A reciprocating engine is an engine that uses one or more pistons in order to convert pressure into rotational motion. They use the reciprocating (up-and-down) motion of the pistons to translate this energy. There are many different types, including the internal combustion engine which is used in most motor vehicles, the steam engine which is a type of external combustion engine, and the Stirling engine. A rotary engine would do the same task as reciprocating engine but in a very different manner due to its triangular rotor.
All types have one or more pistons, which follow the four-stroke cycle visible in Figure 1. Common engine block configurations include a single row of cylinders (in-line), two rows converging to a point (V-engine), a double zigzag (W-engine) and two horizontal rows (opposed engine). The engines mentioned above (internal combustion, steam, Stirling) all use somewhat different processes to complete the cycle, so the general case will be explored (as seen in Figure 2).
Figure 1: 4-stroke internal combustion engine. 1:fuel injection, 2:ignition, 3:expansion(work is done), 4:exhaust.
Figure 2: The crankshaft (red) converts reciprocating motion from the pistons (grey) which is often combined with a flywheel (black).
The four-stroke cycle is what gives the engine its energy, but now it must translate this energy into rotational energy for the transmission, drive shaft and wheels. This is done by the crankshaft, which is seen in Figure 2. The crankshaft converts this up-and-down motion into rotational motion, which is often combined with a flywheel to retain the discontinuous reciprocating energy as rotational energy.