We know that a piston moves to and fro within a cylinder and there are several piston rings inserted in the grooves on the piston which perform multiple tasks including sealing of pressure inside the combustion chamber lest it leaks from below. The speed of rubbing between these piston rings and the cylinder liner is quite high and apart from that there are extreme conditions of temperature, pressure and corrosive gases inside the combustion chamber to which any lubricating fluid between the liner and the piston would be subject to.
If you remember the 3 types of lubrication conditions we had studied in a previous article, the type of lubrication found inside the cylinder is of the thin film or boundary type of lubrication for most parts of the piston motion except for the upper and lower extremes of motion where this changes to imperfect lubrication as the speed reduces to zero at these points and direction of motion is reversed.
It does not require much reflection to imagine that lubricating oil used inside a cylinder liner must be able to withstand conditions of high temperature, pressure and should have good corrosion resistance as well. Just to give you a broad idea, typical lubricating oil used inside the liner should have viscosity in the range of 115-150 cst at temperatures of 50 degrees Celsius and this should not vary too much even at higher temperatures. Special additives could be added to the oil in order to increase its oiliness or stickiness. Apart from that the amount of oil supplied should be ideal because if too little oil will cause the surfaces to tear apart rapidly, too much of oil will also be a problem as it would interfere with normal combustion inside the chamber and also cause damage to various parts such as valves, valve seats and the exhaust pipe.
