Spark plug

Abstract

To provide a spark plug capable of reducing falling of a chip from a base material.SOLUTION: A spark plug comprises: a first electrode having a base material and a chip connected to the base material via a molten part; and a second electrode that is opposite to a discharge surface of the chip. In a cross section that is vertical to the discharge surface including a center of gravity of the discharge surface, when a region at the middle that is obtained by equally divided into three pieces so as to be vertical to a short side of a rectangle having a first long side that is parallel to the discharge surface including a point most separate from the discharge surface existing in a first boundary surface between the chip and the molten part, a second long side that is parallel to the discharge surface including a point that is nearest to the discharge surface existing in a secondary boundary surface between the molten part and the base material, and two short sides positioned at the middle of the molten part so that the length of the two sides becomes the maximum is divided into six ranges so as to be vertical to the long side, ranges where a percentage content of an element of a main component contained in the chip is different by 10% or more are adjacent.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a spark plug in which a chip is connected to a base material via a molten part.

Background Art

[0002] Prior art related to a spark plug in which a chip is connected to a base material via a molten part is disclosed, for example, in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, when a temperature difference occurs at both ends of the molten part, thermal stress is generated. Due to the thermal stress, cracks occur in the interface or the molten part of the molten part, and if the crack progresses along the interface, the chip may fall off from the base material. There is a need for a technique to reduce the chip dropout even if the crack progresses.

[0005] The present invention has been made to meet this requirement, and an object thereof is to provide a spark plug capable of reducing the dropout of the chip from the base material.

Means for Solving the Problems

[0006] To achieve this object, the spark plug of the present invention includes a first electrode including a base material and a chip connected to the base material via a molten portion, and a second electrode facing the discharge surface of the chip. In a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface, the first interface between the chip and the molten portion is cut off by two straight lines perpendicular to the discharge surface passing through both ends of the discharge surface. A first long side parallel to the discharge surface including the point farthest from the discharge surface in the portion, the second interface between the molten portion and the base material is cut off by two straight lines perpendicular to the discharge surface passing through both ends of the discharge surface. A second long side parallel to the discharge surface including the point closest to the discharge surface in the portion, and a rectangle having two short sides located in the molten portion so that the lengths of the two long sides are maximized. When the middle region vertically trisected with respect to the short side is vertically divided into six ranges with respect to the long side, ranges in which the content rates of the elements of the main components contained in the chip differ by 10% or more are adjacent to each other.

Advantages of the Invention

[0007] According to the first aspect, in a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface of the chip, when the middle region vertically trisected with respect to the short side of the rectangle contained in the molten portion is vertically divided into six equal parts with respect to the discharge surface, ranges in which the content rates of the elements of the main components contained in the chip differ by 10% or more are adjacent to each other. Since the linear expansion coefficient of the molten portion is different at the boundary of the adjacent portions of the ranges, cracks are likely to occur in the adjacent portions of the ranges. The generated cracks are likely to progress in a direction intersecting the discharge surface. When the cracks progress and the molten portion is divided, the linear expansion amount of the molten portion becomes smaller and the thermal stress becomes smaller than when the molten portion is not divided, so that it becomes difficult for the cracks to progress along the interface of the molten portion. In addition, the stress of the molten portion is released by the generated cracks, and it becomes difficult for the cracks to progress. Therefore, the dropout of the chip can be reduced.

[0008] According to a second aspect, in the first aspect, in a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface of the chip, among the perpendiculars dropped from the point where the second interface between the molten part and the base material intersects the second surface to the straight line including the discharge surface, the length of the longest perpendicular is defined as Y1, the intersection of the perpendicular of length Y1 and the second interface is defined as the first point, and among the second interfaces, the intersection of the straight line passing through the center of gravity of the discharge surface and perpendicular to the discharge surface and the second interface, and the second point farthest from the first point, when the length of the longest perpendicular among the perpendiculars dropped from the portion between them to the straight line including the discharge surface is defined as Y2, Y1 ≥ Y2. As a result, the relaxation effect of the thermal stress on the molten part near the second surface is greater than that on the molten part far from the second surface. Therefore, breakage of the portion of the molten part near the second surface can be reduced. Since each of the divided molten parts can join the chip to the base material, the joining strength of the chip by the molten part can be prevented from decreasing.

[0009] According to a third aspect, in the first or second aspect, in a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface of the chip, in the portion of the molten part located between two straight lines perpendicular to the discharge surface and passing through both ends of the discharge surface respectively, there is a boundary where ranges with a difference of 10% or more in the content of the main component element contained in the chip are adjacent. When a crack generated near the boundary progresses in a direction intersecting the discharge surface, the molten part is divided into smaller parts compared to the fourth aspect described later, so the linear expansion amount of the molten part becomes smaller and the thermal stress becomes smaller. Since it becomes difficult for the crack to further progress along the interface of the molten part, chip dropout can be further reduced.

[0010] According to a fourth aspect, in the first or second aspect, in a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface of the chip, when the point farthest from the first point among the second interfaces is defined as the second point, in the portion of the molten part including the second point, in the portion partitioned by a straight line perpendicular to the discharge surface and passing through the end of the discharge surface closer to the second point, there is a boundary where ranges with a difference of 10% or more in the content of the main component element contained in the chip are adjacent. Even if a crack generated near the boundary progresses in a direction intersecting the discharge surface, since there is no chip at the tip where the crack progresses, it becomes difficult for the crack to enter the chip.

[0011] According to the fifth aspect, in the first or second aspect, the discharge surface is square. Since the square discharge surface has an extension only at the corner portions as compared with a circular discharge surface having the same area, it is advantageous against the consumption due to the discharge between the discharge surface and the second electrode by the amount corresponding to the corners.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Modes for Carrying Out the Invention

[0013] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a partial cross-sectional view of one side of a spark plug 10 with respect to an axis O in one embodiment. In FIG. 1, the lower side of the paper surface is the tip side of the spark plug 10, and the upper side of the paper surface is the rear end side of the spark plug 10. As shown in FIG. 1, the spark plug 10 includes a center electrode 20 and a ground electrode 40.

