Diesel versus Otto cycle: effect of cut-off ratio on efficiency Fill the blank: “When the cut-off ratio is _____, the efficiency of the Diesel cycle approaches the Otto cycle efficiency.” Choose the correct value.

Concept/Approach

• Cut-off ratio r_c = V₃/V₂ during constant-pressure heat addition in the Diesel cycle.
• As r_c → 1, the heat addition tends to constant volume; Diesel tends to the Otto cycle.

Diesel efficiency (air-standard, schematic)
η_D = 1 − (1/r_k^γ−1) · [(r_c^γ − 1)/(γ(r_c − 1))]Taking r_c → 1 gives the Otto efficiency: η_O = 1 − 1/r_k^γ−1.

Final Answer
1 (unity)