![Turbofan3 Labelled](https://upload.wikimedia.org/wikipedia/commons/e/e5/Turbofan3_Labelled.gif)
The turbofan or fanjet is a type of
airbreathing jet engine
An airbreathing jet engine (or ''ducted jet engine'') is a jet engine that ejects a propelling (reaction) jet of hot exhaust gases after first taking in atmospheric air, followed by compression, heating and expansion back to atmospheric pressure ...
that is widely used in
aircraft propulsion
A powered aircraft is an aircraft that uses onboard propulsion with mechanical power generated by an aircraft engine of some kind.
Aircraft propulsion nearly always uses either a type of propeller, or a form of jet propulsion. Other potential ...
. The word "turbofan" is a
portmanteau
A portmanteau word, or portmanteau (, ) is a blend of words[gas turbine engine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...](_blank)
which achieves
mechanical energy
In Outline of physical science, physical sciences, mechanical energy is the sum of potential energy and kinetic energy. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, t ...
from combustion,
and the ''fan'', a
ducted fan
In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing
(VTOL) applicatio ...
that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
passes through the
combustion chamber
A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process.
Interna ...
and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the
thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
.
The ratio of the mass-flow of air bypassing the engine core to the mass-flow of air passing through the core is referred to as the
bypass ratio
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for ev ...
. The engine produces thrust through a combination of these two portions working together; engines that use more
jet thrust
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, water jet, and hybrid propulsion, the term typicall ...
relative to fan thrust are known as ''low-bypass turbofans'', conversely those that have considerably more fan thrust than jet thrust are known as ''high-bypass''. Most commercial aviation jet engines in use today are of the high-bypass type,
and most modern military fighter engines are low-bypass.
Afterburner
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and comba ...
s are used on low-bypass turbofan engines with bypass and core mixing before the afterburner.
Modern turbofans have either a large single-stage fan or a smaller fan with several stages. An early configuration combined a low-pressure turbine and fan in a single rear-mounted unit.
Principles
The turbofan was invented to improve the fuel consumption of the turbojet. It would do this by increasing the mass and lowering the speed of the propelling jet compared to that of the turbojet. This would be done mechanically by adding a
ducted fan
In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing
(VTOL) applicatio ...
rather than using viscous forces by adding an ejector, as first envisaged by Whittle.
Frank Whittle
Air Commodore Sir Frank Whittle, (1 June 1907 – 8 August 1996) was an English engineer, inventor and Royal Air Force (RAF) air officer. He is credited with inventing the turbojet engine. A patent was submitted by Maxime Guillaume in 1921 for ...
envisioned flight speeds of 500 mph in writing his UK patent 471,368 "Improvements relating to the propulsion of aircraft", filed in March 1936, and in which he describes the principles behind the turbofan, although not called as such at that time. The turbojet uses the gas from its thermodynamic cycle as its propelling jet. There are two penalties for using the cycle gas for the propelling jet for aircraft speeds of 500 mph and they are addressed by the turbofan.
There is energy wasted because the propelling jet is going much faster rearwards than the aircraft is going forwards, leaving a very fast wake. The kinetic energy of the wake is a reflection of the fuel used to produce the wake rather than fuel used to move the aircraft forwards and, as such, is fuel wasted. However,it is a fundamental aspect of producing thrust in a fluid by accelerating some of it rearwards whether by a propeller or a combustor in a duct (ramjet) and, as such, can only be reduced and not eliminated. The turbofan reduces the speed of the propelling jet.
The other penalty is present because any action to reduce the fuel consumption of the engine by increasing its pressure ratio or turbine temperature causes a corresponding increase in pressure and temperature in the exhaust duct which in turn cause a higher gas speed from the propelling nozzle (and higher KE and wasted fuel). Although the engine uses less fuel to produce a pound of thrust more fuel is wasted in the faster propelling jet. In other words, the independence of thermal and propulsive efficiencies, as exists with the piston engine/propeller combination which preceded the turbojet, is lost. In contrast, Roth considers regaining this independence the single most important feature of the turbofan which allows specific thrust to be chosen independently of the gas generator cycle.
The working substance of the thermodynamic cycle is the only mass accelerated to produce thrust in a turbojet which is a serious limitation (high fuel consumption) for aircraft speeds below supersonic. For subsonic flight speeds the speed of the propelling jet has to be reduced because there is a price to be paid in producing the thrust. The energy required to accelerate the gas inside the engine (increase in kinetic energy) is expended in two ways, by producing a change in momentum ( ie a force), and a wake which is an unavoidable consequence of producing thrust by an airbreathing engine (or propeller). The wake velocity, and fuel burned to produce it, can be reduced and the required thrust still maintained by increasing the mass accelerated. A turbofan does this by transferring energy available inside the engine, from the gas generator, to a
ducted fan
In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing
(VTOL) applicatio ...
which produces a second, additional mass of accelerated air.
The transfer of energy from the core to bypass air results in lower pressure and temperature gas entering the core nozzle (lower exhaust velocity) and fan-produced temperature and pressure entering the fan nozzle. The amount of energy transferred depends on how much pressure rise the fan is designed to produce (fan pressure ratio). The best energy exchange (lowest fuel consumption) between the two flows, and how the jet velocities compare, depends on how efficiently the transfer takes place which depends on the losses in the fan-turbine and fan.
The fan flow has lower exhaust velocity, giving much more thrust per unit energy (lower
specific thrust
Specific thrust is the thrust per unit air mass flowrate of a jet engine (e.g. turbojet, turbofan, etc.) and can be calculated by the ratio of net thrust/total intake airflow.
Low specific thrust engines tend to be more efficient of propellant (a ...
). Both airstreams contribute to the gross thrust of the engine. The additional air for the bypass stream increases the ram drag in the air intake stream-tube, but there is still a significant increase in net thrust. The overall effective exhaust velocity of the two exhaust jets can be made closer to a normal subsonic aircraft's flight speed and gets closer to the ideal
Froude efficiency. A turbofan accelerates a larger mass of air more slowly, compared to a turbojet which accelerates a smaller amount more quickly, which is a less efficient way to generate the same thrust (see the
efficiency
Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
section below).
The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the
bypass ratio
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for ev ...
. Engines with more
jet thrust
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, water jet, and hybrid propulsion, the term typicall ...
relative to fan thrust are known as ''low-bypass turbofans'', those that have considerably more fan thrust than jet thrust are known as ''high-bypass''. Most commercial aviation jet engines in use today are high-bypass,
and most modern fighter engines are low-bypass.
Afterburner
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and comba ...
s are used on low-bypass turbofans on combat aircraft.
Bypass ratio
The ''bypass ratio (BPR)'' of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A bypass ratio of 6, for example, means that 6 times more air passes through the bypass duct than the amount that passes through the combustion chamber.
Turbofan engines are usually described in terms of BPR, which together with overall pressure ratio, turbine inlet temperature and fan pressure ratio are important design parameters. In addition BPR is quoted for turboprop and unducted fan installations because their high propulsive efficiency gives them the overall efficiency characteristics of very high bypass turbofans. This allows them to be shown together with turbofans on plots which show trends of reducing
specific fuel consumption (SFC) with increasing BPR. BPR can also be quoted for lift fan installations where the fan airflow is remote from the engine and doesn't flow past the engine core.
Considering a constant core (ie fixed pressure ratio and turbine inlet temperature), core and bypass jet velocities equal and a particular flight condition (ie Mach number and altitude) the fuel consumption per lb of thrust (sfc) decreases with increase in BPR. At the same time gross and net thrusts increase, but by different amounts. There is considerable potential for reducing fuel consumption for the same core cycle by increasing BPR.This is achieved because of the reduction in pounds of thrust per lb/sec of airflow (specific thrust) and the resultant reduction in lost kinetic energy in the jets (increase in propulsive efficiency).
If all the gas power from a gas turbine is converted to kinetic energy in a propelling nozzle, the aircraft is best suited to high supersonic speeds. If it is all transferred to a separate big mass of air with low kinetic energy, the aircraft is best suited to zero speed (hovering). For speeds in between, the gas power is shared between a separate airstream and the gas turbine's own nozzle flow in a proportion which gives the aircraft performance required. The trade off between mass flow and velocity is also seen with propellers and helicopter rotors by comparing disc loading and power loading. For example, the same helicopter weight can be supported by a high power engine and small diameter rotor or, for less fuel, a lower power engine and bigger rotor with lower velocity through the rotor.
Bypass usually refers to transferring gas power from a gas turbine to a bypass stream of air to reduce fuel consumption and jet noise. Alternatively, there may be a requirement for an afterburning engine where the sole requirement for bypass is to provide cooling air. This sets the lower limit for BPR and these engines have been called "leaky" or continuous bleed turbojets (General Electric YJ-101 BPR 0.25) and low BPR turbojets (Pratt & Whitney PW1120). Low BPR (0.2) has also been used to provide surge margin as well as afterburner cooling for the
Pratt & Whitney J58
The Pratt & Whitney J58 (company designation JT11D-20) is an American jet engine that powered the Lockheed A-12, and subsequently the YF-12 and the SR-71 aircraft. It was an afterburning turbojet engine with a unique compressor bleed to the af ...
.
