JP5118758B2 - Spark plug - Google Patents

Description

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

  A spark plug used in a combustion apparatus such as an internal combustion engine includes, for example, a center electrode extending in the axial direction, an insulator provided on the outer periphery of the center electrode, and a cylindrical metal shell assembled on the outside of the insulator And a ground electrode extending from the tip of the metal shell and bent toward the center electrode side. In order to improve ignitability and wear resistance, a technique for joining a noble metal tip made of a noble metal alloy to the tip of the center electrode has been proposed.

  Further, in recent years, it has been proposed to make the noble metal tip longer along the axial direction (see, for example, Patent Document 1). This is due to the following reason, for example.

  That is, the tip surface of the ground electrode is opposed to the tip side surface of the noble metal tip, and a spark discharge is performed along the direction orthogonal to the axis line in a spark discharge gap formed between them (so-called, In the case of a horizontal discharge type spark plug, if the melted part that joins the center electrode and the noble metal tip and the ground electrode are close to each other, an abnormal spark discharge will occur between them and the durability will decrease. There is a risk that. In this respect, if the noble metal tip is made longer, a sufficient distance along the axial direction between the melted portion and the ground electrode can be secured, and the occurrence of abnormal spark discharge and the decrease in durability can be more reliably ensured. Can be prevented.

  In addition, the tip of the ground electrode is opposed to the tip of the noble metal tip, and spark discharge is performed in a direction substantially along the axial direction in a spark discharge gap formed between them (so-called parallel electrode) In the type) spark plug, by making the noble metal tip longer, the ignition position can be projected closer to the center of the combustion chamber, and the ignitability can be improved. That is, from the viewpoint of improving durability and ignitability, it is possible to make the noble metal tip longer along the axial direction in various types of spark plugs.

JP 2009-158343 A

  However, when the noble metal tip is lengthened, a greater stress is applied to the portion of the central electrode located near the base end of the noble metal tip or the boundary portion between the central electrode and the melted portion due to vibration accompanying the operation of the internal combustion engine or the like. Will be added. Therefore, breakage occurs in the center electrode, the boundary portion, and the like, and there is a possibility that the above-described operation effect by providing the noble metal tip may not be sufficiently exhibited.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to more reliably prevent breakage of the center electrode and the like in a spark plug including a relatively long noble metal tip. An object of the present invention is to provide a spark plug capable of sufficiently exhibiting the effect of improving the ignitability, durability and the like due to the provision of.

  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 on the tip side of the shaft hole;
A cylindrical metal shell provided on the outer periphery of the insulator;
A ground electrode disposed at the tip of the metal shell;
A noble metal tip bonded to the tip of the center electrode and forming a gap with the ground electrode;
At the tip of the center electrode, a shoulder is formed that tapers toward the tip in the axial direction.
The noble metal tip is joined to the center electrode by forming a melted part in which the center electrode and the noble metal chip are melted in at least a part of the base end of the noble metal tip,
In the outer surface of the noble metal tip, the shortest distance between the tip surface of the noble metal tip along the axis and the molten part is 0.8 mm to 1.2 mm,
The outer diameter at the most distal portion of the melted portion is smaller than the outer diameter at the most proximal portion,
When the acute angle among the angles formed by the following straight line L1 and straight line L2 is θ1,
θ1 ≦ 72 °
The filling,
The center electrode includes an outer layer and an inner layer provided inside the outer layer and having higher thermal conductivity than the outer layer,
The shorter distance of the distance from the inner layer to the base end face of the noble metal tip or the distance from the inner layer to the molten part is 2 mm or less,
In a cross section including the axis, when an acute angle among the angles formed by two straight lines passing through the intersection of the straight line L1 and the straight line L2 and in contact with the outline of the inner layer is θ3,
(Θ1 × 1/3) ≦ θ3
It is characterized by satisfying.

Straight line L1: A straight line formed by extending one of the two outlines of the shoulder portion located across the axis in the cross section including the axis toward the tip end in the axial direction Straight line L2: the axis In the cross section including the axis, a straight line formed by extending the other external line out of the two external lines of the shoulder located across the axis toward the tip end side in the axial direction. The outer contour line may be curved, or the shoulder outer contour line may be bent. When the contour line of the shoulder is curved, the straight lines L1 and L2 mean straight lines obtained by extending a line segment connecting both ends of the contour line toward the front end side in the axial direction. Further, when the shoulder outline is bent, the straight lines L1 and L2 are formed by extending a line segment located on the distal side of the bent portion of the shoulder outline toward the distal end in the axial direction. Is a straight line.

  According to the said structure 1, the noble metal chip | tip is comprised comparatively long so that the shortest distance along the axis line between the front end surface and a fusion | melting part may be 0.8 mm or more in an outer surface. Therefore, durability and ignitability can be improved.

  On the other hand, if the noble metal tip is made relatively long, there is a concern about breakage of the center electrode or the like as described above, but according to the configuration 1, the acute angle θ1 is 72 out of the angles formed by the straight lines L1 and L2. It is considered to be relatively small at less than °. That is, the stress concentration occurs at a portion where the cross-sectional area changes relatively greatly, and the change rate of the cross-sectional area along the axis is relatively small at the shoulder portion of the center electrode where the breakage is particularly a concern. Yes. Therefore, it can suppress effectively that the stress accompanying a vibration concentrates on a shoulder part, and can prevent the breakage in a shoulder part more reliably.

