Bypass Ratio - Description

Description

Turbojet engines are relatively inefficient as Brayton cycle engines, since it is not their function to provide mechanical power, but instead to provide direct propulsive thrust through expanding combustion gases in a nozzle. In fact the conventional units of power measurement for a turbojet engine are in pounds force or kilo Newtons, unlike propeller aircraft (including turboprops) which are measured in horsepower or kilo-watts. Turbojets convert the thermal energy from combustion directly into kinetic energy in the form of a high-velocity reaction jet. Turbofans, on the other hand, are very efficient Brayton cycle engines. In a turbofan, the gas turbine is optimized to convert as much of the thermal energy from combustion as possible into mechanical shaft power. The essential difference between a turbojet and turbofan gas turbine is that the turbine stage in a turbojet is designed to extract only a fraction of the available thermal energy in the high pressure and temperature exhaust, producing only enough mechanical energy to run the compressor stage as a net-zero mechanical energy system (ignoring very small mechanical outputs to run auxiliary equipment such as generators) and leaving a relatively high temperature and back pressure exhaust at the turbine exit for effective reaction propulsion. The gas turbine on a turbofan has additional turbine disks and stators, sufficient to convert most of the available thermal energy into mechanical work, leaving an exhaust plume of greatly reduced temperature, pressure, and velocity. The back pressure at the turbine exit for a high bypass turbofan should be close to ambient pressure to allow for maximum energy extraction, but at the loss of direct jet propulsive efficiency (which is far more than compensated for by the increased thrust derived from the ducted fan).

Only the limitations of weight and materials (e.g. the strengths and melting points of materials in the turbine), prevent the maximum amount of energy being extracted by a turbofan gas turbine. Note that while the exhaust gases may still have available energy to be extracted, there is a point of diminishing returns where each additional stator and turbine disk retrieves progressively less mechanical energy per unit of increased weight added. Alternately, increasing the compression ratio of the system, by adding to the compressor stage, can increase overall system efficiency at the cost of higher temperatures at the turbine face (the maximum operating temperature of the turbine disk being the limiting factor). Stated concisely, a high bypass turbofan engine may be characterized as a system of two parts: a gas turbine optimized to convert the maximum amount of thermal energy from combustion into mechanical energy, and a ducted fan to use the mechanical power to move a large amount of air through a relatively small change in velocity.

The physics of propulsive efficiency may be stated succinctly as follows. For any given amount of available energy (thermal and mechanical), thrust is optimized by moving the maximum mass flow at the minimum difference in inlet and exhaust velocities. This can be explained by the relationships in an action-reaction propulsion system (which an air-breathing jet engine is an example), thrust is calculated by multiplying the mass flow (in kg/s) by the difference between the inlet and exhaust velocities (in m/s), which is a linear relationship. Whereas the kinetic energy of the exhaust is the same mass flow (kg/s) multiplied by one-half the square of the difference in velocities. By mechanically moving a very large volume (and consequently mass) of air through a relatively small difference in velocity produces a relatively small change in kinetic energy for a very large change in momentum and thrust.

Rolls–Royce came up with a better use of the extra energy in their Conway turbofan engine, developed in the early 1950s. In the Conway, an otherwise normal axial-flow turbojet was equipped with an oversized first compressor stage (the one closest to the front of the engine), and centered inside a tubular nacelle (in effect, a ducted fan arrangement). While the inner portions of the compressor worked "as normal" and provided air into the core of the engine, the outer portion blew air around the engine to provide extra thrust. The Conway had a very small bypass ratio of only 0.3, but the improvement in fuel economy was notable; as a result, it and its derivatives like the Spey became some of the most popular jet engines in the world.

Through the 1960s the bypass ratios grew, making jetliners competitive in fuel terms with piston-powered planes for the first time. Most of the very-large engines in this class were pioneered in the United States by both Pratt & Whitney and General Electric, which for the first time was out-competing the United Kingdom in engine design. Rolls-Royce also started the development of the high-bypass turbofan, and although it caused considerable trouble at the time, the RB.211 would go on to become one of their most successful products.

Today, almost all jet engines include some amount of bypass. For lower speed operations, such as airliners, modern engines use bypass ratios up to 17, while for higher speed operations such as fighter aircraft the ratios are much lower, around 1.5; and around 0-0.5 for speeds up to Mach 2 and somewhat above. For flights consisting mostly of extended supersonic cruise at Mach 2, having no bypass at all was found to be optimal on both Concorde and Tu-144 due to reduction in inlet drag.