Spark plug

Abstract

To provide a spark plug capable of reducing the overheating of a ground electrode.SOLUTION: A spark plug includes: a metal shell that has a cylindrical tip extending along an axis toward a tip side and that is provided with a through hole penetrating in a direction that intersects the axis, at the tip; and a ground electrode having a fixing portion disposed in the through hole. The through hole includes a counterbore portion and a penetrating portion that is connected to the counterbore portion and that extends to an inner circumferential surface of the tip. The fixing portion includes a first portion disposed in the counterbore portion and a second portion that is disposed in the penetrating portion and that is thinner than the first portion. There is a first gap between the first portion and an edge of the penetrating portion connected to the counterbore portion.SELECTED DRAWING: Figure 2

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 ignites fuel gas, a technology is known in which a ground electrode is disposed in a through hole that penetrates a metal shell. In the technology disclosed in Patent Document 1 (FIG. 5), the portion of the ground electrode that is disposed in the through hole is in contact with the through hole over the entire circumference. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-46660 A Summary of the Invention [Problem to be solved by the invention]

[0004] In this type of spark plug, if the ground electrode overheats, it can become a spark that causes pre-ignition.

[0005] The present invention has been made to solve this problem, and has an object to provide a spark plug which is capable of reducing overheating of the ground electrode. [Means for solving the problem]

[0006] To achieve this object, the spark plug of the present invention comprises a metallic shell having a cylindrical tip portion extending along an axis toward the tip side, and a through hole formed in the tip portion that penetrates in a direction intersecting the axis, and a ground electrode having a fixing portion disposed in the through hole, wherein the through hole comprises a countersunk portion and a through portion connected to the countersunk portion and extending to the inner surface of the tip portion, and the fixing portion comprises a first portion disposed in the countersunk portion and a second portion disposed in the through portion and narrower than the first portion, and a first gap is provided between the edge of the through portion connected to the countersunk portion and the first portion. Effect of the Invention

[0007] According to a first aspect, a through hole penetrating a tip end portion of a metal shell includes a countersunk portion and a through portion extending from the countersunk portion to an inner peripheral surface of the tip end portion. The fixed portion of the ground electrode has a first portion disposed in the countersunk portion and a second portion narrower than the first portion disposed in a through portion narrower than the countersunk portion. A first gap is provided between an edge of the through portion connected to the countersunk portion and the first portion. When fuel gas enters the first gap, the first portion is cooled, thereby reducing overheating of the ground electrode.

[0008] According to the second aspect, since the first part is in contact with the countersunk part, the first part is cooled by heat conduction from the first part to the countersunk part. Since the second gap between the penetration part and the second part is connected to the first gap, the first part is cooled by the fuel gas that has entered the second gap. In addition to the effect of the first aspect, overheating of the ground electrode can be further reduced.

[0009] According to the third aspect, since the tip portion and the first portion are joined by the fusion zone, in addition to the effects of the first or second aspect, it is possible to prevent the ground electrode from falling out of the through hole. [Brief description of the drawings]

[0010] [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. [Figure 4] FIG. 11 is a cross-sectional view of a spark plug according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] 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 in a first embodiment. In Fig. 1, the lower side of the paper refers to the leading end side of the spark plug 10, and the upper side of the paper refers to the rear end side of the spark plug 10 (the same applies to Figs. 2 to 4). Fig. 1 shows a cross section including an axis O of the leading end side of the spark plug 10. The spark plug 10 includes a metal shell 20 arranged on the outer periphery of an insulator 11, and a ground electrode 40 connected to the metal shell 20.

[0012] The insulator 11 is a substantially cylindrical member having an axial hole 12 along the axis O, and is made of ceramics such as alumina, which has excellent mechanical properties and insulating properties at high temperatures. A center electrode 13 is disposed in the axial hole 12 of the insulator 11. The center electrode 13 is a rod-shaped member having electrical conductivity. A portion of the center electrode 13 protrudes from the insulator 11 toward the tip side. The center electrode 13 is electrically connected to a terminal fitting 14 in the axial hole 12. The terminal fitting 14 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is made of a metallic material having electrical conductivity (such as low-carbon steel). The terminal fitting 14 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 (such as low carbon steel). The metal shell 20 has a cylindrical tip portion 21 provided with a male thread 22, and a seat portion 23 adjacent to the rear end side of the tip portion 21. The male thread 22 of the tip portion 21 screws into a threaded hole of an engine (not shown). The outer diameter of the seat portion 23 is larger than the outer diameter of the male thread 22. The seat portion 23 receives an axial force when the male thread 22 is tightened into the threaded hole of the engine.

