Patent Application
20130214671
The present invention relates to a spark plug for an internal combustion engine and, more particularly, to a ground electrode of the spark plug.
It is required that a ground electrode (outer electrode) of a spark plug used in an internal combustion engine has heat resistance due to the fact that the ground electrode is subjected to high-temperature conditions. There is thus known a spark plug of the type in which a ground has a multilayer structure of a plurality of layers including a surface layer and a core formed of a higher thermal conductivity material such as copper or copper alloy inside the surface layer in order to promote heat radiation for improvement in heat resistance (see, for example, Patent Document 1). In this type of spark plug, the ground electrode is generally joined to a metal shell by resistance welding.
The heat resistance of the multilayer ground electrode increases with the amount of the core. On the other hand, the thickness of the surface layer decreases with increase in the amount of the core. The highly heat-conductive core does not largely contribute to the resistance welding strength between the ground electrode and the metal shell. Otherwise, the joint strength between the ground electrode and the metal shell decreases according to an increase in the amount of the core because the core is lower in strength than the surface layer.
Further, the surface layer spreads outwardly (as a welding burr) during the resistance welding of the ground electrode and the metal shell due to differences in melting point and strength between the surface layer and the core. A redundant part of such a spread of the surface layer is generally removed. However, the joint strength between the ground electrode and the metal shell further decrease upon removal of the part of the surface layer. The durability of the spark plug may deteriorate as the joint strength between the ground electrode and the metal shell decreases as mentioned above.
In view of at least part of the above problems, it is an object of the present invention to provide a spark plug in which a multilayer ground electrode is joined by resistance welding to a metal shell so as to secure favorable joint strength between the ground electrode and the metal shell.
The present invention has been made to solve at least part of the above problems and can be embodied in the following aspects or application examples.
A spark plug, comprising:
a rod-shaped center electrode extending in an axis direction of the spark plug;
an insulator having an axial hole formed in the axis direction so as to retain the center electrode in the axial hole;
a metal shell circumferentially surrounding the insulator; and
a ground electrode having a base end portion welded to the metal shell and a distal end portion facing an axially front end portion of the center electrode with a gap left between the distal end portion of the ground electrode and the front end portion of the center electrode,
the ground electrode including a surface layer defining a surface of the ground electrode and a core located inside the surface layer and having a higher thermal conductivity than that of the surface layer,
the surface layer having a thickness of 0.2 mm to 0.4 mm at a specific position that is located 1 mm from the base end portion in a direction toward the distal end portion along an outer diameter of the ground electrode,
wherein the spark plug satisfies the following condition: W1≧W2×1.55−(W3+0.25) where W1 (mm) is a width of the metal shell at a weld region of the metal shell joined with the base end portion in a specific direction that extends perpendicular to the axis direction through a center line of the ground electrode; W2 (mm) is a thickness of the ground electrode at the specific position in the specific direction; and W3 (mm) is a thickness of the surface layer at the specific position in the specific direction.
The spark plug according to Application Example 1,
wherein the core includes a first core member located on an inner side thereof and a second core member located on an outer side thereof so as to circumferentially surround the first core member and having a higher thermal conductivity and a lower hardness than those of the first core member;
wherein the first core member has a protruding shape that protrudes to a rear of the second core member in the axis direction;
wherein the weld region has an undulating shape that undulates according to the protruding shape of the first core member; and
wherein a distance between frontmost and rearmost ends of the weld region in the axis direction is 0.15 mm or more.
The spark plug according to Application Example 1,
wherein the core includes a first core member located on an inner side thereof and a second core member located on an outer side thereof so as to circumferentially surround the first core member and having a higher thermal conductivity and a lower hardness than those of the first core member; and
wherein, when viewed in cross section along a plane defined by the axis direction and the specific direction, the spark plug satisfies the following condition: W4/W5≦0.34 where, on the assumption that: a first imaginary line extends in parallel to the specific direction through a midpoint of a spark gap SG, which is defined between the center electrode and the ground electrode in the axis direction; and a second imaginary line passes through an intersection point of the first imaginary line and a plane of the ground electrode located closer to the center electrode and intersecting the first imaginary line at an elevation angle of 45 degrees toward the specific direction, W4 is the sum of widths of the second core member on the secondary imaginary line; and W5 is the sum of widths of the surface layer on the second imaginary line.
