Spark plug for internal combustion engine and related manufacturing method

Abstract

A spark plug for an internal combustion engine and related manufacturing method are disclosed having a metal shell, a porcelain insulator, a center electrode and a ground electrode. The center electrode includes a substantially columnar shaped base material body, and a substantially square shaped rod-like Ir alloy tip bonded to a leading end portion of the base material body. The leading end portion has a diameter D2 smaller than a diameter D1 of the base material body. The Ir alloy tip has a square shape with a long diagonal line supposed to form a diameter A of a circumscribed circle CA of the Ir alloy tip whose inscribed circle CB is coaxial with the circumscribed circle CA and has a diameter B, with four diameters A, B, D1 and D2 lay in the relationship expressed as D1>A>D2>B.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a half cross-sectional view showing an overall structure of a spark plug of a first embodiment according to the present invention.

FIG. 2 is an enlarged cross-sectional view showing a spark discharge portion and its vicinity of the spark plug shown in FIG. 1.

FIG. 3 is a plan view of the spark plug of the first embodiment, shown in FIG. 1, as viewed from a distal end of a center electrode thereof.

FIG. 4 is a perspective view showing the center electrode of the spark plug of the first embodiment shown in FIG. 1.

FIG. 5 is a side view showing the center electrode of the spark plug of the first embodiment shown in FIG. 1.

FIG. 6 is a perspective view showing a base material body of the center electrode forming part of the spark plug of the first embodiment shown in FIG. 1.

FIG. 7 is an enlarged illustrative view showing a joint portion between the center electrode base material body and an Ir alloy tip forming the spark plug of the first embodiment shown in FIG. 1.

FIG. 8 is an enlarged perspective view showing the center electrode under a condition supported with a porcelain insulator.

FIG. 9 is an enlarged illustrative view showing a positional relationship between the porcelain insulator and the center electrode.

FIG. 10 is a plan view of a center electrode of a modified form of the spark plug of the first embodiment shown in FIG. 1 with the center electrode formed in a substantially rectangular in cross section.

FIG. 11 is a plan view of a center electrode of another modified form of the spark plug of the first embodiment shown in FIG. 1 with the center electrode formed in a substantially rhombic shape in cross section.

FIG. 12 is an illustrative view showing the center electrode, as viewed from a distal end thereof, in which the center electrode base material body and the Ir alloy tip are bonded to each other by a laser welding operation using 16-point equiangular laser beams being irradiated to the joint portion.

FIG. 13 is an illustrative view of the center electrode, as viewed from a side area, showing a structure with the center electrode base material body and the Ir alloy tip bonded to each other by the laser welding operation shown in FIG. 12.

FIG. 14 is a perspective view of the center electrode, forming part of the spark plug of the first embodiment shown in FIG. 1, with the center electrode base material body and the Ir alloy tip bonded to each other by laser welding.

FIG. 15 is a half cross-sectional view showing an overall structure of a modified form of the spark plug of the first embodiment shown in FIG. 1 with a ground electrode having a leading end placed in opposition to a distal end of the center electrode.

FIG. 16 is an illustrative view showing the positional relationship between an Ir alloy tip of a center electrode and a noble metal tip of a ground electrode forming a spark plug of a second embodiment according to the present invention.

FIG. 17 is an illustrative view showing the Ir alloy tip of a center electrode with a substantially quadrate shape in cross section.

FIG. 18 is an illustrative view showing the Ir alloy tip of a center electrode with a substantially rectangular shape in cross section.

FIG. 19 is an illustrative view showing one positional relationship between the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode, forming the spark plug of the second embodiment shown in FIG. 16, under a condition where the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode are displaced from each other within an allowable limit.

FIG. 20 is an illustrative view showing another positional relationship between the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode, forming the spark plug of the second embodiment shown in FIG. 16, under a condition where the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode are displaced from each other beyond the allowable limit.

FIG. 21 is an illustrative view of a spark plug of a third embodiment according to the present invention showing a positional relationship between a corner portion of an Ir alloy tip of the center electrode and a center electrode base material body.

FIG. 22A is a plan view of a center electrode, as viewed from a distal end of an Ir alloy tip bonded to a base material leading end portion with a diameter D2 of 2.1 mm, on which a first evaluation test was conducted.

FIG. 22B is a cross-sectional view of the center electrode taken on line K-K of FIG. 22A.

FIG. 22C is a cross-sectional view of the center electrode taken on line N-N of FIG. 22A.

FIG. 23A is a plan view of a center electrode, as viewed from a distal end of an Ir alloy tip bonded to a base material leading end portion with a diameter D2 of 2.4 mm, on which a second evaluation test was conducted.

FIG. 23B is a cross-sectional view of the center electrode taken on line K1-K1 of FIG. 23A.

FIG. 23C is a cross-sectional view of the center electrode taken on line N1-N1 of FIG. 23A.

