Abstract: An economical spark plug for an internal combustion engine which has a long service life and by which a reduction in the discharge voltage and the use of the minimum amount of a noble metal can be achieved. The spark plug has an insulator, a center electrode supported by the insulator, and an earth electrode facing to the center electrode. A narrowed portion is arranged on a tip of an electrode material of at least one of the center electrode and the earth electrode. The narrowed portion includes a projection formed by extending the electrode material and a noble metal chip firmly connected to a terminal end of the projection. The electrode material is a nickel-base heat-resisting alloy. The noble metal chip is made of a Pt--Ir alloy essentially consisting of 90-100% by weight Pt and 0-10% by weight Ir. A diameter D of the narrowed portion is in a range of 0.6-1.2 mm. A thickness t of the noble metal chip is in a range of 0.16-0.8 mm.

Abstract: Method of producing a spark plug having a center electrode 1 provided with precious metal chips 31 and 32 on the tip 10. Production of the center electrode 1 is composed of a flat area formation process wherein a cylindrical center electrode 1 is formed by extrusion of metal material and wherein flat areas 111 and 113 are formed on the sides of the tip 10 of the same, a tip bonding process wherein precious metal chips 31 and 32 are bonded to the flat areas, and a plastic machining process wherein the tip 10 whereupon the precious metal chips are bonded is formed into an abbreviated cylinder by means of plastic machining. By applying an identification mark to the tip surface of the tip 10, the precious metal chips 31 and 32 can accurately and easily be arranged opposite the ground electrodes when mounting the center electrode in a metal housing.

Abstract: A cold forging apparatus for producing a metal tip for use, for example, in an electrode of a spark plug includes an intermediate die for receiving a metal blank, an upper die having a punch for forming a recess in the metal blank, and a lower die movable in a lateral direction. A press member is mounted on the lower die, and is retracted when a projection, extruded from the metal tip by the punch, is brought into pressing engagement with the press member. The punch is driven into the metal blank to form a recess and the projection respectively on upper and lower surfaces of the metal blank, and subsequently in a hydrostatic condition, the lower die is moved laterally to remove the projection from the metal blank.

Abstract: According to the present invention, a spark plug for an internal combustion engine includes a center electrode, a ground electrode so as to form a spark gap with the center electrode, and a tip electrode composed of a material having a superior consumable resistance as compared to the center electrode and disposed on an end face of the center electrode so as to form a spark gap with the ground electrode. An axial length of the discharge portion of the ground electrode where spark discharge occurs is substantially larger than an axial length of the tip electrode where spark discharge occurs. When used for an internal combustion engine, a spark discharge is mainly generated between the peripheral surface of the center electrode and the ground electrode during early operation. Consequently, as the center electrode has been consumed, the spark discharge is mainly generated between the tip electrode and the ground electrode.

Abstract: The tip 10 of the center electrode 1 is opposed to the ground electrodes 21 and 22. The tip 10 has flat faces 111 and 121 opposed to the discharge faces of the ground electrodes 211 and 221, and inclined faces 101 and 102 extending from the flat faces to the base 19. The flat faces have noble metal pieces 31 and 32 at positions opposed to the discharge faces. The discharge faces have a spark gap G between itself and the noble metal piece, a minimum gap A between itself and the inclined face, and a minimum gap B between itself and the tip face 41 of the insulator 4. The spark gap G, minimum gap A, and minimum gap B are related such that the distance of A.gtoreq.1.2.times.the distance of G, and the distance of B.gtoreq.the distance G. The axial length L of the flat face and the area S of a cross section perpendicular to the flat face should, preferably, have the relation S/L.gtoreq.0.7.

Abstract: A composite chip formed by joining a discharging layer and a heat stress relieving layer at a joint interface therebetween beforehand is provided on at least one of a central electrode and a ground electrode in its discharge portion made of an electrode material. The discharging layer is made of a precious metal or a precious metal alloy having superior spark- and wear-resistance, and the heat stress relieving layer is made of a metal or an alloy having a linear expansion coefficient between those of the discharging layer and the electrode material. Formed at the joint interface between both the discharging layer and the heat stress relieving layer through mutual diffusion of those materials developed when the two layers are joined to each other is a diffusion layer, in which concentrations of materials of both the layers are continuously changed. A thickness of the diffusion layer is not less than 3 .mu.m in a state that the composite chip is welded to the discharge portion.

Abstract: There is disclosed a spark plug which can maintain a low spark voltage for a long period of time even if a noble metal tip is used in a small amount. A discharge gap is formed between a central electrode and an earth electrode. The noble metal tip is formed on that end face of the central electrode facing the earth electrode. A cross-shaped groove is formed in that end face of the noble metal tip joined to the central electrode. The cross-shaped groove is exposed when the noble metal tip is consumed by a discharge developing in the discharge gap.