Which statement best describes how efficiency relates to pressure and temperature ratios in the Brayton cycle?

• A. Efficiency decreases with higher pressure ratio.
• B. Efficiency is not affected by pressure ratio.
• C. Efficiency is determined by mass flow.
• D. Efficiency rises with higher pressure and temperature ratios.

Answer: (D)

In a Brayton cycle, efficiency improves as the pressure ratio and the temperature ratio across the cycle increase. A higher pressure ratio means the compressor raises the air pressure more before combustion, which increases the turbine inlet temperature for a given fuel flow and raises the turbine work relative to the compressor work. Coupled with a higher maximum cycle temperature (larger temperature rise during combustion), the cycle delivers more useful work per unit of fuel, boosting thermal efficiency — up to material and hardware limits. This is why increasing both pressure and temperature ratios leads to higher efficiency.

The other statements don’t fit as well: increasing the pressure ratio does not inherently decrease efficiency; efficiency is not independent of pressure ratio; and while mass flow affects the amount of power produced, it doesn’t determine the cycle’s intrinsic efficiency.