Suck, Squeeze, Bang, Blow

[GJB]

Any self respecting pilot will recognise all these essential functions to a good night in.
Discussing the jet engine.

I have read several explanations on how a jet engine works, but they all refer to complex formulae and I don't have a grasp on the variables.

Can anyone come in with an easy-to-follow explanation?

[ickle black box]

Suck, Squeeze, Bang, Blow ...

Isn't that what your girlfriend does ?

ickle :)

------------------
If God had meant man to fly, He would have given him more money.

[Winkiepinkie]

Quite simply (and in principle the jet engine is a very simple device) the 4 stages of a typical internal combustion engine are present in the jet engine. The main difference with the jet engine is that all 4 are operating continuously, all at the same time.

The induction part (suck) occurs where air is drawn into the jet engine (into the carb and cylinder head in the car engine) by the front ‘blades’, the compressor.

Compression (squeeze) takes place when the air is passed through a restricted area/volume by the front blades ( ie. compressor) and is accelerated.

Ignition (bang) happens in the combustion chamber (sometimes referred to as the ‘cans’ ;) where fuel and air are mixed and burned.

Exhaust (blow) is when the hot gas expands and escapes flowing past another set of blades, the turbine (and subsequently turning it). The turbine is attached to the compressor by a shaft, so the hot gases, by escaping turn the turbine turn the compressor and the whole cycle continues. It never actually starts and stops like a car engine.

Actually, now that I think of it, the best book anyone could ever read about the jet engine is called…..wait for it…..’The Jet Engine’. It was produced by Rolls Royce and explains everything from the basics to the more intricate side of the jet engine. A really excellent book for anyone. And a good read.

Good luck, W.

[Princess PP]

Winkiepinkie -

That was a really excellent summary, it has whetted my appetite for studying that very book I have just purchased!

GJB -

You can get it in the Transair catalogue (01273 466 000) for 30 quid, soft back.

[Superfly]

Completely true, This book "the Jet engine" is brilliant, I bought it and it helped me a lot, Definitely a reference for whoever wants to get the hang of how it all works.


Happy Landings, Superfly

[GJB]

Thanks for the help, but what I am really after, is info on how this process creates thrust?

Would anyone care to enlighten me?

The book is on order - ta.

Ickle - you are a dirty boy and your remarks are inappropriate ;)
For this discussion at least.

[M13]

It's Newtons law of motion:

:) "Every action has an equal and opposite reaction" :)


M13 :) :)

[This message has been edited by M13 (edited 22 March 2001).]

[Winkiepinkie]

Exactly the same way a propellor creates thrust, or a wing lift.....etc

W

[GJB]

M13 - This is where I get lost and have read some conflicting data.

Winkie - Aer you suggesting that it is to do with differential pressure?

As I understand it, the thrust is not derived from the burnt gases pushing the aircraft forward?
So, does the engine 'suck' the aircraft forward?



[This message has been edited by GJB (edited 22 March 2001).]

[M13]

What by creating a pressure gradient using an aerofoil?
I never knew that.
I'll have to invest in that book pretty sharpish. ;)

LL13 :) :)

[This message has been edited by M13 (edited 22 March 2001).]

[M13]

I think that there are differences between a Jet engine and a turbofan engine and this is why there may be some confusion. A turbofan must develop thrust using aerofoils.???

Mmmm?? I'm off to find out.

[M13]

Ok I`m back and I`ve got something for you. Have a look at this it is very simplistic probably for kids but it shows you the differences between the types of jet engines.

http://howstuffworks.com/turbine2.htm

Sorted M13.

[GJB]

very useful - thanks.

Good link to RR.

[Winkiepinkie]

All the blades of a fan are (compressor), are simply the equivelent to the rotors on a helicopter, or the prop on your C150. With a high by-pass engine such as those of the 747 or the 767 (eg RB 211) over 70% of the thrust is derived from this front disc (fan). On smaller a/c such as fighters, especially old stuff from the 1960's such as the J79 from the F-4 this figure is much lower.

If you take a look at a 747 engine fan (diagram or real) you'll see its angled to the on coming air (ie, AoA, just as a wing to the air). This angle decreases the further from the centre you go as the tips travel faster than the inner areas. Also, if you cut a blade in half you see it has an aerofoil section, albeit very small. The faster the blade (or what is effectively a wing) the less the aerofoil section.

I hope this helps, but I doubt it will, I'm crap at teaching things. Its like singing, its a gift you're born with. You either can or can't teach/sing. And I can't teach.

I used to be in the choir though when I was a little boy.........hey, maybe I'm a really good teacher.