FIG. 24A is a plan view of a center electrode, as viewed from a distal end of an Ir alloy tip bonded to a base material leading end portion with a diameter D2 of 2.9 mm, on which a third evaluation test was conducted.

FIG. 24B is a cross-sectional view of the center electrode taken on line K2-K2 of FIG. 24A.

FIG. 24C is a cross-sectional view of the center electrode taken on line N2-N2 of FIG. 24A.

FIG. 25 is a graph showing the relationship between a diameter D2 of a base material leading end portion and an Ir ratio (wt %) of the center electrode.

FIG. 26 is a graph showing the relationship between a diameter D2 of a base material leading end portion and a fusing depth (mm) of the center electrode.

FIG. 27 is a graph showing the relationship between an Ir ratio (%) of a fused layer and a thermal expansion coefficient of the fused layer.

FIG. 28 is a graph showing the relationship between a flaking incidence rate (%) of an Ir alloy tip from a base material body of a center electrode and a diameter D2 of a base material leading end portion.

FIG. 29 is a perspective view showing a spark plug of the related art.

FIG. 30 is a side view showing the spark plug of the related art shown in FIG. 29.

FIG. 31 is a plan view of the spark plug of the related art, shown in FIG. 29, as viewed from a distal end of a center electrode.

FIG. 32 is an enlarged illustrative view showing the center electrode encountered with cracking occurred in a fused layer.

Claims

1. A spark plug for an internal combustion engine, comprising: a metal shell having an outer periphery formed with a mounting thread;a porcelain insulator fixedly secured to the metal shell on a central axis thereof;a center electrode retained within the porcelain insulator along a central axis thereof; anda ground electrode extending from the metal shell and having a leading end placed in face-to-face relationship with the center electrode to provide a spark discharge gap;the center electrode including a center electrode base material body of a substantially columnar shape having a base material leading end portion, and a substantially square shaped rod-like Ir alloy tip bonded to the base material leading end portion of the center electrode base material body; andthe base material leading end portion of the center electrode base material body having a columnar shape with a diameter D2 smaller than a diameter D1 of the center electrode base material body;wherein the square shaped rod-like Ir alloy tip has a square shape in cross section on a plane perpendicular to an axis of the Ir alloy tip with diagonal lines among which a long diagonal line is supposed to be a diameter A of a circumscribed circle CA of the Ir alloy tip and an inscribed circle CB is supposed to internally touch at least two sides of the square shape coaxially with the circumscribed circle CA of the Ir alloy tip and have a diameter B, with four diameters A, B, D1 and D2 lay in the relationship expressed as D1>A>D2>B.

2. The spark plug for the internal combustion engine according to claim 1, wherein: the substantially square shape of the Ir alloy tip in cross section has corners formed in curved portions, respectively, and straight portions interconnecting the curved portions to each other; andwherein each straight portion has a length L and the Ir alloy tip has a width W under the relationship expressed as 0.8×W≦L<W.

3. The spark plug for the internal combustion engine according to claim 2, wherein: the ground electrode includes a noble metal ship having an opposing surface, facing a sidewall of the Ir alloy tip to provide the spark discharge gap, which has a width M in relation to the length L of the straight portion under the relationship expressed as M<L.

4. The spark plug for the internal combustion engine according to claim 3, wherein: the opposing surface of the noble metal tip of the ground electrode is placed in face-to-face relationship with the sidewall of the Ir alloy tip such that the opposing surface of the noble metal tip does not protrude from the sidewall of the Ir alloy tip.

5. The spark plug for the internal combustion engine according to claim 2, wherein: the opposing surface of the noble metal tip of the ground electrode is placed in face-to-face relationship with a sidewall of the Ir alloy tip so as not to protrude from the sidewall of the Ir alloy tip in a widthwise direction thereof; andeach of the curved portions of the Ir alloy tip has a depth less than 0.3 mm along a direction in which each curved portion faces the noble metal tip of the ground electrode.

6. The spark plug for the internal combustion engine according to claim 1, wherein: the substantially square shape of the Ir alloy tip in cross section thereof has one of a substantially quadrate shape and a rectangular shape.

7. The spark plug for the internal combustion engine according to claim 1, wherein: the substantially square shape of the Ir alloy tip in cross section has a profile including corners formed in curved portions, respectively, and straight portions interconnecting the curved portions to each other; andwherein a square shape is supposed to be defined with four extended lines of the straight portions with a length E of a long diagonal line among diagonal lines of the substantially square shape of the Ir alloy tip under which a virtual circumscribed circle CC is supposed to have a diameter C, corresponding to the long diagonal line among diagonal lines of the substantially square shape of the Ir alloy tip, which has the relationship expressed as D1>C>D2≧E.

8. The spark plug for the internal combustion engine according to claim 1, wherein: the diameter D1 of the center electrode base material body and the diameter D2 of the base material leading end portion of the center electrode base material body lies in the relationship expressed as 0.5×D1≦D2≦0.95×D1.

