Bypass Ratio - Principles

Principles

In a gas turbine engine, the stoichiometry of the fuel-air mixture is limited to a fairly narrow range, with a tendency to a "leaner" fuel mixture to limit the maximum temperatures in the engine. In a pure (zero-bypass) jet engine, the majority of the thrust occurs from the high temperature and high pressure exhaust gas being accelerated by expansion through a propelling nozzle (the lesser part of the thrust is obtained by accelerating air in the compressor stage). Note that in a zero-bypass engine all the air taken in is involved in combustion. In a pure jet engine the net mechanical energy produced by the compressor-turbine system is essentially zero, i.e. all the mechanical energy produced by the turbine is consumed in the compressor stage. In a design featuring bypass, the gas turbine component (or engine core) is designed to produce a large net positive power output, i.e. the turbine now produces far more power than the compressor consumes. This excess power is used to drive a ducted fan to accelerate air from the front of the engine rearwards. Turbofan engines are closely related to turbo-prop designs in concept, since 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). When a gas turbine engine is optimized for shaft power output, the exhaust pressure and temperature are minimized for maximum thermal efficiency within the limits of a Brayton cycle engine. This is in contrast to pure jet designs where a high pressure and temperature are required features to allow thrust derived by expansion through a nozzle. Note that in a bypass design there are actually 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). 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.