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In addition to my previous question, I am mainly concerned with burning all the fuel.

  • Why isn't the combustion process using all of the Oxygen in the air? Is it because other gases are in the way and it isn't in the right place to help the reaction?

  • Are engineers limiting the amount of fuel into the process because they know some of the Oxygen won't be available for combustion due to its proximity(lack thereof) to the fuel?

  • How much fuel remains un-burnt in a modern jet engine? I thought that an after-burner was meant to use the fuel that somehow got through the whole process without combusting?

  • Wouldn't using pure O2 allow for a complete fuel burn? I do get that the added weight may not make it worthwhile since we need to weigh the value of complete burn against other non-monetary concerns(like pollution).

Finally, I assume the 2000-2500 degree C temp mentioned (in this answer) in the last post is to actually burn Oxygen. Although I know Oxygen is not flammable, I am sure everything does burn at some point. I do not want to actually burn the Oxygen.

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Jon Rausch
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  • So far as I can tell this is covered by this answer to your previous question. If you want to better understand how oxygen and flammability relate, it might be better to ask that specific question in Physics.se or Chemistry.se – RedGrittyBrick Dec 08 '14 at 14:55
  • @RedGrittyBrick: It seems Jon was not completely happy with the answer, and he should ask until we can provide a satisfying explanation. Besides, the answer you cite did not get all the facts right. – Peter Kämpf Dec 08 '14 at 15:46
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    Ugh, voting to re-open against my better judgement - There are too many questions here, perhaps making this too broad. – CGCampbell Dec 09 '14 at 14:59
  • @CGCampbell If you believe that the question should be closed, don't vote to re-open it! Reopening a question that was closed for the wrong reason, just so it can be closed for the right reason isn't a productive use of anyone's time. – David Richerby Dec 10 '14 at 08:58
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    If you're going to carry all your propellant and oxidizer, well, you now have a rocket engine... – Brian Knoblauch Dec 08 '15 at 17:50
  • Oxygen cannot burn as burning (combustion) is the chemical reaction of a material with Oxygen. Basically, all the fuel that goes into the combustion chamber is burnt. If all the Oxygen was used for combustion there could be no afterburners without adding fresh air into the exhaust. Most of the air that goes into a straight jet is actually used to cool the combustion chamber, to protect it from the flame which burns at about 2100°C (https://en.wikipedia.org/wiki/Adiabatic_flame_temperature) (fuel oil flame temperature) – Chris V Dec 21 '15 at 22:09

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The compression in the forward part of a jet engine heats the air, and combustion raises its temperature again. In order to achieve efficient operation, this heating must be restricted. If air is heated above approx. 2,000 K, adding more energy will result in dissociation of the gas with less further heat increase. Since thrust is produced by expanding air through heating, raising combustion temperatures above 2,000 K will result in less thrust increase for the amount of fuel consumed. The most advanced engines (F-119 and EJ-200) have turbine entry temperatures of 1800 K, and civilian engines operate a few 100 K below that. The number of 2000 - 2500°C (2300 - 2800 K) you got from a previous answer is way too high.

Oxygen starts to dissociate already between 2,000  and 4,000 K, depending on pressure, while Nitrogen will dissociate mainly above 8,000 K. The 2,000 K figure above is a soft limit, but this temperature is also a challenge for the materials of the combustion chamber and the turbine, and techniques like film cooling are mandatory. As you can see, using pure oxygen would greatly increase the efficiency loss due to dissociation once technology allows to run the turbine at higher temperatures than 1800 K. It would be better for jet engines if air had a lower oxygen content.

To make the best use of the fuel, much more air than would be needed for stochiometric combustion is used in a modern turbojet or turbofan core. The idea is to heat the gas just as much as technically and economically possible.

Too much oxygen does create problems, mainly by raising the amount of nitrous oxides a jet engine produces. However, it helps to achieve largely complete combustion, even if some of the reaction happens not in the combustor, but at later stages. Combustion is never fully complete, and the remaining unburnt constituents are proportional to an inverse exponential function over time.

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    " It would be better for jet engines if air had a lower oxygen content." - Does that mean it would help to feed back part of the exhaust gas to the intake air? – Volker Siegel Jun 02 '15 at 05:45
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    "Too much oxygen does create problems" Not least of which is that it converts your "jet engine" into a "rocket engine". – Aron Sep 07 '15 at 08:21
  • @VolkerSiegel, the exhaust gas is already hot and therefore useless. More cold gas would help, but the only cold gas practically available is ambient air. – Jan Hudec Dec 20 '15 at 22:25