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O2 Sensors

Regarding our previous thread on O2 sensors, and building your own
"bargraph" display of mixture, and what not: here is some general
background info.

Have fun...



Excerpted from the Bosch "Mechanical Gasoline Fuel-Injected System with
Lambda Closed-Loop Control" K-Jetronic Technical Instruction:

Lambda closed-loop control circuit

   By means of the Lambda closed-loop control, the air-fuel ratio can
be maintained precisely at Y=1.00.  ["Y" == "Lambda"] 

   This control principle is based upon the measurement of the ex-
haust-gas oxygen by the Lambda sensor. The exhaust-gas oxygen is a
measure for the composition of the air-fuel mixture supplied to the
engine.  The Lambda sensor acts as a probe in the exhaust pipe and
delivers the information as to whether the mixture is richer or leaner
than Y=1.00.

   In case of a deviation from this Y=1.00 figure, the voltage of the
sensor output signal changes abruptly. This pronounced change is eval-
uated by the ECU which is provided with a closed-loop control circuit
for this purpose.

   Tolerances and the ageing of the engine have no effect whatsoever.
At values above Y=1.00, move fuel is metered to the engine, and at
values below Y=1.00, less.

   This continuous, almost lag-free adjustment of the air-fuel mix-
ture to Y=1.00, is one of the prerequisites for the efficient after-
treatment of the exhaust gases by the downstream catalytic converter.

   The Lambda closed-loop control operates in a range between
Y=0.8...1.2, in which normal disturbances (such as the effects of
altitude) are compensated for by controlling Lambda to 1.00 with an
accuracy of +-1%.

Lambda sensor

   The Lambda sensor inputs a voltage signal to the ECU which repre-
sents the instantaneous composition of the air-fuel mixture.

   The Lambda sensor is installed in the engine exhaust manifold at a
point which maintains the necessary temperature for the correct func-
tioning of the sensor over the complete operating range of the engine.

   The sensor protrudes into the exhaust gas stream and is designed so
that the outer electrode is surrounded by exhaust gas, and the inner
electrode is connected to the atmospheric air.

   Basically, the sensor is constructed from an element of special
ceramic, the surface of which is coated with microporous platinum
electrodes. The operation of the sensor is based upon the fact that
ceramic material is porous and permits diffusion of the oxygen present
in the air (solid electrolyte). At higher termperatures, it becomes
conductive, and if the oxygen concentration on one side of the elec-
trode is different to that on the other, then a voltage is generated
between the electrodes. In the area of stoichiometric air-fuel mixture
(Y=1.00), a jump takes place in the sensor voltage output curve. This
voltage represents the measured signal.

        Voltage curve of the Lambda sensor at
        an operating temperature of 600C.

	      0.6   0.8   1.0   1.2   1.4
              |            .
	 1000 | -------\   .
     V	      |         \  .
     o	      |          \ .
     l	  800 |          | .
     t	      |          | .
     a	      |           \.
     g	  600 |           |.
     e	      |           |.
	      |           |.
	  400 |           |.
    (mv)      |           |.
	      |            \
	  200 |            .|
	      |            .|
	      |            .|
	    0 |            . \-----------
              |            .
	      0.6   0.8   1.0   1.2   1.4
                    Excess-air factor