Efficiency
Propeller
A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
engines are most efficient for low speeds,
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
engines – for high speeds, and turbofan engines – between the two. Turbofans are the most efficient engines in the range of speeds from about , the speed at which most commercial aircraft operate.
In a turbojet (zero-bypass) engine, the high temperature and high pressure exhaust gas is accelerated when it undergoes expansion through a
propelling nozzle
A propelling nozzle is a nozzle that converts the internal energy of a working gas into propulsive force; it is the nozzle, which forms a jet, that separates a gas turbine, or gas generator, from a jet engine.
Propelling nozzles accelerate the av ...
and produces all the thrust. The compressor absorbs the mechanical power produced by the turbine. In a bypass design, extra turbines drive a
ducted fan
In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing
(VTOL) applicatio ...
that accelerates air rearward from the front of the engine. In a high-bypass design, the ducted fan and nozzle produce most of the thrust. Turbofans are closely related to
turboprop
A turboprop is a turbine engine that drives an aircraft propeller.
A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle. Air enters the intake and is compressed by the compressor. Fuel ...
s in principle because both transfer some of the gas turbine's gas power, using extra machinery, to a bypass stream leaving less for the hot nozzle to convert to kinetic energy. Turbofans represent an intermediate stage between
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
s, which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less).
[The turbofan engine]
", p. 7. SRM Institute of Science and Technology
SRM Institute of Science and Technology (SRMIST), formerly SRM University, is a private higher education institute deemed to be university, located in Kattankulathur, Chengalpattu (near Chennai), Tamil Nadu, India. Founded in 1985 as SRM Engin ...
, Department of aerospace engineering. Extracting shaft power and transferring it to a bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency. The turboprop at its best flight speed gives significant fuel savings over a turbojet even though an extra turbine, a gearbox and a propeller were added to the turbojet's low-loss propelling nozzle. The turbofan has additional losses from its greater number of compressor stages/blades, fan and bypass duct.
Froude, or Propulsive, Efficiency can be defined as:
:
where:
Thrust
While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30% and 70% of the total thrust produced by a turbofan system.
The thrust (''F
N'') generated by a turbofan depends on the
effective exhaust velocity
Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...
of the total exhaust, as with any jet engine, but because two exhaust jets are present the thrust equation can be expanded as:
:
where:
Nozzles
The cold duct and core duct's nozzle systems are relatively complex due to the use of two separate exhaust flows. In high bypass engines, the fan is situated in a short duct near the front of the engine and typically has a convergent cold nozzle, with the tail of the duct forming a low pressure ratio nozzle that under normal conditions will choke creating supersonic flow patterns around the core. The core nozzle is more conventional, but generates less of the thrust, and depending on design choices, such as noise considerations, may conceivably not choke. In low bypass engines the two flows may combine within the ducts, and share a common nozzle, which can be fitted with afterburner.
Noise
![GEnx-1B on Air India B787 (2)](https://upload.wikimedia.org/wikipedia/commons/2/20/GEnx-1B_on_Air_India_B787_%282%29.jpg)
Most of the air flow through a high-bypass turbofan is lower-velocity bypass flow: even when combined with the much-higher-velocity engine exhaust, the average exhaust velocity is considerably lower than in a pure turbojet. Turbojet engine noise is predominately
jet noise
In aeroacoustics, jet noise is the field that focuses on the noise generation caused by high-velocity jets and the turbulent eddies generated by shearing flow. Such noise is known as broadband noise and extends well beyond the range of human heari ...
from the high exhaust velocity. Therefore, turbofan engines are significantly quieter than a pure-jet of the same thrust, and jet noise is no longer the predominant source.
[Kempton, A.]
"Acoustic liners for modern aero-engines"
15th CEAS-ASC Workshop and 1st Scientific Workshop of X-Noise EV, 2011. Win.tue.nl. Turbofan engine noise propagates both upstream via the inlet and downstream via the primary nozzle and the by-pass duct. Other noise sources are the fan, compressor and turbine.
Modern commercial aircraft employ high-bypass-ratio (HBPR) engines with separate flow, non-mixing, short-duct exhaust systems. Their noise is due to the speed, temperature, and pressure of the exhaust jet, especially during high-thrust conditions, such as those required for takeoff. The primary source of jet noise is the turbulent mixing of shear layers in the engine's exhaust. These shear layers contain instabilities that lead to highly turbulent vortices that generate the pressure fluctuations responsible for sound. To reduce the noise associated with jet flow, the aerospace industry has sought to disrupt shear layer turbulence and reduce the overall noise produced.
Fan noise may come from the interaction of the fan-blade wakes with the pressure field of the downstream fan-exit stator vanes. It may be minimized by adequate axial spacing between blade trailing edge and stator entrance.
At high engine speeds, as at takeoff, shock waves from the supersonic fan tips, because of their unequal nature, produce noise of a discordant nature known as "buzz saw" noise.
All modern turbofan engines have
acoustic liner
Acoustic may refer to:
Music Albums
* ''Acoustic'' (Above & Beyond album), 2014
* ''Acoustic'' (Deine Lakaien album), 2007
* ''Acoustic'' (Everything but the Girl album), 1992
* ''Acoustic'' (John Lennon album), 2004
* ''Acoustic'' (Love Amo ...
s in the
nacelle
A nacelle ( ) is a "streamlined body, sized according to what it contains", such as an engine, fuel, or equipment on an aircraft. When attached by a pylon entirely outside the airframe, it is sometimes called a pod, in which case it is attached ...
to damp their noise. They extend as much as possible to cover the largest surface area. The acoustic performance of the engine can be experimentally evaluated by means of ground tests
or in dedicated experimental test rigs.
In the
aerospace
Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and astrona ...
industry, chevrons are the "saw-tooth" patterns on the trailing edges of some
jet engine
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
nozzles
that are used for
noise reduction
Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an und ...
. The shaped edges smooth the mixing of hot air from the engine core and cooler air flowing through the engine fan, which reduces noise-creating turbulence.
[ Chevrons were developed by GE under a ]NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
contract. Some notable examples of such designs are Boeing 787
The Boeing 787 Dreamliner is an American wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes.
After dropping its unconventional Sonic Cruiser project, Boeing announced the conventional 7E7 on January 29, 2003, ...
and Boeing 747-8
The Boeing 747-8 is a wide-body airliner formerly developed by Boeing Commercial Airplanes, and the largest variant of the 747.
After introducing the 747-400, Boeing considered larger 747 versions as alternatives to the proposed double-deck ...
on the Rolls-Royce Trent 1000
The Rolls-Royce Trent 1000 is a high-bypass turbofan engine produced by Rolls-Royce plc, one of the two engine options for the Boeing 787 Dreamliner, competing with the General Electric GEnx.
It first ran on 14 February 2006 and first flew on ...
and General Electric GEnx
The General Electric GEnx ("General Electric Next-generation") is an advanced dual rotor, axial flow, high-bypass turbofan jet engine in production by GE Aviation for the Boeing 787 and 747-8. The GEnx is intended to succeed the CF6 in GE's pr ...
engines.
History
Early turbojet engines were not very fuel-efficient because their overall pressure ratio and turbine inlet temperature were severely limited by the technology and materials available at the time.
The first turbofan engine, which was only run on a test bed, was the German Daimler-Benz DB 670
The Daimler-Benz DB 007, ( RLM (''Reichsluftfahrtministerium'' - Reich Air Ministry) designation ZTL 109-007, company designation ZTL6001), was an early German jet engine design stemming from design work carried out by Karl Leist from 1939. This ...
, designated the 109-007 by the Nazi Ministry of Aviation, with a first run date of 27 May 1943, after the testing of the turbomachinery using an electric motor, which had been undertaken on 1 April 1943. Development of the engine was abandoned with its problems unsolved, as the war situation worsened for Germany.
Later in 1943, the British ground tested the Metrovick F.3 turbofan, which used the Metrovick F.2
The Metropolitan-Vickers F.2 is an early turbojet engine and the first British design to be based on an axial-flow compressor. It was an extremely advanced design for the era, using a nine-stage axial compressor, annular combustor, and a two- ...
turbojet as a gas generator with the exhaust discharging into a close-coupled aft-fan module comprising a contra-rotating LP turbine system driving two co-axial contra-rotating fans.
Improved materials, and the introduction of twin compressors, such as in the Bristol Olympus
The Rolls-Royce Olympus (originally the Bristol B.E.10 Olympus) was the world's second two- spool axial-flow turbojet aircraft engine design, first run in May 1950 and preceded only by the Pratt & Whitney J57, first-run in January 1950. It is ...
, and Pratt & Whitney JT3C engines, increased the overall pressure ratio and thus the thermodynamic
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of ther ...
efficiency of engines. They also had poor propulsive efficiency, because pure turbojets have a high specific thrust/high velocity exhaust, which is better suited to supersonic flight.
The original low-bypass turbofan engines were designed to improve propulsive efficiency by reducing the exhaust velocity to a value closer to that of the aircraft. The Rolls-Royce Conway
The Rolls-Royce RB.80 Conway was the first turbofan engine to enter service. Development started at Rolls-Royce in the 1940s, but the design was used only briefly, in the late 1950s and early 1960s, before other turbofan designs replaced it. H ...
