JP5723250B2 - Spark plug and manufacturing method thereof - Google Patents

Description

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

  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, a cylindrical insulator provided on the outer periphery of the center electrode, and a cylindrical metal shell provided on the outer periphery of the insulator And a ground electrode having a base end portion joined to a tip end portion of the metal shell. The ground electrode has its substantially middle portion bent back so that the tip of the ground electrode faces the tip of the center electrode, and there is a spark discharge gap between the tip of the center electrode and the tip of the ground electrode. It is formed.

  By the way, in recent years, direct injection, high EGR combustion, lean combustion, etc. are performed in order to improve the efficiency of the combustion apparatus, and a large-diameter intake / exhaust valve or the like is used to improve volumetric efficiency and reduce pumping loss. An engine having a variable valve mechanism has been proposed. However, an engine equipped with a large-diameter intake / exhaust valve and a variable valve mechanism has very large vibration during operation. For this reason, in the ground electrode, there is a possibility that breakage may occur at a bent portion where stress accompanying vibration is particularly concentrated.

  Therefore, in order to suppress breakage of the ground electrode, a technique has been proposed in which the ground electrode is formed into a straight bar (straight) shape without providing a bent portion (see, for example, Patent Document 1). Further, it is conceivable to reduce the stress applied to the bent portion during vibration and suppress breakage of the ground electrode by making the tip of the ground electrode thin (see, for example, Patent Document 2).

JP 2003-59618 A JP 2009-295569 A

  However, in the technique described in Patent Document 1, not only the tip portion of the ground electrode but also the middle portion of the ground electrode approaches the center electrode. For this reason, the presence of the ground electrode inhibits the growth of the flame generated in the spark discharge gap, which may lead to a decrease in ignitability.

  Further, when a thin wall portion is provided at the tip of the ground electrode as in the technique described in Patent Document 2, an improvement in breakage resistance can be expected, but the thin wall portion may be overheated. . If the thin-walled portion is overheated, the thin-walled portion is likely to be oxidized, and the durability of the ground electrode may be reduced. Moreover, there is a concern that pre-ignition may occur using the thin-walled portion as a heat source.

  The present invention has been made in view of the above circumstances, and the object thereof is to achieve excellent ignitability while suppressing breakage of the ground electrode. Furthermore, it has excellent durability and pre-ignition resistance. It is an object of the present invention to provide a spark plug capable of obtaining the characteristics and a manufacturing method thereof.

  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 shaft hole;
A cylindrical metal shell provided on the outer periphery of the insulator;
A spark plug that is fixed to the tip of the metal shell and is bent toward the center electrode at a bent portion and includes a ground electrode that forms a gap with the center electrode,
When the radius of curvature of the inner surface located on the central electrode side in the bent portion is R (mm), R ≦ 3.0 is satisfied,
The ground electrode is
A base fixed to the metal shell and including at least a part of the bent portion;
Extending from the tip of the base along the extending direction of the tip of the base, and having a thin portion formed thinner than the base,
The acute angle among the angles formed by the plane orthogonal to the axis is 75 °, and the thin-walled portion is positioned closer to the tip side of the ground electrode than the reference plane passing through the center of the radius of curvature of the inner surface. And
When the length along the central axis direction of the thin portion is A (mm) and the average cross-sectional area of the thin portion in a cross section orthogonal to the central axis is dGH (mm ² ), A / dGH ≦ 3 .03 (mm ^-1 )
Between the front end surface of the thin-walled portion and each side surface of the thin-walled portion adjacent to the thin-walled portion and between the side surfaces adjacent to each other, a curved surface that protrudes outward is formed.

  According to the configuration 1, the ground electrode is provided with the bent portion, and the curvature radius R of the bent portion is set to 3.0 mm or less. Therefore, a relatively large space is formed between the ground electrode and the center electrode, and flame growth inhibition by the ground electrode can be more reliably prevented. As a result, good ignitability can be realized.

  On the other hand, when the bent portion is provided, a large stress may be applied to the bent portion due to vibration, and there is a concern that the ground electrode may be broken at the bent portion. In particular, when the radius of curvature R of the bent portion is as small as 3.0 mm or less as in the above-described configuration 1, the stress applied to the bent portion is also greater, so there is a greater concern about breakage of the ground electrode. .

  In this regard, according to the above configuration 1, since the thin portion is provided at the tip of the ground electrode, the weight of the tip of the ground electrode can be reduced, and the stress applied to the bent portion due to vibration can be effectively reduced. Can be reduced. Furthermore, in view of the fact that breakage of the ground electrode is likely to occur in the range of the base end side of the ground electrode with respect to the reference plane, the thin portion that is thin and slightly inferior in strength is located outside the range, and is thick and strong in strength. The superior base is configured to be located within the above range. Thus, the strength of the bent portion can be sufficiently maintained by setting the formation position of the thin portion or the like. As a result, the breakage of the ground electrode can be extremely effectively suppressed in combination with the effect of reducing the stress by providing the thin portion.

