In a turbo fan engine, what makes bypassed air more efficient at creating engine thrust?

In a turbo fan engine, what makes bypassed air more efficient at creating engine thrust?

• A. It is hotter than exhaust air
• B. It is cooler and denser than hot exhaust air
• C. It travels faster than exhaust air
• D. It contains more fuel than exhaust air

Answer: (B)

Bypassed air in a turbo fan engine is more efficient at producing thrust primarily because it is cooler and denser than the hot exhaust air. This characteristic is critical for a couple of reasons. When air is bypassed around the engine core, it is compressed and accelerated before being expelled. Its cooler and denser nature means that, when it is accelerated, it produces a greater mass flow rate compared to the hotter exhaust air. Thrust generation in a jet engine is fundamentally about the change in momentum of the air being expelled; therefore, denser air can contribute to a more effective thrust-to-weight ratio. Moreover, because cooler air holds more oxygen molecules per unit volume than hotter air, it can potentially enhance combustion efficiency in the broader context of the engine's operation, indirectly benefiting overall performance. In turbo fan engines, particularly in the case of high bypass ratio designs, the majority of thrust is generated by this bypassed air rather than the core airflow, accounting for the efficiency and capability of modern commercial aircraft to operate quietly and with less fuel consumption. The other options do not accurately capture why bypassed air is more efficient for thrust generation. Hot exhaust air, while produced at high speed, does not contribute to thrust as efficiently due to its lower density.

Bypassed air in a turbo fan engine is more efficient at producing thrust primarily because it is cooler and denser than the hot exhaust air. This characteristic is critical for a couple of reasons.

When air is bypassed around the engine core, it is compressed and accelerated before being expelled. Its cooler and denser nature means that, when it is accelerated, it produces a greater mass flow rate compared to the hotter exhaust air. Thrust generation in a jet engine is fundamentally about the change in momentum of the air being expelled; therefore, denser air can contribute to a more effective thrust-to-weight ratio. Moreover, because cooler air holds more oxygen molecules per unit volume than hotter air, it can potentially enhance combustion efficiency in the broader context of the engine's operation, indirectly benefiting overall performance.

In turbo fan engines, particularly in the case of high bypass ratio designs, the majority of thrust is generated by this bypassed air rather than the core airflow, accounting for the efficiency and capability of modern commercial aircraft to operate quietly and with less fuel consumption.

The other options do not accurately capture why bypassed air is more efficient for thrust generation. Hot exhaust air, while produced at high speed, does not contribute to thrust as efficiently due to its lower density.