It is necessary to cool the cylinder walls so they may be at a sufficiently low temperature to retain the lubricant, and the head and exhaust valve seat must also be cooled to prevent undue thermal stresses and distortion, softening, or burning of parts.
Is used on almost all motor-cycle engines, aero radials, and a few industrial engines, particularly those operating in low temperatures. It is found that increased powers obtainable from modern fuels necessitate a proportional increase in the fin surfaces, which for aero engines, are now usually fully machined in the form, depth and pitch found most effective.
A few aero engines use a high boiling point liquid like ethylene glycol on account of the reduction of radiator area rendered possible by the higher temperature (boiling point 197° C). Steam cooling in radiators fitted with load safety valves is also used. The water and steam are separated at the engine outlet, the steam going through to the cooling surfaces so as to be condensed, while the water is circulated rapidly through the jackets again. Most engines. however are water-cooled.
Radiator on aero, vehicle and small commercial engines return the same water to the jacket repeatedly, and are to be preferred to the method employed in other cases of supplying new water continuously, for two reasons: (a) The new water tends to deposit more and more lime, etc., in the jackets if fresh. (b) If sea water it must be kept below about 55°C to prevent deposition of salt. In either case the new water usually enters the cylinder jacket first and keeps it cold, the worst possible condition for bore wear, as it causes the condensation of small quantities of acid products on the cylinder bore.
It is better to use "fresh"(non salt) water in a closed circuit with the sea water passing through a "heat interchanger". In this way a very rapid circulation may be kept up in the cylinder head where the greatest differences of heat input exist between the inlet and exhaust valve seats, etc., and where the directing of a large quantity of fairly hot water in the right places prevents great temperature differences in the various parts of metal.
The jacket round the cylinder should be kept as hot as possible without boiling. This reduces wear by preventing deposition of water vapor and acid products on the walls, and also reduces the viscosity of the cylinder lubricant and thus the frictional losses. The necessary safety valve being incorporated in the radiator cap. In large engines the pistons are also cooled,in the old days sometimes by water, but increasingly by oil circulation, as corrosion fatigue of piston rods has been experienced with water.