In addition, when the thin portion is provided, overheating of the thin portion is a concern. However, according to Configuration 1, the length of the thin portion is A (mm), and the average cross-sectional area of the thin portion is dGH (mm ² ), A / dGH ≦ 3.03 (mm ⁻¹ ) is satisfied. That is, the average cross-sectional area dGH corresponding to the heat conduction capability of the thin portion is sufficiently large with respect to the length A corresponding to the amount of heat received by the thin portion. Therefore, the amount of residual heat per unit length of the thin-walled portion (the amount of heat received per unit length minus the amount of heat conducted from the thin-walled portion to the base through the thin-walled portion) can be made sufficiently small. The overheating of the part can be suppressed. As a result, oxidation of the thin-walled portion can be more reliably prevented, and excellent durability can be realized.

  Furthermore, when the tip portion and the side surfaces of the thin wall portion or between the adjacent side surfaces are formed in an angular shape, when the internal combustion engine or the like is used, the angular portion is There is a risk that pre-ignition may occur due to local overheating. In view of this point, according to the above-described configuration 1, a curved surface that protrudes outward is formed between the distal end surface and each side surface of the thin portion and between the adjacent side surfaces. Therefore, local overheating in the ground electrode can be suppressed, and combined with the effect of suppressing overheating of the thin portion by A / dGH ≦ 3.03, the pre-ignition resistance can be dramatically improved. Can do.

Configuration 2. The spark plug of this configuration is the above-described configuration 1, wherein the thin portion is formed on a surface side of the ground electrode that is positioned on the center electrode side, and on the side of the thin portion on the opposite side of the center electrode. The connecting portion between the positioned surface and the distal end surface of the base is tapered or has a concave curved surface with a curvature radius of 0.5 mm or more.

  According to the configuration 2, the thin wall portion is configured to approach the center electrode, and as a result, the thin wall portion is closer to the fixed portion of the ground electrode with respect to the metal shell (base end of the ground electrode). It is configured as follows. Therefore, the force (moment) applied to the fixed portion of the ground electrode due to vibration can be further reduced, and breakage of the ground electrode at the fixed portion can be more reliably prevented.

  Further, the connecting portion between the surface on the side opposite to the center electrode of the thin-walled portion and the distal end surface of the base portion is tapered or has a concave curved surface with a radius of curvature of 0.5 mm or more. Yes. Therefore, it is possible to more reliably prevent stress concentration on the connecting part due to vibration, and to effectively suppress the occurrence of cracks in the connecting part. As a result, breakage of the ground electrode can be prevented more reliably.

  Configuration 3. The spark plug of this configuration is characterized in that, in the above configuration 1 or 2, 1.0 ≦ A ≦ 5.0 is satisfied.

  According to the said structure 3, the length A of a thin part is made into 1.0 mm or more. Therefore, the breakage resistance of the ground electrode can be further reliably improved.

  In addition, when the length A of the thin portion is larger than 5.0 mm, the improvement rate of the breakage resistance with respect to the increase amount of the length A is slightly lowered. Further, when the length A is large, it is necessary to increase the average cross-sectional area (dGH) of the thin-walled portion in order to suppress overheating of the thin-walled portion. There is a possibility that the weight cannot be reduced sufficiently. Therefore, in view of these points, it is preferable to set the length A of the thin portion to 5.0 mm or less.

Configuration 4. The spark plug manufacturing method of this configuration is the spark plug manufacturing method according to any one of the above configurations 1 to 3,
A thin-walled portion forming step for forming the thin-walled portion at the tip of the ground electrode;
Bending the ground electrode to the center electrode side and forming the bent portion, and a bending portion forming step,
The bent portion forming step is performed after the thin portion forming step.

  If the thin part is formed by cutting or the like after bending the ground electrode, the ground electrode will be deformed during processing, and the gap between the center electrode and the ground electrode will be at an appropriate position and an appropriate size. There is a possibility that it cannot be formed.

  In this regard, according to the configuration 4, the ground electrode is bent after the thin portion is formed. Therefore, the gap between the center electrode and the ground electrode can be formed at a more appropriate position and with a more appropriate size.