The spark plug according to Application Example 2,
wherein, when viewed in cross section along a plane defined by the axis direction and the specific direction, the spark plug satisfies the following condition: W4/W5≦0.34 where, on the assumption that: a first imaginary line extends in parallel to the specific direction through a midpoint of a spark gap SG, which is defined between the center electrode and the ground electrode in the axis direction; and a second imaginary line passes through an intersection point of the first imaginary line and a plane of the ground electrode located closer to the center electrode and intersecting the first imaginary line at an elevation angle of 45 degrees toward the specific direction, W4 is the sum of widths of the second core member on the secondary imaginary line; and W5 is the sum of widths of the surface layer on the second imaginary line.
The spark plug according to any one of Application Examples 1 to 4,
wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, a center line of the ground electrode extending at the specific position in the axis direction is located closer to the center electrode than a center line of the metal shell extending at the weld region in the axis direction.
The spark plug according to any one of Application Examples 1 to 5,
wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, the metal shell has a protruding shape that protrudes at the center thereof toward the front in the axial direction, and the core has an undulating shape that undulates according to the protruding shape of the metal shell, and an outer thickness of the ground electrode in the specific direction at axially rear end points of the core is larger than a thickness of the distal end portion of the ground electrode.
In the spark plug of Application Example 1, the ground electrode includes the surface layer and the core having a higher thermal conductivity than that of the surface layer. It is therefore possible to improve the heat resistance of the ground electrode. It is also possible to secure favorable joint strength between the ground electrode and the metal shell by keeping the surface layer and the core in balance with each other.
In the spark plug of Application Example 2, the core includes the first core member located in the inner side thereof and the second core member located in the outer side thereof. Further, the first core member is higher in hardness than the second core member. It is therefore possible to improve the joint strength between the ground electrode and the metal shell. The joint strength can be further improved by forming the welding face between the ground electrode and the metal shell into a predetermined undulating shape.
In the spark plug of Application Example 3, the surface layer and the core can be kept in balance so that it is possible to prevent the ground electrode from becoming deformed by heating/cooling cycles during actual use of the spark plug on the internal combustion engine.
In the spark plug of Application Example 4, the surface layer and the core can be kept in balance so that it is possible to prevent the ground electrode from becoming deformed by heating/cooling cycles during actual use of the spark plug on the internal combustion engine.
In the spark plug of Application Example 5, the joint strength between the ground electrode and the metal shell can be improved.
In the spark plug of Application Example 6, the joint strength between the ground electrode and the metal shell can be improved by securing the thickness of the ground electrode at the base end part of the core.
A first embodiment of the present invention will be described below.
The center electrode 20 is a rod-shaped electrode protruding from a front end of the ceramic insulator 10. The terminal rod 40 is inserted in a rear side of the ceramic insulator 10; and the center electrode 20 is electrically connected to the terminal rod 40 within the ceramic insulator 10. An outer circumference of the center electrode 20 is retained by the ceramic insulator 10, whereas an outer circumference of the ceramic insulator 10 is retained by the metal shell 50 at a position apart from the terminal rod 40.
The ceramic insulator 10 is a cylindrical insulator having, in the center thereof, an axial hole 12 in which the center electrode 20 and the terminal rode 40 are inserted. The ceramic insulator 10 is formed by sintering a ceramic material such as alumina. The ceramic insulator 10 includes a middle body portion 19 located at an axially middle position thereof and having an enlarged outer diameter, a rear body portion 18 located rear of the middle body portion 19 so as to provide an insulation between the terminal rod 40 and the metal shell 50, a front body portion 17 located front of the middle body portion 19 and having an outer diameter made smaller than that of the rear body portion 18 and a leg portion 13 located front of the front body portion 17 and having an outer diameter made smaller than that of the front body portion 17 in such a manner that the outer diameter of the leg portion 13 gradually decreases toward the center electrode 20.
