores both strength and dimensional accuracy.

(b) Common forms of seal failure in a stern tube

1. Loss of elasticity in seal material – Nitrile rubber seals may lose their elastic properties over time, reducing their sealing effectiveness.
2. Surface damage to chrome liner – Grooving or scoring of the chrome liner can impair sealing surfaces, leading to leakage.
3. Excessive shaft vibration – Heavy vibration of the propeller shaft can cause uneven wear and seal deformation.
4. Insufficient cooling – Inadequate cooling can cause rubber sealing elements to harden and eventually fail.
5. Exceeding running hour limits – Operating beyond the manufacturer’s recommended service life increases the risk of seal failure.
6. Deterioration of oil quality – Contaminated or degraded oil reduces lubrication and protection, accelerating seal wear.
7. Incorrect header tank level adjustment – Failure to adjust the header tank according to vessel draft can cause oil loss, leading to inadequate lubrication and seal damage.

(c) Restricting oil loss due to seal failure whilst on passage

1. Use of high-viscosity oil – Recharge the system with a thicker oil to reduce leakage rate through damaged seals.
2. Temporary oil supply arrangement –
   • Disconnect the existing oil supply line.
   • Connect a 45-gallon drum supported by a block and tackle arrangement.
   • Adjust the drum height to vary the oil head pressure, matching it to the surrounding water pressure and minimizing leakage.
3. Fresh water introduction – Supply fresh water to the gravity tank to emulsify with any leaked oil. The resulting emulsion helps coagulate around the damaged seal area while the oil is circulated to maintain lubrication and sealing.

The aft stern tube bearing is designed to distribute the propeller load uniformly and prevent excessive pressure from being concentrated at one location. The main design features are:

1. Slope Boring (Taper Boring) (Most Important Exam Point)

The aft bearing is machined with a slight taper (slope bore) to match the natural deflection of the propeller shaft caused by the weight of the propeller.

Reason:

This ensures that the load is distributed over the entire length of the bearing instead of being concentrated at the aft end, thereby reducing wear and increasing bearing life.

2. Long Bearing Length

The aft bearing is made longer than the forward bearing, providing a larger contact area between the shaft and the bearing.

Reason:

The increased bearing area reduces the unit bearing pressure and distributes the load more evenly.

3. Large Bearing Surface Area

The bearing is provided with a large diameter and a long white-metal or composite bearing surface.

Reason:

The larger bearing surface spreads the propeller load over a greater area, reducing localised stresses and wear.

4. Proper Bearing Clearance and Hydrodynamic Oil Film

The bearing is designed with the correct clearance to maintain a continuous hydrodynamic oil film between the shaft and the bearing during operation.

Reason:

The oil film prevents metal-to-metal contact and supports the shaft hydraulically, ensuring uniform load distribution and reducing friction and wear.

VIT is a system which advances the fuel injection timing (i.e. makes the start of injection earlier relative to TDC) as the engine load (and hence the fuel charge per cycle) is reduced below about 85 percent of rated load, so that the maximum combustion pressure (Pmax) is kept at a high, nearly constant value over a wide power range. The purpose is to improve fuel economy and reduce specific fuel oil consumption (SFOC). At high load the injection is retarded to keep Pmax within the safe design limit; at part load the timing is advanced to restore Pmax, which increases the indicated work per cycle and the thermal efficiency, giving a flatter and lower SFOC curve. It is achieved on mechanical engine by a device (e.g. the VIT cam / adjustable fuel pump cone or the VIT mechanism on the fuel pump plunger) which alters the effective timing by changing the position of the delivery valve or the timing piston in proportion to the fuel pump index (load). On electronic engines VIT is achieved by software control of the injection start.

Two-stroke propulsion engine: the end of injection is determined by the fuel injection pump plunger - the delivery valve closes when the plunger uncovers the spill port (or when the pump's fixed geometry ends injection). On a cam-driven pump the injection ends by the mechanical cut-off of the helical edge of the plunger when the spill is opened at a point related to the rack position (load). On electronically controlled two-stroke engines the end of injection is set by the injection duration commanded by the ECU controlling the fuel injection quantity.

Four-stroke medium-speed auxiliary engine: end of injection is controlled by the fuel pump geometry - in a jerk pump, by when the plunger's spill helix/spill port is uncovered as the plunger rotates/traverses during injection, which is set by the rack (load). On common-rail engines the end of injection is determined by when the ECU closes the solenoid valve / triggers the injector needle to close, so the pulse width of the electrical command sets the end of injection. So in both cases the end of injection is effectively set by the amount of fuel injected (load), because injection always starts at a given crank angle and the quantity of fuel determines the duration.

A draw card (or indicator card) is a pressure-volume (P-V) diagram taken from the cylinder by a mechanical (indicator) or electronic (draw card/priman) device. Its purpose is to show the actual pressure variation in the cylinder through the cycle, from which the engineer can assess: the maximum combustion pressure (Pmax), the compression pressure, the mean indicated pressure (MIP) and hence the indicated power of the cylinder, the pressure rise and ignition point, and detect faults such as late/early injection, dribble, blow-by, valve leakage, compression loss, and the presence of excessive back-pressure. It is used to balance the load between cylinders, to check combustion quality, to optimise injection timing/VIT, and to detect developing faults, so the engine can be run efficiently and safely.

(a) Purpose of Auxiliary Blower

In a 2-stroke turbocharged marine diesel engine, the auxiliary blower is essential for supplying scavenge air when the turbocharger cannot produce sufficient air pressure. Its functions include:

1. Low-Load Operation: At low engine speeds (e.g., maneuvering), exhaust gas energy is insufficient to drive the turbocharger efficiently. The auxiliary blower provides the required scavenge air pressure for proper scavenging and combustion.
2. Engine Starting: Before starting, it supplies fresh air to purge exhaust gases from the cylinders, ensuring clean and smooth ignition when fuel is introduced.
3. Improved Cooling: The additional air flow helps cool engine components such as pistons, liners, and cylinder heads.
4. Uniform Cylinder Conditions: Ensures even distribution of air across all cylinders for balanced combustion.
5. Emergency Backup: Provides limited air supply in case of turbocharger malfunction, allowing controlled engine operation.

(b) Safeties Provided for the Scavenge Air Receiver

The scavenge air receiver stores pressurized air for delivery to the cylinders, so several safety devices are installed:

1. Pressure Relief Valves (PRV): Protect the receiver from over-pressurization by releasing excess air.
2. Water/Oil Level Alarms or Switches: Detect water or oil accumulation in the receiver to prevent carry-over into the engine, which can cause damage or scavenge fires.
3. High Temperature Alarms: Monitor scavenge air temperature and warn of issues such as cooler malfunction, fouling, or abnormal compression.
4. Drain Cocks (Manual or Automatic): Remove condensate (water/oil) from the receiver to prevent contamination and fire hazards.

