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The obvious answer to this question would be that you are no longer supplying fuel to cause combustion and the frictional losses slows down the rotating and reciprocating parts of engine causing engine rpm to drop.

But I guess it has more to do with the process involved in engine braking than frictional losses. Engine braking technically refers to the process of passively slowing down the vehicle using the engine itself by putting the vehicle in gear and letting go off the accelerator pedal. In gasoline engines this is done by closing the throttle valve while in diesels its done by closing a valve in the exhaust system and in trucks Jake brakes are used.

But doesn't the same braking process happen (not referring to the braking of vehicle rather referring to slowing down of the engine) even if you don't put the vehicle in gear i.e say when you put it in neutral or disengage the clutch and let go off accelerator pedal? Now it's just that engine components themselves are only slowing down instead of slowing down the whole vehicle as its not hooked to the transmission.

So my question is "Is it the engine braking (or the process involved in it) along with frictional losses that causes the rpm to drop or is it just due to frictional loss alone? (I don't think there is that much frictional losses in engine to cause such a quick drop in rpm. If there was so much frictional loss that would be a poorly designed engine).

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    I have some difficulties understanding your question: Do you want to know why the RPM drops on disengaging the clutch or the relation on friction vs. engine breaking?
    – Martin
    Jul 2, 2021 at 8:08
  • @Martin I want to know why the rpm drops "fast" when you disengage the clutch. While cutting down power and frictional losses are definitely contributing the drop in rpm, does engine spent/waste energy on suction and compression of air alone? Also if so , isn't this suction and compression that causes the major loss of energy when you disengage clutch?
    – Abhishek P G
    Jul 2, 2021 at 10:44

2 Answers 2

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The engine braking comes from the energy needed to compress the gases in the cylinders, and to lift the valves.

These resistances to motion are considerably greater than the friction from moving parts.

Some engines with manual starting have a device to prevent compression, so that a person is physically capable of turning over the engine. Then when the engine is turning the device is released to allow compression and firing.

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  • So it is pretty much the engine braking that drops the rpm even when you are on neutral or clutch is disengaged right?
    – Abhishek P G
    Jul 2, 2021 at 8:05
  • It is those resistances to motion that slow down the engine. "Engine braking" is the name of the effect, not its cause. When the drive train is engaged, the weight isn't braking the engine. It's the other way round – the engine is acting as a brake.
    – Weather Vane
    Jul 2, 2021 at 8:14
  • Resistance to motion? I didn't get that quite correctly. Friction is always a resistance to motion. In most online sites they refer it as engine braking only when your are on a gear so that engine puts a braking effort on the vehicle as a whole along with engine i.e they refer to the braking effort applied by engine on the vehicle as engine braking. They don't mention rpm drop while you disengage clutch as engine braking. Maybe it's because they don't want to explicitly mention it. I hope we both are agreeing to the same thing. If not would you make the answer more clearer.
    – Abhishek P G
    Jul 2, 2021 at 10:14
  • I don't really understand your question. When you shut off the fuel supply, the resistances to motion (friction, compression stroke, lifting the valves, viscosity of the oil, driving the alternator) are greater than the energy gained by burning the fuel, and the engine slows down (to the point where the idling setup controls it, when there is no load, or it stalls, if under load). If the engine is being "pushed" by the momentum of the vehicle through the drive train, it does not slow down as quickly (or at all, if going downhill).
    – Weather Vane
    Jul 2, 2021 at 10:18
  • "they refer it as engine braking only when your are on a gear" That is what engine braking means: using the force that is required to turn the engine to brake the vehicle. Just letting the engine stop by removing power (when the drive is disengaged) isn't "braking". Why would the engine not lose revs when you shut off the fuel?
    – Weather Vane
    Jul 2, 2021 at 10:27
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When testing an engine on a test bed, one of the tests was to turn the engine using the dynamometer to measure the total friction and pumping losses, and this result was called "pumping losses". This was done at WOT (Wide Open Throttle).

Another test was the Morse test to see if the loss per cylinder was similar for all cylinders.

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  • Isn't pumping loss higher at less throttle opening? So the test must be done at low throttle opening right?
    – Abhishek P G
    Jul 3, 2021 at 1:08
  • @AbhishekPG we did all our testing making a set of assumptions. What test bench / dynamometer are you using? Does it have a balance arm?
    – Solar Mike
    Jul 3, 2021 at 5:37
  • I haven't done any testing. I just read it online that you have more pumping loses when the throttle is more closed than WOT.
    – Abhishek P G
    Jul 3, 2021 at 6:15
  • @AbhishekPG the methods of providing sufficient air for low throttle ie idle are different - so if you want tests under similar conditions you decide the assumptions. Good experience if you test engines: single cylinder large bore diesels, multicylinder engines, engines with variable compression ratio are also interesting ie holding most variables constant to see the effect of changing compression ratio on power output...
    – Solar Mike
    Jul 3, 2021 at 6:28

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