Jet engine performance: Difference between revisions

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A visual introduction to jet engine performance, from the fuel efficiency point of view, is the Temperature~entropy (T~s) diagram. The diagram originated in the 1890s for evaluating the thermal efficiency of steam engines. At that time entropy was introduced in graphical form in the T~s diagram which gives thermal efficiency as a ratio of areas of the diagram. The diagram also applies to air-breathing jet engines with an area representing kinetic energy<ref name="Propulsion and Power">{{Cite journal |last1=Kurzke |first1=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|isbn=978-3-319-75977-7 }}</ref> added to the air flowing through the engine. A propulsion device, a nozzle, has to be added to a gas turbine engine to convert its energy into thrust. The efficiency of this conversion (Froude or propulsive efficiency) reflects work done in the 1800s on ship propellers. The relevance for gas turbine-powered aircraft is the use of a secondary jet of air with a propeller or, for jet engine performance, the introduction of the bypass engine. The overall efficiency of the jet engine is thermal efficiency multiplied by propulsive efficiency ( <math>\eta_o = \eta_{th} \eta_{pr}</math>).

There have been rapid advances in aero-engine technology since jet engines entered service in the 1940s. For example, in the first 20 years of commercial jet transport from the Comet 1 Ghost engine to the 747 JT9D Hawthorne<ref>{{Cite journal |last=Hawthorne |first=William |date= 1978|title=Aircraft propulsion from the back room |url=https://www.cambridge.org/core/journals/aeronautical-journal/article/abs/aircraft-propulsion-from-the-back-room/771675086CDE0E766BE700CD6B3198E7 |journal=The Aeronautical Journal |language=en |volume=82 |issue=807 |pages=93–108 |doi=10.1017/S0001924000090424 |s2cid=117522849 |issn=0001-9240}}</ref> scales up the Ghost to give JT9D take-off thrust and it is four and a half times as heavy. Gaffin and Lewis<ref>{{Cite journal |last1=Gaffin |first1=William O. |last2=Lewis |first2=John H. |date= 1968|title=Development of the High Bypass Turbofan |url=https://nyaspubs.onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.1968.tb15216.x |journal=Annals of the New York Academy of Sciences |language=en |volume=154 |issue=2 |pages=576–589 |doi=10.1111/j.1749-6632.1968.tb15216.x |bibcode=1968NYASA.154..576G |s2cid=84722218 |issn=0077-8923}}</ref> make an assessment using one company's design knowledge. Using JT3D-level technology (1958) to produce a JT9D cycle (1966), with its higher bypass ratio and pressure ratio, an hypothetical engine came out 70% heavier, 90 % longer and with a 9 % bigger diameter than the JT9D engine.

==Conversion of fuel into thrust==

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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 report |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 |language=en}}</ref>

Entropy, identified as 's', is introduced here because, although its mathematical meaning is acknowledged as difficult,<ref>{{Cite journal |last1=Smith |first1=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 |journal=Physical Review Special Topics - Physics Education Research |volume=11 |issue=2 |page=020116 |doi=10.1103/PhysRevSTPER.11.020116|doi-access=free |arxiv=1508.04104 |bibcode=2015PRPER..11b0116S }}</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, p. 237</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}}</ref>

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:

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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=Britannica |language=en}}</ref>

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= September 1966|publisher=American Institute of Aeronautics and Astronautics |via=Internet Archive |language=English}}</ref>

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:Marquardt RJ43-MA-9 Ramjet Engine - sectioned.jpg|[[Marquardt RJ43]] ramjet cutaway museum exhibit. A ramjet is a propulsive duct in which high velocity air is converted into pressure in a diffuser, heat is added and the air leaves with a higher velocity. For this particular supersonic ramjet compression takes place starting at the tip of the inlet spike and ending at the red-coloured high-blockage grid, this length constitutes the diffuser. Combustion occurs from the beginning of the cylindrical section to the nozzle and expansion takes place in the convergent-divergent nozzle.

File:Pratt & Whitney JT3.jpg|[[Pratt & Whitney J57]] turbojet (1/4 scale model). A turbojet uses its thermodynamic cycle gas as its propelling jet. The jet velocity exceeds the speed of a subsonic aircraft by too great an amount to be an economical method of subsonic aircraft propulsion. The purpose behind the jet engine is to convert fuel energy into kinetic energy of the cycle air but after the thrust-producing momentum has appeared the unwanted byproduct is the wake velocity which results in kinetic energy loss, known as residual velocity loss (RVL). The wake velocity behind a turbojet-powered aircraft at subsonic speed is about 600 mph. At maximum propeller-driven speeds, the wake velocity behind the propeller it replaced as a thrust producer is about 10 mph with an insignificant RVL.<ref>{{Cite book |last=Smith G. Geoffrey |url=http://archive.org/details/in.ernet.dli.2015.19428 |title=Gas Turbines And Jet Propulsion For Aircraft |date=1946}}</ref> It is impossible to transform completely the kinetic energy acquired inside the engine into thrust work. The whole increase in kinetic energy obtained inside the engine is expended in thrust work and losses of kinetic energy outside the engine. There is thus kinetic energy inside the engine which will remain unused. In the case of the stationary engine before take-off the whole kinetic energy turns into losses since the thrust force does no work.<ref>{{Cite report |url=https://apps.dtic.mil/sti/citations/AD0722283 |title=Theory of Jet Engines |language=en}}</ref>

