1. Field of the Invention
The present invention relates to a spark plug used for providing ignition of an internal combustion engine.
2. Description of the Related Art
A known spark plug for providing ignition of an internal combustion engine such as an automotive engine, comprises: an insulator having an axial hole in the axial direction of the spark plug, a center electrode disposed in a tip end side of the axial hole of the insulator; a metal shell surrounding the insulator; a first ground electrode in which one end is bonded to the metal shell and another end portion opposes a tip end face of the center electrode; and a second ground electrode in which one end is bonded to the metal shell, and another end opposes a side peripheral face of the center electrode or that of the insulator. In such a spark plug, spark discharge is caused in a first discharge gap formed by the center electrode and the first ground electrode to ignite an air-fuel mixture. On the other hand, carbon or the like may adhere to the tip end face of the insulator to result in so-called “fouling.” In this case, spark discharge creeping on the surface of the insulator occurs in a second discharge gap which is formed by the second ground electrode and the side peripheral face of the center electrode. Therefore, the contaminant is burned out, so that an anti-contamination property can be attained (for example, see JP-A-2001-237045).
Recently, the demand for further enhancing the performance of an engine is increasing, and there is a need to further improve the ignitability of a spark plug. In a spark plug in which only a first ground electrode is disposed (a second ground electrode is not disposed), the method described below is effective in improving the ignitability. A method in which, in a noble metal tip (precious metal tip) joined to an inner side face of the another end portion of a first ground electrode body, the distance in the axial direction between a face opposing the center electrode and the inner side face (hereinafter, also referred to as protrusion amount) is increased, is also effective. The reason is as follows. A flame kernel produced in a first discharge gap which is formed by the center electrode and the first ground electrode is caused to grow by swirling or the like. When the protrusion amount of the noble metal tip is small, however, the distance between the first discharge gap and the first ground electrode body is so small that, in an early stage of the growing process of the flame kernel, the flame kernel makes contact with the first ground electrode body. As a result, the growth of the flame kernel may be impeded (hereinafter, this is also referred to as a flame quenching effect). Therefore, a structure in which the protrusion amount of a noble metal tip is made as large as possible so as to expedite growth of a flame kernel is often employed.
In order to further improve the ignitability, the inventors have studied a configuration in which a noble metal tip having a larger protrusion amount is joined to the first ground electrode body of the spark plug of JP-A-2001-237045. However, the spark plug of JP-A-2001-237045 has a structure in which, although having a second ground electrode, a flame kernel makes contact with the second ground electrode when the flame kernel grows. Therefore, there is a possibility that the flame quenching effect will impede the growth of the flame kernel. Also in the first ground electrode body in which the noble metal tip protrudes by a large amount, moreover, there is a possibility that, when the flame kernel further grows to exceed the above-mentioned state, the growth of the flame kernel is impeded. As a result, a problem arises in that ignitability cannot be sufficiently ensured.