Insulator strength by seat geometry

1. 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 )].

2. The spark plug ( 20 ) of claim 1 wherein said insulator seat angle (α i ) is not greater than 300% of the boundary value.

3. The spark plug ( 20 ) of claim 2 wherein said insulator seat angle (α i ) is not greater than 200% of the boundary value.

4. The spark plug ( 20 ) of claim 3 wherein said insulator seat angle (α i ) is not greater than 150% of the boundary value.

5. The spark plug ( 20 ) of claim 1 including a shell ( 58 ) presenting a shell inner surface ( 76 ) facing said insulator inner surface ( 32 ) and extending from a shell upper end ( 72 ) to a shell lower end ( 74 );

said shell ( 58 ) including a shell body region ( 80 ) between said shell upper end ( 72 ) and said shell lower end ( 74 );

said shell ( 58 ) presenting a seventh radius (R 7 ) at said shell body region ( 80 ) extending from said center axis (A) to said shell inner surface ( 76 );

said shell ( 58 ) including a rib ( 82 ) extending radially toward said center axis (A) and disposed between said shell body region ( 80 ) and said shell lower end ( 74 );

said rib ( 82 ) including a shell seat ( 84 ) facing said insulator seat ( 28 ) and extending from said shell body region ( 80 ) radially inwardly toward said center axis (A) at a shell seat angle (α s ) to a rib inner surface ( 86 ); and

said shell ( 58 ) presenting a third radius (R 3 ) at said rib inner surface ( 86 ) extending from said center axis (A) to said shell inner surface ( 76 ), said third radius (R 3 ) being less than said seventh radius (R 7 ).

6. The spark plug ( 20 ) of claim 5 wherein said shell seat angle (α s ) is within +/−1° of said insulator seat angle (α i ).

7. The spark plug ( 20 ) of claim 5 wherein said third radius (R 3 ) is 0.121 inches (3.073 mm).

8. The spark plug ( 20 ) of claim 5 including a first gasket ( 60 ) compressed between said insulator seat ( 28 ) and said shell seat ( 84 );

said first gasket ( 60 ) having a gasket inner surface ( 90 ) facing toward said insulator ( 22 ) and a gasket outer surface ( 92 ) facing toward said shell ( 58 ), said gasket inner surface ( 90 ) and said gasket outer surface ( 92 ) each extending from a gasket top surface ( 94 ) to a gasket bottom surface ( 96 );

said first gasket ( 60 ) presenting an outer gasket thickness (t g1 ) extending from said gasket top surface ( 94 ) to said gasket bottom surface ( 96 ) at said gasket outer surface ( 92 ) and an inner gasket thickness (t g2 ) extending from said gasket top surface ( 94 ) to said gasket bottom surface ( 96 ) at said gasket inner surface ( 90 ); and

said outer gasket thickness (t g1 ) being greater than said inner gasket thickness (t g2 ).

9. The spark plug ( 20 ) of claim 8 wherein said inner gasket thickness (t g2 ) is greater than or equal to 70% of said outer gasket thickness (t g1 ).

10. The spark plug ( 20 ) of claim 8 wherein said gasket top surface ( 94 ) and said gasket bottom surface ( 96 ) have a friction coefficient less than or equal to 0.15 and said insulator seat angle (α i ) is from 35° to 45°.

11. The spark plug ( 20 ) of claim 1 wherein said insulator seat angle (α i ) is 45°±/−2°.

12. The spark plug ( 20 ) of claim 1 wherein said insulator seat angle (α i ) is 45°, R 1 is 0.145 inches (3.683 mm), R 2 is 0.105 inches (2.667 mm), R 4 is 0.080 inches (2.032 mm), R 5 is 0.020 inches (0.508 mm), and said insulator seat angle (α i ) is equal to 122% of the boundary value.

13. The spark plug ( 20 ) of claim 1 wherein said insulator seat angle (α i ) is 45°, R 1 is 0.145 inches (3.683 mm), R 2 is 0.095 inches (2.431 mm), R 4 is 0.120 inches (2.048 mm), R 5 is 0.020 inches (0.508 mm), and said insulator seat angle (α i ) is equal to 112% of the boundary value.

