The Otto cycle is the ideal air-standard cycle for a spark-ignition (petrol) engine. It is made up of four reversible processes acting on the working fluid inside the cylinder:

1. 1→2 Isentropic (reversible adiabatic) compression: the piston compresses the charge with no heat transfer, so entropy stays constant.
2. 2→3 Constant-volume heat addition: heat is added at top dead centre (idealising the rapid combustion) while volume is fixed.
3. 3→4 Isentropic (reversible adiabatic) expansion: the high-pressure gas pushes the piston down with no heat transfer, again at constant entropy — this is the power stroke.
4. 4→1 Constant-volume heat rejection: heat is rejected at fixed volume, returning the fluid to its initial state.

Why compression and expansion is correct: An isentropic process is both adiabatic and reversible, meaning entropy is unchanged. In the Otto cycle the only two such processes are the compression (1→2) and the expansion (3→4) strokes. Hence the pair that are both isentropic is compression and expansion.

Why the other pairs are wrong: The heat addition and heat rejection processes are constant-volume processes involving heat transfer, so they are not isentropic. Any pairing that includes heat addition or heat rejection (such as compression with heat rejection, or expansion with heat addition) therefore cannot be a pair of isentropic processes.