Patent Grant
9935430
This application is a National Stage of International Application No. PCT/JP2013/083450 filed Dec. 13, 2013, which claims the benefit of Japanese Patent Application No. 2012-275110 filed Dec. 17, 2012.
The present invention relates to a spark plug and particularly to a spark plug having a noble metal portion provided in at least one of a ground electrode and a center electrode.
A spark plug used for ignition of an internal combustion engine, such as an automobile engine, generally includes: a tubular metallic shell; a tubular insulator disposed in a bore of the metallic shell; a center electrode disposed in a bore of the insulator at the forward end of the bore; and a ground electrode with one end joined to the forward end of the metallic shell and the other end forming a spark discharge gap between the other end and the center electrode. With the spark plug, spark discharge occurs in the spark discharge gap formed between the distal end of the center electrode and the forward end of the ground electrode within a combustion chamber of an internal combustion engine to thereby ignite fuel injected into the combustion chamber.
For the purpose of improving the durability of spark plugs, noble metal tips formed from a noble metal alloy have conventionally been provided on the discharge surface of the ground electrode and the discharge surface of the center electrode that face each other. However, increasing the degree of compression within combustion chambers and lean burn have recently become mainstream practices, and the use environment of spark plugs has become increasingly severe. Therefore, there is demand for further development of spark plugs that can maintain durability even in such a severe environment.
Even when a tip formed of high-melting point Ir alloy is used as the material of a spark discharge electrode, the resistance of the tip to spark wear is insufficient. To address this problem, Japanese Patent Application Laid-Open (kokai) No. 2002-93547, for example, has as its object to improve the heat dissipation of an Ir alloy tip (see paragraphs 0003 to 5 in Japanese Patent Application Laid-Open (kokai) No. 2002-93547). The solution to the problem described in Japanese Patent Application Laid-Open (kokai) No. 2002-93547 is “the spark plug being characterized in that the Ir alloy tip is embedded in an end portion of the ground electrode with part of the Ir alloy tip exposing from the discharge surface and that, when the discharge surface is viewed from the top, a side edge (47) of the Ir alloy tip coincides with an outer circumferential edge of the discharge surface or is located inward of the outer circumferential edge of the discharge surface” (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. 2002-93547).
One possible method of ensuring sufficiently high wear resistance of a noble metal tip of a spark plug to extend its service life even in the recent severe use environment of the spark plug is to increase the diameter of the noble metal tip. However, when a noble metal tip with an increased diameter is laser-welded to an electrode in an ordinary manner, the noble metal tip may be easily separated from the electrode. Therefore, when a noble metal tip with an increased diameter is used, the energy of the applied laser beam must be increased to ensure sufficiently high separation resistance of the noble metal tip. However, when the energy of the applied laser beam is increased, the exposed area of a fused portion between the noble metal tip and the electrode increases, and therefore the surface area of the noble metal tip decreases; i.e., the height of the noble metal tip from a discharge surface to the end of the fused portion decreases. In this case, a consumable portion of the noble metal tip decreases, so that the effect of extending the service life of the spark plug by increasing the diameter of the noble metal tip is reduced.
An advantage of the present invention is a spark plug in which a noble metal tip is provided on at least one electrode of a ground electrode and a center electrode (the at least one electrode may hereinafter be referred to simply as an electrode) and which is excellent in durability by virtue of the noble metal tip having a sufficiently high wear resistance and a sufficiently high separation resistance.
(1) In accordance with a first aspect of the present invention, there is provided a spark plug comprising a center electrode, a ground electrode, and a noble metal tip laser-welded to at least one electrode of the center electrode and the ground electrode (the at least one electrode being hereinafter referred to as an electrode), the noble metal tip having a gap forming surface that forms a gap between the gap forming surface and the other electrode, wherein
(2) In accordance with a second aspect of the present invention, there is provided a spark plug as described in paragraph (1), wherein the noble metal tip and the electrode to which the noble metal tip is joined have respective facing surfaces facing each other.
(3) In accordance with a third aspect of the present invention, there is provided a spark plug as described in paragraph (2), wherein the ratio of the area of the fused portion in a second region to the area of a first region is at least 60%, the first region being a region that is surrounded by the side circumferential surface of the noble metal tip and located in a virtual plane extending in a radial direction of the noble metal tip and including a point of the second fused portion that is closest to the gap, the second region being a projection of the first region onto the first electrode surface.
(4) In accordance with a fourth aspect of the present invention, there is provided a spark plug as described in paragraph (3), wherein a surface of the noble metal tip that is opposite the gap forming surface is joined entirely through the fused portion to the electrode to which the noble metal tip is joined.
