Editing Turbofan

{{short description|Airbreathing jet engine designed to provide thrust by driving a fan}}
{{distinguish|propfan|turboprop}}
{{About|the mechanism used in jets|the turbo like air pressure blower sometimes misnamed Turbo fan|Centrifugal fan}}
[[File:Turbofan3 Labelled.gif|thumb|360px|alt=Animation of turbofan, which shows flow of air and the spinning of blades.|Animation of a 2-spool, high-bypass turbofan {{ordered list
    |list_style_type=upper-alpha
    |1=<!--A-->Low-pressure spool
    |2=<!--B-->High-pressure spool
    |3=<!--C-->Stationary components
}}{{ordered list
    |1=Nacelle
    |2=Fan
    |3=Low-pressure compressor
    |4=High-pressure compressor
    |5=Combustion chamber
    |6=High-pressure turbine
    |7=Low-pressure turbine
    |8=Core nozzle
    |9=Fan nozzle
}}]]
{{Seriesbox aircraft propulsion}}

A '''turbofan''' or '''fanjet''' is a type of [[airbreathing jet engine]] that is widely used in [[aircraft engine|aircraft propulsion]]. The word "turbofan" is a combination of references to the preceding generation engine technology of the [[turbojet]] and the additional fan stage. It consists of a [[gas turbine engine]] which achieves [[mechanical energy]] from combustion,<ref name=stuffworks>{{cite web|url= http://science.howstuffworks.com/turbine.htm |title= How Gas Turbine Engines Work |publisher= howstuffworks.com |author= Marshall Brain |date= April 2000 |access-date=2010-11-24}}</ref> and a [[ducted fan]] that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a [[turbojet]] passes through the [[combustion chamber]] 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]].

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 engine produces thrust through a combination of these two portions working together. Engines that use more [[Propelling nozzle|jet thrust]] 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 are of the high-bypass type,<ref name="Hall">{{cite web|url=https://www.grc.nasa.gov/www/k-12/airplane/aturbf.html|title=Turbofan Engine|last1=Hall|first1=Nancy|date=May 5, 2015|website=Glenn Research Center|publisher=NASA|access-date=October 25, 2015|quote=Most modern airliners use turbofan engines because of their high thrust and good fuel efficiency.}}</ref><ref name="HackerBurghardt2009">{{cite book|author1=Michael Hacker|author2=David Burghardt|author3=Linnea Fletcher |author4=Anthony Gordon |author5=William Peruzzi |title=Engineering and Technology|url=https://books.google.com/books?id=0-xuCgAAQBAJ&pg=PT336|access-date=October 25, 2015|date=March 18, 2009|publisher=Cengage Learning|isbn=978-1-285-95643-5|page=319|quote=All modern jet-powered commercial aircraft use high bypass turbofan engines [...]}}</ref> and most modern [[Fighter aircraft|fighter]] engines are low-bypass.<ref name="Verma2013"/><ref name="Magill">{{cite book|editor=Frank Northen Magill|title=Magill's Survey of Science: Applied science series, Volume 3|date=1993|publisher=Salem Press|isbn=9780893567088|page=1431|quote=Most tactical military aircraft are powered by low-bypass turbofan engines.}}</ref> [[Afterburner]]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==
[[File:tfan-schematic-kk-20090106.png|thumb|450px|Schematic diagram illustrating a modern 2-spool turbofan engine installation in a nacelle. The low-pressure spool is colored blue and the high-pressure one orange.]]
The turbofan was invented to improve the fuel consumption of the turbojet. It achieves this by pushing more air, thus increasing the mass and lowering the speed of the propelling jet compared to that of the turbojet. This is done mechanically by adding a [[ducted fan]] rather than using viscous forces.<ref>Thrust Augmentation with Mixer/Ejector systems, Presz, Reynolds, Hunter, AIAA 2002-0230, p.3</ref>  A [[vacuum ejector]] is used in conjunction with the fan as first envisaged by inventor [[Frank Whittle]].<ref>Gas Turbine Aerothermodynamics With Special Reference To Aircraft propulsion, Sir Frank Whittle 1981,{{ISBN|0 08 026719 X}}, p.217</ref>

Whittle envisioned flight speeds of 500&nbsp;mph in his March 1936 UK patent 471,368 "Improvements relating to the propulsion of aircraft", in which he describes the principles behind the turbofan,<ref>Gas Turbine Aerothermodynamics With Special Reference To Aircraft propulsion, Sir Frank Whittle 1981,{{ISBN|0 08 026719 X}}, p.218</ref> although not called as such at that time. While the turbojet uses the gas from its thermodynamic cycle as its propelling jet, for aircraft speeds below 500&nbsp;mph there are two penalties to this design which are addressed by the turbofan.

First, the energy required for a given thrust increases as the exhaust air is propelled at ever greater speeds, so the efficiency can be improved by diverting energy to propel larger quantities of air at lower speeds than the core. A turbofan achieves this by using an additional turbine to drive a ducted fan to blow air that bypasses the high speed core. With a lower core thrust, most of the thrust now comes from the large mass of low speed bypass air, providing the same thrust with a reduced fuel burn.

The other penalty is that combustion is less efficient at lower speeds. Any action to reduce the fuel consumption of the engine by increasing its pressure ratio or turbine temperature to achieve better combustion 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 would use 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.<ref>{{Cite journal |last=Rubert |first=Kennedy F. |date=1945-02-01 |title=An analysis of jet-propulsion systems making direct use of the working substance of a thermodynamic cycle |url=https://ntrs.nasa.gov/citations/19930093532 |journal=NASA|language=en |page=}}</ref> In contrast, Roth<ref>{{Cite thesis |title=A theoretical treatment of technical risk in modern propulsion system design |url=https://ui.adsabs.harvard.edu/abs/2000PhDT.......101R |date=2000-09-01 |first=Bryce Alexander |last=Roth|bibcode=2000PhDT.......101R }} p.76</ref> 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 ( i.e. a force), and a wake which is an unavoidable consequence of producing thrust by an airbreathing engine<ref>{{Cite book |url=http://archive.org/details/sim_journal-of-aircraft_september-october-1966_3_5 |title=Journal of Aircraft September-October 1966: Vol 3 Iss 5 |date=September 1966 |publisher=American Institute of Aeronautics and Astronautics |pages=386 |language=English}}</ref> (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]] 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 higher pressure and temperature bypass-air 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.<ref>{{Cite book |url=http://archive.org/details/sim_journal-of-aircraft_september-october-1966_3_5 |title=Journal of Aircraft September-October 1966: Vol 3 Iss 5 |date=September 1966 |publisher=American Institute of Aeronautics and Astronautics |pages=387 |language=English}}</ref>

The fan flow has lower exhaust velocity, giving much more thrust per unit energy (lower [[specific thrust]]). 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 [[Aquatic locomotion#Efficiency|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]] 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]]. Engines with more [[Propelling nozzle|jet thrust]] 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 are high-bypass,<ref name="Hall"/><ref name="HackerBurghardt2009"/> and most modern fighter engines are low-bypass.<ref name="Verma2013">{{cite book| first = Bharat | last = Verma |title= Indian Defence Review: Apr–Jun 2012 |url= https://books.google.com/books?id=IvAzNhvLK6AC&pg=PA18!|access-date=October 25, 2015 |date=January 1, 2013|publisher=Lancer Publishers|isbn= 978-81-7062-259-8 |page= 18|quote= Military power plants may be divided into some major categories – low bypass turbofans that generally power fighter jets...}}</ref><ref name="Magill" /> [[Afterburner]]s are used on low-bypass turbofans on combat aircraft.

