Bypass ratio

Schematic turbofan engines; the high-bypass engine (middle) has a large fan that routes much air around the turbine, the low-bypass engine (upper) has a smaller fan routing more air into the turbine; turbojets (bottom) have zero bypass, all air goes through the turbine

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 that is involved in combustion to produce mechanical energy. For example, a 10:1 bypass ratio implies that 10 kg of air passes around the combustion chamber through the ducted fan for every 1 kg of air passing through the combustion chamber.

The ducted fan, rather than combustion gases expanding in a nozzle, produces the vast majority of the thrust in high-bypass designs. A high bypass ratio 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 most commercial jet aircraft; however, the lower exhaust velocities of high-bypass designs also figure strongly in lower noise output, which is a decided advantage over earlier low or zero bypass designs. This also qualifies for turboprops, using a propeller rather than an enclosed fan.^[1][2][3][4] High bypass designs are by far the dominant type for all 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. A good example of the differences between a pure jet engine and a low-bypass turbofan may be seen in the Spey turbofan used in the F-4 Phantom.

Principles

The stoichiometry of the fuel-air mixture in a gas turbine engine is limited to a fairly narrow range and tends to a "leaner" fuel mixture to limit the maximum temperatures in the engine. 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 turbo-prop 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).^[5] 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).^[6]

Description

Turbojet engines are relatively inefficient as Brayton cycle engines because they directly convert thermal energy from combusting fuel into kinetic energy in the form of a high-velocity reaction jet directed through an expansion nozzle instead of producing mechanical power; therefore, pounds force or kilonewtons—not horsepower or kilowatts, as in propeller or turboprop engines—measure the power of a turbojet. Turbofans, conversely, are very efficient Brayton cycle engines because their gas turbine convert thermal energy from combustion 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, which are sufficient to convert most of the available thermal energy into mechanical work and leave an exhaust plume of greatly reduced temperature, pressure, and velocity. The back pressure at the turbine exit of a high bypass turbofan that maximally converts thermal energy into mechanical energy should be close to ambient pressure because the increased thrust derived from the ducted fan more than compensates the low direct jet propulsive efficiency of such an engine. In a high-bypass turbine engine, the gas turbine uses thermal energy from combustion to turn a ducted fan that slightly increases the velocity of a large amount of air.

Only the limitations of weight and materials (e.g., the strengths and melting points of materials in the turbine) reduce the efficiency at which a turbofan gas turbine converts this thermal energy into mechanical energy, for while the exhaust gases may still have available energy to be extracted, each additional stator and turbine disk retrieves progressively less mechanical energy per unit of weight, and increasing the compression ratio of the system by adding to the compressor stage to increase overall system efficiency increases temperatures at the turbine face. Nevertheless, high-bypass engines have a high propulsive efficiency because even slightly increasing the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust: thrust is the engine's mass flow (the amount of air flowing through the engine) multiplied by the difference between the inlet and exhaust velocities in—a linear relationship—but the kinetic energy of the exhaust is the mass flow multiplied by one-half the square of the difference in velocities.^[7][8] A low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use.^[9][10][11]

The Rolls–Royce Conway turbofan engine, developed in the early 1950s, better uses this energy. 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.

The growth of bypass ratios during the 1960s gave jetliners fuel efficiency that could compete with that of piston-powered planes. Pratt & Whitney and General Electric developed most of the very large high-bypass engines in the United States, which for the first time was besting 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 have some bypass. Modern engines in slower aircraft, such as airliners, have bypass ratios up to 12:1; in higher-speed aircraft, such as fighters, bypass ratios are much lower, around 1.5; and craft designed for speeds up to Mach 2 and somewhat above have bypass ratios below 0.5. Concorde and Tu-144 had no bypass to reduce intake ramp aerodynamic drag during extended supersonic cruise at Mach number 2.

Turboprops may have bypass ratios up to 50-100,^[1][2][3] although the propulsion airflow is less clearly defined for propellers than for fans^[12] and propeller airflow is slower than for turbofans.^[13][11]

Engine bypass ratios

Turbofans, unless otherwise noted

References