The Otto cycle
The four-stroke engine was first patented by Alphonse Beau de Rochas in 1861. Before, in about 1854-57, two italians (Eugenio Barsanti & Felice Matteucci) invented an engine that was rumored to be very similar, but the patent was lost. And no one will really know who invented it.
The first person to actually build a car with this engine was German engineer Nicolaus Otto. Which is why the four-stroke principle today is commonly known as the Otto cycle and four-stroke engines using spark plugs often are called Otto engines. The Otto Cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion and rejection of heat at constant volume.