Turbofan/Turbojet

Nebzor

Nebzor

Nebzor

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After reading up on the differences (vague curiosity stemming from the 'rod' thread) I've got a question hopefully one of you can answer.

Am I correct in thinking that of two engines with the same air intake size, a turbojet will create more thrust but use more fuel than the equivalent turbofan engine? (i.e. therefore turbojet has higher thrust/weight ratio but less economy?)

Also, why would you bother with a turboprop when you could have a turbofan?
 
Yes, turbofans are more economical than turbojets, they are also quieter.

Turboprops are more economical than turbofans at lower altitudes and over short distances (Hence why they are used by regional airlines doing short hops).
 
Turbojets need to be flown at high speeds to be vaguely efficient. Turbofans are more efficient as they have a main fan that produces thrusts more efficient at lower speeds (and speeds here I'm talking sub-sonic) Turboprops are very efficient at lower speeds (sub-500knots ish) and hence are used for commuter a/c for lower altitude/short flights.
 
Turbojets are very inefficient, and see very little (if any) use these days. Turbofans have taken over pretty much every role.

Things get more complicated than turbojet or turbofan though. You have different sorts of turbofans. High bypass turbofans, as used on civil aircraft (typical airliner engine) are very, very fuel efficient and produce high thrust. But they do not like high speed (transonic or above) flight and they are very large & heavy, this means they are only really viable for large civil aircraft.

Low & medium bypass turbofans however are a different story, and are what you'll find in all modern fighter aircraft and large bombers. Obviously we don't have to worry about fuel economy as much as civil airlines, weight and bulk are much more of a concern. And while low bypass turbofans produce less power than high bypass, we solve that by adding an afterburner system. There are other factors at work, such as military engines not needing to last as long between services etc and therefore they can be more highly tuned to produce more power etc, but that's another story.
 
Check out the Rolls-Royce website, we have some good stuff on there.

Quick question: when you say 'air intake size', do you mean the mass flow rate through both engines is the same? Or do you mean the surface area of the intake is the same?

If you add more fuel, you will fly faster. The efficiency reduction comes from ram drag (the resistance air entering the turbine experiences as it slows down to enter the first stages).

Short answer to the turboprop v turbofan question is that props are lighter!
 
Thanks for the answers, that really cleared it up.

Could you point out why in particular certain engines are more efficient at certain speeds/altitudes?

And why are turbojets so inefficient? Surely they have applications similar to rocket engines but without the cost of one?
 
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Nebzor said:
Could you point out why in particular certain engines are more efficient at certain speeds/altitudes?
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Can't find my lecture notes from last year anywhere but I remember something to do with the % loss in thrust for a given throttle setting with a given increase in altitude. High comp engines lose a smaller % in thrust compared to a prop engine. Air density is lower the higher you go making them more efficient.


Should really remember it properly but first years are always a bit blurry... lol
 
Ah okay, that clears up altitude (I presume the less dense the air the less thrust you get from the fan/prop just throwing it backwards whereas the turbojet can still compress it relatively efficiently?)

What about the speed?
 
The limits for prop engines is because once you get to a certain speed the speed of the free stream air exceeds the maximum speed that the props can accelerate it too.

Turbo jets are better at high speed because they don't have the bypass system that turbo fan engines do (brings down the overall exhaust gas speed).

I think lol, aircraft ops module this year rather than aeronautics so if you need to know how to operate an airline I can probably help better ha
 
So in summary...

In terms of efficiency:

Turboprop > turbofan > turbojet (> rocket engine)

However, in terms of exhaust gas velocity:

(Rocket engine >) turbojet > turbofan > turboprop (wholly negligible exhaust flow as most comes from the propeller accelerating air.)

So when choosing which to use, it becomes simply a choice of the most efficient engine for whatever velocity range you plan on doing (maximum exhaust gas velocity/propeller output velocity dictates the maximum air velocity that engine can perform at.)

Much thanks to all.
 
