Compression brakes, also known as compression release engine brakes, are engine braking mechanisms installed on some diesel engines. They are frequently called Jacobs brakes or Jake Brakes, which are actually specific brands of compression brakes manufactured by Jacobs Vehicle Systems. When activated, compression brakes open the exhaust valves near the top of the compression stroke, releasing the compressed air in the cylinder so that the air compression energy is not returned to the crankshaft. This can assist a vehicle to maintain or even reduce speed with minimal use of friction brakes. The power of this type of brake can be around the same as the engine power.

The typical compression brake consists of a hydraulic system using engine oil which transfers the motion of the fuel injector rocker arm to the engines exhaust valve(s) . The controls for a compression brake typically consist of an on/off switch and often a selector that controls the number of cylinders on which the brake is active. Throttle and clutch switches are integral with the system, and activation occurs when both the clutch and the throttle are released.

In comparison to exhaust brakes, which use a valve to create backpressure in the exhaust system, compression brakes are generally considered to be more effective at slowing down a vehicle. However, they can be very loud, which is why some areas have signs prohibiting their use.

In summary, compression brakes are engine braking mechanisms that can assist a vehicle to maintain or even reduce speed with minimal use of friction brakes. They are frequently called Jacobs brakes or Jake Brakes, and they work by releasing the compressed air in the cylinder so that the air compression energy is not returned to the crankshaft. They are generally considered to be more effective than exhaust brakes, but they can be very loud.