(c) Why a Scavenge Air Cooler Is Required

The scavenge air cooler is placed after the turbocharger to cool the compressed air before entering the scavenge receiver. Its purposes are:

1. Increase Air Density: Cooling compressed air raises its density, allowing more oxygen mass to enter the cylinder.
2. Increase Power and Efficiency: Denser air allows more fuel to be burnt efficiently, improving power output and reducing specific fuel consumption.
3. Reduce Engine Operating Temperatures: Lowers cylinder, piston crown, and exhaust gas temperatures, protecting components from thermal stress.
4. Improve Scavenging Efficiency: Cooler, denser air enhances the purging of exhaust gases from the cylinder, ensuring cleaner combustion.
5. Control NOx Emissions: Cooler charge air reduces peak combustion temperatures, thereby reducing NOx formation.
6. Maintain Engine Reliability: Ensures components operate within safe temperature limits, improving longevity and reducing risk of failure.

A crankshaft in a two-stroke engine is subjected to several types of stresses during normal operation. These stresses arise due to gas pressure, inertia forces, centrifugal forces, and torsional effects, and they influence the strength, durability, and fatigue life of the crankshaft.

Types of Stresses Acting on the Crankshaft

1. Torsional (Shear) Stresses: The turning moment produced by gas pressure acting on the piston causes torsional shear stresses in the crankpin, webs, and journals. These stresses may be further increased by torsional vibrations, particularly when the engine operates at or near resonant speeds.
2. Bending Stresses: The crankshaft is subjected to bending moments between the main bearing supports due to gas forces and inertia forces of the reciprocating parts. These bending moments cause alternating tensile and compressive stresses in the crankshaft as it rotates.
3. Centrifugal Stresses: Centrifugal forces generated by rotating masses produce:
   • Tensile stresses in the crank webs,
   • Shear stresses in the crankpin and journals, and
   • Additional bending stresses in the journals.

Generation and Variation of Stresses During One Cycle

• Inertia forces act on both rotating and reciprocating masses. For a constant engine speed, inertia forces on rotating masses remain constant in magnitude.
• Gas forces acting on the crankshaft cause alternating bending of the crank webs and crankpin. During the upper part of the crank travel, the webs tend to spread, while during the lower part they tend to close.
• Gas forces can be resolved into two components:
  • Radial component: Causes bending and twisting of the crankpin.
  • Tangential component: Causes bending of the crank webs and tensile stress in the journals due to torque transmission.
• In a two-stroke engine, the gas force is minimum at bottom dead centre (BDC), reaches a maximum at top dead centre (TDC), and then reduces again to a minimum at BDC.
• Repeated flexing of the crank webs, combined with the reaction to propeller thrust, causes alternate lengthening and shortening of the crankshaft, resulting in axial thrust stresses.

Accommodation of Stresses by Good Design and Operation

• Material selection: The crankshaft material should possess high fatigue strength, good resistance to wear and corrosion, and reliable performance under all operating conditions.
• Grain flow control: Continuous and favourable grain flow improves the strength and fatigue resistance of the material.
• Manufacturing process: Forging is preferred as it produces a sound workpiece free from sub-surface defects.
• Design geometry: Sharp changes in cross-section are avoided to prevent stress concentration points.
• Balancing: Proper balancing of the engine’s rotating and reciprocating masses reduces dynamic stresses.
• Vibration control: Installation of vibration dampers and detuners helps control torsional vibrations and associated stresses.

Thin shell bearings are made of a layer of white metal (or bimetal/tri-metal overlay) bond to a steel back in the form of two half shells. Compared with the older method of cast/white-metal-lined bearing housing (a "white metal flooded" bearing), they offer:

1. Greater fatigue strength: the thin layer of white metal (~0.3 to 0.5 mm) bonded to a rigid steel backing gives a much higher fatigue/load capacity, so higher specific loads and higher peak pressures can be sustained, allowing higher MEP engines.
2. Less white metal required and lower cost of the expensive overlay material.
3. Ease of replacement: shells are standardised and easily interchangeable without remetalling/re-machining the bearing housing; worn shells are simply discarded and new ones fitted.
4. Improved heat flow: the steel backing conducts heat away rapidly from the loaded zone to the housing/oil, reducing bearing temperature.
5. Better dimensional accuracy and conformity (precision bored) giving correct oil film, clearance and alignment; less tendency to fatigue cracking and wiping that plague thick white metal.
6. They can be made of composite materials (e.g. trimetal with a soft overlay on a bronze/lead-bronze) giving a surface tolerant of misalignment plus a strong backing.
7. Reduced maintenance and the ability to use higher oil pressure and load.

The shells are prevented from rotating by locating them firmly in the bearing housing/housing bore. Because the shells are a shrink/interference fit (fit with a small amount of surface pressure when the two shells are tightened - the "crush"), half shells are held in the bearing housing. In addition, a projection/lug or dowel (a small rectangular or semi-circular "nip/locator" tang) at one end of each shell engages with a corresponding groove in the bearing housing bore, so the shells cannot rotate or move axially (each half is restrained by the tang in its groove on the parting face). The correct crush/tightening of the bearing cap also clamps the shells in place, together with the tang preventing rotation and axial movement.

Axial location of the bearing (locating the shells along the shaft axis) is important so that the bearing half-shells cannot slide along the housing, which would misalign the oil holes/grooves with the oil supply passages, cut off lubrication, cause overheating, and lead to wiping/seizure of the journal. It is achieved by the locating tang/lug of each shell fitting into a machined groove in the housing bore, and by the shells being clamped axially (with correct end clearance) between the housing shoulders. The shells are also designed so that the oil groove and hole coincide with the supply passages in the housing, and their axial position is fixed so lubrication is always delivered to the oil groove. Incorrect axial location would starve the bearing of oil and cause failure.

Working of a typical Diesel Electrical Propulsion system:

Electrical propulsion systems in merchant vessels replace the conventional main diesel engine with electric motors as the prime movers for the propeller. The system generally involves diesel generators (or gas turbines) producing electrical power, which is then supplied through transformers, converters, and advanced control systems to high-voltage propulsion motors. These motors directly drive the propeller shaft, thereby eliminating the need for a direct mechanical link between the prime mover and the propeller.

Components of an Electrical Propulsion System:

• Diesel generators or gas turbines (prime movers for electricity generation).
• High-voltage switchboards.
• Transformers (to step voltage up or down).
• Power converters and inverters (for variable speed control of propulsion motors).
• High-voltage propulsion motors (typically synchronous or asynchronous motors).
• Propeller shaft and coupling arrangements.
• Control and monitoring systems (including automation and protection devices).
• Cooling systems for generators, motors, and converters.

