smartt1982
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Vent At The Back Of CFM 56 Engine

Sat Apr 24, 2010 11:39 am

Hi

Hoping someone can help me with this. My understanding of bypass engines is that mass of cold air goes around the hot core with the minority and smaller hot air coming out the middle section. What I am confused about and what I cannot seem to find an answer to is the purpose behind the long thin tube that extends out from the middle of what I would call the hot exhaust section.

http://www.airliners.net/photo/Xiame...d=5af7ea12208d8c801e99b8eb91bddf9c

I have only noticed this on CFM engines and my thought that all the exhaust/hot air would be extended through the turbines and cannot understand the purpose of this exhaust or vent tube

Many thanks

steve
 
Tristarsteve
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RE: Vent At The Back Of CFM 56 Engine

Sat Apr 24, 2010 12:29 pm

It is the air breather vent from the gearbox.
GE engines are similar.
RR engines have a hole on the left side of the fan cowl for this, which sticks out from the cowl on later engines and Trents.
 
smartt1982
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RE: Vent At The Back Of CFM 56 Engine

Sat Apr 24, 2010 1:03 pm

Ah right ok, so it doesn't provide any thrust at all then? how does it work anyway, sometimes I see smoke coming out of it
 
MD11Engineer
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RE: Vent At The Back Of CFM 56 Engine

Sat Apr 24, 2010 3:30 pm

All gas turbine engines use pressurised seals for their shaft bearing compartments to keep the oil in. The result is that the scavenge oil returning to the oil tank via the scavenge pump, chip detectors and scavenge filter contains a lot of air, it is more a foam than a liquid.
Engines use a centrifugal oil / air separator in the circuit to remove the air from the oil and to vent the system to the ambient to avoid overpressure. PW and RR use centrifuges attached to their main gear box, which vent overboard through a hole in the cowling. GE and CFM use the hollow N1 shaft as a centrifuge. The heavier oil sticks to the inside of the shaft and gets returned into the circuit via holes, while the lighter air stays in the center and gets blown out through the pipe in the back. The air blown out is not completely free of oil, so when the engine has just been shut down, you´ll see some oil vapour coming out from this pipe.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
smartt1982
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Posts: 226
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RE: Vent At The Back Of CFM 56 Engine

Sat Apr 24, 2010 4:29 pm

that is absoutly fantastic guys, many thanks for your help. It was driving me mad
 
maddogjt8d
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RE: Vent At The Back Of CFM 56 Engine

Sat Apr 24, 2010 6:43 pm

Great post. I've seen a similar center shaft with the whole in the center like the CFM56 on the CF6-80A and C series, never knew what the difference was about (just through observation, not working on the engines).

What about PW? The PW4000 has what looks like a closed compressor exhaust cone similar to the back of the RR. How do PW engines vent the centrifugal air/oil separator?
 
MD11Engineer
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RE: Vent At The Back Of CFM 56 Engine

Sat Apr 24, 2010 7:31 pm

Quoting MadDogJT8D (Reply 5):

What about PW? The PW4000 has what looks like a closed compressor exhaust cone similar to the back of the RR. How do PW engines vent the centrifugal air/oil separator?

Pratts use a centrifuge attached to the accessory gearbox and vent overboard through a hole in the cowling.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
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jetmech
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RE: Vent At The Back Of CFM 56 Engine

Sun Apr 25, 2010 2:29 am

Quoting MD11Engineer (Reply 3):
use the hollow N1 shaft as a centrifuge.

How clever! I never knew that. Very smart way of doing things!

Quoting smartt1982 (Thread starter):
purpose behind the long thin tube that extends out from the middle of what I would call the hot exhaust section.

As others have noted, the tube dumps sealing air overboard. If you look inside this tube, you can often see a thin layer of coked up oil on the inside surface. More info on bearing seals and air / oil separators can be found here.

What Is This "Smoke" From Under A340 Engines? (by Ambanmba Aug 22 2009 in Tech Ops)

Regards, JetMech

[Edited 2010-04-24 19:31:13]
JetMech split the back of his pants. He can feel the wind in his hair :shock: .

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