[0014] The insulator 11 is a substantially cylindrical member having a shaft hole 12 formed along the axis O, and is formed of ceramics such as alumina that is excellent in mechanical properties and insulation properties at high temperatures. The insulator 11 is provided with a rear-end facing surface 13, which is an annular surface facing the rear-end side, on the tip side of the inner peripheral surface formed by the shaft hole 12. The rear-end facing surface 13 has a reduced diameter toward the tip side.

[0015] The center electrode 20 is a rod-shaped member locked to the rear-end facing surface 13. The tip of the center electrode 20 protrudes from the tip of the insulator 11 toward the tip side. The center electrode 20 has a core material mainly composed of copper covered with a bottomed cylindrical base material 21. The base material 21 has a chemical composition containing 50 wt% or more of Ni. It is possible to omit the core material.

[0016] A chip 23 is connected to the tip of the base material 21 via a molten part 22. The chip 23 has a chemical composition containing 50 wt% or more of one or more of noble metals such as Pt, Rh, Ir, and Ru. The discharge surface 24 of the chip 23 faces the ground electrode 40. In this embodiment, the discharge surface 24 is circular. The center electrode 20 is electrically connected to a terminal fitting 25 within the shaft hole 12.

[0017] The terminal fitting 25 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is formed of a conductive metal material (for example, low-carbon steel, etc.). The terminal fitting 25 is fixed on the rear-end side of the insulator 11 with the tip side inserted into the shaft hole 12.

[0018] A main body fitting 30 is fixed to the outer periphery on the tip side of the insulator 11. The main body fitting 30 is a substantially cylindrical member formed of a conductive metal material (for example, low-carbon steel, etc.). The main body fitting 30 includes a seat portion 31 that projects outward in the radial direction in a flange shape, and a screw portion 32 provided on the outer peripheral surface on the tip side of the seat portion 31. The main body fitting 30 is fixed by fastening the screw portion 32 to a screw hole (not shown) of an engine (cylinder head). A ground electrode 40 is connected to the tip portion of the main body fitting 30.

[0019] The grounding electrode 40 is a rod-shaped member formed of a conductive metal material. The grounding electrode 40 includes a rectangular columnar curved base material 41 whose end is joined to the main fitting 30 and extends toward the axis O, and a chip 44 disposed on the upper surface 42 of the base material 41 facing the center electrode 20 and connected via a molten portion 43. The base material 41 has a chemical composition containing 50 wt% or more of Ni. The chip 44 has a chemical composition containing 50 wt% or more of one or more of noble metals such as Pt, Rh, Ir, and Ru. The upper surface 42 of the base material 41 is the surface on the side to which the chip 44 is connected. The discharge surface 45 of the chip 44 faces the center electrode 20. A spark gap G is formed between the discharge surface 45 of the chip 44 and the center electrode 20.

[0020] FIG. 2(a) is a plan view of the grounding electrode 40 (first electrode) viewed from the direction of the axis O. The base material 41 includes two side surfaces 46 located adjacent to the upper surface 42 (first surface), and an end surface 47 (second surface) located adjacent to the upper surface 42 and connecting the two side surfaces 46. If the grounding electrode 40 is the first electrode, the center electrode 20 is the second electrode. In the present embodiment, the base material 41 has a substantially rectangular cross-section and is rod-shaped, and the chip 44 has a cuboid shape.

[0021] The discharge surface 45 is quadrilateral. The area of the discharge surface 45 is larger than the area of the discharge surface 24 of the center electrode 20. Since the quadrilateral discharge surface 45 has an extension only at the corner portions as compared with a circular discharge surface having the same area, it is advantageous for the consumption due to the discharge between the discharge surface 45 and the center electrode 20 by the amount corresponding to the corners. The quadrilateral of the discharge surface 45 includes a rectangle, a square, a parallelogram, a rhombus, a trapezoid, and the like.

[0022] FIG. 2(b) is a cross-sectional view of the grounding electrode 40 taken along line IIb-IIb of FIG. 2(a). The line IIb-IIb is a straight line passing through the centroid 48 of the discharge surface 45 of the chip 44 and extending along the extending direction of the base material 41. The centroid 48 of the discharge surface 45 is the geometric center calculated by well-known means when the discharge surface 45 is regarded as a planar figure. The cross-section shown in FIG. 2(b) is a cross-section perpendicular to the discharge surface 45 including the centroid 48 of the discharge surface 45, and intersects the upper surface 42 (first surface) and the end surface 47 (second surface).

[0023] The base material 41 includes a bottom surface 49 located on the side opposite to the upper surface 42. The bottom surface 49 is connected to two side surfaces 46 and an end surface 47. The molten part 43 that connects the chip 44 to the base material 41 is formed by melting the chip 44 and the base material 41 together. The molten part 43 has a function of joining the chip 44 to the base material 41 and buffering the difference in linear expansion between the base material 41 and the chip 44.

[0024] The ground electrode 40 includes a first interface 50 between the molten part 43 and the chip 44, a second interface 51 between the molten part 43 and the base material 41, a first surface 51a of the molten part 43 connecting the end surface 47 of the base material 41 and the chip 44, and a second surface 51b of the molten part 43 connecting the upper surface 42 of the base material 41 and the chip 44. The second interface 51 includes a first point 52 and a second point 53 at both ends of the second interface 51. The second point 53 is the point farthest from the first point 52 on the second interface 51. The second interface 51 intersects the end surface 47 at the first point 52 and intersects the upper surface 42 at the second point 53. The first surface 51a of the molten part 43 includes the first point 52, and the second surface 51b of the molten part 43 includes the second point 53.