Diagram of a typical Diesel Electrical Propulsion system:

Advantages of Electrical Propulsion systems over conventional two-stroke engine propulsion systems:

1. Instead of a large main engine, ships use comparatively small electrical motors of equal or greater output power, saving space and weight for cargo capacity.
2. Electrical motors require much less maintenance compared to main diesel engines.
3. The bottom platform remains almost empty without main diesel engines, allowing greater flexibility for locating machinery.
4. No scavenge space waste oil production, and no cylinder oil or main lube oil usage.
5. Ship construction can omit main engine sump space for lube oil and associated cofferdams.
6. No requirement for starting air pipelines or starting air compressors for the main engine.
7. No HFO/MDO purifiers running as in diesel ships, leading to reduced maintenance and cost.
8. Sludge production is minimal, and condensate from the main engine and associated tanks can be neglected.
9. Watchkeeping is easier since there are fewer parameters to monitor in the absence of a main propulsion engine.
10. No vibrations and noise compared to diesel engines.
11. Electric motors can operate even at 1 or 2 RPM, providing significant navigational advantages.
12. No limitations on the number of starts, unlike diesel engines.
13. Departure or arrival notice time is much shorter compared to diesel engine-propelled ships.
14. Complex electrical and electronic circuits, including transformers and speed control devices, are required (a disadvantage but part of the system design).
15. Sea water piping and cooling circuits are smaller in size compared to diesel ships.
16. Regular transfer of fuel is unnecessary, as the ship’s generators may run on gas.
17. Such ships are generally classed as high-voltage vessels, operating around 6.6 kV to 11 kV.
18. Electric motor starting problems are far fewer compared to diesel engine reversing and starting difficulties.
19. Electrical braking of the propulsion shaft is more effective than mechanical braking in diesel systems.
20. The total operational cost of the ship is significantly lower than that of diesel-engine-propelled ships.

Starting air overlap (or air-overlap) refers to the crank angle range over which starting air is admitted to a cylinder during the starting of a two-stroke engine. In a two-stroke engine starting air must be available to each cylinder during the upper part of its expansion stroke, i.e. shortly after TDC and while the piston is moving down, so that the air pressure acting on the piston crown helps to turn the engine. The overlap is the period when the starting air valve is open. In practice, for a cylinder to be started at any crankshaft position, the starting air valve is arranged to open a small angle before TDC and to close a certain angle after TDC, giving a range (overlap) of admission. To ensure smooth starting at any position, the valve timing is set so the air is admitted to a cylinder shortly after the piston passes TDC (for a two-stroke the air should act on the down-going piston) and the distributor/starting sequence provides air to each cylinder in turn. The overlap prevents air being admitted continuously and is set to give a compact starting torque. The starting air supply must be admitted once per revolution to the cylinders in firing order during the starting stroke.

For a two-stroke engine the starting air valve for each cylinder is opened by the air distributor shortly after top dead centre of the cylinder (typically opening about 5 to 15 degrees after TDC, depending on design) and closed before the exhaust ports are passed or before the firing stroke, i.e. the valve is closed well before the piston reaches bottom dead centre and the exhaust blows down. The duration is set so the air push contributes to rotation without wasting excessive air. In practice the injection of starting air into a particular cylinder is timed to begin after the top dead centre position of the same cylinder and before the commencement of firing/ignition; some designs open a little before TDC to build up pressure. The exact opening/closing angles are set by the cam or by the electronic control of the distributor.

Slow turning is a feature by which the engine is first turned over very slowly (on turning gear or by a special low-speed air supply) for a short period before normal starting, without firing. The purpose is to:

• ensure the engine can be turned over freely before the actual start, detecting any seizure, hydraulic lock (e.g. water in cylinders), or foreign objects;
• expel any liquid (condensate, water, accumulated oil) from the cylinders and combustion space to avoid hydrostatic damage;
• lubricate the cylinder and running gear before the engine starts;
• check the engine is safe to start, particularly after standing idle or after maintenance;
• on engines reversing, to check the direction control.

The slow turning is achieved either by the turning gear (slow mechanical rotation) or by giving a very short burst of starting air to turn the engine a fraction of a revolution (air-driven slow turn), after which the control system checks that the crankshaft is turning and then proceeds to the normal start. It also verifies the starting air system is operational.

VIT is a system which advances the fuel injection timing (i.e. makes the start of injection earlier relative to TDC) as the engine load (and hence the fuel charge per cycle) is reduced below about 85 percent of rated load, so that the maximum combustion pressure (Pmax) is kept at a high, nearly constant value over a wide power range. The purpose is to improve fuel economy and reduce specific fuel oil consumption (SFOC). At high load the injection is retarded to keep Pmax within the safe design limit; at part load the timing is advanced to restore Pmax, which increases the indicated work per cycle and the thermal efficiency, giving a flatter and lower SFOC curve. It is achieved on mechanical engine by a device (e.g. the VIT cam / adjustable fuel pump cone or the VIT mechanism on the fuel pump plunger) which alters the effective timing by changing the position of the delivery valve or the timing piston in proportion to the fuel pump index (load). On electronic engines VIT is achieved by software control of the injection start.

Two-stroke propulsion engine: the end of injection is determined by the fuel injection pump plunger - the delivery valve closes when the plunger uncovers the spill port (or when the pump's fixed geometry ends injection). On a cam-driven pump the injection ends by the mechanical cut-off of the helical edge of the plunger when the spill is opened at a point related to the rack position (load). On electronically controlled two-stroke engines the end of injection is set by the injection duration commanded by the ECU controlling the fuel injection quantity.

Four-stroke medium-speed auxiliary engine: end of injection is controlled by the fuel pump geometry - in a jerk pump, by when the plunger's spill helix/spill port is uncovered as the plunger rotates/traverses during injection, which is set by the rack (load). On common-rail engines the end of injection is determined by when the ECU closes the solenoid valve / triggers the injector needle to close, so the pulse width of the electrical command sets the end of injection. So in both cases the end of injection is effectively set by the amount of fuel injected (load), because injection always starts at a given crank angle and the quantity of fuel determines the duration.

A draw card (or indicator card) is a pressure-volume (P-V) diagram taken from the cylinder by a mechanical (indicator) or electronic (draw card/priman) device. Its purpose is to show the actual pressure variation in the cylinder through the cycle, from which the engineer can assess: the maximum combustion pressure (Pmax), the compression pressure, the mean indicated pressure (MIP) and hence the indicated power of the cylinder, the pressure rise and ignition point, and detect faults such as late/early injection, dribble, blow-by, valve leakage, compression loss, and the presence of excessive back-pressure. It is used to balance the load between cylinders, to check combustion quality, to optimise injection timing/VIT, and to detect developing faults, so the engine can be run efficiently and safely.

The bottom end bearing is a split-type bearing consisting of two semi-circular precision-insert shells housed within the "big end" of the connecting rod.

• Connecting Rod Big End: The larger, lower part of the rod that is split into two halves.
• Bearing Shells: Usually tri-metal (steel back, lead-bronze interlayer, and a thin white-metal or tin-aluminum overlay).
• Bearing Cap (Keep): The removable lower half that secures the assembly around the crankshaft journal.
• Connecting Rod Bolts: High-tensile bolts that clamp the cap to the rod.
• Oil Hole: A "rifle-bored" hole through the center of the connecting rod shank allows lubricating oil to flow from the big end up to the small end.

Serrations (teeth-like structures) are machined into the mating surfaces where the connecting rod meets the bearing cap. Their purposes include

• Positive Location: They ensure perfect alignment between the two halves, preventing any relative movement or "shuffling" during operation.
• Shear Load Resistance: They take the high transverse (shear) loads generated by the oscillating motion of the rod, preventing these forces from being transferred directly to the bolts.
• Fretting Prevention: By arresting relative movement, they reduce fretting corrosion on the mating faces.

