tek@CS.UCLA.EDU (02/22/90)
From: tek@CS.UCLA.EDU
Here is a question for you aero experts:
What are the important differences between turbojet and turbofan
airplane engines?
My limited understanding is that a turbofan is a turbojet with a
bypass added. So some of the air goes around the ignition area and
then rejoins the exhaust flow. Is there any other basic difference?
Why is bypassing a "good thing"? What does it accomplish?
How does changing the "bypass ratio" affect the engine performance?
Does turbofan performance tend to be limited by different factors than
turbojet performance?
-ted
Ted Kim
UCLA Computer Science Department Internet: tek@penzance.cs.ucla.edu
3804C Boelter Hall UUCP: ...!{uunet|ucbvax}!cs.ucla.edu!tek
Los Angeles, CA 90024 Phone: (213) 206-8696henry@zoo.toronto.edu (Henry Spencer) (02/25/90)
From: henry@zoo.toronto.edu (Henry Spencer) >Here is a question for you aero experts: (This probably would have been better in sci.aeronautics, actually.) >What are the important differences between turbojet and turbofan >airplane engines? >My limited understanding is that a turbofan is a turbojet with a >bypass added... Is there any other basic difference? No. The sole difference, in principle, is that the turbofan adds one or more "fan" stages ahead of the compressor, with some of the air passing through those stages and then going straight out the back, not passing through the rest of the engine. >Why is bypassing a "good thing"? What does it accomplish? >How does changing the "bypass ratio" affect the engine performance? Basically, if you look at the equations for the physics of propulsion, you get more push per penny :-) at low speed if you push a lot of air out at low velocity, rather than a little at high velocity. The fan pushes more air through. On the big turbofans used on subsonic transports, the fan is large, most of the air does not go through the engine core (i.e. the bypass ratio is large), and the result is a relatively massive exhaust at fairly low velocity. The engine core is mostly there to spin the fan, and makes only a small thrust contribution itself. In the low-bypass turbofans used in a lot of fighters, only a little bit of air bypasses the core, but it's still enough to help fuel economy (and hence range and endurance) at subsonic speed. Which is better depends on requirements, notably speed. At very low speeds you're better off enlarging the fan until it becomes, surprise surprise, a propellor, and you end up with a turboprop. At very high speeds you forget the fan altogether. >Does turbofan performance tend to be limited by different factors than >turbojet performance? It depends on what you mean by "performance". Different customers care about different things. The underlying physics say that the most basic performance parameter is how hot the combustion gas is when it hits the turbine, and that applies to both forms. (The limit is set by turbine materials, mostly.) Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu
gwh%typhoon.Berkeley.EDU@ucbvax.Berkeley.EDU (George William Herbert) (02/25/90)
From: gwh%typhoon.Berkeley.EDU@ucbvax.Berkeley.EDU (George William Herbert) In article <24932@ut-emx.UUCP> rdd@walt.cc.utexas.edu (Robert Dorsett) writes: > >> What are the important differences between turbojet and turbofan >> airplane engines? >> >>My limited understanding is that a turbofan is a turbojet with a >>bypass added. So some of the air goes around the ignition area and >>then rejoins the exhaust flow. Is there any other basic difference? > >All jet engines in service work more or less as follows: incoming air is >compressed via a set of "low-pressure" compressor blades. Airflow leaving >the low-pressure compressor ultimately constitutes the effective thrust of >the engine. The air leaving the low-pressure compressor then either goes >straight into the high-pressure compressor (after which it is used to >maintain the turbine) or "bypassed" around the high-pressure compressor >and turbine. Turbine exhaust does *not* comprise the thrust of the engine; >the engine thrust comes from the "bypassed" air (after-burning is a special >case). In a turbojet, this is not the case at all. In most turbofans, yes most of the thrust is from the low pressure compressor, but not all by a longshot. You need to do some more reading on jet engines... >Early engines ("turbojets") had relative low bypass ratios (1:1, 2:1). >These would also generally recombine exhaust and bypass air into a common >exhaust port. "Turbofans" have much higher bypass ratios (generally 5:1 >through 10:1). In these engines, a far higher proportion of air from the >low-pressure compressor is bypassed. Not quite right: A turbojet puts all of the air into the combustion chamber at one point or another. Turbofans duct some of this around and [barring an afterburner] never combust it. A Turbojet has a bypass ratio of zero. Turbofans can come with a bypass ratio of whatever they want. The 10 or 15:1 is for commercail engines...most military ones are much lower. >Fans provide much better fuel economy than turbojets, and are much quieter; >hence, this is where nearly all civil R&D is headed. The current "hot" >research topics are development of the unducted propfan, and development >of "small fans" (such as the International Aero Engines V.2500, which is being >sold as an option on the Airbus A320). Fans, however, tend to have poor >performance at very high speeds. Military aircraft tend to use turbojets. Fans have low performance at supersonic speeds relative to turbojets. Most military high-performance planes use low bypass turbofans, like the TF30 or F100 or F404. Pure turbojets are too fuel-guzzling, and the only plane [modern] that i know of using one was the MiG-25, which was designed for such high speeds [mach 3+] that it NEEDED a turbojet, because the turbofan effeciency drop really hits after mach two. ******************************************************************************* George William Herbert JOAT For Hire: Anything, Anywhere: My Price UCB Naval Architecture undergrad: Engineering with a Bouyant Attitude :-) ------------------------------------------------------------------------------- gwh@ocf.berkeley.edu ||||||||||| "What do I have to do to convince you?"-Q gwh@soda.berkeley.edu ||||||||||||||||||||||||||||||||||||||||| "Die."-Worf maniac@garnet.berkeley.edu||"Very good, Worf. Eaten any good books recently?"-Q
