Air is the working substance for an internal combustion engine and can be considered to be an ideal gas. An internal combustion engine cycle consists of four steps: (1) reversible adiabatic compression from A to B, (2) reversible constant-volume pressure increase from B to C due to the combustion of a small amount of fuel, (3) reversible adiabatic expansion from C to D, and (4) reversible and constant-volume pressure decrease back to state A.