Because these bolts are subjected to intense cyclic stress and load reversal, they incorporate several design features:

• Waisted Shank: The bolt diameter is reduced (waisted) between the head and the threads. This increases elasticity, allowing the bolt to stretch more effectively under load and better resist fatigue.
• Rolled Threads: Threads are formed by rolling rather than cutting to improve grain structure and increase fatigue strength.
• High Tensile Material: Usually made of high-grade alloy steel (e.g., chrome-molybdenum) to withstand extreme tensile loads.
• Stress-Reducing Fillets: Large, smooth radii are provided at the head-to-shank transition to minimize stress concentration

[Note: a sketch shows the flyweights/spring governor head connected through hydraulics to the fuel rack; a speed-setting (speeder) spring and a droop feedback linkage.]

Droop is the characteristic where the governor's set-point speed reduces slightly as the load increases, expressed as percentage: Droop = (no-load speed - full-load speed) / (rated speed) x 100.

Droop mechanism: the governor compares the engine speed (via a speed-sensing element - mechanical flyweights whose centrifugal force opposes a speeder spring, or an electrical speed pick-up) with the desired speed. In an electro-hydraulic governor the flyweight/speed sensor generates a "droop" signal. A droop feedback is produced by a linkage connected to the fuel rack/output which, on load increase, moves a spring adjusting the effective speed set-point downward by an amount proportional to the load (output). Thus, as the alternator load increases, more fuel is needed, and the governor slightly reduces the speed set-point. This gives a governor whose speed is proportional to load; several such alternators in parallel can share load stably.

Mechanism step: at no load the governor holds the engine at, say, 100% speed. As load is applied the fuel rack moves to increase fuel; the droop linkage (tied to the output/fuel rack) pulls against the speeder spring, raising the speed setting demand effectively so that the flyweights balance at a slightly lower speed. The engine stabilises at, e.g., 97% speed at full load. Altering the droop (adjusting the droop linkage/percentage droop knob) changes the slope of the speed-load line.

Isochronous governor: has zero speed droop; it maintains constant speed irrespective of load. The speeder spring force is not changed by load, so when load increases the governor opens the fuel rack to hold speed exactly at set-point. It is used where constant frequency/speed is essential, e.g. a single engine driving an alternator feeding a grid: the engine speed and hence the alternator frequency remain constant at every load.

Droop governor: the speed falls slightly with load (droop given as percentage). This is necessary for the stable parallel operation of two or more alternators (generating units) so that they share the load. Without droop, minor differences between two isochronous governors would make the machines "hunt" and fight each other, one taking all the load and the other overloading. With droop, if one machine tends to take more load its speed falls, it takes less, and load is shared proportionally among the machines set at the same droop.

Therefore: single machine + isochronous (constant speed); multiple parallel alternators + droop governors to share load. In practice the droop is commonly set (e.g. 3-4%) and adjustable.

A Variable Geometry Turbocharger (VGT) uses a mechanism to alter the flow area of the exhaust gas entering the turbine, so the turbocharger's performance can be matched to the engine over a wide load and speed range, instead of being fixed at one design point as in a Fixed Geometry Turbocharger (FGT).

Working principle: In a FGT the turbine nozzle area is fixed, so the turbocharger is matched to the engine at one speed/load point (usually around full load). At low load the small exhaust energy makes the turbocharger spin slowly and deliver inadequate boost, giving poor air/fuel ratio, black smoke and high pumping work; at high load a fixed nozzle can cause the turbocharger to over-speed or the turbine to choke. In a VGT the turbine entry/nozzle area is varied by rotating vanes (or a moving ring/sleeve) that surround the turbine wheel. By adjusting the nozzle area the exhaust gas velocity (and hence the energy extracted) is controlled:

• At low load/speed, the nozzle vanes are closed (area reduced), increasing the exhaust gas velocity onto the turbine blades so the turbocharger speeds up earlier and delivers higher boost, improving the air/fuel ratio and reducing smoke.
• At high load/speed, the vanes are opened (area increased), limiting the gas velocity to prevent turbocharger over-speed and provide more flow.

This is controlled electronically/hydraulically based on engine speed and load to hold a target boost (charge air pressure).

How it differs from fixed geometry: the FGT has a fixed nozzle so its match is a compromise; it gives poor part-load performance, poor transient response and smoke. The VGT can maintain good boost and air/fuel ratio across the load range, giving better part-load fuel economy, lower smoke, faster response, and protection against turbocharger surging/over-speeding.

Importance for marine engines and maintaining the right air/fuel ratio: Marine engines, especially for auxiliary power and for variable-speed propulsion (and for complying with emission limits), must operate over a wide load range. The VGT keeps the scavenge/charge air pressure and the air/fuel ratio correct at all power levels. At low engine power a FGT would deliver too little air, causing excess fuel-to-air ratio, incomplete combustion, soot/smoke, high exhaust temperature and poor combustion - the VGT closes to deliver more air. At high power the FGT might over-speed; the VGT opens to limit boost and protect the turbocharger, and to limit peak cylinder pressures. By maintaining the correct air/fuel ratio the VGT ensures complete combustion, low smoke and particulate emissions, lower fuel consumption, and controlled NOx - which is important for Tier II/Tier III compliance. It also gives better load acceptance and manoeuvring response which a fixed-turbocharger engine cannot achieve.

A crankshaft in a two-stroke engine is subjected to several types of stresses during normal operation. These stresses arise due to gas pressure, inertia forces, centrifugal forces, and torsional effects, and they influence the strength, durability, and fatigue life of the crankshaft.

Types of Stresses Acting on the Crankshaft

1. Torsional (Shear) Stresses: The turning moment produced by gas pressure acting on the piston causes torsional shear stresses in the crankpin, webs, and journals. These stresses may be further increased by torsional vibrations, particularly when the engine operates at or near resonant speeds.
2. Bending Stresses: The crankshaft is subjected to bending moments between the main bearing supports due to gas forces and inertia forces of the reciprocating parts. These bending moments cause alternating tensile and compressive stresses in the crankshaft as it rotates.
3. Centrifugal Stresses: Centrifugal forces generated by rotating masses produce:
   • Tensile stresses in the crank webs,
   • Shear stresses in the crankpin and journals, and
   • Additional bending stresses in the journals.

Generation and Variation of Stresses During One Cycle

• Inertia forces act on both rotating and reciprocating masses. For a constant engine speed, inertia forces on rotating masses remain constant in magnitude.
• Gas forces acting on the crankshaft cause alternating bending of the crank webs and crankpin. During the upper part of the crank travel, the webs tend to spread, while during the lower part they tend to close.
• Gas forces can be resolved into two components:
  • Radial component: Causes bending and twisting of the crankpin.
  • Tangential component: Causes bending of the crank webs and tensile stress in the journals due to torque transmission.
• In a two-stroke engine, the gas force is minimum at bottom dead centre (BDC), reaches a maximum at top dead centre (TDC), and then reduces again to a minimum at BDC.
• Repeated flexing of the crank webs, combined with the reaction to propeller thrust, causes alternate lengthening and shortening of the crankshaft, resulting in axial thrust stresses.

