Otto Cycle (1876):

This cycle consists of the two reversible adiabatic processes and the two constant volume processes as shown in the figure given below on P-V and T-S diagrams.

The process 1-2 is showing reversible adiabatic compression, the process 2-3 is heat addition at the constant volume, the process 3-4 is reversible adiabatic expansion and the process 4-1 is heat rejection at the constant volume. 

The cylinder is supposed to contain air as the working substance and heat is supplied at end of compression, and heat is rejected at end of expansion to sink and cycle is repeated.

Process

0-1 = suction

1-2 = isentropic compression

2-3 = heat addition at constant volume

3-4 = isentropic expansion

4-1 = constant volume heat rejection of the cycle1-0 = exhaust