JP2008204882A - Spark plug for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spark plug for an internal combustion engine which has a grounding electrode of a circular cross-section and can control wearing of the electrode due to oxidation corrosion and sparking wear and can improve a durability of the grounding electrode and moreover can prolong a life of the spark plug. <P>SOLUTION: The spark plug 1 is provided with an insulator 2 with an axial hole 4, a central electrode 5 inserted into the axial hole 4, a main metal fitting 3 provided around an outer circumference of the insulator 2 and a grounding electrode 27 provided on the main metal fitting 3. A precious metal chip 31 is jointed on a top end of the central electrode 5 and between the precious metal chip 31 and the grounding electrode 27, there is formed a spark discharging gap 33. A cross-section of the grounding electrode 27 is circular and a surface of the top end portion of a base metal matrix composing the grounding electrode 27 is covered by a precious metal covered portion 34 composed of at least either of the precious metal or an alloy containing the precious metal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a spark plug used for an internal combustion engine.

  A spark plug for an internal combustion engine is attached to an internal combustion engine (engine) and is used for ignition of an air-fuel mixture in a combustion chamber and combustion of the air-fuel mixture. In general, a spark plug is composed of an insulator having a shaft hole, a center electrode inserted through the shaft hole, a metal shell provided on the outer periphery of the insulator, and a front end surface of the metal shell. And a ground electrode that forms a spark discharge gap therebetween. Here, a high voltage is applied to the center electrode, whereby a spark can be discharged from the center electrode to the ground electrode. By discharging the spark, the air-fuel mixture located in the spark discharge gap is ignited, and a flame nucleus is generated in the spark discharge gap. As the flame kernel grows and the flame propagates through the combustion chamber, the air-fuel mixture can be burned. Conventionally, a noble metal tip made of a noble metal (platinum or the like) excellent in spark wear resistance has been provided on the discharge surface of the center electrode or the ground electrode facing the spark discharge gap.

By the way, in recent years, it has been proposed that the shape of the ground electrode having a prismatic shape (rectangular cross section) is a columnar shape (circular cross section) (see, for example, Patent Document 1). As a result, even when the air-fuel mixture is sent from the back surface of the ground electrode in accordance with the state of attachment of the spark plug to the internal combustion engine, the air-fuel mixture easily flows from the back surface of the ground electrode to the spark discharge gap. It can reliably ignite the mixture. Further, by making the ground electrode have a circular cross section, the ground electrode can be made thinner than the case where the ground electrode has a prismatic shape. As a result, heat pulling from the ground electrode that hinders the growth of flame nuclei can be suppressed, and the growth of flame nuclei can be promoted. That is, by making the ground electrode have a circular cross section, the ignitability and flame propagation can be improved at once.
Japanese Patent Laid-Open No. 11-121142

  However, although the above technique can surely improve the ignitability and the flame propagation property, an edge where spark discharge tends to concentrate does not exist on the surface of the ground electrode. Therefore, in combination with the reduction in the diameter of the ground electrode, sparks can easily reach the surface (back surface) opposite to the noble metal tip provided on the surface facing the center electrode of the ground electrode. turn into. As a result, there is a concern that spark discharge occurs over a relatively wide range of the surface of the ground electrode, and electrode wear such as oxidative corrosion and spark wear of the base metal itself constituting the ground electrode becomes more significant. .

  The present invention has been made in view of the above circumstances, and an object of the present invention is to improve ignitability and flame propagation in an internal combustion engine spark plug, as well as oxidation corrosion and sparks of a base material constituting a ground electrode. It is intended to improve the durability of the ground electrode by suppressing the consumption of the electrode such as the consumption, thereby extending the life of the spark plug.

  Hereinafter, each configuration suitable for solving the above-described object will be described in terms of items. In addition, the effect specific to the corresponding structure is added as needed.

Configuration 1. The spark plug of this configuration includes a cylindrical insulator having an axial hole penetrating in the axial direction, a center electrode inserted in the axial hole, and a cylindrical metal shell provided on the outer periphery of the insulator. , The tip portion of the metal shell is provided on the tip surface of the metal shell so that the tip portion of the metal electrode faces the tip surface of the center electrode, and a spark discharge gap is formed between the tip portion of the center electrode and at least the spark discharge gap. A spark plug for an internal combustion engine comprising a ground electrode having a circular cross section on the tip side from the center,
The distance of the spark discharge gap is 0.5 mm or more, and
The base material constituting the ground electrode is made of an alloy mainly composed of a base metal, and only the surface of the tip portion is covered with at least one of a noble metal and an alloy containing the noble metal.