sman@prandtl.nas.nasa.gov (Sekong Man) (02/25/90)
From: sman@prandtl.nas.nasa.gov (Sekong Man) In <14242@cbnews.ATT.COM> Ted Kim asked about the difference between turbofan and turbojet. He was right. The only difference is By-pass. (Although this has a bearing on the choice of thermodynamic cycle, design arrangement and structure etc.) By-pass is good only in certain operation condition. By moving a larger air mass, the propulsion efficiency (which depends on the Froude efficiency which in turn is a function of the jet exit velocity) is increased. It is like the difference between throwing a small stone fast or a big stone slow. You can get the same reaction force as long as the product of mass and velocity give you the same value of momentum. However the energy carried away by the small stone is higher, in proportion to its speed. So for the same thrust, by-pass reduces fuel consumption. All modern airliners use turbofans, some with by-pass ratio as high as 5. The fans get to be like shrouded propellers. Increase by-pass increases engine thurst at LOW speed. So if you look at the static thrust of F-100, it may seem so much more powerful than the J79. However, the thrust from the by-pass falls off quickly with aircraft speed since it is limited by the amount of energy transferrable to the fan air flow and thrust times velocity is energy. In other words, the by-pass air velocity has to go down at high speed. Meanwhile the core part of the exhaust derives its velocity from expansion at the expense of the heat from combustion and it is high at any speed. So supersonic performance of the re-engined Super Phantom is not that much better. Too much by-pass can incur aircraft performance penalty, mainly from the increase of engine(core + by-pass) air flow and frontal area. Larger air flow means larger intake and hence drag. Larger frontal area implies larger fuselage cross-section and also drag, especially at supersonic speed. Of course, larger air flow give more for reheat boost. But this is at the expense of fuel burn. The F-100 (for F15) with 0.5 by-pass gives like 50% reheat thrust and fuel burn goes up like 5 times. Supersonic fighters rarely have engines with by-pass larger than unity and the higher the Vmax usually the lower the by-pass. But with low by-pass, turbofan is very much like turbojet, except perhaps a little bit heavier, more complicated, slower in throttle response, more prone to surge and overheat, more maintenance problems, and the host of things you hear about the F-15 engine troubles. Turbofan will probably not be used above Mach 2.5 while turbojet is good for Mach 3+.
rdd@rascal.ics.utexas.edu (Robert Dorsett) (02/25/90)
From: Robert Dorsett <rdd@rascal.ics.utexas.edu> > What are the important differences between turbojet and turbofan > airplane engines? > >My limited understanding is that a turbofan is a turbojet with a >bypass added. So some of the air goes around the ignition area and >then rejoins the exhaust flow. Is there any other basic difference? > >Why is bypassing a "good thing"? What does it accomplish? > >How does changing the "bypass ratio" affect the engine performance? > >Does turbofan performance tend to be limited by different factors than >turbojet performance? All jet engines in service work more or less as follows: incoming air is compressed via a set of "low-pressure" compressor blades. Airflow leaving the low-pressure compressor ultimately constitutes the effective thrust of the engine. The air leaving the low-pressure compressor then either goes straight into the high-pressure compressor (after which it is used to maintain the turbine) or "bypassed" around the high-pressure compressor and turbine. Turbine exhaust does *not* comprise the thrust of the engine; the engine thrust comes from the "bypassed" air (after-burning is a special case). Early engines ("turbojets") had relative low bypass ratios (1:1, 2:1). These would also generally recombine exhaust and bypass air into a common exhaust port. "Turbofans" have much higher bypass ratios (generally 5:1 through 10:1). In these engines, a far higher proportion of air from the low-pressure compressor is bypassed. The casings for turbofan engines vary. Some mix the turbine exhaust and bypass air before ejecting it . Other engines get rid of the bypass air well in front of the turbine exhaust port; however, this is largely a function of how the engine is to be mounted (a side effect is that this affects how the engine is operated--via N1 or EPR, but that's another story). Ironically, as jet engine technology improves, the low-pressure compressor is getting pared down; one version of the RB.211 (a large fan) has just one fan stage providing the lion's share of thrust. The critical difference between this and a conventional propeller, however, is how the airflow interacts with the en- gine casing. Unducted propfans promise bypass ratios in the 15:1 to 30:1 range. This is still mostly experimental technology (both McDonnell-Douglas and Boeing have considered and rejected UDF technology, since fuel's too cheap). Fans provide much better fuel economy than turbojets, and are much quieter; hence, this is where nearly all civil R&D is headed. The current "hot" research topics are development of the unducted propfan, and development of "small fans" (such as the International Aero Engines V.2500, which is being sold as an option on the Airbus A320). Fans, however, tend to have poor performance at very high speeds. Military aircraft tend to use turbojets. This subject ideally belongs on sci.aeronautics. I've posted a copy of the original post (and this reply) there. If any of you don't carry the newsgroup, I run a moderated mailing list. Robert Dorsett Internet: rdd@rascal.ics.utexas.edu UUCP: ...cs.utexas.edu!rascal.ics.utexas.edu!rdd