Bypass ratio

Schematic turbofan engines; the high-bypass engine (lower) 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.

The bypass air is shown in pink, whilst the core gases are shown in red.

The term bypass ratio (BPR) relates to the design of turbofan engines, commonly used in aviation. It is defined as the ratio between the mass flow rate of air drawn through a fan disk which bypasses the engine core (un-combusted air), to the mass flow rate passing through the engine core which is involved in combustion to produce mechanical energy. For example, with a 10:1 bypass ratio, for every 1 kg of air passing through the combustion chamber, 10 kg of air passes around the combustion chamber through the ducted fan alone.

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.

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. They are by far the dominant type for all commercial passenger aircraft and both civilian and military jet transports. Lower exhaust velocities also figure strongly in lower noise output which is a decided advantage over earlier low or zero bypass designs.

Low bypass ratios tend to be favored for military combat aircraft as a compromise between improved fuel economy and the requirements of combat, which values higher power-to-weight ratios, supersonic performance, and the ability to use afterburners 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

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 intaked 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.

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. They are by far the dominant type for all commercial passenger aircraft and both civilian and military jet transports. Lower exhaust velocities also figure strongly in lower noise output which is a decided advantage over earlier low or zero bypass designs.

Lower bypass ratios tend to be favored for military combat aircraft as a compromise between improved fuel economy and the requirements of combat, which values higher power-to-weight ratios, supersonic performance, and the ability to use afterburners 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.

In spite of this, it turns out that for jet engines in general, at optimum bypass ratios, the fuel burnt to travel any particular distance is largely independent of airspeed, but with supersonic jet engines being slightly more efficient in practice, at their design point.^{[citation needed]}

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/sec) by the difference in inlet and exhaust velocities (in m/sec), which is a linear relationship. Whereas the kinetic energy of the exhaust is the same mass flow (kg/sec) 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.

Engine bypass ratios

Engine	Aircraft	Bypass ratio
Rolls-Royce/Snecma Olympus 593	Concorde (turbojet)	0:1
Rolls-Royce Tay	Gulfstream IV, Fokker 70, Fokker 100	3.1:1
SNECMA M88	Dassault Rafale	0.30:1
Pratt & Whitney JT8D	DC-9, MD-80, Boeing 727, Boeing 737 100 and 200 series	0.96:1
Pratt & Whitney F100	F-16, F-15	0.36:1
General Electric F404	F/A-18, T-50, F-117, X-29, X-31	0.34:1
Eurojet EJ200	Eurofighter Typhoon	0.4:1
Klimov RD-33	MiG-29, Il-102	0.49:1
Saturn AL-31F	Su-27, Su-30, Chengdu J-10	0.59:1
Kuznetsov NK-321	Tu-160	1.4:1
PowerJet SaM146	Sukhoi Superjet 100	4.43:1
Pratt & Whitney PW2000	Boeing 757, C-17 Globemaster III	5.9:1
Progress D-436	Yak-42M, Beriev Be-200, An-148	6.2:1
General Electric GEnx	Boeing 787	8.5:1
Rolls-Royce Trent 900	Airbus A380	8.7:1
General Electric GE90	Boeing 777	9:1
Rolls-Royce Trent 1000	Boeing 787	11:1