Real vs. air-standard cycles — effect of variable specific heats: Compared with the ideal air-standard assumption (constant specific heats), in an actual engine cycle the variation of specific heats with temperature tends to ________ the maximum pressure and maximum temperature for the same heat addition.

Introduction / Context:
Air-standard cycle analysis simplifies thermodynamics by assuming constant specific heats. In real engines, however, specific heats of gases increase with temperature. Recognizing this difference helps explain why real peak pressures and temperatures are lower than predicted by simplistic models for the same heat addition.

Given Data / Assumptions:

• Idealized air-standard analysis assumes constant cp and cv.
• Real working fluid (combustion products) exhibits temperature-dependent specific heats that generally rise with temperature.
• Heat addition is compared on the same basis to evaluate state-point differences.

Concept / Approach:
When cp and cv rise with temperature, the same amount of added heat causes a smaller temperature rise because the gas can absorb more energy per kelvin. A lower temperature rise at constant mass and volume/pressure conditions leads to a lower resulting pressure rise as well (via ideal gas relationships), thus reducing both the maximum temperature and maximum pressure compared with constant-specific-heat predictions.

Step-by-Step Solution:

Verification / Alternative check:
Detailed combustion gas tables and cycle simulations show noticeable reductions in predicted peak values when variable specific heats are used, aligning closer to measured engine data.

Why Other Options Are Wrong:

• Increase p_max/T_max: Opposite of the thermodynamic effect.
• Mixed increase/decrease options: Not consistent with the coupled p–T reduction caused by higher effective heat capacity at elevated temperatures.

Common Pitfalls:
Assuming R changes significantly; R remains constant for a given gas mixture, while cp and cv change, altering gamma and temperature rise for a given heat input.

Final Answer:
reduce maximum pressure and maximum temperature