Compression release

A black lever (decompression valve) within an engine compartment
The decompression valve (black lever) on a Yanmar YSB8 marine diesel engine.

A compression release is a mechanism to ease the starting of an internal combustion engine by allowing it to rotate to starting speed without having to work against the compression of the pistons. It does this via either a release valve that is incorporated within the cylinder head that vents the cylinder pressure to the outside atmosphere until the engine has sufficient momentum to overcome it, or by venting straight through one of the main valves.

US patent US5375570 A of 1994 describes an automated compression release valve actuated by engine oil pressure.

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