Spark Plugs
Chapter 2
Study Tips
The spark plugs are critical parts that dominate
the engine combustion and bear a major
responsibility for higher engine performance.
Spark Plugs
Chapter 2 Spark Plugs
27
5 Service Life
Service Life
Electrode wear
The electrode wears from the locations that discharge
easily with spark discharge.
In particular, since the center electrode reaches high
temperatures, it oxidizes and wears.
The amount of electrode wear varies with the electrode
material melting point, strength, hardness, etc. In order
to reduce the amount of this wear, nickel alloys,
platinum, iridium, and other such materials are used for
the electrodes and service lives are also extended with
fine electrodes.
Also, the wear varies with the engine type and usage
conditions, but for normal nickel alloy plugs, it is
approximately in the range of 0.10-0.15 mm for each
10,000 km driven.
Electrode wear
Rise in required voltage
The required voltage rises in proportion to the distance
driven.
This rise in the required voltage is large until the sharp
section at the end of the center electrode is worn round
to some degree (about 4,000 km). After that, the main
factor is the enlargement of the gap due to electrode
wear and the rise in the required voltage is smaller.
Shape change
Gap wear
D: Mileage driven (km) V: Required voltage (kV)
2
2
1
1
Misfiring and its cause
Air-Fuel mixture
does not burn.
Failure due to sparks not
dischanging between the
electrodes of the spark plug
This occurs when the voltage
generated by the ignition coil
is lower than the voltage
required by the spark plug.
Sparks dischange between
the electrodes of the spark
plug but the air-fuel mixture
does not ignite and burn.