[0014] A cap 24 is connected to the tip portion 21 of the metallic shell 20 on the tip side of the male thread 22. The cap 24 is a cylindrical or semispherical member with a bottom, and is formed of a metal material containing one or more of Fe, Ni, Cu, etc. as a main component. The tip portion 21 of the metallic shell 20 is closed by the cap 24. In this embodiment, the cap 24 is a semispherical member, and is joined to the tip portion 21 by a welded portion (not shown). An auxiliary chamber 25 is formed by being surrounded by the tip portion 21 and the cap 24. The cap 24 is provided with an injection hole 26 that penetrates the cap 24 in the thickness direction. The injection hole 26 communicates between the combustion chamber of the engine (not shown) and the auxiliary chamber 25.

[0015] A ground electrode 40 is connected to the tip 21 of the metallic shell 20. The ground electrode 40 is a rod-shaped metal member whose main component is, for example, one or more of Pt, Ni, Ir, etc. The material of the ground electrode 40 is different from the material of the metallic shell 20. A spark gap is provided between the center electrode 13 and the ground electrode 40. In this embodiment, a spark gap is provided on the axial tip side of the center electrode 13. It is of course possible to place a tip on the center electrode 13 or the ground electrode 40 in contact with the spark gap.

[0016] When the spark plug 10 is attached to an engine (not shown), operation of the engine valves and pistons causes fuel gas to flow from the engine's combustion chamber through the nozzle hole 26 into the pre-chamber 25. The spark plug 10 generates a flame kernel in the spark gap by discharging between the center electrode 13 and the ground electrode 40. As the flame kernel grows, it ignites the fuel gas in the pre-chamber 25 and the fuel gas burns. Due to the expansion pressure generated by this combustion, the spark plug 10 injects a gas flow containing a flame from the nozzle hole 26 into the combustion chamber. The fuel gas in the combustion chamber is combusted by the flame jet.

[0017] Fig. 2 is a cross-sectional view of the spark plug 10, enlarging a portion indicated by II in Fig. 1. As shown in Fig. 2, a through hole 30 is provided in the tip portion 21 of the metallic shell 20, penetrating from the outer peripheral surface 27 to the inner peripheral surface 28 of the tip portion 21. In this embodiment, the through hole 30 is provided at the position of the male thread 22 in the tip portion 21. The through hole 30 includes a countersunk portion 31 and a through portion 32, in this order from the outer peripheral surface 27 to the inner peripheral surface 28 of the tip portion 21.

[0018] The countersunk portion 31 opens to the outer peripheral surface 27 of the tip portion 21. The depth of the countersunk portion 31 is deeper than the valley 29 of the male thread 22. The through portion 32 is a hole narrower than the countersunk portion 31, and extends from the countersunk portion 31 to the inner peripheral surface 28 of the tip portion 21. One edge 33 of the through portion 32 is connected to the countersunk portion 31, and the other edge 34 of the through portion 32 is connected to the inner peripheral surface 28 of the tip portion 21. In this embodiment, the countersunk portion 31 and the through portion 32 are cylindrical surfaces with a circular cross-sectional shape. An annular step 35 is provided between the countersunk portion 31 and the through portion 32.

[0019] The ground electrode 40 has a fixed portion 41 disposed in the through hole 30. The fixed portion 41 has a first portion 42 disposed in the counterbore portion 31 and a second portion 43 disposed in the through hole 32. The second portion 43 is thinner than the first portion 42. In this embodiment, the first portion 42 and the second portion 43 are cylindrical. The first portion 42 is disposed radially inside the valley 29 of the male thread 22. The first portion 42 is joined to the counterbore portion 31 by a fusion portion 46. Therefore, the first portion 42 is in contact with the counterbore portion 31 via the fusion portion 46. Since the tip portion 21 and the first portion 42 are joined by the fusion portion 46, the ground electrode 40 is prevented from falling off from the through hole 30.