9. The spark plug for the internal combustion engine according to claim 1, wherein: the diameter D1 of the center electrode base material body and the diameter D2 of the base material leading end portion of the center electrode base material body lies in the relationship expressed as 0.7×D1≦D2≦0.9×D1.

10. The spark plug for the internal combustion engine according to claim 1, wherein: the diameter D1 of the base material body of the center electrode base material body is greater than 2.5 mm.

11. The spark plug for the internal combustion engine according to claim 1, wherein: the Ir alloy tip is bonded to the center electrode base material body at a joint portion in which a fused layer is formed with an unfused portion being formed inside the fused layer in a width F in a correlation with the diameter A of the circumscribed circle CA of the Ir alloy tip under the relationship expressed as F≦0.2×A.

12. The spark plug for the internal combustion engine according to claim 1, wherein: the Ir alloy tip is bonded to the center electrode base material body at a joint portion formed with a fused layer having an entire circumference spaced from a bore end of a through-bore of the porcelain insulator by a distance greater than 0.1 mm.

13. The spark plug for the internal combustion engine according to claim 12, wherein: the center electrode has an annular space defined between an insulator leading end portion and an outer periphery of the joint portion between the Ir alloy tip and the center electrode base material body; andthe joint portion has an inward boundary edge exposed to the annular space of the center electrode.

14. The spark plug for the internal combustion engine according to claim 1, wherein: the ground electrode comprises a ground electrode base body having a trailing end connected to the metal shell and a leading end to which a noble metal tip is bonded in face-to-face relationship with a sidewall of the center electrode to define the spark discharge gap.

15. The spark plug for the internal combustion engine according to claim 1, wherein: the porcelain insulator has an insulator leading end portion axially extending through the metal shell and ended at a position axially inward of a leading end of the metal shell; andthe base material leading end portion of the base material body is spaced from the insulator leading end portion of the porcelain insulator through an annular space;wherein the joint portion between the Ir alloy tip and the base material leading end portion is exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator.

16. The spark plug for the internal combustion engine according to claim 15, wherein: the Ir alloy tip and the base material leading end portion are bonded to each other at the joint portion through a fused layer having an inward boundary edge exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator.

17. The spark plug for the internal combustion engine according to claim 16, wherein: the fused layer of the joint portion has an outer circumferential periphery spaced from the leading end portion of the porcelain insulator through the annular space by a given distance.

18. The spark plug for the internal combustion engine according to claim 1, wherein: the Ir alloy tip of the center electrode has side surfaces, each having a corner and a straight portion L, and a width W that lies in the relationship expressed as 0.8×W≦L<W.

19. The spark plug for the internal combustion engine according to claim 18, wherein: the curved portion of the Ir alloy tip of the center electrode has a depth falling in a value less than 0.3 mm.

20. A method of manufacturing a spark plug for an internal combustion engine, comprising the steps of: preparing a metal shell having an outer periphery formed with a mounting thread;preparing a porcelain insulator;preparing a center electrode having a substantially columnar shaped center electrode base material body having a base material leading end portion and a substantially square shaped rod-like Ir alloy tip bonded to the base material leading end portion of the center electrode base material body by laser welding;inserting the center electrode through the porcelain insulator in a fixed place; andinserting the porcelain insulator, with which the center electrode is supported, in the metal shell;wherein the base material leading end portion of the center electrode base material body has a columnar shape with a diameter D2 smaller than a diameter D1 of the center electrode base material body; andwherein the square shaped rod-like Ir alloy tip has a square shape in cross section on a plane perpendicular to an axis of the Ir alloy tip with diagonal lines among which a long diagonal line is supposed to be a diameter A of a circumscribed circle CA of the Ir alloy tip and an inscribed circle CB is supposed to internally touch at least two sides of the square shape coaxially with the circumscribed circle CA of the Ir alloy tip and have a diameter B, with four diameters A, B, D1 and D2 lay in the relationship expressed as D1>A>D2>B.

21. The method of manufacturing the spark plug for the internal combustion engine according to claim 20, wherein: the porcelain insulator has an insulator leading end portion axially extending through the metal shell and ended at a position axially inward of a leading end of the metal shell; andthe base material leading end portion of the base material body is spaced from the insulator leading end portion of the porcelain insulator through an annular space;wherein the joint portion between the Ir alloy tip and the base material leading end portion is exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator.

22. The method of manufacturing the spark plug for the internal combustion engine according to claim 20, wherein: the Ir alloy tip and the base material leading end portion are bonded to each other at the joint portion through a fused layer having an inward boundary edge exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator.

23. The method of manufacturing the spark plug for the internal combustion engine according to claim 22, wherein: the fused layer of the joint portion has an outer circumferential periphery spaced from the leading end portion of the porcelain insulator through the annular space by a given distance.