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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.

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    $\begingroup$ @CuriousOne Are you giving me that link because the answer is in the article? $\endgroup$ – B T Feb 29 '16 at 8:35
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    $\begingroup$ 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? $\endgroup$ – CuriousOne Feb 29 '16 at 9:30
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    $\begingroup$ I'm voting to close this question as off-topic because it shows insufficient effort and would better be asked in engineering. $\endgroup$ – CuriousOne Feb 29 '16 at 9:38
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    $\begingroup$ 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? $\endgroup$ – B T Feb 29 '16 at 9:42
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    $\begingroup$ Engineering is based on sciences. why cant it be explained simply using classical physics? $\endgroup$ – user43794 Feb 29 '16 at 12:00
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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.

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    $\begingroup$ 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. $\endgroup$ – B T Feb 29 '16 at 9:37
  • $\begingroup$ @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 $\endgroup$ – RedGrittyBrick Feb 29 '16 at 10:45
  • $\begingroup$ Sudden compression leads to adiabatic heating of gases inside engine. PV = nRT is not applicable in this case. $\endgroup$ – user43794 Feb 29 '16 at 11:56
  • $\begingroup$ 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. $\endgroup$ – Jon Custer Feb 29 '16 at 14:06
  • $\begingroup$ Ah interesting. And then all that heat exits out the exhaust? $\endgroup$ – B T Feb 29 '16 at 22:14
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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.

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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.

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  • $\begingroup$ "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 $\endgroup$ – B T Nov 16 '17 at 22:40
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    $\begingroup$ 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. $\endgroup$ – niels nielsen Nov 16 '17 at 22:48
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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/

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

protected by Qmechanic Apr 20 '16 at 9:31

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