  Note that “circular shape” does not necessarily mean a circular shape in a strict sense. Therefore, the shape may be somewhat oval, oval, teardrop (however, the arcuate surface is the center electrode side), and the like. Further, a noble metal tip may be provided at the tip of the center electrode. In this case, the “distance of the spark discharge gap” refers to the distance between the noble metal tip provided at the tip of the center electrode and the ground electrode (hereinafter the same).

  According to the above configuration 1, since the ground electrode has a circular cross section at least on the tip side from the center of the spark discharge gap, it is possible to improve ignitability and spark propagation. On the other hand, if the cross section of the ground electrode is circular, a spark can easily reach the surface (back surface) on the opposite side of the surface facing the center electrode of the ground electrode, and a relatively wide range of ground electrodes. There is a risk of spark discharge. In this regard, in Configuration 1, only the surface of the tip portion of the base material constituting the ground electrode is covered with at least one of the noble metal and the alloy containing the noble metal (referred to as “noble metal or the like”). For this reason, even if a spark discharge occurs over a relatively wide range, the base material constituting the ground electrode is protected by a noble metal, etc., and suppresses electrode consumption such as oxidative corrosion and spark consumption of the base material. Can do. As a result, the durability of the ground electrode can be improved, and the life of the spark plug can be extended. In addition, an increase in cost can be suppressed as compared with the case where the entire ground electrode is made of noble metal or the like, or the entire surface of the base material of the ground electrode is covered with the noble metal or the like.

  When the distance of the spark discharge gap is less than 0.5 mm, the spark discharge tends to concentrate on the surface of the ground electrode facing the center electrode, so that the spark discharge occurs over a wide range of the ground electrode. It is hard to happen. Therefore, it can be said that it is particularly effective to coat the surface of the tip of the base material constituting the ground electrode with a noble metal or the like by setting the distance of the spark discharge gap to 0.5 mm or more. In this case, the growth of the flame kernel can be further promoted, and the flame propagation property can be improved.

  Configuration 2. The spark plug of this configuration is the above-described configuration 1, wherein the distance of the spark discharge gap is 0.5 mm or more and 1.5 mm or less, and the diameter of the section of the ground electrode having a circular cross section is 2 mm or less. Yes, the diameter of the tip of the center electrode is 0.4 mm or more and 0.8 mm or less, and the diameter of the tip of the center electrode is subtracted from the diameter of the section of the ground electrode having a circular cross section. The measured value is less than 1.5 mm.

  The “diameter of the tip of the center electrode” refers to the diameter of the noble metal tip when the tip of the center electrode is provided with the noble metal tip (hereinafter the same).

  According to the configuration 2, the distance of the spark discharge gap is set to 0.5 mm or more and 1.5 mm or less, and the diameter of the tip of the center electrode is determined from the diameter of the cross section of the ground electrode having a circular cross section. The subtracted value (referred to as “electrode diameter difference”) is set to less than 1.5 mm. Here, when the distance of the spark discharge gap exceeds 1.5 mm, the spark discharge between the two electrodes hardly occurs. In addition, when the electrode diameter difference is 1.5 mm or more, the diameter (width) of the ground electrode is relatively larger than the diameter of the tip of the center electrode, so that a spark wraps around the back surface of the ground electrode. It becomes difficult. In other words, the spark plug having the above-described configuration 2 has a configuration in which sparks are more likely to reach the back portion of the ground electrode, and spark discharge is likely to occur over a relatively wide range of the ground electrode. Therefore, it can be said that the operation effect by the said structure 1 is show | played more effectively in the spark plug of such a structure.

  Further, the diameter of the cross section of the ground electrode having a circular cross section is set to 2 mm or less, and the diameter of the tip portion of the center electrode is set to 0.4 mm or more and 0.8 mm or less. Thereby, the heat pull by an electrode can be suppressed and the growth of a flame nucleus can be aimed at further. If the diameter of the cross section of the ground electrode having a circular cross section is made larger than 2 mm, or if the diameter of the tip of the center electrode is made larger than 0.8 mm, the heat sinking will be worsened. There is a risk that. Moreover, when the diameter of the front-end | tip part of a center electrode shall be less than 0.4 mm, there exists a possibility that durability of a center electrode may fall.

