A compression release mechanism works to ease the starting of

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

s by allowing them to rotate to starting speed without having to work against the compression of the

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...

s. It does this via either a release valve that is incorporated within the

cylinder head In a piston engine, the cylinder head sits above the cylinders, forming the roof of the combustion chamber. In sidevalve engines the head is a simple plate of metal containing the spark plugs and possibly heat dissipation fins. In more modern ...

that vents the cylinder pressure to the outside atmosphere until the engine has sufficient

momentum In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...

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.

Motorcycles

Early large displacement motorcycles with

kick start Kick start is a method of starting an internal combustion engine (usually that of a motorcycle) by pushing a ratcheting lever with the rider's foot. Kick start mechanisms were almost universally a part of motorcycle engines before the mid-1970s ...

ers provided riders with a manual compression release while later models linked them to the kick start lever with a cable for automatic operation. The 2012

Ducati 1199 The Ducati 1199 Panigale is a Ducati sport bike introduced at the 2011 Milan Motorcycle Show. The motorcycle is named after the small manufacturing town of Borgo Panigale. Ducati had announced a larger displacement 1299 Panigale for the 2015 m ...

, equipped with an electric starter, uses a compression release that is automatically activated at low engine speed by a centrifugal flyweight on each exhaust cam. This reduces the work needed to spin the high 12.5:1

compression ratio The compression ratio is the ratio between the maximum and minimum volume during the compression stage of the power cycle in a piston or Wankel engine. A fundamental specification for such engines, it can be measured in two different ways. Th ...

engine, allowing for a smaller battery and starter motor, amounting to a total weight saving of . From the mid-1960s to the late-1970s, engine compression releases were also used to supplement rear wheel braking on many two-stroke motorcycles, primarily those used off-road or for various types of dirt-track racing, such as motocross and flat-tracking. Since two-stroke engines normally lack the compression braking effect of four-stroke engines when the throttle is shut off, the addition of a compression release brake on two-strokes essentially mimicked the four-stroke compression braking effect. Using throttle-off engine compression for at least some braking on both two-stroke and four-stroke motorcycles was especially important during that era, since most motorcycles used

drum brakes A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating bowl-shaped part called a brake drum. The term ''drum brake'' usually means a brake in which shoes press on the inner surface o ...

, which fade more quickly than modern

disc brakes Disc or disk may refer to: * Disk (mathematics), a two dimensional shape, the interior of a circle * Disk storage * Optical disc * Floppy disk Music * Disc (band), an American experimental music band * ''Disk'' (album), a 1995 EP by Moby Other ...

Diesel engines

Small

diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

s may be fitted with compression release valves if they are to be started by hand cranking.

Small engines

Most small gasoline engines used in power equipment use an automatic compression release system of some kind, designed to reduce load on the starter motor or user manually starting the engine. The most common kind of small engine compression release uses a camshaft mounted mechanical lever that pushes the exhaust valve

tappet A tappet or valve lifter is a valve train component which converts rotational motion into linear motion in activating a valve. It is most commonly found in internal combustion engines, where it converts the rotational motion of the camshaft into ...

slightly, venting engine compression through the exhaust valve. Once the engine spins to starting speed, centrifugal force causes the lever to move out of the way, and the exhaust valve is allowed to seat fully. Older compression release systems used a small bump on the cam that would bump the valve very slightly at low speeds. Once the engine gained speed, the pressure would push the exhaust valve against its seat. These systems were phased out as mechanical compression releases became common.

Notes

{{Reflist, 30em Internal combustion engine technology Starting systems

Motorcycles

Diesel engines

Small engines

See also

Notes