The big idea
An engine burns a little fuel in a closed space and lets the hot, high-pressure gas push a piston. It does this in four strokes, two turns of the crankshaft: draw in air and fuel, squeeze it, burn it so it pushes, and push out what is left. Cams opened by the crankshaft time the valves, so each door opens at just the right moment.
Memory trick: “In, squeeze, boom, out.” One word for each stroke: intake, compression, power, exhaust.
A bit of history: In 1862 the French engineer Alphonse Beau de Rochas described the four-stroke cycle in a patent, but he did not build an engine. In 1876 the German engineer Nikolaus Otto built a working four-stroke gas engine at the Deutz works near Cologne, and the cycle is still often called the Otto cycle.