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The meaning of jet engine performance has been phrased as 'the end product that a jet engine company sells'<ref>Gas Turbine Performance, Second Edition,Walsh and Fletcher 2004,{{ISBN|0 632 06434-X}}, Preface </ref> and, as such, criteria include thrust and fuel consumption, life, weight, emissions, diameter and cost. Performance criteria reflect the level of technology used in the design of an engine and the technology has been advancing continuously since the jet engine entered service in the 1940s. Categories of performance include performance improvement, performance deterioration, performance retention, bare engine performance (uninstalled) and performance when part of an aircraft powerplant (installed).
Jet engine performance (thrust and fuel consumption) for a pilot is displayed in the cockpit as engine pressure ratio (EPR) and exhaust gas temperature (EGT) or fan speed (N1) and EGT. EPR and N1 are indicators for thrust. EGT is an indicator for fuel flow but more importantly is a health monitor<ref>"EGT margin indicates engine health"' pp.
The performance of an engine is calculated using a thermodynamic analysis of the engine cycle. It works out what happens inside the engine. The conditions inside the engine, together with the fuel used and thrust produced, may be shown in a convenient tabular form summarising the analysis.<ref>{{Cite web |title=A Variable Cycle Engine for Subsonic Transport Applications - PDF Free Download |url=https://docplayer.net/140309337-A-variable-cycle-engine-for-subsonic-transport-applications.html |access-date=2023-11-16 |website=docplayer.net}}</ref>
==Introduction==
An introductory look at jet engine performance may be had in a cursory but intuitive way with the aid of diagrams and photographs which show features that influence the performance. An example of a diagram is the velocity triangle which in everyday life tells cyclists why they struggle against wind from certain quarters (and where head-on is worst) and in the engine context shows the angle air is approaching compressor blades (head-on is best for low losses). The use of velocity triangles in compressors and turbines to show the all-important angle at which air approaches the blading goes back to early steam turbines.<ref>https://arc.aiaa.org/doi/abs/10.2514/1.9176?journalCode=jpp,"Ideas and Methods of Turbomachinery Aerodynamics: A Historical View", Cumpsty and Greitzer, Fig. 1</ref>
Photographs show performance-enhancing features such as the existence of bypass airflow (increased propulsive efficiency) only visually obvious on engines with a separate exit nozzle for the bypass air. They are also used to show rarely seen internal details such as honeycomb seals which reduce leakage and save fuel (increased thermal efficiency), and degrading details such as the rub marks on centrifugal impeller blades which indicate loss of material, increased air leakage and fuel consumption.
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File:Marquardt RJ43-MA-9 Ramjet Engine - sectioned.jpg|[[Marquardt RJ43]] supersonic ramjet. This cutaway museum exhibit shows the 3 components of a ramjet, diffuser, combustion chamber and nozzle. At supersonic airspeeds air compression starts at the tip of the diffuser cone and continues internally due to internal air passage contours between the black centerbody and duct inner wall as far as the red high-blockage grid<ref>{{Cite book |url=https://discovery.nationalarchives.gov.uk/details/r/530c316d-1e47-4593-afd4-cf603ac01b27 |title=AGARD (Advisory Group for Aerospace Research and Development), Lecture Series No.136: Ramjet and Ramrocket Propulsion Systems for Missiles |date= |language=English}}</ref> then combustion in the cylindrical section after the yellow fuel nozzles and as far as the nozzle entry, then expansion through the convergent/ divergent nozzle.<ref name=":0">{{Cite
File:Meccanismo della spinta dell'autoreattore.png|This purpose of this sketch is to show that there are forward acting pressure forces and rearward acting forces inside the engine and the forward are greater than the rearward so forward thrust is the result. A typical ramjet pressure distribution over all the internal surfaces is shown by Thomas.<ref name=":0" /> Combustion of the fuel in a ramjet, in area shown red, causes the air to expand. The ramjet is shown moving to the left and the ram pressure rise (P1) in the diffuser (diffusore) is maintained by the expanding gas which can only accelerate rearwards in the presence of the ram rise. Thrust (Sd) comes from the pressure acting on the rear-facing diffuser surfaces. If a nozzle (ugello) restriction is included, as shown but not necessary for the production of thrust,<ref>{{Cite
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==Conversion of fuel into thrust and waste==
[[File:F-GSTF Beluga Airbus 5 (8138504167).jpg|thumb|Visual evidence of jet engine waste is the distorted view through the high temperature jet wakes from the core of the engine. "The efficiency of a gas turbine can be increased by reducing the proportion of heat that goes to waste, that is, by reducing the temperature of the exhaust."<ref>"Gas Turbines And Their Problems", Hayne Constant, Todd Reference Library, Todd Publishing Group Ltd., 1948, p. 46</ref> Less waste is involved in producing most of the thrust (~ 90%) of a modern civil bypass engine since the bypass air is barely warm, only 60
The waste leaving a jet engine is in the form of a wake which has 2 constituents, one mechanical, called the residual velocity loss (RVL) due to its kinetic energy, and the other thermodynamic, due to its high temperature. The waste heat in the exhaust of a jet engine can only be reduced at source by addressing the loss-making processes and entropy generated as the air flows through the engine. For example, a more efficient compressor has lower losses, generates less entropy and contributes less to the temperature of the exhaust leaving the engine. Another example is the transfer of energy from an engine to air bypassing the engine. In the case of a high bypass engine there is a large proportion (~90%) of barely-warm (~60 degF warmer than ambient) thrust-producing air with only a 10% contribution from the much hotter exhaust from the power-producing core engine. As such, Struchtrup et al.<ref>https://www.researchgate.net/publication/252167474_External_losses_in_high-bypass_turbo_fan_air_engines, Introduction</ref> show the benefit of the high bypass turbofan engine from an entropy-reducing perspective instead of the usual propulsive efficiency advantage.
