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5
.6
SPARK
PLUG
77
5.6
SP
PUG
::: -;;
performs
one
o=
·-e
-:::s-
-
~portent
-
-~
--
-
-
·
ne
engin
e
igni
io
s,
_
e":l.
The
high
o ·:::;;;e
:::r
oduced
by the
mogne·o o·
·
e
ignition
coil
·s
re::e··
ed
by
the
spark
plug
O"O
causes
the
high
e sio
n
spark
to
jump
across
from
he
cente
r
electrode
of
he
spark
plug to
the side
electrode
within
the
engin
e
combustion
cha
mber.
The
spa
rk
ignites the
compressed
fuel
mixture
in the
combustion chamber
a nd
produces on
explosion w
hich operates
the
engine.
Even
under
various adverse
conditions,
durability and
reliability
is required.
(Fig
.
5-21)
1.
SPARK
PLUG
The
operation o
f
the
engine
ca
n
be determined
by
the
condition of the plug.
The firing
area
of
the
insulator colored white, gray or
light
gray indicates
good
condition
and
is
performing
satisfacto
rily.
2.
HEAT RANGE OF THE SPARK PLUG
The
firing
area of the
spa
rk plug
insulator
is
exposed
to
ca rbon
and
oil while the
engine is
o perat-
ing,
and to prevent its
build-up,
plugs are designed
to burn off any deposits
by
the
heat of
combustion.
Spark
plugs
which are
too
cold
wil
l
not burn
off
the
carbon
and
oil
deposits and
wi
ll
cause
shorting
of
the high tension
voltage
resulting
in
the
engine
to
mis-
fire
.
In
the opposite case, the power
output
will
be drastically reduced
.
In
order to prevent
the
above troubles, the surface
of
the insulator firing area must be maintained at
approximately 500-870°C
1932-1600°
Fl
range.
This
temperature
is
referred
to
as
the
self-cl
eaning
temperature.
The temperature of
the
spark plug will
vary
to a
considerable
degree with
the
type engine and de-
sign,
riding
condition,
and fuel.
In
order for the
plugs to
function properly under
the different
conditions,
it
is
necessary
for the
plugs
to
properly dissipate
temperature
of the plug caused
by the
heat
of
combustion.
The rate of
heat
dissipation of the spark plug is
the heat range of the spark
plug.
A
spark plug which
readily dissipates
the
heat and which is difficult to
overheat is referred to as a "Cold Type",
A
spark
plug which retains the
heat
and which will burn readily
is referred to
as
a "Hot
Type".
On engine
oper-
ating
at high temperature, a
spark
plug which
is
difficult
to
overheat,
in
other words,
the
cold type spark
plug
is
used and for engine operating at low temperature
a hot type spark plug
is
used.
(Fig. 5-
22,
5-
23)
Figure
5-21.
Spork plug
Metal
packing
gap
Side electrode
Figure
5-
22.
Construction of
spark plug
Hot
type
c
-4
C
I
c
-
6
c
-
-s
st?
a
o JJse
,
.,
c
-
g
-
~
--
--
-
~
c
-
~-=~---·~
-~
c
-
1;-
---
---
g
--
~
c
-
~-
______
::-___ }
~
I
C
-
,
_
-----
..
----
.iij
Cold
tpe
cr:
Hot
type
(
low
te~era
ra se
Cold
type
(
high
temperature
use)
Figure
5-23.
Hea range of
spark
plug
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