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Function Of Jet Engines?

Mon Feb 23, 2004 1:53 pm

How do turbine engines provide air conditioning, electricity, and pressurized air for an aircrafts cabin?

I know that basic schematic of the jet engine and how bleed air is taken from the N1 compressor section. However, I do not know what happens after that. Where does that air go, and how does it transition into the three items above?
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RE: Function Of Jet Engines?

Mon Feb 23, 2004 8:03 pm

There is something called the accessory drive gear box which is driven by the spinning of the engine. Normally mounted below the compressor section this turns a constant speed drive. Because aircraft electrical power is rated at AC 115v 400Hz we need a CSD to keep the generator turning irrespective of the speed of the engine.

As for pressurisation this is done by air from the engine if you put air into the fuselage and dont let any escape the pressure goes up. if you let air out and dont put alot in, the pressure goes down.
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RE: Function Of Jet Engines?

Mon Feb 23, 2004 10:17 pm

Bleed air from a jet engine is used to drive the air cycle machines for pressurization and airconditioning and for anti-icing of the wing and empennage leading edges. On engines with inlet guide vanes, bleed air is also used to deice them.
The accessory gear box on the engine is used to operate engine driven hydraulic pumps, the Constant Speed Drive (CSD) for the generators (or an Integrated Drive Generator IDG, which includes the CSD and generator in one unit), engine driven fuel pumps, and an engine tachometer.
Bleed air, depending on the power setting of the engine and the type of engine may be bleed off of the low pressure, intermediate pressure (in case of a 3 spool engine) or the high pressure compressor.
The bleed source is determined usually by the temperature of the bleed air.
The engine cases, from which bleed air maybe drawn, have slots in their outer diameter covered by a circumferential manifold. The bleed valve is attached to a port on the manifold and opens and closes as demand requires. Often these valves are call PRSOV's or Pressure Regulating Shut-Off Valves.
Topic Author
Posts: 382
Joined: Mon Oct 06, 2003 11:28 am

RE: Function Of Jet Engines?

Mon Feb 23, 2004 11:27 pm

Thank you for that broke!

You mentioned 3 spool common are those? Would those have a N1, N2, and N3 compartment?

I hear a lot about the PACKS...can anybody explain what this is?
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RE: Function Of Jet Engines?

Tue Feb 24, 2004 12:49 am

Since the RB211-22 engine was developed, all Rolls-Royce engines are 3 spool. The N1 spool includes the Fan and the low pressure turbine; the N2 spool includes the intermediate pressure compressor and intermediate pressure turbine; the N3 spool includes the high pressure compressor and the high pressure turbine. There are three concentric shafts that connect each compressor with its respective turbine.
There are not any compartments.
A "pack" is the term for a single aircycle system, which includes the aircycle machine (includes an engine bleed air driven turbine and a centrifugal compressor), the heat exchangers, and the associated pneumatic valves and temperature sensors. A L-1011, for instance, has 3 packs mounted in the forward fuselage under the cockpit. Most Boeing airplanes have their packs in the wing fillet fairing under the fuselage and center fuel tank.
One theory on TWA 800 has the packs heating the residual fuel in the center tanks to a level that created a volatile combustible fuel air mixture in the tank.
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RE: Function Of Jet Engines?

Tue Feb 24, 2004 1:13 am

A pack cools engine bleed air by sending it through a compressor and heat exchanger.

When the air is bled off the engine it is hot due to its compression. When it enters the pack, it goes through a compressor (which is itself powered by a turbine downstream). This compression further heats the air. At this point the air goes through a heat exchanger. This is similar in principle to the radiator on a car- the air goes through tubing with a high surface area that is exposed to outside air (let in through air inlets ahead of the pack). Because the air has been pressurised and is very hot, there is a large difference between it and the temperature of the outside air. This causes the compressed air to cool (and the outside air to be warmed, but we don't care about that). The air is then routed to the turbine, where it drives the compressor, and it expands. As the air pressure drops, so does the temp. The end result is cold air, which can be mixed with hot bleed air to attain the correct temp, before being sent into the cabin.

So basicly, the air is compressed to increase the temperature differential between it and the outside air.
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