Spark plug

Abstract

To provide a spark plug that can extend time until a fusion part is exposed.SOLUTION: In a cross section of an electrode including an axial line and a center of gravity of a discharge surface of a chip in a spark plug, when the length in a direction orthogonal to the axial line of the discharge surface is D, the length of a line obtained by cutting the axial line by a border of the chip is L1, the length of a line whose distance from the axial line is D / 4 and that is obtained by cutting a first line parallel to the axial line by the border of the chip is L2, and the length of a line whose distance from the axial line is 3D / 8 and that is obtained by cutting a second line parallel to the axial line by the border of the chip is L3, the relation of L1<L2<L3 is satisfied.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a spark plug having an electrode that includes a tip. [Background technology]

[0002] In an electrode including a tip that is joined to a base material via a fusion zone, an electric field tends to concentrate at the edge of the discharge surface of the tip, and discharges tend to occur at the edge of the tip, so the edge of the tip is easily worn away by the discharge. In the prior art disclosed in Patent Document 1, as the tip wears out, the fusion zone near the edge of the tip becomes exposed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-119818 Summary of the Invention [Problem to be solved by the invention]

[0004] Because the rate at which the molten part wears away due to discharge is faster than the rate at which the tip wears away, there is a need for technology that can extend the time until the molten part becomes exposed in order to reduce the frequency of spark plug replacement.

[0005] The present invention has been made to meet this demand, and has as its object to provide a spark plug that can extend the time until the molten portion is exposed. [Means for solving the problem]

[0006] A first aspect of the present invention for achieving this object includes a first electrode including a base material and a chip joined to the base material via a molten part, and a second electrode facing the discharge surface of the chip in the axial direction. In a cross-section of the first electrode including the center of gravity of the discharge surface and the axis, when the length in the direction orthogonal to the axis of the discharge surface is D, a first straight line having a distance of D / 4 from the axis and parallel to the axis, a second straight line having a distance of 3D / 8 from the axis and parallel to the axis, and a third straight line located on the opposite side of the base material with respect to the center of gravity of the discharge surface and orthogonal to the axis are drawn. Let the length of the line segment formed by cutting the axis by the boundary of the chip on the side other than the discharge surface and the third straight line be L1, the length of the line segment formed by cutting the first straight line by the boundary of the chip on the side other than the discharge surface and the third straight line be L2, and the length of the line segment formed by cutting the second straight line by the boundary of the chip on the side other than the discharge surface and the third straight line be L3. Then, in at least one of the two regions of the adjacent cross-sections with the axis as the boundary, the relationship L1 < L2 < L3 is satisfied. <00000,35> A second aspect is that in the first aspect, in the cross-section, the molten part is continuous from one end to the other end of the chip across two regions.

[0008] A third aspect is that in the first or second aspect, in the cross-section, when the length of the line segment formed by cutting the axis by the boundary between the molten part and the base material and the third straight line is L4, and the length of the line segment formed by cutting the second straight line by the boundary between the molten part and the base material and the third straight line is L5, in at least one region, the relationship L4 < L5 is satisfied.

[0009] A fourth aspect is that in the third aspect, in the two regions, the relationship L4 < L5 is satisfied.

[0010] A fifth aspect is that in any of the first to fourth aspects, the length obtained by subtracting L1 from L3 is 0.05 mm or more.

[0011] A sixth aspect is that in any of the first to fourth aspects, the length obtained by subtracting L1 from L3 is 0.08 mm or more.

Advantages of the Invention

[0012] According to the present invention, since the length L1 of the chip on the axis of the discharge surface with length D, the length L2 of the chip at a distance D / 4 from the axis, and the length L3 of the chip at a distance 3D / 8 from the axis satisfy the relationship L1 < L2 < L3, the distance between the end of the discharge surface and the melting part can be made longer than the distance between the center of the discharge surface of the chip and the melting part. Therefore, while ensuring the bonding strength near the center of the chip, the time until the melting part is exposed can be extended.

Brief Description of the Drawings

[0013] [Figure 1] It is a partial cross-sectional view of one side of the spark plug in the first embodiment. [Figure 2] It is a cross-sectional view of the spark plug. [Figure 3] It is a cross-sectional view of the spark plug in the second embodiment.

