# Where does the energy go when engine braking?

If you're in gear in a car and not accelerating, the car slows down faster than it would from just air resistance and tire deformation. In normal braking, the energy is turned into heat from the brake pad rubbing on something connected to the tire. Where does your car's kinetic energy go when engine braking? IE how does energy get transferred between the road and the car to remove the car's kinetic energy?

The wikipedia article on this doesn't seem to explain it, and google searching didn't turn up anything enlightening.

• @CuriousOne Are you giving me that link because the answer is in the article?
– B T
Feb 29 '16 at 8:35
• Then you should have said what you miss in the answers that you have found. To me the Wikipedia answer is sufficient and if I wanted to know more, I could certainly find more in the internet. May I suggest you refine your question? Feb 29 '16 at 9:30
• I'm voting to close this question as off-topic because it shows insufficient effort and would better be asked in engineering. Feb 29 '16 at 9:38
• I honestly don't understand what additional effort you think I should have had. If the answer is in the wikipedia article, what section has the answer? If you're so sure the answer is so easy to find, why not give at least a link with the answer?
– B T
Feb 29 '16 at 9:42
• Engineering is based on sciences. why cant it be explained simply using classical physics? Feb 29 '16 at 12:00

I don't think a direct answer is given in the wiki. Transmission is connected if one does not apply clutch. With engine brakes on, wheels drive the engine not the other way round. I believe the energy will be released via the heat in the engine produced by piston, gearbox and skidding (if your gears are lowered further) though mostly via adiabatic heating of the gases inside the engine by the pistons.

• How does the energy get transferred from the road into heat in the engine? I suppose I could imagine the wheels driving the piston and gearbox causing friction, but from what I read, the vacuum inside the engine causes braking. Its still not clear to me how that action produces the majority of the heat.
– B T
Feb 29 '16 at 9:37
• @BT Your supposition is correct and also, compressing gases in the pistons heats the gas - so that's another way kinetic energy is converted to thermal energy Feb 29 '16 at 10:45
• Sudden compression leads to adiabatic heating of gases inside engine. PV = nRT is not applicable in this case. Feb 29 '16 at 11:56
• Truck 'Jake' brakes vary the valve timing to turn the engine in to an air compressor, resulting in hot air and a hot engine. In my car I just get the drag of my engine being spun without gas going to the cylinders. Feb 29 '16 at 14:06
• Ah interesting. And then all that heat exits out the exhaust?
– B T
Feb 29 '16 at 22:14

In a gasoline engine, engine braking dissipates the shaft power input as frictional heat, which warms the coolant and the motor oil. If the torque converter is unlocked, then some of the shaft work heats up the fluid in it as well. In each case, that frictional heat gets dumped via oil coolers for the engine and transmission, and the radiator.

in the engine brakes used in large diesel engines (the so-called "Jake Brake", manufactured by the Jacobs Corp.), the shaft power input is used to compress air in the cylinders of the engine, which is then wasted by valving it out of the cylinder under pressure.

• "valving it out of the cylinder under pressure" - Oh! That's that loud "chsssht" you hear when a big rig stops huh? Cool! Had no idea that was related to breaking
– B T
Nov 16 '17 at 22:40
• actually, that's the loud BLAB-BLAB-BLAB-BLAB noise the diesel makes as it slows down with the jake brake on. The CHSSSHT noise is the sound of air pressure working the brakes on and off. Nov 16 '17 at 22:48

I always wondered the same thing, my guesses are all kinetic energy in tires turns into KE in engine and then lost through heat, as you may notice engine reving up when you down shift , the engine isn't getting any energy from fuel it must from the tires, so its the opposite the tires move the engine.

I don't want to get into the energy aspect of what happens. But basically when you put your car in gear and you let go off the throttle, then the vehicle is now moving under the influence of the momentum it gained so far. Now the only difference is that the momentum is driving the vehicle and in turn drives the engine because now you have hooked the transmission to engine by putting it in gear. So the engine is basically absorbing the energy from the vehicle's kinetic energy.In terms of how it works there are different scenarios

1. In gasoline engines during engine braking the butterfly valve in throttle body would be almost closed. So when piston moves down during suction stroke it's similar to what happens when you close the end of a syringe partially and try to suck in air, it will be very hard and absorbs a lot of energy to do it. Then you compress whatever air is drawn in and and some heat will be produced. A portion of heat generated is removed to surrounding while the remaining energy of compressed air pushes piston back during power stroke (except there is no power being generated). Then at exhaust stroke you push the air out to atmosphere. Here you lose energy during suction mainly along with some energy being lost during compression and exhaust stroke in addition to friction.

2. In diesel engines (cars,small trucks) there isn't a throttle body to control the amount of air sucked in. So you can't waste energy on suction stroke. So here instead you place a butterfly valve in the exhaust manifold. This valve will be almost closed during exhaust stroke, so that the engine has to do a lot of work to push out the air through a small area. Again you can take analogy of a syringe where you close the end almost completely and try to push out the fluid in it. Thus in diesel engines most of the energy is lost during exhaust stroke.

3. When coming to trucks (large diesel engines) jake brakes are used. Jake brakes basically employ a cam to override the exhaust valve opening. It makes use of the fuel injection timing cam to open the exhaust valve right near to the end of compression stroke. This will cause most of the compressed air to be removed from cylinder. Thus all the energy that engine spent on compressing air is wasted during/at the end of compression stroke. When you are at quite high rpm you could here large brrrrrrr sound, this is actually the sound produced when you release the compressed air during compression stroke quickly over each cycle of the engine. This is also why jake brakes are called as compression release brakes. In jake brakes most of the energy is removed during compression stroke.

Now you could mention all other small losses that occur like the frictional, heat loss between road and tyre, in meshing gears, in the various moving parts of engine and air drag too. In fact air drag will be more than frictional losses. While these are not the losses that i think the op is actually expecting to be addressed.

Kinetic energy is transformed into the thermal energy of the brakes. Check out a similar question/answer: https://www.physicsforums.com/threads/transforming-kinetic-energy-into-thermal-energy.634380/

• The question is not about brakes, but about using the engine to slow down.
– mpv
Feb 29 '16 at 9:20
• You're right. But the answer remains the same, no matter friction takes place in brakes, tires, gears, oil, etc. Feb 29 '16 at 14:14