  Further, for the melted portion formed on the distal end side of the shoulder portion, the outer diameter at the most distal portion is smaller than the outer diameter at the most proximal portion (that is, the outer peripheral portion has a tapered shape). To make up). Therefore, it is possible to prevent the boundary portion between the shoulder portion and the melted portion from having a steep bent shape (a shape in which the cross-sectional area changes rapidly), and the stress caused by the vibration is It can prevent more reliably that it concentrates with respect to the vicinity. As a result, breakage at the boundary portion and its vicinity can be more reliably suppressed.

  As described above, according to the present configuration 1, it is possible to improve the breakage resistance in the shoulder portion, the boundary portion, and the like, and more reliably improve the durability and ignitability by providing the noble metal tip, and more It can be exhibited over a long period of time.

  For the noble metal tip, if the shortest distance along the axis between the tip surface and the melted portion is larger than 1.2 mm, the stress applied to the shoulder portion and the like will increase extremely, and the noble metal tip Heating gets worse. Therefore, it is preferable that the shortest distance is set to 1.2 mm or less in order to prevent breakage in the shoulder portion or the like and decrease in wear resistance in the noble metal tip.

  Moreover, it is preferable to make angle (theta) 1 smaller from the point of aiming at the further improvement of breakage resistance. However, if the angle θ1 is reduced, the axial length of the shoulder portion is further increased, so that the noble metal tip protrudes excessively toward the tip end side with respect to the tip end of the insulator, resulting in heat resistance. Etc. may be reduced. On the other hand, if the amount of protrusion of the noble metal tip with respect to the tip of the insulator is suppressed, a large annular space will be formed between the base end side outer peripheral portion of the shoulder and the shaft hole of the insulator, and the insulator There is a risk that the heat resistance of the will be reduced. Therefore, when the angle θ1 is relatively small, the outer diameter of the rear end of the shoulder is relatively small (for example, 2.6 mm or less or 2.1 mm so that the axial length of the shoulder does not excessively increase). Or less).

  Further, the heat of the noble metal tip is drawn from the noble metal tip directly or through the melted portion to the center electrode side. According to the above configuration 1, the noble metal from the inner layer excellent in thermal conductivity provided inside the center electrode is used. At least one of the distance to the base end surface of the chip or the distance from the inner layer to the molten part is 2 mm or less (that is, the inner layer is disposed at a position relatively close to the noble metal chip or the molten part). ) In addition, it is configured to satisfy θ1 × 1/3 ≦ θ3, that is, the inner layer front end portion has a sufficient volume corresponding to the thickness of the center electrode front end portion accompanying the change in the angle θ1. For this reason, the heat of the noble metal tip can be efficiently drawn by the inner layer, and the wear resistance of the noble metal tip can be further improved.

Configuration 2. The spark plug of this configuration is the same as that of the above configuration 4, in which θ3 ≦ (θ1 × 3/4)
It is characterized by satisfying.

  As in the configuration 2, the wear resistance of the noble metal tip can be improved by making θ3 relatively large. However, if θ3 is excessively increased, the ratio of the inner layer to the center electrode tip is excessively increased in the cross section orthogonal to the axis, while the outer layer is excessively thin. As a result, the amount of thermal expansion of the inner layer increases, the strength of the outer layer becomes insufficient, and as a result, the surface of the center electrode may crack due to repeated cooling and heating cycles.

  In this regard, according to the above configuration 5, since it is configured to satisfy θ3 ≦ (θ1 × 3/4), an appropriate volume corresponding to the thickness of the tip of the center electrode accompanying the change in the angle θ1. An inner layer and an outer layer having an appropriate thickness are set. As a result, the outer layer has sufficient strength against the thermal expansion of the inner layer, and the occurrence of cracks in the center electrode can be more reliably prevented.

Configuration 3. The spark plug of this configuration is the above configuration 1 or 2, in the cross section including the axis,
The outline of the shoulder portion is linear.

  “Linear” means that the outline of the shoulder is not bent (no corners are formed) or is not excessively curved, and the outline of the shoulder is strictly It does not mean that it is a straight line.

  According to the configuration 3, since the outline of the shoulder portion is linear, it is possible to more reliably prevent stress concentration at the shoulder portion. As a result, breakage resistance can be further improved.

Configuration 4. In the spark plug of this configuration, in any one of the above configurations 1 to 3, the ground electrode is disposed such that a tip surface thereof faces an outer surface of the noble metal tip,
In the gap, spark discharge is performed substantially along a direction orthogonal to the axis.

  In the spark plug of the type (so-called lateral discharge type) in which spark discharge is performed substantially along the direction orthogonal to the axis as in the configuration 4, the abnormal spark discharge between the melted portion and the ground electrode is prevented. Therefore, further lengthening of the noble metal tip is desired. However, if the noble metal tip is made longer, the risk of breakage in the center electrode and the like will increase.

  In this respect, by adopting the above-described configuration 1 or the like, it is possible to more reliably prevent breakage of the center electrode or the like in a horizontal discharge type spark plug that requires a longer noble metal tip. That is, the above-described configuration 1 and the like are particularly significant in a horizontal discharge type spark plug.

  Configuration 5. The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 4, the noble metal tip has a columnar shape and an outer diameter of a tip surface thereof is 0.7 mm or less.

  In order to suppress the flame extinguishing action due to the noble metal tip and improve the ignitability, it is preferable that the noble metal tip has a relatively small diameter. However, when the diameter of the noble metal tip is reduced, the shoulder portion to which the noble metal tip is joined can also have a relatively small diameter. When the diameter of the shoulder portion is reduced, the strength of the shoulder portion is lowered, and therefore there is a further concern about breakage in the shoulder portion and the like.

