Compression release engine brake

A compression release engine brake, compression brake, or decompression brake is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves to the cylinders, right before the compression stroke ends, releasing the compressed gas trapped in the cylinders. The compression followed by the "wasteful" release consumes a great amount of energy, effectively slowing the vehicle.

Clessie Cummins was granted U.S. patent 3,220,392 for the engine compression brake in 1965. The first company to manufacture them was Jacobs Vehicle Systems, resulting in the term Jake brake.