Does a Richer Mixture Always Cool the Engine

Martin:

““Excess fuel will absorb heat and cool the engine” – we have all read this in ground school textbooks or even in FAA knowledge test questions.

But this statement is an oversimplification. It is true that an engine running with a full rich mixture is cooler (measured in cylinder head temperature, or CHT) than a slightly leaned engine; however, that difference has less to do with extra fuel serving as coolant, and more with the fact that the engine with a full rich mixture develops less horsepower to begin with than the slightly leaned one, so it also develops less heat. And that results in cooler CHTs.

Maybe a better way to think about it is that the results of combustion inside a cylinder depend on multiple variables, including manifold pressure, induction air temperature, ignition timing, and also the fuel/air ratio. It turns out that CHTs are hottest at fuel/air mixtures in the mid-range, just slightly on the rich side of peak EGT. Thus, when flying rich of peak, the opening statement correctly suggest that enrichening the mixture will make the engine cooler. However, when we cruise on the lean side of peak, adding fuel will actually increase CHTs – meaning the additional fuel will make the engine hotter.