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Engine Bleed Air  
User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 25
Posted (11 years 5 months 1 day 3 hours ago) and read 8604 times:

In the latest edition on Airliner World an article was talking about how the 7E7 would have a bleedless engine, it got me wondering, what is a bleed air valve?

What does it do and why?

 Smile

Sorry to be a pain Big grin


Whoever said "laughter is the best medicine" never had Gonorrhea
26 replies: All unread, showing first 25:
 
User currently offlineBR715-A1-30 From , joined Dec 1969, posts, RR:
Reply 1, posted (11 years 5 months 1 day 3 hours ago) and read 8511 times:

Engine Bleed Air comes from either an outside air source or the APU. A Bleedless engine would mean that the engine could start by itself and would not require the APU to be running for it to start.

User currently offlineM717 From United States of America, joined Dec 2002, 608 posts, RR: 4
Reply 2, posted (11 years 5 months 1 day 3 hours ago) and read 8512 times:

Actually, engine bleed air comes from the engine. It's true that either an outside air source or the APU can supply pneumatic pressure for various systems; however, normally these systems receive pneumatic pressure from the engine through a pressure regulator and shutoff valve (or similar).

Systems which use bleed air include air conditioning, pressurization, thermal anti-ice, engine start, hydraulic reservoir pressurization...and maybe others which don't immediately come to mind.


User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 3, posted (11 years 5 months 23 hours ago) and read 8465 times:

Someone expand upon the concept of a bleedless engine. To me this sounds like an engine that has no bleed air taps to run aircraft systems. This would increase thrust and decrease fuel burn, but you still need an air source to supply the aircraft. Will there be a constant duty APU installed? Will it be redundant?

User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 4, posted (11 years 5 months 20 hours ago) and read 8434 times:

Actually, the largest consumers of bleed air are the air conditioning units and various air-driven pumps, generators, etc. The need for this bleed air results in having to greatly oversize the fan/compressor stage of the engine, costing much weight and power loss.

The bleedless engine will, instead, use electric motors to drive these devices, and the electric power to support these motors will come from generators driven off the shaft through gearboxes. The unferlying principle is that taking power off the drive shaft is more efficient and economical than using bleed air.

I work in hydraulic systems, and for us, the major challenge is replacing the air turbine driven pumps (for example, as used on the 767 and 777) by large brushless DC motor driven pumps. We're talking 100+ hp, which would be by far the biggest electric motors ever put on an aircraft.

The bleedless engine will still use bleed air for pressurizing the hydraulic fluid reservoirs and other small consumers. However, moving the large consumers away from bleed air will result in a significant portion of the efficiency increase Boeing projects for the 7E7.

Pete  Smile


User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 5, posted (11 years 5 months 20 hours ago) and read 8405 times:

If they were to do that, they would have to have a muc more reliable & much more powerful APU than those in service today.
That APU would have to be able to supply bleed air for Pack operation (pretty big load for an aircraft of its size), engine anti-ice and wing anti-ice.
I'd love to read more about this.


User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 6, posted (11 years 5 months 19 hours ago) and read 8413 times:

Cdfmx... everything will be driven electrically. The packs will have electrically driven compressors to replace the bleed air supply from the engine. I'm not sure how the APU will be affected, but I suspect they will not grow.

Pete


User currently offlineAloges From Germany, joined Jan 2006, 8764 posts, RR: 42
Reply 7, posted (11 years 5 months ago) and read 8342 times:

Wouldn't the APU have a very different main task in that case, which would be providing electric power to the aircraft? If almost everything is driven electrically, what do you need bleed air from the APU for, except starting the engines?

And if the APU was mainly used as a generator, wouldn't it be somewhat redundant since electric power could be provided at/by almost every airport?