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 enlarged front view which shows the structure of a ground electrode. It is a partial expanded bottom view which shows structures, such as a thin part. It is a partial expanded side view which shows structures, such as a thin part. It is a partial enlarged front view which shows structures, such as a thin part. It is a graph which shows the test result of the ignitability evaluation test in the sample which changed the curvature radius R of the bending part variously. It is a graph which shows the test result of the vibration resistance improvement rate evaluation test in the sample which changed the curvature radius R of the bending part variously. It is explanatory drawing for demonstrating the method at the time of specifying the broken position of a ground electrode in a desktop vibration test. It is a graph which shows the breakage number of the ground electrode in each angle. It is a graph which shows the test result of the heat resistance evaluation test in the sample which changed the curvature radius X of the curved surface part variously. It is a graph which shows the test result of the vibration resistance improvement rate evaluation test in the sample which changed variously the length A of the thin part. It is a partial expanded front view which shows the structure of the ground electrode in another embodiment. It is a partial expanded front view which shows the structure of the ground electrode in another embodiment. It is a partial expanded bottom view which shows the structure of the ground electrode in another embodiment. It is a partial expanded front view which shows the structure of the ground electrode in another embodiment. It is a partial expanded front view which shows the structure of the ground electrode 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 a cylindrical 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. A tapered step portion 14 is formed at the 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 extending along the axis CL <b> 1, and a center electrode 5 is inserted and fixed to the tip side of the shaft hole 4. The center electrode 5 includes an inner layer 5A made of a metal having excellent thermal conductivity [copper, copper alloy, pure nickel (Ni)] and an outer layer 5B made of a Ni alloy containing Ni as a main component. The center electrode 5 has a rod shape (cylindrical shape) as a whole, and a tip portion of the center electrode 5 projects from the tip of the insulator 2. In addition, the tip of the center electrode 5 is provided with a cylindrical tip 31 made of a metal having excellent wear resistance (for example, an iridium alloy or a platinum alloy).

  In addition, a terminal electrode 6 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 through 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. Further, a seat portion 16 is formed on the rear end side of the screw portion 15 so as to protrude toward the outer peripheral side, and a ring-shaped gasket 18 is fitted into the screw neck 17 at the rear end of the screw portion 15. Further, 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 combustion device is provided on the rear end side of the metal shell 3. A caulking portion 20 that bends inward in the radial direction is provided at the rear end portion of the metal shell 3.

  In addition, 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 to the metal shell 3 by caulking the opening on the side inward in the radial direction, 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 talc 25 powder. 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, a ground electrode 27 made of a metal containing Ni as a main component is joined to the front end surface 26 of the metal shell 3. As shown in FIG. 2, the ground electrode 27 is bent back by a bent portion 27B formed at a substantially intermediate portion of the ground electrode 27, and the tip side surface faces the tip portion (chip 31) of the center electrode 5. Yes. A spark discharge gap 33 is formed as a gap between the tip of the center electrode 5 (chip 31) and the tip of the ground electrode 27, and the spark discharge gap 33 substantially extends along the axis CL1. Spark discharge is performed in the direction of the direction.

  Further, in the present embodiment, as shown in FIG. 3, when the radius of curvature of the inner surface of the bent portion 27B located on the center electrode 5 side is R (mm), R ≦ 3.0 is satisfied. It is configured.

  The ground electrode 27 includes a base portion 28 fixed to the distal end portion 26 of the metal shell 3 and a thin portion 29 extending from the distal end of the base portion 28.

  The base portion 28 includes a bent portion 27B, and has a relatively large thickness (for example, 0.9 mm or more) from the surface on the center electrode 5 side to the back surface thereof. Further, by appropriately adjusting the amount of bending of the bent portion 27B, the distal end portion of the base portion 28 is configured to extend substantially along the direction orthogonal to the axis line CL1.

  The thin portion 29 extends from the distal end of the base portion 28 along the extending direction of the distal end portion of the base portion 28, and is formed thinner than the base portion 28. The thin portion 29 is formed on the surface side of the ground electrode 27 located on the center electrode 5 side, and the surface of the thin portion 29 on the center electrode 5 side and the surface of the base portion 28 on the center electrode 5 side are surfaces. It is one.

  In addition, in the present embodiment, the thickness H of at least the tip side of the thin portion 29 is configured to be substantially constant along the longitudinal direction of the ground electrode 27, and the thickness H is in a predetermined range. (For example, 0.6 mm or more and 2.0 mm or less). Further, as shown in FIG. 4, the thin portion 29 has a width smaller than that of the base portion 28. At least the distal end side of the thin portion 29 is configured to have a substantially constant width G along the longitudinal direction of the ground electrode 27, and the width G is within a predetermined range (for example, 0.6 mm or more 3 0.0 mm or less).

  Returning to FIG. 3, the thin-walled portion 29 is located on the tip side of the ground electrode 27 with respect to a reference plane BS described below. That is, the thin portion 29 is provided at a position sufficiently separated from the center position of the bent portion 27 </ b> B in the central axis direction of the ground electrode 27. The reference plane BS is a plane in which the acute angle θ among the angles formed by the virtual plane VS orthogonal to the axis CL1 is 75 ° and passes through the center CP of the curvature radius R.

Furthermore, in the present embodiment, the thin-walled portion 29 has a length along its own central axis direction as A (mm), and an average cross-sectional area in a cross section orthogonal to the central axis as dGH (mm ² ), A / dGH ≦ 3.03 (mm ⁻¹ ) [more preferably, A / dGH ≦ 0.98 (mm ⁻¹ )] is satisfied. The “average cross-sectional area dGH” means that a plurality of cross-sectional areas of the thin-walled portion 29 are obtained at equal intervals along the extending direction of the thin-walled portion 29 and an average of the obtained cross-sectional areas is calculated. Can be obtained.

