Ideal Otto Cycle

The four-stroke spark-ignited internal
combustion engine operates in an
approximation of the Air Standard Otto
Cycle represented by the (p,v) and (T,s)
diagrams shown opposite. In location 1 the piston is at the bottom of the cylinder (max. volume; BDC) and the inlet valve has just closed.

From: Moran & Shapiro, "Fundamentals of Engineering Thermodynamics," Wiley (1992)

The air/fuel mixture is then compressed adiabatically, work being done on the mixture along the path 1-2. At 2 the piston is at the top dead center (min. volume). Ignition occurs at 2 and the combustion reaction adds heat at constant volume along the path 2-3. The hot reaction products expand adiabatically and do external work along the path 3-4. At 4 the heat is rejected to the surroundings at constant volume along the path 4-1. The exhaust valve opens at 1 and the gases are rejected to the atmosphere. The cycle then restarts.