From: ravalent@mothra.syr.EDU (Bob Valentine)
Subject: EGR/Fuel Economy [Was: Re: Poor Economy--318 Mopar]
Organization: Syracuse University, Syracuse, NY
Date: Tue, 26 Jan 93 12:11:44 EST

In article <1k14s6INN6or@fmsrl7.srl.ford.com> tgl@slee01.srl.ford.com (Tom Leone) writes:
>Bob Valentine (ravalent@mothra.syr.EDU) wrote:
>: In article <1jpakqINN7up@fmsrl7.srl.ford.com> tgl@slee01.srl.ford.com (Tom Leone) writes:
>: >
>: >For that matter, EGR helps fuel economy, too.  Was it removed
>: >or disabled before the engine was installed?
>: 
>:     NOT!  Diluting the intake mixture with burned/partialy burned
>: exhaust gas will only serve to reduce economy, since the fuel system
>: will have to compensate by adding more fuel.  Just disconnecting the
>: EGR _WILL_ reduce mileage, since the system will then tend to run rich.
>
>First of all, the fuel system is designed to add only enough
>fuel for the air coming in (especially with feedback from the
>exhaust gas oxygen sensor in this car and all modern cars).
>
>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 airOA/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.  

    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.....

>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.

   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.....

>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).
                                       ^^^^^^^^^^^^^^^^
    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....

    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.....   

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

                      -->   Bob Valentine  <--  
                  --> ravalent@mailbox.syr.edu <--