How do turbofan engines differ from turbojets in terms of thrust production and efficiency at subsonic speeds?

• A. Turbojets rely on engine exhaust for thrust and are less efficient at subsonic speeds.
• B. Turbofans generate thrust solely through the engine core.
• C. Turbofan engines use bypass air to generate additional thrust, increasing propulsive efficiency and reducing specific fuel consumption and noise.
• D. Turbofan engines produce no noise.

Answer: (C)

The key idea is that turbofan engines gain efficiency at subsonic speeds by using bypass air to generate most of the thrust. In a turbojet, all the air goes through the engine core and is expelled at a very high velocity. That high jet speed provides thrust, but it’s not very energy-efficient for an airplane moving at subsonic speeds, so fuel use and overall propulsion efficiency suffer.

Turbofans, on the other hand, have a large fan at the front and route a big portion of incoming air around the core through a bypass duct. This bypass air is accelerated by the fan to a moderate speed, producing a large amount of thrust with a much lower exhaust velocity. Because moving the airplane forward with a slower, higher-mass flow is more efficient, the propulsive efficiency improves and the engine’s specific fuel consumption drops. The bypass flow also tends to be quieter than the core exhaust, contributing to the reduced noise.

So the best answer reflects that turbofan engines use bypass air to add thrust, which raises propulsive efficiency and lowers fuel consumption and noise at subsonic speeds.