Spark plug

Abstract

To provide a spark plug that can reduce the occurrence of preignition.SOLUTION: A spark plug comprises: a center electrode; a subject fitting that insulates and holds the center electrode; and a columnar ground electrode that is electrically connected with the subject fitting and faces the center electrode at one end. The subject fitting includes a cylindrical leading end having one end of the ground electrode located therein, and the leading end has a hole into which the other end of the ground electrode is inserted. Dividing the surface area of a portion of the ground electrode located on an inner peripheral side of the leading end by the side area of a portion of the ground electrode located in the hole, yields a value of 13.1 or less. The value is preferably 6.0 or less, and more preferably 5.5 or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a spark plug having a ground electrode connected to a metallic shell. [Background technology]

[0002] In a spark plug that includes a center electrode, a metal shell that insulates and holds the center electrode, and a ground electrode connected to the metal shell, a prior art in which the ground electrode is disposed in a hole provided at the cylindrical tip end of the metal shell is disclosed in Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-145018 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the prior art, the ground electrode surrounded by the tip is difficult to cool by fuel or intake air, so there is a risk that the overheated ground electrode will become a spark and cause pre-ignition.

[0005] The present invention has been made to solve this problem, and has an object to provide a spark plug that can reduce the occurrence of pre-ignition. [Means for solving the problem]

[0006] In order to achieve this object, an embodiment of the present invention is a spark plug comprising a center electrode, a metal shell insulating and holding the center electrode, and a columnar ground electrode electrically connected to the metal shell and having one end facing the center electrode, the metal shell including a cylindrical tip end portion inside which one end of the ground electrode is located, the tip end portion having a hole into which the other end of the ground electrode is inserted, wherein a value obtained by dividing a surface area of ​​a portion of the ground electrode located on the inner periphery side of the tip end portion by a side area of ​​a portion of the ground electrode located within the hole is 13.1 or less, preferably 6.0 or less, more preferably 5.5 or less. Effect of the Invention

[0007] According to the present invention, the surface area of ​​the portion of the ground electrode located on the inner side of the tip, i.e., the portion that receives heat from the combustion gas, divided by the side area of ​​the portion of the ground electrode located within the hole, i.e., the portion that dissipates heat to the metallic shell, is 13.1 or less, so overheating of the ground electrode can be prevented and the occurrence of pre-ignition can be reduced. [Brief description of the drawings]

[0008] [Figure 1] 1 is a partial cross-sectional view of a spark plug according to a first embodiment. [Diagram 2] 2 is an enlarged cross-sectional view of the spark plug shown in FIG. 1. [Diagram 3] FIG. 11 is a cross-sectional view of a spark plug according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a partial cross-sectional view of a spark plug 10 according to a first embodiment. Fig. 1 shows a cross section including an axis O of a tip end portion of the spark plug 10. In Fig. 1, the lower side of the paper surface refers to the tip end side of the spark plug 10, and the upper side of the paper surface refers to the rear end side of the spark plug 10 (the same applies to Figs. 2 and 3). As shown in Fig. 1, the spark plug 10 includes an insulator 11, a center electrode 15, a metal shell 20, and a ground electrode 25.

[0010] The insulator 11 is a substantially cylindrical member having an axial hole 12 extending along the axis O. The insulator 11 is made of ceramics such as alumina, which has excellent mechanical properties and insulating properties at high temperatures. The insulator 11 includes a locking portion 13 and a tip portion 14 adjacent to the tip side of the locking portion 13. The outer diameter of the tip portion 14 is smaller than the outer diameter of the locking portion 13.

[0011] A center electrode 15 is disposed along the axis O in the axial hole 12 from at least the engaging portion 13 to the tip portion 14 of the insulator 11. The center electrode 15 includes a rod-shaped base material 16 mainly composed of Ni, a tip 17 mainly composed of one or more of precious metals such as Pt, Rh, Ru, Ir, etc., disposed at the tip of the base material 16, and a fusion zone 18 that joins the tip 17 and the base material 16. The tip 17 and the fusion zone 18 may be omitted.