14. The spark plug ( 20 ) of claim 1 wherein said insulator ( 22 ) includes an insulator inner surface ( 32 ) facing toward said center axis (A), said insulator inner surface ( 32 ) and said insulator outer surface ( 30 ) presenting an insulator thickness (t i ) therebetween;

said insulator inner surface ( 32 ) extending annularly around said center axis (A) and presenting a bore along said center axis (A);

said insulator inner surface presenting ( 32 ) an insulator inner diameter (D 1 ) surrounding said bore and said insulator outer surface ( 30 ) presenting an insulator outer diameter (D 2 ), wherein the ratio of said insulator inner diameter (D 1 ) to said insulator outer diameter (D 1 ) along said insulator body region ( 24 ) adjacent said insulator seat ( 28 ) is from 0.12 to 0.45;

said insulator thickness (t i ) along said insulator nose region ( 26 ) being less than said insulator thickness (t i ) along said insulator body region ( 24 ) and said insulator thickness (t i ) decreasing along said insulator nose region ( 26 ) toward said insulator nose end ( 36 ); and

said insulator thickness (t i ) along said insulator seat ( 28 ) decreasing from said insulator body region ( 24 ) to said insulator nose region ( 26 ).

15. The spark plug ( 20 ) of claim 1 wherein said insulator ( 22 ) includes an insulator inner surface ( 32 ) extending from said insulator upper end ( 34 ) to said insulator nose end ( 36 );

said insulator inner surface ( 32 ) and said insulator outer surface ( 30 ) presenting an insulator thickness (t i ) therebetween;

said insulator inner surface ( 32 ) extending annularly around said center axis (A) and presenting a bore extending longitudinally along said center axis (A);

said insulator ( 22 ) including an insulator terminal region ( 38 ) extending from said insulator upper end ( 34 ) toward said insulator nose end ( 36 );

said insulator thickness (t i ) along said insulator terminal region ( 38 ) being constant;

said insulator ( 22 ) including an insulator transition region ( 40 ) between said insulator terminal region ( 38 ) and said insulator nose end ( 36 );

said insulator thickness (t i ) along a portion of said insulator transition region ( 40 ) being greater than said insulator thickness (t i ) along said insulator terminal region ( 38 );

said insulator thickness (t i ) along a portion of said insulator transition region ( 40 ) being less than said insulator thickness (t i ) along said insulator terminal region ( 38 );

said insulator thickness (t i ) along a portion of said insulator transition region ( 40 ) decreasing toward said insulator nose end ( 36 );

said insulator ( 22 ) including an insulator upper shoulder ( 42 ) extending from said insulator terminal region ( 38 ) to said insulator transition region ( 40 );

said insulator thickness (t i ) along said insulator upper shoulder ( 42 ) increasing from said insulator terminal region ( 38 ) to said insulator transition region ( 40 );

said insulator ( 22 ) including said insulator body region ( 24 ) between said insulator transition region ( 40 ) and said insulator nose end ( 36 );

said insulator ( 22 ) including an insulator lower shoulder ( 44 ) extending from said insulator transition region ( 40 ) to said insulator body region ( 24 );

said insulator thickness (t i ) along said insulator lower shoulder ( 44 ) decreasing from said insulator transition region ( 40 ) to said insulator body region ( 24 );

said insulator thickness (t i ) along said insulator body region ( 24 ) being less than said insulator thickness (t i ) along said insulator terminal region ( 38 ) and less than said insulator thickness (t i ) along said insulator transition region ( 40 );

said insulator inner surface ( 32 ) presenting an insulator inner diameter (D 1 ) surrounding said bore and said insulator outer surface ( 30 ) presenting an insulator outer diameter (D 2 ), wherein the ratio of said insulator inner diameter (D 1 ) to said insulator outer diameter (D 1 ) along said insulator body region ( 24 ) adjacent said insulator seat ( 28 ) is from 0.12 to 0.45;

said insulator thickness (t i ) along a portion of said insulator body region ( 24 ) being constant;

said insulator inner surface ( 32 ) along said insulator body region ( 24 ) presenting an electrode seat ( 46 );

said insulator thickness (t i ) along a portion of said insulator body region ( 24 ) increasing toward said center axis (A) and toward said insulator nose end ( 36 ) to present said electrode seat ( 46 );

