Many large diesel trucks (and even some larger RVs) are equipped with "Jake Brakes," also known as compression release engine braking systems. They are called Jake Brakes because Jacobs Vehicle Systems is the original maker of this sort of braking system.
The basic idea behind a Jake Brake is to use the engine to provide braking power. If you own a stick shift car and have ever downshifted to provide braking, you understand part of the idea. When you brake a car by downshifting, you are using engine vacuum to slow the car down.
A Jake Brake goes a step further, and actually turns the engine into an air compressor to provide a great deal more braking power. If you have read How Car Engines Work, then you know that the engine goes through a compression stroke. Compressing the air in the cylinder takes power. If the engine's drive shaft is turning the engine to brake the truck, the power used to compress the air is braking power. However, that power is stored in the cylinder, so if you let it, the compressed air simply pushes the piston back down. Therefore, you don't really get any braking at all from the compression stroke on an unmodified engine.
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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