[0012] The tip of the center electrode 15 protrudes from the insulator 11 toward the tip side. The center electrode 15 is electrically connected to a terminal fitting 19 inside the axial hole 12. The terminal fitting 19 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 19 is fixed to the rear end of the insulator 11.

[0013] The metal shell 20 is a substantially cylindrical member made of a conductive metal material (e.g., copper, copper alloy, low carbon steel, etc.). The metal shell 20 is disposed on the outer periphery of the insulator 11. The metal shell 20 includes a cylindrical tip portion 21 located on the outer periphery side of at least the locking portion 13 and the tip portion 14 of the insulator 11. The tip portion 21 is provided with a male thread 22 on the outer periphery and a shelf portion 23 on the inner periphery. The male thread 22 fits into a female thread provided in a plug hole of an engine (not shown). The shelf portion 23 is located on the tip side of the locking portion 13 of the insulator 11 and locks the locking portion 13. The tip portion 21 extends further toward the tip side than the tip portion 14 of the insulator 11.

[0014] A hole 24 is provided in the tip portion 21 at a portion closer to the tip end than the shelf portion 23, where a portion of the inner circumference of the tip portion 21 is recessed. In this embodiment, the hole 24 penetrates the tip portion 21 in the radial direction, has a circular cross section, and causes a portion of the male thread 22 to be missing due to the hole 24. A portion of the ground electrode 25 is disposed in the hole 24. The ground electrode 25, with a portion thereof disposed in the hole 24, protrudes from the tip portion 21 toward the center electrode 15.

[0015] The cap 26 closes the tip end side of the metallic shell 20, and provides a space 29 inside the cap 26. The material of the cap 26 is, for example, a metal material containing one or more of Fe, Ni, Cu, etc. as a main component. In this embodiment, the cap 26 is connected to the tip end side of the tip portion 21 via a fusion portion 28. The cap 26 is provided with a through hole 27 that communicates between the inside and outside of the space 29.

[0016] 2 is a cross-sectional view of the spark plug 10, enlarging a portion indicated by II in FIG. 1. The ground electrode 25 is columnar in shape and includes one end 32 that faces the center electrode 15 (see FIG. 1) to form a spark gap, and the other end 33 that is disposed in the hole 24. The one end 32 is located inside the inner circumference 30 of the tip portion 21. The one end 32 is cylindrical and has a circular cross section. The other end 33 is located inside the outer circumference 31 of the tip portion 21 and inside the valley bottom 22a of the male thread 22. The other end 33 is cylindrical and has a circular cross section that fits into the hole 24. The hole 24 that penetrates the tip portion 21 is closed by the other end 33.

[0017] The ground electrode 25 includes a base material 34 mainly composed of, for example, Ni, a tip 35 mainly composed of one or more of precious metals such as Pt, Rh, Ru, and Ir, and a fusion zone 36 that joins the tip 35 to the base material 34. One end 32 of the ground electrode 25 includes a part of the tip 35, and the other end 33 includes a part of the base material 34. The tip 35 and the fusion zone 36 may be omitted.

[0018] The ground electrode 25 is connected to the tip portion 21 through a molten portion 37 in the hole 24. The molten portion 37 is made by irradiating a bottom surface 38 of the ground electrode 25 disposed in the hole 24 with a laser beam. The molten portion 37 is formed by melting a portion including a part of the bottom surface 38 of the ground electrode 25 and a portion including a part of the tip portion 21. In this embodiment, the molten portion 37 is continuous around the entire circumference of the bottom surface 38 of the ground electrode 25, and the radially inner end of the molten portion 37 reaches the inner circumference 30 of the tip portion 21.

[0019] Heat from the ground electrode 25 is transferred to the tip 21 through the interface 39 between the molten portion 37 and the tip 21. This causes the ground electrode 25 to cool. The area of ​​the interface 39 has a significant effect on heat transfer. The molten portion 37 is part of the ground electrode 25. Meanwhile, the side surface 40 and bottom surface 41 of the ground electrode 25, which are the portion of the ground electrode 25 located inside the inner circumference 30 of the tip 21 (the portion located within the space 29), are heat-receiving surfaces that receive the heat generated by the combustion of fuel in the space 29.

