During the Diesel cycle, which process occurs at constant volume to reject heat?

• A. Isothermal
• B. Isochoric
• C. Isobaric
• D. Adiabatic

Answer: (B)

In this cycle, the cooling (heat-rejection) portion happens at constant volume. After the combustion and expansion phases, the gas taxis back to the same cylinder volume, with the piston effectively at a fixed position, so the volume does not change during heat loss. Because the volume is fixed, no boundary work is done (W = ∫P dV = 0). All the energy leaving the gas is heat, which lowers the internal energy and the temperature. This is why heat rejection is described as a constant-volume process.

Why this fits while other descriptions don’t: heat transfer during this stage is not at constant pressure, nor is it at a stage with changing volume, and it isn’t adiabatic (there is heat transfer, just not with changing volume). The heat addition stage, in contrast, occurs at constant pressure as fuel burns, raising the gas temperature while pressure remains nearly constant.