(5) In accordance with a fifth aspect of the present invention, there is provided a spark plug as described in any of paragraphs (1) through (4), wherein the noble metal tip is placed on and joined to a flat surface of the electrode to which the noble metal tip is joined, or the noble metal tip is partially embedded in and joined to a recess formed on a surface of the electrode and the axial length of a portion of the noble metal tip embedded in the recess is 0.15 mm or less.
In the spark plug of the present invention, the noble metal tip is joined to the electrode through the fused portion formed by laser welding. The fused portion includes the first fused portion and the second fused portion. In the first fused portion, the fused portion is exposed at a second electrode surface opposite the gap forming surface and/or a first electrode surface to which the noble metal tip is joined. In the second fused portion, the fused portion is exposed at the side circumferential surface of the noble metal tip. Since the exposed area of the second fused portion that has lower wear resistance than the noble metal tip is reduced as much as possible, the wear resistance is improved. Since the first fused portion is provided, separation resistance is ensured.
When the spark plug is used in a severe environment, it is contemplated that, for example, the diameter of the noble metal tip is increased in order to ensure sufficiently high wear resistance of the noble metal tip. Even in such a case, the first fused portion provided prevents easy separation of the noble metal tip, so that sufficiently high separation resistance can be ensured without increasing the exposed area of the second fused portion. Even when the diameter of the noble metal tip is increased, it is not necessary to increase the exposed area of the second fused portion accordingly in order to ensure the separation resistance, thereby ensuring the surface area of the noble metal tip; i.e., the distance from the gap forming surface to a point in the second fused portion that is closest to the gap. Since the noble metal tip is worn from the gap forming surface; i.e., the discharge surface, in a depth direction, the longer (i.e., the greater) the above distance, the longer the service life of the noble metal tip. Therefore, according to the spark plug of the present invention, the separation resistance of the noble metal tip can be ensured, and the wear resistance can also be improved to an extent equivalent to an increase in the volume of the noble metal tip achieved by increasing its diameter.
In the spark plug of the present invention, the noble metal tip and the electrode to which the noble metal tip is joined may have respective facing surfaces that face each other. In this case, the noble metal tip and the electrode joined by laser welding have non-fused portions that are in direct contact with each other. The thermal conductivity of the electrode is higher than the thermal conductivity of the fused portion formed by fusing the noble metal tip and the electrode. Therefore, when the noble metal tip and the electrode have respective facing surfaces that face each other with no fused portion therebetween, heat generated by spark discharge and heat received by the noble metal tip from a high-temperature combustion chamber can be easily dissipated through the facing surfaces (the heat dissipation may hereinafter be referred to as heat transfer). Therefore, the spark plug having the facing surfaces has much higher wear resistance.
In the spark plug of the present invention, the ratio of the area of the fused portion in the second region to the area of the first region may be at least 60%. Specifically, since the noble metal tip and the electrode are joined to each other through the fused portion at the above area ratio, the separation resistance of the noble metal tip can be sufficiently ensured.
In the spark plug of the present invention, the surface of the noble metal tip that is opposite the gap forming surface may be joined entirely through the fused portion to the electrode to which the noble metal tip is joined. Specifically, the noble metal tip and the electrode do not have facing surfaces which are in direct contact with each other, and the entire noble metal tip is in contact with the electrode through the fused portion. Therefore, the occurrence of separation of the noble metal tip starting from the facing surfaces can prevented, and the separation resistance of the noble metal tip can be further improved.
In the spark plug of the present invention, the noble metal tip may be placed on and joined to a flat surface of the electrode. Alternatively, the noble metal tip may be partially embedded in and joined to a recess formed on a surface of the electrode, and the axial length of the portion of the noble metal tip that is embedded in the recess may be 0.15 mm or less. The portion of the noble metal tip that is embedded in the recess of the electrode does not contribute to the improvement in the wear resistance. When the noble metal tip is placed on and joined to the flat surface of the electrode, the noble metal tip does not have such an embedded portion. Therefore, a wear resistance improving effect corresponding to the volume of the joined noble metal tip is obtained. In the spark plug of the present invention in which the noble metal tip is joined to the electrode through the first fused portion and the second fused portion, it is more effective for improvement of the wear resistance that the noble metal tip is joined to the electrode without being embedded in the electrode. When the noble metal tip is partially embedded in and joined to the recess of the electrode, the second fused portion can be formed by laser welding such that almost no second fused portion is exposed at the side circumferential surface. Thus, the surface area of the noble metal tip can be maximized. Therefore, deterioration of the effect of improving the wear resistance by the noble metal tip, which deterioration is caused by the second fused portion, can be suppressed. When the noble metal tip is partially embedded in and joined to the recess of the electrode, the separation resistance is enhanced as compared with the case where the noble metal tip is placed on and joined to the flat surface of the electrode. As the axial length of the portion of the noble metal tip that is embedded in the recess increases, the volume of the embedded portion that does not contribute to the improvement in the wear resistance increases, and therefore the wear resistance improving effect corresponding to the volume of the noble metal tip cannot be obtained. When the portion of the noble metal tip, the portion being embedded in the recess, is 0.15 mm or less, the effect of improving the wear resistance by the noble metal tip is obtained while the separation resistance between the noble metal tip and the electrode is improved.
The spark plug according to the present invention includes a center electrode, a ground electrode, and a noble metal tip laser-welded to at least one of the center electrode and the ground electrode. No structural limitation is imposed on the spark plug according to the present invention so long as the spark plug has the above-described structure, and any of various known structures may be used.
A spark plug which is an embodiment of the spark plug according to the present invention is shown in
As shown in
The insulator 3 has the axial bore 2 extending in the direction of the axis O. The center electrode 4 is disposed within the axial bore 2 at its forward end, and the metal terminal 5 is disposed within the axial bore 2 at its rearward end. Seal bodies 10 and 11 for fixing the center electrode 4 and the metal terminal 5 within the axial bore 2 and a resistor 12 for reducing radio noise are disposed between the center electrode 4 and the metal terminal 5. A flange portion 13 protruding radially is formed near the center, with respect to the direction of the axis O, of the insulator 3. A rear trunk portion 14 that accommodates the metal terminal 5 and insulates the metal terminal 5 from the metallic shell 6 is formed rearward of the flange portion 13. A forward trunk portion 15 that accommodates the resistor 12 is formed forward of the flange portion 13, and a leg portion 16 that accommodates the center electrode 4 and has an outer diameter smaller than the outer diameter of the forward trunk portion 15 is formed forward of the forward trunk portion 15. The insulator 3 is fixed to the metallic shell 6 with the forward end of the insulator 3 protruding from the forward end surface of the metallic shell 6. Preferably, the insulator 3 is formed from a material having mechanical strength, thermal strength, and electric strength. Examples of such a material include a ceramic sintered body formed mainly of alumina.
The metallic shell 6 has a cylindrical shape and is formed so as to hold the insulator 3 inserted thereinto. A threaded portion 17 is formed on the outer circumferential surface of a forward end portion of the metallic shell 6. The spark plug 1 is attached to the cylinder head of an internal combustion engine (not shown) through the threaded portion 17. A flange-shaped gas seal portion 18 is formed rearward of the threaded portion 17, and a gasket 19 is fitted between the gas seal portion 18 and the threaded portion 17. A tool engagement portion 20 for engagement with a tool such as a spanner or a wrench is formed rearward of the gas seal portion 18, and a crimp portion 21 is formed rearward of the tool engagement portion 20. Ring-shaped packings 22 and 23 and talc 24 are disposed in an annular space formed between the outer circumferential surface of the insulator 3 and the inner circumferential surfaces of the crimp portion 21 and the tool engagement portion 20, and the insulator 3 is thereby fixed to the metallic shell 6. The metallic shell 6 may be formed from a steel material having electrical conductivity such as low-carbon steel.
The metal terminal 5 is used to externally apply to the center electrode 4 a voltage for generating spark discharge between the center electrode 4 and the ground electrode 7. The metal terminal 5 includes an exposed portion 25 and a substantially cylindrical columnar portion 26. The exposed portion 25 has an outer diameter larger than the inner diameter of the axial bore 2 and protrudes from the axial bore 2 with a flange-shaped portion of the exposed portion 25 partially in contact with a rear end surface, with respect to the direction of the axis O, of the insulator 3. The columnar portion 26 extends forward from the forward end surface, with respect to the direction of the axis O, of the exposed portion 25 and is accommodated in the axial bore 2. The metal terminal 5 may be formed from a metal material such as low-carbon steel.
The center electrode 4 has a substantially rod shape and is composed of an outer layer 27 and a core 28 that is formed so as to be embedded coaxially at the center of the outer layer 27. The center electrode 4 is fixed within the axial bore 2 of the insulator 3 with the forward end of the center electrode 4 protruding from the forward end of the insulator 3 and is insulated from the metallic shell 6. The core 28 is formed from a material having a thermal conductivity higher than that of the outer layer 27, and examples of such a material include Cu, Cu alloys, Ag, Ag alloys, and pure Ni. The outer layer 27 may be formed from any well-known material used for center electrodes, and it is preferable to use a Ni alloy such as Inconel 600 to form the outer layer 27.
