From Academic Kids
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. The more cylinders a piston engine has, the more power it is capable of producing, so it is common for such engines to be classified by the number and alignment of cylinders. Single- and two-cylinder engines are common in smaller vehicles such as motorcycles; automobiles, locomotives, and ships may have a dozen cylinders or more. 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 In most applications of steam power, the piston engine has been replaced by the more efficient turbine instead, with pistons being used in cars owing to their requirement for a high level of torque.
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.de:Kolbenmaschine es:Motor alternativo ja:レシプロエンジン pl:Silnik tłokowy zh:往复式发动机