Fire end Appearance
Normal condition
An engine’s condition can be judged by the spark plug’s fi ring end appearance. If the fi ring end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning correctly.
Normal life (Life in case of normal use)
A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap requires higher voltage. It also reduces the engine’s effi ciency, resulting in reduced fuel economy and increased exhaust emissions. Moreover, it contributes to pollution generated by exhaust gas.
Carbon fouling (Wet condition & Dry condition)
Actually, although there are various cases, from the view point of the safety factor, when the insulation resistance is 10MΩ or more, then engine can be started normally. If the insulation resistance drops to 0MΩ, not just dry fouling but mostly both dry fouling and wet fouling occur in combination.
Deposit
The accumulation of deposits on the fi ring end is infl uenced by oil leakage, fuel quality and the engine’s operating period. Especially, on the two stroke engine, deposits are created due to burning of two stroke engine oil added to air/fuel mixture gas.
Overheating
After having been overheated, the insulator tip is glazed or glossy, and deposits accumulated on the insulator tip melt. Sometimes these deposits blister on the insulator tip. In addition, when the plug continues to be overheated, the insulator starts melting.
Breakage of the insulator
This is caused due to internal stress or thermal shock (rapid change in temperature) due to abnormal thermal expansion.
Breakage of the ground electrode
Mechanical fatigue is caused in the electrode due to the high frequency gas vibrations from the high temperature combustion gas. It is also caused by the excessive vibrations of the engine itself.
Melting
Melting is caused by abnormal overheating. Mostly, the electrode surface is rather lustrous and uneven. The nickel alloy for electrodes starts melting at temperature of 1200 to 1300℃ or more.
Fouling by fuel additives
In order to raise the octane number of gasoline to suppress the knocking, MMT or ferrocene is added to the gasoline and due to this there is fouling in the plug. The plug fouled by MMT or ferrocene is diff erent from the plug fouled by carbon (carbon fouling) in that there is no self-cleaning even when the temperature of the plug rises. Moreover, since in this type of fouling, the deposits adhere strongly and cannot be cleaned completely, once this type of fouling occurs, the insulation resistance cannot be recovered.