  Configuration 3. The spark plug of this configuration is the above configuration 1 or 2, wherein at least the distance G from the tip surface of the center electrode to the spark discharge gap and the cross section of the ground electrode are circular in the tip portion of the ground electrode base material. The surface of the part in the range of the distance (D + G) which added the diameter D of the cross section of the part which is a shape is covered with at least one of the noble metal and the alloy containing the noble metal.

  According to Configuration 3, at least the distance G of the spark discharge gap from the tip surface of the center electrode of the tip portion of the base material of the ground electrode and the diameter D of the section of the ground electrode having a circular cross section are added. The surface of the portion within the range of the distance (D + G) is covered with a noble metal or the like. That is, at least the surface of the ground electrode base material within a range where spark discharge can occur is covered with a noble metal or the like. As a result, the amount of noble metal used can be suppressed while improving the durability of the ground electrode, and the increase in cost can be effectively suppressed.

  Hereinafter, an embodiment will be described with reference to the drawings. FIG. 1 is a partially cutaway front view showing a spark plug 1. In FIG. 1, the axis C1 direction of the spark plug 1 is defined as the vertical direction in the drawing, the lower side is described as the front end side of the spark plug 1, and the upper side is described as the rear end side.

  The spark plug 1 includes an insulator 2 as an elongated insulator, a cylindrical metal shell 3 that holds the insulator 2, and the like.

  A shaft hole 4 is formed through the insulator 2 along the axis C1. A center electrode 5 is inserted and fixed on the front end side of the shaft hole 4, and a terminal electrode 6 is inserted and fixed on the rear end side. A resistor 7 is disposed between the center electrode 5 and the terminal electrode 6 in the shaft hole 4, and both ends of the resistor 7 are connected to the center electrode via conductive glass seal layers 8 and 9. 5 and the terminal electrode 6 are electrically connected to each other.

  The center electrode 5 protrudes from the tip of the insulator 2, and the terminal electrode 6 is fixed in a state of protruding from the rear end of the insulator 2. Further, a noble metal tip 31 is joined to the tip of the center electrode 5 by welding (this will be described later).

  On the other hand, the insulator 2 is formed by firing alumina or the like, as is well known, and has a flange-shaped large-diameter portion 11 that protrudes radially outward at a substantially central portion in the direction of the axis C1 in its outer shape. And an inner body portion 12 having a smaller diameter on the distal end side than the large diameter portion 11, and an inner body portion 12 having a smaller diameter on the distal end side than the intermediate body portion 12, and an internal combustion engine (engine). And a leg length portion 13 exposed to the combustion chamber. Among the insulators 2, a part of the distal end side including the large-diameter portion 11, the middle trunk portion 12, and the leg long portion 13 is accommodated in a metal shell 3 formed in a cylindrical shape. A step portion 14 is formed at the connecting portion between the leg long portion 13 and the middle trunk portion 12, and the insulator 2 is locked to the metal shell 3 at the step portion 14.

  The metal shell 3 is formed in a cylindrical shape from a metal such as low carbon steel, and a screw portion (male screw portion) 15 for attaching the spark plug 1 to the engine head is formed on the outer peripheral surface thereof. A seat portion 16 is formed on the outer peripheral surface on the rear end side of the screw portion 15, and a ring-shaped gasket 18 is fitted on the screw neck 17 on the rear end of the screw portion 15. Further, on the rear end side of the metal shell 3, a tool engaging portion 19 having a hexagonal cross section for engaging a tool such as a wrench when the metal shell 3 is attached to the engine head is provided. A caulking portion 20 for holding the insulator 2 is provided.

  Further, a step portion 21 for locking the insulator 2 is provided on the inner peripheral surface of the metal shell 3. The insulator 2 is inserted from the rear end side to the front end side of the metal shell 3, and the rear end of the metal shell 3 is engaged with the step portion 14 of the metal shell 3. It is fixed by caulking the opening on the side radially inward, that is, by forming the caulking portion 20. An annular plate packing 22 is interposed between the step portions 14 and 21 of both the insulator 2 and the metal shell 3. Thereby, the airtightness in the combustion chamber is maintained, and the fuel air entering the gap between the leg long portion 13 of the insulator 2 exposed to the combustion chamber and the inner peripheral surface of the metal shell 3 is prevented from leaking outside.