Modes for Carrying Out the Invention

[0014] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a partial cross-sectional view combining an external view and a full cross-sectional view with the axis O of the spark plug 10 in the first embodiment as a boundary. In FIG. 1, the lower side of the paper is the rear end side of the spark plug 10, and the upper side of the paper is the front end side of the spark plug 10 (the same applies to FIGS. 2 and 3).

[0015] As shown in FIG. 1, the spark plug 10 includes an insulator 11, a center electrode 13 disposed at the center of the insulator 11, a main metal fitting 15 disposed on the outer periphery of the insulator 11, and a ground electrode 16 connected to the main metal fitting 15. The insulator 11 is a cylindrical member made of ceramic such as alumina, which is excellent in mechanical properties and insulation properties at high temperatures. The insulator 11 is provided with an axial hole 12 extending along the axis O.

[0016] The center electrode 13 (first electrode) is a rod-shaped conductor disposed in the axial hole 12 of the insulator 11 and extending along the axis O. The tip of the center electrode 13 protrudes from the tip of the insulator 11 along the axis O. The center electrode 13 is electrically connected to a terminal fitting 14 in the axial hole 12.

[0017] The terminal fitting 14 is a rod-shaped member to which an ignition device (not shown) is connected, and is made of a conductive metal material (such as low-carbon steel). The terminal fitting 14 is fixed to the rear end of the insulator 11 with its front end inserted into the axial hole 12 and its rear end protruding from the insulator 11.

[0018] A metal shell 15 is fixed to the outer periphery of the insulator 11. The metal shell 15 is a cylindrical member made of a metal material (for example, low carbon steel). A ground electrode 16 (second electrode) is connected to the tip of the metal shell 15.

[0019] The ground electrode 16 is a conductor extending from the metallic shell 15 toward the axis O. The ground electrode 16 is, for example, made of a metal mainly composed of Ni with a core material mainly composed of copper embedded therein. The core material may be omitted. In this embodiment, the ground electrode 16 is bent from the metallic shell 15 toward the center electrode 13.

[0020] 2 is a cross-sectional view of the spark plug 10 including the axis O, showing an enlarged view of the tip portion of the center electrode 13. The cross section of the center electrode 13 is divided into a first region 17 and a second region 18 adjacent to each other with the axis O as the boundary. The center electrode 13 includes a base material 19, a fusion zone 20, and a tip 21. The rear end side of the base material 19 is not shown in FIG. 2.

[0021] A copper-based core (not shown) is embedded in the base material 19 to improve thermal conductivity. The core may be omitted. An example of the material for the base material 19 is a Ni-based alloy. A tip 21 is joined to the base material 19 via a fusion zone 20. An example of the material for the tip 21 is a material containing at least one precious metal element, such as Pt, Ir, Ru, or Rh. The fusion zone 20 is formed by melting the base material 19 and the tip 21. The fusion zone 20 is formed, for example, by laser welding. The boundary 22 between the base material 19 and the fusion zone 20 is curved toward the tip at its center, and the boundary 23 between the fusion zone 20 and the tip 21 is also curved toward the tip at its center. The fusion zone 20 is continuous across the axis O across the first region 17 and the second region 18.

[0022] The discharge surface 24 of the tip 21 faces the ground electrode 16 (see Figure 1) in the direction of axis O. A spark gap is formed between the discharge surface 24 of the tip 21 and the ground electrode 16. Spark discharge between the center electrode 13 and the ground electrode 16 mainly occurs in the spark gap. The shape of the discharge surface 24 can be a circle, an ellipse, a polygon, etc., but there is no limitation on the shape. The cross-sectional view of Figure 2 includes the center of gravity 25 of the discharge surface 24 and the axis O. The center of gravity 25 is the geometric center of the discharge surface 24.

[0023] The center electrode 13, which includes the tip 21 joined to the base material 19 via the fusion zone 20, is prone to electric field concentration at the edge of the discharge surface 24 of the tip 21 due to the potential difference between the center electrode 13 and the ground electrode 16. Because discharges are likely to occur at the edge portions of the discharge surface 24 where the electric field concentrates, the edge portions of the discharge surface 24 are more likely to be worn away by the discharge than the center of the discharge surface 24. As the edge portions of the discharge surface 24 are worn away by the discharge, the fusion zone 20 near the edge of the discharge surface 24 becomes exposed.

