The ideal air-standard Otto cycle is the thermodynamic model for spark-ignition (petrol) engines. It consists of four internally reversible processes acting on the working fluid (treated as air):

• 1→2 Isentropic (adiabatic) compression: the piston compresses the charge with no heat transfer; temperature and pressure rise.
• 2→3 Constant-volume heat addition: at top dead centre the piston is momentarily stationary, and combustion is assumed to be instantaneous. Because it happens so fast that the piston barely moves, the volume stays essentially constant while a large amount of heat (energy released by the fuel) is added, sharply raising pressure and temperature.
• 3→4 Isentropic expansion: the high-pressure gas pushes the piston down, doing work, with no heat transfer.
• 4→1 Constant-volume heat rejection: exhaust; heat leaves at constant volume.

Why constant volume is correct: Heat is added during process 2→3, which is defined as an isochoric (constant-volume) process. This is the defining feature that distinguishes the Otto cycle from the Diesel cycle and gives its efficiency formula η = 1 − 1/r^γ−1, which depends only on the compression ratio r.

Why the other options are wrong:

• Isentropic compression is a process in the Otto cycle, but by definition an isentropic process involves no heat transfer — it is where work is done on the gas, not where heat is added.
• Constant pressure heat addition is the characteristic of the Diesel cycle (compression-ignition engines), not the Otto cycle.
• Isothermal expansion belongs to the Carnot and Stirling cycles; it does not appear anywhere in the Otto cycle.