Usually seen in trucks, the jake brake shuts off the exhaust valves Usually seen in trucks, the jake brake shuts off the exhaust valves so that in the exhaust stroke, the burned gasses cannot escape through the exhaust valves. Instead they press against the head of the piston and causes the piston to slow down. When the intake valve opens, some of the exhaust escapes out the intake valve and gives a distinctive loud rapping noise. Jake brakes are used in large truck engines to assist in slowing the vehicle.

Exhaust brakes do not close the exhaust valve causing the gasses to travel across the piston and escape out the intake valve and making that rapping sound. They actually open the exhaust valve prior to combustion. This not only prevents combustion and, in turn, the production of power but causes the braking effect. You see, by doing this the engine has effectively been turned into an air compressor and the power that turns it is now the drive tires. That distinctive rapping sound is really the compressed charge being released on the compression stroke. Before any fuel has been added, the charge is under very high pressure and is being allowed to escape only through a very small hole, causing that rapping sound. Under normal operating conditions, after the fuel has been ignited and the piston forced down (power stroke) the piston would travel back up to top dead center and a few degrees before this the exhaust would begin to open allowing the piston to push the exhaust out the valve rather than having it explode out.

Terrybear you couldnt be more wrong....the only effect keeping the exhaust valves shut all the time would do, would be to blow the head of of your deisel, which in essence, would slow you down...to a dead stop, until the wrecker picked you up... and then, if the latter part of your statement were true, that the jake brake allows some of the combustion back into the intake, you would burn your supercharger to shit. You are the epitamy of a self proclaimed expert, but really you know nothing. The jake brake shut the fuel injectors off, so that there is no combustion period, so you too, Ginger, are wrong...at the top of the compression stroke, it is just compressed air anyway, but as it becomes compressed the motor works to compress it. The exaust valves are then cracked, letting the air escape, causing no work to be done by the compressed air itself when the piston goes back down. This rapid release of about 280 or so PSI causes the popping sound, not the motor backfiring through the intake...

Ok, there seems to be some confusion here between a "Jake (Jacob) Brake" and an "Exhaust Brake" they are NOT the same thing. The Cylinder Head of an engine with a Jake Brake has another Exhaust Valve which is (in simplified terms) activated by a Solenoid when the Accelerator Pedal is raised (if the Jake Brake switch is on). This Extra Valve opens at (or near) TDC of the Compression Stroke (which has used energy to compress the charge) to release the pressure so that it does not help to push the piston back down again, hence slowing the truck (usually) down. An Exhaust Brake shuts of the exhaust AFTER the Exhaust Manifold which increases the Back Pressure and uses more energy to push the piston up to TDC on the "Exhaust Stroke". Typically, the Jake Brake makes the Popping sound and the Exhaust Brake makes a hissing sound as pressure is released through a Pressure Relief Valve. Many people wrongly call Exhaust Brakes "Jake Brakes". If you don't believe me, do some research before commenting on my answer. The above is a VERY simplified explanation of how both types of braking systems work and is on to show the differences between the two.