[0024] Since the rate at which the molten part 20 is consumed by discharge is faster than the rate at which the chip 21 is consumed, the consumption of the molten part 20 progresses from the exposed part. When the molten part 20 that joins the base material 19 and the chip 21 is consumed from the edge and the periphery of the molten part 20 sinks, and the molten part 20 becomes thinner than the chip 21, there is a risk of causing the chip 21 to fall off. In order to reduce the replacement frequency of the spark plug 10, it is preferable that the time until the molten part 20 is exposed is longer.

[0025] In order to increase the time until the molten part 20 is exposed, the spark plug 10 is characterized by the length in the direction of the axis O of each part of the chip 21. In order to specify the length in the direction of the axis O of each part of the chip 21, a plurality of auxiliary lines are drawn on the cross section of the chip 21. First, the length in the direction orthogonal to the axis O of the discharge surface 24 is defined as D, and in the first region 17, a first straight line 26 having a distance of D / 4 from the axis O and parallel to the axis O, and a second straight line 28 having a distance of 3D / 8 from the axis O and parallel to the axis O are drawn. In the second region 18 as well, a first straight line 27 having a distance of D / 4 from the axis O and parallel to the axis O, and a second straight line 29 having a distance of 3D / 8 from the axis O and parallel to the axis O are drawn.

[0026] Next, a third straight line 30 is drawn on the opposite side of the base material 19 with respect to the centroid 25 of the discharge surface 24 and orthogonal to the axis O. The third straight line 30 is a straight line that serves as the starting point of the length in the direction of the axis O of each part of the chip 21, and may be any straight line orthogonal to the axis O as long as it does not intersect the chip 21. The distance between the third straight line 30 and the discharge surface 24 is arbitrary. When the discharge surface 24 is orthogonal to the axis O, the third straight line 30 may be a straight line including the discharge surface 24 (line segment).

[0027] In the first region 17, when the length of the line segment 31 formed by the boundary 23 of the chip 21 and the third straight line 30 cutting the axis O is defined as L1, the length of the line segment 32 formed by the boundary 23 of the chip 21 and the third straight line 30 cutting the first straight line 26 is defined as L2, and the length of the line segment 33 formed by the boundary 23 of the chip 21 and the third straight line 30 cutting the second straight line 28 is defined as L3, the relationship L1 < L2 < L3 is satisfied.

[0028] In the second region 18 as well, when the length of the line segment 31 formed by the axis O being cut off by the boundary 23 of the chip 21 and the third straight line 30 is defined as L1, the length of the line segment 34 formed by the first straight line 27 being cut off by the boundary 23 of the chip 21 and the third straight line 30 is defined as L2, and the length of the line segment 35 formed by the second straight line 29 being cut off by the boundary 23 of the chip 21 and the third straight line 30 is defined as L3, the relationship L1 < L2 < L3 is satisfied.

[0029] Repeated discharges occur at the end portion of the discharge surface 24 in the first region 17. Even if the consumption of the end portion of the discharge surface 24 progresses, since L3 is longer than L1 and L2, the time until the melted portion 20 is exposed can be lengthened. Also, repeated discharges occur at the end portion of the discharge surface 24 in the second region 18. Even if the consumption of the end portion of the discharge surface 24 progresses, since L3 is longer than L1 and L2, the time until the melted portion 20 is exposed can be lengthened. When the time until the melted portion 20 is exposed is about the same as that of the conventional product, since the relationship L1 < L2 < L3 is satisfied in the first region 17 and the second region 18, the volume of the chip 21 can be made smaller than that of the conventional product. That is, the amount of material required for manufacturing the chip 21 can be reduced.

[0030] Since the melted portion 20 is continuous across the first region 17 and the second region 18, the bonding area of the chip 21 by the melted portion 20 can be made larger than when the melted portion is intermittent in the first region 17 or the second region 18. Therefore, even if the melted portion 20 exposed by the discharge is consumed, the bonding of the chip 21 by the melted portion 20 can be ensured over a long period of time.