[0020] In a spark plug 10 (see FIG. 1) attached to an engine (not shown), fuel flows from the engine's combustion chamber into a space 29 through a through hole 27 by operating a valve of the engine. The spark plug 10 generates a flame kernel by discharging between the center electrode 15 and the ground electrode 25. As the flame kernel grows, the fuel in the space 29 is ignited and the fuel is burned. Expansion pressure caused by the combustion of the fuel generates a gas flow containing a flame, and the gas containing the flame is injected into the combustion chamber from the through hole 27. The fuel in the combustion chamber is burned by the flame jet. In other words, the space 29 inside the cap 26 functions as a sub-combustion chamber provided in the combustion chamber of the engine.

[0021] In the spark plug 10, the value S / E obtained by dividing the surface area S of the portion of the ground electrode 25 located on the inner periphery 30 side of the tip portion 21, which is the sum of the areas of the side surface 40 and the bottom surface 41, by the side area E of the portion of the ground electrode 25 located within the hole 24 (the area of ​​the interface 39 in this embodiment), is 13.1 or less. This is to prevent overheating of the ground electrode 25 by a balance between heat reception by the ground electrode 25 and heat transfer from the ground electrode 25 to the tip portion 21, thereby reducing the occurrence of pre-ignition. The value S / E is preferably 6.0 or less, and more preferably 5.5 or less.

[0022] In the spark plug 10, a portion of the ground electrode 25 located on the inner circumference 30 side of the tip portion 21 is covered with a cap 26. Since a space 29 on the inner circumference 30 side of the tip portion 21 is connected to the combustion chamber (not shown) through a through hole 27 in the cap 26, the ground electrode 25 is difficult to cool by the fuel and intake air supplied to the combustion chamber, and the ground electrode 25 is prone to overheating. However, since the value S / E of the spark plug 10 is set to 13.1 or less, overheating of the ground electrode 25 can be prevented and the occurrence of pre-ignition can be reduced. Therefore, the spark plug 10 is suitable for having a cap 26.

[0023] A second embodiment will be described with reference to Fig. 3. In the first embodiment, a case was described in which the molten portion 37 is continuous around the entire circumference of the bottom surface 38 of the ground electrode 25 and reaches the inner circumference 30 of the tip portion 21. In contrast, in the second embodiment, a case will be described in which the molten portion 59 is provided intermittently along the edge of the bottom surface 60 of the ground electrode 53 and there is a gap between the inner circumference 30 of the tip portion 21 and the molten portion 59. In the second embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description will be omitted.

[0024] 3 is a cross-sectional view of a spark plug 50 in a second embodiment. In FIG. 3, the vicinity of a hole 51 provided in the tip portion 21 of the metallic shell 20 is shown enlarged, and other parts are not shown. A part of the inner circumference 30 of the tip portion 21 is recessed by the hole 51. In this embodiment, the hole 51 penetrates the tip portion 21 in the radial direction, and has a circular cross section. The hole 51 has a counterbore 52 provided in the outer circumference 31 of the tip portion 21. The counterbore 52 is located inside the valley bottom 22a of the male thread 22.

[0025] The ground electrode 53 includes a base material 54 mainly composed of, for example, Ni, a tip 55 mainly composed of one or more of precious metals such as Pt, Rh, Ru, and Ir, and a fusion zone 56 that joins the tip 55 and the base material 54. The ground electrode 53 is columnar in shape and includes one end 57 that faces the center electrode 15 (see FIG. 1) to form a spark gap, and the other end 58 that is disposed within the hole 51.

[0026] One end 57 is cylindrical and has a circular cross section. The other end 58 is cylindrical and has a circular cross section that fits into hole 51. Hole 51 penetrating tip 21 is closed by the other end 58. The fit between the other end 58 and hole 51 may be any of an interference fit, a loose fit, and a transition fit.