The power expenditure to produce thrust consists of two parts, thrust power from the rate of change of momentum and aircraft speed, and the power represented by the wake kinetic energy.<ref name=":1">{{Cite
Entropy, identified as 's', is introduced here because, although its mathematical meaning is acknowledged as difficult,<ref>{{Cite journal |last=Smith |first=Trevor I. |last2=Christensen |first2=Warren M. |last3=Mountcastle |first3=Donald B. |last4=Thompson |first4=John R. |date=2015-09-23 |title=Identifying student difficulties with entropy, heat engines, and the Carnot cycle |url=https://link.aps.org/doi/10.1103/PhysRevSTPER.11.020116 |journal=Physical Review Special Topics - Physics Education Research |volume=11 |issue=2 |pages=020116 |doi=10.1103/PhysRevSTPER.11.020116}}</ref> its common representation on a Temperature~entropy (T~s) diagram for a jet engine cycle is graphical and intuitive since its influence is shown as areas of the diagram. The T~s diagram was invented to help engineers responsible for the operation of steam engines to understand the efficiency of their engines. It supplemented the already-existing p~v diagram which only gave half the heat engine efficiency story in only showing the cylinder work done with no reference to the heat supplied and wasted in doing so. The need for an additional diagram, as opposed to understanding difficult theories, recognized the value of graphically representing heat transfers to and from an engine.<ref>Transactions The Manchester Association of Engineers 1904, The Temperature-Entropy Diagram,Mr.G.James Wells,p237</ref> It would show areas representative of heat converted to work compared to heat supplied (thermal efficiency).<ref>{{Cite book |last= |url=http://archive.org/details/reportcommittee05unkngoog |title=Report of the committee appointed on the 31st March, 1896, to consider and report to the Council upon the subject of the definition of a standard or standards of thermal efficiency for steam-engines .. |date=1898 |publisher=London, the Institution
The mathematical meaning of entropy, as applicable to the gas turbine jet engine, may be circumvented to allow use of the term in connection with the T~s diagram:
Quoting [[Frank Whittle]]:<ref>"Gas Turbine Aero-thermodynamics",Sir Frank Whittle,{{ISBN|0-08-026718-1}},p.2</ref> "Entropy is a concept which many students have a difficulty in assimilating. It is a somewhat intangible quantity...". Entropy is generated when energy is converted in to an unusable form analogous to the loss of energy in a waterfall where the original potential energy is converted to unusable energy of turbulence.
