In turbofan engines, a higher bypass ratio increases what?

• A. Propulsive efficiency and reduces SFC and noise.
• B. Decreases efficiency.
• C. Has no effect on efficiency.
• D. Increases core temperature.

Answer: (A)

A higher bypass ratio improves propulsive efficiency by using more air that bypasses the combustor and core and is accelerated by the fan to a modest speed. Because that large mass of air is pushed forward at a relatively low exhaust velocity, more of the engine’s energy goes into useful thrust rather than being wasted as high-speed jet exhaust. This boosts propulsive efficiency, meaning you get more thrust per unit of fuel energy, which lowers the specific fuel consumption.

At the same time, the slower, heavier bypass flow reduces jet noise since noise largely comes from high-velocity exhaust and shear between streams. The core still provides thrust and heat, but the overall fuel per thrust and the perceived noise drop as the bypass ratio rises.

So, the best effect of a higher bypass ratio is increased propulsive efficiency and accompanying reductions in SFC and noise.