Cylinder Liner Lubrication of Marine Two Stroke Crosshead Diesel Engines Part 2 (Page 3 of 4)

Correct running in of an engine should produce a close mating of the piston rings with the cylinder liner surfaces to prevent combustion blow by. In addition it is likely that the metallurgical transformations involved in the smoothing the peaks in the metal surfaces do generate a surface layer whose wear properties are superior to those of the parent materials.

The greatest assistance in the running in of any engine is given by the presence of sulfur in the fuel since it provides corrosive wear to help smooth the initial roughness of the liner surfaces. For this reason it is essential that the cylinder oil used should not have sufficient alkalinity to neutralize the combustion acids.

In the case of engines which are being run in while burning diesel fuel it is customary to use a straight mineral oil of SAE40 or SAE50 viscosity as the cylinder lubricant.

In case of running in while the engine is burning heavy fuel it is customary to use heavy duty oil of TBN around 10 and SAE 50 viscosity as the cylinder lubricant. For detailed running in procedures it is advisable that the particular engine builder recommendations should be observed.

All the above attempts to keep liner and ring wear within acceptable limits will, however be nullified if the oil feed rate, method of application and proper corrective action of troubleshooting is not done properly.

Blow by of combustion products and Scavenge fire

Indications – scavenge temperature high and sticking of piston rings

Causes:

1. Insufficient spreadability, detergency of lube oil and thermal degradation.
2. Insufficient feed rate of cylinder lube oil.
3. Leakage in the cylinder lube oil feed system.
4. Lubricating oil discharge by lubricating quill is not uniform.
5. Excess side clearance between piston ring and groove resulting in pumping action which causes the oil to be lost without providing lubrication.

Sticking of pistons rings and wear down takes place because of the above resulting in blow by of combustion products and eventually causing a scavenge fire.

Corrective actions:

1. Analyze the cylinder lube oil and fuel used by chemical laboratory to find out compatibility or suitability of the cylinder oil for the residual fuel used and accordingly choose the proper grade of cylinder lube oil.
2. Compare the theoretical and the actual feed rate (theoretical feed rate can be found out from the graphs given in the engine manual). Any discrepancy shows leakage or insufficient pressure build up in lubricating mechanism due to wear. So temporarily increase the feed rate but carry out the overhaul of the cylinder lubrication mechanism at the earliest.
3. Locate the leakage and correct it.
4. During maintenance if oil discharge by all quills is found to be non-uniform, then it is to be checked. Check whether the adjusting screw on the cylinder lubricating pump for the lubricating point is differently adjusted with other adjusting screws. Otherwise locate the defective quill and repair it or replace it.
5. If side clearance between the piston ring and the groove is found to be excessive then replace the piston ring.