Compression release
A compression release mechanism works to ease the starting of internal combustion engines by allowing them to spin up to starting speed without having to work against the pumping action of the pistons. It does this through 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. At that point the valve closes and the ignition is engaged.
Motorcycles
Early large displacement motorcycles provided riders with a manual control for this when kick starters were used while later models linked them to the kick lever through a cable for automatic operation. Though it only uses electric start, not kick start, the 2012 Ducati 1199 uses a compression release, which is automatically activated at low engine speed by a centrifugal flyweight on each exhaust cam. This reduces the work of cranking the high 12.5:1 compression ratio engine, allowing a smaller battery and smaller starter motor, for a total weight savings of 3.3 kilograms (7.3 lb).[1]
Diesel engines
Small diesel engines may be fitted with compression release valves if they are to be started by hand cranking.[2]
Automation
US patent US5375570 A of 1994 describes an automated compression release valve actuated by engine oil pressure.[3]
