A gas turbine is most efficient when

• A. both turbine shafts run at the same speed
• B. the combustion chamber is kept cool
• C. the ambient air temperature is low
• D. operated at the highest combustion temperature that the construction materials permit

Answer: (D)

The main idea being tested is how the efficiency of a gas turbine rises with the temperature of the gas entering the turbine, up to the point allowed by the materials. In a Brayton-cycle engine, increasing the turbine inlet temperature means more of the fuel’s energy is converted into useful work rather than wasted in exhaust, so thermal efficiency improves as TIT goes up. The highest efficiency is reached when you push the combustion temperature to the limit that the construction materials can safely withstand, with proper cooling.

If you keep the combustor cooler, the TIT drops and efficiency decreases. A cooler ambient temperature can provide a modest gain in performance by increasing air density, but it doesn’t boost the cycle’s intrinsic efficiency as much as raising the turbine inlet temperature. Having both turbine shafts run at the same speed isn’t a direct factor in maximizing efficiency; while speeds are designed for, they don’t drive the fundamental thermodynamic efficiency like TIT does.