In a

piston engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more Reciprocating motion, reciprocating pistons to convert high temperature and high pressure into a Circular motion, rotating motion. This article ...

, the valve timing is the precise timing of the opening and closing of the valves. In an

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 ...

those are usually

poppet valve A poppet valve (also sometimes called mushroom valve) is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications. It consists of a hole or open-ended ch ...

s and in a

steam engine A steam engine is a heat engine that performs Work (physics), mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a Cylinder (locomotive), cyl ...

they are usually slide valves or piston valves.

Internal combustion engines

Camshaft

In four-stroke cycle engines and some

two-stroke cycle A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston, one up and one down, in one revolution of the crankshaft in contrast to a four-stroke engine which re ...

engines, the valve timing is controlled by the

camshaft A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition syst ...

. It can be varied by modifying the camshaft, or it can be varied during engine operation by

variable valve timing Variable valve timing (VVT) is the process of altering the timing of a Poppet valve, valve lift event in an internal combustion engine, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combina ...

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

Valve overlap

With traditional fixed valve timing, an engine will have a period of "valve overlap" at the end of the exhaust stroke, when both the intake and exhaust valves are open. The intake valve is opened before the

exhaust gas Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through ...

es have completely left the cylinder, and their considerable velocity assists in drawing in the fresh charge. Engine designers aim to close the exhaust valve just as the fresh charge from the intake valve reaches it, to prevent either loss of fresh charge or unscavenged exhaust gas. In the diagram, the valve overlap periods are indicated by the overlap of the red and blue arcs. Key: * TDC = Top dead centre * BDC = Bottom dead centre * IO = Inlet valve opens * IC = Inlet valve closes * EO = Exhaust valve opens * EC = Exhaust valve closes Either valve opens before the piston head reaches top dead centre or bottom dead centre. The amount in crankshaft degrees by which the valves open before top dead centre or bottom dead centre is reached is known as valve lead. The amount in crankshaft by which the valves close after top dead centre or bottom dead centre is reached is known as valve lag. Valve overlap is a secondary means to cool exhaust valves with intake air during valve overlap. Primary cooling is accomplished by dissipating heat to the valve seats.

Variable valve timing

Engines that always run at a relatively high speed, such as race car engines, will have considerable overlap in their valve timings for maximum volumetric efficiency. Road car engines are different because they are required to idle at less than 1000rpm, and excessive valve overlap would make smooth idling impossible because of the mixing of fresh and exhaust gases. Variable valve timing can give both maximum power at high rpm and smooth idling at low rpm by making small changes to the relative angular position of the camshafts and thereby varying the valve overlap.

Ported engines

Many

and all

wankel engine The Wankel engine (, ) is a type of internal combustion engine using an eccentric (mechanism), eccentric Pistonless rotary engine, rotary design to convert pressure into rotating motion. The concept was proven by German engineer Felix Wankel, f ...

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

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Hamburg, Manch ...

s, and/or modifying the piston skirt (two-stroke applications). However, some

supercharged In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement. It is a form of forced induction that is mechanically powered (usually by ...

two-stroke

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 (such as the Wilksch aero-engine) do have a

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 ...

and poppet valves, similar to a four-stroke cycle engine.

Tappet clearance

The valve timing of a diesel engine also depends on tappet clearance of the inlet and exhaust valves. If tappet clearance is less, then valve will open early and close late. If tappet clearance is more, then valve will open late and close early. Tappet clearance is measured by an instrument called feeler gauge.

External combustion engines

In an

external combustion engine An external combustion engine (EC engine) is a Reciprocating engine, reciprocating heat engine where a working fluid, contained internally, is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid t ...

, such as a

, the control of the valve timing is by the

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. It can also serve as a reversing ...

. In a typical

piston valve A piston valve is a device used to control the motion of a fluid or gas along a tube or pipe by means of the linear motion of a piston within a chamber or cylinder. Examples of piston valves are: * The valves used in many brass instruments * ...

arrangement, the timing of the intake and exhaust events for each cylinder are inextricably related as they are governed by the movement of a single piston uncovering two ports. However, the duration of the intake event can be controlled (the "cut-off") using the reversing gear and this reduces steam usage under cruising conditions. Caprotti valve gear is more closely related to that of an internal combustion engine, uses poppet valves, and was developed to allow independent timing of the intake and exhaust events. It was never used as widely as piston valves or the earlier slide valves.

References

{{Aircraft piston engine components Engine valves Steam locomotive technologies