Accommodation of Stresses by Good Design and Operation

• Material selection: The crankshaft material should possess high fatigue strength, good resistance to wear and corrosion, and reliable performance under all operating conditions.
• Grain flow control: Continuous and favourable grain flow improves the strength and fatigue resistance of the material.
• Manufacturing process: Forging is preferred as it produces a sound workpiece free from sub-surface defects.
• Design geometry: Sharp changes in cross-section are avoided to prevent stress concentration points.
• Balancing: Proper balancing of the engine’s rotating and reciprocating masses reduces dynamic stresses.
• Vibration control: Installation of vibration dampers and detuners helps control torsional vibrations and associated stresses.

The protection ring is a separate hardened ring seated in a groove at the top of the cylinder liner. Its function is to protect the upper liner bore where the piston rings reverse direction at top dead centre, where combustion gas pressure is maximum and the top ring is practically stationary. At this position the ring cannot wipe away the products of combustion, so the liner throat would otherwise wear rapidly and crack face erosion by the flame. The protection ring provides a hard, corrosion- and wear-resistant surface which: preserves the liner bore diameter, gives a consistent sealing surface and prevents bore polishing, stops the flame eroding the liner rim, and protects against ring groove hammering and fretting. It is usually made of a special hardened steel, and is a replaceable part which can be renewed when worn, so the liner itself lasts longer.

In a conventional four-stroke engine the fuel pump plunger is driven by a cam on the low-speed camshaft through a follower and roller, with a fixed lift profile and a return spring; injection timing is fixed by the cam angle and the timing of the fuel delivery is adjusted by mechanical means (nozzle/rack). In a modern engine the fuel injection drive has been modified by one or both of two approaches; (1) common rail injection, where a high-pressure rail is charged by a power-cylinder-driven or dedicated pump and each injector is opened electronically (solenoid/hydraulically triggered) so timing, duration and pressure are variable, decoupling injection timing from the mechanical cam; (2) electronic unit injection (or single camshaft-less pumps) where the pump and injector are mounted in one unit and the injection is controlled by an electronic control unit (ECU) using signals of engine speed, load and temperature. The modification removes fixed cam timing, allowing variable injection timing (VIT) and flexible injection control for better combustion, lower smoke/emissions and improved fuel economy, and reduces wear on drive gear/cams.

Multi-hole nozzle tips have a series of injection holes. Staggering means the holes are arranged so that the spray axes of adjacent holes are offset by a small angle in relation to the valve centreline or to each other. This means that no two sprays issue from diametrically opposite/equal angles, so the sprays divide into the combustion chamber space evenly and are less likely to hit the piston crown/bowl lip or liner (wall wetting). The staggered arrangement, together with swirl, gives a more even spatial distribution of fuel, better mixing, avoids overlapping of adjacent sprays that would shield each other, improves atomization, and produces more uniform heat release, reducing smoke and unburnt fuel and increasing efficiency.

Swirl is the rotary motion of the air charge about the cylinder axis generated by a tangential/helical inlet port during the induction stroke: the port vanes impart angular momentum to the air. Effect: high relative velocity between the fuel spray and the air improves atomization and mixing, shortens the ignition delay, promotes fast and complete combustion, and gives more uniform gas temperature and lower smoke and emissions.

Squish is the rapid radial inward movement of the air from the outer piston-cylinder clearance (squish band) into the piston bowl as the piston approaches TDC, generated by the piston crown geometry. Effect: it induces turbulence in the combustion chamber just before and during injection, which thoroughly mixes fuel and air, accelerates the flame front, improves combustion and prevents knock, while reducing unburnt HC. Combined, swirl and squish create the turbulence that yields efficient, clean, rapid combustion.

The thermostatic expansion valve (TEV) controls the flow of refrigerant to the evaporator so that it is completely vaporised at the evaporator outlet. It senses: (1) superheat at the outlet via the sensing bulb (bulb pressures), (2) the spring pressure, and (3) the evaporator pressure at the outlet. The valve holds the diaphragm in balance between these pressures. In a valve with an internal equalising connection, the evaporator pressure acting on the underside of the diaphragm is taken from inside the valve, i.e. at the valve outlet, which is the inlet pressure to the evaporator. In an evaporator with several parallel circuits or with a long distributing header, there is a pressure drop through the evaporator (between the valve outlet and the point where the bulb senses). This pressure drop means the pressure at the evaporator outlet (where the bulb is and where the gas leaves) is below the pressure at the valve outlet. If internal equalising is used, the valve sees the high inlet pressure, so it would under-feed: the effective superheat it senses is larger than the true superheat, and the evaporator would be starved, giving a large superheat and poor cooling.

The external equalising connection takes the evaporator pressure from the outlet (bulb) region via an external tube to the underside of the diaphragm. This cancels out the evaporator pressure drop, so the TEV controls on the true outlet superheat and feeds correctly. It is therefore fitted on evaporators with distributors, multiple circuits, or a significant internal pressure drop, to keep the control accurate. Without it, the evaporator would be under-fed (too little refrigerant), causing freezing of frost at the outlet but poor overall capacity, or even hunting.

A back pressure valve (an evaporator pressure regulator / suction-regulating valve or an outlet regulating valve) is fitted in the suction line to hold the evaporator pressure (and hence the evaporating temperature) at a set minimum value, regardless of the compressor suction pressure. Its purpose is to maintain a constant evaporator temperature in a multi-temperature or cold room system (e.g. to avoid freezing of a water/chilled-commodity chamber), or to protect a high-temperature evaporator from being pulled down by a compressor working to a lower pressure circuit. By throttling the suction gas it holds back pressure and stabilises the box temperature (e.g. to keep a vegetable room above freezing point).

Effect if the back pressure valve leaks: if it has an internal leak or fails to close properly, the evaporator pressure cannot be maintained; suction pressure falls, the evaporating temperature drops, and the box/space can overcool and freeze the product (e.g. damage to cargo or water). It also makes the compressor work harder at lower suction pressure, increasing energy consumption, and can cause ice formation on the evaporator. If it fails fully open, temperature control is lost and the room may go too cold. Leakage also unbalances the multi-temperature system, starving other higher-pressure circuits.

An electronic (electric) expansion valve replaces the thermal sensing bulb and diaphragm of a mechanical TEV with sensors and an electronic controller (e.g. a PLC/ECU) plus a motorised drive valve (the EEV itself). The EEV is a stepper/solenoid-driven needle valve in the liquid line. Function:

1. Temperature sensors (Pt100/thermistor) are fitted at the evaporator inlet and outlet; sometimes pressure sensors at the evaporator.
2. The controller computes the actual superheat at the evaporator outlet (outlet temperature minus the saturation temperature corresponding to the outlet pressure) continuously.
3. It compares this actual superheat with a set-point superheat, and adjusts the opening of the EEV (by stepping the motor) to maintain the set-point.
4. If superheat is too high (starved) it opens the valve more; if too low (flooded) it closes it.

Advantages: very accurate superheat control over a wide range, quick response, no hunting, better energy efficiency, allows lower stable set superheat (utilising the evaporator fully), remote adjustment, and protection functions (e.g. liquid slugging prevention). It is used for refrigeration/freezing and on systems where precise control and efficiency matter.

