Reciprocating Engine

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

The reciprocating engine was first developed as the now largely obsolete steam engine during the eighteenth century, followed by the stirling engine and internal combustion engine in the nineteenth. Today the most common form of reciprocating engine is the internal combustion engine running on the combustion of petrol, diesel or natural gas and used to power motor vehicles.

There may be one or more pistons. Each piston is inside a cylinder, into which a gas is introduced, either already hot and under pressure (steam engine), or heated inside the cylinder either by ignition of a fuel air mixture (internal combustion engine) or by contact with a hot heat exchanger in the cylinder (sterling engine). The hot gases expand, pushing the piston to the bottom of the cylinder. The piston is returned to the cylinder top (Top dead centre) either by a flywheel or the power from other pistons connected to the same shaft. In most types the expanded or "exhausted" gases are removed from the cylinder by this stroke.

The linear movement of the piston is converted to a rotating movement via a connecting rod and a crankshaft or by a swashplate. A flywheel is often used to ensure smooth rotation. The more cylinders a piston engine has, the more vibration-free (smoothly) it can run. The higher the combined piston displacement volume it has the more power it is capable of producing.

It is common for such engines to be classified by the number and alignment of cylinders and the total volume of displacement of gas by the pistons moving in the cylinders. Single- and two-cylinder engines are common in smaller vehicles such as motorcycles. Automobiles typically have between four and eight, while locomotives, and ships may have a dozen cylinders or more.

Cylinders may be aligned in line, in a V configuration, opposite each other , or radially around the crankshaft. Opposed piston engines put 2 pistons working at opposite ends of the same cylinder and this has been extended into triangular arrangements such as the Napier Deltic. Some designs have set the cylinders in motion around the shaft, see the Rotary engine.

Reciprocating engines that are powered by compressed air, steam or other hot gasses are still used in some applications such as to drive many modern torpedoes. In most cases the gas, like that produced by high test peroxide or Otto fuel II, is pressurised without the need of combustion and therefore oxygen. This allows propulsion under water for considerable periods of time and over significant distances. e.g.  Mark 46 torpedo.


Internal combustion engines are most commonly used for mobile propulsion systems. In mobile scenarios internal combustion is advantageous, since it can provide high power to weight ratios together with excellent fuel energy-density. These engines have appeared in almost all automobiles, motorbikes, many boats, and in a wide variety of aircraft and locomotives.