Spark plug

Abstract

To provide an ignition plug which comprises a ground electrode and enables the deformation of the same to be suppressed when the ground electrode becomes high temperature.SOLUTION: A ground electrode 60 of an ignition plug 10, includes: a first linear portion 610 extending linearly from an apical surface S; a bent portion 630 extending from the end portion of the first linear portion 610 in such a way as to be bent; and a second linear portion 620 extending linearly from the end portion of the bent portion 630. In a view along a y direction, when a virtual straight line extending vertically to the apical surface S from an intersection point of a straight line L10 and the apical surface S is denoted as a first straight line L11, a virtual straight line obtained by extending a straight line L20 is denoted as a second straight line L21, a length from the intersection point of the apical surface S to the intersection point of the second straight line L21 in the first straight line L11 is denotes as A, and a length from a boundary of the bent portion 630 and the second linear portion 620 to the intersection point with the first straight line L11 in the second straight line L21 is denoted as B, a value of B / A is 0.35 or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to spark plugs. [Background technology]

[0002] For example, an internal combustion engine mounted on a vehicle is provided with a spark plug for igniting fuel. The spark plug has a center electrode and a ground electrode, and ignites the fuel by generating a spark discharge between these electrodes. In order to generate a normal and stable spark discharge, it is necessary to adjust in advance the gap between the center electrode and the ground electrode to an appropriate size.

[0003] However, when vibrations caused by the operation of an internal combustion engine or the like are applied to the spark plug, the shape of the ground electrode changes due to the vibrations, and the size of the gap may change. Such deformation of the ground electrode is particularly likely to occur in small spark plugs. Therefore, the spark plug described in Patent Document 1 below is designed to suppress deformation of the ground electrode due to vibrations by improving the cross-sectional shape of the ground electrode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-114754 A Summary of the Invention [Problem to be solved by the invention]

[0005] In recent vehicles, internal combustion engines have become smaller and more powerful, and the thermal load applied to the spark plug has become larger. The inventors have found that when a large thermal load is applied to the spark plug, the ground electrode becomes deformed when the ground electrode becomes hot. Such deformation of the ground electrode is caused by the distortion remaining in the ground electrode during processing, and is considered to occur when the hardness of the ground electrode becomes hot. The deformation of the ground electrode caused by the thermal load is different from the deformation caused by vibration in terms of its direction, etc. For this reason, it is considered difficult to sufficiently suppress the deformation even if the configuration described in the above Patent Document 1 is adopted.

[0006] An object of the present disclosure is to provide an ignition plug capable of suppressing deformation of the ground electrode when the temperature becomes high. [Means for solving the problem]

[0007] The spark plug according to the present disclosure includes a center electrode (30), an insulator (20) that holds the center electrode from the outer periphery, a metal shell (50) that holds the insulator from the outer periphery, and a ground electrode (60) having one end connected to a tip surface (S) of the metal shell and the other end facing the center electrode. The ground electrode has a first straight portion (610) that extends linearly from the tip surface, a bent portion (630) that extends from an end of the first straight portion opposite the tip surface toward the center electrode, and a second straight portion (620) that extends linearly from an end of the bent portion opposite the first straight portion toward the center electrode. When viewed along a third direction perpendicular to both a first direction, which is a direction along the central axis of the center electrode, and a second direction, which is a direction in which the center electrode and the ground electrode are aligned, an imaginary line extending perpendicular to the tip face from the intersection of a line indicating the outline of the part of the first straight line portion on the center electrode side with the tip face is defined as a first line (L11), an imaginary line extending a line indicating the outline of the part of the second straight line portion on the center electrode side toward the first straight line is defined as a second straight line (L21), the length of the first line from the intersection with the tip face to the intersection with the second line is defined as A, and the length of the second line from the boundary between the bent portion and the second straight line portion to the intersection with the first line is defined as B, in this ignition plug, the value of B / A is 0.35 or less.

[0008] In the spark plug having the above-mentioned configuration, the degree of work hardening at the bent portion of the ground electrode is relatively large. As a result, even when the ground electrode becomes hot, the hardness of the bent portion is maintained to a certain extent, so that deformation at the bent portion can be suppressed. Furthermore, in the base portion of the ground electrode, the amount of deformation during processing is kept small and the residual strain is also small, so that deformation at this portion can also be suppressed. In this way, in the spark plug having the above-mentioned configuration, deformation at the portions that are particularly susceptible to deformation at high temperatures, i.e., the bent portion and the base portion of the ground electrode, can be suppressed compared to the conventional case. Effect of the Invention

