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I'm reading this article on afterburners:

https://www.airspacemag.com/flight-today/how-things-work-afterburners-18481403/

It says in the article:

A typical jet engine uses only about half the oxygen it ingests, leaving a large amount of potential energy. The afterburner, which is a long extension at the back of the engine, combines much of the remaining oxygen with jet fuel, squirted into the high-speed exhaust stream from the engine’s turbine, and ignites the mixture.

I've searched many places but I've never really understood why the engine only uses about half of the oxygen it ingests. Instead, the afterburner is needed to burn more oxygen. But why is not possible to design jet engines that burn more oxygen instead of adding another separate component, the afterburner? Why do jet engines only burn about a half of the oxygen ingested?

S. Rotos
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2 Answers2

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After burner primary task is to add extra thrust to a jet engine by heating the jet stream after the turbine via injecting and burning fuel in it.

The reason they just don't add more fuel in the jet engine before turbine even though it would be more economical is the very high temperatures of the engine just before the turbine.

The fuel injection into the jet engine is restricted by the temperatures not available oxygen consumption.

kamran
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    To further quantify "very high temperatures", usually the air going past the turbine blades is a good 20 deg hotter than the melting point of the alloy. The only reason the blades don't melt is because of an intricate cooling system (e.g. https://netl.doe.gov/File%20Library/Research/Coal/energy%20systems/turbines/handbook/4-2-2-1.pdf). Getting the turbine any hotter than it already is would be very difficult. – Daniel K Jan 28 '18 at 01:13
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There are some parts to your question:

  • Part 1: The highest temperature is achieved with a air-fuel-ratio below 1.
  • Part 2: The temperature limit of available materials cooling processes is even lower.
  • Part 3: Given an upper temperature limit the thermodynamic-cycle of a jet-engine can only deliver more power by re-heating the exhaust-gases.
  • Part 4: Every design-process is a trade-off between multiple factors. For the after-burner two factors are efficiency and complexity of the engine.

Part 1: Combustion Reactions
Depending on the air-fuel-ratio the combustion-temperature will vary. The following figure shows the relation. Observe that the maximum power (think temperature) is not achieved for a air-fuel-ratio of 1 (, i.e. stoichiometric combustion, $\lambda = 1$). This is due to dissociation of the combustion products.
enter image description here

Part 2: Temperature Limit of Materials
Due to the high mechanical loads on the rotating turbine parts (aero- and centrifugal-loads) the maximum allowable temperature is below the theoretical maximum limit. The following figure shows the how the development of new materials did allow for higher temperatures in the jet-engine1. enter image description here

Part 3: Thermodynamic Cycle
The basic (idealised) thermodynamic cycle of a jet-engine is called Brayton-Cycle. The following figure shows this cycle. enter image description here The enthalpy-difference between state 5 and 6 is what is converted into thrust. Given the temperature limits the only way to increase the power output of the thermodynamic cycle (vertical distance between state 5 and 6) is to re-heat the turbine-exhaust. enter image description here Observe that the temperature limit for stationary parts is higher, this is also due to the lower operating hours of the after-burner.

Part 4: Engineering considerations, trade-offs
Usually the additional thrust produced by the after-burner is not needed throughout the whole mission. Therefore it is acceptable to sacrifice efficiency for mechanical simplicity. enter image description here The shown EJ-200 also show the engine stations. It is easily seen the difference in mechanical complexity between stations 4-5 and stations 5-6.

Conclusion:

But why is not possible to design jet engines that burn more oxygen instead of adding another separate component, the afterburner?

It is theoretical possible to design an engine which would burn all oxygen. However, until now this was not necessary.

Why do jet engines only burn about a half of the oxygen ingested?

Because fuel is burned for maximum heat not to deplete oxygen. The maximum temperature is limited by material and dissociation.

[1]: engineering.virginia.edu

rul30
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