Thermal cycles — relative efficiency of Otto and Diesel cycles at the same maximum pressure and temperature For equal peak (maximum) pressure and maximum temperature limits, which ideal cycle is more thermally efficient?

Introduction / Context:
Otto and Diesel cycles are idealised models for spark-ignition and compression-ignition engines, respectively. Comparing their efficiencies under matching constraints reveals why SI engines can be thermally competitive despite lower compression ratios in practice.

Given Data / Assumptions:

• Air-standard analysis with constant specific heats (idealised comparison).
• Both cycles limited to the same maximum pressure and maximum temperature.
• Compression and expansion are isentropic; heat addition follows the ideal processes for each cycle.

Concept / Approach:
For the same peak pressure and temperature, the Otto cycle (heat addition at constant volume) produces a larger pressure rise during heat addition than the Diesel cycle (heat addition partly at constant pressure). This leads to a higher average temperature of heat addition relative to exhaust heat rejection, improving thermal efficiency.

Step-by-Step Solution:
Recognise constraint: identical Pmax and Tmax for both cycles.In the Otto cycle, heat is added at constant volume → greater pressure rise for the same temperature limit.In the Diesel cycle, part of heat addition occurs at constant pressure → lower peak pressure rise for the same Tmax.Hence, the Otto cycle attains higher thermal efficiency under the stated constraints.

Verification / Alternative check:
At the same compression ratio, Otto is also more efficient than Diesel. The present constraint (same Pmax and Tmax) similarly favours Otto due to the thermodynamic path of heat addition and resulting mean temperatures.

Why Other Options Are Wrong:

• Diesel more efficient: not under equal Pmax and Tmax; its constant-pressure portion reduces efficiency here.
• Equally efficient or none: contradict thermodynamic comparisons of the ideal cycles.

Common Pitfalls:
Confusing real-engine data (influenced by friction, pumping, and mixture control) with ideal cycle results.

Final Answer:
Otto cycle is more efficient than Diesel cycle