[0025] FIG. 3 is a cross-sectional view of the ground electrode 40 obtained by enlarging the view shown in FIG. 2(b). The procedure for drawing a rectangle 58 on the molten part 43 of the ground electrode 40 revealed in the cross-section will be described. First, draw two straight lines 68 and 69 perpendicular to the discharge surface 45 passing through both ends 66 and 67 of the discharge surface 45 respectively. Next, find the point located at the position farthest from the discharge surface 45 among the portions of the first interface 50 cut off by the two straight lines 68 and 69, and draw a first long side 54 passing through (including) that point. Similarly, find the point located at the position closest to the discharge surface 45 among the portions of the second interface 51 cut off by the two straight lines 68 and 69, and draw a second long side 55 passing through (including) that point. The first long side 54 and the second long side 55 are line segments parallel to the discharge surface 45. Then, draw two short sides 56 and 57 located in the molten part 43 so that the lengths of the two long sides 54 and 55 are maximized. Thereby, a rectangle 58 having the long sides 54 and 55 and the short sides 56 and 57 is formed.

[0026] Among the four vertices 59, 60, 61, and 62 of the rectangle 58, the vertices 59, 61, and 62 are located at the boundary of the melted portion 43, and the vertex 60 exists inside the boundary of the melted portion 43. When the middle region 63 obtained by dividing the rectangle 58 into three equal parts perpendicular to the short sides 56 and 57 is divided into six ranges 64 perpendicular to the long sides 54 and 55, ranges 64 in which the content rates of the elements of the main components included in the chip 44 differ by 10% or more are adjacent to each other. The reason for performing elemental analysis on the middle region 63 obtained by dividing the rectangle 58 into three equal parts is to exclude the portions near the interfaces 50 and 51 from the object of elemental analysis. Since there may be many elements contained in the chip 44 and the base material 41 in the portions near the interfaces 50 and 51, the accuracy of quantitative analysis can be improved by excluding the portions near the interfaces 50 and 51 from the object of elemental analysis.

[0027] The elements of the main components included in the chip 44 refer to the elements with the highest content among the elements constituting the chip 44. Quantitative analysis of the elements of the main components constituting the chip 44 and the elements of the melted portion 43 can be performed by wavelength-dispersive X-ray analysis (WDS) using an electron probe microanalyzer (EPMA). Elemental analysis (surface analysis) is performed for each range 64 by WDS using EPMA, and the presence or absence of a boundary 65 where ranges 64 in which the content rates of the elements of the main components of the chip 44 differ by 10% or more are adjacent to each other is determined. When there is a boundary 65, the position of the boundary 65 is also specified. In the present embodiment, the boundary 65 where ranges 64 in which the content rates of the elements of the main components of the chip 44 differ by 10% or more are adjacent to each other is located between two straight lines 68 and 69.

[0028] Among the six ranges 64, the range 64 closest to the first surface 51a in contact with the end face 47 of the base material 41 is partially located between the two straight lines 68 and 69. This indicates that the chip 44 is arranged in an open state near the end face 47 of the base material 41. Since the propagation of the flame generated in the spark gap G is less likely to be obstructed by the ground electrode 40, the ignitability can be improved.

[0029] For the second interface 51 of the melting portion 43, the length of the perpendicular line 71 dropped from the first point 52 to the straight line 70 including the discharge surface 45 is defined as Y1, and among the second interface 51, the length of the longest perpendicular line 74 dropped from the straight line 70 from the portion between the intersection point 73 of the straight line 72 passing through the centroid 48 of the discharge surface 45 and perpendicular to the discharge surface 45 and the second interface 51 and the second point 53 farthest from the first point 52 is defined as Y2, and Y1 ≥ Y2. This means that the distance between the straight line 70 and the second interface 51 becomes shorter from the first point 52 toward the second point 53.

[0030] When a high voltage is applied between the terminal fitting 25 and the main body fitting 30 of the spark plug 10 and a discharge occurs in the spark gap G, the chip 44 is heated. Since the end face 47 of the base material 41 of the ground electrode 40 is open, in the ground electrode 40, the portion closer to the end face 47 has a faster heating rate due to discharge and a faster cooling rate due to the combustible mixture than the portion farther from the end face 47. As a result, a large thermal strain occurs in the portion of the melting portion 43 closer to the end face 47, and a large thermal stress is generated. Since the melting portion 43 has the relationship of Y1 ≥ Y2, the relaxation effect of the thermal stress in the portion of the melting portion 43 closer to the end face 47 is greater than that in the portion of the melting portion 43 farther from the end face 47. Thereby, breakage of the portion of the melting portion 43 closer to the end face 47 can be reduced.

[0031] Since the first surface 51a of the melting portion 43 connects the end face 47 of the base material 41 and the chip 44, the chip 44 is arranged in an open state in the vicinity of the end face 47 of the base material 41. Since the propagation of the flame generated in the spark gap G is less likely to be obstructed by the ground electrode 40, the ignitability can be improved.

[0032] An example of a method for manufacturing the grounding electrode 40 will be described. After placing the chip 44 on the upper surface 42 of the base material 41, a laser beam is irradiated from a processing head (not shown) facing the end surface 47 of the base material 41. The processing head is moved while irradiating the beam along the portion where the base material 41 and the chip 44 are in contact to melt the base material 41 and the chip 44. After the melted portion has hardened, the processing head is moved while irradiating the beam along the hardened portion to remelt and form the melted portion 43. By adjusting the time until remelting, the power density of the laser beam, etc., and setting the range of remelting and the degree of melting, a boundary 65 is formed where a range 64 in which the content rate of the element of the main component of the chip 44 differs by 10% or more is adjacent.

