Name the Otto cycle and its key processes.

• A. Otto cycle includes two adiabatic (isentropic) processes and two constant-volume heat addition/rejection processes
• B. Otto cycle includes four isothermal processes
• C. Otto cycle includes two adiabatic and two constant-pressure heat transfers
• D. Otto cycle includes one adiabatic and three constant-volume processes

Answer: (A)

The Otto cycle is the idealized cycle for a spark-ignition engine. It has two adiabatic (isentropic) processes—compression and expansion—and two constant-volume heat transfer processes, corresponding to heat addition during combustion at essentially fixed volume and heat rejection at essentially fixed volume during the exhaust phase. This means the cycle steps are: compression with no heat transfer, rapid heat addition at constant volume, expansion with no heat transfer, and heat rejection at constant volume.

Why this is the best description: during ignition, the piston is near its top position so the volume hardly changes as fuel burns, making the heat addition occur at roughly constant volume. The compression and expansion strokes are treated as adiabatic since they occur quickly with negligible heat transfer. The other options propose isothermal processes or heat transfers at constant pressure, which do not reflect how a typical Otto-cycle engine operates.