When the engine is running, but the vehicle is not moving fuel is still being consumed, where does the energy go?. I am guessing that it goes in rotating the flywheel and crankshaft and stuff, but I am not so sure. Any detailed explanation would be appreciated. Also, when we release the clutch the rpm seem to go down, why does it happen?

  • It turns into heat. The engine is experiencing less load than if it were in gear / moving, so it uses less fuel, but the excess energy is released as heat. Google 'entropy' or 'heat death of the universe' if you're bored. The same thing happens to the "excess" power produced by the alternator when the battery is fully charged and the electrical system isn't pulling as much as the alternator is producing.
    – 3Dave
    Commented May 31, 2017 at 20:07

6 Answers 6


Combustion raises the temperature of the gases in the cylinder. Not all of this can be used to do mechanical work, some of it is wasted in heating the engine itself and the rest is lost when the heated gases are exhausted from the cylinder.

The engine contains a lot of moving parts. Whenever you have moving parts in contact you have friction, which takes away some of the engine's energy in wear and heat generation.

In a typical car, the engine is the source of power for the whole vehicle, so a lot of other equipment has to be powered by the engine.

  • The alternator takes some of the energy away to charge the battery.
  • Power from the battery is used to power engine electronics including spark ignition for fuels that require it, the engine management electronics and to start the engine.
  • Further power is taken to power all of the electronics in the rest of the vehicle; radio, dashboard lights, interior lights, exterior lights, etc.
  • The engine must be cooled, so in most engines some power is used to drive a mechanical or electric water pump to circulate cooling water. This water is passed through a radiator, which works best with good airflow, so where there is little airflow (e.g. in a stationary vehicle) a mechanical, electric or hydraulic fan is driven to force air over the radiator.
  • The aforementioned friction can be reduced by lubricating parts, so lubricating oil needs to be circulated to these parts, often with a mechanical oil pump.
  • If the vehicle is fitted with air-conditioning this will either be electrically or mechanically driven.
  • Engines may be fitted with turbo- or superchargers to increase their power at higher rpms, but at idle these act as a small additional load.
  • The fuel is often situated some distance from the engine, so must be pumped from the tank to the engine.

In regards to the engine speed dropping when the clutch is engaged:

Many components taking power from the engine take more power when the engine is spinning faster. As such, there is a particular speed where the power taken from the engine is equal to the power generated. The engine therefore idles at a constant speed and if you apply a little throttle it'll idle slightly higher. If you start to engage the clutch, you add some more load to the engine as there is now friction between the plates in the clutch, transferring some energy into trying to accelerate the vehicle or overcoming friction in the brakes, etc. The engine is still producing the same amount of power, so it slows down unless you also press the throttle to tell it to give more power.

  • Okay, so from the actual bhp quoted for the car is not utilized fully and a small part of the power goes in maintaining other stuff as you mentioned above. Does that mean that for gauging the performance of the car we must look at actual power output to the wheels rather than accessories.Is there any way to do this? Commented May 31, 2017 at 8:18
  • @NitinTiwari That is what a dyno meter is for. It measures the power the wheels can generate on a set of test-rollers. The actual power a car can deliver usually varies somewhat from the manufacturer supplied figures. Sometimes it varies wildly.
    – Tonny
    Commented May 31, 2017 at 12:05
  • Power measured at the wheels (with a dyno as @Tonny states) is a more accurate way of measuring the power available for performance. It's worth noting however that the figures that are quoted are usually bhp at the flywheel so most of the power used for ancillaries has already been "taken" and the difference between that and "at the wheels" figures is essentially the power lost in the transmission (so gearbox, differentials, driveshafts etc) Commented May 31, 2017 at 12:28

In an idling engine, a few things are happening. The rotating parts of the engine (pistons, rods, crankshaft) are needed to be kept in motion, and a good deal of the energy goes into that.

Another good chunk of energy is expended in the accessories (alternator, coolant pump, AC compressor, power steering pump). While many of these accessories usually aren't applying a load while idling, the water pump and alternator do, and maintaining these at operating levels is critical.

Based on the engine's design, there is also a minimum RPM that the engine can remain stable at. Any lower and the internal friction of the engine overwhelms the moving parts.

Finally, when the engine is put into gear, that applies a significant load to the engine. For the first time, the engine needs to fight the inertia of the transmission and the entire vehicle, so some power goes into that.

