Bypass ratio
Encyclopedia
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 in by the fan 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 thrust specific fuel consumption, but reduces the top speed and gives a heavier engine, however there is noise reduction from this arrangement.
A lower bypass ratio gives a higher exhaust speed, which is needed to sustain higher, usually supersonic, airspeeds. This increases the thrust specific fuel consumption.
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.
face, and that is a function of the total amount of fuel burned. Increasing airflow, and thus thrust
, 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 efficiency
. 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–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.
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 Thrust specific fuel consumption 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 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.
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 aircraft
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.
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
engines, commonly used in aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
. It is defined as the ratio between the mass flow rate of air drawn in by the fan 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 thrust specific fuel consumption, but reduces the top speed and gives a heavier engine, however there is noise reduction from this arrangement.
A lower bypass ratio gives a higher exhaust speed, which is needed to sustain higher, usually supersonic, airspeeds. This increases the thrust specific fuel consumption.
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 turbineTurbine
A turbine is a rotary engine that extracts energy from a fluid 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 move and...
face, and that is a function of the total amount of fuel burned. Increasing airflow, and thus thrust
Thrust
Thrust 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 force of equal magnitude but opposite direction on that system....
, 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 efficiency
Fuel efficiency
Fuel 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 is...
. 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–Royce
Rolls-Royce Limited
Rolls-Royce Limited was a renowned British car and, from 1914 on, 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....
came up with a better use of the extra energy in their Conway
Rolls-Royce Conway
The 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...
turbofan
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
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
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
(in effect, a ducted fan
Ducted fan
A ducted fan is a propulsion arrangement whereby a fan, which is a type of propeller, is mounted within a cylindrical shroud or duct. The duct reduces losses in thrust from the tip vortices of the fan, and varying the cross-section of the duct allows the designer to advantageously affect the...
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.
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 Thrust specific fuel consumption 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 jetliners
Jet airliner
A jet airliner is an airliner that is powered by jet engines. This term is sometimes contracted to jetliner or jet.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 States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
by both Pratt & Whitney
Pratt & Whitney
Pratt & Whitney is a U.S.-based aerospace manufacturer with global service operations. It is a subsidiary of United Technologies Corporation . Pratt & Whitney's aircraft engines are widely used in both civil aviation and military aviation. Its headquarters are in East Hartford, Connecticut, USA...
and General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
, which for the first time was out-competing the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
in engine design. Rolls-Royce
Rolls-Royce Limited
Rolls-Royce Limited was a renowned British car and, from 1914 on, 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....
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 aircraft
Fighter aircraft
A 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...
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
| Engine | Aircraft | Bypass ratio |
|---|---|---|
| Rolls-Royce/Snecma Olympus 593 Rolls-Royce/Snecma Olympus 593 The Rolls-Royce/Snecma Olympus 593 was a reheated turbojet which powered the supersonic airliner Concorde. Initially a joint project between Bristol Siddeley and Snecma based on Bristol's Olympus engine, Rolls-Royce Limited acquired Bristol making it a division of Rolls-Royce.Until Concorde's... |
Concorde Concorde Aérospatiale-BAC Concorde 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... (turbojet) |
0:1 |
| Rolls-Royce Tay | Gulfstream IV Gulfstream IV The Gulfstream IV and derivatives are a family of twin-jet aircraft, mainly for private or business use. The aircraft was designed and built by Gulfstream Aerospace, a General Dynamics company based in Savannah, Georgia, United States from 1985 until 2003.-Design and development:Gulfstream, in... , Fokker 70, Fokker 100 |
3.1:1 |
| SNECMA M88 SNECMA M88 |-See also:-External links:* * Snecma M88's pdf* *... |
Dassault Rafale Dassault Rafale The Dassault Rafale is a French twin-engine delta-wing multi-role 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 the French Navy... |
0.30:1 |
