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[edit]
Internal combustion engines[edit]
Internal combustion engine piston, sectioned to show the gudgeon pin.
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.
Gas sealing is achieved by the use of
piston rings. 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
cast from
aluminium alloys. For better strength and fatigue life, some racing pistons may be
forged instead. 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 alloyand
Hiduminium, specifically for use as pistons.
Trunk pistons[edit]
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 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.
Crosshead pistons[edit]
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.
Because of the additional weight of these pistons, they are not used for high-speed engines.
Slipper pistons[edit]
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. 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, the benefit is reduced.
Deflector pistons[edit]
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.
[1] 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.
[2]
Steam engines[edit]
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.
For liquids[edit]
For gases[edit]
Air cannons[edit]
There are two special type of pistons used in
air cannons: close tolerance pistons and double pistons. In close tolerance pistons
O-rings serve as a valve, but O-rings are not used in double piston types.
See also[edit]
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