  Further, in order to make the sealing by caulking more complete, annular ring members 23 and 24 are interposed between the metal shell 3 and the insulator 2 on the rear end side of the metal shell 3, and the ring member 23 , 24 is filled with powder of talc (talc) 25. That is, the metal shell 3 holds the insulator 2 via the plate packing 22, the ring members 23 and 24, and the talc 25.

  A ground electrode 27 having a substantially L shape is joined to the front end surface 26 of the metal shell 3. That is, the ground electrode 27 is welded at its proximal end to the distal end surface 26 of the metal shell 3, and the distal end portion is bent back, and its side surface is the surface on the distal end side of the center electrode 5 (the noble metal tip 31. ). A spark discharge gap 33 is formed in the gap between the noble metal tip 31 and the main body of the ground electrode 27. In this embodiment, the distance of the spark discharge gap 33 is 0.5 mm or more and 1.5 mm or less (1.0 mm in this embodiment).

  The center electrode 5 includes an inner layer 5A made of copper or a copper alloy and an outer layer 5B made of a nickel (Ni) alloy. In addition, as shown in FIG. 2, the center electrode 5 has a reduced diameter at the tip side, has a rod shape (cylindrical shape) as a whole, and has a flat tip surface. The columnar (disc-shaped) noble metal tip 31 having a diameter of 0.4 mm or more and 0.8 mm or less (in this embodiment, 0.6 mm) is superposed here, and laser welding is performed along the outer edge of the joint surface. The noble metal tip 31 is joined by forming a welded portion by electron beam welding, resistance welding or the like and fixing it. The noble metal tip 31 may be omitted. In this case, a spark discharge gap 33 is formed between the tip of the center electrode 5 and the main body of the ground electrode 27.

  In the present embodiment, the noble metal tip 31 is composed of a noble metal containing platinum (Pt) as a main component and containing 20% by mass of iridium (Ir). Note that the noble metal tip 31 may be composed of another noble metal.

  Further, the ground electrode 27 is made of a Ni-based alloy, which is a base metal, and has a cylindrical shape (circular cross-section), and a cross-sectional diameter of 2 mm or less (1.3 mm in this embodiment). ). Further, among the base materials constituting the ground electrode 27, the surface of the tip portion thereof is at least one of the noble metal and the alloy containing the noble metal (hereinafter referred to as “noble metal covering portion 34” for convenience, and in this example, for example, as described above The noble metal covering portion 34 is covered with a molten alloy of a Pt-20Ir alloy and a Ni-based alloy which is a base material of the ground electrode 27. In the present embodiment, the case where the surface of the tip portion of the base material constituting the ground electrode 27 is covered with the noble metal covering portion 34 is illustrated, but the entire surface of the tip portion of the base material is not necessarily limited. It doesn't have to be covered. That is, as shown in FIGS. 3A and 3B, among the base materials constituting the ground electrode 27, at least the distance D between the diameter D of the cross section of the ground electrode 27 and the spark discharge gap 33 from the tip surface of the noble metal tip 31. It is preferable that the surface of the part within the range of the distance R (= D + G) obtained by adding G is covered with the noble metal covering portion 34.

  Next, the manufacturing method of the spark plug 1 comprised as mentioned above is demonstrated. First, the metal shell 3 is processed in advance. That is, a cylindrical metal material (for example, an iron-based material such as S17C or S25C or a stainless steel material) is formed by forming a through-hole by cold forging to produce a rough shape. Thereafter, the outer shape is adjusted by cutting to obtain a metal shell intermediate.

  Subsequently, the ground electrode 27 is formed. That is, first, a cylindrical metal member made of the above-described Ni-based alloy is manufactured as a base material of the ground electrode 27. And a small diameter part (groove part) is formed by giving press work or cutting with respect to the front-end | tip part surface of the said metal member. Next, a noble metal powder (particles) containing Pt as a main component and containing 20% by mass of Ir is disposed so as to cover the small diameter portion. Next, the noble metal powder is irradiated to the noble metal powder from a direction substantially orthogonal to the metal member to bond the noble metal powder to the surface of the tip portion of the metal member. As a result, the noble metal covering portion 34 in which the noble metal powder and the metal member are melted is formed at the tip portion of the metal member, and the ground electrode 27 is obtained. Of course, after joining, it is good also as performing a swaging process etc. in order to make a diameter uniform, and arranging precious metal powder without providing a small diameter part etc., and performing a swaging process etc. after joining. It is good. For joining the noble metal powder and the metal member, other joining methods (for example, resistance welding) may be used. Furthermore, it is good also as providing the cyclic | annular or cylindrical noble metal coating | coated part 34 in a metal member, after winding a noble metal around the said small diameter part or carrying out external fitting arrangement | positioning of the noble metal ring (precious metal cylinder).

