CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of application serial number

61/568,889 filed December 9, 201 1, the entire contents of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] This invention relates generally to spark plugs, and more particularly to insulator geometry of the spark plugs, and methods of manufacturing the same.

2. Related Art

[0003] Spark plugs for use in combustion chambers of automotive or industrial engines include a center electrode and a ground electrode providing a spark gap therebetween. During operation, a spark forms across the spark gap to ignite a combustible mixture of fuel and air. An insulator surrounds and electrically isolates the central electrode, and also provides mechanical support to the central electrode. The insulator is surrounded by a metal shell which is threaded into a cylinder head of the engine. According to one spark plug design, the insulator includes a body region and a tapering nose region which are separated by an insulator seat. A gasket is compressed between insulator seat and shell to maintain the insulator in position. The preload on the gasket should be high enough to seal under all operating conditions. However, the high preload causes tensile stress around the gasket and along the insulator seat.

[0004] The insulator of the spark plug also experiences significant bending stress around the insulator seat when used in a high-output engine. These engines generate "mega-knock" or "super-knock" causing high pressure transient shock waves which create a force transverse to the insulator nose region. SUMMARY OF THE INVENTION

[0005] One aspect of the invention provides a spark plug including an insulator geometry providing reduced tensile stress during installation and increased bending strength during use in a high-output engine. The insulator extends along a center axis and presents an insulator outer surface extending from an insulator upper end to an insulator nose end. An insulator body region extends between the insulator upper end and the insulator nose end. The insulator presents a first radius (Ri) at the insulator body region extending from the center axis to the insulator outer surface. The insulator also includes an insulator nose region between the insulator body region and the insulator nose end. The insulator presents a sixth radius (R ₆ ) at the insulator nose region extending from the center axis to the insulator outer surface. The sixth radius is less than the first radius.

[0006] An insulator seat is disposed between the insulator body region and the insulator nose region. The insulator seat extends radially toward the center at an insulator seat angle. The insulator includes a convex first transition extending from the insulator body region to the insulator seat. The insulator presents a fifth radius (R ₅ ) at the first transition, and the fifth radius is a spherical radius. The insulator also presents a concave second transition extending from the insulator seat to the insulator nose region. The insulator presents a second radius (R ₂ ) extending from the center axis to a point at the intersection of the insulator outer surface of the insulator seat and the insulator outer surface of the insulator nose region adjacent the second transition. The insulator presents a fourth radius (R4) at the second transition, and the fourth radius is a spherical radius. The insulator seat angle is from 35° to 50°, and the insulator seat angle is greater than or equal to a boundary value provided by the equation: 90° - acos[ 1 - (Rj - R ₂ ) ÷ (R4 + R5) ].

[00071 Another aspect of the invention provides a method of forming the spark plug. The method includes selecting a value for the insulator seat angle between 35° to

Ί 50°; obtaining values for Ri , R2, R ₄ , and R5; and determining whether the selected insulator seat angle (<¾) is greater than or equal to a boundary value provided by the equation: 90° - acos[ l - (Ri - R ₂ ) ÷ (R4 + R ₅ ) ]-

[0008] The geometry of the insulator seat provides reduced tensile stress along and around the insulator seat during assembly of the spark plug, particularly reduced tensile stress caused by compressing the gasket between the insulator and shell. The geometry of the insulator seat also provides increased bending strength along and around the insulator seat when the spark plug is used in a high-output engine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

[0010] Figure 1 is a cross-sectional view of a spark plug in accordance with one embodiment of the invention;

[0011] Figure 2 is an enlarged view of a portion of Figure 1 around the insulator seat;

[0012] Figure 2A is an enlarged view of a portion of Figure 2;

[0013] Figure 3 is an enlarged view of a portion of a spark plug according to a second embodiment of the invention;

[0014] Figure 4 is a cross-sectional view of a comparative spark plug; and

[0015] Figure 5 is a graph illustrating the bending strength of the spark plugs of Figures 1 , 3, and 4.

DETAILED DESCRIPTION

[0016] One aspect of the invention provides a spark plug 20 for use in an internal combustion engine, as shown in Figure 1. The spark plug 20 includes an insulator 22 with reduced tensile stress during assembly and increased bending strength when subjected to shock wave forces that occur due to mega-knock or super- knock in a high-output engine. The insulator 22 includes an insulator body region 24 and an insulator nose region 26 with an insulator seat 28 therebetween. The insulator 22 is designed to include an insulator seat angle oi- _t of 35° to 50° and an increased insulator thickness t; in selected areas around the insulator seat 28.