[0009] According to the present disclosure, there is provided an ignition plug capable of suppressing deformation of the ground electrode when it becomes hot. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the configuration of an ignition plug according to this embodiment. [Diagram 2] FIG. 2 is a diagram showing the configuration of the ground electrode and its vicinity in the spark plug according to this embodiment. [Diagram 3] FIG. 3 is a cross-sectional view showing the internal configuration of the ground electrode. [Figure 4] FIG. 4 is a diagram for explaining a method for forming the ground electrode. [Diagram 5] FIG. 5 is a diagram for explaining a method for forming the ground electrode. [Figure 6] FIG. 6 is a diagram for explaining a method for forming the ground electrode. [Figure 7] FIG. 7 is a diagram for explaining a method for forming the ground electrode. [Figure 8] FIG. 8 is a diagram showing the relationship between the value of B / A and the degree of deformation of the ground electrode. [Figure 9] FIG. 9 is a diagram showing the relationship between the radius of curvature of the bent portion and the degree of deformation of the ground electrode. [Figure 10] FIG. 10 is a diagram showing the relationship between the angle of the ground electrode and the degree of deformation of the ground electrode. [Figure 11] FIG. 11 is a diagram showing the Vickers hardness at each position on the ground electrode. [Figure 12] FIG. 12 is a diagram showing the configuration of the ground electrode and its vicinity in an ignition plug according to a comparative example. [Figure 13] FIG. 13 is a diagram showing the relationship between the hardness ratio of the ground electrode and the degree of deformation of the ground electrode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the present embodiment will be described with reference to the accompanying drawings. In order to facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals as much as possible, and duplicated description will be omitted.

[0012] The configuration of an ignition plug 10 according to this embodiment will be described with reference to Fig. 1. In Fig. 1, a cross section of the ignition plug 10 taken along a plane including a central axis CX (described later) is shown on the left side. However, of the components constituting the ignition plug 10, the center electrode 30 and the terminal fitting 40 are shown not in cross section but in their respective external appearances.

[0013] The ignition plug 10 is a device provided in each cylinder of an internal combustion engine (not shown) for igniting fuel in the combustion chamber of the cylinder. The ignition plug 10 includes an insulator 20, a center electrode 30, a terminal metal fitting 40, a metal shell 50, and a ground electrode 60.

[0014] The insulator 20 is a cylindrical member made of an insulating material such as alumina. An axial hole 200 is formed in the insulator 20. The axial hole 200 is a through hole formed to pass through the insulator 20 along its central axis. The central axis of the axial hole 200 coincides with the central axis of the insulator 20. The central axis of the axial hole 200 is hereinafter also referred to as the "central axis CX." In a cross section of the insulator 20 cut perpendicularly to the central axis CX, the shape of the axial hole 200 is circular.

[0015] The center electrode 30 is a metal member held by the insulator 20 at a position that is one end of the axial hole 200 along the central axis CX (the lower end in FIG. 1). The center electrode 30 is a rod-shaped member, most of which is disposed inside the axial hole 200. A part of the center electrode 30 protrudes from the axial hole 200 to the outside of the insulator 20, and a discharge tip 31 is attached to the tip of the protruding part. The insulator 20 described above can be said to be a member that holds the center electrode 30 from the outer periphery.

[0016] The terminal fitting 40 is a metal member held by the insulator 20 at a position that is the other end of the axial hole 200 along the central axis CX (the upper end in FIG. 1). The terminal fitting 40 is a rod-shaped member, most of which is disposed inside the axial hole 200. A part of the terminal fitting 40 protrudes from the axial hole 200 to the outside of the insulator 20. This protruding part serves as an electrode terminal to which a voltage is applied from an external power source (not shown).

[0017] In FIG. 1, the z-direction is a direction along the central axis CX from the terminal fitting 40 toward the central electrode 30. The x-direction is perpendicular to the z-direction, and extends from the left side to the right side in FIG. 1. The y-direction is perpendicular to both the z-direction and the x-direction, and extends from the front side to the back side of the paper in FIG. 1. In the following description, the configuration of each part will be described using the z-direction, x-direction, and y-direction. The z-direction corresponds to the "first direction" in this embodiment. The x-direction corresponds to the "second direction" in this embodiment. The y-direction corresponds to the "third direction" in this embodiment.

[0018] A resistor 71 is disposed in the axial hole 200 between the terminal metal fitting 40 and the center electrode 30. The resistor 71 is a member disposed to adjust the electrical resistance of the electrical path from the terminal metal fitting 40 to the center electrode 30. The resistor 71 is formed of a material in which a predetermined amount of carbon powder is added to powdered glass and zirconia. The electrical resistance of the resistor 71 is adjusted by the amount of carbon added. By disposing the resistor 71 in the electrical path from the terminal metal fitting 40 to the center electrode 30, the generation of electromagnetic noise caused by the spark discharge of the ignition plug 10 is suppressed. The resistor 71 and the center electrode 30 are electrically connected via a conductive seal layer 72. Similarly, the terminal metal fitting 40 and the resistor 71 are electrically connected via a conductive seal layer 73. Both of the conductive seal layers 72 and 73 are layers having electrical conductivity formed of a material in which copper powder is added to powdered glass.

