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It is very common to find in the AFM/POH, Section 5 of most piston engine airplanes (C152, C172, PA28, BE76, etc) charts that indicate the fuel consumption for cruise/economy power settings.

You will notice that in all cases, for a given set of pressure altitude, RPM and manifold pressure (if constant speed propeller) the fuel consumption will depend on the OAT. The question is: why does the fuel flow (in PPH or GPH) decrease for higher temperatures?

I should make sense to me that higher temperatures (hot atmosphere) would decrease the overall performance of the engine, as we all know, since the air is less dense which directly impacts in the engine's operation. If the engine's performance is degraded in a hot day, why does it burn less fuel?

All manufacturers publish similar results, but I can't figure out a physical explanation to it. If you could point out an official reference in any textbook I would really appreciate.

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    $\begingroup$ You still need to maintain the Fuel/Air mixture to get the optimal burn. If you are running hot and there is less air (oxygen) for a given cycle, it follows that you would need less fuel to maintain the same a/f ratio. It may be less fuel but it is also producing less power too. $\endgroup$
    – Ron Beyer
    Commented Jan 12, 2017 at 19:30
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    $\begingroup$ Please define "better". I guess all you see is a lower consumption at the same power percentage, but the reference for that percentage goes down with rising temperature, so the consumption/power ratio gets actually worse. $\endgroup$
    – Peter Kämpf
    Commented Jan 12, 2017 at 19:47
  • $\begingroup$ I just modified the title to properly address the question: "better fuel consumption" I meant "lower fuel consumption". $\endgroup$
    – Francesco C
    Commented Jan 13, 2017 at 22:57

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I don't have a reference, but the physics behind is not that complicated.

The key is that you have constant RPM, constant pressure altitude and constant manifold pressure.

A motor sucks in lots of air, but since the throttle restricts the air flow in the intake, the air pressure between throttle and motor is low. This is the manifold pressure1. At idle, i.e. almost closed throttle, the pressure is very low, while at full throttle, it ideally equals ambient pressure. And in turbocharged motors, it is even higher than ambient pressure.

Finally, manifold pressure together with RPM and displacement of the motor is a measure of how much air volume is aspirated by the motor per time interval.

Since you keep everything constant, this volume also is constant. But warmer air is less dense and contains less oxygen molecules per volume. Less oxygen molecules means less fuel can be burned. This is why fuel consumption is lower.
But it also means less power. To compensate, the throttle could be opened a little more to let in a little more air volume. This would also increase the manifold pressure. But if the throttle is already fully opened to get max. power, you can't open it further. So, max. power is lower at higher temperature.

1) NOTE: For simplicity, I'm speaking about absolute pressure here. The gauge in the cockpit displays the under-pressure with respect to ambient pressure, so a high displayed value means a low absolute pressure. And since ambient pressure changes with altitude, manifold pressure does, too, even if the displayed value is constant. The benefit of this is that the displayed value is a good measure of the power with respect to the max. power achievable at that altitude.

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  • $\begingroup$ maybe you should add that a higher ambient temperature means that the thermodynamic cycle is less efficient. The physics is quite the same as that for turbo machines. So relative to power, fuel consumption even goes up with temperature. $\endgroup$
    – Peter Kämpf
    Commented Jan 14, 2017 at 1:21
  • $\begingroup$ The problem is, those performance tables show that the airspeed is the same while the fuel consumption and the power is lower. This means that the same amount of work is performed in the same time interval, thus the same net power is output by the engine with better energy efficiency (at least at first glance, fuel consumption), while the thermodynamics tells us otherwise. $\endgroup$
    – RedGlyph
    Commented Nov 26, 2017 at 18:36
  • $\begingroup$ @RedGlyph, I don't see the question mention same speed. Speed will be lower, but in GA you usually have defined cruise power and the speed is whatever it is. $\endgroup$
    – Jan Hudec
    Commented May 25, 2021 at 5:16
  • $\begingroup$ @JanHudec It's an old question so I'm not going to read all that again, I think it was a side comment to the more general consumption issue. I can just say that any manual I've seen provides those tables in function of the airspeed, which makes sense because the important point is fuel consumption when you prepare a flight. $\endgroup$
    – RedGlyph
    Commented Jun 7, 2021 at 9:07
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It's much more fundamental then that, and also much simpler.

These fossil fuel engines work by burning a fuel air mixture in a confined space. A fixed amount of burned fuel releases a fixed amount of energy. If you extract energy by cooling, then there is less energy left over to do actual work. (If you cool the hot gasses, the pressure gets lower).

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  • $\begingroup$ Welcome to aviation.se! $\endgroup$
    – MD88Fan
    Commented May 3, 2021 at 17:13
  • $\begingroup$ You start well, but the second half you wrote makes no sense: the power stroke happens so fast that there is no appreciable cooling. And in fact, the engine is more efficient when the outside air is colder. The problem with warm air and confined space is that warm air is less dense, so less of it fits in that space. $\endgroup$
    – Jan Hudec
    Commented May 25, 2021 at 4:15
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Let's try it from the aerodynamics side. Hotter air at a given altitude is thinner. It's the same thing as flying higher: more TAS for the same IAS.

As long as you do not overheat, and your engine/prop can develop enough thrust, the same benefits are gained as with flying higher: better miles per gallon.

Note the airspeed on the Cessna 152 fuel consumption chart is listed as KTAS. So as you move across the chart from cold to warm, similar KTAS is lower IAS. It is the same as you read fuel consumption (Gallons per Hour) at higher altitudes for the same TAS.

Notice that poor prop can't even make 2400 RPM at 2000 feet and 20 degrees colder than Standard Temperature. Its the same thing for your wing and fuselage: trying to push through air at a higher density.

From this we can gather that flying fish are on the right track!

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