Cumpsty says<ref>{{Cite book |last=Cumpsty |first=N. A. |url=http://archive.org/details/jetpropulsionsim0000cump |title=Jet propulsion : a simple guide to the aerodynamic and thermodynamic design and performance of jet engines |date=1997 |publisher=Cambridge
Denton compares it with aircraft drag, which is intuitive, "For an aircraft the ultimate measure of lost performance is the drag of its components....entropy creation reflects loss of efficiency in jet engines".<ref>Entropy Generation In Turbomachinery Flows"'Denton, SAE 902011, p. 2251</ref> He uses an analogy which imagines any inefficiency mechanism, such as the creation of whirls in the airflow, as producing smoke. Once created it cannot be destroyed and the concentration at the exit of the engine includes contributions from all loss-producing sources in the engine. The loss of efficiency is proportional to the concentration of the smoke at the exit.<ref>"Loss mechanisms in Turbomachines"'Denton,ASME 93-GT-435,p.4</ref>
Thrust is generated inside a jet engine by internal components as they energize a gas stream.<ref>{{Cite web |date=2023-10-24 |title=Jet engine {{!}} Engineering, Design, & Functionality {{!}} Britannica |url=https://www.britannica.com/technology/jet-engine |access-date=2023-11-16 |website=
Fuel energy released in the combustor is accounted for in two main categories: acceleration of the mass flow through the engine and residual heat.<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= |publisher=American Institute of Aeronautics and Astronautics |
Acceleration of the flow through the engine causes simultaneous production of kinetic energy accompanying the thrust-producing backward momentum. The kinetic energy is left behind the engine without contributing to the thrust power<ref>'Jet Propulsion For Airplanes',Buckingham,NACA report 159,p.85</ref> and is known as residual velocity loss. The thrust force from a stationary engine becomes thrust power when an aircraft is moving under its influence.
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File:Schematic diagram of a heat engine02.jpg|This depiction of a jet engine as a [[heat engine]] shows that significant energy is wasted in the production of work, the energy balance being W=QH - Qa.<ref>{{Cite book |last=rayner joel |url=http://archive.org/details/heatengines0000rayn |title=heat engines |date=1960 |others=Internet Archive}}</ref> There is heat transfer QH from continuous combustion at TH to the airflow in the combustor, and simultaneous kinetic energy production W and energy dissipation with heat transfer Qa on leaving the engine to the surrounding atmosphere at Ta.
File:Joule-T-s-diagram.jpg|The T~s diagram (absolute temperature, T, and entropy, s,) is a graphic representation of two heat transfers, represented by areas of the diagram, and an area (blue-lined) representing mechanical work but in heat units. Heat transfer to the engine Qzu is area between line 2-3 and x-axis. Heat transferred to atmosphere Qab is area between line 1-4 and x-axis and the difference between the areas is the thermal energy converted to kinetic energy Wi.<ref>{{Cite journal |last=Kurzke |first=Joachim |last2=Halliwell |first2=Ian |date=2018 |title=Propulsion and Power |url=https://link.springer.com/book/10.1007/978-3-319-75979-1 |journal=SpringerLink |language=en |doi=10.1007/978-3-319-75979-1}}</ref> For a real engine, with flow losses (entropy-producing processes), the area of Wi (useful output) shrinks within the heat added area since less heat is converted to work and more is rejected in the exhaust. <ref>{{Cite
File:Ts Real Brayton Cycle 2.png|The black-line diagram represent a jet engine cycle with maximum pressure p2 and temperature T3. When component inefficiences are incorporated for a real engine the blue-lined area is the result which shows that entropy is increased in each process, including the combustion pressure loss from p3 tp p3', by the loss-making characteristics of air flow, such as friction, through each.<ref name=":2">{{Cite book |last=Mattingly |first=Jack D. |url=https://arc.aiaa.org/doi/book/10.2514/4.103711 |title=Elements of Propulsion: Gas Turbines and Rockets, Second Edition |last2=Boyer |first2=Keith M. |date=2016-01-20 |publisher=American Institute of Aeronautics and Astronautics, Inc. |isbn=978-1-62410-371-1 |___location=Reston, VA |language=en |doi=10.2514/4.103711}}</ref> Afterburning adds area to the cycle beyond line 3-4. The diagram also applies to a turbofan core cycle and an additional, smaller diagram<ref name=":2" /> is required for the bypass compression, bypass duct pressure loss and fan nozzle expansion.<ref>{{Cite journal |last=Lewis |first=John Hiram |date= |title=Propulsive efficiency from an energy utilization standpoint |url=https://arc.aiaa.org/doi/10.2514/3.44525 |journal=Journal of Aircraft |language=en |volume=13 |issue=4 |pages=299–302 |doi=10.2514/3.44525 |issn=0021-8669}}</ref>
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Since the introduction into service of the bypass principle in xx a progressively greater proportion of bypass air compared to that passing through the power-producing core has been enabled by increases in core power per pound a second of core airflow (specific core power).
