JP7582018B2 - Spark plug - Google Patents

Description

This disclosure relates to spark plugs.

A conventional spark plug is described in the following Patent Document 1. This spark plug includes a center electrode inserted into a cylindrical insulator, a housing provided on the outer periphery of the insulator, and a ground electrode fixed to the housing. The base end of the ground electrode is fixed to the housing, and is bent toward the center electrode at a bent portion, and is disposed so that a gap is formed between the base end and the center electrode. In the bent portion of the ground electrode, the strength of the portion located on the center electrode side of the center of gravity of the cross section of the ground electrode in a cross section perpendicular to the center axis of the ground electrode is greater than the strength of the portion located on the opposite side to the center electrode. This suppresses the phenomenon in which the tip of the ground electrode rises in a direction away from the center electrode, in other words, the phenomenon in which the spark gap formed between the center electrode and the ground electrode expands, even in an environment in which the spark plug is subjected to vibrations associated with the operation of the vehicle engine, etc.

JP 2013-114754 A

When manufacturing a spark plug, a joining process is performed in which the base end of the ground electrode is joined to the end face of the housing, followed by a bending process in which the ground electrode is bent inward so that its tip faces the center electrode, and then a gap adjustment process in which the tip of the ground electrode is struck from the outside to adjust the size of the spark gap. As the ground electrode is shaped through these processes, a compressive force is applied to the inner surface of the base end of the ground electrode. This is the cause of internal stress remaining in the inner surface of the base end of the ground electrode.

On the other hand, in recent engines that have improved thermal efficiency, the combustion temperature is higher than before. In such engines, the base end of the ground electrode is continuously exposed to high-temperature combustion heat, which may cause the base end of the ground electrode to deform over time due to residual stress, a phenomenon known as creep. The creep phenomenon that occurs at the base end of the ground electrode causes the tip end of the ground electrode to approach the center electrode, in other words, the spark gap to shrink. If the spark gap shrinks, when the mixture is ignited by the spark discharge formed between the center electrode and the ground electrode, the flame that is generated is more likely to come into contact with the ground electrode, etc., and there is a concern that the heat of the flame will be taken away by the ground electrode, etc., and ignition performance will deteriorate.

This disclosure was made in light of these circumstances, and its purpose is to provide a spark plug that is capable of suppressing deformation of the ground electrode caused by creep.

A spark plug (10) that solves the above problem includes an insulator (12) formed in a cylindrical shape around a predetermined central axis, a center electrode (13) inserted into the insulator, a cylindrical housing (11) provided on the outer periphery of the insulator, and a ground electrode (14) joined to the tip face of the housing. The ground electrode has a root portion (51) joined to the tip face of the housing and formed to extend from the tip face of the housing in a direction along the central axis, and a facing portion (53) formed to extend from the tip (511) of the root portion to a position facing the center electrode. When the end of the root portion joined to the tip surface of the housing is defined as the base end, the outer wall portion of the root portion located in the circumferential direction of the central axis is defined as the side wall portion, and the width of the root portion in the radial direction of the central axis is defined as the plate thickness, the side wall portion of the root portion is formed in a trapezoidal shape such that when viewed in the circumferential direction of the central axis, the portion on the back surface (512) side of the root portion located on the outer side in the radial direction of the central axis extends upright from the tip surface of the housing, and the plate thickness becomes thinner from the base end (510) of the root portion toward the tip.

According to this configuration, when the ground electrode is bent toward the inner side through the bending process and the gap adjustment process, the inner surface of the base portion functions like a tension rod, making the inner surface of the base portion less likely to deform in the compression direction. As a result, deformation that generates stress is less likely to occur in the inner surface of the base portion, making it possible to suppress deformation of the ground electrode due to creep.

Note that the symbols in parentheses in the above means and claims are examples showing the correspondence with the specific means described in the embodiments described below.

The spark plug disclosed herein can suppress deformation of the ground electrode caused by creep.

