A jake brake is a braking system used primarily on semi-trucks or other large vehicles that modifies engine valve operation to use engine compression to slow the vehicle. They are also known as compression brakes. The name comes from Jacobs Vehicle Systems.
When the accelerator is released on a truck its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses. A compression brake uses a solenoid to open the exhaust valve at the top of the compression stroke. This releases the compressed air in the cylinder preventing it from returning its energy back to the piston.
The driver controls consist of an on/off switch and a three position switch that actuates the compression brake on two, four or all six cylinders. When the compression brake is turned on, it will activate when the driver releases the accelerator. There is also a switch on the clutch pedal that will deactivate the compression brake when the clutch is disengaged.
Jake brakes often make a loud chattering or machine gun noise while being used, which has led many communities to ban them. There are signs on the side of some freeway offramps, or at municipal boundaries, that will say "Engine brakes restricted", etc. They are referring to jake/compression brakes. This has led to special mufflers on trucks that also muffle the sound of the compression brake.
External links
Jacobs Vehicle Systems
