The efficiency of a gas turbine engine at altitude

• A. decreases
• B. remains constant
• C. increases
• D. varies unpredictably

Answer: (C)

At the heart of engine efficiency is the Brayton cycle, where the thermodynamic efficiency improves as the pressure ratio across the compressor increases. As altitude rises, the ambient pressure drops, so for the same compressor speed the engine can develop a higher pressure ratio. In cruise, aircraft speed also provides ram pressure at the inlet, further increasing the inlet total pressure. This higher pressure ratio makes the cycle more efficient because a larger portion of the energy added in combustion goes into useful pressure energy rather than wasted in compression losses, raising the thermodynamic efficiency of the engine.

Keep in mind that while thermodynamic efficiency can improve with altitude, thrust generally decreases due to lower air density, so overall propulsion performance behaves differently from the cycle’s thermal efficiency.