said insulator thickness (t i ) being constant from said insulator transition region ( 40 ) to said electrode seat ( 46 );

said insulator ( 22 ) including said insulator nose region ( 26 ) disposed between said insulator body region ( 24 ) and said insulator nose end ( 36 );

said insulator nose region ( 26 ) tapering toward said insulator nose end ( 36 );

said insulator thickness (t i ) along said insulator nose region ( 26 ) being less than said insulator thickness (t i ) along said insulator body region ( 24 ) and said insulator thickness (t i ) decreasing toward said insulator nose end ( 36 );

said insulator seat angle (α i ) being relative to a plane extending perpendicular to said center axis (A) and intersecting said insulator seat ( 28 );

said insulator thickness (t i ) along said insulator seat ( 28 ) decreasing from said insulator body region ( 24 ) to said insulator nose region ( 26 );

said insulator seat angle (α i ) being not greater than 200% of the boundary value;

said first radius (R 1 ) presented by said insulator ( 22 ) being constant from said insulator lower shoulder ( 44 ) to said second transition ( 50 );

said insulator ( 22 ) formed of an electrically insulating material having a dielectric strength of 14 to 30 kV/mm and a relative permittivity of 2 to 12 and a coefficient of thermal expansion (CTE) between 2×10 −6 /° C. and 18×10 −6 /° C.;

said electrically insulating material including alumina;

a center electrode ( 52 ) received in said bore of said insulator ( 22 ) and extending longitudinally along said center axis (A) from an electrode terminal end ( 66 ) past said insulator nose end ( 36 ) to a center electrode firing end ( 100 );

said center electrode ( 52 ) including a head at said electrode terminal end ( 66 ) resting on said electrode seat ( 46 ) of said insulator ( 22 );

a ground electrode ( 64 ) extending from said shell lower end ( 74 ) parallel to said center axis (A) and curving toward said center axis (A) to a ground electrode firing end ( 102 );

said ground electrode ( 64 ) presenting a ground spark surface ( 98 ) facing parallel to and spaced from said center electrode firing end ( 100 );

said center electrode firing end ( 100 ) and said ground spark surface ( 98 ) presenting a spark gap therebetween;

a terminal ( 54 ) received in said bore of said insulator ( 22 ) and extending longitudinally along said center axis (A) from an energy input end ( 68 ) to an energy output end ( 70 ) spaced from electrode terminal end ( 66 );

a seal ( 56 ) contained in said bore and extending continuously between said energy output end ( 70 ) of said terminal ( 54 ) and said electrode terminal end ( 66 ), said seal ( 56 ) being resistive or non-resistive;

a shell ( 58 ) formed of a steel material disposed annularly around said insulator ( 22 ) and extending longitudinally from a shell upper end ( 72 ) along said insulator transition region ( 40 ) and said insulator body region ( 24 ) to a shell lower end ( 74 );

said shell ( 58 ) presenting a shell inner surface ( 76 ) facing said insulator inner surface ( 32 ) and a shell outer surface ( 78 ) facing opposite said shell inner surface ( 76 ), said shell inner surface ( 76 ) and said shell outer surface ( 78 ) each extending from said shell upper end ( 72 ) to said shell lower end ( 74 ), said shell inner surface ( 76 ) and said shell outer surface ( 78 ) presenting a shell thickness (t s ) therebetween;

said shell ( 58 ) including a shell body region ( 80 ) extending along said center axis (A) between said shell upper end ( 72 ) and said shell lower end ( 74 );

said shell ( 58 ) presenting a seventh radius (R 7 ) at said shell body region ( 80 ) and extending from said center axis (A) to said shell inner surface ( 76 );

said shell upper end ( 72 ) being disposed along said insulator upper shoulder ( 42 ) and said shell lower end ( 74 ) being disposed along said insulator nose region ( 26 ) such that said insulator nose end ( 36 ) is disposed outwardly of said shell lower end ( 74 );

said shell ( 58 ) including a rib ( 82 ) extending radially toward said center axis (A) between said shell body region ( 80 ) and said shell lower end ( 74 );

said rib ( 82 ) presenting a shell seat ( 84 ) facing said insulator seat ( 28 ) and extending from said shell body region ( 80 ) radially inwardly toward said center axis (A) at a shell seat angle (α s ) to a rib inner surface ( 86 ), said rib inner surface ( 86 ) being disposed at the innermost point of said shell inner surface ( 76 );

