Piston

For other uses, see Piston (disambiguation).

Components of a typical, four-stroke cycle, DOHC piston engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug, (V) Valves, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow.

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.

Piston engines

Main article: Reciprocating engine

Internal combustion engines

The expansion of the high-temperature and high pressure gases produced by combustion apply direct force to the piston.

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  A piston and its connecting rod.

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  CAD drawing of crankshaft and pistons.

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  Large pistons (over 0.5 m incl. connecting rod).

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  Simplified piston animation.

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  Two-stroke engine with a tuned expansion pipe

Steam engines

Steam engines are usually double-acting (i.e. steam pressure acts alternately on each side of the piston) and the admission and release of steam is controlled by slide valves, piston valves or poppet valves. Consequently, steam engine pistons are nearly always comparatively thin discs: their diameter is several times their thickness. (One exception is the trunk engine piston, shaped more like those in a modern internal-combustion engine.)

Pumps

Piston pumps can be used to move liquids or compress gases.

For liquids

Main article: Reciprocating pump

For gases

Main article: Reciprocating compressor

Air cannons

There are two special type of pistons used in air cannons: close tolerance pistons and double pistons. While in close tolerance pistons, O-rings serve as a valve, O-rings are not used in in double piston types.

Close-tolerance pistons have a number of disadvantages: They can swell and stick, their properties alter due to atmospheric changes, and they fit tightly in the cylinder with close tolerances. Backlash may suck some of the bin material into the valve which can cause the piston to stick.

Common features of double piston construction: They cannot swell and stick, they fit loosely in the cylinder (no tight tolerances), atmospheric changes do not affect them, and foreign material entering the cylinder doesn't cause sticking.

Drawbacks

Since the piston is the main reciprocating part of an engine, its movement creates an imbalance. This imbalance generally manifests itself as a vibration, which causes the engine to be perceivably harsh. The friction between the walls of the cylinder and the piston rings eventually results in wear, reducing the effective life of the mechanism.

The sound generated by a reciprocating engine can be intolerable and as a result, many reciprocating engines rely on heavy noise suppression equipment to diminish droning and loudness. To transmit the energy of the piston to the crank, the piston is connected to a connecting rod which is in turn connected to the crank. Because the linear movement of the piston must be converted to a rotational movement of the crank, mechanical loss is experienced as a consequence. Overall, this leads to a decrease in the overall efficiency of the combustion process. The motion of the crank shaft is not smooth, since energy supplied by the piston is not continuous and it is impulsive in nature. To address this, manufacturers fit heavy flywheels which supply constant inertia to the crank. Balance shafts are also fitted to some engines, and diminish the instability generated by the piston's movement.

See also

• Air gun
• Flamethrower
• Fire piston
• Fruit press
• Hydraulic cylinder
• Knurling
• Slide whistle
• Wankel engine (an internal combustion engine with a rotary 'piston')
• Steam locomotive components
• Rocket engine nozzle effectively forms the piston for rockets

External links

• Piston Engines Essay
• How Stuff Works - Basic Engine Parts
• Piston highlight: Hypervideo of construction and operation of four-cylinder internal combustion engine courtesy of Ford Motor Company