[0019] The metal shell 50 is a tubular member that holds a part of the insulator 20 from the outer periphery. The metal shell 50 is entirely made of metal. The metal shell 50 is fixed to the insulator 20 by crimping, and holds the insulator 20 in this state. The metal shell 50 has a fitting portion 52, a flange portion 55, and an insertion portion 56.

[0020] The fitting portion 52 is a portion that is fitted with a tool such as a plug wrench when the spark plug 10 is attached to an internal combustion engine. The shape of the fitting portion 52 when viewed along the central axis CX is, for example, hexagonal.

[0021] The flange portion 55 is a portion that comes into contact with the outer surface of the internal combustion engine via the gasket GK when the spark plug 10 is attached to the internal combustion engine. The flange portion 55 is provided at a position that is on the z-direction side of the fitting portion 52, and protrudes toward the outer periphery.

[0022] The insertion portion 56 is a portion located further toward the z-direction side than the flange portion 55, and is a portion to be inserted into an insertion hole (not shown) formed in the internal combustion engine. A male screw 561 is formed on the outer circumferential surface of the insertion portion 56. When the spark plug 10 is attached to the internal combustion engine, the fitting portion 52 rotates about the central axis CX due to the force received from a tool. As a result, a female screw formed on the inner circumferential surface of the insertion hole and the male screw 561 of the insertion portion 56 are screwed together. As a result, the spark plug 10 is fastened and fixed to the internal combustion engine. When the spark plug 10 is attached to the internal combustion engine, the potential of the metallic shell 50 becomes the same ground potential as that of the internal combustion engine.

[0023] The ground electrode 60 is a metal member formed so as to extend from the tip surface S of the metallic shell 50 on the z-direction side further toward the z-direction side. The tip surface S is a surface perpendicular to the central axis CX. The previously described x-direction (i.e., the second direction) is the direction in which the center electrode 30 and the ground electrode 60 are aligned. The ground electrode 60 is bent, and a part of it faces the discharge tip 31 of the center electrode 30 along the central axis CX. In other words, one end side of the ground electrode 60 is connected to the tip surface S of the metallic shell 50, and the other end side faces the center electrode 30. The discharge tip 61 is attached to a part of the ground electrode 60 facing the center electrode 30. As a result, the discharge tip 61 and the discharge tip 31 are aligned along the central axis CX and face each other. The gap formed between the discharge tip 61 and the discharge tip 31 facing each other is a discharge gap GP where a spark discharge occurs.

[0024] When the internal combustion engine is in operation, a pulsed high voltage is applied between the terminal metal fitting 40 of the spark plug 10 and the body of the internal combustion engine. This high voltage is applied between the opposing discharge tips 61 and 31, causing a spark discharge in the discharge gap GP. Note that either or both of the discharge tips 61 and 31 may be omitted.

[0025] The specific shape of the ground electrode 60 will be described with reference to Fig. 2. As shown in Fig. 2, the ground electrode 60 has a first straight portion 610, a bent portion 630, and a second straight portion 620.

[0026] The first straight portion 610 is the portion of the ground electrode 60 closest to the -z direction and extends linearly from the tip surface S. The direction in which the first straight portion 610 extends is generally perpendicular to the tip surface S, but does not have to be strictly perpendicular to the tip surface S. The end of the first straight portion 610 in the -z direction is welded to the tip surface S.

[0027] The bent portion 630 is a portion of the first straight portion 610 that is bent in an arc shape and extends from an end portion opposite to the tip surface S toward the center electrode 30 side (i.e., the -x direction side). The bent portion 630 is bent at approximately 90 degrees toward the center electrode 30 side, but the angle may be different from 90 degrees. In Fig. 2, the boundary between the first straight portion 610 and the bent portion 630 is indicated by a dotted line DL1.

[0028] The second straight portion 620 is a portion that extends linearly from the end of the bent portion 630 opposite to the first straight portion 610 toward the center electrode 30 (i.e., the -x direction side). The direction in which the second straight portion 620 extends is generally parallel to the tip surface S, but does not have to be strictly parallel to the tip surface S. The second straight portion 620 extends to a position directly above the center electrode 30. The discharge tip 61 described above is attached to a portion of the second straight portion 620 through which the central axis CX passes. In FIG. 2, the boundary between the bent portion 630 and the second straight portion 620 is indicated by a dotted line DL2.

[0029] When the spark plug 10 is viewed in the y direction as in Fig. 2, the outline of the portion of the first straight portion 610 on the side of the center electrode 30 is a straight line. This straight line will hereinafter be referred to as "straight line L10". Point P1 shown in Fig. 2 is the intersection of the straight line L10 and the tip end surface S. Point P2 shown in Fig. 2 is the intersection of the boundary (dotted line DL1) between the first straight portion 610 and the bent portion 630 and the straight line L10. Point P11 shown in Fig. 2 is a point indicating the center position between points P1 and P2 along the straight line L10.

