Spark plug with noble metal tip

#The present invention relates to a spark plug.

#Conventionally, a spark plug is used for providing ignition in an internal combustion engine such as a gasoline engine. In a spark plug, a spark discharge gap is formed between a center electrode and a ground electrode. Incidentally, there is known a spark plug in which a noble metal tip is attached on the electrode base metal of a ground electrode via an intermediate member (refer to, for example, International Publication WO02009/084565). The intermediate member is used for reducing the occurrence of a defect which could otherwise result from the noble metal tip being attached directly onto the electrode base metal. For example, through use of the intermediate member therebetween, while the amount of use of a noble metal is reduced, the area of joining to the electrode base metal can be increased. According to the technique mentioned in International Publication WO02009/084565, the intermediate member to which the noble metal tip is attached is joined to the electrode base metal by welding.

#However, as a result of recent tendency toward higher engine outputs, the working environment of a spark plug is becoming more severe than before. Accordingly, the spark plug manufactured on the basis of the technique disclosed in International Publication WO02009/084565 has potentially suffered separation of the intermediate member from the electrode base metal as a result of the welding strength between the intermediate member and the electrode base metal failing to endure a working environment. Such a problem is not limited to the ground electrode, but is in common with a spark plug in which the noble metal tip is attached on the electrode base metal of the center electrode via the intermediate member.

#FIG. 1 is a sectional view of a spark plug 100 according to an embodiment of the present invention, showing essential portions thereof.

#FIGS. 2(A) and 2(B) are a set of views for explaining the details of a center electrode 20 and a ground electrode 30.

#FIG. 3 is a sectional view taken along line A-A of FIG. 2(B).

#Next, embodiments of the present invention will be described in the following order. A. Embodiment, and B. Modified embodiments.

#FIG. 1 is a sectional view of a spark plug 100 according to an embodiment of the present invention, showing essential portions thereof. For convenience of explanation, the upper side of the spark plug 100 in FIG. 1 may be referred to as one end side (rear side), and the lower side in FIG. 1 may be referred to as the other end side (forward side). The spark plug 100 includes a ceramic insulator 10, a center electrode 20, a ground electrode 30, a metal terminal 40, and a metallic shell 50. The rodlike center electrode 20 protrudes from the other end of the ceramic insulator 10 and is electrically connected to the metal terminal 40 provided at one end of the ceramic insulator 10, through the interior of the ceramic insulator 10. The center electrode 20 is held by the ceramic insulator 10. The ceramic insulator 10 is held by the metallic shell 50. The ground electrode 30 is electrically connected to the metallic shell 50 and forms a spark gap for generating sparks in cooperation with the forward end of the center electrode 20. The spark plug 100 is mounted, through the metallic shell 50, to a mounting threaded hole 601 provided in an engine head 600 of an internal combustion engine. For example, when a high voltage of 20,000 to 30,000 volts is applied to the metal terminal 40, a spark is generated across the spark gap formed between the center electrode 20 and the ground electrode 30.

#The ceramic insulator 10 is an insulator formed from a ceramic material such as alumina by firing. The ceramic insulator 10 is a tubular member having an axial bore 12 formed at the center for accommodating the center electrode 20 and the metal terminal 40. The ceramic insulator 10 has a central trunk portion 19 formed at its center with respect to the direction of an axis CL of the spark plug 100 and having an outside diameter greater than that of the remaining portion. The ceramic insulator 10 has a rear trunk portion 18 formed on a side toward the metal terminal 40 (one end side) of the central trunk portion 19 for electrically insulating the metal terminal 40 and the metallic shell 50 from each other. The ceramic insulator 10 has a forward trunk portion 17 formed on a side toward the center electrode 20 of the central trunk portion 19 and having an outside diameter smaller than that of the rear trunk portion 18. The ceramic insulator 10 further has a leg portion 13 formed on the forward side of the forward trunk portion 17 and having an outside diameter which is smaller than that of the forward trunk portion 17 and which reduces toward the side toward the center electrode 20 (the other end side).