Bypass ratio is one of the most important numbers used to describe a turbofan jet engine. It compares how much air flows around the engine core to how much air flows through the core where fuel is burned. Modern airliners use high-bypass turbofans because they can produce the needed thrust while using less fuel and making less noise.
This is a major reason today’s commercial jets are quieter than many older jet aircraft.
Key Facts
- Bypass ratio = mass flow rate of bypass air ÷ mass flow rate of core air
- BPR = m_bypass ÷ m_core
- Total air mass flow rate = m_bypass + m_core
- Thrust increases when the engine changes the momentum of air: F = Δp/Δt
- For the same thrust, moving a large mass of air slowly is usually quieter than moving a small mass of air very fast.
- High-bypass turbofans are common on airliners, while low-bypass engines are more common in older jets and some high-speed military aircraft.
Vocabulary
- Bypass ratio
- The ratio of the mass of air that flows around the engine core to the mass of air that flows through the engine core.
- Turbofan
- A jet engine that uses a large fan to send some air through the core and some air around the core.
- Core flow
- The air that passes through the compressor, combustion chamber, turbine, and exhaust nozzle of a jet engine.
- Bypass flow
- The air pushed by the fan that travels around the engine core and usually provides much of the thrust in a high-bypass turbofan.
- Mass flow rate
- The amount of mass passing a point each second, often measured in kilograms per second.
Common Mistakes to Avoid
- Treating bypass ratio as a percentage is wrong because it is a ratio of two mass flow rates, not a fraction of the total flow unless it is converted carefully.
- Thinking all thrust comes from burning fuel in the core is wrong because high-bypass turbofans get much of their thrust from fan-driven bypass air.
- Assuming a higher exhaust speed is always better is wrong because very fast exhaust can waste energy and create more noise, especially for subsonic airliners.
- Ignoring the difference between bypass air and core air is wrong because they follow different paths and contribute to thrust, efficiency, and noise in different ways.
Practice Questions
- 1 A turbofan has 320 kg/s of bypass air and 40 kg/s of core air. What is its bypass ratio?
- 2 An engine has a bypass ratio of 9 and a core mass flow rate of 35 kg/s. What is the bypass mass flow rate and the total mass flow rate?
- 3 Two engines produce the same thrust. One accelerates a small amount of air to a very high speed, while the other accelerates a large amount of air to a lower speed. Explain which is likely to be quieter for an airliner and why.