A four-stroke engine uses the four-stroke combustion cycle, which converts fuel, in most cases gasoline, into movement and energy. The four-stroke combustion cycle is made up of the intake stroke, the compression stroke, the combustion stroke and the exhaust stroke. The combustion cycle is generated through the movement of a piston connected by a rod to a crankshaft. With every revolution of the crankshaft, the combustion cycle is started.
At the start of the combustion cycle, the piston starts at the top of an engine with an open intake valve through which fuel and air can travel through. Once the piston begins movement, the engine cylinder fills with gasoline and air, finishing the intake stroke. As the piston moves back towards the top, the air and gasoline mixture becomes compressed, which increases its combustion force. Once the piston nears the top again, a spark plug connected to the cylinder sends an electric spark which ignites the gasoline and creates combustive power for the engine. At the end of the four-stroke combustion cycle, the exhaust valve opens to let out the waste left by the first combustion.
The four-stroke engine was first developed by Nikolaus Otto in 1867 and is also sometimes referred to as the Otto cycle.