, the world's first production turbofan, had a bypass ratio of 0.3, similar to the modern General Electric F404
The General Electric F404 and F412 are a family of afterburning turbofan engines in the class (static thrust). The series is produced by GE Aviation. Partners include Volvo Aero, which builds the RM12 variant. The F404 was developed into the la ...
fighter engine. Civilian turbofan engines of the 1960s, such as the Pratt & Whitney JT8D
The Pratt & Whitney JT8D is a low-bypass (0.96 to 1) turbofan engine introduced by Pratt & Whitney in February 1963 with the inaugural flight of the Boeing 727. It was a modification of the Pratt & Whitney J52 turbojet engine which powered th ...
and the Rolls-Royce Spey
The Rolls-Royce Spey (company designations RB.163 and RB.168 and RB.183) is a low-bypass turbofan engine originally designed and manufactured by Rolls-Royce that has been in widespread service for over 40 years. A co-development version of the ...
, had bypass ratios closer to 1 and were similar to their military equivalents.
The first Soviet airliner powered by turbofan engines was the Tupolev Tu-124
The Tupolev Tu-124 (NATO reporting name: Cookpot) was a 56-passenger short-range twinjet airliner built in the Soviet Union. It was the first Soviet airliner powered by turbofan engines.
Design and development
Developed from the medium-range ...
introduced in 1962. It used the Soloviev D-20
The Soloviev D-20P, built by the Soloviev Design Bureau, was a low-bypass turbofan engine rated at 52.9 kN (11,900 lbf) thrust used on the Tupolev Tu-124. A later derivative with increased bypass ratio, the D-20P-125, was developed in ...
. 164 aircraft were produced between 1960 and 1965 for Aeroflot
PJSC AeroflotRussian Airlines (russian: ПАО "Аэрофло́т — Росси́йские авиали́нии", ), commonly known as Aeroflot ( or ; russian: Аэрофлот, , ), is the flag carrier and the largest airline of Russia. The ...
and other Eastern Bloc
The Eastern Bloc, also known as the Communist Bloc and the Soviet Bloc, was the group of socialist states of Central and Eastern Europe, East Asia, Southeast Asia, Africa, and Latin America under the influence of the Soviet Union that existed du ...
airlines, with some operating until the early 1990s.
The first General Electric turbofan was the aft-fan CJ805-23, based on the CJ805-3 turbojet. It was followed by the aft-fan General Electric CF700
The General Electric CF700 (military designation TF37) is an aft-fan turbofan development of the CJ610 turbojet. The fan blades are an extension of the low-pressure turbine blades.
Variants
;CF700-2B:Baseline aft-fan CJ610 variant rated at for ...
engine, with a 2.0 bypass ratio. This was derived from the General Electric J85/CJ610 turbojet to power the larger Rockwell Sabreliner 75/80 model aircraft, as well as the Dassault Falcon 20
The Dassault Falcon 20 is a French business jet developed and manufactured by Dassault Aviation. The first business jet developed by the firm, it became the first of a family of business jets to be produced under the same name; of these, both ...
, with about a 50% increase in thrust to . The CF700 was the first small turbofan to be certified by the Federal Aviation Administration
The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...
(FAA). There were at one time over 400 CF700 aircraft in operation around the world, with an experience base of over 10 million service hours. The CF700 turbofan engine was also used to train Moon-bound astronauts in Project Apollo as the powerplant for the Lunar Landing Research Vehicle
The Bell Aerosystems Lunar Landing Research Vehicle (LLRV, nicknamed the Flying Bedstead) was a Project Apollo era program to build a simulator for the Moon landings. The LLRVs were used by the FRC, now known as the NASA Armstrong Flight Resear ...
.
Common types
Low-bypass turbofan
A high-specific-thrust/low-bypass-ratio turbofan normally has a multi-stage fan behind inlet guide vanes, developing a relatively high pressure ratio and, thus, yielding a high (mixed or cold) exhaust velocity. The core airflow needs to be large enough to ensure there is sufficient core power to drive the fan. A smaller core flow/higher bypass ratio cycle can be achieved by raising the inlet temperature of the high-pressure (HP) turbine rotor.
To illustrate one aspect of how a turbofan differs from a turbojet, comparisons can be made at the same airflow (to keep a common intake for example) and the same net thrust (i.e. same specific thrust). A bypass flow can be added only if the turbine inlet temperature is not too high to compensate for the smaller core flow. Future improvements in turbine cooling/material technology can allow higher turbine inlet temperature, which is necessary because of increased cooling air temperature, resulting from an overall pressure ratio
In aeronautical engineering, overall pressure ratio, or overall compression ratio, is the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine. The terms ''compression ratio'' and ''pressure ...
increase.
The resulting turbofan, with reasonable efficiencies and duct loss for the added components, would probably operate at a higher nozzle pressure ratio than the turbojet, but with a lower exhaust temperature to retain net thrust. Since the temperature rise across the whole engine (intake to nozzle) would be lower, the (dry power) fuel flow would also be reduced, resulting in a better specific fuel consumption (SFC).
Some low-bypass ratio military turbofans (e.g. F404, JT8D
The Pratt & Whitney JT8D is a low-bypass (0.96 to 1) turbofan engine introduced by Pratt & Whitney in February 1963 with the inaugural flight of the Boeing 727. It was a modification of the Pratt & Whitney J52 turbojet engine which powered th ...
) have variable inlet guide vanes to direct air onto the first fan rotor stage. This improves the fan surge
Surge means a sudden transient rush or flood, and may refer to:
Science
* Storm surge, the onshore gush of water associated with a low-pressure weather system
* Surge (glacier), a short-lived event where a glacier can move up to velocities 100 ...
margin (see compressor map A compressor map is a chart which shows the performance of a turbomachinery compressor. This type of compressor is used in gas turbine engines, for supercharging reciprocating engines and for industrial processes, where it is known as a dynamic comp ...
).
File:Pratt & Whitney JT8D-17A 1.JPG, The widely produced Pratt & Whitney JT8D
The Pratt & Whitney JT8D is a low-bypass (0.96 to 1) turbofan engine introduced by Pratt & Whitney in February 1963 with the inaugural flight of the Boeing 727. It was a modification of the Pratt & Whitney J52 turbojet engine which powered th ...
used on many early narrowbody jetliners
A jet airliner or jetliner is an airliner powered by jet engines (passenger jet aircraft). Airliners usually have two or four jet engines; three-engined designs were popular in the 1970s but are less common today. Airliners are commonly clas ...
. The fan is located behind the inlet guide vanes.
File:Solowjow D-30 III.jpg, Soloviev D-30
The Soloviev D-30 (now the Aviadvigatel PS-30) is a Soviet two-shaft low-bypass turbofan engine, officially referred to as a "bypass turbojet". It is probably the single most important turbofan engine developed in the Soviet Union. Development o ...
which powers the Ilyushin Il-76
The Ilyushin Il-76 (russian: Илью́шин Ил-76; NATO reporting name: Candid) is a multi-purpose, fixed-wing, four-engine turbofan strategic airlifter designed by the Soviet Union's Ilyushin design bureau. It was first planned as a comm ...
& Il-62M; Mikoyan MiG-31
The Mikoyan MiG-31 (russian: link=no, Микоян МиГ-31; NATO reporting name: Foxhound) is a supersonic interceptor aircraft that was developed for use by the Soviet Air Forces. The aircraft was designed by the Mikoyan design bureau as a ...
; Xian H-6
The Xian H-6 () is a twin-engine jet bomber of the Chinese People's Liberation Army Air Force (PLAAF). The H-6 is a license-built version of the Soviet Tupolev Tu-16 and remains the primary bomber aircraft of the People's Republic of China.
Del ...
K & Y-20
The Xi'an Y-20 Kunpeng () is a large military transport aircraft developed by the Xi'an Aircraft Industrial Corporation for the People's Republic of China (PRC).
The aircraft is nicknamed "Chubby Girl" () in the Chinese aviation industry becaus ...
File:AL-31FN.jpg, Saturn AL-31
The Saturn AL-31 is a family of axial flow turbofan engines, developed by the Lyulka design bureau in the Soviet Union, now NPO Saturn in Russia, originally as a 12.5-tonne (122.6 kN, 27,560 lbf) powerplant for the Sukhoi Su-27 long ran ...
which powers the Chengdu J-10
The Chengdu J-10 Vigorous Dragon (; NATO reporting name: Firebird), is a medium-weight, single-engine, multirole combat aircraft capable of all-weather operations, configured with a delta wing and canard design, with fly-by-wire flight contr ...
& J-20; Shenyang J-11
The Shenyang J-11 (Chinese: 歼-11; NATO reporting name Flanker-B, -L) is a twin-engine jet fighter of the People's Republic of China whose airframe is derived from the Soviet-designed Sukhoi Su-27. It is manufactured by the Shenyang Aircraft Co ...
, J-15 & J-16
The Shenyang J-16 ( Chinese: 歼-16) is a Chinese advanced fourth-generation, tandem-seat, twinjet, multirole strike fighterBronk, page 38 developed from the Shenyang J-11 (itself derived from the Sukhoi Su-27) and built by Shenyang Aircraft Co ...
; Sukhoi Su-30
The Sukhoi Su-30 (russian: Сухой Су-30; NATO reporting name: Flanker-C/G/H) is a twin-engine, two-seat supermaneuverable fighter aircraft developed in the Soviet Union by Russia's Sukhoi Aviation Corporation. It is a multirole fighte ...
