In a turbofan engine, bypass ratio is defined as?

• A. The ratio of air that bypasses the engine to air that goes through the core
• B. The ratio of air that goes through the core to bypass air
• C. The ratio of fuel to air
• D. The ratio of exhaust to intake

Answer: (A)

The main idea is that a turbofan moves a large amount of air around the engine core without going through combustion. Bypass ratio measures how much of that air bypasses the core compared to how much goes through the core for combustion. It’s defined as the mass flow rate of bypass air divided by the mass flow rate through the core. In practice, a higher bypass ratio means more air is being moved by the fan around the core, which tends to improve fuel efficiency and reduce noise at subsonic cruise, while a lower bypass ratio means more air goes through the core, yielding higher specific thrust but lower efficiency.