[0030] When the spark plug 10 is viewed in the y direction as in Fig. 2, the outer shape of the portion of the bent portion 630 on the side of the center electrode 30 is an arc-shaped curve. Hereinafter, this curve will also be referred to as "curve L30." Point P3 shown in Fig. 2 is the intersection point of the boundary (dotted line DL2) between the second straight portion 620 and the bent portion 630 and the curve L30. Point P12 shown in Fig. 2 is a point indicating the center position between points P2 and P3 along the curve L30.

[0031] When the spark plug 10 is viewed in the y direction as in Fig. 2, the outer shape of the part of the second straight portion 620 on the side of the center electrode 30 is a straight line. Hereinafter, this straight line will also be referred to as "straight line L20." Point P4 shown in Fig. 2 is a point indicating the end of straight line L20 on the opposite side to the bent portion 630. Point P13 shown in Fig. 2 is a point indicating the center position between points P3 and P4 along straight line L20.

[0032] In other words, the straight line L10 extends from point P1 to point P2 in Figure 2, with its center at point P11. The curve L30 extends from point P2 to point P3 in Figure 2, with its center at point P12. The straight line L20 extends from point P3 to point P4 in Figure 2, with its center at point P13.

[0033] 2, the straight line labeled "L11" is an imaginary straight line extending perpendicularly to the tip surface S from point P1, which is the intersection point between the straight line L10 and the tip surface S. Hereinafter, this straight line will also be referred to as a "first straight line L11."

[0034] The straight line marked with the symbol "L21" in Fig. 2 is a virtual straight line obtained by extending the straight line L20 toward the first straight line portion 610 (i.e., the x-direction side). This straight line will also be referred to as a "second straight line L21" below. Point P5 shown in Fig. 2 is the intersection point between the first straight line L11 and the second straight line L21.

[0035] "A" shown in FIG. 2 represents the length from point P1, which is the intersection point of the first straight line L11 with the tip surface S, to point P5, which is the intersection point of the second straight line L21. Also, "B" shown in FIG. 2 represents the length from point P3 on the boundary (dotted line DL2) between the bent portion 630 and the second straight line portion 620, to point P5, which is the intersection point of the second straight line L21 with the first straight line L11. The value obtained by dividing B by A (i.e., B / A) is also referred to as the "bend ratio" below. In the ground electrode 60 of this embodiment, the dimensions of each portion are set so that the bend ratio is 0.35 or less. The reason for this will be described later.

[0036] When the spark plug 10 is viewed in the y direction as shown in FIG. 2, the angle between the straight line (i.e., straight line L10) showing the outline of the part of the first straight portion 610 on the side of the center electrode 30 and the tip surface S is also referred to as "angle θ" hereinafter. The angle θ represents the angle between the straight line L10 and the tip surface S in the range on the opposite side of the center electrode 30 as viewed from the first straight portion 610, that is, in the range to the left of the straight line L10 in FIG. 2. Therefore, as the angle θ increases, the ground electrode 60 leans in a direction approaching the center electrode 30. In the ground electrode 60 of this embodiment, the angle θ is adjusted to be 88 degrees or more and 96 degrees or less. The reason for this will be described later.

[0037] 3 shows a cross section of the ground electrode 60 taken along a plane passing through the central axis thereof. As shown in the figure, the ground electrode 60 is not entirely made of a single material, but has a first layer 641, a second layer 642, and a third layer 643 made of different materials.

[0038] The first layer 641 is the innermost layer of the ground electrode 60 and is made of pure nickel. The second layer 642 is a layer that covers the first layer 641 from the outside and is made of copper. The third layer 643 is a layer that further covers the second layer 642 from the outside and is made of a nickel-based alloy such as Inconel.

[0039] Thus, the first layer 641 made of pure nickel and the second layer 642 made of copper are provided inside the ground electrode 60. By embedding copper, a material with relatively high thermal conductivity, it is possible to suppress a rise in temperature of the ground electrode 60 when the ignition plug is in operation.

[0040] A method for forming the ground electrode 60 will be described with reference to Fig. 4 to Fig. 7. As shown in Fig. 4, immediately after the ground electrode 60 is welded to the tip surface S, the ground electrode 60 is a rod-shaped member, i.e., a member having a linear shape. At this time, the longitudinal direction of the ground electrode 60 coincides with the direction of the central axis CX (i.e., the z direction). The discharge tip 31 is attached to the ground electrode 60 in advance by welding.

[0041] The linear ground electrode 60 is bent using a support base 910 and rollers 920 into the shape shown in Fig. 2 and other figures. The support base 910 is a cylindrical member. As shown in Fig. 4, the support base 910 is in contact with the side surface of the ground electrode 60 on the side of the center electrode 30 with its central axis aligned in the y direction. At this time, the positions of the spark plug 10 including the ground electrode 60 and the support base 910 are both fixed.