  In this respect, according to the above-described configuration 5, the noble metal tip is reduced in diameter so that the outer diameter of the tip end surface is 0.7 mm or less, so that it can be expected to improve the ignitability, while being resistant to breakage. Although there is a concern about the decrease, the concern can be eliminated by adopting the configuration 1 or the like. In other words, the above configuration 1 and the like are particularly effective in a spark plug having a noble metal tip whose outer diameter of the tip end surface is reduced to 0.7 mm or less.

  Configuration 6. The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 5, the noble metal tip has a cylindrical shape, and an outer diameter of a tip surface thereof is 0.5 mm or less.

  According to the configuration 6, the noble metal tip has an outer diameter of 0.5 mm or less at the front end surface, and thus can be expected to further improve the ignitability, but breakage at the shoulder portion or the like can be expected. More concerned. In this respect, by adopting the above-described configuration 1 or the like, it is possible to suppress the concentration of stress in the shoulder, and it is possible to achieve excellent breakage resistance while maintaining good ignitability. In other words, the above configuration 1 or the like is more effective in a spark plug having a noble metal tip whose outer diameter of the distal end surface is reduced to 0.5 mm or less.

  Configuration 7. The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 6, the noble metal tip is formed of an alloy containing iridium (Ir) or platinum (Pt) as a main component.

  According to the configuration 7, the noble metal tip is formed of an alloy having Pt or Ir as a main component and having excellent wear resistance, and therefore, the durability can be further improved.

  Further, by using such an alloy, it is possible to relatively easily form a long and narrow noble metal tip as in the above-described configurations 5 and 6.

It is a partially broken front view which shows the structure of a spark plug. It is a partially broken expanded front view which shows the structure of the front-end | tip part of a spark plug. It is a partial expanded sectional schematic diagram which shows structures, such as a shoulder part and a fusion | melting part. It is a partial expanded sectional view of a fusion part etc. for explaining another example of a fusion part. It is a graph which shows the test result of the fracture resistance evaluation test about the sample which changed various (theta) 1. It is a graph which shows the test result of the fracture resistance evaluation test at the time of changing variously (theta) 1-theta2 about the sample which made (theta) 1 72 degrees. It is a graph which shows the test result of the fracture resistance evaluation test at the time of changing variously (theta) 1-theta2 about the sample which set (theta) 1 to 60 degrees. (A), (b) is a partial expanded cross-section schematic diagram which shows structures, such as a shoulder part, in another embodiment. It is a partially broken enlarged front view which shows the structure of the spark plug in another embodiment. (A), (b) is a partially broken enlarged front view which shows the structure of the spark plug in another embodiment.

  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 direction of the axis CL <b> 1 of the spark plug 1 is the vertical direction in the drawing, the lower side is the front end side of the spark plug 1, and the upper side is the rear end side.

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

  As is well known, the insulator 2 is formed by firing alumina or the like, and in its outer portion, a rear end side body portion 10 formed on the rear end side, and a front end than the rear end side body portion 10. A large-diameter portion 11 that protrudes radially outward on the side, a middle body portion 12 that is smaller in diameter than the large-diameter portion 11, and a tip portion that is more distal than the middle body portion 12. The leg length part 13 formed in diameter smaller than this on the side is provided. In addition, of the insulator 2, the large diameter portion 11, the middle trunk portion 12, and most of the leg long portions 13 are accommodated inside the metal shell 3. In addition, a tapered step portion 14 is formed at a connecting portion between the middle body portion 12 and the long leg portion 13, and the insulator 2 is locked to the metal shell 3 at the step portion 14.

  Further, the insulator 2 is formed with a shaft hole 4 penetrating along the axis CL1, and a center electrode 5 is inserted and fixed to the tip end side of the shaft hole 4. The center electrode 5 includes, in order from the distal end side, a shoulder portion 51 that tapers toward the distal end side in the axis CL1 direction, a main body portion 52 that extends from the rear end of the shoulder portion 51 along the axial line CL1, and a rear portion of the main body portion 52. And a flange 53 that bulges radially outward at the end, and the flange 53 is locked to a tapered portion formed in the shaft hole 4. In the present embodiment, the main body 52 has a small diameter, and the outer diameter of the base end is relatively small (for example, 2.6 mm or less or 2.1 mm or less). Further, a small-diameter portion 52A (refer to FIG. 2) having substantially the same outer shape called a thermo is provided at the distal end portion of the main body portion 52A.

  In addition, the center electrode 5 is made of an outer layer 5B made of a Ni alloy containing nickel (Ni) as a main component, and a metal material (for example, copper, copper alloy, pure Ni, etc.) having better thermal conductivity than the outer layer 5B. And an inner layer 5A. Moreover, the front-end | tip part of the center electrode 5 protrudes from the front-end | tip of the insulator 2, The noble metal tip 31 is joined to the front-end | tip part of the center electrode 5 through the fusion | melting part 35 formed by laser welding. .

  The noble metal tip 31 has a cylindrical shape and is made of an alloy mainly composed of iridium (Ir) or platinum (Pt). Further, the melting portion 35 is formed by melting the metal constituting the center electrode 5 and the metal constituting the noble metal tip 31, and is formed on at least a part of the base end side of the noble metal tip 31. (The configuration of the center electrode 5, the noble metal tip 31, and the melting portion 35 will be described in detail later).

  A terminal electrode 6 made of low carbon steel or the like is inserted and fixed on the rear end side of the shaft hole 4 in a state of protruding from the rear end of the insulator 2.