[0031] In the first region 17, when the length of the line segment 36 formed by cutting the axis O by the boundary 22 of the base material 19 and the third straight line 30 is defined as L4, and the length of the line segment 37 formed by cutting the second straight line 28 by the boundary 22 of the base material 19 and the third straight line 30 is defined as L5, the relationship L4 < L5 is satisfied. Since the relationships L1 < L2 < L3 and L4 < L5 are satisfied, the thickness of the molten part 20 can be ensured throughout the first region 17. Since the effect of the molten part 20 buffering the thermal stress of the chip 21 can be increased, the breakage of the molten part 20 caused by the thermal stress can be reduced.

[0032] In the second region 18 as well, when the length of the line segment 36 formed by cutting the axis O by the boundary 22 of the base material 19 and the third straight line 30 is defined as L4, and the length of the line segment 38 formed by cutting the second straight line 29 by the boundary 22 of the base material 19 and the third straight line 30 is defined as L5, the relationships L1 < L2 < L3 and L4 < L5 are satisfied. Since the thickness of the molten part 20 can be ensured throughout the first region 17 and the second region 18, the effect of the molten part 20 reducing the thermal stress of the chip 21 becomes even greater. Therefore, the breakage of the molten part 20 caused by the thermal stress can be further reduced.

[0033] The second embodiment will be described with reference to FIG. 3. In the first embodiment, the case where the molten part 20 is continuous across the first region 17 and the second region 18 was described. In contrast, in the second embodiment, a center electrode 50 including intermittent molten parts 51 in the first region 17 and the second region 18 will be described. The center electrode 50 is arranged in place of the center electrode 13 of the spark plug 10 described in the first embodiment. Parts that are the same as those described in the first embodiment are denoted by the same reference numerals, and the following description thereof is omitted.

[0034] FIG. 3 is a cross-sectional view of a spark plug in the third embodiment including the axis O with the tip of the center electrode 50 enlarged. The center electrode 50 includes a base material 19, a molten part 51, and a chip 21. The molten part 51 is formed by melting the base material 19 and the chip 21. The molten part 51 is provided in the first region 17 and the second region 18, respectively. Since the molten part 51 is separated with the axis O as a boundary, the boundary 52 between the chip 21 and the base material 19 appears.

[0035] In the first region 17, when the length of the line segment 31 formed by the axis O being cut off by the boundary 52 of the chip 21 and the third straight line 30 is L1, the length of the line segment 32 formed by the first straight line 26 being cut off by the boundary 52 of the chip 21 and the third straight line 30 is L2, and the length of the line segment 33 formed by the second straight line 28 being cut off by the boundary 52 of the chip 21 and the third straight line 30 is L3, the relationship L1 < L2 < L3 is satisfied.

[0036] In the second region 18 as well, when the length of the line segment 31 formed by the axis O being cut off by the boundary 52 of the chip 21 and the third straight line 30 is L1, the length of the line segment 34 formed by the first straight line 27 being cut off by the boundary 52 of the chip 21 and the third straight line 30 is L2, and the length of the line segment 35 formed by the second straight line 29 being cut off by the boundary 52 of the chip 21 and the third straight line 30 is L3, the relationship L1 < L2 < L3 is satisfied.

[0037] Repeated discharges occur at the end portion of the discharge surface 24 in the first region 17. Even if the consumption of the end portion of the discharge surface 24 progresses, since L3 is longer than L1 and L2, the time until the molten portion 51 is exposed can be lengthened. Also, repeated discharges occur at the end portion of the discharge surface 24 in the second region 18. Even if the consumption of the end portion of the discharge surface 24 progresses, since L3 is longer than L1 and L2, the time until the molten portion 51 is exposed can be lengthened. When the time until the molten portion 51 is exposed is about the same as that of the conventional product, since the relationship L1 < L2 < L3 is satisfied in the first region 17 and the second region 18, the volume of the chip 21 can be made smaller than that of the conventional product.

Example

[0040] The tester attached the samples to an engine (model CAT3526A) and conducted a test in which the engine was operated for 2000 hours with the intake throttle valve fully open. The test time of 2000 hours was determined based on experience as the time when the tip 21 was moderately worn without the melting part 20 being exposed.