& Su-27
The Sukhoi Su-27 (russian: Сухой Су-27; NATO reporting name: Flanker) is a Soviet-origin twin-engine supermaneuverable fighter aircraft designed by Sukhoi. It was intended as a direct competitor for the large US fourth-generation jet ...
File:Williams Research F107.jpg, Williams F107
The Williams F107 (company designation WR19) is a small turbofan engine made by Williams International. The F107 was designed to propel cruise missiles. It has been used as the powerplant for the AGM-86 ALCM, and BGM-109 Tomahawk, as well as the ...
which powers the Raytheon
Raytheon Technologies Corporation is an American multinational aerospace and defense conglomerate headquartered in Arlington, Virginia. It is one of the largest aerospace and defense manufacturers in the world by revenue and market capitaliza ...
BGM-109 Tomahawk
The Tomahawk () Land Attack Missile (TLAM) is a long-range, all-weather, jet-powered, subsonic cruise missile that is primarily used by the United States Navy and Royal Navy in ship and submarine-based land-attack operations.
Under contract f ...
cruise missile
File:AL-55 at the MAKS-2011 (01).jpg, NPO Saturn AL-55
The NPO Saturn AL-55 is a high performance turbofan engine manufactured by NPO Saturn Russia, for powering advanced trainers, unmanned aerial vehicles (UAV) and light attack aircraft. A variant of the AL-55I powers the HAL HJT-36 Sitara Indian jet ...
which powers certain HAL HJT-36 Sitara
The HAL HJT-36 ''Sitara'' ( Sitārā: "''star''") is a subsonic intermediate jet trainer aircraft designed and developed by Aircraft Research and Design Centre (ARDC) and built by Hindustan Aeronautics Limited (HAL) for the Indian Air Force and ...
File:Eurojet EJ200 for Eurofighter Typhoon PAS 2013 01 free.jpg, Eurojet EJ200
The Eurojet EJ200 is a military low-bypass 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 Eu ...
which powers the Eurofighter Typhoon
The Eurofighter Typhoon is a European multinational twin-engine, canard delta wing, multirole fighter. The Typhoon was designed originally as an air-superiority fighter and is manufactured by a consortium of Airbus, BAE Systems and Leonardo ...
File:XF3 KASM001.jpg, Ishikawajima-Harima F3
The Ishikawajima-Harima Heavy Industries (IHI) F3 is a low bypass turbofan engine developed in Japan by Ishikawajima-Harima Heavy Industries for the Kawasaki T-4 jet trainer aircraft. The first prototype engine, the XF3, was manufactured in ...
which powers the Kawasaki T-4
The Kawasaki T-4 is a Japanese subsonic intermediate jet trainer aircraft developed and manufactured by the commercial conglomerate Kawasaki Heavy Industries. Its sole operator is the Japan Air Self-Defense Force (JASDF), in part due to historic ...
File:GTX-35VS Kaveri.jpg, GTRE GTX-35VS Kaveri
The GTRE GTX-35VS Kaveri is an afterburning turbofan project developed by the Gas Turbine Research Establishment (GTRE), a lab under the Defence Research and Development Organisation (DRDO) in Bengaluru, India. An Indian design, the Kaveri was o ...
developed by GTRE
Gas Turbine Research Establishment (GTRE) is a laboratory of the Defence Research and Development Organisation (DRDO). Located in Bengaluru, its primary function is research and development of aero gas-turbines for military aircraft. As a spin- ...
Afterburning turbofan
Since the 1970s, most jet fighter
Fighter aircraft are fixed-wing aircraft, fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air supremacy, air superiority of the battlespace. Domination o ...
engines have been low/medium bypass turbofans with a mixed exhaust, afterburner
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and comba ...
and variable area exit nozzle. An afterburner is a combustor located downstream of the turbine blades and directly upstream of the nozzle, which burns fuel from afterburner-specific fuel injectors. When lit, large volumes of fuel are burnt in the afterburner, raising the temperature of exhaust gases by a significant degree, resulting in a higher exhaust velocity/engine specific thrust. The variable geometry nozzle must open to a larger throat area to accommodate the extra volume and increased flow rate when the afterburner is lit. Afterburning is often designed to give a significant thrust boost for take off, transonic acceleration and combat maneuvers, but is very fuel intensive. Consequently, afterburning can be used only for short portions of a mission.
Unlike in the main engine, where stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
temperatures in the combustor have to be reduced before they reach the turbine, an afterburner at maximum fuelling is designed to produce stoichiometric temperatures at entry to the nozzle, about . At a fixed total applied fuel:air ratio, the total fuel flow for a given fan airflow will be the same, regardless of the dry specific thrust of the engine. However, a high specific thrust turbofan will, by definition, have a higher nozzle pressure ratio, resulting in a higher afterburning net thrust and, therefore, a lower afterburning specific fuel consumption (SFC). However, high specific thrust engines have a high dry SFC. The situation is reversed for a medium specific thrust afterburning turbofan: i.e., poor afterburning SFC/good dry SFC. The former engine is suitable for a combat aircraft which must remain in afterburning combat for a fairly long period, but has to fight only fairly close to the airfield (e.g. cross border skirmishes). The latter engine is better for an aircraft that has to fly some distance, or loiter for a long time, before going into combat. However, the pilot can afford to stay in afterburning only for a short period, before aircraft fuel reserves become dangerously low.
The first production afterburning turbofan engine was the Pratt & Whitney TF30
The Pratt & Whitney TF30 (company designation JTF10A) is a military low-bypass turbofan engine originally designed by Pratt & Whitney for the subsonic F6D Missileer fleet defense fighter, but this project was cancelled. It was later adapted with a ...
, which initially powered the F-111 Aardvark
The General Dynamics F-111 Aardvark is a retired supersonic, medium-range, Multirole combat aircraft, multirole combat aircraft. Production variants of the F-111 had roles that included attack aircraft, ground attack (e.g. Air interdiction, i ...
and F-14 Tomcat
The Grumman F-14 Tomcat is an American carrier-capable supersonic aircraft, supersonic, twinjet, twin-engine, two-seat, twin-tail, variable-sweep wing fighter aircraft. The Tomcat was developed for the United States Navy's Naval Fighter Experi ...
. Current low-bypass military turbofans include the Pratt & Whitney F119
The Pratt & Whitney F119, company designation PW5000, is an afterburning turbofan engine developed by Pratt & Whitney for the Advanced Tactical Fighter (ATF) program, which resulted in the Lockheed Martin F-22 Raptor. The engine delivers thrust ...
, the Eurojet EJ200
The Eurojet EJ200 is a military low-bypass 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 Eu ...
, the General Electric F110
The General Electric F110 is an afterburning turbofan jet engine produced by GE Aviation. The engine is derived from the General Electric F101 and shares its core design, and primarily powers tactical fighter aircraft. The F118 is a non-afterbu ...
, the Klimov RD-33
The Klimov RD-33 is a turbofan jet engine for a lightweight fighter jet that is the primary engine for the Mikoyan MiG-29 and CAC/PAC JF-17 Thunder. It was developed in OKB-117 led by S. P. Izotov (now OAO Klimov) from 1968 with production start ...
, and the Saturn AL-31
The Saturn AL-31 is a family of axial flow turbofan engines, developed by the Lyulka design bureau in the Soviet Union, now NPO Saturn in Russia, originally as a 12.5-tonne (122.6 kN, 27,560 lbf) powerplant for the Sukhoi Su-27 long ran ...
, all of which feature a mixed exhaust, afterburner and variable area propelling nozzle.
High-bypass turbofan
To further improve fuel economy and reduce noise, almost all of today's jet airliners and most military transport aircraft (e.g., the C-17) are powered by low-specific-thrust/high-bypass-ratio turbofans. These engines evolved from the high-specific-thrust/low-bypass-ratio turbofans used in such aircraft in the 1960s. Modern combat aircraft tend to use low-bypass ratio turbofans, and some military transport aircraft use turboprops
A turboprop is a turbine engine that drives an aircraft propeller
A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch t ...
.
Low specific thrust is achieved by replacing the multi-stage fan with a single-stage unit. Unlike some military engines, modern civil turbofans lack stationary inlet guide vanes in front of the fan rotor. The fan is scaled to achieve the desired net thrust.
The core (or gas generator) of the engine must generate enough power to drive the fan at its rated mass flow and pressure ratio. Improvements in turbine cooling/material technology allow for a higher (HP) turbine rotor inlet temperature, which allows a smaller (and lighter) core, potentially improving the core thermal efficiency. Reducing the core mass flow tends to increase the load on the LP turbine, so this unit may require additional stages to reduce the average stage loading
Stage Loading is a measure of the load on a turbomachinery stage, be it a part of a compressor, fan or turbine.
The parameter, which is non-dimensional, is defined as:
L = \frac
where:
Imperial Units (SI Units)
g accelerat ...
and to maintain LP turbine efficiency. Reducing core flow also increases bypass ratio. Bypass ratios greater than 5:1 are increasingly common; the Pratt & Whitney PW1000G
The Pratt & Whitney PW1000G, also called the Geared Turbofan (GTF), is a high-bypass geared turbofan engine family produced by Pratt & Whitney.
After many demonstrators, the program was launched with the Mitsubishi MRJ's PW1200G in March 2008, a ...
