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What can cause the engine temperature to be high in piston engine aircraft?

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    $\begingroup$ What engine temperature, exactly? Oil temp? Cylinder head temp? Exhaust gas temp? Coolant temp for a liquid cooled engine? $\endgroup$ – Fred Larson Sep 10 at 14:16
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Low oil quantity, improper cooling, running excessively lean of peak. Excessive boost from a turbo or supercharger at high altitudes where sufficient cooling is not possible. Ther are quite a few reasons.

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  • $\begingroup$ Running lean of peak should actually cause lower CHTs, assuming proper fuel mix (calibrated fuel-injectors, for ex). Martin Pauly has an excellent video on this using his bonanza as an example: youtube.com/watch?v=h3bATVXMHQg $\endgroup$ – Tyzoid Sep 10 at 18:36
  • $\begingroup$ To a degree, yes. $\endgroup$ – Carlo Felicione Sep 10 at 21:41
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There could be any number of reasons, and it really depends on the aircraft's systems to determine how it is set up. As always - if you suspect a problem in your aircraft, it's always a safe choice to temporarily ground the aircraft and have an A&P or engine mechanic take a look.

Typically, piston engines (o-320, io-360, etc.) control temperature through two primary means - fuel mix, and oil cooling.

Oil is usually air-cooled as it passes through a radiator, and the cooled oil is used to lubricate the engine. Oil temperature is regulated via the use of a thermal bypass valve to avoid it becoming too cold.

The second mean, by regulating the fuel mixture, is controlled by the pilot. AOPA's 2016 article, "Temperature, Temperature, Temperature" talks at length about how mixture can be used to affect both the Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT):

There are basically three different strategies for keeping CHTs low during cruise:

  • Baby the engine
  • Operate very rich
  • Operate lean-of-peak

Essentially, when you put fuel and air together with a spark, the fuel will oxidize rapidly (burn). This oxidization has an ideal ratio of fuel to air, where both will be used up completely. Excess fuel will lead to incomplete combustion, and excess air will lead to a reduction in available power. This ideal ratio is what we refer to when we say "peak", since it causes the highest temperatures to occur within the engine cylinder.

Operating rich of peak, i.e. with excess fuel, decreases temperatures due to the cooling effect of the uncombusted or incompletely-combusted fuel absorbing excess heat from the combustion reaction. In order for these effects to be visible, however, engines need to be run well rich of peak. This is because adding more fuel does react, even if incompletely, and so the temperature reduction to the cylinder head does not fall off as fast as the EGT might.

Operating lean of peak, i.e. with excess air, decreases temperatures due to a lessening of the combustion happening within the cylinder, along with the excess ambient air causing a cooling effect within the cylinder. Because reducing the amount of fuel available to the engine causes less combustion within the combustion chamber, this causes a decrease in performance.

As the article notes:

In any of these cases, you’re trading power and airspeed for reduced temperatures and increased longevity. For most of us, that’s a reasonable tradeoff to make.

In an engine that's overheating, the forces that cause the engine to cool aren't effectively counteracting those things that cause excess heat. As for what might cause the temperatures to be high in the first place, it could be any number of things. A miscalibrated fuel injector or carburetor,, a problem in the oil circulation that causes poor regulation, a bad seal causing oil to combust, running at too high a power or improper leaning technique, or some other mechanical or electrical issue.

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