FIG. 1 is a cross-sectional view showing a broken cross-sectional structure of a spark plug according to an embodiment. FIG. 2 is a perspective view showing a perspective structure of the ground electrode according to the embodiment. FIG. 3 is a side view showing a right side structure of a base portion of an electrode base material of a ground electrode according to the embodiment. FIG. 4 is a side view showing a left side structure of a base portion of an electrode base material of a ground electrode according to an embodiment. FIG. 5 is a cross-sectional view showing a cross-sectional structure of a base portion of an electrode base material of a ground electrode according to an embodiment. FIG. 6 is a cross-sectional view showing a cross-sectional structure of an opposing portion of an electrode base material of a ground electrode according to an embodiment. FIG. 7 is a perspective view showing a structure of a primary molded product of an electrode base material of the ground electrode according to the embodiment. FIG. 8 is a perspective view showing a secondary formed product of an electrode base material of the ground electrode according to the embodiment. 9A and 9B are diagrams showing a part of the manufacturing process of the spark plug of the embodiment. 10A and 10B are diagrams showing a part of the manufacturing process of the spark plug of the embodiment. FIG. 11 is a cross-sectional view showing a cross-sectional structure of a joint portion between a housing and a ground electrode of the reference example. FIG. 12 is an enlarged view showing the structure around the ground electrode of the spark plug of the reference example. FIG. 13 is a cross-sectional view showing a cross-sectional structure of a base portion of an electrode base material of a ground electrode according to another embodiment. FIG. 14 is a cross-sectional view showing a cross-sectional structure of a base portion of an electrode base material of a ground electrode according to another embodiment. FIG. 15 is a cross-sectional view showing a cross-sectional structure of a base portion of an electrode base material of a ground electrode according to another embodiment. FIG. 16 is a perspective view showing a perspective structure of a ground electrode according to another embodiment. FIG. 17 is a cross-sectional view showing a cross-sectional structure of a base portion of an electrode base material of a ground electrode according to another embodiment. FIG. 18 is a cross-sectional view showing a cross-sectional structure of a base portion of an electrode base material of a ground electrode according to another embodiment. FIG. 19 is a cross-sectional view showing a cross-sectional structure of a base portion of an electrode base material of a ground electrode according to another embodiment.

Hereinafter, an embodiment of a spark plug will be described with reference to the drawings. In order to facilitate understanding of the description, the same components in the various drawings are denoted by the same reference numerals as much as possible, and duplicated description will be omitted.
First, a schematic configuration of a spark plug 10 according to the present embodiment shown in Fig. 1 will be described. The spark plug 10 is mounted, for example, on an engine head. The spark plug 10 ignites an air-fuel mixture in a cylinder of the engine by forming a spark discharge based on the application of a voltage. The spark plug 10 includes a housing 11, an insulator 12, a center electrode 13, and a ground electrode 14.

The housing 11 is formed in a cylindrical shape centered on the central axis m10 of the spark plug 10. The housing 11 is formed of a metal material such as carbon steel. The lower end of the insulator 12 is inserted coaxially inside the housing 11. In the following, the direction along the central axis m10 is referred to as the "plug axial direction Da."

The insulator 12 is formed in a cylindrical shape centered on the central axis m10. The insulator 12 is made of an insulating material such as alumina. In this embodiment, the insulator 12 corresponds to the insulator. The housing 11 is integrally assembled to the outer periphery of the insulator 12. An axial hole 120 is formed inside the insulator 12. The axial hole 120 is formed so as to penetrate the insulator 12 from the tip to the base along the central axis m10. The center electrode 13, the first seal body 15, the resistor 16, the second seal body 17, and the terminal fitting 18 are inserted into the axial hole 120 in this order from the tip side.

The center electrode 13 has an electrode base material 30 and an electrode tip 40. The electrode base material 30 is formed in a cylindrical shape centered on the central axis m10. The electrode base material 30 is formed of a nickel (Ni) alloy or the like, which has excellent heat resistance. The electrode tip 40 is joined to the tip of the electrode base material 30. The electrode tip 40 is formed in a cylindrical shape centered on the central axis m10. The electrode tip 40 is formed of an iridium alloy or the like. A first seal body 15, a resistor 16, and a second seal body 17 are sandwiched between the base end of the center electrode 13 and the tip of the terminal metal fitting 18.

