A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to 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. An internal combustion engine is acted upon by the pressure of the combustion gases in the combustion chamber space at the top of the cylinder. This force then acts downwards through the rod and onto the crankshaft. The connecting rod is attached to the piston by a swivelling gudgeon pin and this pin is mounted within the piston, unlike the steam engine, there is no piston rod or crosshead. The pin itself is of hardened steel and is fixed in the piston, 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, gas sealing is achieved by the use of piston rings. These are a number of iron rings, fitted loosely into grooves in the piston. 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 rings have solid faces and provide gas sealing, lower rings have narrow edges. 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, a few early gas engines had double-acting cylinders, but otherwise effectively all internal combustion engine pistons are single-acting. During World War II, the US submarine Pompano was fitted with a prototype of the infamously unreliable H. O. R, although compact, for use in a cramped submarine, this design of engine was not repeated. Media related to Internal combustion engine pistons at Wikimedia Commons Trunk pistons are long relative to their diameter and they act both as a piston and cylindrical crosshead. As the connecting rod is angled for much of its rotation, a longer piston helps to support this
Pistons within a sectioned petrol engine
Internal combustion engine piston, sectioned to show the gudgeon pin.