In a turbofan engine, the bypass ratio is defined as the ratio of:

• A. bypass air to core air
• B. core air to bypass air
• C. total air to engine weight
• D. exhaust velocity to intake velocity

Answer: (A)

Bypass ratio quantifies how much air passes around the engine core relative to the air that goes through the core. It is defined as the mass flow rate of bypass air divided by the mass flow rate of core air, making it a dimensionless number. This ratio controls the engine’s performance characteristics: a higher bypass ratio means more air is moved by the fan with less energy per unit mass through the core, leading to greater propulsive efficiency, cooler and slower exhaust, and quieter operation. For example, if the bypass stream is 100 kg/s and the core stream is 20 kg/s, the bypass ratio is 5. The concept is not about a ratio of core to bypass air, not about engine weight, and not about velocities, which is why the correct description is bypass air to core air.