Darryn said:
When you say "we" does that mean you work for Rolls? I myself work at the Barlick site, so know a little about fan blades.
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Aye! I'm abroad at the mo, but I'm due to move to Brizzle in September, something Airbus-related I think seeing as the shop there is practically next door to the 'bus. They do like to keep one moving about, eh?

OP: dead right, the engine you choose will be mission-dependent. Remember as well, if you're looking into this for some sort of parametric study, that there are hybrids: ramjets that need rockets to bring them up to high Mach for combustion springs to mind.

Some thrust-to-weight-ratio vs propulsive efficiency graphs would be great right bout now. Cripes I'm a hooj nerd.
 
Nope, I'm just asking purely out of curiosity. I'd love to get into that sort of work but I highly doubt I'm clever enough to be of much use. Things take a fair amount of explaining :p

Next stop, de Laval nozzles!
 
So, I understand that in a de Laval nozzle supposedly the speed of the fluid increases and therefore pressure decreases (i.e. the Venturi effect.)

But why exactly does the fluid speed up rather than just get stuck and begin flowing backwards/giving terrible turbulence?

And why does it not instantly start slowing down again the second the chamber begins to expand?

Surely if it doesn't slow down again (as it appears to me to be doing, somehow) you could shove venturi after venturi after venturi and continue speeding up the fluid?

I know I'm missing something crucial here, so please point it out, I beg you :p
 
If I understand correctly (which I probably don't), it's to do with what you stated in your first sentence. The pressure of the fluid in the first chamber causes it to speed up when it passes through the middle section. This means that the fluid in the second chamber is at a higher velocity, but a lower pressure. This means that the next de Laval nozzle would not be effective, as the pressure would not be high and would therefore not speed the fluid up as much.

Please someone correct me if I'm wrong, since this is all useful to me (Aerospace engineering student).
 
Nebzor said:
So in summary...

In terms of efficiency:

Turboprop > turbofan > turbojet (> rocket engine)

However, in terms of exhaust gas velocity:

(Rocket engine >) turbojet > turbofan > turboprop (wholly negligible exhaust flow as most comes from the propeller accelerating air.)

So when choosing which to use, it becomes simply a choice of the most efficient engine for whatever velocity range you plan on doing (maximum exhaust gas velocity/propeller output velocity dictates the maximum air velocity that engine can perform at.)

Much thanks to all.
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I had a very quick read around this subject last night after it was raised in another thread. There is a reliabilty/maintenance factor as well. If you are designing a military jet the cost of fuel and frequency of service is not a major issue. If you are designing a commercial engine they are pretty major issues.
 
jr1104 said:
If I understand correctly (which I probably don't), it's to do with what you stated in your first sentence. The pressure of the fluid in the first chamber causes it to speed up when it passes through the middle section. This means that the fluid in the second chamber is at a higher velocity, but a lower pressure. This means that the next de Laval nozzle would not be effective, as the pressure would not be high and would therefore not speed the fluid up as much.

Please someone correct me if I'm wrong, since this is all useful to me (Aerospace engineering student).
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That slightly makes sense to me but at the same time makes no sense at all :p

Surely when forced into a smaller pipe either:

a) pressure should increase and therefore velocity should decrease
or
b) the fluid builds up back pressure and turbulence and the whole flow cocks up.

So how come it does speed up and pressure decrease, boffins? :p

(P.S. I've tried the usual sources but I'm really not getting it :/)
 
This may also be incorrect, I'm just trying to work it out in my head. Pressure is equal to force over area, which means that as the area is decreased, the force increases due to the pressure of the fluid. Then, because acceleration is proportional to force, the fluid is then accelerated to a higher velocity.
 
Wow, hehe I'm suprised my comments last night started such an interesting thread! Fascinating things, jet engines.

Alex_6n2 said:
Can't find my lecture notes from last year anywhere but I remember something to do with the % loss in thrust for a given throttle setting with a given increase in altitude. High comp engines lose a smaller % in thrust compared to a prop engine. Air density is lower the higher you go making them more efficient.


Should really remember it properly but first years are always a bit blurry... lol
Click to expand...

What course if you don't mind me asking?
 
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