[0033] Since the melted portion 43 is formed by melting together the chip 44 and the base material 41, the difference in the content rate of the element of the main component of the chip 44 means that the ratio of melting together the chip 44 and the base material 41 is different. Since the linear expansion coefficient of the chip 44 is smaller than that of the base material 41, in the melted portion 43, the portion where the ratio of the component of the chip 44 is large has a smaller linear expansion coefficient than the portion where the ratio of the component of the chip 44 is small. Near the boundary 65 where the range 64 in which the content rate of the element of the main component of the chip 44 differs by 10% or more is adjacent, due to the difference in the linear expansion coefficient, cracks are likely to occur due to thermal stress caused by temperature changes in the grounding electrode 40. Setting the boundary 65 based on 10% of the content rate of the element of the main component of the chip 44 is based on an empirical rule found from the ease of crack generation.

[0034] Since the boundary 65 extends in the direction intersecting the discharge surface 45, when a crack occurs near the boundary 65, the crack is likely to progress in the direction intersecting the discharge surface 45. When the crack progresses in the direction intersecting the discharge surface 45 and the melted portion 43 is divided, compared to the case where the melted portion 43 is not divided, the linear expansion amount of the melted portion 43 becomes smaller and the thermal stress becomes smaller. As a result, it becomes difficult for the crack to progress along the interfaces 50, 51 of the melted portion 43. Also, the stress of the melted portion 43 is released by the generated crack, making it difficult for the crack to progress. As a result, it becomes difficult for the melted portion 43 to break along the interfaces 50, 51, so the dropout of the chip 44 from the base material 41 can be reduced.

[0035] Since the melting part 43 has the relationship of Y1≥Y2, even if the melting part 43 is divided, breakage of the part of the melting part 43 close to the end face 47 can be reduced. Since the chip 44 is joined to the base material 41 by each of the divided melting parts 43, it is possible to prevent the joining strength of the chip 44 by the melting part 43 from decreasing.

[0036] A boundary 65 exists in a part of the melting part 43 located between two straight lines 68, 69 perpendicular to the discharge surface 45 passing through both ends 66, 67 of the discharge surface 45 of the melting part 43. When a crack generated near the boundary 65 propagates in the direction in which the boundary 65 extends, compared with the case where the boundary 65 exists in the part of the melting part 43 including the second point 53 and partitioned by the straight line 69, the melting part 43 is divided into smaller parts, so the linear expansion amount of the melting part 43 becomes smaller and the thermal stress becomes smaller. As a result, it becomes difficult for the crack to further propagate along the interfaces 50, 51 of the melting part 43, so that the dropout of the chip 44 can be further reduced.

[0037] The size of the melting part 43 for joining the chip 44 having the rectangular discharge surface 45 to the base material 41 is larger than that of the melting part for joining a chip having a circular discharge surface with the same area to the base material 41, so the linear expansion amount of the melting part 43 also becomes larger. By dividing the melting part 43 with a crack generated near the boundary 65, the linear expansion amount of each of the divided melting parts 43 becomes smaller, so the melting part 43 of the chip 44 having the rectangular discharge surface 45 has a particularly large effect of relaxing stress.

[0038] The second embodiment will be described with reference to FIG. 4. The shape of the melting part 81 in the second embodiment is different from that in the first embodiment. In the second embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description is omitted. FIG. 4 is a cross-sectional view perpendicular to the discharge surface 45 including the centroid 48 of the discharge surface 45 of the ground electrode 80 of the spark plug in the second embodiment. The ground electrode 80 is provided in the spark plug 10 instead of the ground electrode 40 in the first embodiment. The ground electrode 80 includes a melting part 81 that connects the chip 44 to the base material 41.

[0039] The ground electrode 80 includes a first interface 82 between the molten part 81 and the chip 44, a second interface 83 between the molten part 81 and the base material 41, a first surface 83a of the molten part 81 connecting the end face 47 of the base material 41 and the chip 44, and a second surface 83b of the molten part 81 connecting the upper surface 42 of the base material 41 and the chip 44. The second interface 83 includes a first point 84 and a second point 85 at both ends of the second interface 83. The second point 85 is the point farthest from the first point 84 on the second interface 83. The second interface 83 intersects the end face 47 at the first point 84 and intersects the upper surface 42 at the second point 85. The first surface 83a includes the first point 84, and the second surface 83b includes the second point 85.

[0040] Since the first surface 83a of the molten part 81 connects the end face 47 of the base material 41 and the chip 44, the chip 44 is arranged in an open state near the end face 47 of the base material 41. The propagation of the flame generated in the spark gap G (see FIG. 1) is less likely to be blocked by the ground electrode 81, so the ignitability can be improved.

[0041] On the cross-section of the ground electrode 80, draw a rectangle 90 having a first long side 86 and a second long side 87 parallel to the discharge surface 45, and two short sides 88, 89 perpendicular to the discharge surface 45, and provide a middle region 95 that is trisected perpendicularly to the short sides 88, 89. The rectangle 90 is entirely included in the molten part 81.

[0042] The first long side 86 passes through the point farthest from the discharge surface 45, which is the part where the first interface 82 is cut off by the two straight lines 68, 69. The second long side 87 passes through the point closest to the discharge surface 45, which is the part where the second interface 83 is cut off by the two straight lines 68, 69. Among the four vertices 91, 92, 93, 94 of the rectangle 90, three vertices 91, 92, 93 are included in the boundary of the molten part 81. The vertex 94 exists inside the boundary of the molten part 81. When the molten part 81 is equally divided into six ranges 96 perpendicular to the long sides 86, 87, the boundary 97 where the ranges 96 with a difference in the content rate of the main component elements contained in the chip 44 of 10% or more are adjacent is present in the part of the molten part 81 that includes the second point 85 and is partitioned by the straight line 69.

