INSIDE AN ENGINE;
 

Hi,
I had a doubt on this, inside an engine, there is a compressor and a com-bustor, but on ground we still give compressed air to the turbine with the APU BLEED, so does the compressor and the com-bustor don't work when the engines are not on? We give compressed air to the turbine but why? During the flight, it takes the air from outside, so why not before start?

Thanks to explain...

The only time APU bleed air goes to the Engine is during start to turn the starter motor via the start valve. It doesn't go into the Compressor or Turbine section it only turns the starter motor.

Apart from that I don't know what you are on about?

I suggest reading a few good books or Google Jet Engines.

I'm having a little problem understanding what you wrote, but I'll take a stab at an explanation that might address your question.

APU air does not get inside the engine you are starting, it only drives are tiny turbine in a starter connected to the engine gearbox.

This external starter motor is connected to the compressors and turbine through the gear box and spins the engine compressors just fast enough to allow fuel to be burned in the combustor

Thanks,
See this photo: http://upload.wikimedia.org/wikipedi...engine.svg.png
Where is the motor? I didn't really get what you meant by motor...
What is the engine gear box?

EDIT: http://upload.wikimedia.org/wikipedi...ion-en.svg.png

Do you mean that the apu bleed is normal air which enters in the gearbox, and then goes to compressor and then the combustion chamber to finally turn the turbine, which will turn the prop? But why can't it take air from "outside"?
Thanks to help (very important for me)

The accessory gearbox is wrapped around the front section of a modern turbofan. It includes things like the electrical generators (to supply the aircraft electrical systems) hydraulics pumps to power flight controls, the engine oil sump and pumps and start motor. On the ground, the engine is started using typically an electrical motor for smaller engines or an air start nmotor, which is like a car turbo, which takes high pressure apu air and converts it into rotation. Tis rotation is fed, via a clutch, into the engine gear box via a shaft.

THis then starts the engine turning. Once the engine is rotating fast enough, usually at around 20% core speed, fuel is introduced. The air that is being sucked through the engine is used, the engine does not use the apu air for combustion. The starter typically disengages once the engine is 'self sustaining', his usually occurs at around 50-60% core speed.

Wikipedia helps:

http://en.m.wikipedia.org/wiki/Accessory_drive

That image you have on is a much simplified version of a turbojet,it is good for explaining how a jet works but jot much else!

I think reading this may help.

How are turbine engines started? - Aviation Stack Exchange

AF330-----no no no

The Bleed Air is only used to run the starter motor.
The Starter motor is physically connected to the Engine and turns it over. During start the Engine turns over faster and faster sucking fresh Air in the front mixed with fuel and ignited. The Engine then runs at Idle all on it own with the starter motor now disconnected.

Start bleed air is NOT blown in the Engine to turn it over for start.

AF330, this could also be a good source of information for you: http://www.valentiniweb.com/piermo/m...t%20Engine.pdf

I'd suggest to start by watching this little video:

Just like to point out that AF330 is type-rated on Airbus aircraft. makes you think, eh?:ugh:

Now read the poster's 'previous posts'................................

...but I fully believe his quoted age of 13. And reading his previous posts suggests that if AF330 spent more time learning english he might actually be able to answer his own questions.

Starting alternative system
 

All the above is not applicable to Boeing's newest, the 787.

Its engines are started via an electric motor. Once started, the starter functions as a generator.

And this is hardly the first time this starting system has been used. The CT7 turboprop uses it, and long before that, the ancient J47.

Here's a video that should help:

BOAC-------thanks I just looked through his previous posts. I hope to hell he's not a "real" Pilot flying an Airbus..........:mad:

I don't think any of us begrudge lending some technical (or quasi-tech) knowledge to an interested novice. That was my profession for a decade or so, and I've learned to quickly assess a student's initial knowledge.

But trying to pass oneself off as a typerated pilot is a quick way to wear a whole heap of scorn in the real world... :*

Young lads have to learn a bunch of lessons on their pathway through the Industry .. some perhaps are a bit more like Walter at the start than others ?

Best result is to provide the information - there will be others reading who, for whatever reason, chose not to ask the question themselves.

Missed this one, John?

.. and elsewhere they go if they move too far from what is useful here ..

So, apart from the loss of the advertising 'pay per view' revenue from your Christmas bonus, John what loss would that be to we professional aviators?

I wouldn't fuss too much about the bonus .. X% of zero doesn't add up to much.

