Jet engine performance: Difference between revisions

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Jet engines perform in two basic ways, the combined effect of which determines how much waste they produce as a byproduct of burning fuel to do thrust work on an aircraft.<ref>An engine applies a thrust force to a stationary aircraft and thrust work is done on the aircraft when it moves under the influence.</ref> First is an energy conversion as burning fuel speeds up the air passing through which at the same time produces waste heat from component losses (thermal efficiency). Second, part of the power which has been given to the air by the engine is transferred to the aircraft as thrust work with the remaining part being kinetic energy waste in the wake (propulsive efficiency). The two efficiencies were first formulated in the 19th century for the steam engine (thermal efficiency <math>\eta_{th}</math>) and the ship's propeller (propulsive or Froude efficiency <math>\eta_{pr}</math>).

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 |lastlast1=Kurzke |firstfirst1=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 |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 |lastlast1=Gaffin |firstfirst1=William O. |last2=Lewis |first2=John H. |date= 1968|title=DEVELOPMENTDevelopment OFof THEthe HIGHHigh BYPASSBypass TURBOFANTurbofan |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 |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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==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&nbsp;°F above ambient at take-off. Only ~10% comes from the visible much hotter core exhaust, 900 deg above ambient.<ref>{{cite report |author=Kiran Siddappaji |title=Benefits of GE 90 representative turbofan through cycle analysis |date=November 2008 |doi=10.13140/RG.2.2.25078.50243 |url=https://www.researchgate.net/publication/323790787_Benefits_of_GE_90_representative_turbofan_through_cycle_analysis323790787}}</ref>]]

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&nbsp;°F 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>{{cite journal |author1=Henning Struchtrup |author2=Gwynn Elfring |title=External losses in high-bypass turbo fan air engines |date=June 2008 |journal=International Journal of Exergy |volume=5 |number=4 |page=400 |doi=10.1504/IJEX.2008.019112 |url=https://www.researchgate.net/publication/252167474_External_losses_in_high-bypass_turbo_fan_air_engines252167474}}</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 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 |lastlast1=Smith |firstfirst1=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 |page=020116 |doi=10.1103/PhysRevSTPER.11.020116|doi-access=free }}</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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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.

Zhemchuzhin et al.<ref>{{Cite book |lastlast1=Zhemchuzhin |firstfirst1=N. A. |url=http://archive.org/details/nasa_techdoc_19770023121 |title=Soviet aircraft and rockets |last2=Levin |first2=M. A. |last3=Merkulov |first3=I. A. |last4=Naumov |first4=V. I. |last5=Pozhidayev |first5=O. A. |last6=Frolov |first6=S. P. |last7=Frolov |first7=V. S. |date=1977-01-01 |others=NASA}}</ref> show an energy balance for a turbojet engine in flight in the form of a [[Sankey diagram]]. Component losses leave the engine as waste heat and add to the heat rejected area on a T~s diagram reducing the work area by the same amount.<ref name=":1" />

The engine does work on the air going through it and this work is in the form of an increase in kinetic energy. The increase in kinetic energy comes from burning fuel and the ratio of the two is the thermal efficiency which equals increase in kinetic energy divided by the thermal energy from the fuel (fuel mass flow rate x lower calorific value). The expansion following combustion is used to drive the compressor-turbine and provide the ram work when in flight, both of which cause the initial rise in temperature in the T~s diagram. The remainder of the T~s diagram expansion work is available for propulsion, but not all of which produces thrust work since it includes the residual kinetic energy<ref name="Aircraft">{{Cite journal |last=Lewis |first=John Hiram |date= 1976|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> or RVL.

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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 name="Propulsion and Power" /> 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 report |lastlast1=Weber |firstfirst1=Richard J. |last2=Mackay |first2=John S. |date=1958-09-01 |title=An Analysis of Ramjet Engines Using Supersonic Combustion |url=https://ntrs.nasa.gov/citations/19930085282 |language=en}}</ref>

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 inefficiencies 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 |lastlast1=Mattingly |firstfirst1=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 name="Aircraft" />

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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 |lastlast1=Liu |firstfirst1=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}}</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).