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.

Turbofan

The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the turbo portion refers to a gas turbine engine which takes mechanical energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to accelerate air rearwards. Thus, whereas all the air taken in by a turbojet passes through the turbine (through the combustion chamber), in a turbofan some of that air bypasses the turbine. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of those contributing to the thrust. The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working in concert; engines that use more jet thrust relative to fan thrust are known as low bypass turbofans, conversely those that have considerably more fan thrust than jet thrust are known as high bypass. Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass.Afterburners are not used on high-bypass turbofan engines but may be used on either low-bypass turbofan or turbojet engines.