Fuel combustion takes place in a confined space, and produces expanding gases that are used to provide mechanical power. The four-stroke reciprocating engine that is used in automobiles is the most common internal-combustion engine. In this mechanism, mechanical power is supplied by a piston that is fitted inside a cylinder. On a downstroke of the piston, the first stroke, fuel that has been mixed with air (by fuel injection or using a carburetor) enters the cylinder through an intake valve; the piston moves up to compress the mixture at the second stroke. At ignition, the third stroke, a spark from a spark plug ignites the mixture, forcing the piston down; in the exhaust stroke, an exhaust valve opens to vent the burned gas as the piston moves up. The piston is connected to a crankshaft by means of a rod. The reciprocating (up and down) movements of the piston rotate the crankshaft, which is connected by gearing to the drive wheels of the automobile. The ignition spark is provided by an electrical system whose power comes from a battery. This also supplies power to the starting system, which is a small electric motor that turns the crankshaft until the engine takes over. Water circulating around the cylinders cool the engine, and lubrication occurs motor oil that is driven around the moving engine parts by an oil pump. Small vehicles like lawn mowers and motorcycles use the two-stroke engine, which combines intake and compression in the first stroke and power and exhaust in the second.

Diesel engine

This type of internal combustion engine was patented in 1892 by the German engineer Rudolph Diesel. It burns fuel oil instead of gasoline and is heavier and more powerful than the gasoline engine. It differs from the gasoline engine in that the ignition of fuel is caused by compression of air in its cylinders instead of by a spark. By varying the amount of fuel injected in the cylinder, the speed and power of the diesel are controlled. Industrial and municipal electric generators, continuously operating pumps such as those used in oil pipelines, and ships, trucks, locomotives, and other such automobiles widely use diesel to power.