=== Bypass ratio ===
{{main|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.<ref>{{Citation | encyclopedia= Britannica | title=Bypass ratio | url = https://www.britannica.com/technology/bypass-ratio}}</ref> 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 [[Thrust-specific fuel consumption|specific fuel consumption]] (SFC) with increasing BPR.<ref>{{Citation | archive-date = 2013-05-28 | publisher = MIT | url = http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node84.html | title = Thermodynamics | archive-url = https://web.archive.org/web/20130528034153/http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node84.html}}</ref> 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 (i.e. fixed pressure ratio and turbine inlet temperature), core and bypass jet velocities equal and a particular flight condition (i.e. 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.<ref>Jet Propulsion, Nicholas Cumpsty 2003,{{ISBN|978 0 521 54144 2}}, Figure 7.3 Predicted variation in thrust and sfc with bypass ratio for a constant core</ref> 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).<ref>"Practical considerations in designing the engine cycle", M G Philpot, AGARD LS 183, Steady and Transient Performance Prediction,{{ISBN|92 835 0674 X}}, p.2-12</ref>

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.<ref>{{Cite web|title = Flight global | url = https://www.flightglobal.com/pdfarchive/view/1964/1964%20-%202596.html|format=PDF|website=Flightglobal.com}}</ref> 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<ref>{{Citation | publisher = Jane's | title = All The World's Aircraft 1975–1976 | editor-first = John W.R. | editor-last = Taylor | place = Paulton House, 8 Sheperdess Walk, London N1 7LW | page = 748}}</ref> (General Electric YJ-101 BPR 0.25) and low BPR turbojets<ref>{{Citation | title = Proceedings | date = 15 April 2015 | publisher = ASME | doi = 10.1115/84-GT-230 | url = http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=2275853| doi-access = free }}</ref> (Pratt & Whitney PW1120). Low BPR (0.2) has also been used to provide [[Compressor stall#Axi-symmetric stall or compressor surge|surge margin]] as well as afterburner cooling for the [[Pratt & Whitney J58]].<ref>{{Citation | title = PW tales | work = Road runners Internationale | url = http://roadrunnersinternationale.com/pw_tales.htm}}</ref>

===Efficiency===
[[File:Gas turbine efficiency.png|thumb|Propulsive efficiency comparison for various gas turbine engine configurations]]

[[Propeller (aeronautics)|Propeller]] engines are most efficient for low speeds, [[turbojet]] engines for high speeds, and turbofan engines between the two. Turbofans are the most efficient engines in the range of speeds from about {{convert|500|to|1000|km/h|kn mph|abbr= on}}, the speed at which most commercial aircraft operate.<ref name= grc_nasa>{{cite web|url= http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html |title= Turbofan Engine |publisher= GRC NASA |access-date= 2010-11-24}}</ref><ref name="Neumann_2004_1984_pp228-230">{{cite book | last = Neumann | first = Gerhard | author-link = Gerhard Neumann | year = 2004 | orig-year = first published by Morrow 1984<!--removed excess additional detail--> | title = Herman the German: Just Lucky I Guess |publisher = Authorhouse | location = Bloomington, Indiana, US | isbn = 1-4184-7925-X | pages = 228–30}}</ref>

In a turbojet (zero-bypass) engine, the high temperature and high pressure exhaust gas is accelerated when it undergoes expansion through a [[propelling nozzle]] 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]] 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]]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]]s, which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less).<ref name= srm>[http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf "The turbofan engine"] {{Webarchive|url=https://web.archive.org/web/20150418181832/http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf |date= 2015-04-18}}, p. 7. [[SRM Institute of Science and Technology]], Department of Aerospace Engineering.</ref> 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 are added to the turbojet's low-loss propelling nozzle.<ref>{{cite book | title = Gas Turbine Theory | edition = 2nd | last1 = Cohen | last2 = Rogers | last3 = Saravanamuttoo | publisher = Longmans | year = 1972 |isbn = 0-582-44927-8 |page = 85}}</ref> The turbofan has additional losses from its greater number of compressor stages/blades, fan and bypass duct.{{clarify|date=May 2021}}

Froude, or propulsive, efficiency can be defined as:

<math display="block">\eta_f = \frac {2}{1 + \frac {V_j}{V_a}}</math>

where:
* {{math|{{var|V}}{{sub|{{var|j}}}}}} = thrust equivalent jet velocity
* {{math|{{var|V}}{{sub|{{var|a}}}}}} = aircraft velocity

===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.<ref>{{cite book |url= http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/FAA-H-8083-3B.pdf |title= FAA-H-8083-3B Airplane Flying Handbook Handbook|publisher=Federal Aviation Administration |year= 2004 |url-status=dead|archive-url= https://web.archive.org/web/20120921094453/http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/FAA-H-8083-3B.pdf |archive-date=2012-09-21}}</ref>

The thrust ('''''F<sub>N</sub>''''') generated by a turbofan depends on the [[effective exhaust velocity]] of the total exhaust, as with any jet engine, but because two exhaust jets are present the thrust equation can be expanded as:<ref>{{Cite web|url=https://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.html|title=Turbofan Thrust|website=Grc.nasa.gov|access-date=1 March 2022}}</ref>

<math display="block">F_N = \dot{m}_e v_{he} - \dot{m}_o v_o + BPR\, (\dot{m}_c) v_f</math>

where:
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|e}}}}}} = the mass rate of hot combustion exhaust flow from the core engine
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|o}}}}}} = the mass rate of total air flow entering the turbofan = {{math|{{var|{{overset|•|m}}}}{{sub|{{var|c}}}} + {{var|{{overset|•|m}}}}{{sub|{{var|f}}}}}}
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|c}}}}}} = the mass rate of intake air that flows to the core engine
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|f}}}}}} = the mass rate of intake air that bypasses the core engine
* {{math|{{var|v}}{{sub|{{var|f}}}}}} = the velocity of the air flow bypassed around the core engine
* {{math|{{var|v}}{{sub|{{var|he}}}}}} = the velocity of the hot exhaust gas from the core engine
* {{math|{{var|v}}{{sub|{{var|o}}}}}} = the velocity of the total air intake = the true airspeed of the aircraft
* {{mvar|BPR}} = bypass ratio

===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{{Citation needed|date=August 2020}}. 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.<ref>{{cite journal|url=https://dspace.lib.cranfield.ac.uk/bitstream/handle/1826/12476/Civil_turbofan_engine_exhaust_aerodynamics-2017.pdf|title=Civil Turbofan Engine Exhaust Aerodynamics: Impact of Bypass Nozzle After-body Design|journal=Aerospace Science and Technology|volume=73|date=February 2018|pages=85–95|doi=10.1016/j.ast.2017.09.002 |access-date=1 March 2022|last1=Goulos |first1=Ioannis |last2=Stankowski |first2=Tomasz |last3=MacManus |first3=David |last4=Woodrow |first4=Philip |last5=Sheaf |first5=Christopher |bibcode=2018AeST...73...85G |hdl=1826/12476 }}</ref> In low bypass engines the two flows may combine within the ducts, and share a common nozzle, which can be fitted with afterburner.

