Patent ID: 8575828
Filing Date: 2013-11-05
Classification: H01T

Abstract:
1. A spark plug comprising: a center electrode extending in an axial direction of the spark plug; a center-electrode-side noble metal tip joined to a leading end of the center electrode; a fused portion is formed between the center electrode and the center-electrode-side noble metal tip, an obtuse angle achieved in a boundary portion between the center-electrode-side noble metal tip and the fused portion is taken as θ an obtuse angle achieved in a boundary portion between the fused portion and the center electrode is taken as θ a relational expression of θ a ground electrode opposing a side surface of the center-electrode-side noble metal tip; and a ground-electrode-side noble metal tip joined to a distal end of the ground electrode, wherein: of corners of a distal end of the ground-electrode-side noble metal tip, a corner closest toward a leading end direction of the center-electrode-side noble metal tip is taken as a first corner; of corners of the leading end of the center-electrode-side noble metal tip, a corner closest to the ground-electrode-side noble metal tip is taken as a second corner; of corners formed at a starting point where a diameter of the center electrode is reduced, a corner closest to the ground electrode is taken as a third corner; of the corners of the ground electrode, a corner closest to the third corner is taken as a fourth corner; of the corners on the distal end of the ground-electrode-side noble metal tip, a corner closest to the third corner is taken as a fifth corner; of corners formed in a boundary portion between the center electrode and the fused portion, a corner closest to the ground electrode is taken as a sixth corner; a virtual flying spark path established between any two of the corners is defined, on condition that: a line passing through a vertex of one corner and bisecting the one corner is taken as a first line, a line passing through a vertex of another corner and bisecting the another corner is taken as a second line, a circular arc tangent to the first line and having end points at the vertex of the one corner and the vertex of the another corner is taken as a first circular arc, and a circular arc tangent to the second line and having end points at the vertex of the one corner and the vertex of the another corner is taken as a second circular arc, as a combination of: a path extending along the first circular arc which connects the vertex of the one corner to a middle point of the first circular arc; a straight path that connects the middle point of the first circular arc to a middle point of the second circular arc; and a path extending along the second circular arc which connects the middle point of the second circular arc to the vertex of the another corner; and a relational expression of L a length of a virtual flying spark path defined between the first corner and the second corner is taken as L a length of a virtual flying spark path defined between the third corner and the fourth corner is taken as L a length of a virtual flying spark path defined between the third corner and the fifth corner is taken as L L a relational expression of L a length of a virtual flying spark path defined between the fourth corner and the sixth corner is taken as L a length of a virtual flying spark path defined between the fifth corner and the sixth corner is taken as L L