Honda CB160 Shop Manual page 92

88
5.
ELECTRICAL PARTS
Fig.
5.
28 Spark plug
SIDE ELECTRODE - - -...
.::::=r--T
Fig. 5. 29 Construction of spark plug
ORRUGATED
INSULATOR
FILL
HEX SECTION
METAL
SPECIAL NICKEL
ALLOY MATERIAL
IRON LINE
MATERIAL
Fig. 5. 30 Electrode construction
F. Spark plug
The spark plug plays 'the most important role·in the
ignition system of a engine. That is, it receives the
high voltage generated in the magneto or ignition coil
and generates sparks between the center electrode
and side electrode in the engine combusion chamber,
igniting ,the mixed_ compressed gas in the combustion
chamber, accelerating combustion, and applying com·
bustion pressure to the piston.
Spark plugs operate under severe conditions ; hence,
high strength and reliability are required. (Refer to
Fig.
5.
281
1.
Spark plug requirements
To function properly, a spark plug should overcome
the following conditions :
a. Current
Current takes the path of least resistance, in pre·
fence to a spark gap. At normal temperatures, the
insulation quality of an insulator is high, but at high
temperatures the insulation quality is reduced. Hence,
an insulator in which the insulation quality does not
deteriorate at high temperature is required.
b. Combustion
Combustion pressure in the cylinder is between
35° to 45° at atmospheric pressure. Combustion
pressure also takes the path of least resistance. If
the airtightness is insufficient, high temperature
combustion gas enters the plug, the plug overheats
and its performance is decreased. Therefore,
sufficient mechanical strength is required for high
temperature, high pressure, vibration, and shock.
c. Combustion temperature
Combustion temperatures of mixed compressed
gas ·attain 2,000°C. If temperature is not dissipated
quickly the plug overheats and causes early ignition
or burn loss of the electrode, reducing performance
of the engine. Therefore, the spark plug should
provide suitable heat characteristics, withstand sud-
den temperature changes, and dissipate heat gene·
rated by combustion gas and spark discharge.
d. Deposit due to imperfect combustion
When the ignition section of the insulation is
stained, part of the high voltage is loss, resulting
in a poorer spark.
e. Lead alloy
In gasoline, tetraethylene lead is mixed as com·
bustion retarding agent, lead oxide being generated