Editing Piston (section)

==Piston engines==
{{main|Reciprocating engine}}
[[File:Engine movingparts.PNG|thumb|Four-piston engine (3D render)]]
[[File:Piston.stl|thumb|Piston]]

===Internal combustion engines===
[[File:BHB Hiduminium piston (Autocar Handbook, 13th ed, 1935).jpg|thumb|upright|[[Internal combustion piston engine|Internal combustion engine]] piston, sectioned to show the [[gudgeon pin]].]]
An [[internal combustion piston engine|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 typical piston design is on the picture. This type of piston is widely used in car [[diesel engines]]. According to purpose, supercharging level and working conditions of engines the shape and proportions can be changed.

High-power diesel engines work in difficult conditions. Maximum pressure in the combustion chamber can reach 20 MPa and the maximum temperature of some piston surfaces can exceed 450&nbsp;°C. It is possible to improve piston cooling by creating a special cooling cavity. [[Injector]] supplies this cooling cavity «A» with oil through oil supply channel «B». For better temperature reduction construction should be carefully calculated and analysed. Oil flow in the cooling cavity should be not less than 80% of the oil flow through the injector.
[[File:Cooling chanel.png|thumb|A – cooling cavity; B – oil supply channel]]

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 [[circlip]]s.

Gas sealing is achieved by the use of [[piston ring]]s. These are a number of narrow iron rings, fitted loosely into grooves in the piston, just below the crown. The rings are split at a point in the rim, allowing them to press against the cylinder with a light spring pressure. Two types of ring are used: the upper rings have solid faces and provide gas sealing; lower rings have narrow edges and a U-shaped profile, to act as oil scrapers. There are many proprietary and detail design features associated with piston rings.

Pistons are usually [[casting (metalworking)|cast]] or [[forging|forged]] from [[aluminium alloy]]s. For better strength and fatigue life, some racing pistons<ref>{{Cite news|url=http://blog.wiseco.com/-what-makes-a-racing-piston|title=What Makes A Racing Piston?|last=Magda|first=Mike|access-date=2018-04-22|language=en}}</ref> may be [[forging|forged]] instead. Billet pistons are also used in racing engines because they do not rely on the size and architecture of available forgings, allowing for last-minute design changes. Although not commonly visible to the naked eye, pistons themselves are designed with a certain level of ovality and profile taper, meaning they are not perfectly round, and their diameter is larger near the bottom of the skirt than at the crown.<ref>{{Cite news|url=https://blog.wiseco.com/full-round-vs.-strutted-piston-forging-designs-and-skirt-styles-explained|title=Full-Round vs. Strutted: Piston Forging Designs and Skirt Styles Explained|last=Bailey|first=Kevin|access-date=2018-07-15|language=en}}</ref>

Early pistons were of [[cast iron]], but there were obvious benefits for engine balancing if a lighter alloy could be used. To produce pistons that could survive engine combustion temperatures, it was necessary to develop new alloys such as [[Y alloy]] and [[Hiduminium]], specifically for use as pistons.

A few early [[gas engine]]s{{efn-lr|'Gas' here refers to a [[fuel gas]], not [[gasoline]].}} had [[double-acting cylinder]]s, but otherwise effectively all internal combustion engine pistons are [[single-acting cylinder|single-acting]]. During [[World War II]], the US submarine [[USS Pompano (SS-181)|''Pompano'']]{{efn-lr|A handful of submarines in the following class used a similar engine, with almost equally poor results.}} was fitted with a prototype of the infamously unreliable [[Hooven-Owens-Rentschler|H.O.R.]] double-acting two-stroke diesel engine. Although compact, for use in a cramped submarine, this design of engine was not repeated.

{{Commons category-inline|Internal combustion engine pistons }}

====Trunk pistons====
Trunk pistons are long relative to their diameter. They act both as a piston and cylindrical [[crosshead]]. As the connecting rod is angled for much of its rotation, there is also a side force that reacts along the side of the piston against the cylinder wall. A longer piston helps to support this.

Trunk pistons have been a common design of piston since the early days of the reciprocating internal combustion engine. They were used for both petrol and diesel engines, although high speed engines have now adopted the lighter weight [[#Slipper pistons|slipper piston]].

A characteristic of most trunk pistons, particularly for diesel engines, is that they have a groove for an oil ring below the [[gudgeon pin]], in addition to the rings between the gudgeon pin and crown.

