uego

[Mike Klopfer]

I've been looking at the SAE paper 890299 "Wide-Range Air-Fuel Ratio
Sensor". From this it looks like even an ordinary sensor contains 
information about the A/F ratio down into the lambda .6 region. From
their graphs there's a change from 20ma to 14ma in the current 
generated by a sensor at 0V in going from lambda=.6 to .7. The
difficulty is that at 0V the change in current from lambda= 1 to
1.5 is 0. To detect a change in this region of lambda you need
to be at .25V or greater and at this voltage and lambda the sensor 
draws current instead of outputting it. So to keep the sensor
from saturating a circuit is needed that draws 25 ma at 0V and
generates 5ma at .5V.

Perhaps a sensor could be mapped out by using a variable resistor
in series with sensor in series with another fixed resistor in
series with a 1.5 battery. A high impedance measuring
device could measure the voltage across the fixed and varaible
resistors to determine the voltage and current for each setting of
the variable resistor at a given lambda.

You get the following curves of V (horizontal) vs I (vertical) for 
various values of lambda ( the numbers above the corresponding
line).


                  1.5
              _______________
             /    1.2
            /---------------
           / .9      _______
          /______---/
         / .8   __ /
        /____---  /
       /.7  ____ /
      /-----    /
     / .6    __/
    /_____--- 
   /