Semi-surface ignition type spark plugs
These spark plugs provide for a reliable cold start when there is carbon deposit clogging on the insulator and even clean the insulator at temperatures below the self-cleaning temperature.
For this purpose, they have at least two earth electrodes arranged on the side with a tapered point. If the insulator is clean, the arc-over takes place from the tip of the middle electrode to the tip of one of the earth electrodes at the top end.
If the insulator is clogged with carbon deposits, the spark initially creeps over to the insulator tip in order to flash over to the lower end of the earth electrode. To do so, it has to overcome lower resistance than for creeping over on the insulator.
With this process, not only is the fuel-air mixture ignited, the insulator tip is also cleaned of carbon deposits with every spark. If this process has been completed, a normal arc-over from the middle electrode takes place.