Spark plug

Abstract

To provide a spark plug capable of reducing a bending stress of a tip end part of a center electrode.SOLUTION: A spark plug comprises: an insulation body into which a shaft hole extended from a tip end to a rear end side is provided; and a center electrode that is arranged to the shaft hole of an insulation body and is extended along a shaft line. The center electrode contains a tip end part projected from the tip end of the insulation body, and the tip end part contains a root positioned to the tip end of the insulation body, and an electrode tip end of which a width is smaller than that of the root. An outer shape line connecting the root of the tip end part appeared in a cross section and the electrode tip end contains one or more concave parts that is concaved toward the shaft line. A value R / A obtained by dividing a curvature radius R of the rear end concave part that is closest to the root of the concave part with a distance A of a shaft line direction between the rear end concave part and the electrode tip end is 0.12 or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a spark plug in which a center electrode is disposed on an insulator.

Background Art

[0002] Prior art related to a spark plug in which a tip portion of a center electrode protrudes from a tip of an insulator is disclosed in Patent Document 1.

Prior Art Document

Patent Document

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, a force to bend the center electrode is applied to the tip portion of the center electrode by the combustion pressure of the engine. In order to improve the durability etc. of the spark plug, it is preferable that the bending stress at the tip portion of the center electrode is small.

[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 bending stress at the tip portion of the center electrode.

Means for Solving the Problems

[0006] To achieve this objective, a first aspect of the present invention provides an insulator having an axial hole extending from the front end to the rear end, and a central electrode disposed in the axial hole of the insulator and extending along the axis, wherein the central electrode includes a tip portion protruding from the front end of the insulator, and the tip portion includes a base located at the front end of the insulator in a cross-section including the axis, and an electrode tip having a width smaller than the width of the base. The outline line connecting the base of the tip portion and the electrode tip that appears in the cross-section includes one or more recesses that are recessed toward the axis, and the value R / A obtained by dividing the radius of curvature R of the rear end recess closest to the base by the axial distance A between the electrode tip and the rear end recess is 0.12 or more.

[0007] In the second embodiment, the value R / A is 0.22 or greater, as in the first embodiment.

[0008] In the third embodiment, in the first or second embodiment, the thickness C of the tip portion in the rear end recess in a direction perpendicular to the axis is 0.8 mm or more.

[0009] The fourth embodiment is the third embodiment, wherein the distance A is 0.9 mm or less and the thickness C is 0.9 mm or less.

[0010] The fifth aspect is that, in the third or fourth aspect, the value C / A obtained by dividing the thickness C by the distance A is 1.42 or greater.

[0011] The sixth aspect is that, in any of the first to fifth aspects, the tip portion includes a base material including a root, a tip including an electrode tip and containing a precious metal, and a molten portion formed between the tip and the base material. The base material includes a rear end recess, and the molten portion is continuous between two outline lines located on either side of the axis.

[0012] The seventh aspect is that, in any of the first to sixth aspects, the tip portion includes a first portion including a base, a second portion adjacent to the tip side of the first portion and narrower than the first portion, a tip containing a precious metal and including the electrode tip, and a molten portion formed between the tip and the second portion. The rear end recess is located at the boundary between the first portion and the second portion.

[0013] The eighth aspect is that in any of the first to seventh aspects, the value R / A is 0.3 or less. [Effects of the Invention]

[0014] The value R / A, obtained by dividing the radius of curvature R of the rear end recess closest to the base of the tip among the recesses of the outer contour of the tip by the axial distance A between the rear end recess and the electrode tip, is 0.12 or greater, thus reducing the bending stress at the tip. [Brief explanation of the drawing]

[0015] [Figure 1] This is a cross-sectional view of one side of a spark plug in one embodiment. [Figure 2] This is a cross-sectional view of the tip of the central electrode. [Figure 3] This is a cross-sectional view of the rear end recess, enlarged to the portion indicated by III in Figure 2. [Figure 4] This figure shows the relationship between the R / A value and the bending stress in the rear end recess. [Figure 5] This figure shows the relationship between distance A and the bending stress in the rear end recess. [Figure 6] This figure shows the relationship between the C / A value and the bending stress in the rear end recess. [Modes for carrying out the invention]

[0016] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Figure 1 is a one-sided cross-sectional view combining an external view and a full cross-sectional view of the spark plug 10 in one embodiment, with the axis O as the boundary. In Figure 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 Figures 2 and 3).

