Page 226 - Understanding Automotive Electronics

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SENSORS AND ACTUATORS 6

Figure 6.22
Typical Voltage
Switching
Characteristics of
EGO Sensor

FPO

The HEGO sensor includes a section of resistance material. Electrical
power from the car battery is applied at start-up, which quickly warms the
sensor to usable temperatures. This heating potentially shortens the time
interval until closed-loop operation is possible, thereby minimizing the time
during warm-up that air/fuel ratio deviates from stoichiometry and
correspondingly reducing undesirable exhaust gas emissions.

Knock Sensors
Another sensor having applications in closed-loop engine control is the
so-called knock sensor. As explained in Chapter 7, this sensor is employed in
closed-loop ignition timing to prevent undesirable knock. Although a more
detailed explanation of knock is given in Chapter 7, for the purposes of this
chapter it can be described generally as a rapid rise in cylinder pressure during
combustion. It does not occur normally, but only under special conditions. It

UNDERSTANDING AUTOMOTIVE ELECTRONICS 213