=== Noise ===
[[File:Boeing 747-8 VQ-BSK at LSZH (17754867239).jpg|thumb|Chevrons on a 747-8's [[General Electric GEnx|GEnx-2B]]]]
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]] 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.<ref name="Kempton2011">Kempton, A., [http://www.win.tue.nl/ceas-asc/Workshop15/CEAS-ASC_XNoise-EV_K1_Kempton.pdf "Acoustic liners for modern aero-engines"], 15th CEAS-ASC Workshop and 1st Scientific Workshop of X-Noise EV, 2011. Win.tue.nl.</ref> 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.<ref>{{Cite journal| title = Softly, softly towards the quiet jet | first = Michael J. T. | last = Smith | journal=[[New Scientist]] | date = 19 February 1970 | at = fig. 5}}</ref>

Modern commercial aircraft employ high-bypass-ratio (HBPR) engines with separate flow, non-mixing, short-duct exhaust systems. Their noise at takeoff is primarily from the fan and jet.<ref>{{cite book |last1=Huff |first1=Dennis |last2=Envia |first2=Edmane |date=October 2007 |editor-last=Crocker |editor-first=Malcolm |title=Handbook of Noise and Vibration Control |publisher=Wiley |pages=1096–1108 |chapter=Chapter 89: Jet Engine Noise Generation, Prediction, and Control |url= https://www.wiley.com/en-us/Handbook+of+Noise+and+Vibration+Control-p-9780471395997 |isbn= 978-0-471-39599-7}}</ref> 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.<ref name="chevron technology">{{cite journal |url= https://www.researchgate.net/publication/273550214 | journal= Proceedings of the 13th Asian Congress of Fluid Mechanics 17–21 December 2010, Dhaka, Bangladesh | title=Evolution from 'Tabs' to 'Chevron Technology'–a Review | format= PDF | last1 =Zaman | first1 = K. B. M. Q. | last2 =Bridges | first2 = J. E. | last3 =Huff | first3 = D. L. | date=17–21 December 2010 | publisher=b[[NASA Glenn Research Center]] | place = Cleveland, [[Ohio|OH]] | access-date= January 8, 2025}}</ref>

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.<ref>{{cite journal |title=Designing the JT-9D Engine to meet Low Noise Requirements for Future Transports |last1=Kester |first1=JD |last2=Slaiby |first2=TG |journal=SAE Transactions |date=1968 |volume=76 |issue=2 |id=paper 670331 |page=1332 |jstor=44565020 |doi=10.4271/670331}}</ref>
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.<ref>{{cite magazine |title=Quiet Propulsion |first=M.J.T. |last=Smith |magazine=Flight International |date=17 August 1972 |page=241}}</ref><ref name= buzz_saw>{{Citation | first = A. | last = McAlpine | url = http://www.southampton.ac.uk/engineering/research/projects/buzz_saw_noise_and_non_linear_acoustics.page | title = Research project: Buzz-saw noise and nonlinear acoustics | publisher = University of Southampton}}</ref>

All modern turbofan engines have [[acoustic liner]]s in the [[nacelle]] 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<ref name="Schuster2010">{{Citation | last1 = Schuster | first1 = B. | last2 = Lieber | first2 = L. | last3 = Vavalle | first3 = A. | title = Optimization of a seamless inlet liner using an empirically validated prediction method | work = 16th AIAA/CEAS Aeroacoustics Conference | year = 2010 | place = Stockholm, [[Sweden|SE]] |doi=10.2514/6.2010-3824| isbn = 978-1-60086-955-6 | s2cid = 113015300 }}</ref> or in dedicated experimental test rigs.<ref name="Ferrante2011">{{Citation | last1 = Ferrante | first1 = P. G. | last2 = Copiello | first2 = D. | last3 = Beutke | first3 = M. | title = Design and experimental verification of 'true zero-splice' acoustic liners in the universal fan facility adaptation (UFFA) modular rig | work = 17h AIAA/CEAS Aeroacoustics Conference | year = 2011 | id = AIAA-2011-2728 | place = Portland, OR |doi=10.2514/6.2011-2728| isbn = 978-1-60086-943-3 }}</ref>

In the [[aerospace]] industry, <!--link target-->{{anchor|Chevrons}}'''chevrons''' are the "saw-tooth" patterns on the trailing edges of some [[jet engine]] nozzles<ref name=NASA>{{cite web |url= http://www.nasa.gov/topics/aeronautics/features/bridges_chevron_events.html |title= NASA Helps Create a More Silent Night |last1=Banke |first1=Jim |date= 2012-12-13 |publisher=[[NASA]] |access-date= January 12, 2013}}</ref> that are used for [[noise control|noise reduction]]. 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.<ref name=NASA/> Chevrons were developed by GE under a [[NASA]] contract.<ref name="chevron technology" /> Some notable examples of such designs are [[Boeing 787]], [[Boeing 737 Max]] and [[Boeing 747-8]]{{snd}} on the [[Rolls-Royce Trent 1000]] (787), [[General Electric GEnx]] (787, 748-8) and [[CFM LEAP]] (1B variant only; 737 Max) engines.<ref>{{Citation | archive-date = 2014-03-25 | place = [[China|CN]] | work = 13th ACFM | title = Invited | url = http://www.afmc.org.cn/13thacfm/invited/201.pdf | publisher = AFMC | archive-url = https://web.archive.org/web/20140325205124/http://www.afmc.org.cn/13thacfm/invited/201.pdf}}</ref>

==History==
[[File:Rolls Royce Conway Mk508 (1959) used in Boeing 707-420 at Flugausstellung Hermeskeil, pic1.JPG|thumb|[[Rolls-Royce Conway]] low-bypass turbofan from a [[Boeing 707]]. The bypass air exits from the fins, while the exhaust from the core exits from the central nozzle. This fluted jetpipe design is a noise-reducing method devised by Frederick Greatorex at Rolls-Royce]]
[[File:Outer nozzle of GEnx-2B turbofan engine.jpg|thumb|[[General Electric GEnx|General Electric GEnx-2B]] turbofan engine as used on a [[Boeing 747|Boeing 747–8]]. View into the bypass duct looking forward from the bypass nozzle and showing fan exit stators/fan blades]]
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 007|Daimler-Benz DB 670]], designated the 109-007 by the German RLM ([[Ministry of Aviation (Nazi Germany)|Ministry of Aviation]]), with a first run date of 27&nbsp;May 1943, after the testing of the turbomachinery using an electric motor, which had been undertaken on 1&nbsp;April 1943.<ref>"Turbojet History And Development 1930–1960 Volume 1", The Crowood Press Ltd. 2007, {{ISBN|978 1 86126 912 6}}, p.&nbsp;241.</ref> 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<ref>{{cite web|url=http://www.flightglobal.com/airspace/media/aeroenginesjetcutaways/metrovick-f3-cutaway-5614.aspx |title=Metrovick F3 Cutaway – Pictures & Photos on FlightGlobal Airspace |publisher=Flightglobal.com |date=2007-11-07 |access-date=2013-04-29}}</ref> turbofan, which used the [[Metropolitan-Vickers F.2|Metrovick F.2]] 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.<ref>{{cite web|url=https://www.flightglobal.com/pdfarchive/view/1946/1946%20-%202270%20-%200145.html|work=Flight international|date=1946 |title= page 145}}</ref>

Improved materials, and the introduction of twin compressors, such as in the [[Rolls-Royce Olympus|Bristol Olympus]],<ref>{{cite web|url=http://www.flightglobal.com/pdfarchive/view/1954/1954%20-%200985.html |title=1954 &#124; 0985 &#124; Flight Archive |publisher=Flightglobal.com |date=1954-04-09 |access-date=2013-04-29}}</ref> and [[Pratt & Whitney JT3C]] engines, increased the overall pressure ratio and thus the [[thermodynamics|thermodynamic]] 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 world's first production turbofan, had a bypass ratio of 0.3, similar to the modern [[General Electric F404]] fighter engine. Civilian turbofan engines of the 1960s, such as the [[Pratt & Whitney JT8D]] and the [[Rolls-Royce Spey]], 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]] introduced in 1962. It used the [[Soloviev D-20]].<ref>The Development Of Jet And Turbine Aero Engines 4th edition, Bill Gunston 2006, {{ISBN|0 7509 4477 3}}, p.&nbsp;197.</ref> 164 aircraft were produced between 1960 and 1965 for [[Aeroflot]] and other [[Eastern Bloc]] airlines, with some operating until the early 1990s.

