A compression release is a device used on combustion engines to allow for easier starting. As the piston moves up in the cylinder, it builds pressure known as compression. This pressure can make turning the engine over nearly impossible. The compression release allows the operator to purge the built-up compression or pressure out of the cylinders, making the engine very easy to turn over and start. Upon engine start-up, the compression release re-engages, and the engine once again builds up the required compression to allow it to run and produce power.
A compression release is most common on motorcycle, snowmobile and other small engines that are typically started by kicking them over with a kick starter or pulling them over with a pull rope. On some very high-compression engines such as the larger V-twin designs, a compression release is also used with an electric starter to prevent burning the starter up while attempting to turn the large engine over. This same system is also found on some very large diesel engines. Some of the small, two-stroke engine-powered machines such as chainsaws and outboard boat motors also utilize a compression release to assist the starting process.
The typical compression release device is activated by a lever much like a manual choke switch. Other types are released by simply pushing down on a small button-like switch on the engine's cylinder head. In either case, once the compression release has been activated, it allows the engine to be turned over easily for one or two revolutions before the compression begins to build up again. If the engine fails to start on the first try, the compression must once again be released before making another attempt at starting it. Once the engine fires, there is no need for the operator to take further action.
Occasionally, the small device will malfunction and require replacement. This is typically a minor operation and can often be accomplished with ease by the home mechanic with simple hand tools. On some engines, the diesel included, the repair is more extensive and is best accomplished by a licensed mechanic. The typical compression release requires no specialized maintenance or lubrication and is relatively trouble-free in most cases. Occasionally, a manufacturer will recommend the addition of a top-side engine oil treatment additive to keep the seals on the device lubricated and eliminate any problematic sticking that will sometimes plague the release device.