[0017] As shown in Figure 1, the spark plug 10 comprises an insulator 11 and a central electrode 14 held by the insulator 11. The insulator 11 is a substantially cylindrical member made of ceramic such as alumina, which has excellent mechanical properties and insulation properties at high temperatures. The insulator 11 is provided with an axial hole 13 that extends along the axis O from the tip 12 toward the rear end of the insulator 11.

[0018] The center electrode 14 is a rod-shaped conductor extending along the axis O, which is disposed in the axial hole 13 of the insulator 11. The center electrode 14 is covered with a bottomed cylindrical metal having a core material mainly composed of copper. It is possible to omit the core material. The metal constituting the center electrode 14 is exemplified by a Ni-based alloy. The center electrode 14 includes a tip portion 15 protruding from the tip 12 of the insulator 11. The tip portion 15 is provided on the axis O.

[0019] The center electrode 14 is electrically connected to the terminal fitting 16 in the axial hole 13. The terminal fitting 16 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, etc.). The tip side of the terminal fitting 16 is inserted into the axial hole 13, and the rear end protrudes from the insulator 11 and is fixed to the rear end side of the insulator 11.

[0020] The main body fitting 17 is fixed to the outer periphery of the insulator 11. The main body fitting 17 is a cylindrical member made of a metal material (such as low-carbon steel). The main body fitting 17 is provided with an external thread 18 that engages with the internal thread of the plug hole of an engine (not shown). The tip 12 of the insulator 11 is located on the tip side of the tip portion of the external thread 18. A ground electrode 19 is connected to the tip portion of the external thread 18.

[0021] The ground electrode 19 is a conductor extending from the main body fitting 17 toward the axis O. The ground electrode 19 has a core material mainly composed of copper embedded therein. It is possible to omit the core material. In the present embodiment, the ground electrode 19 is bent from the main body fitting 17 toward the tip side of the tip portion 15 of the center electrode 14.

[0022] Figure 2 is a cross-sectional view of the tip portion 15 of the central electrode 14, including the axis O. The tip portion 15 is the part of the central electrode 14 located closer to the tip 12 of the insulator 11. The tip portion 15 includes a base 20 located at the tip 12 of the insulator 11 in the direction of axis O, and an electrode tip 21 whose width is smaller than the width of the base 20. The axis O passes through the centroid of the electrode tip 21. The centroid of the electrode tip 21 is the geometric center calculated by a well-known method when the surface of the electrode tip 21 is treated as a planar figure.

[0023] In this embodiment, the tip portion 15 includes a tip 22 including the electrode tip 21, a base material 23 including the base 20, and a molten portion 24 formed by the melting of the base material 23 and the tip 22. The tip 22 contains one or more noble metal elements selected from Pt, Ir, Ru, Rh, etc. The base material 23 is the metal material to which the tip 22 is welded. In this embodiment, the molten portion 24 is created by laser welding. It is of course possible to create the molten portion 24 by resistance welding or the like.

[0024] In this embodiment, the tip portion 15 includes a first portion 25 including the base 20, a second portion 28 adjacent to the tip side of the first portion 25, a tip 22, and a molten portion 24 formed by the melting of the tip 22 and the second portion 28. The second portion 28 is thinner than the first portion 25. The first portion 25 and the second portion 28 are metallic materials that constitute the base material 23. The first portion 25 and the second portion 28 are made by plastic deformation of the metallic material.

[0025] In this embodiment, the first part 25 includes a cylindrical portion 26 including a base 20 and a conical portion 27 adjacent to the tip side of the cylindrical portion 26. The cylindrical portion 26 is fitted into the axial hole 13. The cylindrical portion 26 is cylindrical with substantially the same thickness along its entire length in the axial direction, and the conical portion 27 is conical, gradually tapering towards the tip. The second part 28 is adjacent to the tip side of the conical portion 27. The second part 28 is cylindrical with substantially the same thickness along its entire length in the axial direction. The first part 25 and the second part 28 are integrally formed by plastic deformation, but are not limited to this, and it is of course possible to join the first part 25 and the second part 28 by welding, diffusion bonding, etc.