  And the ground electrode 27 mentioned above is resistance-welded to the front end surface of the metal shell intermediate body. When the welding is performed, so-called “sag” is generated. After the “sag” is removed, the threaded portion 15 is formed by rolling at a predetermined portion of the metal shell intermediate body. Thereby, the metal shell 3 to which the ground electrode 27 is welded is obtained. The metal shell 3 to which the ground electrode 27 is welded is galvanized or nickel plated. In order to improve the corrosion resistance, the surface may be further subjected to chromate treatment.

  On the other hand, the insulator 2 is formed separately from the metal shell 3. For example, by using a raw material powder mainly composed of alumina and containing a binder or the like, a green granulated material for molding is prepared, and rubber press molding is used to obtain a cylindrical molded body. The obtained molded body is ground and shaped. Then, the shaped one is put into a firing furnace and fired. The insulator 2 is obtained by performing various grinding | polishing processes after baking.

  Separately from the metal shell 3 and the insulator 2, the center electrode 5 is manufactured. That is, the above-described Ni-based alloy is forged, and an inner layer 5A made of a copper alloy is provided at the central portion to improve heat dissipation. And the noble metal tip 31 mentioned above is joined to the front-end | tip part by resistance welding, laser welding, etc.

  Then, the insulator 2 and the center electrode 5, the resistor 7, and the terminal electrode 6 obtained as described above are sealed and fixed by the glass seal layers 8 and 9. The glass seal layers 8 and 9 are generally prepared by mixing borosilicate glass and metal powder, and the prepared material is injected into the shaft hole 4 of the insulator 2 with the resistor 7 interposed therebetween. Then, the terminal electrode 6 is pressed from the rear and then baked in a firing furnace. At this time, the glaze layer may be fired simultaneously on the body surface on the rear end side of the insulator 2, or the glaze layer may be formed in advance.

  Thereafter, the insulator 2 provided with the center electrode 5 and the terminal electrode 6 and the metal shell 3 provided with the ground electrode 27 are assembled as described above. More specifically, it is fixed by caulking the opening on the rear end side of the metal shell 3 formed relatively thin inward in the radial direction, that is, by forming the caulking portion 20.

  Finally, the ground electrode 27 is bent to adjust the spark discharge gap 33 between the noble metal tip 31 provided at the tip of the center electrode 5 and the ground electrode 27 body.

  Thus, the spark plug 1 having the above-described configuration is manufactured through a series of steps.

  Next, while making the ground electrode into a circular cross section, samples of spark plugs with various spark discharge gap distances G were prepared, and the surface of the ground electrode located on the opposite side of the surface facing the center electrode (" In order to investigate whether or not a spark discharge has occurred up to “back part”), a discharge presence / absence determination test was performed on each sample. The evaluation results are shown in Table 1. Here, the outline of the discharge presence / absence determination test is as follows. That is, after a spark plug sample is attached to a 4-cylinder engine with a displacement of 2000 cc, the engine speed is changed in order, such as idling, cruise, and fully open (full load), for a total of 100 hours. The engine was turned over. As a result, if a spark discharge trace is confirmed at the back part of the ground electrode, the evaluation of “○” will be made, whereas if a spark discharge trace is not confirmed at the back part of the ground electrode In this case, an evaluation of “x” was made. Here, since the back portion of the ground electrode is generally a portion where it is difficult for spark discharge to occur, when a spark discharge trace is confirmed in the back portion, spark discharge occurs over a relatively wide range of the ground electrode. It can be said that. The diameter of the cross section of the ground electrode is 1.3 mm, and the diameter of the noble metal tip provided on the center electrode is 0.6 mm.