  Further, a cylindrical resistor 7 is disposed between the center electrode 5 and the terminal electrode 6 of the shaft hole 4. Both ends of the resistor 7 are electrically connected to the center electrode 5 and the terminal electrode 6 via conductive glass seal layers 8 and 9, respectively.

  In addition, the metal shell 3 is formed in a cylindrical shape from a metal such as low carbon steel, and a screw for attaching the spark plug 1 to a combustion device such as an internal combustion engine or a fuel cell reformer on the outer peripheral surface thereof. A portion (male screw portion) 15 is formed. In addition, 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 spark plug 1 is attached to the combustion device is provided. A caulking portion 20 for holding the insulator 2 is provided. In the present embodiment, in order to reduce the size of the spark plug 1, the metal shell 3 is formed with a relatively small diameter, and as a result, the screw diameter of the screw portion 15 is M12 or less (for example, M10 or less). Yes.

  A tapered 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 gas 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.

  Further, as shown in FIG. 2, a substantially middle portion of the metal shell 3 is bent back at the front end portion of the metal shell 3, and a ground electrode 27 whose front end surface is opposed to the outer surface of the noble metal tip 31 is joined. . In addition, a prismatic noble metal made of a predetermined noble metal material (for example, a Pt alloy or an Ir alloy) is projected on the side surface of the tip of the ground electrode 27 so as to protrude from both the tip and side surfaces of the ground electrode 27. The part 32 is joined. A spark discharge gap 33 is formed as a gap between the tip of the noble metal tip 31 and the noble metal part 32, and the spark discharge gap 33 is substantially along a direction orthogonal to the axis CL1. Spark discharge is performed.

In addition, a thermo pocket portion 28 is formed between the outer periphery of the insulator 2 and the tip end portion of the shaft hole 4 to form an annular space with the axis line CL1 as the center. With the thermo pocket portion 28, the distance along the surface of the insulator 2 from the center electrode 5 to the metal shell 3 and the distance between the center electrode 5 and the tip of the insulator 2 can be made relatively large. For this reason, it is possible to more surely prevent an abnormal spark discharge that has hit the surface of the insulator 2 such as a so-called side-fire. The central electrode 5 and the like may be configured without providing the small diameter portion 52A and thus the thermo pocket portion 28.

Furthermore, in this embodiment, in order to improve the ignitability, the noble metal tip 31 has a relatively small diameter, while the length along the axis CL1 is relatively long. Specifically, as shown in FIG. 3 (however, in FIG. 3, hatching generally given in the cross-sectional view is omitted for convenience of explanation), the outer diameter DC of the noble metal tip 31 is 0.7 mm. On the other hand, the shortest distance LC between the front end surface of the noble metal tip 31 and the melted portion 35 along the axis CL1 is 0.8 mm or more on the outer surface of the noble metal tip 31. It is set to 1.2 mm or less.

  The shoulder 51 of the center electrode 5 has a tapered shape, and the tip thereof is formed with a relatively small diameter corresponding to the noble metal tip 31 with a relatively small diameter. In addition, in the cross section including the axis CL1, the outlines OL1 and OL2 of the shoulder 51 are linear (the shoulder 51 is a portion that is tapered toward the tip side in the axis CL1 direction, The small diameter portion 52A having substantially the same outer shape provided at the tip of the portion 52 does not constitute the shoulder portion 51). And in the cross section including the axis line CL1, a straight line L1 formed by extending one of the outer shape lines OL1 and OL2 of the shoulder portion 51 located across the axis line CL1 toward the front end side in the direction of the axis line CL1. The shoulder 51 is formed so that θ1 ≦ 72 ° is satisfied, where θ1 is an acute angle among the angles formed with the straight line L2 formed by extending the other outer shape line OL2 toward the front end side in the axis CL1 direction. ing.

  In addition, the melting portion 35 has an annular shape centered on the axis CL1, and is configured such that the distal end surface of the center electrode 5 and the proximal end surface of the noble metal tip 31 are in contact with each other on the axis CL1. In addition, the shape of the fusion | melting part 35 is not limited to this, For example, as shown in FIG. 4, the front end surface of the center electrode 5 and the base end surface of the noble metal tip 31 do not contact between both. It is good also as forming the fusion | melting part 45 over the whole region.

  Returning to FIG. 3, the melting portion 35 has a shape in which the outer peripheral portion tapers toward the tip side in the direction of the axis CL <b> 1, and the outer diameter DM <b> 1 of the portion located on the most tip side of the melting portion 35 is It is smaller than the outer diameter DM2 of the portion located on the most proximal side. Further, in the cross section including the axis line CL1, a straight line L3 passing through both ends of the outline line OL3 located on one side of the outline line OL3, OL4 of the melted portion 35 exposed on the outer surface across the axis line CL1, and the axis line When θ2 is an acute angle among the angles formed with the straight line L4 passing through both ends of the outline OL4 located on the other side across CL1, θ1> θ2 and (θ1−θ2) ≦ 50 ° are satisfied. Thus, the melting part 35 and the like are formed.

  The melting portion 35 has a depth (in the cross section including the axis CL1 from the outlines OL3 and OL4 of the melting portion 35 to the melting portion 35 in order to sufficiently secure the bonding strength of the noble metal tip 31 to the center electrode 5. Of these, the distance along the direction orthogonal to the outlines OL3 and OL4 to the part located on the innermost side is 0.2 mm or more.

