What is the name of the thermodynamic cycle used in gasoline engines?

The thermodynamic cycle used in gasoline engines is the Otto cycle. It is an idealized model of how a spark-ignition piston engine converts heat from burning fuel into mechanical work.

The cycle is named after German engineer Nicolaus Otto, who built a successful working four-stroke gas engine. In the ideal model, the piston first compresses the fuel–air mixture, combustion adds heat at nearly constant volume, and the expanding gases drive the power stroke. Heat is then rejected before the next cycle begins. A real four-stroke engine also includes intake and exhaust strokes.

The four ideal stages are isentropic compression, constant-volume heat addition, isentropic expansion, and constant-volume heat rejection. The pressure–volume diagram forms a loop, whose enclosed area represents the net work produced. Actual engines lose energy through friction, heat transfer, incomplete combustion, and pumping work.

The main mix-up is with the Diesel cycle. Gasoline engines normally use spark ignition and are modeled by the Otto cycle; diesel engines use compression ignition and are modeled by the Diesel cycle. The Carnot cycle is a theoretical efficiency benchmark, not the standard model for gasoline engines.