The metal shell 50 is a cylindrical metal fixture surrounding and retaining therein a part of the ceramic insulator 10 extending from some point of the rear body portion 18 to the leg portion 13. In the present embodiment, the metal shell 50 is formed of low carbon steel. The metal shell 50 includes a tool engagement portion 51, a mounting thread portion 52, a cylindrical portion 53 and a seal portion 54. The tool engagement portion 51 of the metal shell 50 is engageable with a tool for mounting the spark plug 100 onto an engine head. The mounting thread portion 52 of the metal shell 50 has a screw thread screwed into a mounting thread hole of the engine head. The seal portion 54 of the metal shell 50 is formed into a flange shape at a bottom of the mounting thread portion 52. An annular gasket 5, which is formed by bending a plate material, is disposed between the seal portion 54 and the engine head (not shown). A front end face 57 of the metal shell 50 is formed into a hollow circle shape so that the center electrode 20 protrudes from the leg portion 13 of the ceramic insulator 10 through the center of the front end face 57 of the metal shell 50.
The center electrode 20 has a rod shape including a bottomed cylindrical electrode body 21 and a core 25 having a higher thermal conductivity than that of the electrode body 21 and embedded in the electrode body 21. In the present embodiment, the electrode body 21 is formed of a nickel alloy containing nickel as a main component; and the core 25 is formed of copper or an alloy containing copper as a main component. The center electrode 20 is inserted in the axial hole 12 of the ceramic insulator 10, with a front end of the electrode body 21 protruding from the axial hole 12 of the ceramic insulator 10, and is electrically connected to the terminal rod 40 via a ceramic resistor 3 and a seal member 4.
The ground electrode 30 is joined at one end portion, i.e., a base end portion 37 thereof to the front end face 57 of the metal shell 50 and is bent in such a manner that the other end portion, i.e., a distal end portion 38 of the ground electrode 30 faces a front end portion of the center electrode 20. In the present embodiment, the ground electrode 30 has a two-layer structure. The inner structure of the ground electrode 30 will be explained in detail later. The base end portion 37 of the ground electrode 30 and the front end face 57 of the metal shell 50 are joined together by resistance welding. There is a spark gap defined between the distal end portion 38 of the ground electrode 30 and the front end portion of the center electrode 20.
An end face of the base end portion 37, i.e., a base end face 39 of the ground electrode 30 is located at the center of the front end face 57 of the metal shell 50 in a specific direction PD that extends perpendicular to the axis OL through a center line CA1 of the base end portion 37 of the ground electrode 30. As a result of the burr D being formed around the base end face 39, the weld region 58 of the front end face 57 of the metal shell 50 joined with the base end portion 37 of the ground electrode 30 is made larger than the base end face 30. Further, the weld region 58 is formed throughout the width of the front end face 57 in the specific direction PD. In the case where the burr D largely extends off from the front end face 57 during the joining of the ground electrode 30 and the metal shell 50, at least some of the extending part is generally removed. Herein, the width of the front end face 57 at the weld region 58 may be referred to as a width W1; the length and width of the base end face 39 of the ground electrode 30 before the joining (welding) may be referred to as a length L and a width W, respectively.
The surface layer 31 has a thickness of 0.2 to 0.4 mm a position 1 mm from the base end portion 37 (base end face 39) of the ground electrode 30 in a direction toward the distal end portion 38 of the ground electrode 30 along the outer diameter of the ground electrode 30 (hereinafter, also referred to as “specific position PP”). The specific position PP can be determined by appearance inspection of the ground electrode 30 in the case where the base end face 39 undulates due to the welding of the ground electrode 30 and the metal shell 50. The specific position PP specifies a site where the burr D does not occur during the welding of the ground electrode 30 and the metal shell 50. Further, the following formula (1) holds between W1, W2 and W3 where W1 (mm) is a width of the weld region 58 (front end face 57) of the metal shell 50 in the specific direction PD; W2 (mm) is a thickness of the ground electrode 30 at the specific position PP in the specific direction PD; and W3 (mm) is a thickness of the surface layer 31 at the specific position PP in the specific direction PD. The width W1 can also given by the following formula (2). In the present embodiment, the dimensions of the ground electrode 30 are constant from the base end portion 37 to the distal end portion 38. Namely, the length L and width W of the ground electrode 30 are constant. The width W can be thus also given by the following formula (3).