| Pratt & Whitney JT8D | DC-9, MD-80, Boeing 727 Boeing 727 The Boeing 727 is a mid-size, narrow-body, three-engine, T-tailed commercial jet airliner, manufactured by Boeing. The Boeing 727 first flew in 1963, and for over a decade more were built per year than any other jet airliner. When production ended in 1984 a total of 1,832 aircraft had been produced... , Boeing 737 Boeing 737 The Boeing 737 is a short- to medium-range, twin-engine narrow-body jet airliner. Originally developed as a shorter, lower-cost twin-engine airliner derived from Boeing's 707 and 727, the 737 has developed into a family of nine passenger models with a capacity of 85 to 215 passengers... 100 and 200 series |
0.96:1 |
| Pratt & Whitney F100 Pratt & Whitney F100 -External links:* * *... |
F-16, F-15 F-15 Eagle The McDonnell Douglas F-15 Eagle is a twin-engine, all-weather tactical fighter designed by McDonnell Douglas to gain and maintain air superiority in aerial combat. It is considered among the most successful modern fighters with over 100 aerial combat victories with no losses in dogfights... |
0.36:1 |
| General Electric F404 | F/A-18, T-50 T-50 Golden Eagle The T-50 Golden Eagle is a family of South Korean supersonic advanced trainers and multirole fighters, developed by Korea Aerospace Industries beginning in the late 1990s. The T-50 is South Korea's first indigenous supersonic aircraft and one of the world's few supersonic trainers... , F-117, X-29, X-31 |
0.34:1 |
| Eurojet EJ200 Eurojet EJ200 |-See also:-External links:* * * * *... |
Eurofighter Typhoon Eurofighter Typhoon The Eurofighter Typhoon is a twin-engine, canard-delta wing, multirole combat aircraft, designed and built by a consortium of three companies: EADS, Alenia Aeronautica and BAE Systems; working through a holding company, Eurofighter GmbH, which was formed in 1986... |
0.4:1 |
| Klimov RD-33 Klimov RD-33 |- Specifications :thrust=49.4 kN Dry, 81.4 - 88.4 kN Afterburning. -See also:- External links :* * * * * * * * * *... |
MiG-29, Il-102 | 0.49:1 |
| Saturn AL-31F | Su-27, Su-30, Chengdu J-10 Chengdu J-10 The Chengdu J-10 is a multirole fighter aircraft designed and produced by the People's Republic of China's Chengdu Aircraft Industry Corporation for the People's Liberation Army Air Force... |
0.59:1 |
| Kuznetsov NK-321 | Tu-160 | 1.4:1 |
| PowerJet SaM146 PowerJet SaM146 |-See also:-External links:* * * * *... |
Sukhoi Superjet 100 Sukhoi Superjet 100 The Sukhoi Superjet 100 is a modern, fly-by-wire regional jet in the 75- to 95-seat category. With development starting in 2000, the plane was designed by the civil aircraft division of the Russian aerospace company Sukhoi in co-operation with Western partners... |
4.43:1 |
| Pratt & Whitney PW2000 Pratt & Whitney PW2000 |-See also:-External links:... |
Boeing 757 Boeing 757 The Boeing 757 is a mid-size, narrow-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. Passenger versions of the twinjet have a capacity of 186 to 289 persons and a maximum range of , depending on variant and cabin configuration... , C-17 Globemaster III C-17 Globemaster III The Boeing C-17 Globemaster III is a large military transport aircraft. Developed for the United States Air Force from the 1980s to the early 1990s by McDonnell Douglas, the C-17 is used for rapid strategic airlift of troops and cargo to main operating bases or forward operating bases throughout... |
5.9:1 |
| Progress D-436 Progress D-436 |-See also:-Bibliography:* Ivchenko-Progress D-436 , Aero-engines - Turbofan.Jane's. http://www.janes.com/articles/Janes-Aero-Engines/Ivchenko-Progress-D-436-Ukraine.html. Retrieved Aug 12 2009.... |
Yak-42M Yakovlev Yak-42 The Yakovlev Yak-42 is a 100/120-seat three-engined mid-range passenger jet. It was designed as a replacement for several obsolete Aeroflot jets as a mid-range passenger jet... , Beriev Be-200 Beriev Be-200 The Beriev Be-200 Altair is a multipurpose amphibious aircraft designed by the Beriev Aircraft Company and manufactured by Irkut. Marketed as being designed for fire fighting, search and rescue, maritime patrol, cargo, and passenger transportation, it has a capacity of 12 tonnes of water, or up... , An-148 |
6.2:1 |
| General Electric GEnx General Electric GEnx The General Electric GEnx is an advanced dual rotor, axial flow, high-bypass turbofan jet engine in production by GE Aviation for the Boeing 787 and 747-8... |
Boeing 787 Boeing 787 The Boeing 787 Dreamliner is a long-range, mid-size wide-body, twin-engine jet airliner developed by Boeing Commercial Airplanes. It seats 210 to 290 passengers, depending on the variant. Boeing states that it is the company's most fuel-efficient airliner and the world's first major airliner to use... |
8.5:1 |
| Rolls-Royce Trent 900 Rolls-Royce Trent 900 The Rolls-Royce Trent 900 is a series of turbofan engines, developed from the RB211 and is one of the family of Trent engines.-Design and development:... |
Airbus A380 Airbus A380 The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it... |
8.7:1 |
| General Electric GE90 General Electric GE90 General Electric GE90 is a family of high-bypass turbofan aircraft engines built by GE Aviation for the Boeing 777, with thrust ratings ranging from 74,000 to 115,000 lbf . It was first introduced in November 1995 on British Airways' 777s, and is available only on the 777... |
Boeing 777 Boeing 777 The Boeing 777 is a long-range, wide-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet and is commonly referred to as the "Triple Seven". The aircraft has seating for over 300 passengers and has a range from , depending on model... |
9:1 |
| Rolls-Royce Trent 1000 | Boeing 787 Boeing 787 The Boeing 787 Dreamliner is a long-range, mid-size wide-body, twin-engine jet airliner developed by Boeing Commercial Airplanes. It seats 210 to 290 passengers, depending on the variant. Boeing states that it is the company's most fuel-efficient airliner and the world's first major airliner to use... |
11:1 |