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When carb heat is applied, the mixture enriches due to hotter and less-dense air mixing with the fuel. But a richer mixture has the side effect of cooling the engine down. So why does carb heat cause the engine temp to rise?

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  • $\begingroup$ You need to be specific about which engine temperature you mean, oil temperature, cylinder head or exhaust gas temperature. Is this a theoretical question or are you actually seeing this on an airplane? $\endgroup$
    – GdD
    May 16, 2022 at 12:53
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    $\begingroup$ You mean cylinder head temperature? $\endgroup$
    – John K
    May 16, 2022 at 12:54
  • $\begingroup$ @gdd It's a theoretical question that I got after reading that carb heat reduces the RPM and increases the engine temp. I'm not quite sure which temperature is being referred to, but am now curious how it affects each one of them. Thanks! $\endgroup$
    – ADK
    May 16, 2022 at 13:41
  • $\begingroup$ Carb heat is recycling exhaust heat back through the cylinders, so all temps will rise some small amount, eventually. EGT right away, then CHT after maybe a minute or two, then oil temp. $\endgroup$
    – John K
    May 16, 2022 at 15:37

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Switching Carb Heat to ON sends air to the carburetor that was heated by being near the exhaust pipe. While it is correct the fuel/air mixture will be richer, the amount of fuel entering the engine does not increase. A slight drop in RPMs may have a some cooling tendency but the heating of the incoming air off-sets this. Gasoline absorbs much less energy during evaporation than water$^1$.

Richer mixtures do burn with a lower temperature, but again the heating of incoming air results in a net gain of engine temperature at the cylinder head.

$^1$ water/methanol injection can be used help control engine temperature, but may be corrosive to engine parts

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  • $\begingroup$ Loss of power with Carb Heat ON may actually cause the pilot to increase throttle. Also, as the exhaust gets hotter, cylinder head and oil temp increases will follow. $\endgroup$
    – Robert DiGiovanni
    May 16, 2022 at 18:26

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