Walk together, talk together all ye peoples of the earth. Then, and only then, shall ye have peace.
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 8, posted (11 years 4 months 4 weeks 1 day 23 hours ago) and read 8328 times:

For the example of the APU providing power for the packs on the ground, it doesn't matter (as far as the size of the APU is concerned) whether the APU provides bleed air or electric power to the aircraft -- the power to run the packs is essentially the same. Only the APU design would change so that it would put out the necessary power in the form of electricity instead of compressed air. Ultimately, the electric scenario will be more efficient, so all else being equal, the APU can shrink in size. But I suspect the ever-increasing onboard electronic gadgetry will force the generators to grow and grow in size!  Smile

Pete


User currently offlineFrancoflier From France, joined Oct 2001, 3845 posts, RR: 11
Reply 9, posted (11 years 4 months 4 weeks 1 day 20 hours ago) and read 8297 times:

One aircraft I know that didn't use bleed air was the F-27. At least not for the ACM. Instead it used a separate air compressor (2 actually, 1 for the ACM and another for the dreaded pneumatic system for gears, steering, brakes etc...) driven through a shaft. It still took a lot of power from the engine and had to be shut off for t/o.

One can easily understand why systems will shift to all elecrtic in the future as it is WAY more reliable, flexible and would tap les power from the engine. Electric hyd pumps, electric instruments, electric deicing, and I can reasonably imagine an electric pressurization system, working along with a freon cycle machine for cabin temp control. You'll need very reliable and powerfull generators though.



Looks like I picked the wrong week to quit posting...
User currently offlineFDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 34
Reply 10, posted (11 years 4 months 4 weeks 1 day ago) and read 8262 times:

I don't like it. I don't trust putting all your eggs in one basket, that basket being electrical generation supplying all essential requirements.

The mere fact the engine is operating ensures bleed air, if electrical power is lost, the pneumatic system is failsafe in providing bleed air to user systems. The fact the engine is operating means it's turning its gearbox which will ensure the hydraulic pumps are putting out pressure, even during electical failure.




You're only as good as your last departure.
User currently offlineAloges From Germany, joined Jan 2006, 8764 posts, RR: 42
Reply 11, posted (11 years 4 months 4 weeks 1 day ago) and read 8259 times:

FDXmech,

the point you raise makes me ask yet another question:

What would happen if an engine failed, could the generator still supply electricity, with the engine working as windmill?



Walk together, talk together all ye peoples of the earth. Then, and only then, shall ye have peace.
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 12, posted (11 years 4 months 4 weeks 1 day ago) and read 8259 times:

I am not familiar with the proposed electrical architecture, but I am sure it will be as safe as today's pneumatic systems. All systems have to meet the same safety requirement .... i.e., probability of a failure resulting in a loss of control less than 1E-9.

Pete


User currently offlineFDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 34
Reply 13, posted (11 years 4 months 4 weeks 23 hours ago) and read 8265 times:

>>>What would happen if an engine failed, could the generator still supply electricity, with the engine working as windmill?<<<

Hello Aloges,

Technically no. Generally speaking, when the engine RPM slows down enough due to being shutdown by the crew or by malfunction, the generator control system detects an underspeed and trips off the generator before an under-voltage develops.

Though there was an incident in which a BA 747 lost all four engines when it flew thru a cloud of volcanic dust. One generator, ironically due to a generator system malfunction, did not trip offline when the engine was windmilling. But that was an anomaly.



You're only as good as your last departure.
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 14, posted (11 years 4 months 4 weeks 23 hours ago) and read 8264 times:

When you think about it, it really isn't that far fetched at all.
APUs have load compressors which are nothing more than another accessory. Using APU bleed doesn't steal any bleed air from the APU engine, but does apply a load to the engine, just as a generator does. They could do the same on a main engine.
- Use fan air for ram air
- There would still be a need for regulators and PRSOVs to compensate for variations in engine speed.

I'm not sure if this is how they are going to do it, but this would be the smartest way to do it. Like FDXmech said, I don't think it's a good idea to take something as essential as pneumatics totally away from the engines.


User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 15, posted (11 years 4 months 4 weeks 20 hours ago) and read 8244 times:

.....I don't think it's a good idea to take something as essential as pneumatics totally away from the engines.

As I said earlier, taking compressed air off the engine is more costly in terms of weight and $$$ than taking the same amount of power off the drive shaft and converting to electric power. That's one of the significant energy-saving (= greater efficiency) strategies. There were a tremendous amount of trade study activity which lead to that conclusion -- Boeing has been working on it for 3 years, starting with the NAPD.

