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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 °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}}</ref>]]
The waste leaving a jet engine is in the form of a wake which has two 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 °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 |bibcode=2008IJExe...5..400S |url=https://www.researchgate.net/publication/252167474}}</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>
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