  Furthermore, as described above, the inner layer 5A having excellent thermal conductivity is provided inside the center electrode 5, and the inner layer 5A is set to satisfy the following configuration. That is, the inner layer 5A is provided so that the shorter distance of the base end surface of the noble metal tip 31 or the molten portion 35 is 2 mm or less and is sufficiently close to the noble metal tip 31 and the molten portion 35. . Furthermore, in the cross section including the axis CL1, when the acute angle among the angles formed by the two straight lines L5, L6 passing through the intersection CP of the straight line L1 and the straight line L2 and in contact with the outline of the inner layer 5A is θ3, The shape of the inner layer 5A is set so as to satisfy θ1 × 1/3) ≦ θ3 ≦ (θ1 × 3/4).

  As described in detail above, according to the present embodiment, the noble metal tip 31 has the shortest distance LC along the axis CL1 between the front end surface and the melted portion 35 on the outer side surface of 0.8 mm or more. Yes. Therefore, durability and ignitability can be improved.

  On the other hand, if the noble metal tip 31 is made relatively long, there is a concern about breakage in the center electrode 5 or the like. However, according to the present embodiment, the angle θ1 is set to 72 ° or less. Therefore, it can suppress effectively that the stress accompanying a vibration concentrates on the shoulder part 51, and can prevent the breakage in the shoulder part 51 more reliably.

  Further, the melting part 35 is configured such that the outer diameter DM1 at the most distal portion is smaller than the outer diameter DM2 at the most proximal portion. Therefore, it is possible to prevent the boundary portion between the shoulder portion 51 and the melted portion 35 from having a steep bent shape, and stress accompanying vibration is concentrated on the boundary portion and the vicinity thereof. Can be suppressed. As a result, breakage at the boundary portion and the vicinity thereof can be prevented more reliably.

  As described above, according to this embodiment, it is possible to improve the breakage resistance in the shoulder portion 51, the boundary portion, and the like. As a result, it is possible to more reliably improve the durability and ignitability by providing the noble metal tip 31, and It can be exhibited over a longer period of time.

  In addition, the angle θ2 formed by the straight line L3 and the straight line L4 is configured to satisfy θ1−θ2 ≦ 50 °. Therefore, the rate of change in the cross-sectional area along the axial direction can be further reduced at the portion from the shoulder 51 to the melted portion 35, and as a result, the concentration of stress on the shoulder 51 and the melted portion 35 can be more reliably prevented. can do. As a result, the breakage resistance can be further improved.

  Furthermore, since the outlines OL1 and OL2 of the shoulder 51 are linear, it is possible to more reliably prevent stress concentration in the shoulder 51 and to further improve the breakage resistance.

  In addition, at least one of the distance from the inner layer 5A to the base end surface of the noble metal tip 31 or the melting portion 35 is 2 mm or less and satisfies (θ1 × 1/3) ≦ θ3 (that is, the inner layer). The tip of 5A is configured to have a sufficient volume corresponding to the thickness of the tip of center electrode 5 accompanying the change in angle θ1. For this reason, the heat of the noble metal tip 31 can be efficiently drawn by the inner layer 5A, and the wear resistance of the noble metal tip 31 can be further improved.

  On the other hand, the angle θ3 is configured to satisfy θ3 ≦ (θ1 × 3/4), and the inner layer 5A of an appropriate volume corresponding to the thickness of the tip of the center electrode 5 accompanying the change of the angle θ1. And an outer layer 5B having an appropriate thickness. As a result, the outer layer 5B has sufficient strength against the thermal expansion of the inner layer 5A, and the occurrence of cracks in the center electrode 5 can be more reliably prevented.

  Note that the length along the axis CL1 of the shoulder 51 can be relatively large by reducing the diameter of the noble metal tip 31 to a relatively small value of θ1 ≦ 72 °. Here, considering the heat resistance of the center electrode 5 and the noble metal tip 31, there is a limit to projecting the tip of the center electrode 5 (noble metal tip 31) toward the tip of the axis CL <b> 1 with respect to the tip of the insulator 2. For this reason, as the length of the shoulder portion 51 increases, the volume of the thermo pocket portion 28 formed between the insulator 2 and the shaft hole 4 increases. However, if the volume of the thermo pocket portion 28 is excessively increased, the tip portion of the insulator 2 is overheated, which may cause problems such as pre-ignition. In order to prevent overheating of the insulator 2, for example, it is conceivable to shorten the leg length portion 13 of the insulator 2, but in this case, since the surface area of the leg length portion 13 is reduced, the antifouling property is reduced. May decrease. In this regard, according to the present embodiment, since the main body 52 has a relatively small diameter, the length of the shoulder 51 along the direction of the axis CL1 can be made relatively short. Therefore, even if the small diameter noble metal tip 31 is used while θ1 ≦ 72 °, the volume of the thermo pocket portion 28 does not increase excessively. As a result, overheating of the insulator 2 can be suppressed without shortening the leg length portion 13 (that is, without causing deterioration of the stain resistance).

