US Patent 7554253 Spark plug with increased durability and carbon fouling resistance

A spark plug for an internal combustion engine is provided which includes a metal shell, a porcelain insulator, a center electrode, a main ground electrode, and auxiliary ground electrodes. Each of the auxiliary ground electrodes has an end face facing the insulator to define an auxiliary spark gap. The end face includes an increasing-radial distance surface which is located at a distance from a longitudinal center line of the spark plug in a radial direction thereof which increases as approaching to the metal shell. The insulator nose has a wall thickness T meeting a relation of 0.3 mm≦T≦0.7 mm. This avoids a great local increase in electrical field strength on the auxiliary ground electrode to minimize excessive discharge within the auxiliary spark gap to enhance carbon fouling resistance and durability of the spark plug.