[0026] Figure 2 shows two outline lines 29 and 30 representing the outer shape of the tip portion 15, appearing on either side of the axis O. The outline lines 29 and 30 connect the base 20 of the tip portion 15 to the electrode tip 21, respectively. Since the outline lines 29 and 30 are approximately symmetrical with respect to the axis O, we will explain one of the outline lines 29, 30, and omit the explanation of outline line 29.

[0027] The outline 30 consists of a convex portion 31 projecting away from the axis O, a concave portion 32 concave towards the axis O, a convex portion 33 projecting away from the axis O, and a concave portion 34 concave towards the axis O, in that order from the base 20 toward the electrode tip 21. The convex portion 31 is the part where the cylindrical portion 26 and the conical portion 27 are connected, and the concave portion 32 is the part where the first portion 25 (conical portion 27) and the second portion 28 are connected. The convex portion 33 is near the rear end of the molten portion 24, and the concave portion 34 is the part where the molten portion 24 and the tip 22 are connected.

[0028] The molten portion 24 is continuous between the two outline lines 29 and 30 located on either side of the axis O. Because the molten portion 24 is continuous, the interface 35 between the tip 22 and the molten portion 24, and the interface 36 between the base material 23 and the molten portion 24, connect the outline line 29 and the outline line 30. The recess 32 is located between the interface 36 and the base 20. Because the molten portion 24 is continuous, the bonding area between the tip 22 and the base material 23 can be increased compared to cases where the molten portion 24 is separated between the outline lines 29 and 30. Therefore, the bonding strength of the tip 22 can be increased.

[0029] Of the recesses 32 and 34, recess 32, which is closest to the base 20, is located further from the electrode tip 21 than recess 32. Therefore, when a force that tries to bend the central electrode 14 (see Figure 1) is applied to the tip 15 by the combustion pressure of the engine (not shown), the bending stress in recess 32 tends to be greater than that in recess 34. Thus, the value R / A, obtained by dividing the radius of curvature R of recess 32 (rear end recess), which is closest to the base 20, by the axial distance A between the rear end recess 32 and the electrode tip 21, is used as an indicator of the bending stress in the tip 15.

[0030] Figure 3 is a cross-sectional view of the rear end recess 32, enlarged to the portion shown as III in Figure 2. The starting point 37 on the rear end recess 32 side where distance A between the rear end recess 32 and the electrode tip 21 begins is the point where a straight line 38, obtained by extending the straight portion of the outline of the cone portion 27 toward the tip, intersects with a straight line 39, obtained by extending the straight portion of the outline of the second portion 28 toward the rear end. In this embodiment, since the electrode tip 21 (see Figure 2) and the axis O are perpendicular, the endpoint on the electrode tip 21 side where distance A ends can be either the ends 40 or 41 of the electrode tip 21. When the electrode tip 21 and the axis O intersect at an angle, the endpoint of distance A is taken from the end 40 or 41 of the electrode tip 21 that results in a longer distance A.

[0031] The radius of curvature R of the rear end recess 32 is the radius of a circle 46 that includes a circular arc 45 passing through three points: point 42 just before the outline 30 and the straight line 38 separate, point 43 just before the outline 30 and the straight line 39 separate, and point 44, which is the midpoint between points 42 and 43. The distance between point 42 and the midpoint 44 is equal to the distance between point 43 and the midpoint 44. If the portion of the outline 30 between points 42 and 43 is a smooth curve (circular arc) without an inflection point, the curve tracing the outline 30 is defined as the circular arc 45.

[0032] If the portion of the outline 30 between points 42 and 43 is not an arc but a line with an inflection point, tracing the outline 30 will not result in an arc. Therefore, instead of tracing the outline 30, an intermediate point 44 is established on or near the outline 30, and then an arc 45 is drawn passing through points 42, 43, and the intermediate point 44. The intermediate point 44 is determined in a position such that the arc 45 passing through points 42, 43, and the intermediate point 44 becomes an approximate curve that represents the outline 30.