said shell ( 58 ) presenting a third radius (R 3 ) at said rib inner surface ( 86 ) extending from said center axis (A) to said shell inner surface ( 76 ), said third radius (R 3 ) being less than said seventh radius (R 7 );

said shell thickness (t s ) being constant along said insulator body region ( 24 ) and increasing adjacent said insulator seat ( 28 ) of said insulator ( 22 ) to present said rib ( 82 );

said shell seat ( 84 ) facing and parallel to said insulator seat ( 28 );

said shell seat angle (α s ) being relative to a plane extending perpendicular to said center axis (A) and intersecting said shell seat ( 84 );

said shell seat angle (α s ) being equal to said insulator seat angle (α) or within +/−1° of said insulator seat angle (α i );

said rib ( 82 ) including a rib lower surface ( 88 ) facing toward said shell lower end ( 74 ) and extending radially outwardly from said rib inner surface ( 86 ) at an angle toward said shell lower end ( 74 );

said shell thickness (t s ) increasing gradually along said shell seat ( 84 ) to said rib inner surface ( 86 ) and being constant along said rib inner surface ( 86 ) and decreasing along said rib lower surface ( 88 ) toward said shell lower end ( 74 );

said shell outer surface 78 including threads along at least a portion of said shell body region 80 and adjacent said rib 82 ;

a first gasket ( 60 ) compressed between said insulator seat ( 28 ) and said shell seat ( 84 ),

said first gasket ( 60 ) having an gasket inner surface ( 90 ) facing generally toward said insulator ( 22 ) and a gasket outer surface ( 92 ) facing generally toward said shell ( 58 ) and extending from a gasket top surface ( 94 ) to a gasket bottom surface ( 96 );

said gasket top surface ( 94 ) and said gasket bottom surface ( 96 ) having a friction coefficient;

said first gasket ( 60 ) presenting an outer gasket thickness (t g1 ) extending from said gasket top surface ( 94 ) to said gasket bottom surface ( 96 ) at said gasket outer surface ( 92 ) and an inner gasket thickness (t g2 ) extending from said gasket top surface ( 94 ) to said gasket bottom surface ( 96 ) at said gasket inner surface ( 90 );

said outer gasket thickness (t g1 ) being greater than said inner gasket thickness (t g2 );

said inner gasket thickness (t g2 ) being greater than or equal to 70% of said outer gasket thickness (t g1 ); and

a second gasket ( 62 ) compressed between said insulator upper shoulder ( 42 ) and said shell upper end ( 72 ).

16. A method of manufacturing a spark plug ( 20 ), wherein the spark plug ( 20 ) comprises:

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 );

the insulator ( 22 ) including an insulator body region ( 24 ) extending between the insulator upper end ( 34 ) and the insulator nose end ( 36 );

the insulator ( 22 ) presenting a first radius (R 1 ) at the insulator body region ( 24 ) and extending from the center axis (A) to the insulator outer surface ( 30 );

the insulator ( 22 ) including an insulator nose region ( 26 ) between the insulator body region ( 24 ) and the insulator nose end ( 36 );

the insulator ( 22 ) presenting a sixth radius (R 6 ) at the insulator nose region ( 26 ) and extending from the center axis (A) to the insulator outer surface ( 30 ), the sixth radius (R 6 ) being less than the first radius (R 1 );

the insulator ( 22 ) including an insulator seat ( 28 ) disposed between the insulator body region ( 24 ) and the insulator nose region ( 26 ), the insulator seat ( 28 ) extending radially toward the center axis (A) at an insulator seat angle (α i );

the insulator ( 22 ) including a first transition ( 48 ) extending from the insulator body region ( 24 ) to the insulator seat ( 28 ), the first transition ( 48 ) being convex;

the insulator ( 22 ) presenting a fifth radius (R 5 ) at the first transition ( 48 ), the fifth radius (R 5 ) being a spherical radius at the first transition ( 48 );

the insulator ( 22 ) presenting a second transition ( 50 ) extending from the insulator seat ( 28 ) to the insulator nose region ( 26 ), the second transition ( 50 ) being concave;