The first General Electric turbofan was the aft-fan [[General Electric CJ805|CJ805-23]], based on the CJ805-3 turbojet. It was followed by the aft-fan [[General Electric CF700]] engine, with a 2.0 bypass ratio. This was derived from the [[General Electric J85|General Electric J85/CJ610]] turbojet {{convert|2,850|lbf|N|abbr=on}} to power the larger [[North American Sabreliner|Rockwell Sabreliner]] 75/80 model aircraft, as well as the [[Dassault Falcon 20]], with about a 50% increase in thrust to {{convert|4,200|lbf|N|abbr=on}}. The CF700 was the first small turbofan to be certified by the [[Federal Aviation Administration]] (FAA). There were at one time over 400 CF700 aircraft in operation around the world, with an experience base of over 10&nbsp;million service hours. The CF700 turbofan engine was also used to train Moon-bound astronauts in [[Project Apollo]] as the powerplant for the [[LLRV|Lunar Landing Research Vehicle]].

==Common types==
===Low-bypass turbofan===

[[File:Turbofan operation lbp.svg|thumb|Schematic diagram illustrating a 2-spool, low-bypass turbofan engine with a mixed exhaust, showing the low-pressure (green) and high-pressure (purple) spools. The fan (and booster stages) are driven by the low-pressure turbine, whereas the high-pressure compressor is powered by the high-pressure turbine.]]

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]] 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 [[Thrust specific fuel consumption|specific fuel consumption]] (SFC).

Some low-bypass ratio military turbofans (e.g. [[General Electric F404|F404]], [[Pratt & Whitney JT8D|JT8D]]) have variable inlet guide vanes to direct air onto the first fan rotor stage. This improves the fan [[compressor stall|surge]] margin (see [[compressor map]]).

<gallery mode="packed" heights="120">
File:Pratt & Whitney JT8D-17A 1.JPG|The widely produced [[Pratt & Whitney JT8D]] used on many early narrowbody [[jetliners]]. The fan is located behind the inlet guide vanes.
File:Solowjow D-30 III.jpg|[[Soloviev D-30]] which powers the [[Ilyushin Il-76]] and [[Ilyushin Il-62|Il-62M]]; [[Mikoyan MiG-31]]; [[Xian H-6]]K and [[Xian Y-20|Y-20]]
File:AL-31FN.jpg|[[Saturn AL-31]] which powers the [[Chengdu J-10]] and [[Chengdu J-20|J-20]]; [[Shenyang J-11]], [[Shenyang J-15|J-15]] and [[Shenyang J-16|J-16]]; [[Sukhoi Su-30]] and [[Sukhoi Su-27|Su-27]]
File:Williams Research F107.jpg|[[Williams F107]] which powers the [[Raytheon]] [[Tomahawk (missile family)|BGM-109 Tomahawk]] cruise missile
File:AL-55 at the MAKS-2011 (01).jpg|[[NPO Saturn AL-55]] which powers certain [[HAL HJT-36 Sitara]]
File:Pratt & Whitney TF30 at Oakland Aviation Museum, June 2022.jpg|[[Pratt & Whitney TF-30]] which powers the [[Grumman F-14 Tomcat]]
File:Eurojet EJ200 for Eurofighter Typhoon PAS 2013 01 free.jpg|[[Eurojet EJ200]] which powers the [[Eurofighter Typhoon]]
File:XF3 KASM001.jpg|[[Ishikawajima-Harima F3]] which powers the [[Kawasaki T-4]]
File:GTX-35VS Kaveri.jpg|[[GTRE GTX-35VS Kaveri]] developed by [[Gas Turbine Research Establishment|GTRE]]
</gallery>

=== Afterburning turbofan ===
{{further|Afterburner}}
[[File:Pratt & Whitney F119.JPEG|thumb|[[Pratt & Whitney F119]] afterburning turbofan on test]]

Since the 1970s, most [[jet fighter]] engines have been low/medium bypass turbofans with a mixed exhaust, [[afterburner]] 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]] 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 {{cvt|2100|K|R °F °C|sigfig=2}}. 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]], which initially powered the [[General Dynamics F-111 Aardvark|F-111 Aardvark]] and [[Grumman F-14 Tomcat|F-14 Tomcat]]. Low-bypass military turbofans include the [[Pratt & Whitney F119]], the [[Eurojet EJ200]], the [[General Electric F110]], the [[Klimov RD-33]], and the [[Saturn AL-31]], all of which feature a mixed exhaust, afterburner and variable area propelling nozzle.

=== High-bypass turbofan ===
{{More citations needed section|date=December 2024}}
{{further|Bypass ratio}}
<!-- Old non-animated image.....[[File:tfan-schematic-kk-20090106.png|thumb|Schematic diagram illustrating a modern 2-spool, high-bypass turbofan engine in nacelle with an unmixed exhaust. The low-pressure spool is colored blue and the high-pressure one orange.]] -->

[[File:Turbofan operation.svg|thumb|Schematic diagram illustrating a 2-spool, high-bypass turbofan engine with an unmixed exhaust. The low-pressure spool is coloured green and the high-pressure one purple. Again, the fan (and booster stages) are driven by the low-pressure turbine, but more stages are required. A mixed exhaust is often employed.]]

To further improve fuel economy and reduce noise, almost all jet airliners and most military transport aircraft (e.g., the [[C-17 Globemaster III|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 [[turboprop]]s.

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]] 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]], 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 [[Axial compressor#Bleed air, variable stators|variable geometry stators]] enable high-pressure-ratio compressors to work surge-free at all throttle settings.

[[File:CF6-6 engine cutaway.jpg|thumb|Cutaway diagram of the [[General Electric CF6]]-6 engine]]

The first (experimental) high-bypass turbofan engine was the [[Lycoming Engines|AVCO-Lycoming]] PLF1A-2, a [[Honeywell T55]] turboshaft-derived engine that was first run in February 1962. The PLF1A-2 had a {{cvt|40|in|cm|adj=mid|diameter}} geared fan stage, produced a static thrust of {{cvt|4,320|lb}},<ref name="Boyne2002">{{cite book |url={{GBurl|FW_50wm8VnMC|p=235}} |title=Air warfare: An international encyclopedia: A–L |editor-first=Walter J. |editor-last=Boyne |publisher=ABC-CLIO |year=2002 |page=235 |isbn=978-1-57607-345-2}}</ref> and had a bypass ratio of 6:1.<ref name="NASM-PLF1A-2">{{cite web |title=Lycoming PLF1A-2 turbofan engine |url=https://airandspace.si.edu/collection-objects/lycoming-plf1a-2-turbofan-engine/nasm_A19890042000 |work=[[Smithsonian National Air and Space Museum]] |access-date=December 31, 2021}}</ref> The [[General Electric TF39]] became the first production model, designed to power the [[Lockheed Corporation|Lockheed]] [[C-5 Galaxy]] military transport aircraft.<ref name="Neumann_2004_1984_pp228-230"/> The civil [[General Electric CF6]] engine used a derived design. Other high-bypass turbofans are the [[Pratt & Whitney JT9D]], the three-shaft [[Rolls-Royce RB211]] and the [[CFM International CFM56]]; also the smaller [[TF34]]. More recent large high-bypass turbofans include the [[Pratt & Whitney PW4000]], the three-shaft [[Rolls-Royce Trent]], the [[General Electric GE90]]/[[GEnx]] and the [[GP7000]], produced jointly by GE and P&W. The Pratt & Whitney JT9D engine was the first high bypass ratio [[jet engine]] to power a wide-body airliner.<ref>{{cite book | url=https://books.google.com/books?id=kWJBDAAAQBAJ&dq=first+high+bypass+turbofan&pg=PA997 | title=Fundamentals of Aircraft and Rocket Propulsion | isbn=978-1-4471-6796-9 | last1=El-Sayed | first1=Ahmed F. | date=25 May 2016 | publisher=Springer }}</ref>

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.