[0033] Since there are two outline lines 29 and 30 on either side of axis O, the rear end recess 32 lies on the outline lines 29 and 30, respectively. If the radius of curvature R of the rear end recess 32 on outline line 29 is different from the radius of curvature R of the rear end recess 32 on outline line 30, the smaller radius of curvature R is used to calculate the value R / A.

[0034] Let's return to Figure 2 for explanation. The diameter C is the diameter of the tip portion 15 in the direction perpendicular to the axis O in the rear end recess 32. In this embodiment, the diameter C is equal to the diameter of the rear end of the second portion 28. When a force that tries to bend the central electrode 14 is applied to the tip portion 15, the bending stress in the rear end recess 32 decreases as the radius of curvature R of the rear end recess 32 increases and as the distance A decreases. The bending stress in the rear end recess 32 decreases as the diameter C increases. In other words, for reducing the bending stress in the rear end recess 32, it is advantageous for the value R / A and the diameter C to be larger.

[0035] Figure 4 shows the relationship between the value R / A and the bending stress σ of the rear end recess 32. Figure 4 is a graph showing the bending stress σ calculated for samples No. 1-5 shown in Table 1, which have different distances A and diameters C, when the radius of curvature R is changed, with the value R / A on the horizontal axis and the bending stress σ on the vertical axis. The numbers on the graph are the sample numbers No. 1-5. The scale on the vertical axis shows the reference value of the bending stress as 1, and the reference value and 1 / 10 of the reference value.

[0036] [Table 1]

[0037] As shown in Figure 4, the bending stress σ decreases as the value R / A increases, and it was confirmed that the slope of the tangent line to the bending stress σ graph also decreases as the value R / A increases. In sample No. 1, which had the longest A and the highest bending stress among the five samples, it was found that when R / A is 0.12 or higher, the bending stress can be reduced to 66% or less compared to when R / A = 0.045 (R = 0.14 mm). In samples No. 1-5, when R / A is 0.12 or higher, the slope of the graph is gentler compared to when R / A is less than 0.12, indicating that when R / A is 0.12 or higher, the bending stress of the rear end recess 32 can be reduced.

[0038] In samples No. 1-5, the slope of the graph when the R / A value is 0.22 or higher is even gentler than the slope of the graph when the R / A value is less than 0.22, indicating that an R / A value of 0.22 or higher is more preferable.

[0039] When comparing the graphs of samples No. 1-3 and 5, where the distance C is 0.85 mm but the distance A is different, the bending stress σ was smaller in the order of No. 5, 3, 2, and 1 as the distance A became shorter. Therefore, it was confirmed that a shorter distance A is preferable to reduce the bending stress σ of the rear end recess 32. A distance A of 2.13 mm or less is preferable, 1.87 mm or less is more preferable, and 1.73 mm or less is even preferable.

[0040] Comparing the graphs for sample No. 4 and No. 5, both with a distance A of 1.73 mm, the graph for No. 5, with a thickness C of 0.85 mm, showed a smaller bending stress σ compared to the graph for No. 4, with a thickness C of 0.70 mm. Therefore, to reduce the bending stress σ of the rear end recess 32, it is preferable for the thickness C of the tip portion 15 in the rear end recess 32 to be larger, and particularly preferable for it to be 0.80 mm or larger.

[0041] Figure 5 shows the relationship between distance A and the bending stress σ of the rear end recess 32. Figure 5 is a graph showing the bending stress σ calculated for samples No. 6-11 shown in Table 2, where the distance A is different, with distance A on the horizontal axis and bending stress σ on the vertical axis. For sample No. 6-11, the thickness C was kept constant at 0.85 mm. For sample No. 6-9, where the distance A is 0.8 mm or less, the radius of curvature R was set to 0.10 mm. For samples 10 and 11, where the distance A is 0.9 mm or more, the radius of curvature R was set to 0.14 mm. The scale on the vertical axis shows the ratio of bending stress.

[0042] [Table 2]

[0043] As shown in Figure 5, it was confirmed that the bending stress σ decreases as the distance A decreases. To reduce the bending stress σ, a distance A of 1.00 mm or less is more preferable, and 0.90 mm or less is particularly preferable.

