Patent ID: 8643263

Claim:
A spark plug ( 20 ), comprising: an insulator ( 22 ) extending along a center axis (A) and presenting an insulator outer surface ( 30 ) extending from an insulator upper end ( 34 ) to an insulator nose end ( 36 ); said insulator ( 22 ) including an insulator body region ( 24 ) extending between said insulator upper end ( 34 ) and said insulator nose end ( 36 ); said insulator ( 22 ) presenting a first radius (R 1 ) at said insulator body region ( 24 ) extending from said center axis (A) to said insulator outer surface ( 30 ); said insulator ( 22 ) including an insulator nose region ( 26 ) between said insulator body region ( 24 ) and said insulator nose end ( 36 ); said insulator ( 22 ) presenting a sixth radius (R 6 ) at said insulator nose region ( 26 ) extending from said center axis (A) to said insulator outer surface ( 30 ), said sixth radius (R 6 ) being less than said first radius (R 1 ); said insulator ( 22 ) including an insulator seat ( 28 ) disposed between said insulator body region ( 24 ) and said insulator nose region ( 26 ), said insulator seat ( 28 ) extending radially toward said center axis (A) at an insulator seat angle (α i ); said insulator ( 22 ) including a first transition ( 48 ) extending from said insulator body region ( 24 ) to said insulator seat ( 28 ), said first transition ( 48 ) being convex; said insulator ( 22 ) presenting a fifth radius (R 5 ) at said first transition ( 48 ), said fifth radius (R 5 ) being a spherical radius at said first transition ( 48 ); said insulator ( 22 ) presenting a second transition ( 50 ) extending from said insulator seat ( 28 ) to said insulator nose region ( 26 ), said second transition ( 50 ) being concave; said insulator ( 22 ) presenting a second radius (R 2 ) extending from said center axis (A) to a point (P) at the intersection of said insulator outer surface ( 30 ) of said insulator seat ( 28 ) and said insulator outer surface ( 30 ) of said insulator nose region ( 26 ) adjacent said second transition ( 50 ); said insulator ( 22 ) presenting a fourth radius (R 4 ) at said second transition ( 50 ), said fourth radius (R 4 ) being a spherical radius at said second transition ( 50 ); said insulator seat angle (α i ) being from 35° to 50°; and said insulator seat angle (α i ) being greater than or equal to a boundary value provided by the equation: 90°−acos [1−(R 1 −R 2 )÷(R 4 +R 5 )].