Lambda, Air/Fuel Ratios, Testing

["Scott Mo." <scottmo@teleport.com>]

While it typically is true in most cases when you decrease the Air/Fuel
ratio below the 12.6 to 1 ratio defined as max power, the unburned fuel
or hydrocarbons (HC for short) existing the engine goes up, this is not
always the case on the Audi Turbo engines as my testing has determined.

As mentioned, when you have high levels of unburned fuel entering the
exhaust due to rich mixture and it enters the catalytic convertor, you
can melt down the cat convertor  when the car then goes lean  during
coast down, throttle closed, decel fuel shut off conditions and all that
extra Oxygen hits the unburned fuel in the cat and the oxidation process
in the cat does its thing.

The same cat convertor melt down can occur if you have a simple ignition
misfire which allows unburned fuel and oxygen to pass through one or
more cylinders into the catalytic convertor while you are cruising down
the road. Extremely lean mixtures can cause this sort of ignition
misfire and are just as hazardous to the cat convertor when the HC and
O2 come down the pipe.

I recently did some 4 gas analysis testing on a 1991 200TQ 20V car with
the stock ECU and also on the same car with a IA Stage III ECU with the
2.5 bar PT. This same ECU code was found in the Hoppen 2.5 bar PT mod
ECU and the Tap 2.0 PT ECU I looked at.  I also tested a Hoppen modified
S4 as well as my own 1989 200TQ 10V car.

The exhaust analyzer was connected "upstream" of the cat convertor at
the sniffer pipe and it measured the exhaust exiting the turbo. 

The 4 gas analyzer is portible and measures, Hydrocarbons (HC) in PPM,
Carbon Dioxide (CO2), Carbon Monoxide (CO), Oxygen (O2) and it
calculates the A/F ratio from the readings of the 4 gases mentioned.
This analyzer has the capability to record these readings twice per
second for 3 minutes while doing wide open throttle testing on the road.

I tested the stock 1991 200TQ and found that in 4th gear at wide open
throttle from 2400-6000 RPM, the exhaust gass composition settled in to
have the following readings near the 6000 RPM level. 

HC reading of 72 PPM, CO2 was at 11.3 %, CO was at 6.5%, O2 was at 0.3%
and the Air/Fuel ratio was 12.3 to 1. Ambient air temp was 71F and the
test was done late at night for obvious reasons. The interesting thing
is the very low HC level, this sort of level is often found at idle for
most modern vehicles and indicates that there was very little unburned
fuel existing the turbo. In other words the ignition system was working
very well, even though the CO % was way up at 6.5% and the air/fuel was
down at the 12.3 to 1 level.

I next tested the same car with the modified ECU in 4th gear as before,
and found the following exhaust gas readings as the RPM reached 5800
RPM. 

HC was at 62 PPM, CO2 at 8.9%, CO at 9.7 % (wow!), O2 at 0.6%, and
Air/Fuel was at 11.25 to 1, again the low HC levels indicate very little
unburned fuel is coming out of the turbo. This low A/F ratio and
extremely high CO % was alarming, but the low HC indicates at least to
me that the extra fuel is not hurting anything other than the emissions
would give the EPA a heart attack....the extra fuel may be helping
prevent detonation with its cooling effect....

The Hoppen modified S4 I tested with the stock exhaust manifold and
stock K24 turbo had the following exhaust gas composition in 4th gear
near redline. HC was at 75 PPM, CO2 was at 9.5%, CO was at 9.95% (wow!),
O2 was at 0.2%, Air/Fuel was shown as 11.1 to 1.

My 89 200TQ 10V with modified ECU, single pass 20V intercooler, 11-12psi
boost level, stock fuel system, using stock ECU fuel maps (frequency
valve duty cycle values) had the following exhaust readings in 4th gear
at 6300 RPM.

HC was 66 PPM, CO2 was 10.9%, CO was 6.7%, O2 was 0.3%, Air/Fuel was
shown at 12.2 to 1. In 3rd gear at ~6600 RPM, the following was found.
HC 88 PPM, CO2 was 10.2%, CO was 7.5%, O2 was 0.3%, Air/Fuel was shown
to be 11.8 to 1.

All of this data indicates to me, that rich Air/Fuel mixtures are only
going to be detramental to the cat convertor if the HC levels start to
skyrocket due to poor combustion/ignition system, otherwise, it appears
that our European chip tuners have riched up the mixture beyond the 12.6
to 1, max power range for some reason. More cooling from the extra fuel
and reduced detonation are my guesses....

Here is hoping that any gases you smell, only come out of your
tailpipe....

Cheers

Scott Mo.
1989 200TQ
1988 5000TQ
1966 VW Beetle
http://www.teleport.com/~scottmo