Cylinder Liner Lubrication of Marine Two Stroke Crosshead Diesel Engines

Low speed two stroke crosshead diesel engines are generally used as prime movers on ships and are also used for electricity generation on shore based power plants. The term low speed is applied to engines having a rotational speed of up to 300 rpm and output per cylinder between 400 Kilowatts to 6000 Kilowatts generally. In these types of engines the piston is not connected to the crankshaft directly with the connecting rod but an additional member called the crosshead is provided. The piston has a long piston rod which sits on the crosshead. The crosshead converts the linear motion of the piston into the rotational motion of the crankshaft with the help of the connecting rod. The piston rod reciprocates up and down through the stuffing box which seals the crankcase from the under-piston space. The function of the crosshead is that it absorbs the transverse thrust of the engine. The isolation of the crankcase allows lower cost residual fuels to be burnt without the fear of any fuel contamination of the lubricating oil of the crankcase.

The piston rings on the liner surface mostly operate under boundary or thin film lubrication conditions. The high operating temperature, pressure, and the presence of a corrosive environment make the working condition very harsh. The two stroke crosshead type of marine diesel engines burning residual fuel provide the largest percentage of propulsive power for ships globally. Over eighty percent of total lubricant consumption is on cylinder oil alone and it represents a major expense in the daily operation of the engine. Also over the years, engine developments aimed at giving higher output per cylinder and adapting the engine to burn ever deteriorating fuel grades have worsened the problem of cylinder liner lubrication. The liner average surface temperature has increased from 200 degrees C to 275 degrees C from the 1970s to the 2000s, thus making lubrication more difficult. Similarly the maximum pressure and mean effective pressure also have increased drastically. Please refer to the diagrams for the details.

Low quality Fuel

The use of low quality high sulphur content, viscous and residual fuel to cut down running cost has either created some new problems or aggravated the existing ones.

1. The high sulphur content of fuels up to 4.5 % by weight has greatly increased the risk of cylinder liner and piston ring wear.
2. Ash, either soluble or insoluble, in fuel can increase the abrasive wear of piston rings and liners.
3. Mineral acids formed as a result of condensation of sulphur oxides produced during combustion create a serious hazard if they find their way into the crankcase.
4. Incompletely burnt heavy ends of the fuel can cause excessive deposits in the piston ring zone.
5. Short exhaust valve life due to deposit formation or corrosive attack by vanadium, sodium, sulphur, and other chemical compounds present either in the fuel or in the lube oil.

Apart from aggravating lubrication problems, these fuels can damage injectors and corrode turbocharger nozzles and moving blades. In addition to this, there is a wide variation in marine residual fuel quality according to their crude origin and degree of refining which make difficult the job of choosing proper cylinder lubricant.