[0027] The ground electrode 53 is connected to the tip portion 21 through a fusion zone 59 in the hole 51. A bottom surface 60 of the ground electrode 53 is located on substantially the same plane as the counterbore 52. The fusion zone 59 is created by irradiating the bottom surface 60 of the ground electrode 53 disposed in the hole 51 with a laser beam. The fusion zone 59 is formed by melting a portion including a part of the bottom surface 60 of the ground electrode 53 and a portion including a part of the counterbore 52 of the tip portion 21. In this embodiment, the fusion zone 59 is provided intermittently along the edge of the bottom surface 60 of the ground electrode 53, and there is a gap between the radially inner end of the fusion zone 59 and the inner circumference 30 of the tip portion 21.

[0028] Heat from the ground electrode 53 is transferred to the tip 21 through the interface 61 between the molten portion 59 and the tip 21, and through the side surface 62 inside the hole 51 of the ground electrode 53. This cools the ground electrode 53. The areas of the interface 61 and the side surface 62 have a significant effect on heat transfer. The molten portion 59 is part of the ground electrode 53. Meanwhile, the side surface 63 and bottom surface 64 of the ground electrode 53 in the portion of the ground electrode 53 located inside the inner circumference 30 of the tip 21 are heat-receiving surfaces that receive the heat generated by the combustion of fuel.

[0029] In the spark plug 50, a value S / E obtained by dividing a surface area S, which is the sum of the area of ​​the side surface 63 and the area of ​​the bottom surface 64 of the portion of the ground electrode 53 located on the inner periphery 30 side of the tip portion 21, by a side area E (which is the sum of the area of ​​the interface 61 and the area of ​​the side surface 62 in this embodiment) of the portion of the ground electrode 53 located within the hole 51, is 13.1 or less. This is to prevent overheating of the ground electrode 53 by a balance between heat reception of the ground electrode 53 and heat transfer from the ground electrode 53 to the tip portion 21, and to reduce the occurrence of pre-ignition. EXAMPLES

[0030] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0031] The tester prepared parts such as a metal shell and a ground electrode to fabricate a spark plug. The metal shells were prepared with a radial thickness of 0.3 mm to 1.8 mm at the tip end and a circular hole with a diameter of 0.8 mm to 2.5 mm that penetrated the tip end in the radial direction. The tester inserted ground electrodes of various lengths and with a thickness that fit into the hole of the metal shell, and then irradiated a laser beam onto the bottom surface of the ground electrode to laser-weld the entire circumference of the bottom surface of the ground electrode to the metal shell, thereby fabricating Sample No. 1-12 having the same shape as the spark plug in the first embodiment.

[0032] A three-dimensional image was constructed from the cross-sectional images of sample No. 1-12 obtained using an X-ray CT scanner, and the surface area S (mm 2), the side area E of the part of the ground electrode located inside the hole (the interface area of ​​the fusion zone (mm 2 ) was obtained, and the value S / E was calculated by dividing the surface area S by the side area E. S / E was calculated by rounding off the value obtained by dividing the surface area S by the side area E to one decimal place. Samples No. 1-12 differ in surface area S and side area E, but other dimensions such as the size of the spark gap and the size of the bottom surface of one end of the ground electrode, and materials were the same. The surface area S, side area E, and S / E of Samples No. 1-12 are shown in Table 1.

[0033] [Table 1]

[0034] (Pre-ignition test) The tester measured the advance angle relative to the ignition timing of the genuine spark plug of the test engine in accordance with the pre-ignition test specified in JIS D1606:2020. The larger the crank angle at which pre-ignition occurred, the less likely pre-ignition would occur. Samples where the crank angle at which pre-ignition occurred was advanced by 7° or more relative to the crank angle of the genuine spark plug were rated as A, samples where the crank angle was advanced by 5° to less than 7° were rated as B, samples where the crank angle was advanced by 2° to less than 5° were rated as C, and samples where the crank angle was advanced by less than 2° were rated as D. The results are shown in the plague resistance column in Table 1.

[0035] (Bench durability test) The tester investigated the endurance distance of Sample No. 1-12 according to the bench endurance test specified in JIS D1606:2020. The endurance distance is the distance that corresponds to the spark gap between the center electrode and the ground electrode increasing by 0.2 mm compared to before the test. Samples with an endurance distance of 100,000 km or more were rated as A, and samples with an endurance distance of 50,000 km or more but less than 100,000 km were rated as D. The results are shown in the column for wear resistance in Table 1.