<gallery mode="packed" heights="120">
File:Sam146 1.jpg|[[PowerJet SaM146]] which powers [[Sukhoi Superjet 100]]
File:Ge cf6 turbofan.jpg|[[General Electric CF6]] 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]], 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]]
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]]
File:General Electric GE90 displayed at Farnborough Air Show 2008.jpg|[[GE90]] powering the [[Boeing 777]], the most powerful aircraft engine
</gallery>

==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.<ref>{{Cite web |url=http://www.rolls-royce.com/Images/RB211-535E4%20_tcm92-11348.pdf |title=RB211-535E4 |website= |access-date=1 March 2022 |archive-url=https://web.archive.org/web/20110103084411/http://www.rolls-royce.com/Images/RB211-535E4%20_tcm92-11348.pdf |archive-date=3 January 2011 |url-status=dead}}</ref>

===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 [[General Electric CJ805|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.<ref>{{Cite web|url=http://www.icas.org/ICAS_ARCHIVE/ICAS2002/PAPERS/1.PDF|title=p.01.7|website=Icas.rg|access-date=1 March 2022}}</ref> 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.<ref>{{Cite book |last=Webber |first=Richard J. |title=VARIABLE GEOMETRY AFT-FAN FORTAKEOFFQUIETINGOR THRUST AUGMENTATION OF A TURBOJET ENGINE |publisher=Lewis Research Centre, NASA |year=1971 |location=Ohio}}</ref>

===Basic two-spool===
[[File: Dual-spool axial-flow compressor.png|thumb|A dual-spool [[Axial compressor|axial-flow compressor]].]]

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.{{Citation needed|date=September 2018}} 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 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]], [[GE90]], [[General Electric GE9X|GE9X]] and [[GEnx]] plus [[Pratt & Whitney JT9D]] and [[Pratt & Whitney PW4000|PW4000]]) use booster stages. The Rolls-Royce BR715 is another example. The high bypass ratios used in modern civil turbofans tend to reduce the relative diameter of the booster stages, reducing their mean tip speed. Consequently, more booster 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 booster stages of a boosted two-spool configuration are separated into an 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]] 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===
{{main|Geared turbofan}}
[[File:Geared Turbofan NT.PNG|thumb|Geared turbofan. The gearbox is labeled 2.]]

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]].

===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]] 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.<ref name=":0">{{cite web |url= http://www.mtu.de/en/technologies/engineering_news/others/Riegler_Geared_turbofan_technology.pdf |title= The geared turbofan technology – Opportunities, challenges and readiness status |url-status= dead |archive-url= https://web.archive.org/web/20130520065423/http://www.mtu.de/en/technologies/engineering_news/others/Riegler_Geared_turbofan_technology.pdf |archive-date= 2013-05-20 }} C. Riegler, C. Bichlmaier:, 1st CEAS European Air and Space Conference, 10–13 September 2007, Berlin, Germany</ref> 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 booster 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&nbsp;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:<math display="block">F_n = m \cdot (V_{jfe} - V_a).</math> 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]] 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 must be maintained within close angular limits to keep the air flowing against an increasing pressure. Otherwise air will be rejected back out of the intake.<ref name=LN161021>{{cite web |url= https://leehamnews.com/2016/10/21/bjorns-corner-engine-challenge/ |title= Bjorn's Corner: The Engine challenge |author= Bjorn Fehrm |date= October 21, 2016 |work= Leeham News}}</ref>

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 {{cvt|1700|°C|°F|sigfig=2}} and {{cvt|17|bar|psi}}, for reliable [[temperature sensor|sensor]]s. 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.<ref name=LN161021/>

=== Blade technology ===
A [[turbine]] blade with a weight of {{cvt|100|g|oz}} is subjected to {{cvt|1700|°C|°F|sigfig=2}}, at {{cvt|17|bar|psi}} and a [[centrifugal force]] of {{cvt|40|kN|lbf}}, well above the point of [[plastic deformation]] and even above the [[melting point]].<!--<ref name=LN161021/>-->
Exotic [[alloy]]s, sophisticated [[air cooling]] schemes and special mechanical design are needed to keep the [[physical stress]]es within the strength of the material.<!--<ref name=LN161021/>-->
[[Rotating seal]]s must withstand harsh conditions for 10&nbsp;years, 20,000 missions and rotating at 10 to 20,000&nbsp;rpm.<ref name=LN161021/>

=== Fan blades ===
Fan blades have been growing as jet engines have been getting bigger: each fan blade carries the equivalent of nine [[double-decker bus]]es and swallows air the equivalent volume of a [[squash court]] every second.<!--<ref name=MRO28sep2017/>-->
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.<!--<ref name=MRO28sep2017/>-->
Coincidentally, the [[bypass ratio]] grew to achieve higher [[propulsive efficiency]] and the fan diameter increased.<ref name=MRO28sep2017/>

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]].<!--<ref name=MRO28sep2017/>-->
[[GE Aviation]] introduced [[carbon fiber composite]] fan blades on the [[GE90]] in 1995, manufactured since 2017 with a [[carbon-fiber tape|carbon-fiber tape-layer]] process.<!--<ref name=MRO28sep2017/>-->
GE partner [[Safran]] developed a [[3D weaving|3D woven]] technology with [[Albany Engineered Composites|Albany Composites]] for the [[CFM56]] and [[CFM LEAP]] engines.<ref name=MRO28sep2017>{{cite news |url= http://www.mro-network.com/engines-engine-systems/understanding-complexities-bigger-fan-blades |title= Understanding Complexities Of Bigger Fan Blades |author= Ben Hargreaves |date= Sep 28, 2017 |work= Aviation Week Network}}</ref>

=== Future progress ===
Engine cores are shrinking as they operate at higher [[Overall pressure ratio|pressure ratio]]s and become more efficient and smaller compared to the fan as bypass ratios increase.<!--<ref name=AvWeek26Mar2015/>-->
Blade [[tip clearance]]s are more difficult to maintain at the exit of the high-pressure compressor where blades are {{cvt|0.5|in|mm}} 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 space is constrained within the core.<ref name=AvWeek26Mar2015>{{cite news |url= http://aviationweek.com/technology/reversed-tilted-future-pratt-s-geared-turbofan |title= A Reversed, Tilted Future For Pratt's Geared Turbofan? |date= Mar 26, 2015 |author= Guy Norris and Graham Warwick |work= Aviation Week & Space Technology}}</ref>

[[Pratt & Whitney]] VP technology and environment [[Alan H. Epstein|Alan Epstein]] argued "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%".<ref name=AvWeek8Aug2017>{{cite news |url= http://aviationweek.com/technology/turbofans-are-not-finished-yet |title= Turbofans Are Not Finished Yet |date= Aug 8, 2017 |author= Guy Norris |work= Aviation Week & Space Technology}}</ref>

<!-- Advanced low-pressure-ratio fan tested on Pratt & Whitney geared turbofan -->
[[Geared turbofan]]s and further fan [[Overall pressure ratio|pressure ratio]] reductions may continue to improve [[propulsive efficiency]].<!--<ref name=AvWeek8Aug2017/>-->
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&nbsp;dB EPNdb noise compared with the 2000s state-of-the-art.<ref name="faa-cleen">{{cite web |title=Continuous Lower Energy, Emissions, and Noise (CLEEN) Program |url=https://www.faa.gov/about/office_org/headquarters_offices/apl/eee/technology_saf_operations/cleen |website=www.faa.gov |publisher=Federal Aviation Administration |access-date=11 February 2023}}</ref><!--<ref name=AvWeek8Aug2017/>-->
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.<ref name=AvWeek8Aug2017/>

