Page 222 - Understanding Automotive Electronics

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SENSORS AND ACTUATORS 6

Whenever the air/fuel ratio is at stoichiometry the value for λ is 1.
When the air–fuel mixture is too lean, the condition is represented by lambda
greater than one (denoted λ > 1). Conversely, when the air–fuel mixture is
too rich, the condition is represented by an equivalence ratio of lambda less
than one (λ < 1).
The two types of EGO sensors that have been used are based on the use
of active oxides of two types of materials. One uses zirconium dioxide (ZrO )
2
and the other uses titanium dioxide (TiO ). The former is the most
2
commonly used type today. Figure 6.18 is a photograph of a typical ZrO
2
EGO sensor and Figure 6.19 shows the physical structure. Figure 6.18
indicates that a voltage, V , is generated across the ZrO material. This
o
2
voltage depends on the exhaust gas oxygen concentration, which in turn
depends on the engine air/fuel ratio.
The zirconium dioxide In essence, the EGO sensor consists of a thimble-shaped section of ZrO
2
EGO sensor uses zirco- with thin platinum electrodes on the inside and outside of the ZrO . The inside
2
nium dioxide sand- electrode is exposed to air, and the outside electrode is exposed to exhaust gas
wiched between two through a porous protective overcoat.
platinum electrodes. A simplified explanation of EGO sensor operation is based on the
One electrode is exposed distribution of oxygen ions. An ion is an electrically charged atom. Oxygen
to exhaust gas and the ions have two excess electrons and each electron has a negative charge; thus,
other is exposed to nor- oxygen ions are negatively charged. The ZrO has a tendency to attract the
2
mal air for reference. oxygen ions, which accumulate on the ZrO surface just inside the platinum
2
electrodes.
The platinum plate on the air reference side of the ZrO is exposed to a
2
much higher concentration of oxygen ions than the exhaust gas side. The air
reference side becomes electrically more negative than the exhaust gas side;
therefore, an electric field exists across the ZrO material and a voltage, V ,
2
o
results. The polarity of this voltage is positive on the exhaust gas side and
negative on the air reference side of the ZrO . The magnitude of this voltage
2
depends on the concentration of oxygen in the exhaust gas and on the sensor
temperature.

Figure 6.18
Zirconium Dioxide FPO
(ZrO ) EGO Sensor
2

UNDERSTANDING AUTOMOTIVE ELECTRONICS 209