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112
5.
ELECTRICAL PARTS
···---····-------------
F.
Spark Plug
Spark plug performs one of the most important
functions of the engine ignition system.
The high
voltage produced by the ignition coil
is routed
through the high tension lead to the spark plug and
causes the current to discharge across the center
electrode to the side electrodes in a form of a
spark within the combustion chamber of the engine.
This spark ignites the compressed fuel mixture which
produces the energy to operate the engine.
Since
it must perform under different adverse conditions,
durability and reliability are primary requirements.
Honda 450 uses the spark plug type
NGK B-8 ES.
l.
Spark plug requirements
In order for the spark plug to perform satis-
factorily.
it must fulfill the foliowing conditions.
a.
Electr:cal insulation: Electrical current follows
the path of least resistance and, therefore, it
is constantly seeking a path having less than
that of having to jump across the spark plug
gap.
The
resistance of the insulator under
normal temperature is high but it deteriorates
with
increases in temperature, therefore, the
insulator must be made of material that does
not change with the temperature.
b.
Mechanical property:
The pressure within
the cylinder during combustion is
from
35 to
45 kg/cm
2 •
If the
spark plug
is inade-
quately sealed, the pressure will leak through
the spark plug and also causing the plug to
heat up, resulting in loss of efficiency.
The
spark plug must possess superior mechanical
properties so that it is able to withstand high
temperature and pressure as well as vibration
and shock.
c.
Heat conduction: The combustion temperature
of the fuel mixture exceeds 2,000°C (3,632°Fl
within the cylinder and this heat must be dis-
sipated as rapidly as possible or the plug w,11
overheat, causing preignition as well as dam-
aging the electrodes.
This will prevent ef-
fecient performance of the engine.
Spark plug
must, therefore, be able to withstand rapid
temperature changes and further must also be
able to dissipate the heat produced by the
combustion gas and electrical discharge.
d.
Carbon deposit: When carbon deposits build
up around the insulator due to improper com-
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