[0041] After the test, the samples were removed from the engine, a cross-section including the center of gravity of the discharge surface of the tip 21 of the center electrode 13 of the sample and the axis O was fabricated, and the cross-section was observed with a scanning electron microscope (SEM) to obtain a SEM image of the cross-section. According to the SEM image, the entire discharge surface 24 of the tip 21 was worn, but there was no sample in which the tip 21 was worn and the melting part 20 was exposed.

[0042] The axis O was drawn on the SEM image, and after drawing the second straight line 28 in the first region 17 partitioned by the axis O, the length M1 (mm) of the line segment formed by the axis O being cut off by the boundary 23 between the discharge surface of the tip 21 after the test and the tip 21, and the length M2 (mm) of the line segment formed by the second straight line 28 being cut off by the boundary 23 between the discharge surface of the tip 21 after the test and the tip 21 were measured. Further, a third straight line 30 orthogonal to the axis O was drawn on the SEM image, and the length L1 (mm) of the line segment 31 formed by the axis O being cut off by the boundary 23 between the third straight line 30 and the tip 21, and the length L3 (mm) of the line segment 33 formed by the second straight line 28 being cut off by the boundary 23 between the third straight line 30 and the tip 21 were measured, and L3 (mm) - L1 (mm) for each sample was calculated.

[0043] M1 is the thickness of the remaining part at the center of the chip 21, and M2 is the thickness of the remaining part at the periphery of the chip 21. If M2 > M1, it means that more of the periphery of the chip 21 remains compared to the center. Therefore, if the test is continued, it is estimated that the center of the chip 21 will be consumed and the center of the melting part 20 will be exposed before the end of the chip 21 is consumed and the end of the melting part 20 is exposed. On the other hand, if M2 < M1, it means that the remaining amount at the end of the chip 21 is less than the remaining amount at the center of the chip 21. Therefore, if the test is continued, it is estimated that the end of the chip 21 will be consumed and the end of the melting part 20 will be exposed before the center of the chip 21 is consumed and the center of the melting part 20 is exposed.

[0044] If the end of the melting part 20 is exposed before the center of the melting part 20, the melting part 20 is likely to be consumed from the end and become thinner than the chip 21, so the chip 21 is likely to fall off. Therefore, considering the variation in the consumption amount due to the discharge of the chip 21, among the samples (n = 5) with different L3 - L1, all those with M2 > M1 were determined as A, those with 3 ≤ n ≤ 4 and M2 > M1 were determined as B, and those with n ≤ 2 and M2 > M1 were determined as C. There was no sample with all M2 < M1. The length (L3 - L1) obtained by subtracting L1(mm) from L3(mm) of the sample and the determination are shown in Table 1.

[0045]

Table 1

[0046] According to Table 1, for sample Nos. 1 - 7 where L3 - L1 was 0.05 mm or more, the determination was A or B. If L3 - L1 was 0.05 mm or more, it was confirmed that the periphery of the chip 21 of the majority of the samples remained more than the center. Therefore, it was estimated that the time until the melting part 20 was exposed could be made sufficiently long without expanding the volume of the chip 21.

[0047] For Samples No. 1 - 4 where L3 - L1 was 0.08 mm or more, the determination was A. If L3 - L1 was 0.08 mm or more, it was confirmed that more of the periphery of Chip 21 of all the samples remained than at the center. Therefore, it was estimated that the time until the melting part 20 was exposed could be further lengthened without increasing the volume of Chip 21.

[0048] As described above, the present invention has been explained 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 spirit of the present invention.

[0049] In the embodiment, the lengths from L1 to L5 were explained by exemplifying the center electrodes 13 and 50, but it is not necessarily limited to this. When a chip is joined to the base material of the ground electrode 16 via a melting part, similar to the center electrodes 13 and 50 described in the embodiment, L1 to L5 can be set for the chip and the melting part of the ground electrode 16 so as to satisfy the relationship of L1 < L2 < L3 or the relationship of L4 < L5. Since discharge occurs between the center electrodes 13 and 50 and the ground electrode 16, similar to the center electrodes 13 and 50, the time until the melting part is exposed can be lengthened for the ground electrode 16 as well.