No, forget that. Can't teach.

Man, I'm bored, I'm at work and this job sucks. Why won't any of you big airlines pay for my flying??!?!

So long peoples, I'm going back to work.

W

[CREW_INOP]

You've probably found your answers by now but I'll stick in my two pennies anyway.

As you probably know, most airliners have a big fan at the front of the engine, then a series of compressors of smaller diameter behind them in the "core" of the engine. This big fan creates the majority of the thrust (around 80%) in exactly the same way as a prop. Most of the air going through the fan by-passes the engine completely and is simply chucked out the back, the reactive force pushing the aircraft forwards. The rest going through the middle is super compressed as earlier explained and drives the turbine which drives the fan.

Basically the jet core pushes a small amount of air very quickly (burning loads of fuel), and the fan a larger amount of air more slowly (more efficient). Hence concorde and military fighters having low by-pass ratios, going much faster and eating fuel.

Any of that make sense?

[GJB]

Winkie - share those sentiments too.

But hey, we are being paid to PPRUNE - in a sneaky sort of way ;)

[Billy M]

All good info, except one thing. Correct me if I'm wrong (and I hope I'm not as I'm about to sit my ATPL exams on this stuff), but I think that Winkiepinkie's explanation was spot on except for one thing:

As air goes through the compressor it is not accelerated, as was suggested.

The compressor (in an axial flow compressor, the type you would get on a modern bypass jet engine) consists of rotors (which rotate, obviously) and stators (which are stationary). The rotors speed up the air, but the stators then slow it back down again. Both parts compress the air, and the overall effect is no increase in speed but a massive increase in pressure.

What fun it is learning all these little bits of info!?!?!?!?

[Blindside]

and the stators also straighten the airflow don't they??

cheers

(my 2 penneth)

[Winkiepinkie]

Ok, I don't want to mess up anyones ATPL's but from what I remember (and I'm quite sure) the stators actually 'prepare' the air for the compressor/turbine. In that I mean they try and get the air to hit the blade perpendicular, hence making it more efficient. However, I am convinced that air is accelerated of the simple reason that it is passed through a smaller area (the core of the engine). That and the fact that it comes out the other end 2 zillion miles an hour.

I don't mean to sound picky or shoot you down Billy M (I really don't, I just can't write it any other way that seems 'neutral') but in you last post there seems to ba a bit of conflict.

You say the rotors don't speed the air up, but then you say they do. Also if you increase pressure in a 'container' it'll want to escape at a higher velocity than the surrounding air. Dunno, maybe I've got it all wrong. But like I said, I don't mean to shoot you down. Just check up this stuff before you sit the exam :)

Right, I'm bored, someone say something or I'll put a post up saying I'm bored and you'll all have to answer........

[Jimmy Mack]

Best book to get is The Jet Engine. It's a thoroughly detailed (i.e. far more than you will need for your ATPLs) examination of the jet engine. The colour diagrams are much easier to follow than notes from many of the training organisations. Should be a good book...was developed by Rolls Royce...think they know a bit about engines.

Transair sell it.

[SixDemonBag]

HEY! That's my email address!

[email protected]

[Billy M]

I don't want to start an argument here, especially if I'm wrong, but I stand by my original post. I checked up on this today in college, and unless my notes/lecturer/Rolls Royce web site (unfortunately this has been updated in the last week and the apprpopriate graph removed) are all wrong, I beleive my description to be correct.

The compressor consists of many "stages". Each stage is one row of rotors and one row of stators. As I said before the rotors speed the air up and the stators then slow it down to it's initial speed, while both compress it(increasing the pressure).

I also don't believe my post contradicted itself. The rotors DO speed up the air, but after the effect of the stators on that air (and there are an equal amount of rotors and stators) the overall result is NO increase in speed through the compressor.

The fact that the air comes out the bck at a very high speed is due to what happens to it after the compressor (the combustion and a slight convergent lip to the very back of the exhaust/jet pipe).

If I knew how to do it I would scan in the relevant graph and diagrams that would illustrate this, but I don't I'm afraid.

I hope this doesn't all sound like an attempt to start a fight over this, part of the reason for writing this reply is so that someone who knows more about this can tell me if I'm way off the mark, as for obvious reasons I need to know if I am. Maybe we should move this to the Technical Forum so the clever peopel can settle the argument!!!

Cheers

[DeltaTango]

Very interresting discussion.

Billy-the first stages DO speed up the air up in order for it to "catch up" with the air which has entered before it thus creating the pressure we want before the air enters the chambers.

yes?
no?
someone come with a better explanation-I dare you!!!!

:) :) :) :)
DT