[0017] As shown in Figure 1 , the insulator 22 of the spark plug 20 extends along a center axis A and presents an insulator outer surface 30 and an oppositely facing insulator inner surface 32 each extending longitudinally from an insulator upper end 34 to an insulator nose end 36. The insulator inner surface 32 and the insulator outer surface 30 present an insulator thickness tj therebetween, as shown in Figures 2 and 3. The insulator inner surface 32 extends annularly around the center axis A and presents a bore. The insulator inner surface 32 presents an insulator inner diameter Di surrounding the bore and the insulator outer surface 30 presents an insulator outer diameter D ₂ , as shown in Figures 2 and 3.

[0018] In the embodiment of Figure 1 , the insulator 22 includes an insulator terminal region 38, an insulator transition region 40, the insulator body region 24, and the insulator nose region 26. The insulator terminal region 38 extends from the insulator upper end 34 toward the insulator nose end 36. The insulator transition region 40 is disposed between the insulator terminal region 38 and the insulator body region 24. The insulator thickness tj varies along the insulator transition region 40. Along one portion of the insulator transition region 40, the insulator thickness tj is greater than the insulator thickness tj along the insulator terminal region 38. Along another portion of the insulator transition region 40, the insulator thickness tj is less than the insulator thickness t, along the insulator terminal region 38 and decreases toward the insulator body region 24. An insulator upper shoulder 42 extends from the insulator terminal region 38 to the insulator transition region 40, and the insulator thickness along the insulator upper shoulder 42 increases from the insulator terminal region 38 to the insulator transition region 40.

[0019] The insulator body region 24 is disposed between the insulator transition region 40 and the insulator nose region 26. The insulator 22 presents a first radius Ri along the insulator body region 24 extending from the center axis A to the insulator outer surface 30, as shown in Figures 2 and 3. The insulator thickness tj along the insulator body region 24 is less than the insulator thickness tj along the insulator terminal region 38 and less than the insulator thickness tj along the insulator transition region 40. The ratio of the insulator inner diameter Di to the insulator outer diameter Di along the insulator body region (24) adjacent the insulator seat 28 is preferably from 0.12 to 0.45, and more preferably from 0.18 to 0.38. An insulator lower shoulder 44 extends from the insulator transition region 40 to the insulator body region 24, and the insulator thickness tj along the insulator lower shoulder 44 decreases from the insulator transition region 40 to the insulator body region 24.

[0020] The insulator inner surface 32 along the insulator body region 24 presents an electrode seat 46, and the insulator thickness t; along a portion of the insulator body region 24 increases toward the center axis A and toward the insulator nose end 36 to present the electrode seat 46. In the embodiment of Figure 1 , the insulator thickness tj along the insulator body region 24 is generally constant but increases slightly at the electrode seat 46.

[0021] The insulator nose region 26 is disposed between the insulator body region 24 and the insulator nose end 36. The insulator 22 presents a sixth radius Rf, along the insulator nose region 26 extending from the center axis A to the insulator outer surface 30, as shown in Figures 2 and 3. The sixth radius presented by the insulator nose region 26 is less than the first radius Ri presented by the insulator body region 24. In the embodiment of Figure 1 , the sixth radius R _¾ of the insulator nose region 26 tapers toward the insulator nose end 36. The insulator thickness ti along the insulator nose region 26 is less than the insulator thickness tj along the insulator body region 24, and the insulator thickness t, decreases toward the insulator nose end 36.

[0022] As shown in Figures 1 -3, the insulator seat 28 is disposed between the insulator body region 24 and the insulator nose region 26. The insulator seat 28 extends at an insulator seat angle O!i radially inwardly toward the center axis A and downwardly toward the insulator nose end 36. The insulator seat angle G¾ is measured relative to a plane extending perpendicular to the center axis A and intersecting the insulator seat 28, as shown in Figures 2 and 3. The insulator thickness tj along the insulator seat 28 decreases from the insulator body region 24 to the insulator nose region 26.

