If the waste heat was moved to areas of lower pressure such behind the car-green area- enter image description here Could this reduce the drag from these areas of lower pressure. From what I understand this hot air would increase pressure in those areas but equally hot air is less dense so would it increase drag?

  • $\begingroup$ I suspect the time constants involved, plus the fact that there's almost no stagnant air pool, means you'd generate little to no temperature (and thus pressure) change. $\endgroup$ Aug 18, 2017 at 12:52
  • $\begingroup$ The heat is already "moved" into the biggest green blob behind the car. it comes out of the exhaust pipe! And most car manufacturers don't build vehicles that look anything like the shape of your picture anyway - at least, not in countries where motorists care about fuel economy. $\endgroup$
    – alephzero
    Aug 19, 2017 at 2:02
  • $\begingroup$ kinda like F1's F-duct, except just over the whole body and using exhaust? can probably just implement F1's F-duct for slightly cheaper $\endgroup$ Sep 18, 2017 at 17:30

2 Answers 2


The thermic energy output of a car is too small to make a difference. Also the technical implemention would be very difficult.

Did you ever hold your hand near to the exhaust. Its really not that hot compared to a camp fire. But this was in idle so when driving it might be different. On the other hand when driving you also have a lot of air mass to be heated. So when driving the air will be cooler compared to standing in idle.

Secondly the technical solution would be very complicated and counterintuitive. Usually car makers want the exhaust gases to have only a very small amount of energy. They try to get energy out of it with a turbocharger for example. The added weight by some kind of exhaust distribution system would counter the effect of your idea in terms of fuel efficiency.

  • $\begingroup$ I'm not sure how much this is but wouldn't the 3000 horsepower of heat rejection in a bugatti Chiron be able to heat a meaningful amount of air and reduce drag. $\endgroup$ Aug 19, 2017 at 18:24
  • $\begingroup$ @PhysicsPhun, possibly if you drive flat out in 2nd gear all the time. :) $\endgroup$
    – user6335
    Sep 21, 2017 at 7:21

I see how you're thinking: the car is a "piston" and if you heat the air behind this "piston", it will push it forward.

Now think how inefficient this "engine" is - you are adding fractions of a degree to the air (working gas) which makes for horrible thermodynamic efficiency. On top of that, you have no "piston walls".


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