The weight and size of the [[nacelle]] 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.<!--<ref name=AvWeek8Aug2017/>-->
[[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.<ref name=AvWeek8Aug2017/>

<!-- Safran ground-testing open rotor as potential path forward -->
[[Safran]] expects to deliver another 10–15% in fuel efficiency through the mid-2020s before reaching an [[asymptote]], and next will have to increase the [[bypass ratio]] 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.<!--<ref name=AvWeek8Aug2017/>-->
[[Computational fluid dynamics|Modeling]] advances and high [[specific strength]] materials may help it succeed where previous attempts failed.<!--<ref name=AvWeek8Aug2017/>-->
When noise levels are within existing 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.<ref name=AvWeek8Aug2017/>

<!-- GE focusing on thermodynamic boosts through CMC and advanced cycles -->
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.<!--<ref name=AvWeek8Aug2017/>-->
Under the [[U.S. Air Force]]'s [[Adaptive Engine Transition Program]], adaptive [[thermodynamic cycle]]s will be used for the [[sixth-generation jet fighter]], based on a modified [[Brayton cycle]] and [[Constant volume]] combustion.<!--<ref name=AvWeek8Aug2017/>-->
[[Additive manufacturing]] in the [[General Electric Advanced Turboprop|advanced turboprop]] will reduce weight by 5% and fuel burn by 20%.<ref name=AvWeek8Aug2017/>

Rotating and static [[ceramic matrix composite]] (CMC) parts operates {{cvt|500|°F}} hotter than metal and are one-third its weight.<!--<ref name=AvWeek8Aug2017/>-->
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.<!--<ref name=AvWeek8Aug2017/>-->
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.<ref name=AvWeek8Aug2017/>

<!-- Rolls-Royce targeting 60:1 pressure ratios and geared architectures -->
[[Rolls-Royce Plc]] aim for a 60:1 pressure ratio core for the 2020s [[Ultrafan]] and began ground tests of its {{cvt|100,000|hp}} gear for {{cvt|100,000|lbf|kN}} and 15:1 bypass ratios.<!--<ref name=AvWeek8Aug2017/>-->
Nearly [[stoichiometric]] turbine entry temperature approaches the theoretical limit and its impact on emissions has to be balanced with environmental performance goals.<!--<ref name=AvWeek8Aug2017/>-->
Open rotors, lower pressure ratio fans and potentially [[distributed propulsion]] offer more room for better propulsive efficiency.<!--<ref name=AvWeek8Aug2017/>-->
Exotic cycles, [[heat exchanger]]s and pressure gain/constant volume combustion may improve [[thermodynamic efficiency]].<!--<ref name=AvWeek8Aug2017/>-->
Additive manufacturing could be an enabler for [[intercooler]] and [[recuperator]]s.<!--<ref name=AvWeek8Aug2017/>-->
Closer airframe integration and [[Hybrid electric vehicle#Aircraft|hybrid]] or [[electric aircraft]] can be combined with gas turbines.<ref name=AvWeek8Aug2017/>

Rolls-Royce engines have a 72–82% propulsive efficiency and 42–49% thermal efficiency for a {{cvt|0.63|-|0.49|lb/lbf/h|g/kN/h}} [[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]] for a {{cvt|0.35|lb/lbf/h|g/kN/h}} TSFC<ref>{{citation |url= http://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2014_03_20_EnginesTechnology.pdf |title= Rolls-Royce technology for future aircraft engines |date= March 20, 2014 |author= Ulrich Wenger |publisher= Rolls-Royce Deutschland}}</ref>

As teething troubles may not show up until several thousand hours, the latest turbofans' technical problems disrupt [[airline]]s operations and [[aerospace manufacturer|manufacturer]]s deliveries while production rates rise sharply.<!--<ref name=SeattleTimes15jun2018>-->
[[Trent 1000]] cracked blades [[aircraft on ground|grounded]] almost 50 [[Boeing 787]]s and reduced [[ETOPS]] to 2.3 hours down from 5.5, costing [[Rolls-Royce plc]] almost $950 million.<!--<ref name=SeattleTimes15jun2018>-->
[[PW1000G]] knife-edge seal fractures have caused [[Pratt & Whitney]] to fall behind in deliveries, leaving about 100 engineless [[A320neo]]s waiting for their powerplants.<!--<ref name=SeattleTimes15jun2018>-->
The [[CFM LEAP]] introduction had been smoother but a [[ceramic composite]] {{abbr|HP|High-Pressure}} Turbine coating was prematurely lost, necessitating a new design, causing 60 A320neo engine removals for modification and delaying deliveries by up to six weeks late.<ref name=SeattleTimes15jun2018>{{cite news |url= https://www.seattletimes.com/business/boeing-aerospace/troublesome-advanced-engines-for-boeing-and-airbus-jets-disrupt-airlines-and-production-lines/ |title= Troublesome advanced engines for Boeing, Airbus jets have disrupted airlines and shaken travelers |date= June 15, 2018 |author= Dominic Gates |author-link=Dominic Gates |newspaper= The Seattle Times}}</ref>

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]], while [[Rolls-Royce plc]] hopes for up to 5%.<ref>{{cite news |url= https://www.flightglobal.com/news/articles/how-the-future-of-electric-aircraft-lies-beyond-the-460492/ |title= How the future of electric aircraft lies beyond the engines |date= 6 Sep 2019 |author= Kerry Reals |work= Flightglobal}}</ref>

==Manufacturers==
{{main|List of turbofan manufacturers}}
The turbofan engine market is dominated by [[GE Aerospace|General Electric]], [[Rolls-Royce plc]] and [[Pratt & Whitney]], in order of market share. General Electric and [[Safran Aircraft Engines|Safran]] 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 [[airliner]]s 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&nbsp;lb (89&nbsp;kN) fleet will grow from 7,500 to 9,000 and the fleet of [[turboprop]]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.<ref>{{cite news |url= https://www.flightglobal.com/news/articles/insight-from-flightglobal-flight-fleet-forecasts-e-430071/ |title= Flight Fleet Forecast's engine outlook |work= Flight Global |date= 2 November 2016 }}</ref>