Electronically controlled marine diesel engines (e.g. camshaft-less engines such as MAN B&W ME-series and WinGD X-series) replace the mechanical camshaft and fuel pump with an integrated electronic control system that controls fuel injection timing, exhaust valve timing, and cylinder lubrication by hydraulic actuation triggered by solenoid valves commanded by the engine control system (ECU). The three claimed advantages are achieved as follows:

• Variable fuel injection timing: the ECU can advance or retard the start of injection (and vary the injection duration/profile) precisely with load, keeping the maximum combustion pressure (Pmax) at the optimum level over the whole power range, equivalent to an unlimited VIT. This reduces specific fuel consumption (SFOC).
• Precise control of injection quantity/injection pressure: the quantity injected can be set accurately per cylinder, and multi-event injection (pilot/pre/post) can be used to optimise combustion; the injection profile can be shaped to reduce heat loss and improve thermal efficiency.
• Optimised exhaust valve timing: the exhaust valve opening/closing timing can be varied to control the effective compression/expansion and to optimise the Miller effect and scavenging, reducing pumping losses and improving efficiency.
• Load-dependent cylinder deactivation (on some engines): at low load, some cylinders are cut out (no fuel) while the rest take the load, keeping the remaining cylinders at high load where specific consumption is lower, improving part-load economy.
• Balanced cylinder output: load balancing between cylinders to even out temperatures and maximise efficiency and reliability.

• The precise and variable injection timing, injection shaping (pilot injection), and exhaust valve timing allow the combustion to be tuned to lower NOx (by reducing local peak flame temperatures, e.g. by retarding injection or by the Miller effect/late inlet valve closing) and to lower smoke and particulate.
• Fuel injection can be adapted to operating conditions and to exhaust gas treatment (e.g. to keep the exhaust temperature high enough for a downstream SCR system at low load, or to work with the EGB/exhaust waste heat recovery).
• Because injection timing can be set individually per cylinder, the engine can run with consistent low emissions across cylinders and loads.
• Combined with the ECU the engine can be adjusted to meet the required NOx (Tier II/Tier III) and to give lower SOx smoke when burning various fuels.
• Camshaft-less engines also allow flexible cylinder lubrication (electronic lubrication) to minimise oil consumption and deposits, and can be adapted to synthetic/gas fuels.

• Because there is no camshaft to be shifted and no fuel pump drive to be reversed, the direction of rotation can be changed almost immediately: the ECU simply switches the firing order and controls the valves and injectors, so reversal is fast.
• Starting air consumption is reduced because the injection can begin at the correct instant on the down-stroke, and the engine can be started more efficiently using electronic control of the starting sequence.
• Rapid load acceptance: injection timing and quantity can be advanced before the load is applied, giving fast torque response, so acceleration and deceleration (crash manoeuvres) are quick and controllable.
• The hydraulic system provides instant actuation, and the controls avoid the delays of mechanical reversing gear, so the time to go from ahead to astern is minimised and the manoeuvre is safer and smoother.

These features together give better fuel economy, lower emissions and markedly better manoeuvring performance than camshaft-controlled engines.

The thrust bearing transfers the axial thrust of the propeller (driving the ship ahead or astern) to the ship's structure. It consists of a thrust collar (a large collar mounted on or integral with the crankshaft) running against a set of thrust pads (segments). Each pad rests on a pivot/edge or a spherical seat so it can tilt slightly. As the collar rotates, oil is dragged into the wedge-shaped space between the collar face and the tilted pad, building up a hydrodynamic pressure film which carries the load. The pad pivots to generate a converging oil film wedge. The thrust is transmitted through the pads to the bearing ring and then to the engine bedplate/chocks. Ahead pads carry the forward (ahead) thrust; astern pads (on the opposite face of the collar) carry the engine astern thrust and hold the shaft in place.

The critical clearance is the axial (end) float/clearance - the total movement of the shaft collar between the ahead and astern pads (the "end clearance"/thrust bearing axial clearance). This must be within maker's limits because:

• if too small, the pads may bind/overheat and the collar may not build a proper oil film; thermal growth of the shaft could wipe the pads;
• if too large, the shaft can move excessively, causing propeller thrust collar hammering, rapid pad and collar wear, misalignment of the crankshaft (affecting main bearing deflections), the crankshaft web deflections going out of limits, and vibration.

The radial/pad clearances and the oil film thickness to the pads are also critical for load capacity. The complete total clearance is typically of the order of tenths of a millimetre.

The axial clearance is adjusted by fitting/adjusting shims or liners behind the thrust pads (ahead and astern), or by moving the complete thrust bearing housing fore/aft in the chock location. Removing shims reduces clearance; adding shims increases it. In some designs, thin adjusting liners are placed between the support ring and the bearing housing. The clearance is measured using a dial gauge made up on the shaft collar or by feeler gauges, with the method of checking the total end float by levering the shaft. Adjustment is done by dismantling access covers, adding/removing equal shims on each pad to keep pads parallel, and re-checking the clearance after tightening.

• Oil starvation: insufficient oil supply (low pressure/flow, blocked oil passage, oil cooler fouled) so no proper hydrodynamic film.
• Misdistribution of load among pads or misalignment of the thrust collar relative to the pads (e.g. from crankshaft/hull deflection), causing one pad to take excessive load.
• Excessive thrust due to overload, propeller damage, fouled hull/propeller, or wrong astern operation.
• Oil viscosity too low (hot oil, wrong oil) so the film cannot support the load.
• Contamination of oil with abrasive particles causing metal-to-metal wear.
• Incorrect running-in: new or reconditioned pads not bedded in.
• The collar thrust face distorted/out-of-flat, or edge loading due to pad geometry errors.

The oil film is maintained by ensuring a continuous, adequate, filtered supply of oil at the correct pressure and temperature to each bearing pad's leading edge. The pad shape, keel/pivot position and the collar rotation create the wedge; oil is dragged into the convergent gap by viscosity generating the hydrodynamic film. The bearing must be checked for correct oil flow, the oil free of contaminants, correct viscosity (by cooling), proper pad pivot action, and relief of excess thrust. Keeping the thrust loading reasonable and the pads/collar smooth and parallel also helps maintain the film.

When the wear of a cylinder liner (the increase in liner bore diameter at and above the top ring reversal point) is plotted against running hours, the points are scattered because liner wear is not constant. Reasons for the scatter:

1. Wear is highly dependent on load and speed; the engine rarely runs at constant power. Running hours give no account of the actual loading, so at equal running hours different ships produce different wear.
2. The measurement is done at intervals and at slightly different circumferential positions; the point of maximum wear may not coincide with the fixed measurement line each time, and gauge/reading errors occur.
3. Environmental and operating factors vary: fuel quality, fuel sulphur/BN match, cylinder oil feed rate, water temperature (cooling conditions), ambient air temperature, and specific operating events (slow steaming, manoeuvring, scavenge fires, contamination).
4. Thermal and mechanical factors: at low load the liner runs cooler, increasing cold corrosion wear; high load increases abrasive/mechanical wear. The proportion changes.
5. Cold corrosion and scuffing can cause sudden local wear steps which are not linear.
6. Wear is not uniformly distributed around the bore; the most wear is near TDC and on the thrust/anti-thrust sides, so a standard measurement may bounce between maxima and minima.