For that last part, think of it this way. Your arm has some strength, and you typically use it to lift and move objects. Depending on the load you are carrying, you will get tired sooner or later. But if you just raise your arm without any weight, your arm will still get tired, even though you aren't really lifting anything except your arm's own weight. But if you let your arm hang by your side, it won't ever get tired.


Get in your machine shop and devise a hand crank for your car's engine. Crank that baby over. Note that it's fairly difficult to crank over, even at 20-30 RPM.

Now devise a "gear-up" device that lets you hand-crank the engine at its normal idle speed (e.g. 600 rpm).

Seriously I don't expect you to do any of that; if you've ever barred an engine over using a ratchet wrench, you know there's small effort to it, and quadratically more effort at higher speeds. Spinning an engine at 600 RPM takes a great deal of energy. That's where the energy goes.

So if we're cold-cranking an engine like this at 600 RPM (no fuel) - where would that energy go? Heat. It would make the engine warm. If we were able to crank it for 5-10 minutes, it would become warm enough to need to circulate some coolant. This is totally separate from the heat of combustion, which adds to this - but much of that combustion heat goes down the exhaust pipe.

  • 1
    s/exponentially/quadratically/ Commented May 30, 2017 at 22:12
  • Embolden, if not made into a header (or both): Spinning an engine at 600 RPM takes a great deal of energy. As seen when you take your foot off the brake of an automatic at idle: you don't really go anywhere.
    – Mazura
    Commented May 30, 2017 at 23:11
  • 1
    @Mazura: All that shows is the inefficiency of the torque converter at low rpms. Commented May 30, 2017 at 23:23
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    You don't go anywhere very fast even if its a manual transmission. Less than walking pace in first. (Using the clutch carefully, you can move off without pressing the throttle - a useful skill for parking in tight corners). If I'm already up to speed and on a level road, foot off throttle will keep going indefinitely at about 20mph in 3rd. (VW Golf 1.9 Diesel).
    – nigel222
    Commented May 31, 2017 at 13:04

Look at the following animation

Clutch Engaging

The yellow part comes from the engine and it's always rotating. The blue part goes to the wheels.

In simple terms, when you press the clutch pedal you remove the link from the engine to the wheels. When you let the clutch pedal, you link the engine with the wheels and (if you are in gear) the car will move accordingly.

For the second part, imagine you push an empty shopping cart. It's easy, and you will push it with some speed (eg. engine RPM).

At some point someone drops a boulder in the cart (eg. engaging the clutch). In that moment your speed will drop (eg. RPM drop), and for you to walk again with that speed, you will have to put more effort (eg. accelerating an engine).

I hope that answers your question


A significant amount of the energy in the consumed fuel goes into heat. This heat is produced in the cylinders and transferred by the engine coolant into the radiator. When the vehicle is moving air is pushed through the radiator and takes the heat away.

When the vehicle is stationary and the engine is idling the airflow is often not enough to take away the excess heat and so an electric fan is switched on to force air through the radiator. You can often hear this fan noise over the sound of the engine on a hot day when the car is idling.

  • 2
    Actually at idle I believe ALL the energy goes to heat because there is no other product (and the laws of thermodynamics say that it must all go into some different form of energy). This is true of a moving car as well except that there is also potential energy that is stored/released if you end up at a higher or lower altitude and a little energy spent creating new wind eddies/currents behind you. I can't think of any other energy type coming from the operation of a vehicle... I guess some un-burnt gas (chemical) and light (which mostly ends up heat too)?
    – Bill K
    Commented May 30, 2017 at 19:57
  • 1
    @BillK: That's true but the interesting part of energy "consumption"/use is the path by which it eventually ends up as heat, not the fact that it eventually ends up as heat. The path can be boring and useless (as is the case when idle and assuming the accessories aren't doing anything useful) or very useful (cooling the passenger compartment by moving heat, moving the vehicle, etc.) Commented May 30, 2017 at 23:28

At idling RPM, engine power is used to keep the engine running smoothly without any jerk or vibration, with its accessories like water pump, ac compressor, power steering pump still running to use then when needed.

when you release the clutch, it attaches the engine with transmission and the power is going to transmit up to the wheels to move them against the weight of the body. At idling rpm, power developed is not sufficient to move the wheels when the the gear is selected, so the additional load lowers the engine speed(rpm) and the acceleration at the same time with clutch release increases the engine speed to propel the vehicle.

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