Shape of exhaust duct/nozzle.

[NacelleStrake]

Is there a reason why some engines have a cone shaped duct/nozzle over the exhaust pipe. Is it for aesthetics or function?

Ryanair CFM's just have a pipe protruding out the back.

[B2N2]

Esthetics rately takes priority in aviation, look at the A380 :}

Especially engines are all function.
The “pipe” you see sticking out of that particular model CFM engine functions as a vent.
Just after shutdown you can generally see a trickle of smoke coming from there as a little oil seeps past a bearing seal.

http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/3/img/AERO_2012q4-3_fig7.png

Next-Generation 737 Fuel Performance Improvement (http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/3/)

[DaveReidUK]

Some more background on the centrifugal oil/air separator which dumps (mostly) air overboard via the centre vent tube on the CFM56 (and other GE engines) in this thread:

http://www.pprune.org/engineers-technicians/516660-cfm56-5c-oil-leak-enigma.html

[NacelleStrake]

Thanks, chaps. Appreciated and instructive. Thanks for your time.

[B2N2]

You are very welcome.
( chest inflated)
I love answering these types of questions from the ( with all respect) uninitiated but very interested.
You Sir, have risen above the level of Self Loading Cargo ;)

[tdracer]

Also, the length of that tube is determined by the need for the local pressure to be below ambient so that the air flow always goes out the pipe, not into it when the engine is running.
Not a trivial design task - I was involved in that task for a CF6 engine many, many years ago.

[B2N2]

Bigger is better :8

[RVF750]

The modified CFM56-7BE came in in 2014 BTW. That's the one on the right. It appears to have a longer pipe but the shroud around the hot section exhaust is just a lot shorter. About 100kg an hour less in the cruise I've found.