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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.〔〔〔〔(Animated Engines )〕 High bypass designs are the dominant type for commercial passenger aircraft and both civilian and military jet transports.
Military combat aircraft usually use engines with low bypass ratios to compromise between fuel economy and the requirements of combat: high power-to-weight ratios, supersonic performance, and the ability to use afterburners, all of which are more compatible with low bypass engines.
==Principles==
In a pure (zero-bypass) jet engine, all the air taken in is involved in combustion; high temperature and high pressure exhaust gas accelerating by expansion through a propelling nozzle produces all thrust because the compressor stage consumes all the mechanical energy produced by the turbine. In a bypassed design, conversely, the gas turbine component (or engine core) produces a large net positive power output because the turbine produces far more power than the compressor consumes, and this excess power drives a ducted fan that rearward accelerates air from the front of the engine; in a high-bypass design, the ducted fan, rather than combustion gases expanding in the nozzle, produces the vast majority of thrust. Turbofan engines are closely related to turboprop designs in concept because both designs de-couple the gas turbine engines' shaft horsepower from their exhaust velocities. Turbofans represent an intermediate stage between turbojets, which derive almost all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less).〔"(The turbofan engine )", page 7. ''SRM University, Department of aerospace engineering''〕 Optimizing a gas turbine engine for shaft power output minimizes the exhaust pressure and temperature for maximum thermal efficiency within the limits of a Brayton cycle engine; conversely, pure jet designs require high pressure and temperature because they produce thrust by expanding exhaust gas through a nozzle. Bypass designs have two exhaust velocities, one passing through the core (combustion air) and air passing through the ducted fan alone (since in reality, most designs pass combustion air through the ducted fan first before passing into the compressor stage).〔"Fundamentals of Gas Dynamics", Robert D. Zucker, Matrix Publishers, 1977, pp 322-333〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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