What happens inside a car engine's cylinder?

A car engine turns fuel into motion by burning it in a small steel cylinder, over and over, in a four-step routine. A piston slides up and down inside that cylinder, and every full trip from one end to the other is called a stroke. In a four-stroke engine the piston makes four strokes while it turns the crankshaft, the bent shaft that changes the piston's up-and-down push into a spin.

The first stroke is intake: the piston moves down and pulls a mixture of air and fuel into the cylinder. The second is compression: both valves close and the piston rises, squeezing the mixture into a tiny space. The third is the power stroke, the one that does the work. In a petrol engine a spark plug ignites the squeezed mixture, and the blast forces the piston back down. The fourth is exhaust: the piston rises again and pushes the spent gases out through an open valve. Then the whole thing starts over.

That is what makes it an internal combustion engine. The burning happens inside the engine, and the expanding gas pushes directly on the piston. A steam engine works differently: it heats a separate working fluid, which never mixes with the fire.

The man whose name sits on this cycle did not start as an engineer. Nicolaus Otto was a travelling grocery salesman who came across a gas engine built in Paris by Jean Lenoir. By 1876 Otto and his partner Eugen Langen had built the first engine that squeezed the fuel mixture before lighting it, and it was far more efficient than anything before it. Within a decade, the same four-stroke design was powering the first cars.