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
1.1 Internal combustion engines
1.1.1 Trunk pistons
1.2 Steam engines
2.1 For liquids 2.2 For gases
3 Air cannons 4 See also 5 References 6 Bibliography 7 External links
Internal combustion engine
An internal combustion engine is acted upon by the pressure of the
expanding combustion gases in the combustion chamber space at the top
of the cylinder. This force then acts downwards through the connecting
rod and onto the crankshaft. The connecting rod is attached to the
piston by a swivelling gudgeon pin (US: wrist pin). This pin is
mounted within the piston: unlike the steam engine, there is no piston
rod or crosshead (except big two stroke engines).
The pin itself is of hardened steel and is fixed in the piston, but
free to move in the connecting rod. A few designs use a 'fully
floating' design that is loose in both components. All pins must be
prevented from moving sideways and the ends of the pin digging into
the cylinder wall, usually by circlips.
A slipper piston is a piston for a petrol engine that has been reduced in size and weight as much as possible. In the extreme case, they are reduced to the piston crown, support for the piston rings, and just enough of the piston skirt remaining to leave two lands so as to stop the piston rocking in the bore. The sides of the piston skirt around the gudgeon pin are reduced away from the cylinder wall. The purpose is mostly to reduce the reciprocating mass, thus making it easier to balance the engine and so permit high speeds. Reduced inertia also improves mechanical efficiency of the engine: the forces required to accelerate and decelerate the reciprocating parts cause more piston friction with the cylinder wall than the fluid pressure on the piston head. A secondary benefit may be some reduction in friction with the cylinder wall, since the area of the skirt, which slides up and down in the cylinder is reduced by half. However, most friction is due to the piston rings, which are the parts which actually fit the tightest in the bore and the bearing surfaces of the wrist pin, and thus the benefit is reduced. Media related to Slipper pistons at Wikimedia Commons
Two-stroke deflector piston
Deflector pistons are used in two-stroke engines with crankcase compression, where the gas flow within the cylinder must be carefully directed in order to provide efficient scavenging. With cross scavenging, the transfer (inlet to the cylinder) and exhaust ports are on directly facing sides of the cylinder wall. To prevent the incoming mixture passing straight across from one port to the other, the piston has a raised rib on its crown. This is intended to deflect the incoming mixture upwards, around the combustion chamber. Much effort, and many different designs of piston crown, went into developing improved scavenging. The crowns developed from a simple rib to a large asymmetric bulge, usually with a steep face on the inlet side and a gentle curve on the exhaust. Despite this, cross scavenging was never as effective as hoped. Most engines today use Schnuerle porting instead. This places a pair of transfer ports in the sides of the cylinder and encourages gas flow to rotate around a vertical axis, rather than a horizontal axis. Media related to Deflector pistons at Wikimedia Commons
Cast-iron steam engine piston, with a metal piston ring spring-loaded against the cylinder wall.
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.) Another factor is that since almost all steam engines use crossheads to translate the force to the drive rod, there are few lateral forces acting to try and "rock" the piston, so a cylinder-shaped piston skirt isn't necessary.
Early (c. 1830) piston for a beam engine. The piston seal is made by turns of wrapped rope.
^ 'Gas' here refers to a fuel gas, not gasoline. ^ A handful of submarines in the following class used a similar engine, with almost equally poor results.
^ Ricardo (1922), p. 116. ^ Ricardo (1922), p. 149. ^ Ricardo (1922), pp. 119–120,122. ^ Irving, Two stroke power units, pp. 13–15. ^ Irving, Two stroke power units, pp. 15–16.
Irving, P.E. (1967). Two-Stroke Power Units. Newnes. Ricardo, Harry (1922). The Internal Combustion Engine. Vol I: Slow-Speed Engines (1st ed.). London: Blackie.
Wikimedia Commons has media related to Pistons.
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