[0023] The insulator 22 also includes a first transition 48 extending continuously from the insulator body region 24 to the insulator seat 28, and the first transition 48 is convex. The first radius Ri presented by the insulator body region 24 is typically constant from the insulator lower shoulder 44 to the first transition 48. The insulator 22 also presents a fifth radius Rs at the first transition 48, which is a spherical radius at point located along the first transition 48, as shown in Figures 2 and 3. The spherical radius at a particular point is obtained from a sphere having a radius at that particular point. The spherical radius is the radius of the sphere in three dimensions.

[0024] A second transition 50 extends continuously from the insulator seat 28 to the insulator nose region 26, and the second transition 50 is concave. The insulator 22 presents 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, as shown in Figures 2 and 3. A fourth radius R4 is also located at the second transition 50, and the fourth radius R4 is a spherical radius at a point located along the second transition 50.

[0025] The insulator 22 includes an increased insulator seat angle ο¾, compared to spark plug insulators of the prior art. The insulator seat angle a-, of the inventive spark plug is from 35° to 50°, whereas seat angles of the prior art are 30° or less. In one preferred embodiment, the insulator seat angle Q!j is 45°, or within +/- 2° of 45°.

[0026] The insulator 22 also includes an increased insulator thickness tj around the insulator seat 28. The value of the fourth radius R4 is maximized, while maintaining an acceptable value for the second radius R2. The increased insulator seat angle a; and fourth radius R4 provides reduced tensile stress during assembly and increased bending strength when subjected to shock wave forces due to mega-knock or super-knock which occur during use of the spark plug 20 in a combustion engine.

[0027] The insulator seat angle G¾ is also greater than or equal to a boundary value provided by the equation: 90° - acos[ 1- (Ri - R ₂ ) ÷ (R4 + R5) ]. When manufacturing the insulator 22, the method typically includes selecting a desired insulator seat angle o¾ from 35° to 50°, and then using the equation to determine values for Rj, R ₂ , R ₃ , Rt, and R5 that provide a boundary value less than or equal to the desired seat angle. The method typically includes adjusting at least one of the values of R _ti R ₂ , R _3i R^ and R5 to obtain the desired insulator geometry. For example, the value of R4 is typically increased to a maximum value that provides the desired seat angle while maintaining an acceptable value of R ₂ . The insulator seat angle _t is preferably not greater than 300%, more preferably not greater than 200%, and yet more preferably not more than 150% of the boundary value obtained by the equation.

[0028] The insulator 22 is formed of an electrically insulator 22 material, and preferably a material having a dielectric strength of 14 to 30 kV/mm, a coefficient of thermal expansion (CTE) between 2 x 10 ^"6 /°C and 18 x 10 ^"b /°C, and a relative permittivity of 2 to 12. In one embodiment, the electrically insulating material includes alumina. A coating (not shown) can optionally be applied to the insulator outer surface 30. The coating typically includes nickel or copper.

[0029]

[0030] The spark plug 20 of Figure 1 also includes a center electrode 52, a terminal 54, a seal 56, a shell 58, a pair of gaskets 60, 62, and a ground electrode 64. The center electrode 52 is received in the bore of the insulator 22 and extends longitudinally along the center axis A from an electrode terminal end 66 past the insulator nose end 36 to a center electrode firing end 100. The center electrode 52 includes a head at the electrode terminal end 66 resting on the electrode seat 46 of the insulator 22. A terminal 54 is received in the bore of the insulator 22 and extends longitudinally along the center axis A from an energy input end 68 to an energy output end 70 spaced from electrode terminal end 66. A seal 56 is also contained in the bore of the insulator 22 and extends continuously between the energy output end 70 of the terminal 54 and the electrode terminal end 66. The seal 56 can be resistive or non-resistive.

