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BobbyP27 t1_j4uluok wrote

For external combustion, you want the air as hot as possible. The hot gas temperature after combustion depends on the air temperature and the heat released from the fuel. For the same fuel, hotter inlet air means hotter hot gases, or if you have a fixed target hot gas temperature, less fuel is needed.

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kilotesla t1_j4x8ruw wrote

This is specifically for the air supplying the combustion process. There's also a "cold side" of the heat engine, for example a steam condenser or a heat sink on a Stirling engine. If this is cooled by air, that air should be as cold as possible. (It might instead be cooled by water, in which case you want the water as cold as possible.)

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