One thing that is plainly obvious about the design and configuration of jet airliners is how little they tend to differ.

It's surprising then that even on the same airliner, you will see two quite different-looking engines.

I've seen A330s with "fully-enclosed" engines, and "open" engines:

"Enclosed" design(Trent 700 with exhaust mixer, image from Wikipedia).

"Open" design(PW4000 with unmixed exhaust, image from Wikipedia).

The fact that both designs are used suggests that one isn't simply better than the other, but the difference seems fairly significant.

Why would an engine designer or a customer opt for one design over the other, and why do they arrive at such apparently different solutions for the same use-case?

I assume, perhaps incorrectly, that each has its own advantages and that for whatever reasons different manufacturers and customers value different ones.

  • $\begingroup$ @ymb1. The question said to be the duplicate and give the answer, really doesn't, IMHO. The first answer says it due to noise restrictions, but that makes no sense, because both engines are subject to the same noise requirements. The second answer might be in the right direction, but is very very short. So, a good answer is still lacking. What do you think? I realise you are not the only person who voted this question as a duplicate, but I can only tag 1 person. Cheers $\endgroup$
    – Penguin
    May 30, 2018 at 10:21
  • $\begingroup$ @Penguin - I saw this by chance. Tags only notify the author of the post and anyone who has commented -- I'm not sure what you are suggesting, but feel free to join The Hangar if you wish to discuss it. Basically even if the answers aren't good, it is still the same question. And right now there's a 100-point bounty as an incentive for anyone who wants to answer the older [same] question. Also see Have the same problem? section in the help center. $\endgroup$
    – user14897
    May 30, 2018 at 10:47
  • $\begingroup$ @Penguin - The longer answer makes sense IMO, but here is not the place to discuss it. Why not ask the author for clarification over there? It's perfectly fine to request clarification in a comment. $\endgroup$
    – user14897
    May 30, 2018 at 10:51
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    $\begingroup$ I am still hoping that I will discover one day what is being considered when engine designers and customers choose one design over another - what advantages are they trading-off, and what makes different advantages more important for different designers/customers? This is not asked, or answered, in the supposed duplicate. $\endgroup$ Feb 8, 2019 at 9:17
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    $\begingroup$ @DanieleProcida Please update your question with this information you provided in the comment and then it can be reopened. $\endgroup$
    – Farhan
    Feb 8, 2019 at 16:25

1 Answer 1


Sometimes manufacturers will certify with multiple engine types because their customers have their own power plant preferences (or manufacturer preferences, like say an airline's existing fleet is RR engines and they want to keep engine commonality for logistics benefits, or an airline feels one model engine is cheaper to run than another). Providing different powerplant options for an airliner can therefore result in more sales.

  • 2
    $\begingroup$ This explains why there are different engine options, but not why different engines make different design decisions, which is what the question is asking. $\endgroup$
    – fooot
    May 29, 2018 at 20:23
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    $\begingroup$ In that case it would be differences in mission profile. Generally an engine with a lower bypass ratio is better at higher altitudes, so an airline might go for that because of their mission profile has a preference for very high altitude long range performance which favours a lower bypass ratio vs another engine with a higher bypass that is optimized for climbs to 30 something thousand ft on 2 hour legs.. $\endgroup$
    – John K
    May 29, 2018 at 21:40
  • $\begingroup$ The bypass ratio also isn't the aspect being asked about - it's the mixed vs unmixed exhaust design. $\endgroup$
    – Penguin
    May 30, 2018 at 10:11
  • $\begingroup$ The mixed exhaust is a sign of a lower bypass ratio, which is the most significant aspect. Also, shrouding the core tail pipe with fan exhaust may reduce the noise level somewhat, so there's that. $\endgroup$
    – John K
    May 30, 2018 at 11:41
  • $\begingroup$ Hi John. "The mixed exhaust is a sign of a lower bypass ratio". If the exhaust design was changed later in the engines life, yes, it might change the bypass ratio. But, at the time of design, why can't the designer choose the bypass ratio to be x, independently of the nozzle design? If the nozzle design affects it when a mixed nozzle is selected, he just iterates the design to get it back to the same value as it had with an unmixed nozzle. Isn't that possible? Or are you talking about the first scenario? Cheers. $\endgroup$
    – Penguin
    May 31, 2018 at 19:08

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