1. Field of the Invention
The present invention relates to a spark plug for internal combustion engine provided with a ground electrode composed of an Ir alloy element and a Ni or Fe base alloy base element, in particular, applicable to the engine for a co-generation system, a delivery pump for pressurized gas and a vehicle.
2. Description of Related Art
A conventional spark plug has generally a center electrode fitted through an insulator into a housing and a ground electrode fixed to the housing. The center electrode partly exposed out of the end of the insulator faces the ground electrode.
To improve the life time and the performance of the spark plug, a noble metal chip is bonded to the ground electrode (generally, Ni base alloy) to constitute a spark discharge spot at the spark discharge gap.
Conventionally, a platinum (Pt) alloy has been widely used as material for the noble metal chip. However, the Pt alloy has a drawback that the consumption resistance thereof is considered not to be sufficient to meet more sever engine specifications for vehicles in future. Therefor, the use of the iridium (Ir) alloy having a melting point higher than that of the Pt alloy has been recently proposed, as shown in JP-A-8-298178.
For bonding an element made of Ir alloy including more than 50 weight percent Ir (hereinafter merely called Ir alloy) to the ground electrode, a resistance welding is not applicable to secure sufficient bonding strength and a laser beam welding is preferable because the chip and the electrode may be sufficiently molten due to the high density of its energy. However, the portion where Ni and Ir are molten is located near the spark discharge gap and, further, as the Ir content of the molten portion is less than that of the chip of Ir alloy, a spark consumption resistance of the molten portion is worse, compared to that of the chip of Ir alloy itself. Therefore, the molten portion tends to be more rapidly consumed so that the chip of Ir alloy may be left out of the ground electrode.