[0042] The roller 920 is a cylindrical member similar to the support base 910. As shown in FIG. 4, the roller 920 is in contact with a side surface of the ground electrode 60 opposite to the center electrode 30 and a portion near the end opposite to the tip surface S with its central axis aligned along the y direction. The roller 920 is supported by a processing device (not shown) in a state in which it can rotate freely around its central axis. The processing device applies a force to the ground electrode 60 by changing the position of the roller 920, thereby deforming the ground electrode 60.

[0043] From the state of FIG. 4, the processing device moves the roller 920 in the direction of the arrow AR1. The ground electrode 60 is deformed as shown in FIG. 5 by the forces from both the support table 910 and the roller 920. The processing device further moves the roller 920 in the direction of the arrow AR2 while maintaining the roller 920 in contact with the ground electrode 60. At this time, the roller 920 rotates in the direction of the arrow AR3 due to the frictional force received from the ground electrode 60. Finally, the roller 920 performs bending processing until the ground electrode 60 reaches the state of FIG. 6. In the state of FIG. 6, the part of the ground electrode 60 that the roller 920 was in contact with is generally parallel to the tip surface S. At this time, the part in contact with the support table 910 becomes the bent portion 630 described above. In addition, the part on the tip surface S side from the bent portion 630 becomes the first straight portion 610, and the part on the opposite side becomes the second straight portion 620. 6, the discharge tip 61 and the discharge tip 31 face each other, and a discharge gap GP is formed between them. As described above, the ground electrode 60 of the spark plug 10 according to this embodiment is formed by bending a straight member.

[0044] After the bending process is completed, the discharge gap GP is adjusted by using a tool 930 such as a hammer. As shown in Fig. 7, the adjustment of the discharge gap GP is performed with the support base 910 and the rollers 920 removed. The adjustment is performed by hitting the part of the ground electrode 60 that is generally behind the discharge tip 61 with the tool 930.

[0045] However, when the ground electrode 60 is bent and the discharge gap GP is adjusted by the above-mentioned method, distortion occurs in the ground electrode 60, and stress remains in a part of the ground electrode 60. The inventors have found a new finding that when a large thermal load is applied to the ignition plug 10 during the operation of the internal combustion engine and the temperature of the ground electrode 60 increases, the ground electrode 60 is deformed due to the above-mentioned residual stress. It is considered that such deformation of the ground electrode 60 occurs due to a decrease in hardness of the ground electrode 60 that has become hot. The deformation of the ground electrode 60 due to the thermal load is different from the deformation due to vibration in terms of its direction, etc. Specifically, the deformation due to vibration occurs in the direction in which the ground electrode 60 moves away from the center electrode 30 (i.e., in the direction in which the angle θ in FIG. 2 decreases), whereas the deformation due to the thermal load occurs in the direction in which the ground electrode 60 approaches the center electrode 30 (i.e., in the direction in which the angle θ in FIG. 2 increases).

[0046] In recent vehicles, internal combustion engines have become smaller and more powerful, which increases the thermal load applied to the spark plug 10. For this reason, if the ground electrode 60 had a conventional shape, it is believed that the deformation of the ground electrode 60 due to the thermal load would become significant and cannot be ignored.

[0047] Therefore, in the spark plug 10 according to this embodiment, deformation due to the thermal load is suppressed by improving the shape of the ground electrode 60. Specifically, the above-mentioned bending ratio (B / A) is set to 0.35 or less, etc., thereby suppressing deformation of the ground electrode 60 due to the thermal load.

[0048] 8 is a graph showing the relationship between the bending ratio (horizontal axis) and the amount of displacement (vertical axis) for ground electrodes 60 of various shapes. The amount of displacement is an index showing the degree of deformation occurring in the discharge gap GP before and after application of a thermal load.

[0049] Point P21 shown in Fig. 2 indicates the point through which the central axis CX passes on the surface of the ground electrode 60 opposite the discharge tip 61. The above-mentioned displacement amount is the amount of change in the distance from point P21 in Fig. 2 to the tip surface S, and in Fig. 8, each data is plotted with the direction in which the distance decreases being positive. Note that the above-mentioned displacement amount is used as an index showing the degree to which the discharge gap GP has changed, rather than the amount of change in the discharge gap GP itself, in order to eliminate the influence of wear of the discharge tip 61 and the discharge tip 31.

[0050] To obtain the data plotted in Fig. 8, spark plugs 10 having ground electrodes 60 of the same shape were attached to each cylinder of a four-cylinder internal combustion engine. The discharge gap GP before application of a thermal load was adjusted to 0.6 mm for all of the spark plugs 10. The nominal diameter of the male screw 561 of each spark plug 10 was M12.