表１に示すように、火花放電間隙の距離Ｇが０．５ｍｍ以上のとき、接地電極の背面部位において火花の放電跡が確認された。つまり、火花放電間隙の距離Ｇが０．５ｍｍ以上である場合に、接地電極の背面部位にまで火花が回り込んでしまう、つまり接地電極の比較的広い範囲にわたって火花放電が起こってしまうおそれがあることが明らかとなった。

As shown in Table 1, when the distance G of the spark discharge gap was 0.5 mm or more, a spark discharge trace was confirmed at the back surface portion of the ground electrode. In other words, when the distance G of the spark discharge gap is 0.5 mm or more, there is a possibility that the spark may reach the back portion of the ground electrode, that is, the spark discharge may occur over a relatively wide range of the ground electrode. It became clear.

  Next, the ground electrode is circular in cross section, the spark discharge gap distance G is 0.5 mm to 2 mm, the cross section diameter D of the ground electrode is 1.0 mm to 2.0 mm, and the noble metal tip provided on the center electrode Samples of spark plugs having various diameters d in the range of 0.4 mm to 0.8 mm were prepared, and the above-described discharge presence / absence determination test was performed on each sample. The evaluation results are shown in Tables 2-4. Table 2 shows the diameter d of the noble metal tip provided on the center electrode is 0.4 mm, Table 3 shows that the diameter d is 0.6 mm, and Table 4 shows that the diameter d is 0.8 mm. The evaluation results are shown respectively. The numerical value in parentheses in each table represents the electrode diameter difference (D−d) obtained by subtracting the diameter d of the noble metal tip provided on the center electrode from the diameter D of the cross section of the ground electrode.

表２〜４に示すように、火花放電間隙の距離Ｇを２．０ｍｍとした場合には、接地電極の背面部位において火花の放電跡が確認されなかった。これは、火花放電間隙の距離Ｇがあまりにも長いため、火花放電が起きづらく、また、仮に火花放電が起きたとしても、接地電極のうち中心電極と対向する側の面において火花放電が集中しやすくなるためであると考えられる。さらに、表２に示すように、電極径差（Ｄ−ｄ）が１．６ｍｍである場合には、表３，４のように電極径差が１．５ｍｍ未満である場合と異なり、接地電極の背面部位において火花の放電跡が確認されなかった。これは、中心電極に設けられた貴金属チップの直径ｄに対して、接地電極の断面の直径Ｄが大きいため、接地電極の背面部位にまで火花が回り込みづらくなったことが原因であると考えられる。

As shown in Tables 2 to 4, when the distance G of the spark discharge gap was set to 2.0 mm, no spark discharge trace was confirmed at the rear portion of the ground electrode. This is because the spark discharge gap distance G is so long that spark discharge is difficult to occur, and even if spark discharge occurs, spark discharge concentrates on the surface of the ground electrode facing the center electrode. This is thought to be easier. Further, as shown in Table 2, when the electrode diameter difference (D−d) is 1.6 mm, unlike the cases where the electrode diameter difference is less than 1.5 mm as shown in Tables 3 and 4, the ground electrode No spark discharge traces were observed at the rear part. This is considered to be caused by the fact that the spark does not easily reach the back surface portion of the ground electrode because the diameter D of the cross section of the ground electrode is larger than the diameter d of the noble metal tip provided on the center electrode. .

  From the above experiment, in the spark plug in which the cross section of the ground electrode is circular, the distance of the spark discharge gap is 0.5 mm or more and 1.5 mm or less, and the electrode diameter difference is less than 1.5 mm, a relatively wide range of the ground electrode It can be said that spark discharge tends to occur. Therefore, for the spark plug having such a configuration, the surface of the tip portion of the base material of the ground electrode is covered with the above-described noble metal coating portion, so that the electrode wear such as oxidative corrosion and spark consumption of the base material of the ground electrode. It is particularly effective in suppressing the above, and it can be said that the durability of the ground electrode can be improved and the life of the spark plug can be extended.

  In addition, it is not limited to the description content of the said embodiment, For example, you may implement as follows. Of course, other application examples and modification examples not illustrated below are also possible.

  (A) As described above, instead of covering the entire surface of the tip portion of the base material of the ground electrode 27 with the noble metal covering portion 34, from the tip surface of the noble metal tip 31 in the base material of the ground electrode 27. Only the surface of the part within the range of the distance R obtained by adding the distance G of the spark discharge gap and the diameter D of the cross section of the ground electrode 27 may be covered with the noble metal covering portion 34 (FIGS. 3A and 3B). ) And the dotted pattern in FIG. Thereby, while aiming at the improvement of durability of a ground electrode, the quantity of noble metals used etc. can be suppressed to the minimum necessary, and the increase in cost can be controlled more effectively.