The terminal fitting 18 is formed in a generally cylindrical shape centered on a central axis m10. The terminal fitting 18 is made of steel or the like. A terminal portion 180 is formed at the base end of the terminal fitting 18. The terminal portion 180 of the terminal fitting 18 is exposed to the outside from the base end of the insulator 12.
The ground electrode 14 has an electrode base material 50 and an electrode tip 60. The electrode base material 50 is made of a nickel alloy or the like. The electrode base material 50 is fixed to the front end surface of the housing 11 and is formed so as to extend from the front end surface to a position facing the electrode tip 40 of the center electrode 13. The electrode tip 60 is joined to the front end of the electrode base material 50. The electrode tip 60 is made of a precious metal alloy such as an iridium alloy or a platinum alloy. The electrode tip 60 is disposed so as to face the electrode tip 40 of the center electrode 13 with a predetermined gap 19 therebetween. Hereinafter, the gap 19 formed between the electrode tip 40 of the center electrode 13 and the electrode tip 60 of the ground electrode 14 will be referred to as a "spark gap 19".

In this spark plug 10, an external circuit capable of applying a high voltage is connected to the terminal portion 180 of the terminal fitting 18. When a high voltage is applied to the terminal portion 180 of the terminal fitting 18 by the external circuit, a spark discharge is formed between the electrode tip 40 of the center electrode 13 and the electrode tip 60 of the ground electrode 14. This spark discharge ignites the air-fuel mixture in the engine cylinder, forming a flame, and the mixture is burned.

Next, the structure of the joint between the housing 11 and the ground electrode 14 will be described in detail.
2, the electrode base material 50 of the ground electrode 14 has a root portion 51, a bent portion 52, and an opposing portion 53. In addition, in the following figures including FIG. 2, the electrode tip 60 is omitted as necessary.

The root portion 51 is a portion joined to the tip surface 110 of the housing 11. The root portion 51 is formed so as to extend from the tip surface 110 of the housing 11 in the plug axial direction Da. The bent portion 52 is a portion formed so as to bend from the tip 511 of the root portion 51 toward the center electrode 13. Note that in FIG. 2, the outer edge of the tip 511 of the root portion 51 is shown by a two-dot chain line. The facing portion 53 is a portion formed so as to extend from the bent portion 52 to a position facing the center electrode 13. In the spark plug 10 of this embodiment, the hardness of the electrode base material 50 of the ground electrode 14 is lower than the hardness of the housing 11.

In the following, the direction perpendicular to the plug axial direction Da, in other words the radial direction of the central axis m10 shown in FIG. 1, is referred to as the "plug radial direction Dr", and the circumferential direction of the central axis m10 is referred to as the "plug circumferential direction Dc". The end 510 of the root portion 51 joined to the tip surface 110 of the housing 11 is referred to as the "base end 510". Furthermore, one outer wall portion of the root portion 51 located in the plug circumferential direction Dc is referred to as the "right side wall portion 514", and the other outer wall portion of the root portion 51 located in the plug circumferential direction Dc is referred to as the "left side wall portion 515".

3, the right side wall 514 of the root portion 51 is formed so that the portion of the root portion 51 on the back surface 512 side extends upright from the tip surface 110 of the housing 11 when viewed from the plug circumferential direction Dc. When the width of the root portion 51 in the plug radial direction Dr is taken as the plate thickness, the plate thickness of the base end 510 of the root portion 51 is set to "W1", and the plate thickness of the tip 511 of the root portion 51 is set to "W3" which is shorter than "W1". In other words, the right side wall 514 of the root portion 51 is formed so that the plate thickness becomes thinner from the base end 510 toward the tip 511. As a result, the right side wall 514 of the root portion 51 is formed in a trapezoidal shape when viewed from the plug circumferential direction Dc.

As shown in FIG. 4, the left side wall portion 515 of the base portion 51 is also formed in a trapezoidal shape when viewed in the plug circumferential direction Dc.
The cross section of the root portion 51 perpendicular to the plug axial direction Da has a shape as shown in FIG. 5. That is, in the cross section of the root portion 51 perpendicular to the plug axial direction Da, the plate thickness of each of the right side wall portion 514 and the left side wall portion 515 is set to "W1", and the plate thickness of the center portion 516 located between the right side wall portion 514 and the left side wall portion 515 in the plug circumferential direction Dc is set to "W2" smaller than "W1". As a result, the inner surface 513 of the root portion 51 located on the inner side in the plug radial direction Dr is formed in a concave shape such that the plate thickness W2 of the center portion 516 is thinner than the plate thickness W1 of the both side wall portions 514, 515 of the root portion 51. The back surface 512 of the root portion 51 is curved so as to bulge outward in the plug radial direction Dr. In addition, in the cross section of the root portion 51 perpendicular to the plug axial direction Da, the width L1 of the inner surface 513 is longer than the width L2 of the back surface 512. With the above-described structure, the cross section of the base portion 51 perpendicular to the plug axial direction Da spreads out like a fan, and the inner surface 513 has a bent shape like an L. In Fig. 5, the outer edge of the bent portion 52 continuing to the tip 511 of the base portion 51 is indicated by a two-dot chain line.

