The basic idea behind a Jake Brake is to use the engine to provide braking power.
A Jake Brake modifies the timing on the exhaust valves so that, when braking is desired, the exhaust valves open right as the piston reaches the top of the compression stroke. The energy gathered in the compressed air is released, so the compression stroke actually provides braking power.
The main advantage of a Jake Brake is that it saves wear on the normal brakes. This is especially important on long downhill stretches.
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