  (B) In the above-described embodiment, the noble metal covering portion 34 is formed of an alloy in which a noble metal and a Ni-based alloy that is a base metal are melted. However, the noble metal covering portion 34 may be formed of only the noble metal. Further, a part of the noble metal coating portion 34 may be formed of a noble metal, and the other part may be formed of an alloy containing the noble metal. Further, the type of noble metal is not particularly limited to the composition of the above embodiment. For example, a noble metal or the like may be composed of only Pt or only Ir.

  (C) In the above embodiment, the ground electrode 27 has a circular cross section in the entire longitudinal direction. However, if the cross section of the ground electrode 27 has a circular shape at least from the center to the distal end side of the spark discharge gap 33. Good.

  (D) In the above embodiment, the ground electrode 27 has a circular cross section, but the cross sectional shape of the ground electrode 27 is not necessarily limited to a circular shape in a strict sense. Accordingly, the shape may be somewhat oval, oval, teardrop (where the arcuate surface is the center electrode 5 side), or the like.

  (E) In the above embodiment, the case where the ground electrode 27 is joined to the tip of the metal shell 3 is embodied. However, a part of the metal shell (or one of the metal tips previously welded to the metal shell is used. The present invention can also be applied to the case where the ground electrode is formed by cutting out the portion (for example, Japanese Patent Laid-Open No. 2006-236906).

  (F) The center electrode 5 of the above-described embodiment has a reduced diameter at the tip side, but it does not necessarily have to be reduced in diameter, and may have a rod shape (cylindrical shape) as a whole. . The center electrode 5 has a two-layer structure including the inner layer 5A and the outer layer 5B. However, the center electrode 5 may have a single layer.

It is a partially broken front view which shows the structure of the spark plug of this embodiment. It is a fragmentary perspective view which shows the noble metal coating | coated part etc. of the front-end | tip part of a spark plug. (A) is an enlarged side view which shows the range in which a noble metal coating | coated part is formed among ground electrodes, (b) is an enlarged front view which shows the range in which a noble metal coating | coated part is formed among ground electrodes. It is a fragmentary perspective view which shows the noble metal coating | coated part etc. of the front-end | tip part of a spark plug in another embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Spark plug, 2 ... Insulator, 3 ... Main metal fitting, 4 ... Shaft hole, 5 ... Center electrode, 27 ... Ground electrode, 33 ... Spark discharge gap, 34 ... Precious metal coating | coated part, C1 ... Axis line.

Claims (3)

A cylindrical insulator having an axial hole penetrating in the axial direction;
A center electrode inserted in the shaft hole;
A cylindrical metal shell provided on the outer periphery of the insulator;
The tip of the metal shell is provided on the tip surface of the metal shell so that its tip portion faces the tip surface of the center electrode, and forms a spark discharge gap between the tip of the center electrode and at least the center of the spark discharge gap. A spark plug for an internal combustion engine comprising a grounding electrode having a circular cross section on its front end side,
The distance of the spark discharge gap is 0.5 mm or more, and
A spark for an internal combustion engine, characterized in that a base material constituting the ground electrode is made of an alloy mainly composed of a base metal, and only a surface of a tip portion thereof is covered with at least one of a noble metal and an alloy containing the noble metal. plug. The distance of the spark discharge gap is 0.5 mm or more and 1.5 mm or less,
The diameter of the cross section of the portion of the ground electrode having a circular cross section is 2 mm or less,
The diameter of the tip of the center electrode is 0.4 mm or more and 0.8 mm or less,
2. The internal combustion engine according to claim 1, wherein a value obtained by subtracting a diameter of a tip portion of the center electrode from a diameter of a cross section of the ground electrode having a circular cross section is less than 1.5 mm. Spark plug.   A distance obtained by adding at least the distance G of the spark discharge gap from the front end surface of the center electrode and the diameter D of the cross section of the ground electrode having a circular cross section in the front end portion of the base material of the ground electrode ( 3. The spark plug for an internal combustion engine according to claim 1, wherein a surface of a portion within a range of D + G) is covered with at least one of a noble metal and an alloy containing the noble metal.

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