Bypass Ratio

Bypass Ratio

The bypass air is shown in pink, whilst the core gases are shown in red.

The term bypass ratio (BPR) relates to the design of turbofan engines, commonly used in aviation. It is defined as the ratio between the mass flow rate of air drawn through a fan disk which bypasses the engine core (un-combusted air), to the mass flow rate passing through the engine core which is involved in combustion to produce mechanical energy. For example, with a 10:1 bypass ratio, for every 1 kg of air passing through the combustion chamber, 10 kg of air passes around the combustion chamber through the ducted fan alone.

In a high-bypass design, the vast majority of the thrust is derived from the ducted fan, rather than from combustion gases expanding in a nozzle. A high bypass ratio provides a lower thrust specific fuel consumption (grams/sec fuel per unit of thrust in kN using SI units) for reasons explained below, especially at zero velocity (at takeoff) and at the cruise speed of most commercial jet aircraft. They are by far the dominant type for all commercial passenger aircraft and both civilian and military jet transports. Lower exhaust velocities also figure strongly in lower noise output which is a decided advantage over earlier low or zero bypass designs.

Low bypass ratios tend to be favored for military combat aircraft as a compromise between improved fuel economy and the requirements of combat, which values higher power-to-weight ratios, supersonic performance, and the ability to use afterburners which are more compatible with low bypass engines. A good example of the differences between a pure jet engine and a low-bypass turbofan may be seen in the Spey turbofan used in the F-4 Phantom.