File:Klimov VK-1F (1948) used in MiG 17 at Flugausstellung Hermeskeil, pic2.jpg|[[Klimov VK-1]]F turbojet with afterburner. An afterburner is a propulsive duct in which high velocity exhaust from an engine turbine is converted into pressure in a diffuser. Afterburner fuel is burned with the oxygen in the dilution air which was not involved in the engine combustion process. The gas expands in a nozzle with an increase in velocity. The turbojet afterburner has the same three requirements as a ramjet, both being propulsive ducts. These are conversion of high velocity gas into pressure in a diffuser, combustion and expansion to a higher velocity in a nozzle. As such the turbojet/afterburner combination was sometimes considered in the late 1940s a turbo-ramjet.<ref>{{Cite web |last= |first= |date= August 1947|title=Performance and Ranges of Application of Various Types of Aircraft-Propulsion System |url=https://digital.library.unt.edu/ark:/67531/metadc55496/ |access-date=2023-11-16 |website=UNT Digital Library |language=English}}</ref><ref>"Design of Tail Pipes for Jet Engines-Including Reheat", Edwards, ''The Aeronautical Journal'', Volume 54, Issue 472, Fig. 1.</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).

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==Thrust and fuel consumption==

Thrust and fuel consumption are key performance indicators for a jet engine. Improvements in thrust and fuel consumption are widely quoted for a new engine design compared to a previous to show that new technology has been incorporated which reduces fuel consumption. As an example the [[Rolls-Royce BR700|Pearl 10X]] turbofan has been reported as producing 8% more thrust and using 5% less fuel than the [[Rolls-Royce BR700|BR725]].<ref>{{cite web | url=https://aviationweek.com/shownews/ebace/rolls-royces-pearl-10x-set-747-flying-testbed-evaluation | title=Rolls-Royce's Pearl 10X Set for 747 Flying Testbed Evaluation &#124; Aviation Week Network }}</ref> Thrust and fuel consumption are combined in a single measure, specific fuel consumption (SFC), which reflects the level of technology used in the engine since it is fuel needed to produce one pound or Newton of thrust regardless of engine size. Two engines separated by about 50 years of gaining knowledge in jet engine design, the Pratt & Whitney JT3C and the Pratt & Whitney 1100G, illustrate a 50% reduction in SFC from 26 to 13 mg/Ns.<ref>On the design of energy efficient aero engines, Richard Avellan, 2011, {{ISBN|978-91-7385-564-8}}, Figure 6</ref>

Thrust is developed inside the engine as the components energize the gas stream.<ref>{{cite web | url=https://www.britannica.com/technology/jet-engine | title=Jet engine &#124; Engineering, Design, & Functionality &#124; Britannica | date=6 December 2023 }}</ref> The same thrust value manifests itself without consideration of what is happening inside the engine. Treating the engine as a [[black box]] thrust is calculated knowing the mass flow rate and velocity of the air entering the engine and the increased velocity of the exhaust leaving the engine. Observing this increase implies a rearward accelerating force has been applied to the gas inside the engine. Thrust is the equal and opposite reaction on the engine internal parts which is transferred to the aircraft through the engine mounts.

==Engine pressure ratio (EPR), low-pressure compressor speed (N1) and exhaust gas temperature (EGT)==

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=== Fan efficiency ===

Fan blades on modern engines have a wide [[Chord (aircraft)|chord]] which replaced conventional narrow chord blades which needed snubbers, or shrouds, to prevent them vibrating to an unacceptable degree. Increasing the length of the chord by an amount which made the blade stiff enough to not require snubbers also made the blade more resistant to damage caused by bird, hail and ice ingestion,<ref>{{Cite journal |last=Amoo |first=Leye M. |date=2013 |title=On the design and structural analysis of jet engine fan blade structures |journal=Progress in Aerospace Sciences |volume=60 |pages=1–11 |doi=10.1016/j.paerosci.2012.08.002|bibcode=2013PrAeS..60....1A }}</ref> and brought several unrelated benefits of improved efficiency, surge margin and noise reductions.<ref>https://asmedigitalcollection.asme.org/GT/proceedings/GT1988/79191/V002T02A017/236878,"Developing The Rolls-Royce Tay", Wilson, 88-GT-302</ref> There is also a greater axial distance for centrifuging debris away from the compressor inlet to prevent erosion of the airfoil surfaces which lowers compressor efficiency.

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The effects of heat transfer and friction in a combustor, both engine and afterburner, cause a loss of stagnation pressure and an increase in entropy. The loss in pressure is shown on a T~s diagram where it can be seen to reduce the area of the work part of the diagram. The pressure loss through a combustor has two contributions. One due to bringing the air from the compressor into the combustion area including through all the cooling holes (friction pressure loss), that is with air flowing but no combustion taking place. The addition of heat to the flowing gas adds another type of pressure loss (momentum pressure loss).

