Which combination of factors generally increases Brayton cycle efficiency?

• A. Lower pressure ratio and lower turbine inlet temperature.
• B. Higher pressure ratio and lower turbine inlet temperature.
• C. Higher pressure ratio and higher turbine inlet temperature.
• D. Lower pressure ratio and higher turbine inlet temperature.

Answer: (C)

In a Brayton cycle, efficiency improves when you push both how much the air is compressed (pressure ratio) and how hot the gas entering the turbine is (turbine inlet temperature). A higher pressure ratio means more energy can be recovered as work from the expanding gas relative to the work needed to compress it, and, in the ideal cycle, the thermal efficiency rises as the pressure ratio increases. A higher turbine inlet temperature adds more heat energy per unit mass in the combustor, so the turbine can do more work during expansion, increasing net output and efficiency—again within the material limits of the engine.

So, increasing both factors—pressure ratio and turbine inlet temperature—moves the cycle toward higher efficiency. Lowering either one reduces the potential net work and thus the efficiency.