[0044] On the other hand, a smaller diameter C makes it less likely for the energy of the flame kernel generated by the spark discharge to be absorbed by the tip 15, allowing the flame kernel to grow more easily and thus improving ignition performance. To ensure ignition performance, a diameter C of 0.90 mm or less is preferable, and 0.85 mm or less is more preferable. Therefore, to reduce the bending stress σ of the rear end recess 32 and ensure ignition performance, a distance A of 0.90 mm or less is preferable, and a diameter C of 0.90 mm or less is preferable. A diameter C of 0.85 mm or less is more preferable.

[0045] Figure 6 shows the relationship between the value C / A (thickness C divided by distance A) and the bending stress σ of the rear end recess 32. Figure 6 is a graph showing the bending stress σ calculated for samples No. 12-21 shown in Table 3, where distance A and thickness C differ, with the value C / A on the horizontal axis and the bending stress σ on the vertical axis. For sample No. 12-19, where distance A is 0.8 mm or less, the radius of curvature R was set to 0.10 mm, and for samples 20 and 21, where distance A is 0.9 mm or more, the radius of curvature R was set to 0.14 mm. The scale on the vertical axis shows the ratio of bending stress.

[0046] [Table 3]

[0047] The bending stress σ in the rear end recess 32 decreases as the diameter C increases and as the distance A decreases. Therefore, a larger value C / A is preferable for reducing the bending stress σ. As shown in Figure 6, it was confirmed that the value C / A and the bending stress σ are inversely proportional. It was confirmed that the bending stress σ can be sufficiently reduced when the value C / A is 1.42 or higher.

[0048] Furthermore, in order to prevent the energy of the flame kernel generated by the spark discharge from being absorbed by the tip portion 15, a smaller diameter C is preferable, and a certain length of distance A is necessary. Considering these factors, a value of C / A of 1.42 or higher is preferable in order to reduce the bending stress σ of the rear end recess 32 and ensure ignition.

[0049] Since the rear end recess 32 is located in the base material 23 between the interface 36 of the molten portion 24 and the root 20, it is easier to control the radius of curvature R of the rear end recess 32 when manufacturing it, compared to when the rear end recess 32 is located in the molten portion 24. Therefore, it is easier to manage the bending stress σ of the rear end recess 32.

[0050] The rear end recess 32 is located at the boundary between the first part 25 and the second part 28, which is narrower than the first part 25. Since the molten portion 24 is interposed between the second part 28 and the tip 22, the volume from the second part 28 to the tip 22 can be reduced compared to the case without the second part 28. Because the heat capacity from the second part 28 to the tip 22 can be reduced, the energy of the flame kernel generated by the spark discharge is less likely to be absorbed by the tip portion 15, and the flame kernel grows more easily. As a result, ignition performance is improved.

[0051] To reduce the bending stress in the rear end recess 32, it is preferable that the radius of curvature R of the rear end recess 32 be large and the distance A be short. However, considering the ease of manufacturing the rear end recess 32, there is an optimal range for the radius of curvature R. Also, in order to prevent the energy of the flame kernel generated by the spark discharge from being absorbed by the tip portion 15, the distance A needs to be of a certain length. Considering these factors, a value of R / A of 0.3 or less is preferable.

[0052] Although the present invention has been described above based on embodiments, it can be easily inferred that the present invention is not limited in any way to the above embodiments, and that various improvements and modifications are possible without departing from the spirit of the present invention.

[0053] In the embodiment, the case in which the base 20 of the tip 15 of the central electrode 14 is located on the cylindrical portion 26 of the first part 25 has been described, but it is not necessarily limited to this. By shortening the length of the central electrode 14 protruding from the tip 12 of the insulator 11, it is of course possible to provide the base 20 of the tip 15 on the conical portion 27 of the first part 25. In this case as well, the recess 32 closest to the base 20 among the recesses 32 and 34 included in the outer contour lines 29 and 30 of the tip 15 is the rear end recess.