[0043] Of the six ranges 96, the range 96 closest to the first surface 83a in contact with the end face 47 of the base material 41 is entirely located between the two straight lines 68, 69. This indicates that the chip 44 is arranged in an open state near the end face 47 of the base material 41. Since the propagation of the flame generated in the spark gap G is less likely to be obstructed by the ground electrode 80, the ignitability can be improved.

[0044] For the second interface 83 of the molten part 81, let the length of the perpendicular line 98 dropped from the first point 84 to the straight line 70 including the discharge surface 45 be Y1, and for the second interface 83, among the part between the intersection 99 of the straight line 72 passing through the centroid 48 of the discharge surface 45 and perpendicular to the discharge surface 45 and the second interface 83, and the second point 85, let the length of the longest perpendicular line 99a dropped to the straight line 70 be Y2. Then Y1≧Y2. In the molten part 81 near the end face 47, since the relaxation effect of the thermal stress is greater than that in the molten part 81 far from the end face 47, the breakage of the part of the molten part 81 near the end face 47 can be reduced.

[0045] Since the boundary 97 extends in a direction intersecting the discharge surface 45, if a crack occurs near the boundary 97, the crack is likely to progress in the direction intersecting the discharge surface 45. When the crack progresses in the direction intersecting the discharge surface 45 and the molten part 81 is divided, compared with the case where the molten part 81 is not divided, the linear expansion amount of the molten part 81 becomes smaller and the thermal stress becomes smaller. As a result, it becomes difficult for the crack to progress along the interfaces 82, 83 of the molten part 81. Also, since the stress is released by the crack generated near the boundary 97, it becomes difficult for the molten part 81 to break along the interfaces 82, 83. Therefore, the dropout of the chip 44 from the base material 41 can be reduced.

[0046] Since the boundary 97 exists in the part of the molten part 81 including the second point 85 and partitioned by the straight line 69, even if the crack generated near the boundary 97 progresses, it becomes difficult for the crack to reach the interface 82 between the chip 44 and the molten part 81. Therefore, the progress of the crack generated near the boundary 97 is less likely to become the starting point of the breakage of the chip 44.

[0047] Referring to FIGS. 5 and 6, a third embodiment will be described. In the first and second embodiments, the case where the chip 44 is disposed on the upper surface 42 of the base material 41 and the base material 41 and the chip 44 are joined together has been described. In contrast, in the third embodiment, a case where a groove recessed in the upper surface 42 of the base material 41 is provided, the chip 44 is disposed in the groove, and the base material 41 and the chip 44 are joined together will be described. In the third embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description thereof will be omitted.

[0048] FIG. 5(a) is a plan view of the ground electrode 100 of the spark plug in the third embodiment. FIG. 5(b) is a cross-sectional view of the ground electrode 100 taken along line Vb-Vb in FIG. 5(a). The ground electrode 100 is provided in the spark plug 10 instead of the ground electrode 40 in the first embodiment.

[0049] The chip 44 is disposed in a groove provided in the upper surface 42 of the base material 41. The groove extends along the side surface 46 of the base material 41 and opens at the end surface 47. The position of the chip 44 in the direction away from the end surface 47 is restricted by the wall surface 42a of the groove. The chip 44 is connected to the base material 41 via the molten part 101. The ground electrode 100 includes a first interface 102 between the molten part 101 and the chip 44, a second interface 103 between the molten part 101 and the base material 41, and a first surface 103a of the molten part 101 connecting the end surface 47 of the base material 41 and the chip 44.

[0050] FIG. 6 is a cross-sectional view of the ground electrode 100 perpendicular to the discharge surface 45 including the center of gravity 48 of the discharge surface 45. The second interface 103 is continuous from the first point 104 where it intersects the end surface 47 to the chip 44. The second interface 103 includes a second point 105. The second point 105 is the point farthest from the first point 104 on the second interface 103 (the point having the longest linear distance from the first point 104). The first surface 103a includes the first point 104. Since the first surface 103a connects the end surface 47 of the base material 41 and the chip 44, the chip 44 is disposed in an open state in the vicinity of the end surface 47 of the base material 41. The propagation of the flame generated in the spark gap G (see FIG. 1) is less likely to be obstructed by the ground electrode 100, so that the ignitability can be improved.

[0051] On the cross-section of the ground electrode 100, draw a rectangle 110 having a first long side 106 and a second long side 107 parallel to the discharge surface 45, and two short sides 108, 109 perpendicular to the discharge surface 45, and provide a middle region 115 that is equally divided into three parts perpendicular to the short sides 108, 109. The rectangle 110 is entirely included in the melting part 101.

[0052] The first long side 106 includes the point (vertex 112) farthest from the discharge surface 45, which is the part where the first interface 102 is cut off by two straight lines 68, 69. The second long side 107 includes the point (vertex 113) closest to the discharge surface 45, which is the part where the second interface 103 is cut off by two straight lines 68, 69. Among the four vertices 111, 112, 113, 114 of the rectangle 110, three vertices 111, 112, 113 are included in the boundary of the melting part 101. The vertex 114 exists inside the boundary of the melting part 101. When the melting part 101 equally divides the region 115 into six ranges 116 perpendicular to the long sides 106, 107, a boundary 117 where ranges 116 with a difference in the content rate of the main component elements contained in the chip 44 of 10% or more are adjacent exists between the straight lines 68, 69.

[0053] The range 116 closest to the first surface 103a in contact with the end face 47 of the base material 41 among the six ranges 116 is partially located between the two straight lines 68, 69. This indicates that the chip 44 is arranged in an open state near the end face 47 of the base material 41. Since the propagation of the flame generated in the spark gap G is less likely to be obstructed by the ground electrode 100, the ignitability can be improved.

