1. Thermal and Oxidation Stability: The oil must resist degradation under the high temperatures and oxidative conditions within the engine. Deterioration leads to sludge formation, increased viscosity, and reduced lubricating effectiveness. The oil needs to remain stable for extended periods under normal operating conditions.
2. Viscosity: Appropriate viscosity is essential for maintaining an adequate oil film thickness between moving engine components. This prevents metal-to-metal contact, reducing wear and preventing damage. Too high a viscosity hinders oil flow, increasing friction and temperature. Conversely, viscosity that is too low increases the risk of metal-to-metal contact. The viscosity grade must be selected based on the engine's operating temperature and load.
3. Total Base Number (TBN): TBN measures the oil's alkalinity and its ability to neutralise acidic byproducts of combustion. Acids formed during combustion are corrosive and can damage engine parts. Sufficient alkalinity is crucial to neutralise these acids, protecting against corrosion and extending the oil's service life. A minimum TBN is usually specified for the oil to ensure sufficient protection.

• Use the same sampling point every time, preferably from the main supply line just before it enters the main engine.
• Drain a sufficient amount of oil to clear stagnant or contaminated oil from the sampling point.
• Rinse the sampling container with the same oil before collecting the sample.
• Collect the sample only after the engine has been running at normal operating parameters for an adequate period.
• Seal the sample container and label it with essential information, such as the date, vessel name, sampling hours, oil grade, and sampling point identification.

(i) Water is present :

• Identify and eliminate the source of water ingress.
• Run the purifier at a low feed rate.
• Drain water from the crankcase after allowing it to settle.
• Conduct batch purification: Transfer the oil to a settling tank, heat it, allow water to settle, and drain it.
• Continue purification and run the engine at reduced load.

(ii) Alkalinity has fallen :

• Reduce engine load to minimize acidic byproduct formation.
• Raise jacket cooling water temperature to prevent dew formation on liner walls, which leads to acid production.
• Can add some oil of cross head type engine if TBN of trunk piston engine crankcase oil has fallen below. Ensure "MAKER" is same.

(iii) Viscosity has changed appreciably :

• Investigate and rectify the cause of viscosity change, such as fuel contamination.
• Partially drain old oil and replenish it with fresh oil if available.
• Run the engine at a reduced load to limit wear.

(iv) Carbon content has increased :

• Check for improper combustion caused by fuel system issues (e.g., faulty fuel pump, injector, or incorrect fuel temperature).
• Address blow-past due to worn liners or piston rings.
• If an overhaul is not possible, partially replace old oil with fresh oil if available.
• Run the engine at reduced load and maintain continuous purification to manage carbon content.

ALTERNATE ANSWER:

(a) Properties of a Crankcase Oil for a Trunk Piston Engine

1. Alkalinity

The oil must have sufficient alkalinity, often measured by its Total Base Number (TBN), to neutralize acids formed during combustion. The sulfur content in fuel produces sulfuric acid, which can cause corrosive wear on engine components like piston rings and cylinder liners. A typical TBN for trunk piston engines is around 30 to 40.

2. Detergency and Dispersancy

Detergents and dispersants are crucial additives. Detergents have a cleansing action, helping to keep surfaces of components like bearings, piston rings, and liners free of deposits. Dispersants keep contaminants, such as soot and sludge, suspended in the oil, preventing them from accumulating and forming deposits in the crankcase.

3. Anti-Oxidation

The oil must be resistant to oxidation, especially at high operating temperatures. Oxidation can lead to the formation of sludge and varnish, which clog filters and reduce the oil's lubricating properties. Anti-oxidants extend the oil's lifespan and protect engine components from corrosion.

Other important properties include a stable viscosity, low de-emulsification number (to separate water easily), and good thermal conductivity (to dissipate heat).

(b) Obtaining a Representative Sample of Crankcase Oil

• Oil samples should be taken when the engine has been running for at least 30 minutes, ensuring circulation and homogeneity (mixing of sludge, wear particles, etc.).
• Samples must be taken from the designated sampling point, not from vents or drain cocks.
• Use only clean, unused sample bottles.
• Before collecting the sample, open the sampling cock and discard some oil to flush out dirt or accumulated particles.
• After taking the sample, allow the bottle to cool before sealing.
• Properly label the bottle with particulars such as engine details, oil type, and running hours. This information must accompany the sample for analysis.

(c) Action for Oil Analysis Results

(i) Water is present:

• Locate and rectify the source of water ingress.
• If the engine is not required to run:
  • Stop priming pump, allow water to settle, and drain crankcase to remove major water content.
  • Start purifier (for diesel engines); for main engines, ensure the running purifier is working efficiently.
• If the engine must continue running:
  • Ensure proper crankcase venting so water vapour can escape instead of condensing.

  (ii) Alkalinity has fallen:

  • Avoid prolonged low-load operation (as acid formation is higher at low loads).
  • Operate engine at higher load within safe limits.
  • For diesel engines (where purifier cannot run during operation), add fresh surplus oil (with higher TBN) to increase alkalinity. A small amount of used oil may be drained before topping up to maintain oil level and improve alkalinity.
  • If the engine is not running, purify oil to remove sludge; batch purification is most effective.

  (iii) Viscosity has changed appreciably:

  • If viscosity increases:
    • Run purifier continuously to remove sludge and impurities.
    • Heat oil moderately (within safe limits) to lower viscosity. Avoid overheating.
  • If viscosity decreases:
    • Indicates dilution (often due to diesel oil ingress).
    • Partially drain sump and top up with fresh oil of correct grade.

    (iv) Carbon content has increased:

    • Usually due to blowpast.
    • Actions:
      • Run purifier continuously (sump-to-sump circulation).
      • Batch purification and transferring oil to settling tank are recommended.
      • Stop engine, stop priming pump, and allow sump oil to stand still for hours so sludge can settle.
      • Remove sludge, clean crankcase, and then transfer oil back from settling tank to sump via purifier.
      • Investigate and rectify cause of blowpast.
      • In stationary condition, keep oil passing through centrifuge and monitor regularly.