Pete


User currently offlineBroke From United States of America, joined Apr 2002, 1322 posts, RR: 3
Reply 16, posted (11 years 4 months 3 weeks 5 days 9 hours ago) and read 8145 times:

During engine starts, most turbojet/turbofan engines use engine bleed air to reduce the possibility of compressor stalls. A compressor stall in this region would be identified by the inability of the engine to accelerate, rising turbine temperatures, and a chugging sound. High powered stalls result in a very loud bang and usually torching out the inlet and exhaust. These stalls could result in a hung start and overtemping the turbine (not a good thing). As the engine accelerates above the region where stalls could occur, the bleeds close and the engine will accelerate quickly.
I would think that the bleedless engine would be one where the use of bleed air during the start in not necessary.


User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 17, posted (11 years 4 months 3 weeks 5 days 9 hours ago) and read 8114 times:

What kind of engine uses bleed air to reduce the possibility of compressor stalls?
Could you explain a bit further.


User currently offlineM717 From United States of America, joined Dec 2002, 608 posts, RR: 4
Reply 18, posted (11 years 4 months 3 weeks 5 days 7 hours ago) and read 8105 times:

cdfmxtech,

I don't know if this is what he is referring to, but the only reference to "bleed air" reducing compressor stalls I can recall are something like the anti-surge (acceleration) bleed valves that are found on the JT8D-219s which power the MD-80s that I flew at one time.

These valves (3 6th stage, 2 8th stage, and 1 13th stage) open during engine start and spool-up to unload the compressor to prevent compressor stalls. They close during spool-up to 1.4 EPR on takeoff.


User currently offlineE1FAIL From United States of America, joined Dec 2002, 74 posts, RR: 0
Reply 19, posted (11 years 4 months 3 weeks 5 days ago) and read 8088 times:

Ahh....But what about pneumatic anti-ice? The heat from compression is used on the leading edges..I can't imagine that an electric fan could produce the same results. Unlessss...It is one helluva big fan and it runs the air over the electric equipment to pick up it's heat and move it to the leading edges. Hmmm....Two birds with one stone?

User currently offline737doctor From United States of America, joined Mar 2001, 1332 posts, RR: 38
Reply 20, posted (11 years 4 months 3 weeks 5 days ago) and read 8077 times:

During engine starts, most turbojet/turbofan engines use engine bleed air to reduce the possibility of compressor stalls.

Hmmm, I'm scratching my head a little bit on that one too.

Take the CFM56-7 for instance.

There are several systems that are used to increase stall margin, such as the VSV's (Variable Stator Vanes), VBV's (Variable Bleed Valves) and TBV (Transient Bleed Valve), but they don't "use" bleed air. Rather, they are a means to control it to help prevent a compressor stall.

The VSV's control HPC (High Pressure Compressor) airflow by moving the IGV's (Inlet Guide Vanes) and first three stages of HPC stator vanes, making sure that the correct quantity of air flows through to the HPC.

The VBV's allow some LPC discharge air to bypass the engine and mix with the fan discharge air.

The TBV helps prevent HPC stall during engine start and acceleration by controlling the amount of 9th stage air that goes into the stage 1 LPT (Low Pressure Turbine) nozzles.



Patrick Bateman is my hero.
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 21, posted (11 years 4 months 3 weeks 4 days 20 hours ago) and read 8059 times:

There is no bleed air on the 737 engine that prevents compressor stalls. 737doctor, maybe he did have the VBVs that are used in the compressor control mixed up.

Or maybe there is some engine out there on anotehr A/C that uses bleed air for compressor control.



User currently offlineFrancoflier From France, joined Oct 2001, 3845 posts, RR: 11
Reply 22, posted (11 years 4 months 3 weeks 4 days 20 hours ago) and read 8075 times:

Almost all turbojet and truboprop engine use some kind of automatic bleed valve to prevent compressor stall during starting and fast acceleration.

Compressor stall occurs when the compressor sucks too much air for the turbine wheels to handle. Excessive presure then builds aft of the compressor, and the compressor blades stall (also happens with centrifugal type compressor).
Bleeding air from an engine (air is bled just aft of the compressor) relieves that excessive pressure and the compressor can more easily "push" the sucked air behind him.

The above mentionned valve is not controlled by the pilots or anyone, and it does not use bled air to function. It opens or closes with air and/or spring pressure. It is totally separate from the valve used to bleed air for pressurization & the rest...

