Volvo 819 E - Knock Sensor

Group 28 Ignition sys t ems
D esign and function - Compensation funct i ons
B
Knock sensor
c D 1U095
Function
The knock sensor (1) is used to detect engine knock, enabling the con tr ol unit (2) to take the appropriate action by re -
tarding
the igni ti on.
Construct i on
The knock sensor (s ee pic ture C) consis ts of a casing {3) housing a piezoelectric crystal (4) mounted on a threaded
sleeve (5) between two connector strips (6). One side of the assembly is fi tt ed with a damping weigh t (7) and spring
washer (8) secured by a nut (9 ).
Operation
Mounted on the cy lin der blo ck, the sensor detects the vibration caused by knock. Since the dev ice employs a piezo-
electric crys tal lin which a potent ial gradient is set up when the material is subjected to strain), the sensor develops
i ts 'o wn ' signal. The vibrations caused by kno ck produce an ins tantaneous pressure on the bottom of the sensor,
wh ich responds by generating an output voltage.
Signal
The signal delivered by th e piezoelectric crystal consis ts of a con t inuous, variable voltage of a frequency co rrespond-
i ng to the engine vibrations. The signal is fed to the control unit wh ich co mputes a mean or 'reference' value.
Sensing of signals
The control unit is programmed specifically to detect signals of the frequency generated by k nock {approx. 7.5 kH z).
The unit is provided wi th a ' measurement window ' - in effect, a specific range of crankshaft angles wi th in w hich t he
unit is 'o pen' to receive signals from the knock sensor (approx. 15
0
before TOC to app rox. 55
0
after TOC). I mmedia t ely
on detecting knock in a c ylinder , the devi ce generates a signal whi ch deviates significantly from the compu t ed 'refe r-
ence' signal.
This is in terpreted by the control unit as confirmation that knock has occurred. After an interval of th ree
ignit i on pulses (in the case of 4 -cyli nder engines) , the con tro l unit re tards the ignition , thereby correc ti ng the condi-
tion in the individual cylinder concerned.
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