[0054] For the second interface 103 of the melting part 101, let the length of the perpendicular line 118 dropped from the first point 104 to the straight line 70 be Y1, and for the part of the second interface 103 between the intersection point 119 of the straight line 72 and the second interface 103 and the second point 105, let the length of the longest perpendicular line (in this embodiment, the line segment connecting the centroid 48 and the intersection point 119) dropped to the straight line 70 be Y2. When Y1≥Y2, in the melting part 101 near the end face 47, the relaxation effect of the thermal stress is greater than that in the melting part 101 far from the end face 47, so the breakage of the part of the melting part 101 near the end face 47 can be reduced.

[0055] Since the boundary 117 extends in the direction intersecting the discharge surface 45, when a crack occurs near the boundary 117, the crack is likely to propagate in the direction intersecting the discharge surface 45. When the crack propagates in the direction intersecting the discharge surface 45 and the melting part 101 is divided, compared with the case where the melting part 101 is not divided, the linear expansion amount of the melting part 101 becomes smaller and the thermal stress becomes smaller. Also, since the stress is released by the crack generated near the boundary 117, it becomes difficult for the crack to propagate along the interfaces 102, 103 of the melting part 101. Therefore, the dropout of the chip 44 from the base material 41 can be reduced.

[0056] The fourth embodiment will be described with reference to FIGS. 7 and 8. In the first to third embodiments, the ground electrodes 40, 80, 100 whose cross-sections are represented using the cutting plane intersecting the end face 47 of the base material 41 were described. In contrast, in the fourth embodiment, the ground electrode 120 whose cross-section is represented using the cutting plane intersecting the side face 46 of the base material 41 will be described. In the fourth embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description will be omitted.

[0057] FIG. 7(a) is a plan view of the ground electrode 120 of the spark plug in the fourth embodiment. FIG. 7(b) is a cross-sectional view of the ground electrode 120 taken along line VIIb-VIIb in FIG. 7(a). The line VIIb-VIIb is a straight line passing through the centroid 48 of the discharge surface 45 of the chip 44 and intersecting the extending direction of the base material 41. The ground electrode 120 is provided in the spark plug 10 instead of the ground electrode 40 in the first embodiment.

[0058] The chip 44 is disposed in a groove provided in the upper surface 42 (first surface) of the base material 41. The groove extends along the side surface 46 (second surface) of the base material 41 and opens at the end surface 47. The chip 44 is positioned between the side surfaces 46, 46 of the base material 41 by the wall surfaces 42b, 42c of the groove. The chip 44 is connected to the base material 41 via the melting part 121. The ground electrode 120 includes a first interface 122 between the chip 44 and the melting part 121, a second interface 123 between the melting part 121 and the base material 41, and a surface 123a of the melting part 121 that appears on the side surface 46 of the base material 41.

[0059] FIG. 8 is a cross-sectional view perpendicular to the discharge surface 45 including the center of gravity 48 of the discharge surface 45 of the ground electrode 120. The second interface 123 extends from a first point 124 where the side surface 46 (second surface) of the base material 41 intersects the second interface 123 to a second point 125. The second point 125 is the point farthest from the first point 124 on the second interface 123 (the point with the longest linear distance from the first point 124). The surface 123a includes the first point 124.

[0060] On the cross-section of the ground electrode 120, draw a rectangle 130 having a first long side 126 and a second long side 127 parallel to the discharge surface 45, and two short sides 128, 129 perpendicular to the discharge surface 45, and provide a middle region 135 that is equally divided into three parts perpendicular to the short sides 128, 129. The rectangle 130 is entirely included in the melting part 121.

[0061] The first long side 126 includes the point (vertex 132) that is farthest from the discharge surface 45, which is the portion where the first interface 122 is cut off by two straight lines 68, 69. The second long side 127 includes the point (vertex 133) that is closest to the discharge surface 45, which is the portion where the second interface 123 is cut off by two straight lines 68, 69. All four vertices 131, 132, 133, 134 of the rectangle 110 are included in the boundary of the melted portion 121. When the melted portion 121 equally divides the region 135 into six ranges 136 perpendicular to the long sides 126, 127, there is a boundary 137 where ranges 136 with a difference of 10% or more in the content rate of the main component elements contained in the chip 44 are adjacent, between the straight lines 68, 69. The range 136 closest to the surface 123a in contact with the side surface 46 of the base material 41 among the six ranges 136 is located outside between the two straight lines 68, 69.

[0062] If the length of the longest perpendicular line 138 drawn from the point where the interfaces 122, 123 of the melted portion 121 intersect with the side surface 46 of the base material 41 to the straight line 70 is Y1, the first point 124 is the intersection of the perpendicular line 138 and the second interface 123. If the length of the longest perpendicular line (in this embodiment, the line segment connecting the centroid 48 and the intersection 139) drawn from the portion between the intersection 139 of the straight line 72 and the second interface 123 and the second point 125 on the second interface 123 to the straight line 70 is Y2, then Y1 ≥ Y2. In the melted portion 101 near the side surface 46 (left side of FIG. 8), since the relaxation effect of thermal stress is greater than that of the melted portion 121 far from the side surface 46 (left side of FIG. 8), breakage of the portion of the melted portion 121 near the side surface 46 (left side of FIG. 8) can be reduced.

[0063] Since the boundary 137 extends in the direction intersecting the discharge surface 45, if a crack occurs near the boundary 137, the crack is likely to progress in the direction intersecting the discharge surface 45. When the crack progresses in the direction intersecting the discharge surface 45 and the melted portion 121 is divided, compared with the case where the melted portion 121 is not divided, the linear expansion amount of the melted portion 121 becomes smaller and the thermal stress becomes smaller. Also, since the stress is released by the crack generated near the boundary 137, it becomes difficult for the crack to progress along the interfaces 122, 123 of the melted portion 121. Therefore, the detachment of the chip 44 from the base material 41 can be reduced.