[0031] The shell 58 is formed of a metal material, preferably steel, and is disposed annularly around the insulator 22. The shell 58 extends longitudinally from a shell upper end 72 along the insulator transition region 40 and the insulator body region 24 to a shell lower end 74. The shell 58 presents a shell inner surface 76 facing the insulator outer surface 30 and a shell outer surface 78 facing opposite the shell inner surface 76. The shell inner surface 76 and the shell outer surface 78 each extend from the shell upper end 72 to the shell lower end 74, and the shell inner surface 76 and the shell outer surface 78 present a shell thickness t _s therebetween. As shown in Figure 1 , the shell 58 has a shell outer diameter D ₃ , which is typically 12 mm, but can alternatively be from 8 mm to 18 mm. [0032] 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 along the shell body region 80, as shown in Figures 2 and 3. The seventh radius R ₇ extends from the center axis A to the shell inner surface 76. The top of the shell 58 is bent such that the shell upper end 72 rests on the insulator upper shoulder 42. The shell lower end 74 is disposed along the insulator nose region 26 such that the insulator nose end 36 is disposed outwardly of the shell lower end 74.

[0033] The shell 58 includes a rib 82 adjacent the insulator seat 28, as shown in Figures 1-3. The rib 82 extends radially toward the center axis A and is disposed between the shell body region 80 and the shell lower end 74. The shell thickness t _s is constant along the insulator body region 24 and increases adjacent the insulator seat 28 to present the rib 82. The rib 82 includes a shell seat 84 preferably facing parallel to the insulator seat 28 and extending radially inwardly toward the center axis A and downwardly toward the shell lower end 74. The shell seat 84 extends at a shell seat angle which is relative to a plane extending perpendicular to the center axis A and intersecting the shell seat 84, as shown in Figures 2 and 3. The shell seat angle <x _s is preferably equal to the insulator seat angle oi\ or within +/- of the insulator seat angle α¾.

[0034] The shell seat 84 extends from the shell body region 80 to a rib inner surface 86. The shell thickness t _s increases gradually along the shell seat 84 to the rib inner surface 86 and is constant along the rib inner surface 86. In the embodiment of Figure 1 , the rib inner surface 86 is disposed at the innermost point of the shell inner surface 76. The shell 58 presents a third radius R ₃ at the rib inner surface 86 extending from the center axis A to the shell inner surface 76, as shown in Figures 2 and 3. The third radius R ₃ is less than the seventh radius R of the shell body region 80. The rib 82 also includes a rib lower surface 88 facing toward the shell lower end 74. The rib lower surface 88 extends radially outwardly from the rib inner surface 86 at an angle. The shell thickness t _s decreases along the rib lower surface 88 toward the shell lower end 74. The shell outer surface 78 includes threads along at least a portion of the shell body region 80 and adjacent the rib 82, so that the shell 58 can be threaded into a cylinder head.

[0035] The spark plug 20 of Figure 1 includes a first gasket 60 compressed between the insulator seat 28 and the shell seat 84, and can include a second gasket 62 compressed between the insulator upper shoulder 42 and the shell upper end 72. The gaskets 60, 62 are formed of a metal material, such as steel or copper.

[0036] The first gasket 60 has a gasket inner surface 90 facing generally toward the insulator 22 and a gasket outer surface 92 facing generally toward the shell 58. The gasket inner surface 90 and the gasket outer surface 92 both extend from a gasket top surface 94 to a gasket bottom surface 96. A lubricant (not shown) may be applied to the gasket during assembly of the spark plug 20. The gasket top surface 94 and gasket bottom surface 96 present a friction coefficient, which depends on the material used to form the gasket and whether lubricant is applied to the gasket. Reducing friction at this gasket interface, for example by adding a lubricant or by coating the gasket in a low-friction material, leads to a reduction in the tensile stress created by the assembly process; but only for lower seat angles. The friction-reducing coating is preferably located between the gasket and the shell. As the seat angle increases a point is reached where the gasket begins to slide on the shell and the tensile stress increases sharply due to deformation of the insulator seat 28. If the friction coefficient is less than or equal to 0.15, then the insulator seat angle <¾ is preferably from 35° to 45 ". If the friction coefficient is greater than 0.15, then the insulator seat angle a¾ can be up to 50°.

[0037] The first gasket 60 presents an outer gasket thickness t _g i extending from the gasket top surface 94 to the gasket bottom surface 96 at the gasket outer surface 92. The first gasket 60 also presents 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. As shown in Figure 2A, the outer gasket thickness t _g i is greater than the inner gasket thickness t _g2 . The inner gasket thickness t _g2 is preferably greater than or equal to 70% of the outer gasket thickness t _g i .

