Group
28
Ignition sys
t
ems
D
esign and function
Compensation funct
i
ons
B
Knock sensor
c
D
Function
The
knock
sensor (
1
)
is
used to detect engine knock, enabling
t
he con
tr
ol unit (2)
t
o
take
the appropriate
action by
re
-
tard
ing
the
igni
ti
on.
Construct
i
on
The knock
sensor
(s
ee p
ic
ture
C)
consis
t
s
of a
casing
{3)
housing a
p
ie
zoelectric
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
l
in
der blo
c
k
,
the
sensor detects the vibration
c
aused by knock
.
Since the dev
ic
e
employs
a
piezo-
crys
ta
l lin
which
a
potent
i
a
l
gradient
is
set
up when the
material
i
s
subjected
to
strain),
the
sensor
develops
i
ts
'o
wn
'
signal.
The
vibrations
caused
by
kno
ck
produce an
ins
tantaneous
pressure
o
n the bottom
of
the
sensor,
wh
ic
h
responds
by
generating an output
voltage.
Signal
The
signal delivered
by th
e
piezoelectr
ic
crystal consis
ts
o
f
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
i
ch
co
mputes
a
mean
or
'
referen
ce'
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
t
h a
'
measurement window
'
in
effec
t
, a specific
range of cranksha
ft
angles wi
th
in w
h
ic
h t
he
unit
is
'o
pen'
t
o receive
signals
from the kno
c
k
senso
r
(approx.
before TOC to app
r
ox. 55
0
after TOC)
.
I
mmedia
t
ely
on
detecting knock
in a
c
ylinder
,
the devi
ce
generates a
signal whi
c
h deviates
significantly
from the compu
t
ed
'refe
r
-
ence' signal.
This
is
in
terpreted
by the control unit as confirmation
that knock ha
s
occurred.
After an interval
of
th
r
ee
ignit
i
on pulses (in
the
case of
4
-cyli
nder
engines)
,
the
con
tro
l
unit
re
t
ards the
ignition
,
thereby
correc
t
i
ng the
condi-
t
ion i
n the
individual cylinder
concerned.
30
