Patent ID: 7816846

Claim:
A spark plug for an internal combustion engine comprising: a hollow cylindrical metal shell with an open top end, said metal shell having an annular inner shoulder formed on an inner peripheral wall thereof; a porcelain insulator having a length, said porcelain insulator including a first outer shoulder formed on an outer peripheral wall thereof, said porcelain insulator being positioned within said metal shell in abutment of the first outer shoulder with the inner shoulder of said metal shell; a center electrode disposed in said porcelain insulator; a ground electrode joined to said metal shell to define a spark gap between itself and a top end of said center electrode; and a second outer shoulder formed on a portion of the outer peripheral wall of said porcelain insulator which is located between a top end of said porcelain insulator and the first outer shoulder of said porcelain insulator, said second outer shoulder being closer to the first outer shoulder than the open top end of said metal shell in a lengthwise direction of said porcelain insulator, said second outer shoulder tapering toward the top end of said porcelain insulator, wherein said metal shell has a slant peripheral surface formed on the open top end thereof, the slant peripheral surface facing inwardly of said metal shell so as to define an inner diameter of said metal shell increasing outward in an axial direction of said metal shell, wherein the slant peripheral surface is oriented so that a line, defined to extend along the slant peripheral surface toward a longitudinal center line of the porcelain insulator, passes near a boundary between the second outer shoulder of the porcelain insulator and a major body of the porcelain insulator, and wherein the slant peripheral surface is oriented so that an angle that a line, defined to represent an average of directions of a vortex stream of an air-fuel mixture flowing parallel to the slant peripheral surface, makes with a surface of the second outer shoulder of the porcelain insulator is an obtuse angle.