On the face of it, probably not much ...

Consider, though, that the older, experienced, professional aviator may have a responsibility to assist those coming along behind ? Did not you have the benefit of wiser heads when you were a new chum ?

Not to mention that these threads often bring out of the closet videos and other items which may well be of interest to a wider audience ?

Great vid Sidestick n Rudder, not seen that one before.

BOAC. You are not compelled to reply to any thread on PPRUNE, so if the OP's credentials offend you please feel free to move on and let those of us who enjoy passing our knowledge and encouragement on do so.

I'm in the engineering design side of aviation, and our industry is desperate for staff. The more we can get interested the better!

Ok,ok... There is a big problem. I am not at all an airline pilot, just 13 year old, and I really didn't check my profile, just clicked "yes" everywhere. My native language is french, but I am learning english. I AM NOT A PILOT!

Last question: So BLEED AIR is taken from high pressure stage or low pressure stage, depending on what we need...
But has the APU BLEED already got that air? Because at the beginning of the thread, the apu bleed air turned a box to turn the compressor, so does the APU BLEED AIR get's compressed twice, or does it come as normal air and then get's compressed?

It might help You to consider the APU a little jet engine (which it is). Just like its bigger relatives, it has a compressor (usually single-stage radial) that feeds a combustion chamber, which in turn discharges into a turbine (usually single-stage radial as well).

The bleed air is tapped from the APU compressor, diverting some air that would otherwise go to the combustion chamber into the bleed air ducting - plain old pipes basically. And these pipes will lead the APU bleed air into the engine starter: as has been explained, this is a little turbine installed on the accessory gear box that will spin the high pressure compressor of the engine. After having passed the starter turbine, the air will typically just be vented overboard.

What You need is a book on jet engine theory. I am sure that a search on common book dealer pages will return several titles in French as well, should You prefer that language.

Both the engine (the big one) you want to start and the APU (small guy) have compressors and bleed air. Since the APU is running it has air to spare. As said before it uses this air to spin a much smaller device (the starter) which through gears turns the big engine.

The big engine has no bleed air available during start and no direct connection exist in the air between either the APU (small), plus the starter (tiny) and the big engine.

A neat little schematic might focus your mind on this rather than more words

BOAC

You appear to have something of a fixation on putting down posters who are not professional aviators.

Your profile states the following:

Licence Type (eg CPL. Pilots only)
Atpl

Biography
Now retired

Location
UK

Occupation
Sit Vac

If these details are accurate then you are no longer a professional aviator.

You are an EX professional aviator. :hmm:

As such it might be more appropriate if you were to restrict your posts to those threads that are for EX professional aviators.

As JT has intimated, chasing enthusiastic youngsters (some of whom will become tomorrows professional aviators) into the far corners of pprune, will reduce the chances of them getting the information that they are looking for. In some cases this will drive them away from aviation.

"Both the engine (the big one) you want to start and the APU (small guy) have compressors and bleed air. Since the APU is running it has air to spare. As said before it uses this air to spin a much smaller device (the starter) which through gears turns the big engine.

The big engine has no bleed air available during start and no direct connection exist in the air between either the APU (small), plus the starter (tiny) and the big engine.

A neat little schematic might focus your mind on this rather than more words"

Thanks,
But so this is the route of APU BLEED:
APU has it's compressor, combuster. So basically, at the end it will turn a gear which will turn the turbine (so the big fan and the big compressor and the axe, am I right?

"BOAC

You appear to have something of a fixation on putting down posters who are not professional aviators.

Your profile states the following:

Licence Type (eg CPL. Pilots only)
Atpl

Biography
Now retired

Location
UK

Occupation
Sit Vac

If these details are accurate then you are no longer a professional aviator.

You are an EX professional aviator.

As such it might be more appropriate if you were to restrict your posts to those threads that are for EX professional aviators.

As JT has intimated, chasing enthusiastic youngsters (some of whom will become tomorrows professional aviators) into the far corners of pprune, will reduce the chances of them getting the information that they are looking for. In some cases this will drive them away from aviation."

Thanks keith williams, I appreciate, I really didn't want to write that I was a pilot!

AF330,

I've been watching this thread and I am glad to see some posters are attempting to help you with the technical question you have asked. Just ignore some of the old posting dinosaurs, who have nothing to contribute in response to your question. Here is another source of information that could be helpful to you and it is in French.