------

About how you anti-ice without hot bled air? Take a deep look at the thing you iron your clothes with. Plug it in, put an ice cube on the metallic side and see what happens (have an adult supervise you)...

------

About the reliability of the all electric system, well, I can very well imagine two generators or more on each engine, with separate yet interconnectable (for emergencies) circuits for different groups of systems, not unlike the very reliable electric systems of new generation planes. Plus you still have the APU, whose only task now would be to run AC and/or DC generators, which will be enough to maintain all emergency functions, such as pressurization, heating, flight controls and instruments, a bit of lighting in case you lost everything else.
A well thought elecrtical layout would be probably safer than having a different kind of system for everything:

If you lose one bleed air on one side, you very quickly have trouble pressurizing your cabin properly, and it gets tricky when you need it for anti-icing as well. there is no way to bypass that, but there would in an electrical system: just plug it to another available generator.

Bled air and ACM's have never done a good job pressurizing, and especially cooling and renewing cabin air, frankly.

Beed air systems are very often controlled by electric valves and motors anyway!

So why not? Most everything in the cockpit has turned from mechanic and pneumatic to electric & electronic already. And apart from the mechanics, I don't see anyone complaining!




Looks like I picked the wrong week to quit posting...
User currently offline737doctor From United States of America, joined Mar 2001, 1332 posts, RR: 38
Reply 23, posted (11 years 4 months 3 weeks 4 days 19 hours ago) and read 8048 times:

737doctor, maybe he did have the VBVs that are used in the compressor control mixed up.

Perhaps he did; I was just as curious as you concerning his explanation, as I haven't heard of such.

Or maybe there is some engine out there on anotehr A/C that uses bleed air for compressor control.

Once again, maybe there is. But we all know that the systems I mentioned above use servo fuel pressure from the HMU to operate, not bleed air.

 Smile




Patrick Bateman is my hero.
User currently offlineNKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 24, posted (11 years 4 months 3 weeks 4 days 19 hours ago) and read 8050 times:

---"If you lose one bleed air on one side, you very quickly have trouble pressurizing your cabin properly, and it gets tricky when you need it for anti-icing as well. there is no way to bypass that, but there would in an electrical system: just plug it to another available generator."----

What if it's not a source issue, but a user issue?


---"Bled air and ACM's have never done a good job pressurizing, and especially cooling and renewing cabin air, frankly."---


Can't say I agree with you there: It cools it enough to need to warm the cabin air via mix valves or trim air ...which you would need to do anyway regardless of the cooling medium. Same for renewing cabin air: How the air is introduced is less important than how it is regulated/controlled by a pressurization system. Pressurizing? Like gangbusters...that's why we regulate it down on most A/C before sending it to user systems.

My main point though is that bleed air is still a readily availble source that you'll have as long as you have engines turning...it does not have to be transferred from another medium. I'd like to see a practical demonstration on a large ( 120 seat+ ) A/C ...but what about the increasing complexity of the electrical system? How many more CSD's/IDG's will we need for muscle and redundancy? Will we have IDG's that are the size of V8 piston engines ( and a weight to match )? How much ( more ) power will they take to turn and how much space/weight will we save on user systems vs. their old pneumatic counterparts?

---"So why not? Most everything in the cockpit has turned from mechanic and pneumatic to electric & electronic already. And apart from the mechanics, I don't see anyone complaining!"---

Don't freak out too much ( "!" ) as we're not complaining but asking questions...I'm not trying to claim some sort of "halo-effect" but we're out there seeing all this first-hand. I believe they are valid concerns. I'm not some anti-electric luddite, and from a mechanic's standpoint, I really don't care...from a hands-on standpoint. Yes, bleed air leaks ( or the potential for ) create their own need for overheat detection systems/problems, and yes electric systems are fairly reliable ( electronics is a given, and I feel is beyond the scope of this discussion ) but in this case they are better ( IMO ) for acting in a relay function while pneumatics ( bleed air ) do the "heavy lifting".


25 Post contains images Kaddyuk : Thanx for the info
26 Post contains images Francoflier : NKP S2: You are right about the "user" and not "source" issue of the pressurization part. An electric compressor would be quite complex an subject to
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