W1≧W2×1.55−(W3+0.25) (1)
W1=(OD−ID)/2 (2)
W2=W (3)
The reason for satisfaction of the formula (1) between the width W1, the thickness W2 and the thickness W3 will be explained below.
The joint strength test was performed by the following procedure. (1) A bending operation of bending the ground electrode 30 inwardly (toward the center electrode 20) at 90 degrees about a point 2 mm in the direction along the outer diameter of the ground electrode 30 from the base end face 39 of the ground electrode 30 to the distal end portion 38 of the ground electrode 30, and then, bending the ground electrode 30 back to the original position was repeated plural times. (2) The joint strength was evaluated as “normal” (indicated by the symbol “Δ” in
As shown in
For example, the test results of
The relationship of the width W2 of the ground electrode 30 and the width W1 of the front end face 57 of the metal shell 50 was determined for each thickness W3 of the surface layer 31 by plotting the above test results as shown in
The thickness W3 of the surface layer 31 was 0.2 mm in
In the case where the thickness W3 of the surface layer 31 was 0.4 mm, the relationship of the thickness W2 and the width W1 satisfying the formula (1) was specified as the range above line L2 in
A spark plug 200 according to a second embodiment of the present invention will be described below. The spark plug 200 of the second embodiment includes a ground electrode 230 and a metal shell 250 in place of the ground electrode 30 and the metal shell 50 of the first embodiment. The spark plug 200 of the second embodiment is structurally similar to the spark plug 100 of the first embodiment, except that the inner structure of the ground electrode 230 and the cross section of the joint between the ground electrode 230 and the metal shell 250 are different from those of the first embodiment. Hereinafter, differences of the spark plug 200 of the second embodiment from the spark plug 100 of the first embodiment will be explained in detail below.
A base end part of the surface layer 231 of the ground electrode 230 and a front end part of a cylindrical portion 253 of the metal shell 250 spread outwardly as there is a burr formed due to deformation or fusion of the part of the surface layer 231 and the part of the cylindrical portion 253 during the resistance welding of the ground electrode 230 and the metal shell 250.
As shown in
In the spark plug 200, the weld region 258 is formed in such a manner that the distance D1 between the weld ends 258a and 258b in the axis OL direction is 0.15 mm or more. In the present embodiment, the distance D1 is set to 0.20 mm. Although there is a conventional type of spark plug in which a metal shell has an undulating weld region as mentioned above, the distance D1 is of the order of 0.1 mm in the conventional spark plug.
The ground electrode 230 and the metal shell 250 can be formed into such shapes by controlling the application current, application pressure and energization pattern during the resistance welding of the ground electrode 230 and the metal shell 250.
The reason for setting the distance D1 to 0.15 mm or more will be explained below. The setting standard of the distance D1 has been found by vibration test. In the vibration test, the spark plug 200 was subjected to vibrations conditions simulating vibrations applied to the spark plug 200 during actual use on an internal combustion engine for the purpose of evaluating the joint strength between the ground electrode 230 and the metal shell 250. In the present embodiment, the vibration test was performed based on the impact test method according to JIS B 8031. A plurality of samples of different distance D1 was tested for the time to rupture in the joint between the ground electrode 230 and the metal shell 250. The vibration test was performed on five samples for each distance D1; and the average value of the rupture time measurement results was determined as an average rupture time RT. The rupture time measurement was conducted for maximum 60 minutes. The rupture time was indicated as 60 minutes when there occurred no rupture within 60 minutes. Further, the test was performed under heated conditions that the temperature of the front end portion 230 of the ground electrode 230 was set to 900° C. on the assumption of the actual use conditions of the spark plug 200.