  In addition, the length A is set to 1.0 mm or more and 5.0 mm or less, and in the present embodiment, the length A is larger than the width and thickness of the thin portion 29.

  In addition, as shown in FIG. 5, between the front end surface 29F of the thin portion 29 and the side surfaces 29S1, 29S2, 29S3, and 29S4 of the thin portion 29 adjacent thereto, a curved surface that is convex outward is formed. Curved surface portions 29W1, 29W2, 29W3, and 29W4 are provided. Further, curved surfaces 29W5, 29W6, 29W7, and 29W8 are provided between the adjacent side surfaces 29S1 to 29S4 to form curved surfaces that are convex outward.

  Further, as shown in FIG. 6, of the side surfaces 29S1 to 29S4 of the thin portion 29, the connecting portion between the surface 29S2 located on the opposite side of the center electrode 5 and the distal end surface 28F of the base portion 28 has a concave curved shape. A gradually changing portion 27W is formed. And in the cross section containing the central axis of the thin part 29, the curvature radius of the gradually changing part 27W shall be 0.5 mm or more. In the present embodiment, the connecting portion between the thin portions 29S3 and 29S4 and the distal end surface 28F of the base portion 28 is also configured to have a concave curved shape (see FIGS. 4 and 5).

  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 rough shape is formed on a cylindrical metal material (for example, an iron-based material or a stainless steel material) by cold forging or the like, and a through hole is formed. Thereafter, the outer shape is adjusted by cutting to obtain a metal shell intermediate.

  Subsequently, the front end surface of the metal shell intermediate body has a straight bar shape and has a substantially constant width and thickness along the longitudinal direction of the metal shell intermediate body (that is, the thin-walled portion 29 is not provided). The electrode 27 is resistance welded. 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.

  Next, the metal shell 3 to which the ground electrode 27 is welded is subjected to zinc plating or Ni plating by a barrel plating apparatus (not shown). 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 compact for molding is prepared, and a rubber-molded product is used to form a cylindrical molded body. Is obtained. The obtained molded body is ground and shaped, and the shaped product is fired in a firing furnace, whereby the insulator 2 is obtained.

  Separately from the metal shell 3 and the insulator 2, the center electrode 5 is manufactured. That is, the center electrode 5 is produced by forging a Ni alloy in which a copper alloy or the like for improving heat dissipation is arranged at the center. Further, a tip 31 made of a predetermined metal is joined to the tip of the center electrode 5 by laser welding or the like.

  Next, 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 injected into the shaft hole 4 of the insulator 2 with the resistor 7 interposed therebetween, and then from the rear. While being pressed by the terminal electrode 6, it is fired by being heated in a firing furnace. At this time, the glaze layer may be fired simultaneously on the surface of the rear end side body portion 10 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 fixed. More specifically, after the insulator 2 is inserted through the metal shell 3, the opening on the rear end side of the metal shell 3 formed relatively thin is caulked radially inward, that is, the caulking portion 20 is By forming, the insulator 2 and the metal shell 3 are fixed.

  Next, in the thin part forming step, the thin part 29 is formed at the tip of the ground electrode 27. Specifically, the tip of the ground electrode 27 is thinned by applying cutting or pressing to the tip of the ground electrode 27 (at this time, the gradually changing portion 27W is formed). Further, after the cutting or pressing, the edge of the thin portion is subjected to cutting, swaging, or polishing. Thereby, the thin part 29 provided with the curved surface parts 29W1-29W8 is formed. In the case where press working is used when the tip portion of the ground electrode 27 is thinned, a surface corresponding to the curved surface portions 29W1 to 29W8 is provided on the pressing die so that the ground electrode 27 is thinned and the curved surface portion is formed. The formation of 29W1 to 29W8 may be performed simultaneously.

  Next, in the bent portion forming step, the bent portion 27B is formed by bending the ground electrode 27 toward the center electrode 5 side. Finally, the above-described spark plug 1 is obtained by adjusting the size of the spark discharge gap 33 formed between the center electrode 5 (chip 31) and the ground electrode 27.

  As described above in detail, according to the present embodiment, the bent portion 27B is provided in the ground electrode 27, and the curvature radius R of the bent portion 27B is set to 3.0 mm or less. Accordingly, a relatively large space is formed between the ground electrode 27 and the center electrode 5, and flame growth inhibition by the ground electrode 27 can be prevented more reliably. As a result, good ignitability can be realized.

  On the other hand, when the radius of curvature R of the bent portion 27B is as very small as 3.0 mm or less, the stress applied to the bent portion with vibration becomes larger, so there is a further concern about the breakage of the ground electrode 27. In this embodiment, a thin portion 29 is provided at the tip of the ground electrode 27. Therefore, the weight of the tip of the ground electrode 27 can be reduced, and the stress applied to the bent portion 27B can be effectively reduced. Further, considering that the breakage of the ground electrode 27 is likely to occur in the range of the base end side of the ground electrode 27 with respect to the reference plane BS, the thin-walled portion 29 that is thin and slightly inferior in strength is positioned outside the range, The base 28 which is meat and excellent in strength is configured to be located within the above range. By setting the formation position of the thin portion 29 and the like in this way, the strength of the bent portion 27B can be sufficiently maintained. As a result, combined with the effect of reducing the stress by providing the thin portion 29, the breakage of the ground electrode 27 can be extremely effectively suppressed.

