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SENSORS AND ACTUATORS 6

Desirable EGO Characteristics

An ideal EGO sensor The EGO sensor characteristics that are desirable for the type of limit-
would have an abrupt, cycle fuel control system that was discussed in Chapter 5 are as follows:
rapid, and significant 1. Abrupt change in voltage at stoichiometry
change in output voltage 2. Rapid switching of output voltage in response to exhaust gas oxygen
as the mixture passes changes
through stoichiometry. 3. Large difference in sensor output voltage between rich and lean mixture
The output voltage conditions
would not change as 4. Stable voltages with respect to exhaust temperature
exhaust gas temperature
changes. Switching Characteristics
Hysteresis is the difference The switching time for the EGO sensor also must be considered in control
in the switching point of applications. An ideal characteristic for a limit-cycle controller is shown in Figure
the output voltage with 6.20. The actual characteristics of a new EGO sensor are shown in Figure 6.21.
respect to stoichiometry This data was obtained by slowly varying air/fuel ratios across stoichiometry. The
as a mixture passes from arrow pointing down indicates the change in V as the air/fuel ratio was varied
o
lean to rich, as contrasted from rich to lean. The up arrow indicates the change in V as the air/fuel ratio
o
to a mixture that passes was varied from lean to rich. Note that the sensor output doesn’t change at
from rich to lean. exactly the same point for increasing air/fuel ratio as for decreasing air/fuel ratio.
This phenomenon is called hysteresis.
Temperature affects switching times and output voltage. Switching times
at two temperatures are shown in Figure 6.22. Note that the time per division is
twice as much for the display at 350˚C as at 800˚C. This means that the
switching times are roughly 0.1 second at 350˚C, whereas at 800˚C they are

Figure 6.20
Ideal EGO Switching
Characteristics

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 211