In the case of using the ground electrode 230 of W 1.1 mm×L 2.2 mm, for example, the average rupture time RT was 31 minutes when the distance D1 was 0.05 mm. On the other hand, the average rupture time RT was 60 minutes, that is, there occurred no rupture when the distance D1 was 0.14 mm or 0.18 mm.
The relationship of the distance D1 and the average rupture time RT was determined for each dimension type of the ground electrode 230 by plotting the above vibration test results as shown in
A spark plug 300 according to a third embodiment of the present invention will be described below. The spark plug 300 of the third embodiment is structurally similar to the spark plug 100 of the first embodiment, except that the spark plug 300 includes a ground electrode 33 in place of the ground electrode 30 of the first embodiment. Hereinafter, differences of the spark plug 300 of the third embodiment from the spark plug 100 of the first embodiment will be explained in detail below.
The core 332 is located in substantially the center of the ground electrode 330. The core 332 has a shape tapering down toward a distal end portion 338 of the ground electrode 330. Namely, the thickness of the core 332 gradually decreases toward the distal end portion 338. In other words, the thickness of the surface layer 331 gradually increases toward the distal end portion 338. Further, the core 332 is not located in the vicinity of the distal end portion 338 of the ground electrode 330. The above inner structure of the ground electrode 330 results from the production process of the ground electrode 330.
When viewed in cross section, there is a spark gap SG defined between the center electrode 320 and the ground electrode 330 in the axis OL direction. In the cross section of the ground electrode 330, an imaginary line extending in parallel to the specific direction PD through a midpoint MP of the spark gap SG is referred to as a first imaginary line VL1; and an imaginary line passing through an intersection point IP of the first imaginary line VL2 and a plane of the ground electrode 33 located closer to the center electrode 320 and intersecting the first imaginary line VL2 at an elevation angle of 45 degrees toward the specific direction PD is referred to as a second imaginary line VL2. In the present embodiment, the ground electrode 330 is formed in such a manner as to satisfy the following formula (4) where W41 and W42 are widths of the second core member 334 on the second imaginary line VL2; W51 and W52 are widths of the surface layer 31 on the second imaginary line VL2; W4=W41+W42; and W5=W51+W52.
W4/W5≦0.34 (4)
The reason for satisfaction of the formula (4) will be explained below. The formula (4) has been derived by heating/cooling cycle test. When the spark plug 300 with the multilayer ground electrode 330 is mounted to an engine head and subjected to heating/cooling cycles during actual use on an internal combustion engine, a bent portion of the ground electrode 330 becomes deformed outwardly, i.e., toward a side opposite from the center electrode 320 due to a difference in thermal expansion coefficient between the surface layer 331 and the core 332. This causes an increase of the spark gap SG. FIG. 10 is a schematic view showing such a deformed state. In
In the present embodiment, the heating/cooling cycle test was performed by preparing samples of the spark plug 300 in which the value W4/W5 was varied, and then, subjecting each of the samples of the spark plug 300 to heating/cooling cycle conditions where the operation of heating the ground electrode 330 up to maximum 900° C. for 2 minutes with the use of a burner and cooling the ground electrode 300 naturally for 1 minute was assumed as one cycle. The displacement amount DD was measured after repeating 5000 cycles.
The relationship of the value W4/W5 and the displacement amount DD was determined by plotting the above heating/cooling cycle test results as shown in
A spark plug 400 according to a fourth embodiment of the present invention will be described below. The spark plug 400 of the fourth embodiment is structurally similar to the spark plug 100 of the first embodiment, except that the joint position between a ground electrode 430 and a metal shell 450 of the spark plug 400 is different from that of the first embodiment. Hereinafter, differences of the spark plug 400 of the fourth embodiment from the spark plug 100 of the first embodiment will be explained in detail below.
Herein, a center line of the ground electrode 430 extending at the specific position PP in the axis OL direction is referred to as center line CA2; and a center line of the metal shell 450 extending at a weld region 458 in the axis OL direction is referred to as center line CA3. In the spark plug 400, the ground electrode 430 and the metal shell 450 are joined together in such a manner that the center line CA2 is located closer to the center electrode 420 than the center line CA3. It can be said that, by such a positional relationship, the ground electrode 430 is offset toward the center electrode 420. In the above positional relationship of the center lines CA2 and CA3, the distance of separation between the center lines CA2 and CA3 is referred to as offset amount OF.
