Components of a typical, four stroke cycle, DOHC piston engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug, (V) Valves, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow.

A reciprocating engine, also often known as a piston engine, is an engine that utilizes one or more pistons in order to convert pressure into a rotating motion.

The reciprocating engine was introduced with the now obsolete steam engine, but today the most common form of reciprocating engines is the internal combustion engine using the burning of gasoline, diesel fuel, oil or natural gas or to provide pressure. There may be one or more pistons. Each piston is located inside a cylinder, into which a fuel and air mixture is introduced, and then ignited. The now hot gases expand, pushing the piston away. The linear movement of the piston is converted to a circular movement via a connecting rod and a crankshaft. These engines are known collectively as internal-combustion engines, although internal-combustion engines do not necessarily contain pistons.

Though not often used today, steam is another power source for reciprocating engines, in the steam engine. In these cases high pressure steam is used to drive the piston. In most applications of steam power, the piston engine has been replaced by the more efficient turbine instead, with pistons being used in cars owing to their requirement for a high level of torque.

See also:

• Stirling engine
• four-stroke cycle
• two-stroke cycle
• Double overhead cam
• Diesel cycle
• Otto cycle

For a contrasting approach to internal combustion using no pistons, see Rotary combustion engine, not to be confused with the Rotary piston engine formerly very popular in aircraft.