The ground electrode 7 is formed to have, for example, a substantially prism shape. The ground electrode 7 is joined at one end to the forward end of the metallic shell 6 and bent at an intermediate portion into a substantially L-shape, and the other end of the ground electrode 7 is disposed with a gap between the other end and the center electrode 4. As shown in
The noble metal tip 9 is placed on the first electrode surface 31 and disposed such that the gap G is formed between the forward end surface 30 of the center electrode 4 and a gap forming surface 32 that faces the forward end surface 30. The gap G in the spark plug 1 in the present embodiment is the shortest distance between the forward end surface 30 and the gap forming surface 32, and the gap G is generally set to 0.3 to 1.5 mm. In the spark plug 1 in the present embodiment, the noble metal tip 9 is provided only on the ground electrode 7 that tends to have a higher temperature, and no noble metal tip is provided on the center electrode 4. It is only necessary that a noble metal tip be provided on at least one of the center electrode and the ground electrode. For example, a noble metal tip may be provided on each of the ground electrode and the center electrode. In this case, the shortest distance between the noble metal tip disposed on the ground electrode and the noble metal tip disposed on the center electrode is the gap, and spark discharge occurs in the gap.
The noble metal tip 9 is formed from a noble metal alloy, and examples of the noble metal alloy include a noble metal alloy containing Pt or Jr as a main component and at least one none-base noble metal selected from Pd, Rh, Ru, W, Os, Ni, Pt, Jr, etc. The noble metal tip 9 has a cylindrical shape, but no particular limitation is imposed on the shape of the noble metal tip 9. Any appropriate shape such as a disk shape, a polygonal plate shape, a polygonal prism shape, a polygonal pyramid shape, a truncated conical shape, a truncated polygonal pyramid shape, or a combination thereof may be used. Examples of the shape of a combination of a plurality of noble metal tips with different shapes include a shape in which a small disk is stacked on a large disk and a shape in which a quadrangular pyramid is stacked on a quadrilateral plate. The noble metal tip 9 is joined to the first electrode surface 31 through the fused portion 8 formed by laser welding. When the noble metal tip 9 is disposed on at least one of the discharge surfaces located between the center electrode 4 and the ground electrode 7; i.e., on the forward end surface 30 and/or the first electrode surface 31, the durability of the spark plug can be improved because the noble metal tip 9 formed from a noble metal alloy has a higher melting point than the center electrode 4 and the ground electrode 7 formed from, for example, a Ni alloy and is therefore less likely to be worn.
The fused portion 8 includes a first fused portion 34 and a second fused portion 36. In the first fused portion 34, the fused portion 8 is exposed at a second electrode surface 33 opposite the first electrode surface 31 on which the noble metal tip 9 is joined. In the second fused portion 36, the fused portion 8 is exposed at a side circumferential surface 35 of the noble metal tip 9. The noble metal tip 9 is joined through the first fused portion 34 and the second fused portion 36 and therefore has a sufficiently high wear resistance and a sufficiently high separation resistance. In other words, in the spark plug 1, the noble metal tip 9 is joined to the ground electrode 7 through the first fused portion 34 and the second fused portion 36. Therefore, the wear resistance can be improved by reducing as much as possible the exposed area of the second fused portion 36 having lower wear resistance than the noble metal tip 9, and the first fused portion 34 ensures sufficiently high separation resistance. When the exposed area of the second fused portion 36 can be reduced as much as possible, the surface area of the noble metal tip 9; i.e., the distance H, in the direction of a center axis X, from the gap forming surface 32 to a point of the second fused portion 36 that is closest to the gap G, can be maximized. Since the noble metal tip is worn from the gap forming surface 32 in a depth direction, the longer the distance H, the longer the service life of the noble metal tip. As described above, the wear resistance can be improved by reducing the exposed area of the second fused portion 36 as much as possible, and the first fused portion 34 provided in addition to the second fused portion 36 ensures sufficiently high separation resistance.
When the spark plug is used in a severe environment, it is contemplated that, for example, a noble metal tip with an increased diameter is used in order to ensure sufficiently high wear resistance of the noble metal tip. Even in such a case, the first fused portion 34 prevents the noble metal tip 9 from being easily separated, so that sufficiently high separation resistance can be ensured without increasing the exposed area of the second fused portion 36. Even when the diameter of the noble metal tip 9 is increased, it is not necessary to increase the exposed area of the second fused portion 36 in order to ensure sufficiently high separation resistance, so that the surface area of the noble metal tip 9; i.e., the distance H, can be ensured, as described above. Therefore, according to the spark plug of the present invention, while the separation resistance of the noble metal tip 9 is ensured, the wear resistance can be increased to an extent equivalent to an increase in the volume of the noble metal tip 9 achieved by increasing its diameter.
The first fused portion 34 can be formed by applying a laser beam from the second electrode surface 33 side toward the noble metal tip 9. The first fused portion 34 shown in
In
In a region of the first fused portion 34 in the vicinity of the boundary between the noble metal tip 9 and the ground electrode 7, the materials forming the noble metal tip 9 and the materials forming the ground electrode 7 are dissolved into each other, and therefore this region contains these materials. As the distance from the noble metal tip 9 toward the second electrode surface 33 increases, the content of the materials forming the ground electrode 7 increases. A region of the first fused portion 34 in the vicinity of the second electrode surface 33 is formed mostly from the materials forming the ground electrode 7. For example, when the first fused portion is formed so as to appear on the gap forming surface, a region of the first fused portion 34 in the vicinity of the gap forming surface is formed mostly from the materials forming the noble metal tip. Therefore, the region of the first fused portion that is exposed at the gap forming surface has a high wear resistance comparable to the wear resistance of the noble metal tip.