[0036] As shown in Table 1, sample No. 1, which has an S / E ratio greater than 13.1, was rated D for plague resistance, while samples No. 2-12, which have an S / E ratio of 13.1 or less, were rated AC for plague resistance. This shows that pre-ignition can be reduced when the S / E ratio is 13.1 or less.

[0037] Sample No. 2, with an S / E ratio greater than 6.0 and less than 13.1, was given a rating of C for plague resistance, while samples No. 3-12, with an S / E ratio of 6.0 or less, were given a rating of A or B for plague resistance. Therefore, it became clear that pre-ignition can be further reduced if the S / E ratio is 6.0 or less.

[0038] Sample No. 3, with an S / E ratio greater than 5.5 and less than 6.0, was given a B rating for plague resistance, while sample No. 4-12, with an S / E ratio of 5.5 or less, was given an A rating for plague resistance. This shows that pre-ignition can be further reduced if the S / E ratio is 5.5 or less.

[0039] Sample No. 12, with an S / E of 0.5, was rated an A for plague resistance, but a D for wear resistance. This is because when the S / E is 0.5 or less, the volume of the part that functions as the ground electrode where discharge occurs between the center electrode and the electrode becomes smaller, shortening the time it takes to stop functioning as a ground electrode. Therefore, when considering wear resistance, it is desirable for the S / E to be greater than 0.5.

[0040] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention without departing from the spirit of the present invention.

[0041] In the embodiment, the hole 24, 51 is described as being circular, but is not necessarily limited to this. Other shapes of the hole 24, 51 include an ellipse, a semicircle, a polygon such as a triangle, a square, or a hexagon, and a polygon with rounded corners. The cross-sectional shape of the ground electrode 25, 53 disposed in the hole 24, 51 is appropriately set to a shape that fits into the hole 24, 51 according to the shape of the hole 24, 51.

[0042] In the first embodiment, a case where the molten portion 37 is provided continuously around the center line of the hole 24 has been described, and in the second embodiment, a case where the molten portion 59 is provided intermittently around the center line of the hole 51 has been described, but this is not necessarily limited to this. Of course, it is possible to provide the molten portion 37 intermittently around the center line of the hole 24 in the first embodiment, and to provide the molten portion 59 continuously around the center line of the hole 51 in the second embodiment.

[0043] In the embodiment, the ground electrode 25, 53 is joined to the tip portion 21 via the fusion zone 37, 59, but this is not necessarily limited to this. It is of course possible to fix the ground electrode 25, 53 by fitting it into the hole 24, 51 instead of welding. The fit between the hole 24, 51 and the ground electrode 25, 53 is an interference fit or a transition fit. In this case, since there is no interface of the fusion zone, the side area E is the side area of ​​the portion of the ground electrode 25, 53 located inside the hole 24, 51.

[0044] In the embodiment, a case has been described in which a portion of the bottom surface 38, 60 of the ground electrode 25, 53 remains unmelted, but this is not necessarily limited to this. The entire bottom surface 38, 60 of the ground electrode 25, 53 may melt into the molten portion 37, 59, and the bottom surface 38, 60 may disappear. Even when the entire bottom surface 38, 60 is melted into the molten portion 37, 59, the area of ​​the interface 39, 61 between the molten portion 37, 59 and the tip portion 21 is included in the side area E.

[0045] In the first embodiment, the size of the hole 24 is constant in the radial direction of the tip portion 21, but this is not necessarily limited to this. For example, it is of course possible to provide a so-called tapered hole in the tip portion 21, in which the size of the hole decreases from the outside to the inside in the radial direction of the tip portion 21, and to place the ground electrode 25 in the hole.

[0046] In the embodiment, the case where the holes 24, 51 are provided in the male thread 22 of the tip portion 21 has been described, but this is not necessarily limited to this. For example, it is of course possible to provide a cylindrical portion without the male thread 22 in the tip portion 21, and to provide the ground electrodes 25, 53 by drilling holes in the cylindrical portion.