[0038] The ground electrode 64 is attached to the shell 58, as shown in Figure

1 , and extends from the shell lower end 74 to a ground electrode firing end 102. The ground electrode 64 extends parallel to the center axis A and then curves toward the center axis A. The ground electrode 64 presents a ground spark surface 98 facing parallel to and spaced from the center electrode firing end 100 such that the center electrode firing end 100 and the ground spark surface 98 present a spark gap therebetween.

[0039] Another aspect of the invention provides a method of manufacturing the spark plug 20 including an insulator 22 with the insulator seat angle G¾ being from 35° to 50° and the insulator seat angle <¾ being greater than or equal to a boundary value provided by the equation: 90° - acos[ 1 - (Ri - R ₂ ) ÷ (R4 + R ₅ ) ].

[0040] The method first comprises selecting a value for the insulator seat angle α¾ (α¾) between 35° to 50°. The method next includes obtaining values for Ri, R ₂ , R _4, and R5. The values can be calculated using various different methods. The value of R ₄ is preferably maximized while maintaining an acceptable value of R ₂ . Once the values of Ri _, R ₂ , R _4, and R$ are obtained, the method includes determining whether the selected insulator seat angle o¾ is greater than or equal to the boundary value provided by the equation. If the selected insulator seat angle a, is greater than or equal to the boundary value, then the method can include forming the insulator 22 with the selected insulator seat angle <¾ and obtained values of Ri, R ₂ , R4. and R5. [0041] If the selected insulator seat angle a, is less than the boundary value, then the method includes adjusting at least one of the values of R] R ₂ , R4, and R5 so that the boundary value is greater than or equal to the selected insulator seat angle a¾.

[0042] Alternatively, even if the boundary value is greater than or equal to the selected insulator seat angle a¾, the method can include adjusting at least one of the values of Ri _, R ₂ , R4, and R5 so that the boundary value is closer to the selected insulator seat angle G¾. For example, the method could include increasing the selected value of R» and decreasing R ₂ while maintaining the insulator seat angle <¾ greater than or equal to the boundary value. The selected insulator seat angle G¾ is preferably not greater than 300% of the boundary value, more preferably not greater than 200% of the boundary value, and yet more preferably not greater than 150% of the boundary value.

[0043] The method also includes obtaining a value for the third radius R ₃ , which is at the rib inner surface 86 of the shell 58 and extends from the center axis A to the shell inner surface 76. The method next includes determining whether the selected value for R ₃ allows the selected insulator seat angle α¾ to be greater than or equal to the boundary value. If the selected insulator seat angle <¾ is less than the boundary value, then the method includes adjusting at least one of the values of Ri, R ₂ , R ₃ , R>, and R5.

[0044] Once the geometry of the insulator 22 and the shell 58 is determined, the method next includes compressing the first gasket 60 between the insulator seat 28 and the shell seat 84. The outer gasket thickness t _g i is preferably greater than the inner gasket thickness t _g2 after the step of compressing the first gasket 60.

[0045] EXPERIMENT

[0046] Spark plugs of this invention are calculated by Finite Element Analysis

(FEA) to have a lower tensile stress due to plug assembly which leads directly to reduced stress in bending. The geometry changes described here also lead to an additional reduction in stress due to bending loads, due to better distribution of load. An experiment was conducted to compare the bending strength during use of the inventive spark plug 20 having a shell outer diameter D ₃ of 12 mm and an insulator seat angle <¾ of 45° to a comparative spark plug having a shell outer diameter of 12 mm and insulator seat angle of 30°. The insulator 22 of the first inventive embodiment, shown in Figures 1 and 2; the insulator 22 of the second inventive embodiment, shown in Figure 3; and the insulator of the comparative spark plug, shown in Figure 4, were each tested. Table 1 provides Ki - R5 for each of the spark plugs. Table 1 also provides the boundary value for each of the spark plugs, and the insulator seat angle as a percentage of the boundary value.

[0047] Table 1

[0048J The FEA results indicate the average tensile stress during assembly of the inventive spark plug 20 according to the first embodiment and the second embodiment is less than the average tensile stress during assembly of the comparative spark plug and indicate an improvement in bending strength. Table 2 and Figure 5 provides the bending strength test results, and illustrate the average bending strength of the inventive spark plug 20 according to the first embodiment and the second embodiment is greater than the average bending strength of the comparative spark plug.

[0049] Table 2

[0050] Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims. In addition, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting.