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Basic Knowledge
Spark and Ignition
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 plugs, it is approximately in the range of 0.10-0.15mm for each 10,000km driven.
diagram

Rise in required voltage
The required voltage (the voltage required to discharge across the gap) 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 4,000km). After that, the main factor is the enlargement of the gap due to electrode wear and the rise in the required voltage is smaller.
diagram

Misfiring and its cause
diagram
Failure:Air-Fuel mixture does not burn.
Spark failure:Failure due to sparks not discharging 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.
Ignition failure:Sparks discharge between the electrodes of the spark plug but the air-fuel mixture does not ignite and burn.
Economic service life
diagram
The physical service life for a spark plug can be thought of as the number of kilometers driven until the spark plug begins to misfire. Misfiring causes not only wastage of fuel, but also irregular engine vibration and output drop. In particular, for vehicles with emissions controls, misfiring can damage the catalytic converter, so using a spark plug all the way to the end of its physical service life is inexpedient economically.
Therefore, as the economical replacement interval, replacement after the number of kilometers in the figure above is recommended. (However, 100,000km for platinum spark plugs.)


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