the insulator ( 22 ) presenting a second radius (R 2 ) extending from the center axis (A) to a point (P) at the intersection of the insulator outer surface ( 30 ) of the insulator seat ( 28 ) and the insulator outer surface ( 30 ) of the insulator nose region ( 26 ) adjacent the second transition ( 50 );

the insulator ( 22 ) presenting a fourth radius (R 4 ) at the second transition ( 50 ), the fourth radius (R 4 ) being a spherical radius at the second transition ( 50 );

the insulator seat angle (α i ) being from 35° to 50°;

the insulator seat angle (α i ) being greater than or equal to a boundary value provided by the equation: 90°−a cos [1−(R 1 −R 2 )÷(R 1 +R 5 )]; and

comprising the steps of:

selecting a value for the insulator seat angle (α i ) between 35° to 50°;

obtaining values for R 1 , R 2 , R 4 , and R 5 ;

determining whether the selected insulator seat angle (α i ) is greater than or equal to a boundary value provided by the equation: 90°−a cos [1−(R 1 −R 2 )÷(R 4 +R 5 )].

17. The method of claim 16 including adjusting at least one of the values of R 1 , R 2 , R 4 , and R 5 if the selected insulator seat angle (α i ) is less than the boundary value.

18. The method of claim 16 including forming the insulator ( 22 ) with the selected insulator seat angle (α i ) and the obtained values of R 1 , R 2 , R 4 , and R 5 if the selected insulator seat angle (α i ) is greater than or equal to the boundary value.

19. The method of claim 16 including increasing the selected value of R 4 while maintaining the insulator seat angle (α i ) greater than or equal to the boundary value.

20. The method of claim 16 wherein the spark plug ( 20 ) includes a shell ( 58 ) presenting a shell inner surface ( 76 ) facing the insulator inner surface ( 32 ) and extending from a shell upper end ( 72 ) to a shell lower end ( 74 );

the shell ( 58 ) includes a shell body region ( 80 ) extending along the center axis (A) between the shell upper end ( 72 ) and the shell lower end ( 74 );

the shell ( 58 ) presents a seventh radius (R 7 ) extending from the center axis (A) to the shell inner surface ( 76 ) along the shell body region ( 80 );

the shell ( 58 ) presents a rib ( 82 ) extending radially toward the center axis (A) and disposed between the shell body region ( 80 ) and the shell lower end ( 74 ), the rib ( 82 ) including a rib inner surface ( 86 );

the shell ( 58 ) presents a third radius (R 3 ) extending from the center axis (A) to the shell inner surface ( 76 ) along the rib inner surface ( 86 ), the third radius (R 3 ) being less than the seventh radius (R 7 );

the shell ( 58 ) includes a shell seat ( 84 ) facing the insulator seat ( 28 ) and extending from the shell body region ( 80 ) radially inwardly toward the center axis (A) at a shell seat angle (α s ) to the rib inner surface ( 86 ); and including the steps of:

obtaining a value for R 3 ;

determining whether the selected value for R 3 allows the selected insulator seat angle (α i ) to be greater than or equal to the boundary value;

adjusting at least one of the values of R 1 , R 2 , R 3 , R 4 , and R 5 if the selected insulator seat angle (α i ) is less than the boundary value; and

compressing a first gasket ( 60 ) between the insulator seat ( 28 ) and the shell seat ( 84 ).

21. The method of claim 20 wherein the first gasket ( 60 ) has a gasket inner surface ( 90 ) facing toward the insulator ( 22 ) and a gasket outer surface ( 92 ) facing toward the shell ( 58 );

the gasket inner surface ( 90 ) and the gasket outer surface ( 92 ) each extend from a gasket top surface ( 94 ) to a gasket bottom surface ( 96 );

the first gasket ( 60 ) presents an outer gasket thickness (t g1 ) extending from the gasket top surface ( 94 ) to the gasket bottom surface ( 96 ) at the gasket outer surface ( 92 ) and an inner gasket thickness (t g2 ) extending from the gasket top surface ( 94 ) to the gasket bottom surface ( 96 ) at the gasket inner surface ( 90 ); and

the outer gasket thickness (t g1 ) is greater than the inner gasket thickness (t g2 ) after the step of compressing the first gasket ( 60 ).