, which entered commercial service in 2016, attains 12.5:1.
Further improvements in core thermal efficiency can be achieved by raising the overall pressure ratio of the core. Improvements in blade aerodynamics can reduce the number of extra compressor stages required, and variable geometry (i.e., stators) enable high-pressure-ratio compressors to work surge-free at all throttle settings.
The first (experimental) high-bypass turbofan engine was the AVCO-Lycoming PLF1A-2, a T55 turboshaft-derived engine that was first run in February 1962. The PLF1A-2 had a geared fan stage, produced a static thrust of , and had a bypass ratio of 6:1. The General Electric TF39
The General Electric TF39 was a high-bypass turbofan engine that was developed to power the Lockheed C-5 Galaxy. The TF39 was the first high-power, high-bypass jet engine developed. The TF39 was further developed into the CF6 series of engines, a ...
became the first production model, designed to power the Lockheed C-5 Galaxy
The Lockheed C-5 Galaxy is a large military transport aircraft designed and built by Lockheed, and now maintained and upgraded by its successor, Lockheed Martin. It provides the United States Air Force (USAF) with a heavy intercontinental-ran ...
military transport aircraft. The civil General Electric CF6
The General Electric CF6, US military designations F103 and F138, is a family of high-bypass turbofan engines produced by GE Aviation. Based on the TF39, the first high-power high-bypass jet engine, the CF6 powers a wide variety of civilian ai ...
engine used a derived design. Other high-bypass turbofans are the Pratt & Whitney JT9D
The Pratt & Whitney JT9D engine was the first high bypass ratio jet engine to power a wide-body airliner. Its initial application was the Boeing 747-100, the original "Jumbo Jet". It was Pratt & Whitney's first high-bypass-ratio turbofan.
Develo ...
, the three-shaft Rolls-Royce RB211
The Rolls-Royce RB211 is a British family of high-bypass turbofan engines made by Rolls-Royce. The engines are capable of generating of thrust. The RB211 engine was the first production three-spool engine, and turned Rolls-Royce from a signif ...
and the CFM International CFM56
The CFM International CFM56 (U.S. military designation F108) series is a Franco-American family of high-bypass turbofan aircraft engines made by CFM International (CFMI), with a thrust range of . CFMI is a 50–50 joint-owned company of Safran ...
; also the smaller TF34
The General Electric TF34 is an American military turbofan engine used on the A-10 Thunderbolt II and S-3 Viking.
Design and development
Developed by GE Aircraft Engines during the late 1960s, the original engine comprises a single stage fan, ...
. More recent large high-bypass turbofans include the Pratt & Whitney PW4000
The Pratt & Whitney PW4000 is a family of dual-spool, axial-flow, high-bypass turbofan aircraft engines produced by Pratt & Whitney as the successor to the JT9D.
It was first run in April 1984, was FAA certified in July 1986, and was introdu ...
, the three-shaft Rolls-Royce Trent, the General Electric GE90/GEnx and the GP7000, produced jointly by GE and P&W.
The lower the specific thrust of a turbofan, the lower the mean jet outlet velocity, which in turn translates into a high Thrust lapse, thrust lapse rate (i.e. decreasing thrust with increasing flight speed). See technical discussion below, item 2. Consequently, an engine sized to propel an aircraft at high subsonic flight speed (e.g., Mach 0.83) generates a relatively high thrust at low flight speed, thus enhancing runway performance. Low specific thrust engines tend to have a high bypass ratio, but this is also a function of the temperature of the turbine system.
The turbofans on twin-engined transport aircraft produce enough take-off thrust to continue a take-off on one engine if the other engine shuts down after a critical point in the take-off run. From that point on the aircraft has less than half the thrust compared to two operating engines because the non-functioning engine is a source of drag. Modern twin engined airliners normally climb very steeply immediately after take-off. If one engine shuts down, the climb-out is much shallower, but sufficient to clear obstacles in the flightpath.
The Soviet Union's engine technology was less advanced than the West's, and its first wide-body aircraft, the Ilyushin Il-86, was powered by low-bypass engines. The Yakovlev Yak-42, a medium-range, rear-engined aircraft seating up to 120 passengers, introduced in 1980, was the first Soviet aircraft to use high-bypass engines.
File:Sam146 1.jpg, PowerJet SaM146 which powers Sukhoi Superjet 100
File:Ge cf6 turbofan.jpg, General Electric CF6
The General Electric CF6, US military designations F103 and F138, is a family of high-bypass turbofan engines produced by GE Aviation. Based on the TF39, the first high-power high-bypass jet engine, the CF6 powers a wide variety of civilian ai ...
which powers the Airbus A300, Boeing 747, Douglas DC-10 and other aircraft
File:Airbus Lagardère - Trent 900 engine MSN100 (6).JPG, Rolls-Royce Trent 900, powering the Airbus A380
File:PW4000-112 (cropped).jpg, Pratt & Whitney PW4000
The Pratt & Whitney PW4000 is a family of dual-spool, axial-flow, high-bypass turbofan aircraft engines produced by Pratt & Whitney as the successor to the JT9D.
It was first run in April 1984, was FAA certified in July 1986, and was introdu ...
, powering the Boeing 777, MD-11 and Airbus A330
File:CFM56 P1220759.jpg, The CFM International CFM56, CFM56 which powers the Boeing 737, the Airbus A320 and other aircraft
File:Airbus Lagardère - GP7200 engine MSN108 (1).JPG, Engine Alliance GP7000 turbofan for the Airbus A380
File:Engine Il-96 "Aeroflot" (3447358279).jpg, Aviadvigatel PS-90 which powers the Ilyushin Il-96, Tupolev Tu-204, Ilyushin Il-76
The Ilyushin Il-76 (russian: Илью́шин Ил-76; NATO reporting name: Candid) is a multi-purpose, fixed-wing, four-engine turbofan strategic airlifter designed by the Soviet Union's Ilyushin design bureau. It was first planned as a comm ...
File:ALF502.JPG, Lycoming ALF 502 which powers the British Aerospace 146
File:MAKS Airshow 2013 (Ramenskoye Airport, Russia) (524-34).jpg, Aviadvigatel PD-14 which will be used on the Irkut MC-21
File:D-436-148 MAKS-2009.jpg, Three shaft Progress D-436
File:Trent 1000 GoodwinHall VirginiaTech.jpg, Trent 1000 powering the Boeing 787
The Boeing 787 Dreamliner is an American wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes.
After dropping its unconventional Sonic Cruiser project, Boeing announced the conventional 7E7 on January 29, 2003, ...
File:General Electric GE90 displayed at Farnborough Air Show 2008.jpg, GE90 powering the Boeing 777, the most powerful aircraft engine
Turbofan configurations
Turbofan engines come in a variety of engine configurations. For a given engine cycle (i.e., same airflow, bypass ratio, fan pressure ratio, overall pressure ratio and HP turbine rotor inlet temperature), the choice of turbofan configuration has little impact upon the design point performance (e.g., net thrust, SFC), as long as overall component performance is maintained. Off-design performance and stability is, however, affected by engine configuration.
The basic element of a turbofan is a spool, a single combination of fan/compressor, turbine and shaft rotating at a single speed. For a given pressure ratio, the surge margin can be increased by two different design paths:
# Splitting the compressor into two smaller spools rotating at different speeds, as with the Pratt & Whitney J57; or
# Making the stator vane pitch adjustable, typically in the front stages, as with the J79.
Most modern western civil turbofans employ a relatively high-pressure-ratio high-pressure (HP) compressor, with many rows of variable stators to control surge margin at low rpm. In the three-spool Rolls-Royce RB211, RB211/Rolls-Royce Trent, Trent the core compression system is split into two, with the IP compressor, which supercharges the HP compressor, being on a different coaxial shaft and driven by a separate (IP) turbine. As the HP compressor has a modest pressure ratio its speed can be reduced surge-free, without employing variable geometry. However, because a shallow IP compressor working line is inevitable, the IPC has one stage of variable geometry on all variants except the −535, which has none.
Single-shaft turbofan
Although far from common, the single-shaft turbofan is probably the simplest configuration, comprising a fan and high-pressure compressor driven by a single turbine unit, all on the same spool. The Snecma M53, which powers Dassault Mirage 2000 fighter aircraft, is an example of a single-shaft turbofan. Despite the simplicity of the turbomachinery configuration, the M53 requires a variable area mixer to facilitate part-throttle operation.
Aft-fan turbofan
One of the earliest turbofans was a derivative of the General Electric J79 turbojet, known as the CJ805-23, which featured an integrated aft fan/low-pressure (LP) turbine unit located in the turbojet exhaust jetpipe. Hot gas from the turbojet turbine exhaust expanded through the LP turbine, the fan blades being a radial extension of the turbine blades. This arrangement introduces an additional gas leakage path compared to a front-fan configuration and was a problem with this engine with higher-pressure turbine gas leaking into the fan airflow. An aft-fan configuration was later used for the General Electric GE36 UDF (propfan) demonstrator of the early 1980s.
In 1971 a concept was put forward by the NASA Lewis Research Center for a supersonic transport engine which operated as an aft-fan turbofan at take-off and subsonic speeds and a turbojet at higher speeds. This would give the low noise and high thrust characteristics of a turbofan at take-off, together with turbofan high propulsive efficiency at subsonic flight speeds. It would have the high propulsive efficiency of a turbojet at supersonic cruise speeds.
