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I've heard and read a lot of times that when you drive downhill on gear, the engine consumes almost no fuel. In contrast, when you drive down the same hill and press the clutch (or put the car in neutral), the engine consumes normal amount of fuel.

When you leave the car on gear, the pistons are still rotating at about 1-2k RPM. How then is it possible to get so small fuel consumption? What causes the engine to rotate?

On the other hand, why does the car burn so much fuel when put in neutral?

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The answer is really simple.

When in gear, the engine is connected to the wheels. As wheels are rotating, the engine is rotating. The energy to keep the engine rotating comes from the kinetic energy of the car. You can feel this energy lost to friction as engine braking. Of course, if at downhill, the kinetic energy is increased continuously by gravity, so there may not be net kinetic energy loss (i.e. decrease of speed).

When in neutral, there is no connection to the wheels. Thus, the only way for the engine to keep rotating is to inject fuel. In this case, you do not feel engine braking. If at downhill, the car will probably accelerate (unless the speed is so insane that gravity acceleration force equals wind resistance).

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    Mostly correct, but needs an addition: when the engine management system detects you take your foot off the throttle while the engine is in gear, it stops injecting fuel. This is why fuel consumption drops to 0 when you're going downhill (or coasting in general). – Hobbes Feb 8 '17 at 14:14
  • @Hobbes coasting might mean different thing as in the UK coasting referrers to having the clutch pressed while your not breaking. so the opposite of your statement :) – Barkermn01 Feb 8 '17 at 23:19
  • ah, thanks. I meant driving with the car in gear while not pressing the accelerator. – Hobbes Feb 9 '17 at 7:40
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When a car is in neutral and the engine is running, the engine needs fuel because:

  • the engine has internal friction,
  • the engine has to drive ancillaries like the water pump, oil pump, alternator, maybe the A/C, etc.
  • during the compression stroke, air is compressed in the cylinder. This requires energy. You get some of that back during the ignition stroke (even if no fuel is injected), but this process is not 100% efficient.

In addition, petrol engines are really inefficient while idling:

  • the throttle valve is closed so it takes a lot of effort to draw air into the cylinder,
  • the ignition stroke takes a long time so there's lots of heat loss,
  • in a turbo engine, the turbocharger won't be spooled up so no energy recovery.
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