Answer EKM Question 53

Q53. A. State, with reasons, THREE properties required of crankcase oil which is to be used for a trunk piston main engine.

B. Explain how a representative sample of crankcase oil would be obtained from a trunk piston engine.

C. Briefly describe the action to be taken if the crankcase oil charge cannot immediately be replaced and analysis shows –

(i) Water is present;

(ii) Alkalinity has fallen;

(iii) Viscosity has changed appreciably;

(iv) Carbon content has increased.

Answer: A. Three properties of trunk piston engine crankcase lubricating oil
1. Stability: Thermal and oxidation stability ensures that the oil should not readily deteriorate under normal running conditions. Stability is ensured by the use of a good base stock for manufacture of the lubricating oil and by use of additives.
2. Viscosity: Typically SAE 40 which is equivalent to 12.5 to 16.3 centistokes at 100°C. Viscosity is a measure of internal resistance to flow of fluids. Higher the viscosity, the greater will be internal resistance to flow.
The primary function of lubricating oil is to reduce friction and wear between the rubbing elements. For this purpose a thin fluid film must separate the two running surfaces thus avoiding metal to metal contact. Under ideal conditions this is achieved by the hydrodynamic action of the shaft or journal rotating inside the bearing. Too low a viscosity may reduce the oil film thickness with chances of mechanical contact between the parts resulting in increased friction and wear. If viscosity is too high there will be increased internal shear resistance between the layers of oil film, temperature rise and power loss.
3. Total Base Number (TBN) should be 15 to 30. The required TBN is obtained by the use of alkaline additives. Alkalinity of the crankcase oil is essential for neutralizing any acid that may be formed. This is particularly important in a trunk piston type engine where any blow past will result in acidic products of combustion entering the crankcase and contaminating the oil.

B. Obtaining a representative sample for analysis:
When the engine is at stand still, the heavier constituents of the oil including solid particles, oxidation sludge and Water settle down in layers and a sample taken in thin condition will not be representative. A representative sample can be drawn from the system only when the pump is working and preferably the engine has run for about 30 minutes when the oil in circulation will be thoroughly mixed up and, homogeneous.
The sample should be drawn at any convenient point on the oil circulation system e.g. an open valve or cock after the circulating pump at the filter or similar.

C. Action other than replacing the crankcase oil when analyses show:

(i) Water is present:
In The first place, source of water contamination should be traced and  eliminated.
Water should be removed from the system after stopping the engine allowing the circulating oil time to settle down. In case a drain  plug or valve is fitted it may be possible to drain out the water. Alternatively,  consideration may be given to transferring the oil, using a drain or transfer pump or even a portable pump, into a settling tank, applying heating steam, settling and draining water Out. Water may be removed by centrifuging in case the separator is connected to the engine sump.

(ii) Alkalinity has fallen:
Alkalinity can fall if there is acid formation in the cylinder and excessive blow past the liner. By raising the cooling water temperature and avoiding running under tog low a load, dew formation on liner wall may be avoided and this in turn, will prevent acid formation.
If total replacement of oil is not possible, "freshening up" may considered if some new oil is available on board. Trunk piston crankcase oil TBN is usually in the range of 15 to 30 while a crosshead type main engine will probably be using oil with a TBN 8 to 12. If the trunk piston oil TBN has fallen below 8 then consideration may be given to freshening up the oil with some main engine oil. If the two oils are from the same maker the base stock will most likely be similar and there should not be any problem with miscibility.

(iii) Viscosity has changed appreciably:
Viscosity may fall if there are leakages in the fuel pump plunger and guide allowing distillate oil to mix with the lubricating oil. This should be investigated and set right.
If viscosity has fallen appreciably and it is not possible to replace the oil even partially, there is very little that can be done other than partially draining the crankcase oil and topping up with fresh main engine system oil. The engine load should be reduced.
A rise in viscosity may be due to contamination by residual fuel if such fuel is used. This should be investigated and corrected. In this case too, short term remedy would be to partially drain the sump and top with main engine oil which is usually of lower viscosity.

(iv) Carbon Content has increased:
Increase in carbon content is indicative of deterioration of the general condition of the engine e.g. worn-liner, piston rings, poor injection and combustion allowing carbonaceous material to form in the cylinder and find their way into the crankcase. A compete overhaul will probably not be advisable if this cannot be accompanied by an oil change.
Topping up with other oils as suggested above can improve the condition to some extent. The engine may be run with reduced load.