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Its purpose is either to change the volume enclosed by the cylinder, or to exert a force on a fluid inside the cylinder.

A piston in an internal combustion engine usually has two rings as a seal between the piston and the cylinder wall (compression rings) and one or more oil control rings. The head of the piston can be flat or bulged or otherwise shaped. Pistons can be forged or cast. The piston is an important component of a piston engine and of hydraulic and pneumatic systems.

In an Otto or Diesel engine, the head of the piston forms one wall of an expansion chamber inside the cylinder. The opposite wall, called the cylinder head, contains inlet and exhaust valves for gases.

As the piston moves inside the cylinder, it transforms the energy from the expansion of a burning gas (usually a mixture of petrolPetrol gasoline in the United States and Canada) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines. The term gasoline is the common usage within the oil industry, even within companies or dieselThis article is about the fuel. For other uses see diesel (disambiguation). Diesel is a product used as a fuel in a diesel engine invented by Rudolf Diesel, and perfected by Charles F. Petrodiesel One can obtain diesel from petroleum, which is called petr and airAir is a name for the mixture of gases present in the Earth's atmosphere. Compressed air is often used in scuba diving as a shallow water breathing gas and to inflate buoyancy devices. Compressed air is also used as a source of energy for pneumatic tools.) into mechanical power (in the form of a reciprocating linearThe word linear comes from the latin word linearis which means created by lines''. In mathematics, a linear function f ''x is one which satisfies the following two properties (but see below for a slightly different usage of the term): Additivity: f ''x + motionThis article is about motion in physics. See also motion (legal), motion (democracy) and Apple Motion. In physics, motion means a change in the position of a body with respect to time, as measured by a particular observer in a particular frame of referenc). From there the power is conveyed through a connecting rodIn a reciprocal piston engine, the connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves up and down and the crankshaft rotates. The top end, with a small opening for the piston p to a crankshaftThe crankshaft is that part of an engine which translates linear piston motion into rotation. Generally more than one piston is attached to the crank to provide a smoother delivery of power to the rotating part, though many small engines, such as those fo, which transforms it into a rotary motion, which usually drives a gearboxA gearbox is an assembly of gears allowing the rotational speed of an input shaft to be changed to a different speed. Often, more than one gear ratio is provided, and a mechanism for selecting among the ratios is provided. The most common form of gearbox through a clutch.

A steam engine is another type of piston engine. In most steam engines, the pistons are double acting: steam is alternately admitted to either end of the cylinder, so that every piston stroke produces power.