Basic two-spool
Many turbofans have at least basic two-spool configuration where the fan is on a separate low pressure (LP) spool, running concentrically with the compressor or high pressure (HP) spool; the LP spool runs at a lower angular velocity, while the HP spool turns faster and its compressor further compresses part of the air for combustion. The Rolls-Royce BR700, BR710 is typical of this configuration. At the smaller thrust sizes, instead of all-axial blading, the HP compressor configuration may be axial-centrifugal (e.g., CFE CFE738), double-centrifugal or even Mixed flow compressor, diagonal/centrifugal (e.g. Pratt & Whitney Canada PW600).
Boosted two-spool
Higher overall pressure ratios can be achieved by either raising the HP compressor pressure ratio or adding compressor (non-bypass) stages or T-stages to the LP spool, between the fan and the HP compressor, to boost the latter. All of the large American turbofans (e.g. General Electric CF6
The General Electric CF6, US military designations F103 and F138, is a family of high-bypass turbofan engines produced by GE Aviation. Based on the TF39, the first high-power high-bypass jet engine, the CF6 powers a wide variety of civilian ai ...
, GE90, General Electric GE9X, GE9X and GEnx plus Pratt & Whitney JT9D
The Pratt & Whitney JT9D engine was the first high bypass ratio jet engine to power a wide-body airliner. Its initial application was the Boeing 747-100, the original "Jumbo Jet". It was Pratt & Whitney's first high-bypass-ratio turbofan.
Develo ...
and Pratt & Whitney PW4000, PW4000) feature T-stages. The Rolls-Royce BR715 is a non-American example of this. The high bypass ratios used in modern civil turbofans tend to reduce the relative diameter of the T-stages, reducing their mean tip speed. Consequently, more T-stages are required to develop the necessary pressure rise.
Three-spool
Rolls-Royce chose a three-spool configuration for their large civil turbofans (i.e. the Rolls-Royce RB211, RB211 and Rolls-Royce Trent, Trent families), where the T-stages of the boosted two-spool configuration are separated into a separate intermediate pressure (IP) spool, driven by its own turbine. The first three-spool engine was the earlier Rolls-Royce RB.203 Trent of 1967.
The Garrett ATF3, powering the Dassault Falcon 20
The Dassault Falcon 20 is a French business jet developed and manufactured by Dassault Aviation. The first business jet developed by the firm, it became the first of a family of business jets to be produced under the same name; of these, both ...
business jet, has an unusual three spool layout with an aft spool not concentric with the two others.
Ivchenko-Progress, Ivchenko Design Bureau chose the same configuration as Rolls-Royce for their Lotarev D-36 engine, followed by Progress D-18T, Lotarev/Progress D-18T and Progress D-436.
The Turbo-Union RB199 military turbofan also has a three-spool configuration, as do the military Kuznetsov NK-25 and Kuznetsov NK-32, NK-321.
Geared fan
As bypass ratio increases, the fan blade tip speed increases relative to the LPT blade speed. This will reduce the LPT blade speed, requiring more turbine stages to extract enough energy to drive the fan. Introducing a epicyclic gearing, (planetary) reduction gearbox, with a suitable gear ratio, between the LP shaft and the fan enables both the fan and LP turbine to operate at their optimum speeds. Examples of this configuration are the long-established Garrett TFE731, the Honeywell ALF 502/507, and the recent Pratt & Whitney PW1000G
The Pratt & Whitney PW1000G, also called the Geared Turbofan (GTF), is a high-bypass geared turbofan engine family produced by Pratt & Whitney.
After many demonstrators, the program was launched with the Mitsubishi MRJ's PW1200G in March 2008, a ...
.
Military turbofans
Most of the configurations discussed above are used in civilian turbofans, while modern military turbofans (e.g., Snecma M88) are usually basic two-spool.
High-pressure turbine
Most civil turbofans use a high-efficiency, 2-stage HP turbine to drive the HP compressor. The CFM International CFM56
The CFM International CFM56 (U.S. military designation F108) series is a Franco-American family of high-bypass turbofan aircraft engines made by CFM International (CFMI), with a thrust range of . CFMI is a 50–50 joint-owned company of Safran ...
uses an alternative approach: a single-stage, high-work unit. While this approach is probably less efficient, there are savings on cooling air, weight and cost.
In the Rolls-Royce RB211, RB211 and Rolls-Royce Trent, Trent 3-spool engine series, the HP compressor pressure ratio is modest so only a single HP turbine stage is required. Modern military turbofans also tend to use a single HP turbine stage and a modest HP compressor.
Low-pressure turbine
Modern civil turbofans have multi-stage LP turbines (anywhere from 3 to 7). The number of stages required depends on the engine cycle bypass ratio and the boost (on boosted two-spools). A geared fan may reduce the number of required LPT stages in some applications.[ C. Riegler, C. Bichlmaier:, 1st CEAS European Air and Space Conference, 10–13 September 2007, Berlin, Germany] Because of the much lower bypass ratios employed, military turbofans require only one or two LP turbine stages.
Overall performance
Cycle improvements
Consider a mixed turbofan with a fixed bypass ratio and airflow. Increasing the overall pressure ratio of the compression system raises the combustor entry temperature. Therefore, at a fixed fuel flow there is an increase in (HP) turbine rotor inlet temperature. Although the higher temperature rise across the compression system implies a larger temperature drop over the turbine system, the mixed nozzle temperature is unaffected, because the same amount of heat is being added to the system. There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio, causing an increase in the hot mixer entry pressure. Consequently, net thrust increases, whilst specific fuel consumption (fuel flow/net thrust) decreases. A similar trend occurs with unmixed turbofans.
Turbofan engines can be made more fuel efficient by raising overall pressure ratio and turbine rotor inlet temperature in unison. However, better turbine materials or improved vane/blade cooling are required to cope with increases in both turbine rotor inlet temperature and compressor delivery temperature. Increasing the latter may require better compressor materials.
The overall pressure ratio can be increased by improving fan (or) LP compressor pressure ratio or HP compressor pressure ratio. If the latter is held constant, the increase in (HP) compressor delivery temperature (from raising overall pressure ratio) implies an increase in HP mechanical speed. However, stressing considerations might limit this parameter, implying, despite an increase in overall pressure ratio, a reduction in HP compressor pressure ratio.
According to simple theory, if the ratio of turbine rotor inlet temperature/(HP) compressor delivery temperature is maintained, the HP turbine throat area can be retained. However, this assumes that cycle improvements are obtained, while retaining the datum (HP) compressor exit flow function (non-dimensional flow). In practice, changes to the non-dimensional speed of the (HP) compressor and cooling bleed extraction would probably make this assumption invalid, making some adjustment to HP turbine throat area unavoidable. This means the HP turbine nozzle guide vanes would have to be different from the original. In all probability, the downstream LP turbine nozzle guide vanes would have to be changed anyway.
Thrust growth
Thrust growth is obtained by increasing core power. There are two basic routes available:
# hot route: increase HP turbine rotor inlet temperature
# cold route: increase core mass flow
Both routes require an increase in the combustor fuel flow and, therefore, the heat energy added to the core stream.
The hot route may require changes in turbine blade/vane materials or better blade/vane cooling. The cold route can be obtained by one of the following:
# adding T-stages to the LP/IP compression
# adding a zero-stage to the HP compression
# improving the compression process, without adding stages (e.g. higher fan hub pressure ratio)
all of which increase both overall pressure ratio and core airflow.
Alternatively, the core size can be increased, to raise core airflow, without changing overall pressure ratio. This route is expensive, since a new (upflowed) turbine system (and possibly a larger IP compressor) is also required.
Changes must also be made to the fan to absorb the extra core power. On a civil engine, jet noise considerations mean that any significant increase in take-off thrust must be accompanied by a corresponding increase in fan mass flow (to maintain a T/O specific thrust of about 30 lbf/lb/s).
Technical discussion
# Specific thrust (net thrust/intake airflow) is an important parameter for turbofans and jet engines in general. Imagine a fan (driven by an appropriately sized electric motor) operating within a pipe, which is connected to a propelling nozzle. It is fairly obvious, the higher the fan pressure ratio (fan discharge pressure/fan inlet pressure), the higher the jet velocity and the corresponding specific thrust. Now imagine we replace this set-up with an equivalent turbofan – same airflow and same fan pressure ratio. Obviously, the core of the turbofan must produce sufficient power to drive the fan via the low-pressure (LP) turbine. If we choose a low (HP) turbine inlet temperature for the gas generator, the core airflow needs to be relatively high to compensate. The corresponding bypass ratio is therefore relatively low. If we raise the turbine inlet temperature, the core airflow can be smaller, thus increasing bypass ratio. Raising turbine inlet temperature tends to increase thermal efficiency and, therefore, improve fuel efficiency.
# Naturally, as altitude increases, there is a decrease in air density and, therefore, the net thrust of an engine. There is also a flight speed effect, termed thrust lapse rate. Consider the approximate equation for net thrust again: With a high specific thrust (e.g., fighter) engine, the jet velocity is relatively high, so intuitively one can see that increases in flight velocity have less of an impact upon net thrust than a medium specific thrust (e.g., trainer) engine, where the jet velocity is lower. The impact of thrust lapse rate upon a low specific thrust (e.g., civil) engine is even more severe. At high flight speeds, high-specific-thrust engines can pick up net thrust through the ram rise in the intake, but this effect tends to diminish at supersonic speeds because of shock wave losses.