[0050] In the embodiment, the case where the relationship of L1 < L2 < L3 is satisfied in both the first region 17 and the second region 18 has been explained, but it is not necessarily limited to this. It is sufficient if the relationship of L1 < L2 < L3 is satisfied in either the first region 17 or the second region 18. This is because the time until the melting part is exposed can be lengthened in the region where the relationship of L1 < L2 < L3 is satisfied.

[0051] In the first embodiment, the case where the relationship of L4 < L5 is satisfied in both the first region 17 and the second region 18 has been explained, but it is not necessarily limited to this. It is sufficient if the relationship of L4 < L5 is satisfied in either the first region 17 or the second region 18. This is because the effect of the melting part buffering thermal stress can be enhanced in the region where the relationship of L4 < L5 is satisfied.

[0052] In the embodiment, the ground electrode 16 is bent, but this is not necessarily limited to this. It is of course possible to arrange a linear ground electrode 16 at the tip of the metallic shell 15 so that the ground electrode 16 faces the discharge surface 24 of the center electrode 13. The linear ground electrode 16 may be arranged perpendicular to the axis O or may be arranged so as to intersect the axis O at an angle.

[0053] In the embodiment described above, the spark plug 10 has one ground electrode 16 disposed on the metallic shell 15, but this is not necessarily limited to this. It is of course possible to dispose one or more electrodes in addition to the ground electrode 16 on the metallic shell 15, with a portion of the electrode facing a side surface of the center electrode 13. This allows discharge to normally occur between the ground electrode 16 and the center electrode 13, and when carbon adheres to the insulator 11 due to smoldering contamination, discharge occurs between the electrode and the center electrode 13, burning off the carbon, thereby reducing deterioration of insulation between the ground electrode 16 and the center electrode 13. [Explanation of symbols]

[0054] 10 Spark Plugs 13,50 Center electrode (1st electrode) 16 Ground electrode (second electrode) 17 First area 18 Second area 19 Base material 20,51 Welding zone 21 chips 22 Boundary 23 Boundary 24 Discharge surface 25 Center of gravity 26,27 First Line 28,29 Second line 30 The Third Line 31 line segments 32,34 line segments 33,35 line segments 36 line segments 37,38 line segments 52 Boundary O axis

Claims

1. a first electrode including a base material and a tip joined to the base material via a fusion zone; a second electrode facing the discharge surface of the tip in an axial direction, In a cross section of the first electrode including the center of gravity of the discharge surface and the axis, The length of the discharge surface in a direction perpendicular to the axis is defined as D, a first straight line parallel to the axis and at a distance of D / 4 from the axis; a second straight line parallel to the axis and spaced apart from the axis by 3D / 8; a third straight line that is located on the opposite side of the base material with respect to the center of gravity of the discharge surface and is perpendicular to the axis; The length of the line segment obtained by cutting the axis line by the boundary of the tip that is not the discharge surface and the third straight line is defined as L1, The length of the line segment obtained by cutting the first straight line by the boundary of the tip on the side other than the discharge surface and the third straight line is defined as L2, When the length of the line segment obtained by cutting the second straight line by the boundary of the chip that is not the discharge surface and the third straight line is L3, A spark plug wherein at least one of two regions of the cross section adjacent to each other with the axis as a boundary satisfies the relationship L1<L2<L3.

2. 2. The spark plug according to claim 1, wherein said fusion zone is continuous from one end of said tip to the other end of said tip across said two regions in said cross section.

3. In the cross section, The length of the line segment obtained by cutting the axis line by the boundary between the fusion zone and the base material and the third straight line is defined as L4, When the length of the line segment obtained by cutting the second straight line by the boundary between the molten portion and the base material and the third straight line is L5, 3. The spark plug according to claim 2, wherein the relationship L4<L5 is satisfied in at least one of said regions.

4. 4. The spark plug according to claim 3, wherein the relationship L4<L5 is satisfied in the two regions.

5. 5. The spark plug according to claim 1, wherein the length obtained by subtracting L1 from L3 is 0.05 mm or more.

6. 5. The spark plug according to claim 1, wherein the length obtained by subtracting L1 from L3 is 0.08 mm or greater.

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