[0054] In this embodiment, the tip 22 is welded to the base material 23 of the central electrode 14, but this is not necessarily the only option. It is, of course, possible to remove the tip 22 and the molten portion 24. In this case, the shape of the base material 23 described in the embodiment is the shape of the central electrode 14. When the tip 22 and the molten portion 24 are removed, there is one recess 32 included in the outer outlines 29 and 30 of the tip portion 15, and this recess 32 is the rear end recess. To improve spark resistance and other properties, it is, of course, possible to apply a coating containing a noble metal element to the surface of the tip portion 15 of the central electrode 14 from which the tip 22 and the molten portion 24 have been removed.

[0055] In the embodiment, the case where the second part 28 is cylindrical has been described, but it is not necessarily limited to this. It is certainly possible to make the second part 28 conical, similar to the conical portion 27 of the first part 25.

[0056] When the second part 28 is made conical, it may be connected to the conical part 27 of the first part 25, or the conical part 27 of the first part 25 may be removed and the second part 28 may be connected to the cylindrical part 26. When the conical second part 28 is connected to the conical part 27 of the first part 25, a recess is provided between the first part 25 and the second part 28, so the angle between the outline of the second part 28 and the axis O in the cross-section including the axis O is made smaller than the angle between the outline of the conical part 27 and the axis O in the cross-section including the axis O. When the conical part 27 of the first part 25 is removed and the conical second part 28 is connected to the cylindrical part 26, a recess is provided between the first part 25 and the second part 28, so the diameter of the base of the second part 28 is made smaller than the diameter of the cylindrical part 26.

[0057] In the embodiment described, the case in which the cylindrical second part 28 is connected to the conical part 27 of the first part 25 has been explained, but it is not necessarily limited to this. The conical part 27 of the first part 25 may be removed and the cylindrical second part 28 may be connected to the cylindrical part 26 of the first part 25. In this case, in order to provide a recess between the first part 25 and the second part 28, the diameter of the second part 28 is made smaller than the diameter of the cylindrical part 26. [Explanation of Symbols]

[0058] 10 Spark plugs 11 Insulator 12 Tip 13 Shaft hole 14 Center electrode 15 Tip 20 Root 21 Electrode tip 22 chips 23 Base material 24 Molten part 25 Part 1 28 Part 2 29,30 Outline 32 Rear end recess (recess) 34 recess

Claims

1. An insulator having an axial hole extending from the tip towards the rear end, The insulator comprises a central electrode disposed in the axial hole and extending along the axis, The central electrode is a spark plug including a tip portion that protrudes from the tip of the insulator, In a cross-section including the axis, the tip portion includes a base located at the tip of the insulator in the direction in which the axis extends, and an electrode tip that is narrower than the base. The outline of the tip portion appearing in the cross-section, connecting the base of the tip and the electrode tip, includes one or more recesses that are recessed toward the axis. A spark plug in which the value R / A obtained by dividing the radius of curvature R of the rear end recess closest to the root among the recesses by the axial distance A between the rear end recess and the electrode tip is 0.12 or more.

2. The spark plug according to claim 1, wherein the R / A value is 0.22 or greater.

3. The spark plug according to claim 1 or 2, wherein the thickness C of the tip portion in the rear end recess in a direction perpendicular to the axis is 0.8 mm or more.

4. The spark plug according to claim 3, wherein the distance A is 0.9 mm or less and the diameter C is 0.9 mm or less.

5. The spark plug according to claim 3, wherein the value C / A obtained by dividing the diameter C by the distance A is 1.42 or more.

6. The aforementioned tip portion is made of the base material including the root, The tip includes the electrode tip and contains a precious metal, The molten portion formed between the chip and the base material is included, The base material includes the rear end recess, The spark plug according to claim 1 or 2, wherein the molten portion is continuous between the two outline lines located on both sides of the axis.

7. The tip portion comprises a first portion including the base, Adjacent to the tip of the first part is a second part which is thinner than the first part, The tip includes the electrode tip and contains a precious metal, The molten portion formed between the tip and the second part includes, The spark plug according to claim 1 or 2, wherein the rear end recess is located at the boundary between the first part and the second part.

8. The spark plug according to claim 1 or 2, wherein the value R / A is 0.3 or less.

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