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5.
3
IGNITION CIRCUIT
bustion, port of the high voltage is
lost
and
a poor spark is produced at the spark plug
electrodes,
causing engine malfunction.
e.
Lead compound: T etraethyl lead
is
added to
the gasoline as an antiknock additive
.
Lead
oxide
is
formed
during combustion
and
adheres
to
the
insulator of the spark plug.
This be-
comes on electrical
conductor at
high
temper-
ature and causes the partial
loss of
the high
voltage
current; resulting in
engine
malfunction.
It is
required that the insulator and the elec-
trodes
be
free from being
chemically affected
under
high
heat condition.
2.
Spark
Plug
The
construction
of the type
spark
plug most com-
monly used
today
is
shown in cross
section
by Fig.
5. 13
a.
Electrode
The
material
of which
the
electrode
is made
should
be one that is highly resistant
to w
ear.
possess
low discharge voltage, high
heat
con-
ductiv ity, acid resistance
and
must
be a
good
electrical conductor; further, it must be readily
workable.
Nickel alloy and metal having high
heat and corrosion resistant is used
(Fig.
5. 14)
b.
Insulator
The
insulator
is usually mode
of high
qualit
y
alumina.
Since voltage
of
6
to 15 KVA is
applied
to
the electrode
and
exposed
to
temperature exceeding 2000°C,
the insulator
must be able to
withstand
these conditions
.
The chief
benefits
derived from using this type
insulator are:
( 11 The insulating property
under high temper-
ature condition
is
superior
and the possibility
of flashover is reduced due to the ribs
formed
on
the head
of
the insulator body.
Misfire
of the
ignition
under high speed and loaded
condition
is eliminated.
(2)
Due
to its good heat conducting property,
the heat
of
the plug is
rapidly
dissipated, pre-
venting any
overheated condit ion.
(3)
Its high resistance to thermal shock pre-
vents damages to
it
from sudden heating and
cooling.
®----
-
@)
Fig. 5
. 13
Cross-section
of
spark
plug
CD
Center
electrode
@
Wire
pack ing
@
Gasket
®
Plate pocking
®
Spark
clearance
lgap
l
®
Terminal
®
Insulation (with corrugation!
®
Filled powd
er
®
Bonding
@)
Hex nut
@
Metallic
main body
@
Ground electrode
@
Length of thread (reach!
Fig.
5.
14
Electrode construction
CD
Special
nickel alloy material
®
Copper
wire
material
@
Iran
wire
material
113
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