[0020] The first portion 42 of the ground electrode 40 fixed to the through hole 30 by the fusion zone 46 is spaced from the step 35. This provides a first gap 44 between the first portion 42 and the edge 33 of the through hole 32. The first gap 44 may be provided between the entire circumference of the edge 33 and the first portion 42, or may be provided between a part of the edge 33 and the first portion 42. When the first gap 44 is provided between the part of the edge 33 and the first portion 42, the first gap 44 may be provided at one location on the edge 33, or may be provided at multiple locations on the edge 33.

[0021] A second gap 45 is provided between the second portion 43 of the ground electrode 40 and the through portion 32. The second gap 45 is connected from the edge 34 to the edge 33 of the through portion 32. At least a portion of the second gap 45 is connected to the first gap 44. The second gap 45 may be provided between the entire circumference of the through portion 32 and the second portion 43, or may be provided between a portion of the through portion 32 and the second portion 43. When the second gap 45 is provided between a portion of the through portion 32 and the second portion 43, the second gap 45 may be provided at one location of the through portion 32, or may be provided at multiple locations of the through portion 32.

[0022] When unburned fuel gas enters first gap 44 between first portion 42 and edge 33 of through portion 32 connected to counterbore 31 by operation of the engine valves and pistons, first portion 42 is cooled. Since overheating of ground electrode 40 can be reduced by cooling first portion 42, the occurrence of pre-ignition, in which overheated ground electrode 40 becomes a source of fire, can be reduced.

[0023] Since the first portion 42 is in contact with the countersunk portion 31 via the fusion portion 46, the heat of the ground electrode 40 is transferred from the first portion 42 to the engine (not shown) via the countersunk portion 31 (tip portion 21) and the male thread 22. The ground electrode 40 is also cooled by thermal conduction between the first portion 42 and the tip portion 21.

[0024] When the first portion 42 is cooled, the thermal expansion of the first portion 42 is suppressed, and therefore it is possible to reduce the tensile stress generated in the counterbore portion 31 due to the thermal expansion of the first portion 42. This makes it possible to reduce the occurrence of cracks in the counterbore portion 31.

[0025] Since the second gap 45 between the through portion 32 and the second portion 43 is connected to the first gap 44, the unburned fuel gas that has entered the second gap 45 enters the first gap 44, thereby cooling the first portion 42. This further reduces overheating of the ground electrode 40.

[0026] When the second portion 43 is cooled, the thermal expansion of the second portion 43 is suppressed, and this, together with the second gap 45 between the through portion 32 and the second portion 43, reduces the tensile stress generated in the through portion 32 due to the thermal expansion of the second portion 43. Therefore, the occurrence of cracks in the through portion 32 can be reduced.

[0027] When a portion of the edge 33 of the through portion 32 is in contact with the first portion 42, it is possible to expect both the effect that fuel gas enters the first gap 44 and cools the first portion 42, and the effect that the first portion 42 is cooled by thermal conduction at the portion where the edge 33 of the through portion 32 and the first portion 42 contact each other.

[0028] When a portion of the through portion 32 is in contact with the second portion 43, it is possible to expect both the effect that fuel gas enters the second gap 45 and cools the second portion 43, and the effect that the second portion 43 is cooled by thermal conduction at the portion where the through portion 32 and the second portion 43 contact each other.

[0029] The second embodiment will be described with reference to Fig. 3. In the first embodiment, a case where the step 35 between the countersunk portion 31 and the through portion 32 is separated from the first portion 42 of the ground electrode 40 is described. In contrast, in the second embodiment, a case where a part of the first portion 52 of the ground electrode 50 is in contact with the step 35 is described. The same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description will be omitted.

[0030] Fig. 3 is a cross-sectional view of a spark plug according to a second embodiment. Like Fig. 2, Fig. 3 is an enlarged cross-sectional view of a portion indicated by II in Fig. 1 (the same applies to Fig. 4). The ground electrode 50 described in the second embodiment is disposed in the same manner as the ground electrode 40 of the spark plug 10 in the first embodiment.