  Next, the shortest distance (chip length) LC between the tip surface of the noble metal tip along the axis CL1 and the melted portion is changed by changing the noble metal tip in order to confirm the operational effect achieved by the above embodiment. Samples of spark plugs with various changes and various changes in the angle θ1 formed by the straight line L1 and the straight line L2 were prepared, and each sample was subjected to a breakage resistance evaluation test. The outline of the fracture resistance evaluation test is as follows. That is, vibration with a frequency of 27.3 kHz was applied to the sample with an ultrasonic horn, and the time until breakage occurred at the center electrode or the melted part (breakage time) was measured. Here, a sample having a breakage time of 120 seconds or more is evaluated as “◯” as having excellent breakage resistance, and a sample having a breakage time of 180 seconds or more is extremely excellent in breakage resistance. It was decided to give an evaluation of “◎”. On the other hand, a sample having a breakage time of less than 120 seconds was evaluated as “x” because the breakage resistance was insufficient. Table 1 shows the test results of the breakage resistance evaluation test. In Sample 10, the shoulder portion was formed so that the outline of the shoulder portion was linear, and the shoulder portion was formed so that the outer portion had a bent portion (corner portion). Moreover, the fusion | melting part depth of each sample was 0.2 mm. Further, FIG. 5 shows the test results of samples (samples 6 to 9, 11, and 12) in which only the angle θ1 is changed in various conditions, with the chip length LC and the like being the same.

  As shown in Table 1, the samples having the chip length LC of 0.7 mm (Samples 1 to 3) had excellent breakage resistance regardless of the value of the angle θ1, while the chip length It was found that the samples having the LC of 0.8 mm or more (samples 4 to 19) could have insufficient breakage resistance.

  Therefore, when focusing on a sample having a chip length LC of 0.8 mm or more, a sample having a chip length LC of 1.2 mm or less and an angle θ1 of 72 ° or less (samples 5, 9 to 12) ) Has a breakage time of 120 seconds or more and was found to have excellent breakage resistance. This is because the change rate of the cross-sectional area of the shoulder along the axial direction is relatively small by setting the angle θ1 to 72 ° or less, and as a result, the concentration of stress on the shoulder due to vibration can be suppressed. it is conceivable that. Further, as shown in FIG. 5, it was confirmed that the breakage resistance was further improved as the angle θ1 was decreased.

  In addition, a sample (sample 10) in which the outer contour line of the shoulder portion is formed linearly with respect to the sample in which the outer contour line of the shoulder portion has a bent portion, and the outer contour line of the shoulder portion does not exist. It was found to have very good breakage resistance. This is because the cross-sectional area slightly changes along the axial direction at the bent part, so that stress tends to concentrate relatively easily. By eliminating the bent part, the concentration of stress on the shoulder can be further suppressed. It is thought that.

  From the above test results, in the spark plug in which the chip length is 0.8 mm or more and 1.2 mm or less and the breakage in the center electrode or the like is more concerned, θ1 ≦ 72 ° should be set in order to improve the breakage resistance. It can be said that it is significant to form the shoulder so as to satisfy.

  Moreover, it can be said that it is more significant to form a shoulder part linearly or to make angle (theta) 1 smaller (for example, 60 degrees or less) from the point of aiming at the further improvement of breakage resistance.

  Next, the chip length LC is set to 1.2 mm, the angle θ1 is set to 72 ° or 60 °, and the angle θ2 formed by the straight line L3 and the straight line L4 is changed by changing the shape of the melting portion and the angle Spark plug samples in which the difference (θ1−θ2) from θ1 was variously changed were produced, and the above-described breakage resistance evaluation test was performed on each sample. FIG. 6 shows a test result of a sample with an angle θ1 of 72 °, and FIG. 7 shows a test result of a sample with an angle θ1 of 60 °. In FIGS. 6 and 7, the break time is shown as 360 seconds when no breakage occurs in the center electrode or the melted part for a long time of 360 seconds or longer. Moreover, the depth of the fusion | melting part was 0.2 mm with each sample.

  As shown in FIGS. 6 and 7, each sample had a break time of 120 seconds or more and had excellent break resistance, but a sample with θ1-θ2 of 50 ° or less had a break time of 360 seconds. As described above, it was found that the film has extremely excellent breakage resistance. This is because, by making θ1-θ2 relatively small, the change rate of the cross-sectional area along the axial direction becomes relatively small in the portion from the shoulder to the melted portion. This is probably because the concentration of stress was further suppressed.

  From the above test results, it can be said that it is preferable to configure the melted portion or the like so as to satisfy θ1−θ2 ≦ 50 ° from the viewpoint of further improving breakage resistance.

  Next, the chip length LC is set to 1.2 mm and θ1 is set to 45 °, 60 °, or 72 °, and the angle θ3 formed by the straight line L5 and the straight line L6 is changed by changing the configuration of the inner layer. Samples of spark plugs with various changes were prepared, and a heating temperature measurement test was performed on each sample. The outline of the heating temperature measurement test is as follows. That is, in the conventional spark plug having a tip length of 0.4 mm, the tip of each sample was heated by a predetermined burner under the condition that the temperature of the tip of the noble metal tip was 1000 ° C. The temperature at the tip of the noble metal tip was measured. Here, although the tip length LC is 1.2 mm and the noble metal tip is very easily heated, the temperature of the tip of the noble metal tip is 1050 ° C. or lower (ie, the conventional sample). Samples in which the temperature rise during heating was suppressed to 50 ° C. or lower as compared with the spark plug of No. 5) were evaluated as “◯” because they were excellent in heat pulling. On the other hand, a sample in which the temperature of the tip of the noble metal tip exceeded 1050 ° C. was evaluated as “Δ” because it was slightly inferior to heat pulling. Table 2 shows test results for samples with an angle θ1 of 45 °, and Table 3 shows test results for samples with an angle θ1 of 60 °. Table 4 shows the test results of samples with an angle θ1 of 72 °. In each sample, the outer diameter at the base end of the main body of the center electrode was 1.9 mm, and the outer diameter of the noble metal tip was 0.7 mm. In addition, the shortest distance between the inner layer and the noble metal tip or the melted portion was set to 2.0 mm or less.

