Why is it necessary to cool the cylinder heads or covers, cylinder liners and pistons of diesel engines? What is used as the cooling medium?

The temperature inside the cylinders of diesel engines rises to approximately 2000°C during combustion of the fuel and drops to approximately 600°C at the end of expansion. With temperatures in this range the metal of the cylinder covers, cylinder liners and pistons would quickly heat up to the point where its strength would be insufficient to withstand the cylinder pressures; also, no oil film would be able to exist on the cylinder walls, and lubrication of the cylinder and piston rings would break down. Cooling is necessary to maintain sufficient strength in the parts and to preserve the oil film on the cylinder.
The cooling medium for cylinder liners and covers is a flow of distilled or fresh water: the medium for cooling pistons is also distilled or fresh water, or oil from the crankcase system. The amount of heat extracted from the various parts /must be such that they operate at temperatures well within the strength limits of the materials used. The coolant flow patterns must also be arranged so that the surfaces of all parts are as near uniform temperature as possible to prevent large thermal stresses being set up.
With modem highly rated engines the temperatures of the parts subjected to combustion temperature are much lower than in earlier engines. This has been made possible by the availability of better temperature measuring devices and the research carried out by engine builders. The temperature of the combustion chamber surfaces of cylinder covers, piston crowns and cylinder liners varies between 200°C and 350°C in modern highly rated engines. The variation in temperature of the different parts of the surface of cylinder covers will be within about 50°C to 100°C, and for piston crowns the temperature variation will be 75°C to 100°C. Cylinder liners show greater temperature variation throughout their length, but in the highly critical area at the top of the liner the variation is kept to within approximately 100°C.
Small diesel engines with pistons less than about ISO mm (6 in) diameter have ■ only the cylinders and covers cooled by water. The piston crown will be cooled by excess lubricant from the gudgeon bearing and by the heat transfer to the walls of the piston which are then cooled by the cylinder liner. Small high-speed diesel engines may also be cooled by forced air flow passing over fins fitted on the outside of cylinders and cover. It should be noted that air-cooled diesel engines have very low cylinder wear.
Note With pressure-charged engines the air flow during the scavenge period (in two-stroke and four-stroke engines) over the hot internal surfaces of the cylinders, covers and piston crowns helps to maintain low surface temperatures. It also reduces the temperature gradient across the material section and in turn lowers the thermal stresses.

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