What are compression brakes?
by 
dale geiger
ANSWERS: 2
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"Jake brakes" are a mechanism that cuts fuel to the engine and, for all practical purposes, turns the engine into a large air compressor driven by the wheels. The reason you might want to do this is that the act of compressing air requires a bit of effort and thus robs the motive force (in this case, the momentum of the vehicle) of energy. In other words, it slows you down. If you downshift properly, the braking force is quite a bit better than can be managed by conventional brakes. However, it is VERY noisy! You can hear them literally a mile away. In some areas, the use of Jake Brakes violates local noise ordinances.
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Technically there is no such 'device' as compression brakes. The action of compression braking (or exhaust braking) relies on a deliberate obstruction in the exhaust manifold (header) of a large diesel engine. This action dramatically reduces the engine speed due to the work required to compress the inducted air against the exhaust brake butterfly. This reduced engine speed is transferred through the gearbox and transmission of the vehicle to the driven wheels and translates as an overall reduction of vehicle speed (braking). The process of compression braking causes neglegible wear of components compared to conventional braking which wears expensive brake components. Compression braking is prohibited in some built-up areas due to the relatively loud (delicious) noise and, occaisionally, increased emissions.
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