[0051] Thereafter, the internal combustion engine was operated for 1000 hours to simultaneously apply a thermal load to the four spark plugs 10. The temperature of the ground electrode 60 during operation was adjusted to be an actual measured value of 950°C ± 50°C. Each piece of data plotted in Fig. 8 is the average value of the above-mentioned displacement amount measured for each of the four spark plugs 10.

[0052] As shown in Fig. 8, when the bending ratio B / A is in the range of 0.35 or less, the measured displacement is approximately 0. On the other hand, when the B / A exceeds 0.35, the displacement increases sharply. In other words, it was confirmed that the ground electrode 60 deforms in the direction in which the discharge gap GP decreases.

[0053] In this manner, in the spark plug 10 according to this embodiment, by adjusting the dimensions of each portion of the ground electrode 60 so that the B / A value is 0.35 or less, it is possible to suppress deformation of the ground electrode 60 when it becomes hot.

[0054] One of the reasons why the deformation of the ground electrode 60 is suppressed when the value of B / A is 0.35 or less is that the radius of curvature of the bent portion 630 becomes small, and large work hardening occurs in the portion. When large work hardening occurs in the bent portion 630, the hardness of the bent portion 630 increases to a degree that does not cause deformation even when a thermal load is applied. In other words, as a result of large work hardening occurring in the bent portion 630, the limit temperature at which deformation occurs in the ground electrode 60 increases, and the limit temperature becomes higher than the temperature during operation of the internal combustion engine. As a result, it is considered that the deformation of the ground electrode 60 when a thermal load is applied is suppressed.

[0055] In contrast, when the radius of curvature of the portion corresponding to the bent portion 630 is large as in the comparative example of Fig. 12, the hardness does not increase sufficiently due to work hardening. In this case, the hardness of the entire ground electrode 60 is low, so that in the conventional shape, deformation occurs when a thermal load is applied.

[0056] In the spark plug 10 according to this embodiment, as described above, the hardness of the bent portion 630 is increased by work hardening. Therefore, when the adjustment work of the discharge gap GP shown in Fig. 7 is performed, the bent portion 630 is hardly deformed, and the ground electrode 60 is deformed at the second straight portion 620 located further toward the tip side than the bent portion 630, thereby changing the discharge gap GP.

[0057] Another reason why the deformation of the ground electrode 60 is suppressed when the value of B / A is 0.35 or less is that the angle θ, which is the inclination angle of the first straight portion 610, is close to the initial 90 degrees, that is, the amount of deformation at the base of the ground electrode 60 is within a small range. If the angle θ is significantly changed from the initial 90 degrees, distortion and residual stress are generated at the base of the ground electrode 60, and the residual stress causes deformation such that the ground electrode 60 collapses toward the center electrode 30 at high temperatures. In contrast, if the value of B / A is 0.35 or less as in this embodiment, the residual stress is reduced, and the deformation of the ground electrode 60 caused by the residual stress can be suppressed.

[0058] The present inventors have also confirmed the relationship between the radius of curvature of the bent portion 630 and the above-mentioned amount of displacement through experiments. Fig. 9 is a graph showing the relationship between the radius of curvature of the bent portion 630 (horizontal axis) and the above-mentioned amount of displacement (vertical axis) for ground electrodes 60 of various shapes. The above-mentioned radius of curvature specifically refers to the radius of curvature of the curve L30 (see Fig. 2) when viewed along the y direction.

[0059] The ground electrodes 60 corresponding to each data in Fig. 9 have different values ​​of the radius of curvature, but other dimensions are generally the same. Specifically, for all the ground electrodes 60, the length of A in Fig. 2 is standardized to 7 mm, and the angle θ is standardized to 92 degrees.

[0060] As shown in Fig. 9, when the radius of curvature of the curve L30 is in the range of 1.5 mm or less, the measured displacement amount is approximately 0. On the other hand, when the radius of curvature exceeds 1.5 mm, it is confirmed that the value of the displacement amount increases rapidly. In other words, it is confirmed that the ground electrode 60 is deformed in the direction in which the discharge gap GP decreases.

[0061] From the above results, it was confirmed that the deformation of the ground electrode 60 at high temperatures can be further suppressed by setting the value of B / A to 0.35 or less and the radius of curvature of the curve L30 to 1.5 mm or less. It has been confirmed that if the radius of curvature of the curve L30 is smaller than 0.5 mm, the deformation of the bent portion 630 becomes too steep, and cracks are generated in the ground electrode 60 during bending or the like. For this reason, it is preferable that the radius of curvature of the curve L30 is within a range of 0.5 mm or more and 1.5 mm or less.

[0062] The inventors have also confirmed the relationship between the angle θ and the above-mentioned amount of displacement through experiments. Fig. 10 is a graph showing the relationship between the angle θ (horizontal axis) and the above-mentioned amount of displacement (vertical axis) for ground electrodes 60 of various shapes.