The second fused portion 36 is composed of a plurality of fused portions An (n is an integer of 1 or more) that are formed by applying a laser beam, in a direction oblique to the first electrode surface 31, to a region in the vicinity of a line of intersection M of the side circumferential surface 35 of the noble metal tip 9 and the first electrode surface 31, which line is present before the noble metal tip 9 is laser-welded to the ground electrode 7. As shown in
In the noble metal tip 9 shown in
When a plurality of fused portions An are disposed, it is preferable that the fused portions An are disposed so as to be point-symmetric with respect to the center axis when the noble metal tip is viewed from the above. For example, it is preferable that, in a cross section including the center axis X of the noble metal tip 9, at least fused portions An are formed on opposite sides of the center axis X serving as the center, as shown in
Preferably, the second fused portion 36 is formed such that the ratio of the total length of the fused portions An formed on the line of intersection M with respect to the overall length of the line of intersection M is at least 80%. More preferably, the second fused portion 36 is formed over the entire line of intersection M. When the second fused portion 36 is formed as described above, oxidation that occurs from the gap between the noble metal tip 9 and the ground electrode 7 can be suppressed, so that the occurrence of brittle fracture can be suppressed. In this manner, the separation resistance can be further improved.
The second fused portion 36 contains the materials forming the noble metal tip 9 and the materials forming the ground electrode 7 because the materials forming the noble metal tip 9 and the materials forming the ground electrode 7 are dissolved into each other. Therefore, the wear resistance of the second fused portion 36 is lower than that of the noble metal tip 9. As described above, the second fused portion 36 is formed by irradiating the first electrode surface 31 obliquely with the laser beam. In this case, the surface area of the noble metal tip 9 having high wear resistance becomes small, and the exposed area of the second fused portion 36 increases accordingly. As the exposed area increases, the effect of improving the wear resistance obtained by joining the noble metal tip 9 decreases accordingly. As described above, the longer the distance H, the longer the service life of the noble metal tip. It is therefore preferable from the viewpoint of wear resistance that the exposed area is reduced as much as possible. When the exposed area of the second fused portion 36 is reduced in order to increase the effect of improving the wear resistance obtained by joining the noble metal tip 9, the noble metal tip 9 is easily separated. However, the noble metal tip 9 in the present invention is joined to the ground electrode 7 through the second fused portion 36 and the first fused portion 34. Therefore, although the exposed area of the second fused portion 36 is reduced as much as possible in order to improve the wear resistance, sufficiently high separation resistance can be ensured because not only the second fused portion 36 but also the first fused portion 34 is provided.
In the spark plug 1 in the present embodiment, the first fused portion 34 and the second fused portion 36 are separated from each other so as not to be in contact with each other. However, the first fused portion(s) 34 and the second fused portion 36 may be in contact with each other or overlap each other as a result of, for example, formation of a plurality of first fused portions 34 or a first fused portion 34 having a large volume, or formation a second fused portion 36 extending deep into the noble metal tip 9.
The noble metal tip 9 and the ground electrode 7 have a first facing surface 37 and a second facing surface 38, respectively, that face each other (hereinafter these surfaces may be collectively referred to as facing surfaces). Specifically, there is a region in which the noble metal tip 9 and the ground electrode 7 joined to each other by laser welding are in direct contact with each other with no fused portion 8 therebetween. The thermal conductivity of the ground electrode 7 formed from a Ni alloy is higher than the thermal conductivity of the fused portion 8 containing a noble metal alloy and the Ni alloy. Therefore, when the noble metal tip 9 and the ground electrode 7 have the facing surfaces 37 and 38 through which the noble metal tip 9 and the ground electrode 7 are in direct contact with each other with no fused portion 8 therebetween, heat generated by spark discharge and heat received by the noble metal tip 9 from a high-temperature combustion chamber are easily dissipated through the facing surfaces 37 and 38. Therefore, the spark plug including the first facing surface 37 and the second facing surface 38 has higher wear resistance. The spark plug including these facing surfaces 37 and 38 to improve heat transfer in the noble metal tip 9 is preferably used in an environment in which the noble metal tip is particularly required to have high wear resistance.