=== Commercial turbofans in production ===
{| class="wikitable sortable"
|+ Commercial turbofans in production<ref>{{cite book |title= Jane's All the World's Aircraft |issn= 0075-3017 |date=2005 |pages= 850–853}}</ref>
! Model
! Start !! Bypass  !! Length  !! Fan   !! Weight  !! Thrust
! Major applications
|-
| [[General Electric GE90|GE GE90]]
| 1992 || 8.7–9.9 || 5.18–5.40&nbsp;m || 3.12–3.25&nbsp;m || 7.56–8.62&nbsp;t || 330–510&nbsp;kN
| [[Boeing 777|B777]]
|-
| [[Pratt & Whitney PW4000|P&W PW4000]]
| 1984 || 4.8–6.4 || 3.37–4.95&nbsp;m || 2.84&nbsp;m || 4.18–7.48&nbsp;t || 222–436&nbsp;kN
| [[Airbus A300|A300]]/[[Airbus A310|A310]], [[Airbus A330|A330]], [[Boeing 747|B747]], [[Boeing 767|B767]], [[Boeing 777|B777]], [[McDonnell Douglas MD-11|MD-11]]
|-
| [[Rolls-Royce Trent XWB|R-R Trent XWB]]
| 2010 || 9.3  || 5.22&nbsp;m || 3.00&nbsp;m || 7.28&nbsp;t || 330–430&nbsp;kN
| [[Airbus A350|A350XWB]]
|-
| [[Rolls-Royce Trent 800|R-R Trent 800]]
| 1993 || 5.7–5.79 || 4.37&nbsp;m || 2.79&nbsp;m  || 5.96–5.98&nbsp;t || 411–425&nbsp;kN
| [[Boeing 777|B777]]
|-
| [[Engine Alliance GP7000|EA GP7000]]
| 2004 || 8.7  || 4.75&nbsp;m || 2.95&nbsp;m || 6.09–6.71&nbsp;t || 311–363&nbsp;kN
| [[Airbus A380|A380]]
|-
| [[Rolls-Royce Trent 900|R-R Trent 900]]
| 2004 || 8.7  || 4.55&nbsp;m || 2.95&nbsp;m || 6.18–6.25&nbsp;t || 340–357&nbsp;kN
| [[Airbus A380|A380]]
|-
| [[Rolls-Royce Trent 1000|R-R Trent 1000]]
| 2006 || 10.8–11 || 4.74&nbsp;m || 2.85&nbsp;m || 5.77&nbsp;t || 265.3–360.4&nbsp;kN
| [[Boeing 787|B787]]
|-
| [[General Electric GEnx|GE GEnx]]<ref>{{cite web |url= http://www.geaviation.com/commercial/engines/genx/ |title= GEnx |publisher= GE}}</ref>
| 2006
| 8.0–9.3
| {{#expr:169.7*.0254round2}}-{{#expr:184.7*.0254round2}} m
| {{#expr:104.7*.0254round2}}-{{#expr:111.1*.0254round2}} m
| {{#expr:12400*0.00045359237round2}}-{{#expr:12822*0.00045359237round2}} t
| {{#expr:66500*0.00444822162round0}}-{{#expr:76100*0.00444822162round0}} kN
| [[Boeing 747-8|B747-8]], [[Boeing 787|B787]]
|-
| [[Rolls-Royce Trent 700|R-R Trent 700]]
| 1990 || 4.9  || 3.91&nbsp;m || 2.47&nbsp;m || 4.79&nbsp;t || 320&nbsp;kN
| [[Airbus A330|A330]]
|-
| [[General Electric CF6|GE CF6]]
| 1971 || 4.3–5.3 || 4.00–4.41&nbsp;m || 2.20–2.79&nbsp;m || 3.82–5.08&nbsp;t || 222–298&nbsp;kN
| [[Airbus A300|A300]]/[[Airbus A310|A310]], [[Airbus A330|A330]], [[Boeing 747|B747]], [[Boeing 767|B767]], [[McDonnell Douglas MD-11|MD-11]], [[McDonnell Douglas DC-10|DC-10]]
|-
| [[Rolls-Royce Trent 500|R-R Trent 500]]
| 1999 || 8.5  || 3.91&nbsp;m || 2.47&nbsp;m || 4.72&nbsp;t || 252&nbsp;kN
| [[Airbus A340|A340]]-500/600
|-
| [[Pratt & Whitney PW1000G|P&W PW1000G]]<ref>{{cite web |url= http://www.mtu.de/engines/commercial-aircraft-engines/narrowbody-and-regional-jets/pw1000g/ |title= PW1000G |publisher= [[MTU Aero Engines|MTU]] |access-date= 2016-07-01 |archive-date= 2018-08-18 |archive-url= https://web.archive.org/web/20180818154807/https://www.mtu.de/engines/commercial-aircraft-engines/narrowbody-and-regional-jets/pw1000g/ |url-status= dead }}</ref>
| 2008 || 9.0–12.5 || 3.40&nbsp;m || 1.42–2.06&nbsp;m || 2.86&nbsp;t || 67–160&nbsp;kN
| [[Airbus A320neo|A320neo]], [[Airbus A220|A220]], [[E-Jets E2]]
|-
| [[CFM International LEAP|CFM LEAP]]<ref>{{cite web|url=http://www.cfmaeroengines.com/engines/leap |title=The Leap Engine |publisher= CFM International }}</ref>
| 2013 || 9.0–11.0 || 3.15–3.33&nbsp;m  || 1.76–1.98&nbsp;m || 2.78–3.15&nbsp;t || 100–146&nbsp;kN
| [[Airbus A320neo|A320neo]], [[Boeing 737 Max|B737Max]], C919
|-
| [[CFM International CFM56|CFM56]]
| 1974 || 5.0–6.6 || 2.36–2.52&nbsp;m || 1.52–1.84&nbsp;m || 1.95–2.64&nbsp;t || 97.9-151&nbsp;kN
| [[Airbus A320|A320]], [[Airbus A340|A340]]-200/300, [[Boeing 737|B737]], [[KC-135]], [[Douglas DC-8|DC-8]]
|-
| [[IAE V2500]]
| 1987 || 4.4–4.9 || 3.20&nbsp;m || 1.60&nbsp;m || 2.36–2.54&nbsp;t || 97.9-147&nbsp;kN
| [[Airbus A320|A320]], [[McDonnell Douglas MD-90|MD-90]]
|-
| [[Pratt & Whitney PW6000|P&W PW6000]]
| 2000 || 4.90  || 2.73&nbsp;m || 1.44&nbsp;m || 2.36&nbsp;t  || 100.2&nbsp;kN
| [[Airbus A318]]
|-
| [[Rolls-Royce BR700|R-R BR700]]
| 1994 || 4.2–4.5  || 3.41–3.60&nbsp;m || 1.32–1.58&nbsp;m || 1.63–2.11&nbsp;t || 68.9–102.3&nbsp;kN
| [[Boeing 717|B717]], [[Bombardier Global Express|Global Express]], [[Gulfstream V]]
|-
| [[General Electric Passport|GE Passport]]
| 2013 || 5.6  || 3.37&nbsp;m  || 1.30&nbsp;m || 2.07&nbsp;t || 78.9–84.2&nbsp;kN
| [[Global 7000]]/8000
|-
| [[General Electric CF34|GE CF34]]
| 1982 || 5.3–6.3 || 2.62–3.26&nbsp;m || 1.25–1.32&nbsp;m || 0.74–1.12&nbsp;t || 41–82.3&nbsp;kN
| [[Challenger 600]], [[Bombardier CRJ|CRJ]], [[E-jets]]
|-
| [[Pratt & Whitney Canada PW800|P&WC PW800]]
| 2012 || 5.5  ||    || 1.30&nbsp;m ||   || 67.4–69.7&nbsp;kN
| [[Gulfstream G500/G600]]
|-
| [[Rolls-Royce RB.183 Tay|R-R Tay]]
| 1984 || 3.1–3.2 || 2.41&nbsp;m || 1.12–1.14&nbsp;m || 1.42–1.53&nbsp;t || 61.6–68.5&nbsp;kN
| [[Gulfstream IV]], [[Fokker 70]]/[[Fokker 100|100]]
|-
| [[Snecma Silvercrest|Silvercrest]]
| 2012 || 5.9  || 1.90&nbsp;m || 1.08&nbsp;m || 1.09&nbsp;t || 50.9&nbsp;kN
| [[Cessna Citation Hemisphere|Citation Hemisphere]], [[Dassault Falcon 5X|Falcon 5X]]
|-
| [[Rolls-Royce AE 3007|R-R AE 3007]]
| 1991  || 5.0  || 2.71&nbsp;m  || 1.11&nbsp;m || 0.72&nbsp;t || 33.7&nbsp;kN
| [[ERJ]], [[Citation X]]
|-
| [[Pratt & Whitney Canada PW300|P&WC PW300]]
| 1988 || 3.8–4.5 || 1.92–2.07&nbsp;m  || 0.97&nbsp;m || 0.45–0.47&nbsp;t || 23.4–35.6&nbsp;kN
| [[Citation Sovereign]], [[Gulfstream G200|G200]], [[Falcon 7X]], [[Falcon 2000]]
|-
| [[Honeywell HTF7000|HW HTF7000]]
| 1999 || 4.4   || 2.29&nbsp;m || 0.87&nbsp;m || 0.62&nbsp;t || 28.9&nbsp;kN
| [[Challenger 300]], [[Gulfstream G280|G280]], [[Embraer Legacy 500|Legacy 500]]
|-
| [[Garrett TFE731|HW TFE731]]
| 1970 || 2.66–3.9  || 1.52–2.08&nbsp;m || 0.72–0.78&nbsp;m || 0.34–0.45&nbsp;t || 15.6–22.2&nbsp;kN
| [[Learjet 70/75]], [[G150]], [[Falcon 900]]
|-
| [[Williams FJ44]]
| 1985 || 3.3–4.1  || 1.36–2.09&nbsp;m || 0.53–0.57&nbsp;m || 0.21–0.24&nbsp;t || 6.7–15.6&nbsp;kN
| [[Cessna CitationJet|CitationJet]], [[Cessna Citation M2|Citation M2]]
|-
| [[Pratt & Whitney Canada PW500|P&WC PW500]]
| 1993 || 3.90  || 1.52&nbsp;m || 0.70&nbsp;m || 0.28&nbsp;t || 13.3&nbsp;kN
| [[Citation Excel]], [[Phenom 300]]
|-
| [[GE-Honda HF120|GE-H HF120]]
| 2009 || 4.43   || 1.12&nbsp;m  || 0.54&nbsp;m || 0.18&nbsp;t || 7.4&nbsp;kN
| [[HondaJet]]
|-
| [[Williams FJ33]]
| 1998 ||   || 0.98&nbsp;m || 0.53&nbsp;m || 0.14&nbsp;t || 6.7&nbsp;kN
| [[Cirrus SF50]]
|-
| [[Pratt & Whitney Canada PW600|P&WC PW600]]
| 2001 || 1.8–2.8   || 0.67&nbsp;m || 0.36&nbsp;m || 0.15&nbsp;t || 6.0&nbsp;kN
| [[Citation Mustang]], [[Eclipse 500]], [[Phenom 100]]
|-
| [[Aviadvigatel PS-90|PS-90]]
| 1992 || 4.4   || 4.96&nbsp;m || 1.9&nbsp;m || 2.95&nbsp;t || 157–171&nbsp;kN
| [[Il-76]], [[Il-96]], [[Tu-204]]
|-
| [[PowerJet SaM146]]
| 2008  || 4–4.1  || 3.59&nbsp;m || 1.22&nbsp;m || 2.260&nbsp;t || 71.6–79.2&nbsp;kN
| [[Sukhoi Superjet 100]]
|}

