<P> Only the fixed parts of the engine, such as the block and head, are cooled directly by the main coolant system . Moving parts such as the pistons, and to a lesser extent the crank and rods, must rely on the lubrication oil as a coolant, or to a very limited amount of conduction into the block and thence the main coolant . High performance engines frequently have additional oil, beyond the amount needed for lubrication, sprayed upwards onto the bottom of the piston just for extra cooling . Air - cooled motorcycles often rely heavily on oil - cooling in addition to air - cooling of the cylinder barrels . </P> <P> Liquid - cooled engines usually have a circulation pump . The first engines relied on thermo - syphon cooling alone, where hot coolant left the top of the engine block and passed to the radiator, where it was cooled before returning to the bottom of the engine . Circulation was powered by convection alone . </P> <P> Other demands include cost, weight, reliability, and durability of the cooling system itself . </P> <P> Conductive heat transfer is proportional to the temperature difference between materials . If engine metal is at 250 ° C and the air is at 20 ° C, then there is a 230 ° C temperature difference for cooling . An air - cooled engine uses all of this difference . In contrast, a liquid - cooled engine might dump heat from the engine to a liquid, heating the liquid to 135 ° C (Water's standard boiling point of 100 ° C can be exceeded as the cooling system is both pressurised, and uses a mixture with antifreeze) which is then cooled with 20 ° C air . In each step, the liquid - cooled engine has half the temperature difference and so at first appears to need twice the cooling area . </P>

Different types of cooling systems in ic engines