[0063] The ground electrodes 60 corresponding to each data in Fig. 10 are different from each other in the value of the angle θ, but other dimensions are generally the same. Specifically, for all the ground electrodes 60, the length of A in Fig. 2 is standardized to 7 mm, and the radius of curvature of the curve L30 is standardized to 1.0 mm.

[0064] As shown in Fig. 9, when the angle θ is in the range of 96 degrees or less, the measured displacement amount is approximately 0. On the other hand, it was confirmed that the value of the displacement amount increases rapidly when the angle θ exceeds 96 degrees. In other words, it was confirmed that the ground electrode 60 is deformed in the direction in which the discharge gap GP decreases.

[0065] From the above results, it was confirmed that the deformation of the ground electrode 60 at high temperatures can be further suppressed by setting the value of B / A to 0.35 or less and the angle θ to 96 degrees or less. If the angle θ is set to less than 88 degrees, the ground electrode 60 will be tilted too far away from the center electrode 30, and a part of it will protrude outward. As a result, when the ignition plug 10 is attached to the internal combustion engine, the ground electrode 60 may interfere with the edge of the insertion hole. For this reason, it is preferable that the angle θ is set to a range of 88 degrees or more and 96 degrees or less.

[0066] Fig. 11 shows the results of measuring the Vickers hardness at various portions (points P1, P11, P12, and P13 in Fig. 2) of the ground electrode 60. Note that Fig. 11 also shows the results of measuring the Vickers hardness of the ground electrode 60 of the spark plug 10 according to this embodiment, as well as the results of measuring the Vickers hardness of the ground electrode 60 of the spark plug 10A according to the comparative example in Fig. 12.

[0067] The spark plug 10A according to the comparative example of FIG. 12 differs from this embodiment only in the shape of the ground electrode 60. In this spark plug 10A, the radius of curvature of the portion corresponding to the bent portion 630 is larger than that of this embodiment, and the value of B / A is significantly greater than 0.35. In FIG. 12, points P1, P11, P12, and P13 are assigned to the points of the ground electrode 60 corresponding to the points P1, P11, P12, and P13 in FIG. 2, respectively. In each of FIG. 2 and FIG. 12, the lengths from the position of point P1 to each point along the outer shape of the portion of the ground electrode 60 on the center electrode 30 side are approximately equal to each other.

[0068] In the spark plug 10 according to this embodiment, point P1 represents the position of the first straight portion 610 closest to the tip face S and closest to the center electrode 30. The Vickers hardness at point P1 corresponds to the “first hardness” in this embodiment.

[0069] In the spark plug 10 according to this embodiment, the point P11 is the center position along the longitudinal direction of the first straight portion 610, and represents a position on the side of the center electrode 30. The Vickers hardness at the point P11 corresponds to the “second hardness” in this embodiment.

[0070] In the spark plug 10 according to this embodiment, the point P12 is a central position of the bent portion 630 from the first straight portion 610 to the second straight portion 620, and represents a position on the side of the center electrode 30. The Vickers hardness at the point P12 corresponds to the “third hardness” in this embodiment.

[0071] In the spark plug 10 according to this embodiment, the point P13 is the center position along the longitudinal direction of the second straight portion 620, and represents a point on the side of the center electrode 30. The Vickers hardness at the point P13 corresponds to the “fourth hardness” in this embodiment.

[0072] 11, in this embodiment, the first hardness at point P1 and the fourth hardness at point P13 are each within a range of 90% to 110% of the second hardness at point P11. In other words, the Vickers hardness at each of points P1, P11, and P13 is within a relatively narrow range. On the other hand, the third hardness at the remaining point P12 is relatively large and is within a range of 130% to 150% of the second hardness.

[0073] Thus, in the ground electrode 60 of the spark plug 10 according to this embodiment, the Vickers hardness increases significantly locally at the bent portion 630 where bending has been performed (point P12), while the Vickers hardness at other portions (points P1, P11, P13) hardly increases from the initial value. As a result, when the ground electrode 60 becomes hot, no deformation occurs at point P12 because work hardening is sufficient, and no deformation occurs at other points because the residual stress itself is small.

[0074] In contrast, in the ground electrode 60 of the spark plug 10A according to the comparative example, the Vickers hardness increases gradually at point P12, and also increases slightly at points P11 and P13 around point P12. Therefore, when the ground electrode 60 becomes hot, deformation occurs at point P12 due to residual stress due to insufficient work hardening, and deformation also occurs at the surrounding points due to residual stress.

[0075] Here, the value obtained by dividing the third hardness by the second hardness and expressed in units of % is defined as the "hardness ratio". The hardness ratio is an index showing how much the Vickers hardness at point P12, where the effect of work hardening is greatest, is higher than the initial Vickers hardness at point P11, where the effect of work hardening is greatest. As described above, in this embodiment, the hardness ratio falls within the range of 130% to 150%.

