The term
bypass ratio (BPR) relates to the design of
turbofanA turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
engines, commonly used in
aviationAviation is the activity involving man-made air-borne flying devices , including the people, organizations, and regulatory bodies involved with them.- History :...
. It is defined as the ratio between the mass flow rate of air drawn in by the fan but bypassing the engine core to the mass flow rate passing through the engine core.
A high bypass ratio gives a lower (actual) exhaust speed. This reduces the
specific fuel consumptionSpecific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly compared.SFC for thrust engines Specific fuel...
, but reduces the top speed and gives a heavier engine.
A lower bypass ratio gives a higher exhaust speed, which is needed to sustain higher, usually supersonic, airspeeds.
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
turbofanA turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
engines, commonly used in
aviationAviation is the activity involving man-made air-borne flying devices , including the people, organizations, and regulatory bodies involved with them.- History :...
. It is defined as the ratio between the mass flow rate of air drawn in by the fan but bypassing the engine core to the mass flow rate passing through the engine core.
A high bypass ratio gives a lower (actual) exhaust speed. This reduces the
specific fuel consumptionSpecific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly compared.SFC for thrust engines Specific fuel...
, but reduces the top speed and gives a heavier engine.
A lower bypass ratio gives a higher exhaust speed, which is needed to sustain higher, usually supersonic, airspeeds. This increases the
specific fuel consumptionSpecific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly compared.SFC for thrust engines Specific fuel...
.
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.
Description
Jet engines are generally able to create considerably more energy than they can use in moving air through the engine core. This is because the limiting factor is the temperature at the
turbineA turbine is a rotary engine that extracts energy from a fluid or air flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum, with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they...
face, and that is a function of the total amount of fuel burned. Increasing airflow, and thus
thrustThrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system.-Examples:...
, would imply burning more fuel and generating higher temperatures. It is possible to increase the airflow by burning "too much" fuel or adding water in front of the turbine to cool it, but both methods lead to incomplete combustion and very poor
fuel efficiencyFuel efficiency, is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance...
. This was nevertheless commonly practiced in early jet engines because of a need to produce added thrust on takeoff. This is also why the exhaust plumes of older aircraft produce so much visible smoke (which is nothing more than unburned carbon from wasted jet fuel).
Rolls–RoyceRolls-Royce Limited was a British car and, from 1914, aero-engine manufacturing company founded by Charles Stewart Rolls and Henry Royce on 15 March 1906 as the result of a partnership formed in 1904. In 1971, Rolls-Royce was crippled by the development of the advanced RB211 jet engine, resulting...
came up with a better use of the extra energy in their
ConwayThe Rolls-Royce RB.80 Conway was the first by-pass engine in the world to enter service. Development started at Rolls-Royce in the 1940s, but it was used only briefly in the late 1950s and early 1960s before other turbofan designs were introduced that replaced it. The Conway powered versions of...
turbofanA turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
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
nacelleThe nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft or wind turbine. In some cases—most notably the World War II-era P-38 Lightning airplane—an aircraft's cockpit may also be housed in a nacelle...
. 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
SpeyThe Rolls-Royce RB.168 Spey is a low-bypass turbofan engine originally designed and manufactured by Rolls-Royce that has been in widespread service for over 40 years...
became some of the most popular jet engines in the world.
If the fan of a turbofan engine drives two kilograms of air around the engine for every kilogram that passes through the engine's core, the engine is said to have a bypass ratio of 2 (or 2 to 1). Higher bypass ratios generally give better
specific fuel consumptionSpecific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly compared.SFC for thrust engines Specific fuel...
as an increasing amount of thrust is being generated without burning more fuel. This is achieved since the engine propels a larger amount of air rearwards at slower speed, rather than a smaller amount of air at higher speed- because thrust is the momentum given to the air per second the thrust is the same. However energy is a square law on speed, and so it takes less energy to generate the same thrust; and hence less fuel is needed, the specific fuel consumption reduces.
Thus, with the example, for engines with the same thrust, the fuel efficiency would be improved by something less than 50%.
High bypass ratios are also correlated with lower noise, since the large flow of air surrounding the jet exhaust from the engine core helps to buffer the noise produced by the latter.
Through the 1960s the bypass ratios grew, making
jetlinersA jet airliner is an airliner that is powered by jet engines. This term is sometimes contracted to jetliner.In contrast to today's relatively fuel-efficient, turbofan-powered air travel, first generation jet airliner travel was noisy and fuel inefficient...
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 StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
by both
Pratt & WhitneyPratt & Whitney is an American aircraft engine manufacturer of products widely used in both civil and military aircraft. As one of the "big three" aero-engine manufacturers, it competes with General Electric and Rolls-Royce, although it has also formed joint ventures with both of these companies...
and
General ElectricThe General Electric Company, or GE , is a multinational American technology and services conglomerate incorporated in the State of New York. In 2009, Forbes ranked GE as the world's largest company...
, which for the first time was out-competing the
United KingdomThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
in engine design.
Rolls-RoyceRolls-Royce Limited was a British car and, from 1914, aero-engine manufacturing company founded by Charles Stewart Rolls and Henry Royce on 15 March 1906 as the result of a partnership formed in 1904. In 1971, Rolls-Royce was crippled by the development of the advanced RB211 jet engine, resulting...
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.
Turbofans are typically broken into one of two categories: low–bypass and high–bypass ratio. In a low–bypass turbofan, only a small amount of air passes through the fan ducts and the fan is of very small diameter. The fan in a high–bypass turbofan is much larger to force a large volume of air through the ducts. The low–bypass turbofan is more compact, but the high-bypass turbofan can produce much greater thrust, is more fuel efficient, and is much quieter.
