Piston

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
Internal combustion engine piston, sectioned to show the
gudgeon pin.
The piston of 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 alloy and Hiduminium ,
specifically for use as pistons.
A few early gas engines [note 1] had double-acting
cylinders, but otherwise effectively all internal combustion
engine pistons are single-acting . During World War II, the
US submarine Pompano[note 2] was fitted with a prototype
of the infamously unreliable H.O.R. double-acting two-
stroke diesel engine. Although compact, for use in a
cramped submarine, this design of engine was not
repeated.