[0076] Fig. 13 is a graph showing the relationship between the hardness ratio (horizontal axis) and the above-mentioned displacement (vertical axis) for ground electrodes 60 of various shapes. As shown in Fig. 13, when the hardness ratio is in the range of 130% or more, the displacement is approximately 0. On the other hand, it was confirmed that when the hardness ratio becomes small and falls below 130%, as in the comparative example, the displacement becomes large to a degree that cannot be ignored.

[0077] When the hardness ratio exceeded 150, the shape of the bent portion 630 became too steep, causing cracks to occur in parts of the ground electrode 60. In Fig. 13, the experimental results in which cracks occurred in this way are indicated by "X".

[0078] For the above reasons, it is preferable that the hardness ratio falls within the range of 130% to 150%.

[0079] The above-mentioned shape of the ground electrode 60 can be applied to the ground electrode 60 having a plurality of layers as in the present embodiment, as well as to the ground electrode 60 formed entirely from a single material. However, when the second layer 642 made of copper and the like are present inside the ground electrode 60 as in the present embodiment, the thickness of the portion (specifically, the third layer 643) whose hardness increases by bending is small, and it is difficult to cause sufficient work hardening to the extent that deformation does not occur at high temperatures. For this reason, it is particularly necessary to adopt a shape in which the value of B / A is 0.35 or less as in the present embodiment.

[0080] The present embodiment has been described above with reference to specific examples. However, the present disclosure is not limited to these specific examples. Any design modifications made by a person skilled in the art to these specific examples are also included within the scope of the present disclosure as long as they have the features of the present disclosure. The elements of each of the above-mentioned specific examples and their arrangements, conditions, shapes, etc. are not limited to those exemplified and can be changed as appropriate. The combination of each of the elements of each of the above-mentioned specific examples can be changed as appropriate as long as no technical contradiction occurs. [Explanation of symbols]

[0081] 10: Spark plug 20: Insulator 30: Center electrode 50: Main metal fitting 60: Ground electrode 610: 1st straight section 620: 2nd straight section 630: Bend

Claims

1. A central electrode (30); an insulator (20) that holds the center electrode from an outer periphery side; a metal shell (50) that holds the insulator from an outer periphery side; a ground electrode (60) having one end connected to the tip end surface (S) of the metallic shell and the other end facing the center electrode, The ground electrode is A first straight portion (610) that extends straight from the tip surface; a bent portion (630) that is a portion that is bent and extends from an end portion of the first straight portion opposite to the tip surface toward the center electrode side; and a second straight portion (620) which is a portion extending in a straight line from an end of the bent portion opposite to the first straight portion toward the center electrode side, When viewed along a third direction perpendicular to both a first direction along the central axis of the center electrode and a second direction in which the center electrode and the ground electrode are aligned, a first straight line (L11) is a virtual straight line extending perpendicularly to the tip surface from an intersection point between the tip surface and a straight line indicating an outline of the portion of the first straight line portion on the center electrode side, a virtual straight line obtained by extending a straight line showing an outline of a portion of the second straight line portion on the side of the center electrode toward the first straight line portion is defined as a second straight line (L21); A length of the first straight line from an intersection point with the tip surface to an intersection point with the second straight line is defined as A, When the length of the second straight line from the boundary between the bent portion and the second straight line portion to the intersection with the first straight line is B, The value of B / A is 0.35 or less, a Vickers hardness at a position of the first straight portion closest to the tip surface side and at a position on the center electrode side is defined as a first hardness; a Vickers hardness at a center position of the first straight portion along a longitudinal direction thereof and at a position on the side of the center electrode is defined as a second hardness; a Vickers hardness at a position of the bent portion that is a center between the first straight portion and the second straight portion and is on the side of the center electrode is defined as a third hardness; When a Vickers hardness at a center position of the second straight portion along a longitudinal direction thereof and at a position on the side of the center electrode is defined as a fourth hardness, each of the first hardness and the fourth hardness is within a range of 90% to 110% of the second hardness; The third hardness is within the range of 130% to 150% of the second hardness.

2. When viewed along the third direction, 2. The spark plug according to claim 1, wherein a radius of curvature of a curve that defines an outline of a portion of said bent portion on said center electrode side is not less than 0.5 mm and not more than 1.5 mm.

3. When viewed along the third direction, 3. The spark plug according to claim 1, wherein an angle (θ) between a straight line that defines an outline of a portion of said first straight portion on the center electrode side and said tip surface, said angle (θ) being in a range opposite to said center electrode as viewed from said first straight portion, is equal to or greater than 88 degrees and is equal to or smaller than 96 degrees.

4. 4. The spark plug according to claim 1, wherein said ground electrode is formed by bending a straight member.

5. The inside of the ground electrode is A first layer (641) made of nickel; 5. The spark plug according to claim 1, further comprising a second layer (642) which is a layer covering the first layer from the outside and is made of copper.

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