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The intense heat generated when combustion takes place means that all internal combustion engines must be cooled by some means. Air-cooled reciprocating engines are cooled by air passing over fins attached to the cylinders. Liquid-cooled engines, as in an automobile, use a liquid coolant that passes through jackets surrounding the cylinders. In a reciprocating engine, combustion takes place only during every fourth stroke of a four-cycle engine. However, in a gas turbine engine, where the burning process is continuous, nearly all the cooling air must pass through the inside of the engine. If only enough air were admitted to the engine to provide combustion, internal temperatures would increase to more than 4,000 F. Because of this, the amount of air admitted to the engine is in excess of the amount required for combustion only; indeed, about 75 percent of the air is used for cooling and 25 percent for combustion. This large surplus of air (secondary air) cools the hot expanding gases just before they enter the turbines. In some engines, internal air is bled from the engine compressor section and is vented through passages to the bearings and other parts of the engine. This air is then vented into the exhaust stream.



David L. Heiserman, Editor

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Revised: June 06, 2015