Turbofan engines provide which advantage over turbojets?

• A. Lower thrust at maximum speed
• B. Higher fuel consumption in cruise
• C. Heavier structure with no advantages
• D. More efficient cruise thrust and lower fuel consumption

Answer: (D)

The idea being tested is how bypass air in a turbofan changes propulsion efficiency, especially at cruise. A turbofan uses a large amount of air that goes around the engine core (the bypass flow) and is accelerated to a lower speed than the core exhaust. This lower-velocity, high-mass flow produces thrust more efficiently than forcing all air through a high-velocity jet, as in a turbojet.

At cruise, propulsive efficiency depends on keeping jet velocity closer to the aircraft’s speed. The bypass flow achieves this more effectively than the turbojet exhaust, so the engine can produce the same thrust with far less fuel. That’s why cruise thrust is more efficient and fuel consumption is lower with a turbofan.

The other statements don’t fit: turbojets tend to burn more fuel to deliver similar cruise thrust because their exhaust velocity is higher, the turbofan design does provide real advantages, and it isn’t simply about lower thrust at maximum speed.