Page 258 - Understanding Automotive Electronics

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DIGITAL ENGINE CONTROL SYSTEM 7

Figure 7.11
Cylinder Pressure
(Knocking
Condition)

FPO

acoustical energy in the engine block at the sensor mounting point. This
voltage is sent to a narrow bandpass filter that is tuned to the knock frequency.
The filter output voltage V is proportional to the amplitude of the knock
F
oscillations, and is thus a “knock signal.’’ The envelope voltage of these
oscillations, V , is obtained with a detector circuit. This voltage is sent to the
d
controller, where it is compared with a level corresponding to the knock
intensity threshold. Whenever the knock level is less than the threshold, the
spark is advanced. Whenever it exceeds the threshold, the spark is retarded.
Following the detector in the circuit of Figure 7.12 is an electronic gate
that examines the knock sensor output at the time for which the knock
amplitude is largest (i.e., shortly after TDC). The gate is, in essence, an
electronic switch that is normally open, but is closed for a short interval (from 0
to T ) following TDC. It is during this interval that the knock signal is largest
in relationship to engine noise. The probability of successfully detecting the
knock signal is greatest during this interval. Similarly, the possibility of
mistaking engine noise for true knock signal is smallest during this interval.
The final stage in the knock-measuring instrumentation is integration
with respect to time; this can be accomplished using an operational amplifier.
For example, the circuit of Figure 7.13 could be used to integrate the gate
output. The electronic gate actually controls switches S and S . The output
1
2
voltage V at the end of the gate interval T is given by:
K
T
d
⁄
V K = ( – 1 RC) o ∫ V d t() t
This voltage increases sharply (negative), reaching a maximum amplitude at the
end of the gate interval, as shown in Figure 7.13, provided that knock occurs.
However, if there is no knock, V remains near zero.
K

UNDERSTANDING AUTOMOTIVE ELECTRONICS 245