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 most common form of reciprocating engines use gasoline or diesel fuel to provide pressure. The piston is located inside the cylinder, into which a small amount of fuel and air is introduced, and then ignited. The now-hot fuel expands greatly, pushing on the piston. The power is taken via a connecting rod to the crankshaft, which turns to provide rotary power. 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 introduced into the chamber above the piston, the pressure of the steam itself driving in. In most applications of steam power, the piston engine has been replaced by the more efficient turbine instead, with pistons only being used in cars due to its higher torque.