To show the wear rate in a more acceptable form: wear should be plotted as a function of a load/time factor rather than raw hours, e.g. as wear against equivalent hours at a reference load (e.g. wear per 1000 hours at 85% MCR), or normally the mean wear rate in mm per 1000 hours is computed by fitting a best straight line (least-squares) through the points rather than connecting them point-to-point; the slope of the best-fit line gives a uniform average wear rate. Outliers are examined and the wear rate expressed as an average plus its scatter band. Very often the wear is also compared with the maker's allowable rate.

To forecast the length of life of a liner:

• Determine the maximum allowable liner wear (the maximum wear limit) allowed by the manufacturer/society, often quoted as a percentage of the bore (e.g. 0.6 to 1.0% of bore) at which the liner should be renewed/withdrawn.
• Using the well-established average wear rate (from the best-fit line), the life is calculated as: Life (hours) = (allowable wear, mm)/(average wear rate, mm per hour).
• Because wear rate is not constant, use the average worn rate at the reference load and compare with margin; also take into account that wear accelerates once the top ring groove or bore becomes worn, so the forecast is a first estimate, and the liner should be re-measured at intervals and the forecast revised accordingly, with the decision subject to the actual point of maximum wear and to the condition of the running surface, and to the remaining thickness for cylinder wall strength.

Selective Catalytic Reduction is a means of converting nitrous oxides in the exhaust with the help of a catalyst into diatomic nitrogen and water.

A reductant Anhydrous Ammonia (NH3), Aqueous Ammonia (Ammonium Hydroxide) or Urea (Carbamide) solution is added to a stream of exhaust gas and is adsorbed onto a catalyst. Carbon Dioxide (CO2) is a reaction product when urea is used as the reductant.

The chemical equation for the reaction using either anhydrous aqueous ammonia for the process is

4NO + 4NH3 + O2 = 4N2 + 6H2O

2NO2 + 4NH3 + O2 = 3N2 + 6H2O

NO + NO2 + 2NH3 = 2N2 + 3H2O

The reaction for urea instead of anhydrous or aqueous ammonia is

4NO + 2(NH2)2CO + O2 = 4N2 + 4H2O + 2CO2 (in presence of catalyst)

Selective Catalytic Reduction

This exhaust gas after-treatment technology has a NOx abatement capability Of more than 80%. The SCR concept involves injecting a Urea-Water solution into the exhaust gas stream in combination with a special catalyst unit.

The SCR is considered as an additional and independent exhaust treatment system and as such does not interfere with the basic engine design or combustion process.

The process diagram below gives a better understanding of the SCR system wherein the urea interacts with nitrous oxides present in the incoming exhaust gas, in the presence of a catalyst, converting it into free nitrogen and water vapour.

The Maritime Environmental Protection Committee (MEPC) At The IMO has published guidelines for the certification of selective catalytic reduction (SCR) systems, referred to the “SCR Guideline”, namely IMO Resolution MEPC.198(62).

According to their configurations, SCRs can Be Classified into 2 Types- They can be either installed between The Exhaust Gas Manifold & The Turbocharger or between The Turbocharger and The Exhaust Gas Boiler.

1. High-Pressure SCR

In the High-Pressure SCR, the reactor is placed before the turbocharger. A sufficient exhaust gas temperature is to be maintained between 300 to 400 deg Celsius, which might be challenging when the engine is running at low loads and manoeuvring.

Therefore, for two-stroke engines, the most likely location of the SCR unit is before the turbocharger in order to expand the active range of SCR operation. This has little to no effect on the engine combustion process.

It is possible to run high-pressure SCRs on Heavy Fuel Oil.

2. Low-Pressure SCR

In Low-Pressure SCRs, the reactor is placed after the turbine. Pre-heating of the exhaust gas stream may be necessary in order to achieve a sufficient temperature at the reactor inlet for the catalytic reaction. Some power generation may be needed for preheating.

Components of an SCR Dosing Unit

The dosing unit consists of a compact external dosing system having a urea-water solution tank. The tank size depends upon how often the vessel enters NOx Tier III areas and how often the SCR is put in use. Urea Tank capacities range from 4 to 10 cub metres/MW for larger engines.

The urea for marine use is usually dissolved in water having a concentration of 32%-40%. Urea is a non-toxic odourless solution considered safe to transport and store at ambient temperature & pressure. However special caution is required in winter temperatures in order to avoid crystallization.

The dosing handling system provides the reducing agent (urea solution) based on the dosing demand signal provided by the SCR and Engine control and monitoring system.

Vaporizer/ Mixing Unit

The urea from the dosing system is metered and injected into the vaporizer or mixing unit. The injected reducing agent (urea) will vaporise and mix with the incoming exhaust gas.

The mixing unit is in line with the exhaust manifold of the engine and its pipes are designed & constructed after complex flow calculations & intensive testing, to ensure a good mixture of the urea solution & hot exhaust gases. The mixing unit is usually 2 to 6 meters long and 500mm in diameter, however, size may vary as per Engine size.

Injection tubes from the dosing unit penetrate the vaporizer from the bottom, the top of the vaporizer is equipped with an electronic enclosure having a NOx measurement sensor to monitor nitrous oxides in the exhaust gas and Backpressure sensor.

SCR Reactor Chamber

This is where the conversion of NOx in exhaust gas into nitrogen and water takes place in the presence of catalyst material. The SCR reactor contains cassettes of the catalyst substrate material. The substrate elements work in limited temperatures, if exhaust gas temperature is too high, the elements get destroyed.

If the temperature is too low, SCR efficiency is reduced. Catalyst element contains Vanadium Pentoxide (V2O5) which helps the reaction process of converting the urea and exhaust gas into nitrogen and water vapour. The SCR reactor volume is usually 1.5-3 cub metres/MW installed power.

Fuel Oil Quality and SCR technology

The sulphur content in fuel oil and consequent SO2 concentration in the exhaust gas is a critical parameter which has to be observed while operating SCR systems. Urea temperature is to be controlled according to sulphur content in fuel.

A high sulphur content in presence of a low exhaust gas temperature (in case of manoeuvring) will require a higher temperature of urea solution to be injected as a condensation of exhaust gas could result in corrosion and catalyst substrate damage. A lesser content of sulphur in fuel will allow a lesser temperature of urea solution to be injected.

Condensation of water vapour in the presence of sulphur in the exhaust gas during low load operations can cause the formation of solid ammonium bisulphate. Thus, the exhaust inlet temperature is to be kept high enough to avoid condensation of ammonium bisulphate onto catalyst substrate elements.

Condensation would severely affect NOx reduction performance and cause clogging, increasing backpressure due to soot formation in the reactor.

Soot Blowing Unit

To prevent contamination of the reactor elements, a soot blowing system is installed. Soot blowing is done using compressed air of 7 bar.

SCR Control Sensor Unit

NOx sensors measure the NOx concentration before the SCR reactor and the turbocharger.

The reactor chamber also contains outlet NOx sensors and outlet temperature sensors.

Venting System

The venting system vents the SCR reactor when the SCR is bypassed (i.e. when the engine is running in Tier-II mode) to avoid exhaust gas accumulation and soot formation in the reactor. The reactor is vented with Fresh Air during Tier II operation.