In addition to stagnation pressure loss the other measure of combustion performance is incomplete combustion. Combustion efficiency had always been close to 100 % at high thrust levels meaning only small amounts of HC and CO are present, but big improvements had to be made near idle operation. In the 1990s reduction of nitrogen oxides (NOx) became the focus due to its contribution to smog and acid rain for example. Combustor technology for reducing NOx is the Rich burn, Quick mix, Lean burn (RQL)<ref>https://www.researchgate.net/publication/271367881_The_Pratt_Whitney_TALON_X_Low_Emissions_Combustor_Revolutionary_Results_with_Evolutionary_Technology</ref> introduced by Pratt & Whitney with the TALON (Technology for Advanced Low NOx) PW4098 combustor.<ref>{{Cite journal |last1=Liu |first1=Yize |last2=Sun |first2=Xiaoxiao |last3=Sethi |first3=Vishal |last4=Nalianda |first4=Devaiah |last5=Li |first5=Yi-Guang |last6=Wang |first6=Lu |date=2017 |title=Review of modern low emissions combustion technologies for aero gas turbine engines |journal=Progress in Aerospace Sciences |volume=94 |page=15 |doi=10.1016/j.paerosci.2017.08.001|bibcode=2017PrAeS..94...12L |hdl=1826/12499 }}</ref> RQL technology is also used in the Rolls-Royce Phase 5 Trent 1000 combustor and the General Electric LEC (Low Emissions Combustor).<ref>"Engine Technology Development to Address Local Air Quality Concerns", Moran, ICAO Colloquium on Aviation Emissions with Exhibition, 14-16 May 2007</ref>

Engine combustor configurations are reverse-flow separate, straight-through separate, can-annular (all 3 historic because the annular flow chamber gives more area and more even flow to the turbine), and modern annular and reverse-flow annular. Fuel preparation for combustion is either done by converting it into small drops (atomization) or heating it with air in tubes immersed in flame (vaporization).

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=== Tip clearance changes with thrust changes ===

An engine is designed to run steady state at design points such as take-off, climb, and cruise with running clearances which minimize fuel use. Steady state means being at a constant rpm for long enough (several minutes) for all parts to have stopped moving relative to each other from transient thermal growths. During this time clearances between parts may close up to rubbing contact and wear to give larger clearances, and fuel consumption, at the important stabilized condition. This scenario inside the engine is prevented by internal compressor bore cooling<ref>"Jet Engines And Propulsion Systems For Engineers, GE Aircraft Engines 1989, pp. 8–10</ref> and external turbine casing cooling on big fan engines (active clearance control).

<ref>https://ntrs.nasa.gov/citations/20060051674 "Transient tip clearance" fig.1</ref><ref>https://patents.google.com/patent/US6126390A/en "Passive clearance control system for a gas turbine"</ref><ref>{{cite web | url=https://ntrs.nasa.gov/citations/20030003697 | title=Turbine Engine Clearance Control Systems: Current Practices and Future Directions | date=September 2002 }}</ref>

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File:Marbore IV.jpg|[[Turbomeca Marboré]] IV engine showing ___location of leakage between impeller blades and stationary shroud, shown sectioned and painted blue. This is the leak path for a centrifugal impeller equivalent to an axial blade tip to casing clearance.<ref name="AGARD" /> The clearance between the impeller vanes and their shroud is visible and has to be as small as possible without causing rubbing contact. This keeps leakage to a minimum and contributes to the efficiency of the engine.

File:CASING TREATMENT AND DAMAGED BLADES IN LOWER HALF OF J-85 COMPRESSOR CASING - NARA - 17419590.jpg|An example of the appearance of minor compressor blade tip rubs on their shrouds.

File:CFM56 High Pressure Turbine Blade.JPG|A used CFM56 high pressure turbine blade. New blades have 3 different-depth notches at the tip to aid visual assessment (using a borescope) of rubbed away material and consequent increase in tip clearance. 0.25 mm of lost blade-tip causes a 10 deg C loss of EGT margin.<ref>{{cite web | url=https://www.manualslib.com/manual/1589534/Cfm-Cfm56-Series.html?page=142#manual | title=CFM CFM56 Series Training Manual (Page 142 of 217) &#124; ManualsLib }}</ref>

File:CFM56 High Pressure Turbine Vane.JPG|CFM56 turbine nozzle guide vanes. The area for the combustor gas flow for the complete ring of vanes at the narrowest part of the passage is known as the turbine area. When the vane trailing edges deteriorate the area increases and the engine runs hotter, which causes increasingly rapid deterioration, and uses more fuel to reach take-off thrust.<ref>https://www.jstor.org/stable/171375,"The Nozzle Guide Vane Problem", Plante</ref>

File:Repair process for a V2500 high-pressure turbine guide vane (1).jpg|A V2500 vane showing thermal damage at the trailing edge which causes performance loss by altering the flow area.