Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnews!military
From: rdd@rascal.ics.utexas.edu (Robert Dorsett)
Newsgroups: sci.military
Subject: Re: Turbo{jet,fan}s
Message-ID: <14310@cbnews.ATT.COM>
Date: 25 Feb 90 06:12:30 GMT
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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