In a turbofan engine, the bypass ratio is defined as the ratio of bypass air to core air. Which of the following best reflects this definition?

• A. The ratio of bypass air to core air
• B. The ratio of core air to bypass air
• C. The ratio of total air to engine weight
• D. The ratio of engine exhaust to intake

Answer: (A)

The concept being tested is how the bypass ratio is defined in a turbofan: it compares how much air goes around the core to how much air goes through the core. Specifically, bypass ratio is the mass flow rate of bypass air divided by the mass flow rate of core air. Bypass air is the air that goes through the fan and around the engine core without entering the combustion chamber, while core air is the air that enters the core, passes through the compressor, combustor, and turbines, and then exhausts. This ratio is a straightforward way to express how much of the incoming air is used to produce thrust via the bypass stream versus the core stream. A higher bypass ratio means more air bypasses the core, which generally improves propulsive efficiency and reduces noise, though it changes the engine’s thrust characteristics. The other options don’t reflect this division: one would invert the ratio, or refer to quantities like total air to engine weight or exhaust to intake, which aren’t how bypass ratio is defined.