Further, according to the present embodiment, when the length of the thin portion 29 is A (mm) and the average cross-sectional area of the thin portion 29 is dGH (mm ² ), A / dGH ≦ 3.03 (mm ⁻¹ ). That is, the average cross-sectional area dGH corresponding to the heat conduction capability of the thin portion 29 is sufficiently large with respect to the length A corresponding to the amount of heat received by the thin portion 29. Therefore, oxidation of the thin portion 29 can be prevented more reliably, and excellent durability can be realized.

  Further, convex curved surface portions 29W1 to 29W8 are provided on the outer side between the front end surface 29F and the side surfaces 29S1 to 29S4 of the thin portion 29 and between the adjacent side surfaces 29S1 to 29S4. Therefore, local overheating in the ground electrode 27 can be suppressed, and in combination with the effect of suppressing overheating of the thin portion 29 by A / dGH ≦ 3.03, the pre-ignition resistance is drastically improved. Can be increased.

  Further, the thin portion 29 is formed on the surface side of the ground electrode 27 on the side of the center electrode 5, and the thin portion 29 is with respect to a fixed portion of the ground electrode 27 with respect to the metal shell 3 (base end of the ground electrode). It is configured to be closer. Therefore, the force (moment) applied to the fixed portion of the ground electrode 27 due to vibration can be further reduced, and the breakage of the ground electrode 27 at the fixed portion can be more reliably prevented.

  Further, a gradually changing portion 27 </ b> W having a radius of curvature of 0.5 mm or more is provided at a connecting portion between the side surface 29 </ b> S <b> 2 of the thin portion 29 and the distal end surface 28 </ b> F of the base portion 28. Therefore, it is possible to more reliably prevent stress concentration on the connecting part due to vibration, and to effectively suppress the occurrence of cracks in the connecting part. As a result, breakage of the ground electrode 27 can be prevented more reliably.

  In addition, since the length A of the thin portion 29 is 1.0 mm or more, the breakage resistance of the ground electrode 27 can be further reliably improved.

  In addition, in the present embodiment, the ground electrode 27 is bent in the bent portion forming step after the thin portion 29 is formed in the thin portion forming step. Therefore, the spark discharge gap 33 can be formed at a more appropriate position and in a more appropriate size.

  Next, in order to confirm the effects achieved by the above-described embodiment, spark plug samples in which the radius of curvature R (mm) at the bent portion of the ground electrode was variously changed were produced, and an ignitability evaluation test was performed on each sample. . The outline of the ignitability evaluation test is as follows. That is, the sample was mounted on an inline 4-cylinder DOHC engine having a displacement of 2.0 L, and the engine was operated at a rotational speed of 1500 rpm and a suction negative pressure of −60 kPa. Then, the ignition timing was gradually advanced, and the advance angle (limit advance angle) when the variation rate of the average effective pressure reached 20% was determined. FIG. 7 shows the test results of the test. In addition, it can be said that it is excellent in ignitability, so that a limit advance angle is large. In each sample, the thread diameter of the thread portion was M10.

  As shown in FIG. 7, it was confirmed that the sample in which the radius of curvature R is 3.0 mm or less has a critical advance angle exceeding 45 ° CA and has excellent ignitability. This is considered to be because the space formed between the center electrode and the insulator and the ground electrode is increased by reducing the radius of curvature R, and the growth inhibition of the flame kernel by the ground electrode is less likely to occur. It is done.

  From the test results of the above tests, it can be said that the ground electrode is bent and the radius of curvature R at the bent portion is preferably 3.0 mm or less in order to improve the ignitability.

  Next, a thin plug portion is provided at the tip of the ground electrode, the length A of the thin portion is 1.0 mm or 3.0 mm, and a sample of a spark plug in which the radius of curvature R is variously changed is prepared. A vibration resistance improvement rate evaluation test was conducted. The outline of the vibration resistance improvement rate evaluation test is as follows. That is, a spark plug sample (reference sample) having a constant thickness and a variety of curvature radii R without being provided with a thin portion on the ground electrode is attached to a DOHC engine for a motorcycle with a displacement of 600 cc. By operating the engine at a rotational speed of 15000 rpm, intense vibration was applied to the ground electrode, and the time until the ground electrode broke was measured as the breakage reference time. Next, the sample provided with the thin portion was subjected to vibration by the same method as described above, and the time until the ground electrode was broken (breakage time) was measured. Then, based on the breakage reference time in the reference sample having the same radius of curvature R, the ratio of breakage time (vibration resistance improvement rate) in each sample was calculated with the breakage reference time as 100. FIG. 8 shows the test results of the test. In FIG. 8, the test results of the sample with the length A of 1.0 mm are indicated by circles, and the test results of the sample with the length A of 3.0 mm are indicated by triangles. Moreover, in the sample provided with the thin portion, the thin portion was provided on the tip side of the ground electrode with respect to the reference plane.

