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Having read up on WWII air-cooled radials, one thing that struck me is how frequent is the mentioning of external oil cooler overheating, or the need for armored boxes around it, and some even venture to say the so-called air-cooled radials are in reality oil-cooled radials.

This got me thinking. Since the oil in turbofans is cooled by fuel flow, why isn't this the case for radials? Is this because the fuel flow is insufficient, or the oil too thick, or some other reasons?

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    $\begingroup$ piston engine generates much more heat to oil while doen't burn nearly as much fuel. $\endgroup$
    – user3528438
    Commented Jan 9, 2019 at 18:18

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The Fuel/Oil heat exchanger serves two purposes. It helps to dissipate some of the heat from the oil and warm the fuel up for combustion

The fuel-cooled oil cooler acts as a fuel oil heat exchanger in that the fuel cools the hot oil and the oil heats the fuel for combustion. Fuel flowing to the engine must pass through the heat exchanger; however, there is a thermostatic valve that controls the oil flow, and the oil may bypass the cooler if no cooling is needed.

In piston radials you generally dont need this fuel pre-heat. Since Avgas is vaporous at a fairly low temperature you dont really need to pre heat it. On top of that preheating it may vaporize it in the lines which is also a fairly big issue in and of itself.

There are always concerns of cross leaking, and this applies in both cases. A leaky exchanger can add fuel to your oil or oil to your fuel. A potential issue in either direction.

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    $\begingroup$ But the oil in the turbo engines is cooled by nothing other than the fuel flow. Perhaps the oil only cools the bearings and accessory gearboxes in the turbo, whereas it picks up heat all over the interior of the radial, and that results in a more significant fraction of heat going into the oil provided two engines of similar hp, e.g. twin wasp vs pt6? $\endgroup$
    – Meatball Princess
    Commented Jan 9, 2019 at 17:46
  • $\begingroup$ Isn't the fuel-oil heat exchanger even more for heating the fuel in the tank? Jet-A freezes below -40°C and Jet-A1 below -47°C and the average temperature at tropopause is -54°C… Gasoline has lower freezing point and piston aircraft don't have as high service ceiling, so it is not a concern for them. $\endgroup$
    – Jan Hudec
    Commented Jan 10, 2019 at 6:14

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