# Thrust growth on civil turbofans is usually obtained by increasing fan airflow, thus preventing the jet noise becoming too high. However, the larger fan airflow requires more power from the core. This can be achieved by raising the overall pressure ratio (combustor inlet pressure/intake delivery pressure) to induce more airflow into the core and by increasing turbine inlet temperature. Together, these parameters tend to increase core thermal efficiency and improve fuel efficiency.
# Some high-bypass-ratio civil turbofans use an extremely low area ratio (less than 1.01), convergent-divergent, nozzle on the bypass (or mixed exhaust) stream, to control the fan working line. The nozzle acts as if it has variable geometry. At low flight speeds the nozzle is unchoked (less than a Mach number of unity), so the exhaust gas speeds up as it approaches the throat and then slows down slightly as it reaches the divergent section. Consequently, the nozzle exit area controls the fan match and, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake increases nozzle pressure ratio to the point where the throat becomes choked (M=1.0). Under these circumstances, the throat area dictates the fan match and, being smaller than the exit, pushes the fan working line slightly towards surge. This is not a problem, since fan surge margin is much better at high flight speeds.
# The off-design behaviour of turbofans is illustrated under compressor map A compressor map is a chart which shows the performance of a turbomachinery compressor. This type of compressor is used in gas turbine engines, for supercharging reciprocating engines and for industrial processes, where it is known as a dynamic comp ...
and turbine map.
# Because modern civil turbofans operate at low specific thrust, they require only a single fan stage to develop the required fan pressure ratio. The desired overall pressure ratio for the engine cycle is usually achieved by multiple axial stages on the core compression. Rolls-Royce tend to split the core compression into two with an intermediate pressure (IP) supercharging the HP compressor, both units being driven by turbines with a single stage, mounted on separate shafts. Consequently, the HP compressor need develop only a modest pressure ratio (e.g., ~4.5:1). US civil engines use much higher HP compressor pressure ratios (e.g., ~23:1 on the General Electric GE90) and tend to be driven by a two-stage HP turbine. Even so, there are usually a few IP axial stages mounted on the LP shaft, behind the fan, to further supercharge the core compression system. Civil engines have multi-stage LP turbines, the number of stages being determined by the bypass ratio, the amount of IP compression on the LP shaft and the LP turbine blade speed.
# Because military engines usually have to be able to fly very fast at sea level, the limit on HP compressor delivery temperature is reached at a fairly modest design overall pressure ratio, compared with that of a civil engine. Also the fan pressure ratio is relatively high, to achieve a medium to high specific thrust. Consequently, modern military turbofans usually have only 5 or 6 HP compressor stages and require only a single-stage HP turbine. Low-bypass-ratio military turbofans usually have one LP turbine stage, but higher bypass ratio engines need two stages. In theory, by adding IP compressor stages, a modern military turbofan HP compressor could be used in a civil turbofan derivative, but the core would tend to be too small for high thrust applications.
Improvements
Aerodynamic modelling
Aerodynamics is a mix of Speed of sound, subsonic, transonic and supersonic airflow on a single fan/gas compressor blade in a modern turbofan. The airflow past the blades has to be maintained within close angular limits to keep the air flowing against an increasing pressure. Otherwise the air will come back out of the intake.
The FADEC, Full Authority Digital Engine Control (FADEC) needs accurate data for controlling the engine. The critical turbine inlet temperature (TIT) is too harsh an environment, at and , for reliable temperature sensor, sensors. Therefore, during development of a new engine type a relation is established between a more easily measured temperature like exhaust gas temperature and the TIT. Monitoring the exhaust gas temperature is then used to make sure the engine does not run too hot.[
]
Blade technology
A turbine blade is subjected to , at and a centrifugal force of , well above the point of plastic deformation and even above the melting point.
Exotic alloys, sophisticated air cooling schemes and special mechanical design are needed to keep the physical stresses within the strength of the material.
Rotating seals must withstand harsh conditions for 10 years, 20,000 missions and rotating at 10 to 20,000 rpm.[
]
Fan blades
Fan blades have been growing as jet engines have been getting bigger: each fan blade carries the equivalent of nine double-decker buses and swallows air the equivalent volume of a squash court every second.
Advances in computational fluid dynamics (CFD) modelling have permitted complex, 3D curved shapes with very wide Chord (aeronautics), chord, keeping the fan capabilities while minimizing the blade count to lower costs.
Coincidentally, the bypass ratio
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for ev ...
grew to achieve higher propulsive efficiency and the fan diameter increased.[
Rolls-Royce pioneered the hollow, titanium wide-chord fan blade in the 1980s for aerodynamic efficiency and foreign object damage resistance in the RB211 then for the Rolls-Royce Trent, Trent.
GE Aviation introduced carbon fiber composite fan blades on the GE90 in 1995, manufactured today with a carbon-fiber tape, carbon-fiber tape-layer process.
GE partner Safran developed a 3D weaving, 3D woven technology with Albany Engineered Composites, Albany Composites for the CFM56 and CFM LEAP engines.]
Future progress
Engine cores are shrinking as they are operating at higher Overall pressure ratio, pressure ratios and becoming more efficient, and become smaller compared to the fan as bypass ratios increase.
Blade tip clearances are harder to maintain at the exit of the high-pressure compressor where blades are high or less; Structural system, backbone bending further affects clearance control as the core is proportionately longer and thinner and the fan to low-pressure turbine driveshaft is in constrained space within the core.
For Pratt & Whitney VP technology and environment Alan H. Epstein, Alan Epstein "Over the history of commercial aviation, we have gone from 20% to 40% [cruise efficiency], and there is a consensus among the engine community that we can probably get to 60%".
Geared turbofans and further fan Overall pressure ratio, pressure ratio reductions will continue to improve propulsive efficiency.
The second phase of the FAA's Continuous Lower Energy, Emissions and Noise (CLEEN) program is targeting for the late 2020s reductions of 33% fuel burn, 60% emissions and 32 dB EPNdb noise compared with the 2000s state-of-the-art.
In summer 2017 at NASA Glenn Research Center in Cleveland, Ohio, Pratt has finished testing a very-low-pressure-ratio fan on a PW1000G, resembling an open rotor with fewer blades than the PW1000G's 20.[
The weight and size of the ]nacelle
A nacelle ( ) is a "streamlined body, sized according to what it contains", such as an engine, fuel, or equipment on an aircraft. When attached by a pylon entirely outside the airframe, it is sometimes called a pod, in which case it is attached ...
would be reduced by a short duct inlet, imposing higher aerodynamic turning loads on the blades and leaving less space for soundproofing, but a lower-pressure-ratio fan is slower.
UTC Aerospace Systems Aerostructures will have a full-scale ground test in 2019 of its low-drag Integrated Propulsion System with a thrust reverser, improving fuel burn by 1% and with 2.5-3 EPNdB lower noise.[
Safran can probably deliver another 10–15% in fuel efficiency through the mid-2020s before reaching an asymptote, and next will have to introduce a breakthrough: to increase the ]bypass ratio
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for ev ...
to 35:1 instead of 11:1 for the CFM LEAP, it is demonstrating a counterrotating open rotor unducted fan (propfan) in Istres, France, under the European Clean Sky technology program.
Computational fluid dynamics, Modeling advances and high specific strength materials may help it succeed where previous attempts failed.
When noise levels will be within current standards and similar to the Leap engine, 15% lower fuel burn will be available and for that Safran is testing its controls, vibration and operation, while airframe integration is still challenging.[
For GE Aviation, the energy density of jet fuel still maximises the Breguet range equation and higher pressure ratio cores; lower pressure ratio fans, low-loss inlets and lighter structures can further improve thermal, transfer and propulsive efficiency.
Under the U.S. Air Force’s Adaptive Engine Transition Program, adaptive thermodynamic cycles will be used for the sixth-generation jet fighter, based on a modified Brayton cycle and Constant volume combustion.
Additive manufacturing in the General Electric Advanced Turboprop, advanced turboprop will reduce weight by 5% and fuel burn by 20%.][
Rotating and static ceramic matrix composite (CMC) parts operates hotter than metal and are one-third its weight.
With $21.9 million from the Air Force Research Laboratory, GE is investing $200 million in a CMC facility in Huntsville, Alabama, in addition to its Asheville, North Carolina site, mass-producing silicon carbide matrix with silicon-carbide fibers in 2018.
CMCs will be used ten times more by the mid-2020s: the CFM LEAP requires 18 CMC turbine shrouds per engine and the GE9X will use it in the combustor and for 42 HP turbine nozzles.][
Rolls-Royce Plc aim for a 60:1 pressure ratio core for the 2020s Ultrafan and began ground tests of its gear for and 15:1 bypass ratios.
Nearly ]stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
turbine entry temperatures approaches the theoretical limit and its impact on emissions has to be balanced with environmental performance goals.
Open rotors, lower pressure ratio fans and potentially distributed propulsion offer more room for better propulsive efficiency.
Exotic cycles, heat exchangers and pressure gain/constant volume combustion can improve thermodynamic efficiency.
Additive manufacturing could be an enabler for intercooler and recuperators.