The reason for joining the ground electrode 430 and the metal shell 450 with the ground electrode 430 being offset toward the center electrode 420 will be explained below. This positional relationship has been found by vibration test.
The relationship of the offset amount OF and the average rupture time RT was determined by plotting the above vibration test results as shown in
This effect can be obtained according to the fact that, when the ground electrode 430 is offset toward the center electrode 420 due to a difference between the outer diameter OD and inner diameter ID of the weld region 458, the surface area of the burr D formed on the front end face of the metal shell 450, that is, the surface area of the weld region 458 increases. It is feasible to set the offset amount OF as appropriate within the range that, in the positional relationship of the ground electrode 430 and the metal shell 450 before the welding, the offset amount OF has a maximum value at a point where a surface of the metal shell 450 facing the center electrode 420 and a surface of the ground electrode 430 facing the center electrode 420 are located at the same position in the specific direction PD (at the position shown in
A spark plug 500 according to a fifth embodiment of the present invention will be described below. The spark plug 500 of the fifth embodiment includes a ground electrode 530 and a metal shell 550 in place of the ground electrode 30 and the metal shell 50 of the first embodiment. The spark plug 500 of the fifth embodiment is structurally similar to the spark plug 100 of the first embodiment, except that a cross-sectional profile of the joint between the ground electrode 530 and the metal shell 550 is different from that of the first embodiment. Hereinafter, differences of the spark plug 500 of the fifth embodiment from the spark plug 100 of the first embodiment will be explained in detail below.
When the joint between the ground electrode 530 and the metal shell 550 is taken in cross section as shown in
Because of the above-mentioned outwardly spread shape, the outer thickness W6 of the ground electrode 530 at the end points 539 in the specific direction PD is made larger than the thickness W7 of a distal end portion 538 of the ground electrode 530. In the present embodiment, the length L and width W of the ground electrode 530 are constant as in the case of the first embodiment. Any part of the ground electrode 530 that is not formed into the above outwardly spread shape has a thickness of W7 in the specific direction PD.
The ground electrode 530 and the metal shell 550 can be formed into such shapes by adjusting the shape of a jig for chucking inner and outer circumferential surfaces of the ground electrode during the resistance welding of the ground electrode 530 and the metal shell 550.
The reason for forming the above-shaped cross-sectional profile of the joint between the ground electrode 530 and the metal shell 550 will be explained below. The above-shaped cross-sectional profile has been found by vibration test.
The relationship of the value W6/W7 and the average rupture time RT was determined by plotting the above vibration test results as shown in
Although the opposite end points 539 of the core 532 in the specific direction PD are located at the same position in the axis OL direction in the above embodiment, there is a case where the positions of these end points 539 do not strictly coincide with each other because of manufacturing errors etc. In such a case, the thickness W6 can be determined as the outer thickness of the ground electrode 530 in the specific direction PD at the relatively front one of the two end points 539.
Each of the above embodiments has been described as the spark plug of vertical discharge type in which the spark gap SG is defined in the axis OL direction. However, the spark plug of the present invention is not limited to such type. The present invention can be applied to various types of spark plugs. For example, the spark plug of the first, second, fourth or fifth embodiment may be embodied as a spark plug of lateral discharge type in which a spark discharge occurs in a direction perpendicular to the axis OL direction. Further, the spark plug may be provided with a plurality of ground electrodes relative to one center electrode.
Although the present invention has been described with reference to the above embodiments, the constituent features of the present embodiment other than those described in independent claim are additional features. These additional features can be omitted as appropriate or can be adopted in any combination. The present invention is not limited to the above embodiments. Various modification and variations of the above embodiments are possible within the technical scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-247603 | Nov 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/004619 | 8/18/2011 | WO | 00 | 4/25/2013 |