A statement which illustrates the connection between the fan and core engine of a high bypass engine is attributed to Moran.<ref>"Engine Technology Development to Address Local Air Quality Concerns", Moran, ICAO Colloquium on Aviation Emissions with Exhibition,
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File:10+27 German Air Force Luftwaffe Airbus A310-304 MRTT General Electric CF6-80C2 engine ILA Berlin 2016 01.jpg|Turbofan (CF-6) inlet and fan. The core flow area, 1/6th, is visible through the fan. A comparison of how effective the subsonic inlet is at compressing air compared with the fan is given by inlet ram and fan temperature rises for a CFM56 of about 30 and 40 degF at 0.85 Mn cruise.<ref>{{Cite web |title=A Variable Cycle Engine for Subsonic Transport Applications - PDF Free Download |url=https://docplayer.net/140309337-A-variable-cycle-engine-for-subsonic-transport-applications.html |access-date=2023-11-16 |website=docplayer.net}}</ref> Temperature rise is connected to pressure rise by the losses incurred in the way the compression is achieved and all three are visually apparent on a T~s diagram.
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Although EPR is directly related to thrust over the flight envelope American Airlines experience with their first jet engines, [[Pratt & Whitney JT3C]], was marred by instrumentation problems so the cockpit reading was questioned and other parameters, FF and N1, were used by flight personnel in desperation.<ref>"American Airlines Experience with Turbojet/Turbofan Engines",Whatley,ASME 62-GTP-16</ref>
EPR is based on pressure measurements with the sampling tubes vulnerable to getting blocked. [[Air Florida Flight 90]] crashed on take-off in snow and icing conditions. The required take-off thrust was 14,500 lb which would normally be set by advancing the thrust levers to give an EPR reading of 2.04. Due to EPR probe icing the value set, ie 2.04, was erroneous and actually equivalent to 1.70 which gave an actual thrust of only 10,750 lb. The slower acceleration took 15 seconds longer than normal to reach lift off speed and contributed to the crash.<ref>{{cite web|title=
EGT readings can also be misleading. The temperature of the gas leaving the turbine increases with engine use as parts become worn but the [[Strategic Air Command]] approved J57 and TF33 engines for flight without knowing they had bent and broken turbine parts. They were misled by low-reading EGT which indicated, when taken at face value, that the engines were in acceptable condition. It was found that the EGT probes were not positioned correctly to sample a representative gas temperature for the true condition of the engine.<ref>Who needs engine monitoring?, Aircraft Engine Diagnostics, NASA CP2190, 1981, p.214</ref>
==Performance improvement==
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Sealing of the stators was initially accomplished using knife-edge fins on the rotating part and a smooth surface for the stator shroud. Examples are the Avon and Tumansky R-11. With the invention of the honeycomb seal the labyrinth seal has an abrazive honeycomb shroud which is easily cut by the rotating seal teeth without overheating and damaging them.<ref>Selecting a Material For Brazing Honeycomb in Turbine Engines, Sporer and Fortuna, Brazing and Soldering Today February 20014,p.44</ref> Labyrinth seals are also used in the secondary air system between rotating and stationary parts. Example locations for these are shown by Bobo.<ref>https://patents.google.com/patent/US2963307, "Honeycomb seal" Fig.1</ref>
Tip clearance between compressor and turbine blades<ref>https://www.yumpu.com/en/document/view/33920940/8th-israeli-symposium-on-jet-engine-and-gas-turbine, slide 'Effect of tip clearance on turbine efficiency'</ref> and their cases is a significant source of performance loss.
Much of the loss in compressors is associated with tip clearance flow.<ref>Current Aerodynamic Issues For Aircraft Engines,Cumpsty,11th Australian Fluid Mechanics Conference,University of Tasmania,14-18 December 1992,p.804</ref> For a CFM56 engine an increase in high pressure turbine tip clearance of 0.25 mm causes the engine to run 10
Tip clearances have to be big enough to prevent rubbing when they tend to close up during carcase bending, case distortion from thrust transfer, centre-line closure when the compressor case shrinks onto the rotor diameter( rapid reduction in temperature of air entering the engine), thrust setting changes (controlled by Active Clearance Control using compressor rotor cooling and turbine case cooling).
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[[File:P&W JT9D cutaway.jpg|right|The Pratt & Whitney JT9D with a big increase in thrust over the JT8D raised awareness how to transfer engine thrust to the aircraft without bending the engine too much and causing rubs and performance deterioration.<ref>Flight International, 13 November 1969, p.749</ref>.]]
Gas path deterioration and increasing EGT coexist. As the gas path deteriorates the EGT limit ultimately prevents the take-off thrust from being achieved and the engine has to be repaired.<ref>Aircraft Engine Diagnostics,NASA CP 2190, 1981,JT8D Engine Performance Retention, p.64</ref>
The engine performance deteriorates with use as parts wear, meaning the engine has to use more fuel to get the required thrust. A new engine starts with a reserve of performance which is gradually eroded. The reserve is known as its temperature margin and is seen by a pilot as the EGT margin. For a new [[CFM International CFM56]]-3 the margin is 53
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