From: tgl@slee01.srl.ford.com (Tom Leone)
Newsgroups: rec.autos.tech
Subject: Re: EGR/Fuel Economy [Was: Re: Poor Economy--318 Mopar]
Date: 27 Jan 1993 17:14:57 GMT
Organization: Ford Motor Company Research Laboratory
NNTP-Posting-Host: slee02.srl.ford.com
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I wrote:
: >YES, EGR does improve fuel economy.  There are several reasons:
: >
: >(1) Reduced "pumping work" needed to move gases from a low-pressure
: >intake to a high-pressure exhaust.  When you dilute the air/fuel
: >mixture with EGR, you must increase intake pressure (open the throttle
: >more) to get the same mass of air/fuel for the same torque.  Note
: >EGR is automatically turned off at wide-open-throttle, so you get
: >the same maximum torque/power.  

Bob Valentine wrote:
:     Ok, so at part throttle, where a engine spends most of it's time,
: you have to supply more throttle to get the same amount of power as if
: you didn't have EGR.  At WOT and idle, it is off.  
: 
: >(2) Reduced heat transfer losses, due to lower combustion temperatures.
: >
: >(3) Less dissociation in the burned gases, due to lower combustion
: >temperatures (allows fuller use of fuel's energy for useful work).
: 
:     Explain a little further on 2 and 3.....

Well, diluting the mixture lowers combustion temperatures because you
have to heat up some unreactive gases.  That's the main idea of EGR,
because lower temperatures decrease emissions of NOX.

It also reduces heat transfer, because convective heat transfer
is proportional to the difference in temperature (between gas
and walls), and radiative heat transfer is proportional to the
difference of the FOURTH POWER of the temperatures.  Less heat
lost to the walls means a greater percentage of the heat is
going into increased gas pressure, therefore increased work.

Likewise, dissociation of molecules occurs at very high temperatures,
and requires energy.  If you avoid dissociation with lower 
temperatures, you don't waste that energy.  According to the text
I quoted (Heywood), this effect is about half as important as
the pumping and heat transfer effects (which are about equally
important).

I wrote:
: >There may also be some benefit from better fuel evaporation and mixing,
: >due to hot exhaust gas in the intake.  The only problems are a decrease
: >in burn rate, and misfiring for excessively high EGR rates.
: 
Bob Valentine wrote:
: Gee, last time I checked the whole idea was to try and get as
: "cold" a charge of air/fuel into the combustion chamber.  Ducting air
: from in front of the radiatior, "cool cans" used on street/strip and
: race cars.....

Yes, at WIDE OPEN THROTTLE you want cool air, which is more
dense, and therefore allows you to inject more fuel for more
power.  At part throttle, warm air is better to evaporate the
fuel.  

Even the obsolete carbureted cars have "flapper" valves to take
warm air from around the exhaust during part throttle.  They are
vacuum-operated, so they take cold air at wide-open throttle
(less vacuum).  And again, the EGR is automatically off at
wide-open throttle, so it does not heat up the air.  And, I
don't like your "tone of voice", especially when you are wrong.

I wrote:
: >Reference: Heywood, "Internal Combustion Engine Fundamentals", p. 837-8,
: >including graph showing approximately 10% improvement in fuel economy
: >for 20% EGR (for a certain engine and speed/load point).
:                                        ^^^^^^^^^^^^^^^^
Bob Valentine wrote:
:     Exactly.  At some point, there will be a gain.  But at what
: percentage of the time does the engine run at that point, and will the
: losses to either side of that point compensate for the gain at that
: point. Given, that point is not a tiny little peak, but still....

What losses on either side?  The EGR valve is designed to give the
optimum amount of EGR at all times.  The modern systems are
computer-controlled, and even the older systems use a vacuum signal
to get approximately the right amount.  Even a little EGR is better
than none, and the only possible loss is if you have way too much
EGR and hit misfire.  And during typical driving, a huge percentage
of the time is spent at part throttle, where EGR is a great fuel
saver.  When you are accelerating hard, the EGR goes off automatically
and you have good power.  What's the problem?
  
:     BTW, in all this, I'm refering to domestic, carb'd cars.  FI'd
: cars don't really respond to removal of the EGR, and I KNOW that a bad
: EGR on a FI'd car will cause it to run shitty.  Due to the
: fact the the computer is expecting to see it there.   Most of the
: flame directed at EGR is probably due to FAULTY EGR systems.  Engine
: runs rich, bad MPG, bad power.  
:     Ever try to unclog the EGR passages in a V-8 intake (302 Ford)?  
: The next engine I had this happen on (307 Olds), I just jetted the
: carb leaner, and got the same results as the cleaned 302.  The Olds
: even  passed emissions cleaner than the Ford.....   

Agreed that a defective system will hurt performance.  That is true
whether it is a defective EGR valve, carburetor, ignition system,
or whatever.  

What's your problem with EGR when it helps BOTH emissions
and fuel economy (unusual), plus is fairly simple and inexpensive,
and does not hurt performance (even more unusual)?

Tom Leone <tgl@slee01.srl.ford.com>