==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==
<!--Currently unsure how to add this to here currently as my brain isn't working. Please add the engine terms: Flight Idle, Ground Idle, High Idle, TOGA (Take-Off Go-Around)-->

; [[Afterburner]]: jetpipe equipped for afterburning<ref name="auto">The Cambridge Aerospace Dictionary, Bill Gunston 2004,{{ISBN|978 0 511 33833 5}}</ref>
; Augmentor: afterburner for turbofan with burning in hot and cold flows<ref name="auto"/>
; Bypass: that part of the engine as distinct from the core in terms of components and airflow, e.g. 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<ref>Jet Propulsion, Nicholas Cumpsty 1997, {{ISBN|0 521 59674 2}}, p.65</ref>
; Core: that part of the engine as distinct from the bypass in terms of components and airflow, e.g. 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,<ref>{{Cite journal |last1=Roth |first1=Bryce |last2=Mavris |first2=Dimitri |date=2000-07-24 |title=A comparison of thermodynamic loss models suitable for gas turbine propulsion - Theory and taxonomy |url=https://arc.aiaa.org/doi/10.2514/6.2000-3714 |journal=36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit |language=en |location=Las Vegas, NV, U.S.A. |publisher=American Institute of Aeronautics and Astronautics |pages=4–8 |doi=10.2514/6.2000-3714}}</ref> a measure of the energy wasted as a result of producing a force (i.e. thrust) in a fluid by increasing the speed (i.e. 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)<ref>The Cambridge Aerospace Dictionary, Bill Gunston 2004,{{ISBN| 978 0 511 33833 5}}</ref>
; 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").<ref>{{cite web | url=https://skybrary.aero/articles/reduced-thrust-takeoff | title=Reduced Thrust Takeoff | date=30 May 2021 }}</ref>
; 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<ref>Gas Turbine Performance Second Edition, Walsh and Fletcher 2004,{{ISBN|0 632 06434 X}}, p.5</ref>
; HP: high-pressure
; Intake ram drag: Loss in momentum of engine stream tube from freestream to intake entrance, i.e. 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, i.e. 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]]: amount of times the pressure increases due to ram compression and workndone by the compressor stages
; 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)
; Spooling down: decrease 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, i.e. with acceptable losses.<ref>Jet Engines and Propulsion Systems For Engineers, Human Resource Development, GE Aircraft Engines 1989, p.5-9</ref> 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 (i.e. losses) may be regarded as a loading limit.<ref>Aerodynamic Design Of Axial Flow Compressors, N65 23345,1965, NASA SP-36, p.68</ref>
; [[Stagnation pressure]]: also known as total pressure; pressure of the fluid if all the kinetic energy were to be converted into pressure isentropically; sum of static pressure and dynamic pressure
; [[Static pressure]]: pressure of the fluid which is associated not with its motion but with its state<ref>Clancy, L.J., ''Aerodynamics'', page 21</ref> or, alternatively, pressure due to the random motion of the fluid molecules that would be felt or measured if moving with the flow<ref>Introduction To Aerospace Engineering With A Flight Test Perspective, Stephen Corda 2017,{{ISBN|9781118953389}}, p.185</ref>
; [[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: also known as stagnation pressure; sum of static pressure and dynamic pressure; pressure of the fluid if all the kinetic energy were to be converted into pressure isentropically
; Turbine rotor inlet temperature: maximum cycle temperature, i.e. temperature at which work transfer takes place

==See also==
* [[Axial fan design]]
* [[Gas turbine]]
* [[Turbine engine failure]]
* [[Variable cycle engine]]

==References==
{{Reflist}}

==External links==
{{Commons category|Turbofan engines}}
* [[b:Jet Propulsion|Wikibooks: Jet propulsion]]
* {{cite web |url= https://www.hq.nasa.gov/office/aero/ebooks/downloads/nasa_innovation_in_aeronautics.pdf |author= Malcolm Gibson |work= NASA Innovation in Aeronautics NASA/TM-2011-216987 |date= Aug 2011 |title= The Chevron Nozzle: A Novel Approach to Reducing Jet Noise |access-date= 2017-03-24 |archive-date= 2020-03-23 |archive-url= https://web.archive.org/web/20200323200035/https://www.hq.nasa.gov/office/aero/ebooks/downloads/nasa_innovation_in_aeronautics.pdf |url-status= dead }}
* {{cite news |url= https://scribd.com/doc/105381018/Engine-Yearbook |title= The Engine Yearbook |date= 2012 |publisher= [[UBM Aviation]]}}
* {{cite news |url= https://www.flightglobal.com/asset/17069 |title= Commercial engines 2017 |work= Flight Global}}
* {{cite news |url= https://leehamnews.com/2017/04/14/bjorns-corner-aircraft-engines-sum/ |title= Bjorn's Corner: Aircraft engines, sum up |date= April 14, 2017 |work= Leeham Co |author= Bjorn Fehrm}} and previous series

{{Aircraft gas turbine engine components}}
{{Heat engines|state=uncollapsed}}

[[Category:Gas turbines]]
[[Category:Jet engines]]
[[Category:Turbofan engines| ]]