Today, almost all jet engines include some amount of bypass. Lower bypass ratios are appropriate at high speeds because the exhaust velocity must exceed the airspeed to give forward net thrust. For lower speed operations, such as airliners, modern engines use bypass ratios up to 17, while for higher speed operations such as
fighter aircraftA fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets by dropping bombs. Fighters are small, fast, and maneuverable...
the ratios are much lower, around 1.5; and around 0.5 for sustained speeds around Mach 2 and somewhat above.
At transonic and supersonic speeds, very high bypass ratios still present engineering challenges.
Engine bypass ratios
- Rolls-Royce/Snecma Olympus 593
The Rolls-Royce/Snecma Olympus 593 was a reheated turbojet which powered the supersonic airliner Concorde. Rolls-Royce Limited and Snecma developed the engine from the Rolls-Royce Olympus...
- ConcordeThe Aérospatiale-BAC Concorde aircraft was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
- 0:1 (turbojet)
- SNECMA M88
The Snecma M88 is an afterburning turbofan engine developed by Snecma for the Dassault Rafale fighter.Although the M88 engine cycle is similar to that of the Eurojet EJ200, it is smaller and lower in thrust. Other differences are that the M88 has fan variable camber inlet guide vanes, the HP...
- Dassault RafaleThe Dassault Rafale is a French twin-engined delta-wing highly agile multi-role 4.5th-generation jet fighter aircraft designed and built by Dassault Aviation. Introduced in 2000, the Rafale is being produced both for land-based use with the French Air Force and for carrier-based operations with...
- 0.30:1
- Pratt & Whitney F100
The Pratt & Whitney F100 is an afterburning turbofan engine manufactured by Pratt & Whitney which powers the F-15 Eagle and F-16 Fighting Falcon.-Development:...
- F-16, F-15 - 0.34:1
- General Electric F404 - F/A-18, T-50, F-117, X-29, X-31 - 0.34:1
- Eurojet EJ200
The Eurojet EJ200 is a military turbofan, used as the powerplant of the Eurofighter Typhoon The engine is largely based on the Rolls-Royce XG-40 technology demonstrator which was developed in the 1980s. The EJ200 is built by the EuroJet Turbo GmbH consortium.-Rolls-Royce XG-40:Rolls-Royce began...
- Eurofighter TyphoonThe Eurofighter Typhoon is a twin-engine canard-delta wing multirole aircraft. It is being designed and built by a consortium of three separate partner companies: Alenia Aeronautica, BAE Systems, and EADS working through a holding company Eurofighter GmbH which was formed in 1986...
- 0.4:1
- Klimov RD-33
The RD-33 turbofan engine was developed in 1985 to power the Mikoyan MiG-29 fighter. It is an 8000-9000 kgf thrust class turbofan twin-shaft engine with afterburner built by the Klimov company of Russia and has several variants. It features a modular design, individual parts can be replaced...
- MiG-29, Il-102 - 0.49:1
- Saturn AL-31F - Su-27, Su-30, Chengdu J-10
-- This article is a part of Wikipedia:WikiProject Aircraft. Please see -- This article is a part of [[Wikipedia:WikiProject Aircraft]]. Please see -- This article is a part of [[Wikipedia:WikiProject Aircraft]]. Please see [[Wikipedia:WikiProject Aircraft/page...
- 0.59:1
- Kuznetsov NK-321 - Tu-160 - 1.4:1
- Pratt & Whitney PW2000
The Pratt & Whitney PW2000 is a series of high-bypass turbofan aero engines with a thrust range from 37,000 to 43,000 lbf . Built by Pratt & Whitney, they were designed for the Boeing 757. As a 757 powerplant, these engines compete with the RB211-535.-Designe and development:The first PW2000 series...
- Boeing 757The Boeing 757 is a mid-size, narrow-body twinjet airliner manufactured by Boeing Commercial Airplanes. Passenger versions of the 757 can carry between 186 and 279 passengers, and have a maximum range of 3,100 to 3,900 nautical miles depending on variant and seating configuration...
, C-17 Globemaster IIIThe Boeing C-17 Globemaster III is a large military transport aircraft. The C-17 was developed for the United States Air Force from the 1980s to the early 1990s by McDonnell Douglas. The aircraft carries on the name of two previous United States military cargo aircraft, the C-74 Globemaster and...
- 5.9:1
- Rolls-Royce Trent 900
The Rolls-Royce Trent 900 is a turbofan engine, developed from the RB.211 and is one of the family of Trent engines.-Design and development:...
- Airbus A380The Airbus A380 is a double-deck, wide-body, four-engine airliner manufactured by the European corporation Airbus, a subsidiary of EADS. The largest passenger airliner in the world, the A380 made its maiden flight on 27 April 2005 from Toulouse, France, and made its first commercial flight on 25...
- 8.7:1
- General Electric GE90
General Electric GE90 is a family of high-bypass turbofan engines built by GE-Aviation for the Boeing 777, with thrust ranging from 74,000 to 115,000 lb
f . It was first introduced in November 1995 on British Airways' 777s, and is only available on the 777...
- Boeing 777The Boeing 777 is a long-range, wide-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. The world's largest twinjet and commonly referred to as the "Triple Seven", the aircraft can carry between 301 and 368 passengers in a three-class configuration and has a range from...
- 9:1
- Rolls-Royce Trent 1000
The Rolls-Royce Trent 1000 is a turbofan engine, developed from earlier Trent series engines. The Trent 1000 will power the Boeing 787 on its maiden flight.-Design and development:...
- Boeing 787The Boeing 787 Dreamliner is a mid-sized, wide-body, twin-engine jet airliner currently under development by Boeing Commercial Airplanes. Its maximum seating capacity in a one-class configuration is between 290 to 330 passengers depending on variant...
- 11.0:1