[0031] The fixed portion 51 of the ground electrode 50 arranged in the through hole 30 includes a first portion 52 arranged in the counterbore portion 31 and a second portion 53 arranged in the through hole 32. The second portion 53 is thinner than the first portion 52. The first portion 52 and the second portion 53 are cylindrical. The radially outer portion of the first portion 52 protrudes toward the second portion 53 along the axis of the ground electrode 50 and contacts the step 35. The fit between the counterbore portion 31 and the first portion 52 is an interference fit. Therefore, the ground electrode 50 is fixed in the through hole 30 with the first portion 52 contacting the counterbore portion 31. The first portion 52 is arranged slightly inward in the radial direction of the valley 29 of the male thread 22.

[0032] A first gap 54 is provided between a first portion 52 of the ground electrode 50 fixed to the through hole 30 and an edge 33 of the through portion 32. The first gap 54 may be provided between the entire circumference of the edge 33 and the first portion 52, or may be provided between a part of the edge 33 and the first portion 52. When the first gap 54 is provided between a part of the edge 33 and the first portion 52, the first gap 54 may be provided at one location on the edge 33, or may be provided at multiple locations on the edge 33.

[0033] A second gap 55 is provided between the second portion 53 of the ground electrode 50 and the through portion 32. The second gap 55 is connected to the first gap 54. The second gap 55 may be provided between the entire circumference of the through portion 32 and the second portion 53, or may be provided between a part of the through portion 32 and the second portion 53. When the second gap 55 is provided between the part of the through portion 32 and the second portion 53, the second gap 55 may be provided at one location of the through portion 32, or may be provided at multiple locations of the through portion 32.

[0034] In the second embodiment, similarly to the first embodiment, when the fuel gas enters the first gap 54, the first portion 52 is cooled. When the fuel gas enters the second gap 55, the second portion 53 is cooled. Since overheating of the ground electrode 50 can be reduced, the occurrence of pre-ignition caused by the overheated ground electrode 50 can be reduced.

[0035] Since the first portion 52 of the ground electrode 50 is press-fitted into the counterbore portion 31 and the ground electrode 50 is fixed in the through hole 30, welding between the ground electrode 50 and the tip portion 21 can be omitted. The first portion 52 is disposed slightly radially inward of the root 29 of the male thread 22, so that when the male thread 22 is fastened to a screw hole in an engine (not shown), the first portion 52 is located inside the screw hole. Since the movement of the first portion 52 is restricted by the screw hole, the ground electrode 50 cannot come out of the through hole 30, so the ground electrode 50 does not fall out of the through hole 30 even while the spark plug is attached to the engine.

[0036] A third embodiment will be described with reference to Fig. 4. In the second embodiment, the case where the first portion 52 of the ground electrode 50 is pressed into the counterbore portion 31 is described. In contrast, in the third embodiment, the case where the first portion 52 of the ground electrode 60 is joined to the tip portion 21 via a fusion portion 61 is described. The same parts as those described in the first and second embodiments are denoted by the same reference numerals, and the following description will be omitted.

[0037] 4 is a cross-sectional view of a spark plug according to the third embodiment. A ground electrode 60 described in the third embodiment is disposed in the same manner as the ground electrode 40 of the spark plug 10 in the first embodiment.

[0038] The fixed portion 51 of the ground electrode 60 disposed in the through hole 30 includes a first portion 52 and a second portion 53. The fusion portion 61 joins between the tip portion 21 and a portion of the first portion 52 that contacts the step 35. The first portion 52 contacts the counterbore portion 31.

[0039] In the third embodiment, similarly to the first embodiment, when the fuel gas enters the first gap 54, the first portion 52 is cooled. When the fuel gas enters the second gap 55, the second portion 53 is cooled. Since overheating of the ground electrode 50 can be reduced, the occurrence of pre-ignition caused by the overheated ground electrode 50 can be reduced.

[0040] The molten portion 61, which has a lower thermal conductivity than the first portion 52 and the tip portion 21, is separated from the first gap 54 and does not appear in the first gap 54. Since the molten portion 61 is not exposed to the first gap 54, pre-ignition, in which the molten portion 61 is the source of fire, can be prevented.

[0041] Although the present invention has been described based on the embodiment, the present invention is not limited to the above embodiment, and it can be easily assumed that various improvements and modifications are possible within the scope of the present invention. For example, the shape of the through hole 30 and the shapes of the ground electrodes 40, 50, 60 can be appropriately set.

