What Bypass Ratio Means in a Jet Engine

Bypass ratio is the mass of air that flows around a jet engine's core divided by the mass of air that flows through it. A ratio of 10 to 1 means 10 kilograms of air go around the core for every 1 kilogram that goes through it. Airliners use high ratios because moving a lot of air gently is more efficient than moving a little air very fast. This article explains the number, typical values, and the trade-offs.

What the number measures

Most modern jet engines are turbofans. A large fan at the front pulls in air. Some of that air goes into the core, where it is compressed, mixed with fuel and burned. The rest goes through a duct that bypasses the core and leaves at the back. According to the Wikipedia article on the subject, the bypass ratio is the mass flow in that bypass duct divided by the mass flow entering the core.

An older turbojet has no bypass at all, so its ratio is zero. Wikipedia lists early jet aircraft and Concorde as examples of zero-bypass engines.

Why moving more air helps

An engine makes thrust by speeding air backwards. The same thrust can come from speeding a small amount of air by a large amount, or a large amount of air by a small amount. Wikipedia explains the physics: thrust grows in proportion to the change in air speed, but the kinetic energy carried away in the exhaust grows with the square of that change. So a slower, wider stream wastes less energy. The result is lower fuel use for the same thrust, which is why high-bypass designs dominate transport aircraft. The extra bypass air also reduces jet noise.

Typical values

Wikipedia gives these example figures, which vary by engine version:

• Combat aircraft: usually below 1. The F119 in the F-22 is listed at 0.45 and the F414 in the F/A-18E/F at 0.25.
• Business jets: roughly 2 to 6. The Honda HF120 is listed at 2.9 and the PW800 at 5.5.
• Airliners: up to about 12. The CFM56 used on the Airbus A320 and Boeing 737 is listed at 5.0 to 6.6, the GEnx at 8.0 to 9.3, the Rolls-Royce Trent XWB at 9.6, the GE9X at 10.0 and the PW1000G series at 9.0 to 12.5.
• Turboprops: 50 to 100, because a propeller acts like a very large bypass.

Why fighters keep it low

Fighter engines need high power for their weight, supersonic performance and afterburners, which add thrust by burning extra fuel in the exhaust. A low bypass ratio suits those needs better than a wide, slow stream. So the same physics that favours high bypass for airliners favours low bypass for combat aircraft.

The trade-offs of going bigger

A higher ratio generally means a larger fan, and so a larger and heavier engine. A NASA technical memorandum on engine design for single-aisle aircraft of the 737 and A320 class makes an important point: higher bypass ratios improve engine-level measures such as specific fuel consumption, but those benefits may not carry over to the whole aircraft because of what it calls integration penalties. A bigger engine can add weight and drag and is harder to fit under the wing, so the best choice depends on the whole aircraft, not the engine alone.

The same NASA report notes that the trend has been from turbojets to low-bypass first-generation turbofans and then to today's high-bypass designs, mainly to improve efficiency. It also says a geared fan drive makes higher bypass ratios practical, and that such designs produce lower noise. A gearbox lets the large fan turn slowly while the turbine that drives it spins fast, and each can run at a more suitable speed. The report adds that the bypass ratio that minimises fuel use for a whole mission may not need geared fan technology.

Reading the figures carefully

• A bypass ratio is a design figure and can change with the version of an engine.
• It describes air flow, not thrust or size, though higher ratios usually come with larger fans.
• The figures above come from a general reference and are for comparison, not engineering use.

In short, bypass ratio is a simple way to see how an engine is meant to be used. Near zero suggests speed and power at any cost in fuel. In the range of 5 to 12 it suggests a commercial airliner, where fuel and noise matter most.

Sources

• Bypass ratio, Wikipedia
• Engine design study for advanced single-aisle aircraft, NASA/TM-2009-215784, NASA Technical Reports Server