The Reactor Sealing Valve is used to seal the reactor during venting when the SCR is not in use.

Static balance: the balancing of the rotating masses so that the resultant of all centrifugal forces on the crankshaft, when the shaft is not rotating, is zero, i.e. the shaft has no heavy point. Static imbalance is detected when the shaft, supported on knife edges or rollers, always rolls until the heavy side is at the bottom. It is corrected by adding or removing mass on the appropriate crank webs (counterweights) or by drilling/grinding material from the heavy side so the centre of mass lies on the axis of rotation.

Dynamic balance: even a statically balanced shaft can have a couple acting because the unbalanced masses lie in different planes along the shaft, so that when rotating a rocking couple is set up. Dynamic balancing is performed on a balancing machine where the shaft is rotated and the vibrations at the two ends are measured; correction masses are added or removed at calculated positions (usually on the crank webs) in the two end planes so that both the resultant force and the resultant couple are zero. In practice counterweights are attached to, or cast integrally with, the crank webs, and for large shafts the balance is checked during manufacture and correction machining is done on the webs.

The primary reciprocating force arises from the acceleration of the reciprocating masses (piston, rings, small-end part of connecting rod) and varies in magnitude once every revolution (at engine speed, first order). For a multi-cylinder engine, by arranging the firing order and crank positions, the primary forces of different cylinders can be made to cancel to a large extent. The methods used are:

1. Balance (counterweight) on the crank web approximately equal to the fictionally rotating part of the reciprocating mass (a rotating balance weight of about half the reciprocating mass placed on the opposite side of the crank throws) which balances part of the primary force.
2. Arrangement of crank throws so that the out-of-balance primary forces act at different phases and cancel each other in symmetrical multi-cylinder engines (e.g. six-cylinder inline engines), including using an even number of cylinders spaced evenly.
3. The use of balance shafts (counter-rotating shaft pairs) running at engine speed, carrying balance weights, which generate a downward force to cancel the primary force.

They are not completely successful because: (1) the primary force is a force (not a pure couple) that cannot be wholly eliminated for an inline engine unless balance shafts are fitted, and these themselves add weight and complexity; (2) the balance weights cancel only the rotating part of the primary force; the reciprocating part acts along the cylinder axis, so the horizontal component passes through the crank and the vertical component cannot be cancelled by webs alone; (3) remaining combined forces leave a residual imbalance which, though small, is not zero; (4) the counterweight approach only reduces the force, and the exact phase relationship varies with speed.

Secondary imbalance: the secondary reciprocating force varies at twice engine speed (second order) and arises from the finite length of the connecting rod causing the piston acceleration to have a second harmonic. Additions to control implantance are:

1. Counter-rotating balance shafts (Lanchester or reciprocating balance shafts) running at engine speed for primary and at twice engine speed for secondary, with weights arranged to generate cancelling forces. These shafts are gear- or chain-driven from the crankshaft.
2. Addition of counterweights of increased size to the crank webs.
3. For secondary forces, two shafts rotating in opposite senses at twice crankshaft speed with parallel axes are used so their horizontal components cancel and vertical components add to balance the secondary force.
4. Use of a flywheel of correct mass moment of inertia and a torsional damper/detuner to damp out the torque variations and torsional vibrations that such vibration produces.

These additions reduce frame vibration, main-bearing loading, and forces transmitted to the ship's structure, preventing excessive vibration and noise.

(a) Cast Iron Welding is a Challenging Task – Reasons

Welding cast iron is considered a difficult and highly specialized operation because its metallurgical characteristics react unfavorably to the intense heat involved in welding. The following factors explain the challenges:

1. High Carbon Content

• Cast iron contains approximately 2% to 4% carbon, which is nearly ten times higher than that of mild steel. During welding, the base metal is subjected to rapid heating and cooling cycles. This high carbon content promotes the formation of extremely hard and brittle microstructures such as martensite or white iron in the heat-affected zone (HAZ). These structures lack toughness and are highly prone to cracking.

2. Inherent Brittleness

• Unlike steel, cast iron has very low ductility. It cannot deform plastically to relieve stresses created by thermal expansion and contraction during welding. Instead of stretching or yielding under stress, it tends to crack suddenly, especially near the weld area.

3. Thermal Shock and Rapid Cooling

• The welding arc produces intense localized heat, creating steep temperature gradients between the weld zone and the surrounding metal. When cooling occurs rapidly, high internal stresses are generated. These stresses frequently result in immediate or delayed cracking in the HAZ.

4. Presence of Graphite in Gray Cast Iron

• Gray cast iron contains graphite flakes distributed within the iron matrix. These flakes act as internal stress concentrators and weaken the structure. During welding, graphite may dissolve into the weld pool, causing embrittlement and reducing the strength and integrity of the joint.

5. Porosity Due to Oil and Grease Contamination

• Engine casings made of cast iron are often porous and, over years of service, absorb oil and grease. When welding heat is applied, these trapped contaminants vaporize and form gas pockets within the molten weld metal. This leads to porosity, reducing weld strength and reliability.

6. Limited Weldability of Certain Types

• While gray cast iron can be welded with strict preheating and controlled cooling procedures, white cast iron is generally considered unweldable because of its extreme hardness and brittleness. It cannot tolerate the stresses introduced by welding.

Because of all these metallurgical and structural limitations, welding cast iron requires careful temperature control, suitable filler materials, and controlled cooling—yet it still carries a high risk of failure.

(b) Alternative Repair Methods Used by Engine Makers for Cast Iron Casings

Due to the significant risks associated with welding cast iron, engine manufacturers often prefer mechanical or “cold repair” techniques. These methods avoid excessive heat and preserve the original structure and alignment of the engine casing.

1. Metal Stitching and Locking (Metalock Method)

• This is the most widely used industrial repair method for cracked engine casings. Holes are drilled along the length of the crack, and specially designed high-tensile metal “stitching pins” and “locks” are inserted. These components mechanically pull the cracked sections together and restore structural strength. Since no heat is applied, the original metallurgy and alignment of the casing remain unaffected.

2. Braze Welding

• In this method, a filler material such as bronze or nickel alloy—having a lower melting point than cast iron—is used. The base metal is not melted; instead, the filler bonds to it. This significantly reduces thermal stress and minimizes the risk of additional cracking compared to conventional fusion welding.

3. Studding Method

• For major fractures, holes are drilled and tapped along the cracked surfaces. Steel studs are screwed into these holes to provide reinforcement. Weld metal or filler material is then deposited over the studs to secure and anchor the repair. The studs act as mechanical reinforcement, improving the strength of the repaired area.

4. Epoxy Bonding (Cold Weld Compounds)

• For non-structural cracks or minor leakages, metal-filled epoxy compounds can be applied. These adhesives provide a watertight and heat-resistant seal without introducing thermal stresses. This method is suitable for temporary repairs or low-load applications.

5. Patching with Insert Replacement

• In cases of severe damage, such as when a connecting rod breaks through the casing, the damaged section can be completely machined out. A new cast iron insert or patch piece is then fitted into the prepared opening and secured using metal stitching or pinning methods. This restores both strength and dimensional accuracy.