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Old 20th Dec 2002, 23:57   #1 (permalink)
 
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What does an Oil Breather do?

Would someone pls clarify what an oil breather does? I'm brushing up on my oil breather pressure and temp guages, but need to understand in more detail the function of the oil breather first.

Cheers from Chuck.
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Old 21st Dec 2002, 00:08   #2 (permalink)
 
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I assume you are talking about piston engines here. The movement of the pistons and the clearance between the piston rings and the cylinders allows some of the presure created by the explosions in the cylinders to pass into the crankcase. If the crankcase was a sealed chamber then the seals around the crankshaft would blow out dumping the engine's life blood, oil, over the side. This would result in a large silecence from the engine. The breather is the vent to release this pressure. This keeps everybody happy
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Old 21st Dec 2002, 01:55   #3 (permalink)
 
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In addition, piston engines operated in very cold conditions aloft sometimes have the oil breather line iced, because one of the products of combustion is water vapor. Many engines contain a relief port inside the cowling to prevent excessive crankcase pressures from building up in the event the external breather line becomes iced.
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Old 21st Dec 2002, 02:38   #4 (permalink)
 
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What about on a jet engine? Same thing?
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Old 21st Dec 2002, 09:55   #5 (permalink)

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Arrow

Yes! principle is the same. The oil is pressure fed to the bearings at a nominal 50psi (does vary with engine type and temperature, some engines do not have any pressure relief valve). The bearing sealing is done for example by carbon seals or scroll seals backed up by air bled from the engine hp compressor. This bleed air would pressurise the bearing compartments so has to be vented. The return oil and air mix, again depending on the engine type is separated and the excess air vented overboard.
The oil/air separator could be of a centrifugal type.
The breather pressure and/or breather temp guage gives some indication of the health of the engine internals ie sealing.
The P W JT9D had a breather temp guage.
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Old 21st Dec 2002, 13:18   #6 (permalink)
 
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Quote:

The P W JT9D had a breather temp guage.


I believe the idea of the breather temperature is related to determinig if combustion of the oil is taking place in the breather which then has a nasty habit of burning through the breather tubing and setting off the nacelle fire system.

Health is related to wear and deterioration of the sealing characteristics, letting too much hot high pressure air in which lead to high breather pressures and blown out oil loss over many flights.
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Old 21st Dec 2002, 16:22   #7 (permalink)

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Agreed!
Taking the JT9d as a nice simple baseline engine It used 7th stage hp compressor air for cooling and sealing. The no3 (situated at the hottest part of the engine) bearing also had 15th stage air to back up the 7th. If the labrinth seals got excessively worn 15th stage air could enter the (no3) bearing housing and possibly result in an internal fire in the bearing housing and the g/box.
The early JT9 (-3a and 7) had a breather pressure guage, this was eventually modded off in favour of a breather temp guage.
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Old 22nd Dec 2002, 02:03   #8 (permalink)
 
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Arrgh! I don't follow how bleed air is used for sealing as it relates to jet engines.

Does bleed air enterly directly into the oil breather? If so, how is it cooled first (heat exchanger perhaps?).

Is the bearing sealing and oil breather the same thing?

Why do the bearing compartments need to be pressurized?

Does anyone have a pic of an oil breather? Perhaps that would help.

Why didn't I study engineering in school!
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Old 22nd Dec 2002, 11:08   #9 (permalink)

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The air bled from the compressor is not cooled, it is a small tapping from a particular compressor stage or stages.
The bearings are pressurised with bleed air to assist the seals in keeping the oil/air mist in the bearing cavity.
The oil breather is there to vent the bearing cavity otherwise the compressor air would force the oil/air mist back through the labryrinth seals and into the engine. (where it could contaminate the cabin air).This breather air is dumped overboard.
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