A reciprocating engine uses pistons to convert chemical energy into mechanical motion. It does this by burning a fuel and then directing the hot gas so that it pushes on a piston. The piston is connected in such a way that it will begin to turn a circular crankshaft; when the piston has reached the end of its stroke, it will return to its original position without bringing the crankshaft back with it. Historically, the most common types of reciprocating engines have been the steam engine and the internal combustion engine. The amount of power a reciprocating engine delivers is linked to the total internal volume of its cylinders.

The first example of a reciprocating engine to be in widespread use was the steam engine. By the early 19th century, British engineers developed steam engine designs that provided enough power to compete with water wheels. This power could be used for a variety of purposes far away from rivers—steam engines soon found their way into factories, railroad locomotives and ships and were a driving force behind the First Industrial Revolution.

In these steam engines, coal is burned to supply the heat to boil water. The resulting high-pressure steam is then funneled into a cylinder with a piston in it. The piston is pushed to the end of the cylinder, turning a crankshaft via a connecting rod. At this point, a valve directs the steam away from the cylinder so that it doesn’t act on the piston any longer. Now that the crankshaft is moving, its momentum can drag the piston back to its original position, and the cycle can start all over again.

Another type of reciprocating engine is the internal combustion engine. An internal combustion engine gets its heat from a chemical fuel, such as gasoline, that is burned inside its cylinders. Intake valves allow air to flow in before the fuel is combusted. Likewise, exhaust valves allow gases to exit the engine after combustion has taken place. Carburetors or, increasingly, fuel injectors, permit the proper mixture of fuel and air for ideal combustion.

In an internal combustion engine, pistons are mechanically connected to a crankshaft—if one piston moves, all other pistons must move with it. Their positions, however, are staggered so that while one piston is undergoing combustion, another may be exhausting gases. This configuration leads to a less jerky reciprocating engine, because power is being produced at more than one part of the full cycle. In automobiles, internal combustion engines commonly have between four and eight cylinders.