Jake brakes, sometimes called a decompression brake or engine brake, is a feature of some diesel engines that opens exhaust valves to the cylinders to release compressed gas and slow the vehicle.
The feature is mostly used on large trucks, but has been banned in many areas because of the loud noise that it produces during operation.
First, start with an engine. Suck, squeeze, bang, blow.
- draw air and fuel into the engine
- squeeze (compress) it
- light it on fire which drives the piston down forcefully
- push the remains out the exhaust
Now imagine an engine with the fuel removed.
- draw intake air, without fuel, in.
- compress it
- de-compress it (instead of blowing it up)
- push it out the exhaust.
So without fuel, you get some air, the piston goes up, then it gets pushed back down. This is essentially an air spring. The harder you squeeze the air, the harder it pushes back. However, just like a spring, you can’t use this as a brake because all the energy you add to the air by compressing it comes right back when it pushes the piston back down.
A jake brake is a device that opens the exhaust valve prematurely, releasing the compressed air before the piston starts going back down again. Now you’ve converted the engine into an air compressor, which requires energy to operate (unlike the spring, whose net energy consumption is 0). The energy comes from the kinetic energy of the truck, and therefore the truck slows down.
Jake brakes work better on diesel engines because they don’t have a throttle. Gas engines have a throttle which is closed and won’t allow air to enter the engine. Newer electronic throttles and engine computers could allow a jake brake, but it would be a bit pointless.