  Further, the chip length LC was set to 1.2 mm and θ1 was set to 45 °, 60 °, or 72 °, and the shortest distance SD between the inner layer and the noble metal tip or the melted portion was variously changed. Spark plug samples were prepared, and the above-described heating temperature measurement test was performed on each sample. Samples with θ1 of 45 ° have θ3 of 15 °, samples with θ1 of 60 ° have θ3 of 20 °, and samples with θ1 of 72 ° have θ3 of 25 °. The size of the center electrode and the like was the same as described above. Table 5 shows test results for samples with an angle θ1 of 45 °, and Table 6 shows test results for samples with an angle θ1 of 60 °. Table 7 shows the test results of samples with an angle θ1 of 72 °.

  As shown in Tables 2 to 4, it was clarified that the samples in which the angle θ3 was less than (θ1 × 1/3) were slightly inferior in heat pulling. This is presumably because the heat of the noble metal tip could not be efficiently conducted because the volume at the tip of the inner layer adjacent to the noble metal tip was relatively small.

  Furthermore, as shown in Tables 5 to 7, it was found that the samples having the shortest distance SD larger than 2.0 mm were slightly inferior in heat pulling. This is probably because the heat of the noble metal tip is hardly transmitted to the inner layer because the inner layer is relatively separated from the noble metal tip and the melted portion.

  On the other hand, it was clarified that the sample satisfying (θ1 × 1/3) ≦ θ3 while having the shortest distance SD within 2.0 mm is excellent in heat pulling. This is because the inner layer is sufficiently close to the noble metal tip or the like, and the inner layer tip has a sufficient volume corresponding to the thickness of the tip of the center electrode accompanying the change of the angle θ1. This is thought to be because the heat was efficiently conducted.

  From the above test results, in order to efficiently draw the heat of the noble metal tip, the shoulder portion and the inner layer should be configured to satisfy (θ1 × 1/3) ≦ θ3 while keeping the shortest distance SD within 2.0 mm. Is preferable.

  Then, while setting the chip length LC to 1.2 mm and the angle θ1 to 45 °, 60 °, or 72 °, five spark plug samples in which the angle θ3 is variously changed are prepared, A burner cooling test was performed on each sample. The outline of the burner cooling test is as follows. That is, the center electrode tip was heated for 3 minutes under a condition of a temperature of 1000 ° C. with a predetermined burner, and then slowly cooled for 1 minute. After the end of 2500 cycles, the center electrode was observed to check for cracks on the surface. Here, samples in which no cracks were confirmed in all of the five samples were evaluated as “◯” as having excellent expansion resistance, while samples in which cracks were confirmed in at least one of the five samples Therefore, it was decided to give a rating of “Δ” because it was slightly inferior in expansion resistance. Table 8 shows the test results of samples with an angle θ1 of 45 °, and Table 9 shows the test results of samples with an angle θ1 of 60 °. Table 10 shows the test results of samples with an angle θ1 of 72 °. In each sample, the outer diameter at the base end of the main body of the center electrode was 1.9 mm, and the outer diameter of the noble metal tip was 0.7 mm. In addition, the shortest distance between the inner layer and the noble metal tip or the melted portion was set to 2.0 mm or less.

  As shown in Tables 8 to 10, it was found that in the sample in which the angle θ3 was larger than (θ1 × 3/4), the center electrode could crack as the cooling cycle was repeated. This is because when the angle θ1 changes, the thickness of the tip of the center electrode located in the vicinity of the noble metal tip changes (when θ1 is small, it becomes thin, and when θ1 is large, it becomes thick), θ3> (θ1 × 3 / 4), in the cross section orthogonal to the axis, the proportion of the inner layer in the tip of the center electrode becomes excessively large, and the outer layer becomes relatively thin. As a result, the outer layer against the thermal expansion of the inner layer This is considered to be because the strength of the film has become insufficient.

  On the other hand, it was revealed that the sample satisfying θ3 ≦ (θ1 × 3/4) is excellent in expansion resistance without causing a crack in the center electrode. This is because θ3 ≦ (θ1 × 3/4) is set, so that an inner layer of an appropriate volume and an outer layer of an appropriate thickness corresponding to the thickness of the tip of the center electrode accompanying the change of the angle θ1 are set. As a result, it is considered that the outer layer has sufficient strength against the thermal expansion of the inner layer.

  From the above test results, it can be said that it is preferable to configure the shoulder portion and the inner layer so as to satisfy θ3 ≦ (θ1 × 3/4) in order to improve the expansion resistance.

  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) In the above embodiment, the outlines OL1 and OL2 of the shoulder 51 in the cross section including the axis CL1 are formed in a straight line, but the shoulder 51 is tapered toward the front end side in the axis CL1 direction. For example, as shown in FIG. 8A, a bent portion 64 may be formed in the shoulder portion 61, and as shown in FIG. 8B, an outline OL7 of the shoulder portion 71 may be formed. OL8 may be slightly curved so as to form a convex shape on the outside (or on the inside) [In FIGS. 8 (a) and 8 (b), hatching generally applied in the cross-sectional views is provided. It is omitted for convenience of explanation.] In addition, when the bending part 64 is formed in the shoulder part 61, the straight line L1 and the straight line L2 say the straight line which extended the line segment located in the front end side rather than the bending part 64 among the outline lines of the shoulder part 61. . Further, when the outlines OL7 and OL8 of the shoulder portion 71 are curved, the straight lines L1 and L2 extend the line segments connecting both ends of the outlines OL7 and OL8 toward the front end side in the axis CL1 direction. This is a straight line.

