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engine cooling[′en·jən ‚ku̇l·iŋ]
A cooling system in an internal combustion engine that is used to maintain the various engine components at temperatures conducive to long life and proper functioning. Gas temperatures in the cylinders may reach 4500°F (2500°C). This is well above the melting point of the engine parts in contact with the gases; therefore it is necessary to control the temperature of the parts, or they will become too weak to carry the stresses resulting from gas pressure. The lubricating oil film on the cylinder wall can fail because of chemical changes at wall temperatures above about 400°F (200°C). Complete loss of power may take place if some spot in the combustion space becomes sufficiently heated to ignite the charge prematurely on the compression stroke. See Internal combustion engine
A thin protective boundary of relatively stagnant gas of poor heat conductivity exists on the inner surfaces of the combustion space. If the outer cylinder surface is placed in contact with a cool fluid such as air or water and there is sufficient contact area to cause a rapid heat flow, the resulting drop in temperature produced by the heat flow in the inside boundary layer keeps the temperature of the cylinder wall much closer to the temperature of the coolant than to the temperature of the combustion gas.
If the coolant is water, it is usually circulated by a pump through jackets surrounding the cylinders and cylinder heads. The water is circulated fast enough to remove steam bubbles that may form over local hot spots and to limit the water's temperature rise through the engine to about 15°F (8°C). In most engines in automotive and industrial service, the warmed coolant is piped to an air-cooled heat exchanger called a radiator (see illustration). The airflow required to remove the heat from the radiator is supplied by an electric or engine-driven fan; in automotive applications the airflow is also supplied by the forward motion of the vehicle. The engine and radiator may be separated and each placed in the optimum location, being connected through piping. To prevent freezing, the water coolant is usually mixed with ethylene glycol. See Heat exchanger
Engines are often cooled directly by a stream of air without the interposition of a liquid medium. The heat-transfer coefficient between the cylinder and airstream is much less than with a liquid coolant, so that the cylinder temperatures must be much greater than the air temperature to transfer to the cooling air the heat flowing from the cylinder gases. To remedy this situation and to reduce the cylinder wall temperature, the outside area of the cylinder, which is in contact with the cooling air, is increased by finning. The heat flows easily from the cylinder metal into the base of the fins, and the great area of finned surface permits heat to be transferred to the cooling air. See Heat transfer