[0064] The fifth embodiment will be described with reference to FIG. 9. In the first to fourth embodiments, the case where the width of the chip 44 is about half the distance between the side surfaces 46, 46 of the base material 41 (the width of the base material 41) has been described. In contrast, in the fifth embodiment, the case where the width of the base material 41 and the width of the chip 44 are substantially equal will be described. In the fifth embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description will be omitted.

[0065] FIG. 9 is a cross-sectional view perpendicular to the discharge surface 45 including the centroid 48 of the discharge surface 45 of the ground electrode 140. The cross-sectional view of FIG. 9 is represented by using a cutting plane intersecting the side surface 46 of the base material 41. The chip 44 is connected to the base material 41 via the melting part 141.

[0066] The ground electrode 140 includes a first interface 142 between the melting part 141 and the chip 44, a second interface 143 between the melting part 141 and the base material 41, a first surface 143a of the melting part 141 connecting one side surface 46 of the base material 41 and the chip 44, and a second surface 143b of the melting part 141 connecting the other side surface 46 of the base material 41 and the chip 44. The second interface 143 is continuous from a first point 144 intersecting the side surface 46 to a second point 145 intersecting the side surface 46. The second point 145 is the point farthest from the first point 144 on the second interface 143. The first surface 143a includes the first point 144, and the second surface 143b includes the second point 145. The first surface 143a is longer than the second surface 143b.

[0067] On the cross-section of the ground electrode 140, a rectangle 150 having a first long side 146 and a second long side 147 parallel to the discharge surface 45 and two short sides 148, 149 perpendicular to the discharge surface 45 is drawn, and a middle region 155 equally divided into three parts perpendicular to the short sides 148, 149 is provided. The rectangle 150 is entirely included in the melting part 141.

[0068] The first long side 146 passes through (including) the point farthest from the discharge surface 45, which is the portion where the first interface 142 is cut off by the two straight lines 68 and 69. The second long side 147 includes the point (vertex 153) closest to the discharge surface 45, which is the portion where the second interface 143 is cut off by the two straight lines 68 and 69. Among the four vertices 151, 152, 153, and 154 of the rectangle 150, the vertices 151 and 153 are included in the boundary of the molten part 141. The vertices 152 and 154 exist inside the boundary of the molten part 141. When the molten part 141 equally divides the region 155 into six ranges 156 perpendicular to the long sides 146 and 147, a boundary 157 where ranges 156 with a difference of 10% or more in the content rate of the main component elements contained in the chip 44 are adjacent exists between the straight lines 68 and 69.

[0069] Among the six ranges 156, the range 156 closest to the first surface 143a in contact with the side surface 46 of the base material 41 is partially located between the two straight lines 68 and 69. This indicates that the chip 44 is arranged in an open state near the side surface 46 of the base material 41. Since the propagation of the flame generated in the spark gap G is less likely to be blocked by the ground electrode 140, the ignition performance can be improved.

[0070] For the second interface 143 of the molten part 141, when the length of the perpendicular 158 dropped from the first point 144 to the straight line 70 is Y1, and the length of the longest perpendicular (in this embodiment, the line segment connecting the centroid 48 and the intersection point 159) dropped from the straight line 70 from the portion between the intersection point 159 of the straight line 72 and the second interface 143 and the second point 145 in the second interface 143 is Y2, Y1≥Y2. In the molten part 141 of the portion close to the side surface 46 (left side of FIG. 9), since the relaxation effect of the thermal stress is greater than that of the molten part 141 of the portion far from the side surface 46 (left side of FIG. 9), the breakage of the portion of the molten part 141 close to the side surface (left side of FIG. 9) can be reduced.

[0071] Since the boundary 157 extends in a direction intersecting the discharge surface 45, if a crack occurs near the boundary 157, the crack is likely to propagate in the direction intersecting the discharge surface 45. When the crack propagates in the direction intersecting the discharge surface 45 and the molten part 141 is divided, the linear expansion amount of the molten part 141 becomes smaller and the thermal stress becomes smaller than when the molten part 141 is not divided. In addition, since the stress is released by the crack generated near the boundary 157, it becomes difficult for the crack to propagate along the interfaces 142, 143 of the molten part 141. Therefore, the detachment of the chip 44 from the base material 41 can be reduced.

[0072] As described above, the present invention has been described based on the embodiments. However, the present invention is not limited to the above embodiments at all, and it can be easily inferred that various improvements and modifications are possible without departing from the gist of the present invention.

[0073] In the embodiments, the cross-sections of the ground electrodes 40, 80, 100 using a cutting plane parallel to the stretching direction of the base material 41, or the cross-sections of the ground electrodes 120, 140 using a cutting plane perpendicular to the stretching direction of the base material 41 have been described. However, it is not necessarily limited to this. If the cutting plane includes the centroid 48 of the discharge surface 45 of the chip 44, the angle of the cutting plane with respect to the stretching direction of the base material 41 can be set as appropriate.

[0074] In the third and fourth embodiments, the case where the chip 44 arranged in the groove provided in the base material 41 is joined to the base material 41 has been described. However, it is not necessarily limited to this. It is naturally possible to join the chip 44 to the base material 41 without providing a groove in the base material 41 as in the first, second, and fifth embodiments. In addition, it is naturally possible to change the first and second embodiments to the third and fourth embodiments and join the chip 44 arranged in the groove provided in the base material 41 to the base material 41.

[0075] In the fourth embodiment, although the case where the range 136 closest to the surface 123a in contact with the side surface 46 of the base material 41 among the six ranges 136 is located outside the two straight lines 68 and 69 has been described, it is not necessarily limited to this. When the chip 44 is disposed near the side surface 46 in contact with the surface 123a of the molten part 121, at least a part of the range 136 closest to the surface 123a among the six ranges 136 may be located between the two straight lines 68 and 69. In this case, since the propagation of the flame generated in the spark gap G is less likely to be obstructed by the ground electrode 120, the ignitability can be improved.

