Describe the Otto cycle and where it is applied.

The Otto cycle is the ideal thermodynamic cycle for spark-ignition (petrol/gasoline) engines, consisting of two isentropic and two constant-volume processes. It includes isentropic compression, constant-volume heat addition, isentropic expansion, and constant-volume heat rejection. The efficiency depends on compression ratio and is given by η = 1 - (1/r^(γ-1)), where r is the compression ratio and γ is the specific heat ratio.