  As shown in FIG. 8, the improvement in breakage resistance can be expected by providing a thin portion. However, particularly when the curvature radius R of the bent portion is set to 3.0 mm or less, the effect of improving the breakage resistance appears remarkably. I understood that. This is because, as the radius of curvature R is reduced, stress associated with vibration is concentrated on the bent portion, and the ground electrode is easily broken. However, a thin portion is provided on the ground electrode, and the tip of the ground electrode is By reducing the weight, the stress applied to the bent portion can be reduced. As a result, even when the radius of curvature R is 3.0 mm or less and stress is intensively applied to the bent portion, It is thought that this is because it was able to sufficiently resist stress.

  As a result of the above test, in the spark plug in which the curvature radius R of the bent portion is 3.0 mm or less and the ignitability can be improved while the ground electrode is particularly worried, the tip of the ground electrode It can be said that it is particularly effective to provide a thin-walled portion in terms of preventing breakage of the ground electrode.

  Next, 200 spark plug samples having a ground electrode with a constant thickness were prepared without providing a thin portion on the ground electrode, and a desktop vibration test was performed on each sample. The outline of the desktop vibration test is as follows. That is, after attaching the sample to a predetermined vibration testing machine, the sample was vibrated under predetermined conditions to break the ground electrode. 9, between a virtual plane VS extending from the center CP of the radius of curvature R toward the ground electrode and orthogonal to the axis, and a plane CS extending from the center of the radius of curvature R and orthogonal to the virtual plane VS. Was divided into ranges of 5 ° with the center of the radius of curvature R as the center, and it was confirmed in which angle range the breakage position in the ground electrode occurred. Here, when the ground electrode was broken in the range from n ° to n + 5 °, the ground electrode was assumed to be broken at n + 5 °, and the number of breakage of the ground electrode at each angle was obtained. FIG. 10 shows the test results of the test.

  As shown in FIG. 10, it was found that the ground electrode was easily broken in the range of 10 ° to 75 °.

  From the test results of the above test, it can be said that it is preferable to provide the thin part that is relatively thin and easily breaks out of the above-mentioned range on the tip side of the ground electrode. That is, in order to more reliably prevent breakage in the ground electrode, the acute angle of the angle formed with the plane (virtual plane VS) orthogonal to the axis is 75 °, and the reference plane passes through the center of the radius of curvature. It can be said that the thin-walled portion is preferably located closer to the tip side of the ground electrode.

Next, spark plug samples in which the length A (mm) of the thin-walled portion and the average cross-sectional area dGH (mm ² ) were set to different values were prepared, and a heat drawing performance evaluation test was performed on each sample. The outline of the heat drawing performance evaluation test is as follows. That is, the sample was mounted on a 4-cylinder DOHC engine with a displacement of 2.0 L, and the temperature at the tip of the thin wall portion was measured with a thermocouple when the engine was operated in a fully open state (rotation speed: 6000 rpm). Here, the sample with a temperature at the tip of the thin wall portion of 900 ° C. or lower is evaluated as “◎” because it is extremely excellent in heat pulling and can effectively suppress oxidation and pre-ignition of the thin wall portion. A sample having a temperature at the tip of the thin portion of more than 900 ° C. and not more than 1000 ° C. was evaluated as “◯” because it was excellent in heat pulling and could sufficiently suppress oxidation of the thin portion. On the other hand, a sample having a temperature at the tip of the thin portion exceeding 1000 ° C. was evaluated as “x” because there was a concern about oxidation of the thin portion. Table 1 shows the test results of the test. In addition, the sample shown as "-" in the evaluation of Table 1 indicates that the above test was not performed because the temperature at the tip of the thin portion was estimated to be over 1000 ° C. In each sample, the thin portion was configured such that the width and thickness along the central axis direction were constant. Further, Table 2 shows, for reference, the width and thickness of the thin portion when each average cross-sectional area dGH shown in Table 1 is used.

As shown in Table 1, it was revealed that the sample satisfying A / dGH ≦ 3.03 (mm ⁻¹ ) can suppress overheating of the thin-walled portion and is excellent in heat pulling. This is because the average cross-sectional area dGH corresponding to the heat conduction capability of the thin portion is sufficiently large with respect to the length A corresponding to the amount of heat received by the thin portion, so that the residual heat per unit length of the thin portion (thin portion) This is considered to be due to the fact that the amount of heat received from the heat-extracted portion of the heat-excluded heat amount conducted to the base through the thin-walled portion can be made sufficiently small.

In particular, it was found that by setting A / dGH ≦ 0.98 (mm ⁻¹ ), overheating of the thin portion can be more effectively suppressed.

