In a piston engine, the valve timing is the precise timing of the opening and closing of the valves.

In four-stroke cycle

Four-stroke cycle

A four-stroke engine, also known as four-cycle, is an internal combustion engine in which the piston completes four separate strokes—intake, compression, power, and exhaust—during two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.There are two...

 engines and some two-stroke cycle

Two-stroke cycle

A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft...

 engines, the valve timing is controlled by the camshaft

Camshaft

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

. It can be varied by modifying the camshaft, or it can be varied during engine operation by the relatively new technology of variable valve timing

Variable valve timing

In internal combustion engines, variable valve timing , also known as Variable valve actuation , is a generalized term used to describe any mechanism or method that can alter the shape or timing of a valve lift event within an internal combustion engine...

. It is also affected by the adjustment of the valve mechanism, and particularly by the tappet clearance. However, this variation is normally unwanted.

Many two-stroke cycle

Two-stroke cycle

A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft...

 and all wankel engine

Wankel engine

The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle takes place in a space between the inside of an oval-like epitrochoid-shaped housing and a rotor that...

s do not have a camshaft or valves, and the port timing can only be varied by machining the port

Port

A port is a location on a coast or shore containing one or more harbors where ships can dock and transfer people or cargo to or from land....

s, and/or modifying the piston skirt (two stroke applications). Some supercharged

Supercharger

A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...

 two-stroke diesel engine

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s do however have a cylinder head

Cylinder head

In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...

 and camshaft similar to a four-stroke cycle engine.

In a steam engine

Steam engine

A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...

, the control of the valve timing is an important part of the operation of the engine. See valve gear

Valve gear

The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle...

, and also Walschaerts valve gear, Berry accelerator valve gear, Baker valve gear

Baker valve gear

Baker valve gear was the main competitor to Walschaerts valve gear for steam locomotives in the United States. Strictly speaking it was not a valve gear but a variable expansion mechanism adapted to the Walschaerts layout replacing the expansion link and sliding die block...

, Woolf valve gear, Caprotti valve gear

Caprotti valve gear

The Caprotti valve gear is a type of steam engine valve gear invented in the early 1920's by Italian architect and engineer Arturo Caprotti. It uses camshafts and poppet valves rather than the piston valves used in other valve gear...

, Corliss valve gear.

External links

• Berry accelerator valve gear.
• Steam locomotive valve gear simulation software, listing twenty types of locomotive valve gear.
• The impact of valve timing on engine performance and emissions.