[0042] In the embodiment, the case where the tip of the metallic shell 20 is blocked by the cap 24 has been described, but the present invention is not necessarily limited to this. It is of course possible to omit the cap 24 to provide a spark plug 10 without the sub-chamber 25. In this case, too, a flame kernel is generated by discharge between the center electrode 13 and the ground electrodes 40, 50, 60, and the fuel gas is ignited. When the flame kernel grows, the fuel gas in the combustion chamber is burned. Even when the cap 24 is omitted, the fuel gas flows around the ground electrodes 40, 50, 60 due to the operation of the engine valves and pistons, and the fuel gas before combustion enters the first gaps 44, 54 and the second gaps 45, 55, and the ground electrodes 40, 50, 60 are cooled.

[0043] In the embodiment, the ground electrodes 40, 50, 60 are disposed on the metallic shell 20 so as to provide a spark gap on the axial tip side of the center electrode 13, but this is not necessarily limited to this. It is of course possible to dispose the ground electrodes 40, 50, 60 on the metallic shell 20 so as to provide a spark gap on the radial outside of the center electrode 13.

[0044] In the embodiment, the cross-sectional shape of the through hole 30 perpendicular to the axis of the through portion 32 is a circle, but this is not necessarily limited to this. Examples of the cross-sectional shape of the through hole 30 include an ellipse and a polygon. Naturally, it is possible to round or chamfer the corners of the polygon.

[0045] In the embodiment, the case where the countersunk portion 31 has a cylindrical surface has been described, but this is not necessarily limited to this. It is of course possible to make at least the portion of the countersunk portion 31 connected to the through portion 32 a conical surface. In this case, the step 35 may be omitted and the through portion 32 may be connected to the countersunk portion 31, or the step 35 may be provided between the countersunk portion 31 and the through portion 32. It is of course possible to make the step 35 a conical surface.

[0046] In the embodiment, the first parts 42, 52 are cylindrical, but this is not necessarily limited to this. It is of course possible for the shape of the first parts 42, 52 to be, for example, a polygonal column, a polygonal pyramid, or a cone, depending on the shape of the countersunk portion 31.

[0047] In the embodiment, the second parts 43, 53 are described as being cylindrical, but this is not necessarily limited to this. It is of course possible for the second parts 43, 53 to be polygonal prism-shaped. It is of course possible to round or chamfer the corners of the polygonal prism.

[0048] In the embodiment, the through hole 30 to which the ground electrodes 40, 50, 60 are fixed is provided at the position of the male thread 22 of the tip portion 21, but this is not necessarily limited to this. For example, it is of course possible to provide a through hole for fixing the ground electrodes 40, 50, 60 in a portion of the tip portion 21 that is closer to the tip side than the male thread 22.

[0049] In the third embodiment, a case has been described in which the fit between the countersunk portion 31 and the first portion 52 is an interference fit, but this is not necessarily limited to this. It is of course possible for the fit between the countersunk portion 31 and the first portion 52 to be a loose fit. This is because the first portion 52 is joined to the tip portion 21 by the fusion zone 61, and therefore there is no need to press the first portion 52 into the countersunk portion 31 to fix the first portion 52. [Explanation of symbols]

[0050] 10 Spark plug 20 Metal fitting 21 Tip 28 Inner surface of tip 30 Through hole 31 Countersink 32 Penetration 33 Edge of penetration 40,50,60 ground electrode 41,51 Fixed part 42,52 Part 1 43,53 Part 2 44,54 First gap 45,55 Second gap 46,61 Welding section O axis

Claims

1. a metallic shell having a cylindrical tip portion extending along an axis toward a tip side, the tip portion being provided with a through hole passing through in a direction intersecting the axis; a ground electrode having a fixing portion disposed in the through hole, The through hole includes a countersunk portion and a through portion that is connected to the countersunk portion and extends to an inner peripheral surface of the tip portion, the fixing portion includes a first portion disposed in the countersunk portion and a second portion disposed in the through portion and narrower than the first portion, the first portion contacts the counterbore portion, a first gap is provided between an edge of the through portion connected to the countersunk portion and the first portion; The spark plug has a second gap between the through portion and the second portion, the second gap being connected to the first gap.

2. 2. The spark plug according to claim 1, further comprising a fusion zone joining said tip portion and said first portion.

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