  (B) In the above embodiment, the noble metal portion 32 is joined to the side surface of the tip of the ground electrode 27. However, as shown in FIG. 9, the noble metal portion 82 may be joined to the tip of the ground electrode 27. Good.

  (C) In the above embodiment, the technical idea of the present invention is applied to a spark plug 1 of a type in which spark discharge is performed substantially along a direction orthogonal to the axis CL1, but the technical idea of the present invention is applicable. The type of spark plug is not limited to this. Therefore, according to the technical idea of the present invention, for example, as shown in FIG. 10A, spark discharge is performed substantially along the direction of the axis CL1 in the spark discharge gap 83 formed between the noble metal tip 31 and the noble metal portion 92. Type spark plug 1A or a type in which spark discharge is performed obliquely with respect to the axis CL1 in a spark discharge gap 93 formed between the noble metal tip 31 and the noble metal portion 102, as shown in FIG. 10 (b). It is good also as applying to this spark plug 1B. Even in this case, the ignitability and the like can be improved as in the above embodiment.

  (D) Although the noble metal portion 32 is provided on the ground electrode 27 in the above embodiment, the noble metal portion 32 may not be provided. In this case, the spark discharge gap 33 is formed between the noble metal tip 31 and the ground electrode 27.

  (E) In the above-described embodiment, the center electrode 5 has a two-layer structure including the inner layer 5A and the outer layer 5B, but may have a three-layer structure or a multilayer structure of four or more layers. Therefore, for example, an intermediate layer made of copper alloy or pure copper may be provided inside the outer layer 5B, and an innermost layer made of pure nickel may be provided inside the intermediate layer. In the case where the center electrode 5 has a three-layer structure or more, a plurality of layers that are located inside the outer layer 5B and contain a better heat conductive metal than the outer layer 5B correspond to the inner layer 5A. For example, when the above-described configuration in which the intermediate layer and the innermost layer are provided is employed, the intermediate layer and the innermost layer correspond to the inner layer 5A.

  (F) 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 the tip metal fitting previously welded to the metal shell is used. The present invention is also applicable to the case where the ground electrode is formed by cutting out a part of the ground (for example, Japanese Patent Application Laid-Open No. 2006-236906).

  (G) In the above embodiment, the tool engagement portion 19 has a hexagonal cross section, but the shape of the tool engagement portion 19 is not limited to such a shape. For example, it may be a Bi-HEX (deformed 12-angle) shape [ISO 22777: 2005 (E)].

1 ... Spark plug 2 ... Insulator (insulator)
3 ... metal shell 4 ... shaft hole 5 ... center electrode 5A ... inner layer 5B ... outer layer 27 ... ground electrode 31 ... precious metal tip 33 ... spark discharge gap (gap)
35 ... Melting part 51 ... Shoulder part CL1 ... Axis

Claims (7)

A cylindrical insulator having an axial hole penetrating in the axial direction;
A center electrode inserted on the tip side of the shaft hole;
A cylindrical metal shell provided on the outer periphery of the insulator;
A ground electrode disposed at the tip of the metal shell;
A noble metal tip bonded to the tip of the center electrode and forming a gap with the ground electrode;
At the tip of the center electrode, a shoulder is formed that tapers toward the tip in the axial direction.
The noble metal tip is joined to the center electrode by forming a melted part in which the center electrode and the noble metal chip are melted in at least a part of the base end of the noble metal tip,
In the outer surface of the noble metal tip, the shortest distance between the tip surface of the noble metal tip along the axis and the molten part is 0.8 mm to 1.2 mm,
The outer diameter at the most distal portion of the melted portion is smaller than the outer diameter at the most proximal portion,
When the acute angle among the angles formed by the following straight line L1 and straight line L2 is θ1,
θ1 ≦ 72 °
The filling,
The center electrode includes an outer layer and an inner layer provided inside the outer layer and having higher thermal conductivity than the outer layer,
The shorter distance of the distance from the inner layer to the base end face of the noble metal tip or the distance from the inner layer to the molten part is 2 mm or less,
In a cross section including the axis, when an acute angle among the angles formed by two straight lines passing through the intersection of the straight line L1 and the straight line L2 and in contact with the outline of the inner layer is θ3,
(Θ1 × 1/3) ≦ θ3
A spark plug characterized by satisfying.
Straight line L1: A straight line formed by extending one of the two outlines of the shoulder portion located across the axis in the cross section including the axis toward the tip end in the axial direction Straight line L2: the axis In the cross section including the straight line formed by extending the other outline of the two outlines of the shoulder located across the axis toward the tip end side in the axial direction θ3 ≦ (θ1 × 3/4)
The spark plug according to claim 1, wherein: In a cross section including the axis,
The spark plug according to claim 1 or 2, wherein an outline of the shoulder portion is linear. The ground electrode is disposed such that the tip surface thereof faces the outer surface of the noble metal tip,
4. The spark plug according to claim 1, wherein spark discharge is performed in the gap substantially along a direction orthogonal to the axis. 5.   The spark plug according to any one of claims 1 to 4, wherein the noble metal tip has a cylindrical shape, and an outer diameter of a tip surface thereof is 0.7 mm or less.   The spark plug according to any one of claims 1 to 5, wherein the noble metal tip has a cylindrical shape, and an outer diameter of a tip surface thereof is 0.5 mm or less.   The spark plug according to any one of claims 1 to 6, wherein the noble metal tip is formed of an alloy containing iridium or platinum as a main component.

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