The name 'trunk piston' derives from the '[[trunk engine]]', an early design of [[marine steam engine]]. To make these more compact, they avoided the steam engine's usual [[piston rod]] with separate crosshead and were instead the first engine design to place the gudgeon pin directly within the piston. Otherwise these trunk engine pistons bore little resemblance to the trunk piston; they were extremely large diameter and double-acting. Their 'trunk' was a narrow cylinder mounted in the centre of the piston.

{{Commons category-inline|Trunk pistons}}

====Crosshead pistons====
Large slow-speed Diesel engines may require additional support for the side forces on the piston. These engines typically use [[crosshead]] pistons. The main piston has a large [[piston rod]] extending downwards from the piston to what is effectively a second smaller-diameter piston. The main piston is responsible for gas sealing and carries the piston rings. The smaller piston is purely a mechanical guide. It runs within a small cylinder as a trunk guide and also carries the gudgeon pin.

Lubrication of the crosshead has advantages over the trunk piston as its lubricating oil is not subject to the [[heat of combustion]]: the oil is not contaminated by combustion soot particles, it does not break down owing to the heat and a thinner, less viscous oil may be used. The friction of both piston and crosshead may be only half of that for a trunk piston.{{sfnp|Ricardo|1922|page=116}}

Because of the additional weight of these pistons, they are not used for high-speed engines.

{{Commons category-inline|Crosshead pistons}}

====Slipper pistons====
[[File:Ricardo slipper piston (Autocar Handbook, 13th ed, 1935).jpg|thumb|upright|Slipper piston]]
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.{{sfnp|Ricardo|1922|page=149}} In racing applications, slipper piston skirts can be configured to yield extremely light weight while maintaining the rigidity and strength of a full skirt.<ref>{{Citation|title=Piston with improved side loading resistance|date=2009-10-12|url=https://patents.google.com/patent/US20100089358A1/en|access-date=2018-04-22}}</ref>  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.{{sfnp|Ricardo|1922|pages=119–120, 122}} 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.

{{Commons category-inline|Slipper pistons}}

====Deflector pistons====
[[File:Two-stroke deflector piston (Autocar Handbook, 13th ed, 1935).jpg|thumb|left|upright|[[Two-stroke engine|Two-stroke]] deflector piston]]
Deflector pistons are used in [[two-stroke engine]]s with crankcase compression, where the gas flow within the cylinder must be carefully directed in order to provide efficient [[scavenging (automotive)|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]].{{sfn|Irving, Two stroke power units|pages=13–15}}

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.{{sfn|Irving, Two stroke power units|pages=15–16}}

{{Commons category-inline|Deflector pistons}}

====Racing pistons====
[[File:1830s beam engine piston with rope seal, Coalbrookdale Museum of Iron.jpg|thumb|upright|Early (c. 1830) piston for a [[beam engine]]. The piston seal is made by turns of wrapped [[rope]].]]
In racing engines, piston strength and stiffness is typically much higher than that of a passenger car engine, while the weight is much less, to achieve the high engine RPM necessary in racing.<ref>{{Cite news|url=http://www.hotrod.com/articles/ctrp-0708-advanced-piston-technology/|title=Racing Piston Technology – Piston Weight And Design – Circle Track Magazine|date=2007-05-31|work=Hot Rod Network|access-date=2018-04-22|language=en-US}}</ref>

=== Hydraulic cylinders ===
[[File:Hydraulic cylinders used in a particle board machine.jpg|alt=Pistons of Hydraulic cylinders used in a hot press|left|thumb|Pistons of Hydraulic cylinders used in a hot press]]
Hydraulic cylinders can be both [[Single- and double-acting cylinders|single-acting]] or [[Double acting/differential cylinder|double-acting]]. A hydraulic actuator controls the movement of the piston back and/or forth. Guide rings guides the piston and rod and absorb the radial forces that act perpendicularly to the cylinder and prevent contact between sliding the metal parts.

===Steam engines===
[[File:Piston, sprung ring (Heat Engines, 1913).jpg|thumb|Cast-iron [[steam engine]] piston, with a metal piston ring spring-loaded against the [[cylinder (engine)|cylinder]] wall.]]
[[Steam engine]]s 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 [[D slide valve|slide valves]], [[piston valve]]s or [[poppet valve]]s. 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.