Jet engine performance: Difference between revisions

Revision as of 09:47, 19 November 2023

Revision as of 09:50, 19 November 2023

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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 inreaseincrease 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 1940's a turbo-ramjet.<ref>{{Cite web |last= |first= |date= |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).

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, 14–16 May 2007</ref> "The fan provides THRUST(sic.). The Core provides the power to operate the Fan + some thrust." The equivalent may be said of the piston engine/propeller combination. "The propeller provides thrust. The engine provides the power to operate the propeller + some thrust (from the exhaust stubs)." The similarity between the two technologies is that the functions of the power producer and the thrust producer are separated. The thermodynamic and propulsive efficiencies are independent. For the turbojet though, any improvement which raised the cycle pressure ratio or turbine inlet temperature also raised the jetpipejet pipe temperature and pressure giving a higher jet velocity relative to aircraft velocity. As the thermal efficiency went up the propulsive efficiency went down. This interdependence was broken with the bypass engine.

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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&nbsp;°F 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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File:Fig. 14 Museo Motori Unipa GE J47.jpg|Early turbojet, [[General Electric J47]], 1947. The 11 stage compressor has a pressure ratio of 5.4:1.

File:IAE V2500 engine cutaway model 2010 The Sky and Space.jpg|[[IAE V2500]] turbofan (1987) with overall pressure ratio of about 35:1 which is generated by 1 fan, 4 low pressure and 10 high pressure compressor stages. By 2016 overall pressure ratio had reached 60:1 in the [[General Electric GE9X]].<ref>https://drs.faa.gov/browse/excelExternalWindow/DRSDOCID114483736420230203181002.0001?modalOpened=true,"Type Certificate Data Sheet E00095EN"</ref>

File:Pratt & Whitney Canada PW500 (EBACE 2023).jpg|[[Pratt & Whitney Canada PW500]] business jet PW530 turbofan showing HP compressor with 2 axial and centrifugal compressor last stage with backsweepback sweep and pipe diffusers. Overall pressure ratio about 13:1

File:EBACE 2019, Le Grand-Saconnex (EB190665).jpg|[[Honeywell F124]] jet trainer/light combat aircraft turbofan showing HP compressor with 4 axial and centrifugal last stage with high backsweep, splitter blades and leading edge sweep. Overall pressure ratio 19.4:1 from 3 axial fan, 4 axial HP and 1 centrifugal.

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