In a virtual plane K perpendicular to the center axis X of the noble metal tip 9 and including a point P of the second fused portion 36 that is closest to the gap G, let the area of a first region T1 surrounded by the side circumferential surface 35 of the noble metal tip 9 be S1, as shown in
It is particularly preferable that the noble metal tip 9 is configured as follows. As shown in
The area ratio [(S2/S1)×100] can be measured, for example, as follows. First, the noble metal tip 9 is cut along a plane including the point P and orthogonal to the center axis X, and then the area of the obtained cross section is measured, whereby the area S1 can be determined. Then the noble metal tip 9 is cut along a plane including the first electrode surface 31. In the obtained cross section, a virtual circle having the area S1 measured above is drawn with the origin at a point on the center axis of the noble metal tip 9, and the area of the fused portion 8 included in the second region T2 surrounded by the circumference of the virtual circle is measured, whereby the area S2 can be determined. The area S1 and the area S2 can also be measured by CT.
As shown in
The noble metal tip 9 shown in
The noble metal tip 9 shown in
The spark plug 1 is produced, for example, as follows. The noble metal tip is prepared by any of the following methods. In one method, noble metal materials are obtained by mixing at a desired composition ratio and melting. The noble metal materials are, for example, rolled into a plate, and the plate is punched into a prescribed tip shape. In another method, an alloy is rolled, forged, or drawn into a wire-shaped or rod-shaped material, and then the obtained material is cut in its lengthwise direction into a prescribed length. By using any of the above methods, a noble metal tip having a desired shape and a desired composition can be formed. No particular limitation is imposed on the shape of the noble metal tip, and any appropriate shape such as a cylindrical shape, a circular disk shape, a polygonal disk shape, a polygonal columnar shape, or a particle shape can be used.
Electrode base materials forming the outer layer 27 of the center electrode 4 and the ground electrode 7 can be produced as follows. An alloy having a desired composition is melted using, for example, a vacuum melting furnace to prepare a molten alloy, and the molten alloy is subjected to vacuum casting to prepare an ingot. Then the ingot is subjected to hot working, drawing, etc. to appropriately adjust shape and dimensions, whereby an electrode base material having a prescribed shape and prescribed dimensions is produced. The outer layer 27 is formed from a cup-shaped electrode base material made of, for example, a Ni alloy. An inner member made of, for example, a Cu alloy having higher thermal conductivity than the electrode base material is prepared and inserted into the cup-shaped electrode base material. Then the resultant electrode base material is subjected to plastic working such as extrusion, whereby the center electrode 4 with the core 28 disposed inside the outer layer 27 is formed. The ground electrode 7 of the spark plug 1 in the present embodiment is formed from one type of material. However, the ground electrode 7 may include an outer layer and a core embedded at the center of the outer layer, as does the center electrode 4. In this case, as in the case of the center electrode 4, an inner member is inserted into a cup-shaped electrode base material, and the resultant electrode base material is subjected to plastic working such as extrusion. The product formed into a substantially prism shape by plastic working can be used as the ground electrode 7.
Next, one end portion of the ground electrode 7 is joined, by resistance welding or laser welding, to an end surface of the metallic shell 6 formed into a prescribed shape by, for example, plastic working. Then the metallic shell 6 with the ground electrode 7 joined thereto is subjected to Zn or Ni plating. Trivalent chromate treatment may be performed after the Zn or Ni plating.
Next, the noble metal tip 9 produced as described above is joined to the ground electrode 7 by laser welding. First, the noble metal tip 9 is placed on a desired position on the first electrode surface 31, and a laser beam is directed obliquely onto the vicinity of the line of intersection M on which the noble metal tip 9 intersects with the first electrode surface 31 to thereby form a fused portion An. This procedure is repeated a plurality of times over the entire line of intersection M, whereby the second fused portion 36 is formed as shown in
No particular limitation is imposed on the type, power, irradiation direction, number of times of irradiation, spot diameter, etc. of the laser beam used to form the second fused portion 36 and the first fused portion 34. When the second fused portion 36 is formed, it is preferable that the power, etc., of the laser beam are set such that the second fused portion 36 is formed on at least part of the line of intersection M. Specifically, it is preferable to form the second fused portion 36 such that the exposed area of the second fused portion 36 exposed at the side circumferential surface 35 of the noble metal tip 9 is reduced as much as possible within a range within which sufficiently high separation resistance is ensured. When the first fused portion 34 is formed, the power, etc., of the laser beam are set such that the first fused portion 34 is formed with the fused portion 8 exposed at the second electrode surface 33 and biting into at least part of the noble metal tip 9. In the spark plug 1 in the present embodiment, the first fused portion 34 is formed so as to be exposed at the second electrode surface 33. However, the first fused portion may be formed by directing a laser beam onto the gap forming surface 32 so that the fused portion is exposed at the gap forming surface 32. In the method of producing the spark plug 1 in the present embodiment, the second fused portion 36 is first formed, and then the first fused portion 34 is formed. However, no particular limitation is imposed on the order of formation of these fused portions, and the second fused portion 36 may be formed after the first fused portion 34 is formed.