[0047] In the embodiment, the case where the holes 24, 51 penetrate the tip portion 21 has been described, but this is not necessarily limited to this. This is because even if the hole does not penetrate the tip portion 21, as long as a part of the inner circumference 30 of the tip portion 21 is recessed, the other end 33, 58 of the ground electrode 25, 53 can be disposed in the hole.

[0048] In the embodiment, the other end 33, 58 of the ground electrode 25, 53 has substantially the same thickness as the one end 32, 57 of the ground electrode 25, 53, but this is not necessarily limited to this. It is of course possible for the one end 32, 57 of the ground electrode 25, 53 to have a different thickness from the other end 33, 58.

[0049] In the present embodiment, the cross-sectional shape of the one end 32, 57 of the ground electrode 25, 53 is the same as that of the other end 33, 58, but this is not necessarily limited to this. It is of course possible for the cross-sectional shape of the one end 32, 57 to be different from that of the other end 33, 58.

[0050] In the embodiment, a spark gap is provided between the side surface of the center electrode 15 and one end 32, 57 of the ground electrode 25, 53, but this is not necessarily limited to this. A spark gap may be provided between the tip of the center electrode 15 and the side surface of the ground electrode 25, 53 by shifting the position of the hole 24, 51 in the tip portion 21 toward the tip side and making the ground electrode 25, 53 slightly longer.

[0051] In the embodiment, the cap 26 is disposed on the tip side of the tip portion 21 of the metallic shell 20, but this is not necessarily limited to this. It is of course possible to omit the cap 26. This is because, even if the cap 26 is not provided, as long as the one ends 32, 57 of the ground electrodes 25, 53 are surrounded by the tip portion 21 of the metallic shell 20, cooling by the fuel and intake air will be insufficient compared to a spark plug in which the ground electrode is provided outside the metallic shell.

[0052] In the embodiment, the semispherical cap 26 is disposed on the metallic shell 20, but this is not necessarily limited to this. The shape of the cap 26 can be set appropriately. Other examples of the shape of the cap 26 include a bottomed cylindrical shape and a disk shape.

[0053] In the embodiment, the case where the cap 26 is welded to the metallic shell 20 has been described, but the present invention is not necessarily limited to this. It is of course possible to prepare a cylindrical member with a cap at its tip and connect it to the metallic shell 20 to form the space 29. The cylindrical member is a cylindrical member whose tip is closed with a cap, and an internal thread that is connected to the external thread 22 of the metallic shell 20 is provided on its inner circumferential surface. An external thread that is connected to the internal thread of a plug hole of the engine is provided on the outer circumferential surface of the cylindrical member. A cap is disposed on the tip side of the metallic shell 20 by connecting the internal thread of the cylindrical member to the external thread 22 of the metallic shell 20. A through hole 27 is provided in this cap.

[0054] The means for connecting the tubular member to the metal shell 20 and disposing a cap on the tip side of the metal shell 20 is not limited to connecting the female thread on the inner peripheral surface of the tubular member to the male thread 22 of the metal shell 20. It is naturally possible to connect the tubular member to the metal shell by other means. An example of the other means is joining the tubular member and the metal shell by welding or the like. Examples of the material for the tubular member include metal materials such as Ni-based alloys and stainless steel, and ceramics such as silicon nitride. [Explanation of symbols]

[0055] 10,50 Spark plug 15 Center electrode 20 Metal fitting 21 Tip 24,51 holes 25,53 ground electrode 32,57 One end 33,58 Other end

Claims

1. A center electrode; a metal shell that insulates and holds the center electrode; a columnar ground electrode electrically connected to the metallic shell and having one end facing the center electrode, the metallic shell includes a cylindrical tip portion in which the one end portion of the ground electrode is located, The tip portion has a hole into which the other end of the ground electrode is inserted, a surface area of ​​a portion of said ground electrode located on an inner periphery of said tip portion divided by a side area of ​​a portion of said ground electrode located within said hole, said surface area being equal to or less than 13.1;

2. 2. The spark plug of claim 1, wherein said value is less than or equal to 6.

0.

3. 2. The spark plug of claim 1, wherein said value is less than or equal to 5.5.

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