Spark plugs with supplementary spark gap

Spark plugs with supplementary spark gap were developed for engines and operating modes subject to increased amounts of carbon deposits.
NGK spark plugs with supplementary spark gap

These spark plugs were developed for engines and operating modes subject to increased amounts of carbon deposits. Spark plugs with supplementary spark gaps are equipped with a case that is fit close to the insulator.

If the insulator tip is clogged with carbon deposits and the engine is started, the spark voltage first flows up to the height of the fitted case. Since the jump to the case now represents lower resistance than flowing through the insulator, an arc-over to the housing and thus the ignition of the fuel-air mixture occurs.

As soon as the spark plug has reached the self-cleaning temperature of 450 °C, carbon deposits are burned off and the spark can reassume the path through the earth electrode.