Ceramic, for example, is fired into a prescribed shape to produce the insulator 3, and the center electrode 4 is inserted into the axial bore 2 of the insulator 3. Then glass powder forming the seals 10 and 11, a resistor composition forming the resistor 12, and the above glass powder are charged in this order into the axial bore 2 while preliminary compression is performed. Next, while the metal terminal 5 is inserted into the axial bore 2 from an end portion thereof, the resister composition and the glass power are compressed and heated. The resistor composition and the glass powder are thereby sintered, and the resistor 12 and the seals 10 and 11 are formed. Next, the insulator 3 with the center electrode 4 etc. fixed thereto is attached to the metallic shell 6 with the ground electrode 7 joined thereto. Finally, a distal end portion of the ground electrode 7 is bent toward the center electrode 4 such that one end of the ground electrode 7 faces the forward end portion of the center electrode 4, whereby the spark plug 1 is produced.
The spark plug according to the present invention is used as an ignition plug for an automobile internal combustion engine such as a gasoline engine. The spark plug is fixed to a prescribed position with the threaded portion screwed into a threaded hole provided in a head (not shown) that forms a sectioned combustion chamber of the internal combustion engine. The spark plug according to the present invention can be used for any type of internal combustion engine. The use of the noble metal tip having a sufficiently high wear resistance and a sufficiently high separation resistance allows provision of a spark plug which is high in durability. Therefore, the spark plug can be preferably used for a recent internal combustion engine in which the degree of compression within combustion chambers is high and/or lean fuel is used.
The spark plug according to the present invention is not limited to the above-described embodiments, and various modifications are possible so long as the object of the present invention can be achieved. For example, in the spark plug 1, the noble metal tip 9 is provided only on the ground electrode 7, and no noble metal tip is provided on the center electrode 4. However, noble metal tips may be provided on both the ground electrode 7 and the center electrode 4.
In the spark plug 1 described above, the noble metal tip 9 is placed on the first electrode surface 31, which is a side surface of the ground electrode 7. The noble metal tip 9 and the center electrode 4 are disposed such that the noble metal tip 9 and the forward end surface 30 of the center electrode 4 face each other in the direction of the axis O through the gap G. However, in the present invention, a noble metal tip may be provided on the forward end surface 30, and another noble metal tip may be provided on the distal end portion of the ground electrode so as to face a side surface of the noble metal tip provided on the forward end surface 30. In this case, the end surface of the noble metal tip provided on the ground electrode faces the side surface of the noble metal tip provided on the forward end surface 30 in the radial direction of the center electrode with a gap formed therebetween. In this case, one ground electrode having a noble metal tip facing the side surface of the noble metal tip disposed on the center electrode may be provided, or a plurality of such ground electrodes may be provided.
(Production of Test Ground Electrodes)
Evaluation was performed using a cylindrical platinum-rhodium alloy tip having a diameter of 1.0 mm and a height of 1.0 mm as a noble metal tip and a prism-shaped INC601 base of 1.5 mm×2.8 mm as an electrode base. In examples shown below, the cylindrical noble metal tip and the prism-shaped electrode base material having a quadrilateral cross section were used. However, effects similar to those obtained when the noble metal tip had a cylindrical shape were obtained also when the noble metal tip used had a disk-like shape, a polygonal prism shape, a polygonal plate shape, or a combination of these shapes. In addition, effects similar to those obtained when the prism-shaped electrode base material was used were obtained also when a rod-like electrode base material having, for example, a circular, elliptical, or polygonal cross-sectional shape different from that of the prism-shaped electrode base material was used.
The noble metal tip was joined to the circumferential side surface of a distal end portion of the ground electrode by laser welding in the following manner. First, the noble metal tip was placed on the first electrode surface; i.e., the circumferential side surface of the distal end portion of the ground electrode. Then the vicinity of the line of intersection M on which the noble metal tip and the first electrode surface intersected with each other was irradiated with a laser beam a plurality of times in a direction obliquely to the first electrode surface. This operation was repeated a plurality of times over the entire line of intersection M. The noble metal tip and the ground electrode were fused by the irradiation with the laser beam to thereby form a second fused portion, and at least part of the second fused portion was exposed at the side circumferential surface of the noble metal tip. Next, the second electrode surface; i.e., the surface of the ground electrode opposite the surface on which the noble metal tip was placed, was irradiated once with a laser beam along the center axis of the noble metal tip. The power, irradiation time, etc. of the laser beam were controlled such that the laser beam pierced the ground electrode and reached at least part of the noble metal tip. The noble metal tip and the ground electrode were fused by the irradiation with the laser beam to thereby form a first fused portion. The first fused portion was exposed at the second electrode surface. By applying the laser beam in the manner described above, the first fused portion and the second fused portion were formed, and the noble metal tip was joined to the ground electrode, as exemplified in, for example,
Test ground electrodes having different ratios of the fused portion shown in TABLE 1 were produced by appropriately changing the irradiation conditions such as the laser power, the spot diameter of the laser beam, and the number of times of irradiation with the laser beam when the noble metal tip was joined to the ground electrode. The area ratio of the fused portion was determined as follows. First, the noble metal tip was cut along a plane including the first electrode surface of the ground electrode. In the obtained cross section, a virtual circle having a diameter of 1.0 mm corresponding to the diameter of the noble metal tip was drawn with the origin at a point on the center axis of the noble metal tip. Then the total area of the first fused portion and the second fused portion; i.e., the area of the fused portion, included in the region surrounded by the circumference of the virtual circle was measured. Next, the ratio of the area of the fused portion with respect to the area, 0.785 mm2, of the noble metal tip having a diameter of 1.0 mm was computed, and the computed ratio was used as the area ratio of the fused portion.
