Jake brakes work by opening the exhaust valve at the end of a diesel engine’s compression stroke to vent the compressed air through the exhaust so it does not cause a rebound of the piston to begin the next cycle. The system uses hydraulic pressure for opening the valves. While Jake brakes are noisy, they are effective enough to slow the vehicle on a downhill incline without use of its surface brakes.
Diesel engines have no air throttle, so when a driver lifts his foot from the accelerator pedal he only slows the fuel flow into the cylinder. Without an air throttle, diesel vehicles do not experience the engine braking of a gasoline automobile. This loss of engine braking capacity is often disconcerting to drivers accustomed to operating gasoline vehicles, especially if the surface brakes begin overheating. Jake brakes restore this capacity.
While, in theory, Jake brakes could provide enough pressure in the exhaust to lock the drive axles of the vehicles, this is not practical. Locking the drive axles while pulling a trailer downhill is never a good idea. The pressure required to lock the wheels is also more than the engine is able to sustain without causing significant damage. Thus, Jake brake systems provide partial braking that slows the vehicle but requires use of surface brakes to bring it to a stop.