This cycle is so named as it was conceived by ‘Otto’. On this cycle, petrol, gas and many types of oil engines work. It is the standard of comparison for internal combustion engines. Figs. shows the theoretical p-V diagram and T-s diagrams of this cycle respectively. The point 1 represents that cylinder is full of air with volume V1, pressure p1 and absolute temperature T1. Line 1-2 represents the adiabatic compression of air due to which p1, V1 and T1 change to p2,V2 and T2, respectively. Line 2-3 shows the supply of heat to the air at constant volume so that p2 and T2 change to p3 and T3 (V3 being the same as V2). Line 3-4 represents the adiabatic expansion of the air. During expansion p3, V3 and T3 change to a final value of p4, V4 or V1 and T4, respectively. Line 4-1 shows the rejection of heat by air at constant volume till original state (point 1) reaches.