Many Aircraft engines mix the bypass air with the core gases before exhausting to atmospheric pressure through a propelling nozzle. However, some do not mix and have separate nozzles for the flow of each. What are the advantages and disadvantages of each?

PS: I hope that I am clear now. Sorry if the question wasn't clear earlier.

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    $\begingroup$ You've got several very broad questions there. I'd suggest trying to narrow it down to one thing at a time so you can get a good answer, otherwise, this will probably get closed. It sounds like you're working on a design of your own - you might do better to define your current design (with pictures! or at least sketches), then list your alternate(s) for one area, and ask for pros & cons. When you get that sorted, ask the next question as a whole new question, referencing this one for background if necessary. $\endgroup$
    – FreeMan
    Mar 3 '16 at 13:22
  • $\begingroup$ @FreeMan: I hope that now it is short. $\endgroup$ Mar 3 '16 at 13:47
  • $\begingroup$ It is short, but still unclear to me what you're after. Maybe it makes more sense to someone with more of an engineering background. $\endgroup$
    – FreeMan
    Mar 3 '16 at 13:52
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    $\begingroup$ Don't let the close vote scare you off! Keep working at it to define your question better, and it will get reopened and answered. $\endgroup$
    – FreeMan
    Mar 3 '16 at 15:37

In simple terms, mixing the exhaust flow (with any bypass ratio) is good for all the reasons except for construction and weight. It is aerodynamically more efficient; it reduces noise (exhaust noise makes most of the jet noise); it cools combustion chamber (and other downwind parts) more efficiently, etc.

However, making the mixing chamber on a high bypass / big engine would require enormous outer nacelle to cover the whole engine (and extend somewhat beyond), and the weight penalty and complexity/cost negates all the benefits.

  • $\begingroup$ Not sure I follow your argument, since this mixing is commonly done on most if not all "modern" high-bypass ratio engines. Granted, some of them are quite enormous (I'm looking at you Trent XWB with your 118" fan.) $\endgroup$
    – FreeMan
    Apr 19 '16 at 13:15
  • $\begingroup$ @FreeMan, Trent XWB doesn't have a mixing chamber and has classical separate nozzles. Its nacelle is sizable, larger than it could have been in theory (perhaps because the fan pressure ratio is high, or for other aerodynamic reasons), but it would need to be some 50% longer to allow for steam mixing. Similarly, Trent 1000, GEnx, GE90 etc. are not mixing. $\endgroup$
    – Zeus
    Apr 20 '16 at 5:05

Advantage improved propulsion efficiency, stream Mn is affected by increase in temperature ie speed of sound changes. Disadvantages longer nacelle required to mix cold and hot streams, this increases bypass scrubbing loss, nacelle drag and weight & cost.


Early jet engines were called turbojets where all flow went through the turbine. In other words there is no bypass. These are the cigar shaped engines you see in early airliners and military fighters. These types of engines are very loud and whiny and required high fuel flows to achieve the amount of thrust required.

Bypass allows a few things, it provides a cooling flow of air to mix with turbine exhaust and lower the overall exit temperature so you don't overheat your nozzle. The combination of the two exhausts can also see gains in thrust for given fuel flows. One of the most significant gains of bypass flow is noise reduction by having this outer flow "masking" the turbine noise. This is desirable in airliners.

There are two types of bypass:
Low bypass is where there is more turbine flow than bypass flow. This is common in business jets and military fighters with a ratio of around 1.5:1 or 2.0.

High bypass where there is more bypass flow than turbine flow. This is common in modern airlines with bypass ratios upward so 10.0:1.

Low bypass: Low bypass engine
High Bypass: High bypass engine

In low bypass, the flow is remixed prior to exiting the nozzle, giving the cooler exit temperature typically to save the nozzle material. In a high bypass, the bypass air is allowed to flow outside of the engine after its high bypass compressor section, giving a very efficient airflow and adding to the overall thrust by moving a large volume of air outside the combustion and turbine section. This give greater fuel efficiency and much quieter engines for airplanes that can use them due to their size.

Below is an example of a low bypass F-100 engine which is found on F-15 and F-16 fighters.

F-100 engine

This is the GEnx high bypass engine found on 747-8 and 787.

GEnx engine

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    $\begingroup$ So what are the advantages for otherwise comparable engines? E.g. CFM-56 has separate streams and IAE V2500 has mixed stream, but have otherwise very similar parameters and are alternative engines for the A320-family aircraft. $\endgroup$
    – Jan Hudec
    Apr 18 '16 at 12:47
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    $\begingroup$ just a nitpick, if the bypass ratio is over 1.0, then there is more bypass flow than core flow $\endgroup$
    – costrom
    Apr 19 '16 at 15:27

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