Closer airframe integration and Hybrid electric vehicle#Aircraft, hybrid or electric aircraft can be combined with gas turbines.[
Current Rolls-Royce engines have a 72–82% propulsive efficiency and 42–49% thermal efficiency for a Thrust specific fuel consumption, TSFC at Mach 0.8, and aim for theoretical limits of 95% for open rotor propulsive efficiency and 60% for thermal efficiency with stoichiometric turbine entry temperature and 80:1 ]overall pressure ratio
In aeronautical engineering, overall pressure ratio, or overall compression ratio, is the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine. The terms ''compression ratio'' and ''pressure ...
for a TSFC
As teething troubles may not show up until several thousand hours, the latest turbofans technical problems disrupt airlines operations and aerospace manufacturer, manufacturers deliveries while production rates are rising sharply.
Trent 1000 cracked blades aircraft on ground, grounded almost 50 Boeing 787
The Boeing 787 Dreamliner is an American wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes.
After dropping its unconventional Sonic Cruiser project, Boeing announced the conventional 7E7 on January 29, 2003, ...
s and reduced ETOPS to 2.3 hours down from 5.5, costing Rolls-Royce plc almost $950 million.
PW1000G knife-edge seal fractures have caused Pratt & Whitney to fall way behind in deliveries, leaving about 100 engineless A320neos waiting for their powerplants.
The CFM LEAP introduction was smoother but a ceramic composite Turbine coating is prematurely lost, necessitating a new design, causing 60 A320neo engine removal for modification, as deliveries are up to six weeks late.
On a widebody, Safran estimates 5–10% of fuel could be saved by reducing power intake for hydraulic systems, while swapping to electrical power could save 30% of weight, as initiated on the Boeing 787
The Boeing 787 Dreamliner is an American wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes.
After dropping its unconventional Sonic Cruiser project, Boeing announced the conventional 7E7 on January 29, 2003, ...
, while Rolls-Royce plc hopes for up to 5%.
Manufacturers
The turbofan engine market is dominated by GE Aircraft Engines, General Electric, Rolls-Royce plc and Pratt & Whitney, in order of market share. General Electric and SNECMA of France have a joint venture, CFM International. Pratt & Whitney also have a joint venture, International Aero Engines with Japanese Aero Engine Corporation and MTU Aero Engines of Germany, specializing in engines for the Airbus A320 family. Pratt & Whitney and General Electric have a joint venture, Engine Alliance selling a range of engines for aircraft such as the Airbus A380.
For airliners and cargo aircraft, the in-service fleet in 2016 is 60,000 engines and should grow to 103,000 in 2035 with 86,500 deliveries according to Flight Global. A majority will be medium-thrust engines for narrow-body aircraft with 54,000 deliveries, for a fleet growing from 28,500 to 61,000. High-thrust engines for wide-body aircraft, worth 40–45% of the market by value, will grow from 12,700 engines to over 21,000 with 18,500 deliveries. The regional jet engines below 20,000 lb (89 kN) fleet will grow from 7,500 to 9,000 and the fleet of turboprop
A turboprop is a turbine engine that drives an aircraft propeller.
A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle. Air enters the intake and is compressed by the compressor. Fuel ...
s for airliners will increase from 9,400 to 10,200. The manufacturers market share should be led by CFM with 44% followed by Pratt & Whitney with 29% and then Rolls-Royce and General Electric with 10% each.
Commercial turbofans in production
Extreme bypass jet engines
In the 1970s, Rolls-Royce/SNECMA tested a M45SD-02 turbofan fitted with variable-pitch fan blades to improve handling at ultralow fan pressure ratios and to provide thrust reverse down to zero aircraft speed. The engine was aimed at ultraquiet STOL aircraft operating from city-centre airports.
In a bid for increased efficiency with speed, a development of the ''turbofan'' and ''turboprop'' known as a propfan engine was created that had an unducted fan. The fan blades are situated outside of the duct, so that it appears like a turboprop with wide scimitar-like blades. Both General Electric and Pratt & Whitney/Allison demonstrated propfan engines in the 1980s. Excessive cabin noise and relatively cheap jet fuel prevented the engines being put into service. The Progress D-27 propfan, developed in the U.S.S.R., was the only propfan engine equipped on a production aircraft.
Terminology
; Afterburner
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and comba ...
: jetpipe equipped for afterburning[The Cambridge Aerospace Dictionary,Bill Gunston 2004,]
; Augmentor: afterburner for turbofan with burning in hot and cold flows
; Bypass: that part of the engine as distinct from the core in terms of components and airflow, eg that part of fan blading (fan outer) and stators which pass bypass air, bypass duct, bypass nozzle
; Bypass ratio: bypass air mass flow /core air mass flow
; Core: that part of the engine as distinct from the bypass in terms of components and airflow, eg core cowl, core nozzle, core airflow and associated machinery, combustor and fuel system
; Core power: also known as "available energy" or "gas horsepower". It is used to measure the theoretical (isentropic expansion) shaft work available from a gas generator or core by expanding hot, high pressure gas to ambient pressure. Since the power depends on the pressure and temperature of the gas (and the ambient pressure) a related figure of merit for thrust-producing engines is one which measures the thrust-producing potential from hot, high pressure gas and known as "stream thrust". It is obtained by calculating the velocity obtained with isentropic expansion to atmospheric pressure. The significance of the thrust obtained appears when multiplied by the aircraft velocity to give the thrust work. The thrust work which is potentially available is far less than the gas horsepower due to the increasing waste in the exhaust kinetic energy with increasing pressure and temperature before expansion to atmospheric pressure. The two are related by the propulsive efficiency, a measure of the energy wasted as a result of producing a force (ie thrust) in a fluid by increasing the speed (ie momentum) of the fluid.
; Dry: engine ratings/ throttle lever positions below afterburning selection
; EGT: exhaust gas temperature
; EPR: engine pressure ratio
; Fan: turbofan LP compressor
; Fanjet: turbofan or aircraft powered by turbofan (colloquial)
; Fan pressure ratio: fan outlet total pressure/fan inlet total pressure
; Flex temp: At reduced take-off weights commercial aircraft can use reduced thrust which increases engine life and reduces maintenance costs. Flex temperature is a higher than actual outside air temperature (OAT) which is input to the engine monitoring computer to achieve the required reduced thrust (also known as "assumed temperature thrust reduction").
; Gas generator: that part of the engine core which provides the hot, high pressure gas for fan-driving turbines (turbofan), for propelling nozzles (turbojet), for propeller- and rotor-driving turbines (turboprop and turboshaft), for industrial and marine power turbines
; HP: high-pressure
; Intake ram drag: Loss in momentum of engine stream tube from freestream to intake entrance, ie amount of energy imparted to air required to accelerate air from a stationary atmosphere to aircraft speed.
; IEPR: integrated engine pressure ratio
; IP: intermediate pressure
; LP: low-pressure
; Net thrust: nozzle thrust in stationary air (gross thrust) – engine stream tube ram drag (loss in momentum from freestream to intake entrance, ie amount of energy imparted to air required to accelerate air from a stationary atmosphere to aircraft speed). This is the thrust acting on the airframe.
; Overall pressure ratio: combustor inlet total pressure/intake delivery total pressure
; Overall efficiency: thermal efficiency * propulsive efficiency
; Propulsive efficiency: propulsive power/rate of production of propulsive kinetic energy (maximum propulsive efficiency occurs when jet velocity equals flight velocity, which implies zero net thrust!)
; Thrust specific fuel consumption, Specific fuel consumption (SFC): total fuel flow/net thrust (proportional to flight velocity/overall thermal efficiency)
; Spooling up: increase in RPM (colloquial)
; Stage loading: For a turbine, the purpose of which is to produce power, the loading is an indicator of power developed per lb/sec of gas (specific power). A turbine stage turns the gas from an axial direction and speeds it up (in the nozzle guide vanes) to turn the rotor most effectively ( rotor blades must produce high lift), the proviso being that this is done efficiently, ie with acceptable losses. For a compressor stage, the purpose of which is to produce a pressure rise, a diffusion process is used. How much diffusion may be allowed ( and pressure rise obtained) before unacceptable flow separation occurs (ie losses) may be regarded as a loading limit.
; Static pressure: pressure of the fluid which is associated not with its motion but with its state or, alternatively, pressure due to the random motion of the fluid molecules that would be felt or measured if moving with the flow[Introduction To Aerospace Engineering With A Flight Test Perspective,Stephen Corda 2017,, p.185]
; Specific thrust: net thrust/intake airflow
; Thermal efficiency: rate of production of propulsive kinetic energy/fuel power
; Total fuel flow: combustor (plus any afterburner) fuel flow rate (e.g., lb/s or g/s)
; Total pressure: static pressure plus kinetic energy term
; Turbine rotor inlet temperature: maximum cycle temperature, ie temperature at which work transfer takes place
See also
* Jet engine
* Turbojet
* Turboprop
* Turboshaft
* Propfan
* Axial fan design
* Variable cycle engine
* Jet engine performance
* Gas turbine
* Turbine engine failure
References
External links
*Wikibooks:Jet Propulsion, Wikibooks: Jet propulsion
*
*
*
* and previous series
{{Heat engines, state=uncollapsed
Gas turbines
Jet engines
Turbofan engines,