Consider a car going down a large mountain. Instead of braking continuously, the driver shifts down and the car is able to maintain velocity on just engine braking. No fuel is injected into the engine, but the engine is still spinning (assume a manual transmission).

Now, look at a cylinder. Instead of a Diesel or Otto cycle, I would imagine something opposite would happen. Air is compressed, and as such heated up. Usually, we model this as adiabatic compression - however, this is of course not the case in reality; some heat is transferred to the engine block, where it is carried away by the usual means of cooling an engine. Then, the air expands again, but since some energy was carried away by the engine block, the air will inevitably be colder than how it started. Basically, we have created some kind of air cycle machine.

Is this actually the case? Will a Diesel or Otto engine work as ain air cycle machine when it's driven from an external source (i.e., no fuel injected), or am I missing someting in this analysis?

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    $\begingroup$ This depends on how the car is engineered (for one, it is not certain that the fact that the wheels are rolling means that the Otto engine is still being moved by that). What's the physics question here? $\endgroup$
    – ACuriousMind
    Commented Oct 26, 2015 at 19:31
  • $\begingroup$ @ACuriousMind Sorry, I should've included that it's a manual car. The physics question is whether a Diesel or Otto engine will act as an air cycle machine when it's driving without injecting fuel. $\endgroup$
    – Sanchises
    Commented Oct 26, 2015 at 19:36
  • $\begingroup$ @sanchises - Gas is still injected while an internal combustion engine idles though... In fact, if you put most manual transmissions in gear while rolling down a hill, they would automatically start (it can be a "convenient" way to start a car with a dead battery). So I think the exhaust would still be hot, since the energy input rate of the burning fuel is still much higher than any loss due to expansion or conduction. You reach a balance when the exhaust line gets hot enough for radiative and convective effects to balance in heat input. $\endgroup$ Commented Oct 27, 2015 at 12:22
  • $\begingroup$ @SebastianRiese I'm not sure what you're trying to add here - I thought it was pretty clear from my question that I referred to engine braking (also, note that breaking = destroying, braking=slowing down). $\endgroup$
    – Sanchises
    Commented Oct 28, 2015 at 12:26
  • $\begingroup$ @sanchises Sorry, I obviously didn't read very attentively (and, yup, that's a typo). And there is already an answer givin the same link, sorry, my mistake. $\endgroup$ Commented Oct 28, 2015 at 12:34

2 Answers 2


No. Engine braking works by taking the energy of the falling mass of the car and dissipating it into the engine. It only works with throttled engines (no diesels) as much of the energy is lost in pulling the intake air past the mostly-closed throttle valve. Energy is also lost through moving the various parts of the engine back and forth and around. The net result is heating of the intake/exhaust air and engine block. There probably is some heat transfer from the air to the engine block while the air is momentarily compressed, but that is far outweighed by the heating from friction and turbulence.

  • $\begingroup$ Is the correct answer. $\endgroup$
    – Gert
    Commented Oct 26, 2015 at 20:53
  • $\begingroup$ Strictly speaking, cooling also requires external work and would as such also dissipate energy. However, I take your word for it that the friction losses are much higher - perhaps I will make some back-of-the-envelope calculations later and see how negligible the effect I described actually is. $\endgroup$
    – Sanchises
    Commented Oct 28, 2015 at 12:24
  • $\begingroup$ @sanchises Yes, but just because cooling dissipates energy doesn't mean that something that dissipates energy can be assumed to be doing so by cooling. $\endgroup$ Commented Oct 28, 2015 at 15:45

yes . because on breaking a force working between Tyre and ground which is known as friction . in this work energy is used from engine of car so the cold air will exist from car .


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