(Thermal Cycle Test)
For each of the above-produced test ground electrodes, the portion to which the noble metal tip was joined was heated using a gas burner, held at 1,000° C. for 120 seconds, and then allowed to cool for 60 seconds in a room temperature environment. The above procedure was defined as one thermal cycle, and the thermal cycle was repeated 1,000 times. This thermal cycle test is a desk test corresponding to a running of 100,000 km on the market.
(Evaluation of Separation Resistance)
Each test ground electrode after the thermal cycle test was cut along a plane including the axis of the noble metal tip. For example, when, in the obtained cross section, a gap was observed in the joint portion between the noble metal tip and the ground electrode, the noble metal tip was considered to be partially separated from the ground electrode. Then the length of the line segment in the separated portion was measured as a separation length. The ratio of the separation length to the length of the joined portion was computed as a separation ratio. When the separation ratio was 90% or less, the test ground electrode was evaluated as “A.” When the separation ratio was higher than 90%, the test ground electrode was evaluated as “B.” The results are shown in TABLE 1.
As can be seen from TABLE 1, when the area ratio of the fused portion is 60% or higher, the separation resistance of the noble metal tip is improved.
The noble metal tip was joined to the ground electrode in the same manner as in the separation resistance test except that the noble metal tip used was a cylindrical noble metal tip having a diameter of 1.0 mm and a height of 0.8 mm. Specifically, ground electrodes different in the axial length (embedded amount) of the portion of the noble metal tip embedded in the ground electrode as shown in TABLE 2 were produced.
Test spark plugs were produced using the produced ground electrodes and center electrodes formed from Inconel 600 in the manner described above.
In each of the test spark plugs, the distance (the gap G) between the gap forming surface of the noble metal tip joined to the ground electrode and the forward end surface of the center electrode was 0.90 mm. For each of the ground electrodes, the minimum distance between the gap forming surface of the noble metal tip and a point in the second fused portion that was closest to the gap G was measured. The measured value is shown as a “straight length” in TABLE 2.
(Durability Test)
Each of the produced test spark plugs was attached to an engine, and a durability test was performed in which the engine was operated for 200 hours under WOT (Wide-Open Throttle) conditions (rotational speed: 6500 rpm).
(Evaluation of Wear Resistance)
The gap G after the durability test was measured, and the increase in the gap length (the difference between the gap length measured before the durability test and that measured after the durability test) was computed. The results are shown in TABLE 2.
As shown in TABLE 2, the smaller the amount of the noble metal tip embedded in the ground electrode, the smaller the increase in the gap length. When the embedded amount was 0.15 mm or less, the increase in the gap length could be reduced to 0.08 mm or less.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-275110 | Dec 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/083450 | 12/13/2013 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2014/097983 | 6/26/2014 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6337533 | Hanashi et al. | Jan 2002 | B1 |
| 20020003389 | Ishiguro | Jan 2002 | A1 |
| 20030038577 | Hori et al. | Feb 2003 | A1 |
| 20100253203 | Suzuki | Oct 2010 | A1 |
| Number | Date | Country |
|---|---|---|
| 10134671 | Feb 2003 | DE |
| 2000-040577 | Feb 2000 | JP |
| 2002-093547 | Mar 2002 | JP |
| 2003-068421 | Mar 2003 | JP |
| 2004-538617 | Dec 2004 | JP |
| 2005-150011 | Jun 2005 | JP |
| 2007-242456 | Sep 2007 | JP |
| 2007242456 | Sep 2007 | JP |
| Entry |
|---|
| Search report issued in corresponding International Patent Application No. PCT/JP2013/083450, dated Mar. 4, 2014. |
| Number | Date | Country | |
|---|---|---|---|
| 20150372458 A1 | Dec 2015 | US |
