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
- Smokeless Operation
- High injection pressure is maintained throughout the entire operating range, resulting in superior atomization and efficient combustion with significantly reduced smoke emissions.
- Reduced Fuel Consumption
- Electronic control maintains optimum engine settings throughout service life, preventing deterioration in fuel economy due to wear or maladjustment.
- 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.
- 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.
- Improved Combustion
- Precise control of injection timing, pressure and injection pattern results in complete combustion, higher thermal efficiency and lower exhaust temperatures.
- Lower Emissions
- Reduced NOₓ, particulate matter and visible smoke due to optimized injection characteristics.
- Reduced Maintenance
- Elimination of individual jerk pumps, pump timing adjustments and mechanical linkages reduces wear and maintenance requirements.
- 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
- High Initial Cost
- More expensive than conventional jerk-type systems due to electronic control units, sensors, actuators and hydraulic components.
- Greater System Complexity
- Requires sophisticated electronic control systems, hydraulic servo systems and high-pressure fuel equipment.
- Higher Maintenance Skill Requirement
- Troubleshooting and repairs require trained personnel and specialized diagnostic equipment.
- Sensitive to Fuel Cleanliness
- High-pressure components and control valves are susceptible to contamination; excellent fuel filtration is essential.
- 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.