From the test results of the above test, in order to suppress overheating of the thin portion, the length along the central axis direction of the thin portion is A (mm), and the average cross-sectional area of the thin portion in the cross section orthogonal to the central axis is When dGH (mm ² ) is satisfied, it is preferable to satisfy A / dGH ≦ 3.03 (mm ⁻¹ ), and it is more preferable to satisfy A / dGH ≦ 0.98 (mm ⁻¹ ).

  Next, a sample of a spark plug having a curved surface portion in which the curvature radius X (mm) is variously changed between the front end surface and each side surface of the thin portion and between adjacent side surfaces is prepared. A heat resistance evaluation test was conducted. The outline of the heat resistance evaluation test is as follows. That is, a spark plug sample (corresponding to a comparative example) having an angular shape between the front end surface and each side surface of the thin-walled portion and between each adjacent side surface is directly injected into an in-line four-cylinder engine with a displacement of 1600 cc. The engine was operated while being mounted on a turbo engine and at a rotational speed of 2500 rpm and an ignition timing of 30 ° CA. Then, by gradually increasing the supercharging pressure, the temperature in the combustion chamber and thus the temperature of the ground electrode was raised, and the indicated mean effective pressure (reference IMEP) when preignition occurred was measured. Next, the sample (corresponding to the example) provided with the curved surface portion was attached to the same engine, and the engine was operated under the same operating condition as above, and the supercharging pressure was gradually increased, and preignition occurred. The indicated mean effective pressure (IMEP) was measured. Then, the IMEP ratio (IMEP improvement rate) of the sample corresponding to each example with respect to the reference IMEP was calculated with the reference IMEP being 100. IMEP corresponds to the temperature in the combustion chamber when pre-ignition occurs, and it can be said that the larger the IMEP improvement rate, the more normally the engine can be operated without causing pre-ignition in a higher temperature range. In FIG. 11, the graph showing the relationship between the curvature radius X (mm) of a curved surface part and IMEP improvement rate is shown.

  As shown in FIG. 11, each sample provided with the curved surface portion has an IMEP improvement rate of more than 100, and it has been found that the engine can be normally operated in a higher temperature range. This is thought to be due to the fact that the provision of the curved surface portion could prevent a situation in which the temperature between the front end surface and each side surface of the thin-walled portion and between adjacent side surfaces became locally high. It is done.

  From the test results of the above tests, in order to prevent the occurrence of pre-ignition using the ground electrode (thin wall portion) as a heat source, between the front end surface and each side surface of the thin wall portion and between adjacent side surfaces, respectively. It can be said that it is preferable to form a curved surface convex outward.

  Next, a spark having a gradually changing portion with a curved surface in which the radius of curvature Y (mm) is variously changed is provided at the connecting portion between the surface located on the opposite side of the center electrode of the side surface of the thin portion and the tip surface of the base portion Plug samples were prepared and a crack evaluation test was performed on each sample. The outline of the crack evaluation test is as follows. That is, the sample was attached to an in-line four-cylinder DOHC engine with a displacement of 600 cc, and the engine was operated for 100 hours in a fully opened state (rotation speed: 14000 rpm). Then, the presence or absence of the crack in the said connection part was confirmed, the sample in which the crack was confirmed was evaluated as "*", and the sample in which the crack was not confirmed was evaluated as "(circle)". Table 3 shows the test results of the test. In Table 3, the curvature radius Y being 0.0 mm means that the surface located on the opposite side of the central electrode and the tip surface of the base portion are orthogonal to each other in the side surface of the thin portion. .

  As shown in Table 3, it was revealed that the sample in which the gradually changing portion having the curvature radius Y of 0.5 mm or more is provided in the connecting portion can effectively suppress the generation of cracks. This is presumably because the concentration of stress on the connecting portion could be prevented more reliably by providing the gradually changing portion having a relatively large radius of curvature.

  From the test results of the above test, in order to prevent the ground electrode from being broken more reliably, the connecting radius between the surface located on the opposite side of the central electrode and the distal end surface of the base portion of the side surface of the thin wall portion has a curvature radius. It can be said that it is preferable to use a concave curved surface of 0.5 mm or more.

  In terms of preventing stress concentration on the connecting portion and preventing breakage of the ground electrode, it is considered that even if the connecting portion is tapered, the same effect as that of the connecting portion having a curved surface is obtained. It is done.

  Next, the above-mentioned vibration resistance improvement rate evaluation test is performed on a spark plug sample in which the radius of curvature R of the bent portion is set to 1.5 mm or 3.0 mm and the length A (mm) of the thin-walled portion is variously changed. The vibration resistance improvement rate was calculated. FIG. 12 shows the test results of the test. In FIG. 12, the test results of the sample with the curvature radius R of 1.5 mm are indicated by circles, and the test results of the sample with the curvature radius R of 3.0 mm are indicated by triangles. Moreover, in the sample provided with the thin portion, the thin portion was provided on the tip side of the ground electrode with respect to the reference plane.