[0076] In the first to third embodiments, although the case where the tip surface of the chip 44 slightly enters inward from the end surface 47 of the base material 41 has been described, it is not necessarily limited to this. It is of course possible to project the tip surface of the chip 44 beyond the end surface 47 of the base material 41 and project the chip 44 from the end surface 47 of the base material 41.

[0077] In the embodiments, the case where the chip 44 disposed on the upper surface 42 of the base material 41 or the chip 44 disposed in the groove provided on the upper surface 42 is joined to the base material 41 has been described, but it is not necessarily limited to this. As long as the spark gap G is provided between the center electrode 20 and the ground electrodes 40, 80, 100, 120, 140, the chip 44 may be fixed to any surface of the base material 41. For example, it is of course possible to fix the chip 44 to the end surface 47 of the base material 41.

[0078] In the embodiments, the case where the chip 44 of the ground electrode 40 has a rectangular parallelepiped (quadrangular prism) shape has been described, but it is not necessarily limited to this. The shape of the chip 44 can be appropriately set, such as a cylindrical shape or a polygonal prism shape other than a quadrangular prism. Along with the change in the shape of the chip 44, the shape of the discharge surface 45 can also be appropriately set to a polygon other than a circular shape or a square shape.

[0079] In the embodiment, the case where the end portion of the base material 41 is connected to the main fitting 30 has been described, but it is not necessarily limited to this. It is naturally possible to provide a rod-shaped member whose end portion is connected to the main fitting 30, join an intermediate material (base material) to the member, and connect a chip to the base material via a molten portion.

[0080] In the embodiment, the case where the ground electrodes 40, 80, 100, 120, 140 are used as the first electrode and the center electrode 20 is used as the second electrode has been described, but it is not necessarily limited to this. On the contrary, it is naturally possible to use the center electrode as the first electrode and the ground electrode as the second electrode.

Explanation of Reference Numerals

[0081] 10 Spark plug 20 Center electrode (second electrode) 40, 80, 100, 120, 140 Ground electrode (first electrode) 41 Base material 42 Upper surface (first surface) 43, 81, 101, 121, 141 Molten portion 44 Chip 45 Discharge surface 46 Side surface (second surface) 47 End face (second surface) 48 Center of gravity 50, 82, 102, 122, 142 First interface 51, 83, 103, 123, 143 Second interface 52, 84, 104, 124, 144 First point 53, 85, 105, 125, 145 Second point 54, 86, 106, 126, 146 First long side 55, 87, 107, 127, 147 Second long side 56, 57, 88, 89, 108, 109, 128, 129, 148, 149 Short side 58, 90, 110, 130, 150 Rectangle 63, 95, 115, 135, 155 Region 64, 96, 116, 136.156 Range 65, 97, 117, 137, 157 boundaries 66, 67 ends Straight line passing through 68, 69 ends Straight line including 70 discharge surface 71, 98, 118, 138, 158 perpendicular lines Straight line passing through 72 centroid 73, 99, 119, 139, 159 intersection points 74, 99a perpendicular lines

Claims

1. A first electrode including a base material and a chip connected to the base material via a molten part, and a second electrode facing a discharge surface of the chip, the spark plug comprising: In a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface, A first long side parallel to the discharge surface, including the point farthest from the discharge surface, in a portion where a first interface between the chip and the molten part is cut off by two straight lines perpendicular to the discharge surface passing through both ends of the discharge surface respectively; A second long side parallel to the discharge surface, including the point closest to the discharge surface, in a portion where a second interface between the molten part and the base material is cut off by two straight lines perpendicular to the discharge surface passing through both ends of the discharge surface respectively, and A spark plug in which, when a rectangular region having two short sides located in the molten part so that the lengths of the two long sides are maximized is divided into three equal parts perpendicular to the short sides and then divided into six equal ranges perpendicular to the long sides, ranges in which the content ratios of the elements of the main components contained in the chip differ by 10% or more are adjacent to each other.

2. In a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface, The base material includes a first surface which is the surface on the side to which the chip is connected, and a second surface which is located adjacent to the first surface and where the second interface intersects, Let the length of the longest perpendicular line dropped from the point where the second interface intersects the second surface to the straight line including the discharge surface be Y1, and let the intersection point of the perpendicular line of length Y1 and the second interface be the first point, The spark plug according to claim 1, wherein Y1≧Y2 when the length of the longest perpendicular line dropped from a portion between the intersection point of the straight line passing through the center of gravity and perpendicular to the discharge surface and the second interface, and the second point farthest from the first point, to the straight line including the discharge surface is Y2.

3. In a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface, The spark plug according to claim 1 or 2, wherein there is a boundary where ranges in which the content ratios of the elements of the main components contained in the chip differ by 10% or more are adjacent to each other in a portion of the molten part located between two straight lines perpendicular to the discharge surface passing through both ends of the discharge surface respectively.

4. In a cross-section perpendicular to the discharge surface including the center of gravity of the discharge surface, The base material includes a first surface which is the surface on the side to which the chip is connected, and a second surface which is located adjacent to the first surface and where the second interface intersects, Let Y1 be the length of the longest perpendicular line dropped from the point where the second interface intersects the second surface to the straight line including the discharge surface. Let the intersection point of the perpendicular line of length Y1 and the second interface be the first point. Let the point on the second interface that is farthest from the first point be the second point. In the portion of the molten part that includes the second point and is partitioned by a straight line perpendicular to the discharge surface and passing through the end of the discharge surface closer to the second point, there is a boundary where the ranges in which the content rates of the elements of the main components included in the chip differ by 10% or more are adjacent. The spark plug according to claim 1 or 2. **Claim 5** The spark plug according to claim 1 or 2, wherein the discharge surface is quadrilateral.

Images