air-fuel ratio to temp

Ed Lansinger elansi01 at
Tue May 9 21:29:56 GMT 1995

Sven-Erik Tiberg said:

>Hi All.
>There has been a lot of discussion about the correlation of air/fuel ratis
>vs. exhaust temp. This has initialized a grad. work for one of my students.
>The idea was to find out spec. fuelcomsumption vs. torque and rpm. To that
>he will add exhaust temp and air flow.
>As this doesn't seems to be a trival problem, I'm opend for any comments
>and suggestions.

OK, I'll take a stab at this thread.

Having just spent the day on a chassis dyno watching exhaust gas
temperature while measuring torque,  I'd like to point out the
sensitivity of EGT to spark advance.  Retarding spark will increase
EGT.  Naturally, this has an effect on torque output, too, but
that's not to imply that you'll find the same EGT at all operating
points that have the same torque and RPM.  So, one suggestion I have
(of which Sven-Erik is probably already aware) is to account for spark
advance, too, at least relative to maximum brake torque timing.  Some
engines may run retarded from MBT for various reasons, so data both at
and below MBT timing could be interesting.

Heywood, of course, discusses the relationship between EGT and A/F
ratio.  He shows predicted and measured results that indicate that
EGT reaches a maximum at 14.7:1 and falls off as you go rich or lean
(holding RPM and IMEP constant) ("Internal Combustion Engine
Fundamentals", p. 770; data from "A Fundmental Model for Predicting
Fuel Consumption, NOx, and HC Emissions of the Conventional Spark-
Ignited Engine" by Lavoie and Blumberg, printed in Combustion
Science and Technology, Vol. 21, pp. 225-258, 1980).  These results
also show how retarding spark increases EGT.

Assuming these results can be generalized, it looks to me from the
graphs that using EGT to guess at A/F ratio might work as long as you
aren't too concerned about exact values.  In the example, measured
EGT changed only about 3% (absolute scale) from 10.2:1 to 13.2:1.
The curve changes only slightly more rapidly as you approach 14.7:1.
Using EGT to perform tight closed-loop A/F control seems rather
futile, especially when you count in sensor lag.  Unless you have a
tiny, bare thermocouple (not very rugged), the time constant of the
thermocouple is likely to be measured in tenths of a second, if not
seconds.  I'd be interested to know if there is a ruggedized
thermocouple that has a time constant in the <.001 second range.
Such a sensor might be useful for misfire detection at high RPM.

"Steady-state" EGT is invaluable to see if you are about to melt
manifolds (especially turbos) or catalysts, and less accurately
pistons and valves.

Another contributor to the list suggested that the O2 sensor cannot
be used to quantify A/F ratios above and below 14.7:1.  The Bosch
LM511 is a wide-range oxygen sensor which has enough change in output
above and below 14.7:1 to make it useful for doing just this.
Unfortunately, as with all O2 sensors, its output characteristics
change with temperature (although not the rich/lean transition point),
so you need to know EGT.  You'll have to calibrate it yourself,
because the last time I checked the response curves were unavailable
to the public.

Ed Lansinger
General Motors Powertrain
Powertrain Control Center
Premium V Software & Calibration Group
Milford Proving Ground, Milford, MI
elansi01 at  8-341-3049  (810) 684-3049

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