Page 227 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

Figure 6.23
Typical Influence of
Mixture and
Temperature on EGO
Output Voltage

FPO

occurs most commonly with high manifold pressure and excessive spark
advance. It is important to detect knock and avoid excessive knock; otherwise,
there may be damage to the engine.
Some engine knock sen- One way of controlling knocking is to sense when knocking begins and
sors use rods within a then retard the ignition until the knocking stops. A key to the control loop for
magnetic field to detect this method is a knock sensor. A knock sensor using magnetostrictive
the presence of knock. techniques is shown in Figure 6.24. Magnetostriction is a phenomenon whereby
Other use vibration- the magnetic properties of a material depend on stress (due to an applied force).
sensitive crystals or When sensing knock, the magnetostrictive rods, which are in a magnetic field,
semiconductors. change the flux field in the coil due to knock-induced forces. This change in
flux produces a voltage change in the coil. This voltage is used to sense excessive
knock (see Chapter 7). Other sensors use piezoelectric crystals or the
piezoresistance of a doped silicon semiconductor. Whichever type of sensor is
used, it forms a closed-loop system that retards the ignition to reduce the knock
detected at the cylinders. Systems using knock sensors are explained in Chapter
7. The problem of detecting knock is complicated by the presence of other
vibrations and noises in the engine.

214 UNDERSTANDING AUTOMOTIVE ELECTRONICS