  As shown in FIG. 12, it was confirmed that each sample can improve breakage resistance. However, particularly when the length A of the thin portion is 1.0 mm or more, the effect of improving breakage resistance is remarkably exhibited. I found out. This is considered to be due to the fact that the tip of the ground electrode is sufficiently lightened.

  From the test results of the above test, it can be said that the length A of the thin portion is preferably 1.0 mm or more in order to further improve the breakage resistance of the ground electrode.

  In addition, as shown in FIG. 12, the breakage resistance improvement rate in the sample in which the length A of the thin portion is 5.5 mm is similar to the sample in which the length A of the thin portion is 5.0 mm. It has been found that even when the thickness A is further increased within the range of more than 5.0 mm, the breakage resistance is not improved so much. Therefore, considering this point and the like, it can be said that the length A is preferably 5.0 mm or less.

  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 gradually changing portion 27W is provided at the connecting portion between the side surface 29S2 of the thin-walled portion 29 and the distal end surface 28F of the base portion 28. However, as shown in FIG. It is good also as providing the taper part 27T which inclines with respect to the said side surface 29S2 in the connection part with the surface 28F. As shown in FIG. 14, the connecting portion between the side surface 29S2 and the front end surface 28F may be formed in a tapered shape by forming the front end surface 28F in a tapered shape. In these cases, the effect of preventing breakage of the ground electrode 27 can be further enhanced as in the case of providing the gradual change portion 27W.

  (B) In the above embodiment, the width of at least the tip side of the thin portion 29 is substantially constant, but the width of the thin portion 29 may be changed along the longitudinal direction of the ground electrode 27. Therefore, for example, as shown in FIG. 15, the width of the thin portion 41 may be gradually reduced toward the tip of the ground electrode 27.

  (C) In the above-described embodiment, the thickness of at least the tip side of the thin portion 29 is substantially constant, but the thickness of the thin portion 29 may be changed along the longitudinal direction of the ground electrode 27. . Therefore, for example, as shown in FIGS. 16 and 17, the thickness of the thin portions 42 and 43 may be configured to gradually decrease toward the tip of the ground electrode 27.

  (D) In the above embodiment, the thin portion 29 is formed on the surface side of the ground electrode 27 located on the center electrode 5 side, but the formation position of the thin portion 29 is not limited to this. Therefore, for example, the thin portion may be formed on the surface of the ground electrode 27 opposite to the center electrode 5.

  (E) In the above embodiment, the case where the ground electrode 27 is joined to the distal end portion 26 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). The present invention can also be applied to the case where the ground electrode is formed so as to cut out a part of (see Japanese Patent Laid-Open No. 2006-236906, etc.).

  (F) In the above embodiment, the tool engaging portion 19 has a hexagonal cross section, but the shape of the tool engaging 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)
DESCRIPTION OF SYMBOLS 3 ... Metal fitting 4 ... Shaft hole 5 ... Center electrode 27 ... Ground electrode 27B ... Bending part 27W ... Gradual change part 28 ... Base part 28F ... Tip surface 29 (thin part) 29 ... Thin part 29F ... (Thin part) tip surface 29S1 -29S4 ... (thin part) side surface 29W1-29W8 ... curved surface part BS ... reference plane CL1 ... axis

Claims (4)

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;
A spark plug that is fixed to the tip of the metal shell and is bent toward the center electrode at a bent portion and includes a ground electrode that forms a gap with the center electrode,
When the radius of curvature of the inner surface located on the central electrode side in the bent portion is R (mm), R ≦ 3.0 is satisfied,
The ground electrode is
A base fixed to the metal shell and including at least a part of the bent portion;
Extending from the tip of the base along the extending direction of the tip of the base, and having a thin portion formed thinner than the base,
The acute angle among the angles formed by the plane orthogonal to the axis is 75 °, and the thin portion is located on the tip side of the ground electrode with respect to the reference plane passing through the center of the radius of curvature of the inner surface,
When the length along the central axis direction of the thin portion is A (mm) and the average cross-sectional area of the thin portion in a cross section orthogonal to the central axis is dGH (mm ² ), A / dGH ≦ 3 .03 (mm ^-1 )
A spark having a convex curved surface outwardly between the front end surface of the thin portion and each side surface of the thin portion adjacent thereto and between the adjacent side surfaces. plug. The thin-walled portion is formed on the surface side of the ground electrode that is located on the center electrode side, and the surface of the thin-walled portion that is located on the opposite side of the center electrode is connected to the distal end surface of the base portion. The spark plug according to claim 1, wherein the portion has a tapered shape or a concave curved surface shape with a radius of curvature of 0.5 mm or more.   The spark plug according to claim 1, wherein 1.0 ≦ A ≦ 5.0 is satisfied. A method for manufacturing a spark plug according to any one of claims 1 to 3,
A thin-walled portion forming step for forming the thin-walled portion at the tip of the ground electrode;
Bending the ground electrode to the center electrode side and forming the bent portion, and a bending portion forming step,
The method for manufacturing a spark plug, wherein the bent portion forming step is performed after the thin portion forming step.

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