Keep asking questions, it is the way to learn:ok:

Groupe auxiliaire de puissance ? Wikipédia

Thanks a lot Turbine D!

Regarding my 'qualifications', at least I don't lie.

The OP might like to work his/her way through these.

Engines

http://www.scribd.com/doc/51618999/R...ngine#page=217

I'm sorry to see this thread getting personal.

Sure, all opinions are valid, but lets' keep it simple.

Many OPs are confusing for a variety of reasons. Maybe the answers should be simplified down to communicating with the large numbers who read this forum rather than trying to personalize the posters.

Well, see BOAC, I really didn't "lie", It was just an error. I really wanted to post my first question (about SELCAL I think...) and I pressed "YES- CONFIRM". If I really wanted to lie, I would not show "13" as my age.

I hope you understand. I am sorry if you didn't appreciate it. Again, sorry...
Thanks to everyone to have helped me!

Hi AF330,

I thought maybe a couple of engine cutaways might help you to visualize the location of the starter in a large fan engine.
http://i1166.photobucket.com/albums/...psb1b26446.jpg
Just at the rear of the accessory drive section is the starter. In the next cutaway, you will see how that connects to the main engine rotor shaft to turn it beginning the process of engine starting.
http://i1166.photobucket.com/albums/...ps19307b43.jpg
Now you can see the starter and the shaft that goes from the starter upward to the engine rotor shaft. Note the gear at the end of the starter shaft meshes with a gear that is attached to the main engine shaft. So, as the power coming from the APU in the tail of the aircraft energizes the starter to start turning the main engine shaft, rotational speed of the engine rotors will increase. At a certain rotational speed, fuel will be sprayed into the combustor and ignited. As rotor speeds continue to increase the starter is disengaged and the engine becomes self sufficient with no need for external support. In case you are wondering, the depicted engine is the GE CF6-50 that was used to power the A-300, Boeing 747 and also, the DC-10.
Hope this really helps you.

So we have learnt that the main engine is started by a starter motor, that is spun up by the air from the APU...


That leaves just one question... How does the APU start?

It uses bleed air from an even smaller jet engine called the AAPU :E (sorry couldn't resist).

AFR 330.
Another way to explain, hope it helps.

1. A/C on ground, no power. You switch batteries (28vdc) and start APU.
2. APU ON. APU generator is powering whole a/c electricall system (115 vac) and part of the air compressed by APU compressor, necessary for APU own operation, is BLEEDED to supply a/c Pneumatic system (+/- 30 psig)
3. A/C pneumatic system feed air conditioning packs,,ENGINE STARTERS and several minors users. Depend of cockpit selections
4. MAIN ENGINE START: APU keep supplying electrical power to whole aircraft. All APU pneumatic supply is dedicated to one at a time engine start, (air conditioning is switched of during Engine Start). APU Bleed air is directed though air valves to a PNEUMATIC ENGINE STARTER where pneumatic pressure energy is transformed into mechanical torque that, through drive box, makes main engine rotor turn. When MAIN ENGINE ROTOR reaches certain speed, MAIN ENGINE Compressor is able to compress inlet air to a certain pressure at which combustion make take place, so fuel is inyected, ignition plugs are switched on and voilá -combustion. After a short time, MAIN ENGINE is capable of turning and acelerating on his own power, so Pneumatic Starter is mechanically disconected and no more pneumatic power is directed to this starter

Engine Ignition System - DutchOps.com powered

Of course there are differences, this is just a basic explanation. And sorry, I'm not an instructor

Keep asking!!!

The APU is started by means of an onboard battery, or an electrical external power source or, depending on the kind of APU, a hydraulic accumulator that provides compressed gas to the APU. Some APUs aren't capable of being started in the air, but for ETOPs aircraft the APU must have the capability of being started up to an altitude of 40K feet.

TurbineD, you said, "As rotor speeds continue to increase the starter is disengaged......"

If I may elaborate a little to make it a bit clearer for AF330: The start air valve, which is adjacent to the starter, is closed off during the start cycle (circa 50% N3 on most R-R engines) thus shutting off bleed air to the starter motor. The starter motor shaft continues to rotate as it's driven by the gearbox once the engine is self sufficient and cannot actually be mechanically disengaged (unlike the IDG drive), but pawls within the starter mechanism mean that the starter turbine itself ceases to rotate.

Thanks DevX:ok: