Bypass ratio

The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of air drawn through the fan disk that bypasses the engine core (un-combusted air) to the mass flow rate passing through the engine core. For example, a 10:1 bypass ratio means that 10 kg of air passes around the core for every 1 kg of air passing through the core.

The flow from the fan nozzle, rather than the core exhaust from the hot nozzle, produces most of the thrust in high-bypass designs. Bypass 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 the aircraft; also, the lower exhaust velocities reduce jet noise. Lower fuel consumption that comes with high bypass ratios applies to turboprops, using a propeller rather than an enclosed fan. High bypass designs are the dominant type for commercial passenger aircraft and both civilian and military jet transports.