Re: O2 sensor readout (Air/Fuel ratio indicator)

[walter@quattro.corp.inmet.com (Walter Meares)]

> I have done some testing with a portable 4 - gas (O2, HC, CO, CO2) 
> exhaust analyzer with Air/Fuel ratio readout and have found under 
> sustained full throttle acceleration the air fuel ratio is closer to 
> 13.0 to 1 for more power instead of the ideal 14.7 to 1 ratio for 
> lowest emissions. I know that the O2 sensor output is a flat DC voltage 

On many cars, Audis among them, full throttle is a special sensed condition
that disables the closed loop and provides enriched mixture for maximum power.
On my 4000SQ, this is a microswitch on top of the throttle body.

> topped out at the upper limit (around 0.9 volts) under sustained full 
> throttle on my 86 5000CS Turbo. Under deceleration the fuel ratio goes 
> up to around 20 to 1 and the O2 voltage goes flat (bottoms out) at the 

There's a decel fuel cutoff on many EFI's also, sensing the closed throttle
vs engine speed and shutting off the injectors to save fuel.

> lower limit (around 0.1V).  If you are trying to get accurate air/fuel 
> ratio information I don't think the O2 sensor is capable of giving you 
> the actual degree of richness or leaness you have.

A look at a spec sheet for one of these sensors will confirm your test result.
They are highly non-linear, and swing very abruptly to near the extreme values
for slight deviations from stoich.  So the meter will provide some gross 
information and probably provides value-added for driving entertainment, but
it is not a precise measuring instrument.  My understanding is that linear
sensors are pretty expensive (hundreds) and not long-lived (few hundred hrs).

> Scott 
> 
> 

walter

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Walter Meares		Intermetrics, Inc.	walter@inmet.inmet.com
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