Page 225 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

Figure 6.21
Typical EGO Sensor
Characteristics

FPO

about 0.05 second. This is a 2:1 change in switching times due to changing
temperature.
The temperature dependence of the EGO sensor output voltage is very
important. The graph in Figure 6.23 shows the temperature dependence of an
EGO sensor output voltage for lean and rich mixtures and for two different load
resistances—5 megohms (5 million ohms) and 0.83 megohm. The EGO sensor
output voltage for a rich mixture is in the range of about 0.80 to 1.0 volt for an
exhaust temperature range of 350˚C to 800˚C. For a lean mixture, this voltage
is roughly in the range of 0.05 to 0.07 volt for the same temperature range.
EGO sensors are not Under certain conditions, the fuel control using an EGO sensor will be
used for control when operated in open-loop mode and for other conditions it will be operated in
exhaust gas temperature closed-loop mode (as will be explained in Chapter 7). The EGO sensor should
falls below 300˚C not be used for control at temperatures below about 300˚C because the
because the voltage dif- difference between rich and lean voltages decreases rapidly with temperature in
ference between rich and this region. This important property of the sensor is partly responsible for the
lean conditions is mini- requirement to operate the fuel control system in the open-loop mode at low
mal in this range. exhaust temperature. Closed-loop operation with the EGO output voltage used
as the error input cannot begin until the EGO sensor temperature exceeds
about 300˚C.

Heated EGO Sensors
The increasingly stringent exhaust emission requirements for automobiles
in the 1990s have forced automakers to shorten the time from engine start to
the point at which the EGO sensor is at operating temperature. This
requirement has led to the development of the heated exhaust gas oxygen
(HEGO) sensor. This sensor is electrically heated from start-up until it yields
an output signal of sufficient magnitude to be useful in closed-loop control.

212 UNDERSTANDING AUTOMOTIVE ELECTRONICS