As shown in FIG. 6, the cross section of the facing portion 53 perpendicular to the direction in which the electrode base material 50 of the ground electrode 14 extends is rectangular with R-shaped corners. Although not shown, the cross section of the bent portion 52 perpendicular to the direction in which the electrode base material 50 of the ground electrode 14 extends is also rectangular with R-shaped corners.

Next, a method of manufacturing the spark plug 10 of this embodiment, particularly the method of manufacturing the ground electrode 14, will be described.
When manufacturing the ground electrode 14, first, a primary molded product 70a of the electrode base material 50 of the ground electrode 14 is manufactured by extrusion molding the material. The primary molded product 70a is formed so as to extend about a predetermined axis m20. The cross section of the primary molded product 70a perpendicular to the predetermined axis m20 is formed into a rectangular shape having R-shaped corners.

Following the manufacture of the primary molded product 70a, the root portion 71 is embossed to form a recess on the inner surface 713 of the root portion 71 of the primary molded product 70a. This produces a secondary molded product 70b of the electrode base material 50 as shown in FIG. 8. The root portion 71 of the secondary molded product 70b has a shape similar to the root portion 51 of the electrode base material 50 shown in FIGS. 3 and 4. After the secondary molded product 70b as shown in FIG. 8 is manufactured, the electrode tip 60 is joined to the end opposite the root portion 51, thereby completing the manufacture of the ground electrode 14.

After completing the manufacturing of the ground electrode 14 in the above manner, a step of joining the ground electrode 14 to the housing 11 and a step of deforming the ground electrode 14 so as to face the center electrode 13 are performed.
Specifically, first, as shown in Fig. 9(A), a root portion 51 of an electrode base material 50 of the ground electrode 14 is joined to a tip surface 110 of the housing 11 by resistance welding, laser welding, or the like. After such a joining process is performed, a bending process is performed in which an outer surface of the tip portion of the electrode base material 50 is pressed by a bending roller 81 while an inner surface of the electrode base material 50 of the ground electrode 14 is pressed by a cylindrical support stand 80. By this bending process, the tip portion of the electrode base material 50 is bent toward the center electrode 13 as shown in Fig. 9(B). As a result, a bent portion 52 and an opposing portion 53 are further formed in the electrode base material 50 as shown in Fig. 10(A).

Following the bending process, a gap adjustment process is performed. In the gap adjustment process, as shown in FIG. 10(B), the size of the spark gap 19 formed between the electrode tip 60 of the ground electrode 14 and the electrode tip 40 of the center electrode 13 is adjusted by tapping the outer surface of the facing portion 53 of the electrode base material 50 with an adjustment jig 83.

Next, the operation and effects of the spark plug 10 of this embodiment will be described.
Regarding the electrode base material 50 of the ground electrode 14, for example, a method of simply forming the base portion 51 into a rectangular parallelepiped shape is conceivable. However, when the electrode base material 50 has such a shape, internal stress is likely to remain particularly in the portion of the base portion 51 of the electrode base material 50 on the inner surface 513 side.

Specifically, when the ground electrode 14 is formed through the manufacturing process shown in Figures 9(B), 10(A), and (B), an external force is applied to the opposing portion 53 of the ground electrode 14 by the bending roller 81 and the adjustment jig 83, and a force is applied to the root portion 51 of the electrode base material 50 of the ground electrode 14. In detail, as shown in Figure 11, a force F1 in the extension direction, in other words, a force in the direction of peeling from the tip surface 110 of the housing 11, is applied to the part on the back surface 512 side of the root portion 51 of the electrode base material 50. When the part on the back surface 512 side of the root portion 51 of the electrode base material 50 is deformed so as to extend due to this extension direction force F1, a force F2 in the compression direction, in other words, a force in the direction toward the tip surface 110 of the housing 11, is generated in the part on the inner surface 513 side of the root portion 51 of the electrode base material 50. This compressive force F2 causes residual stress in the part of the electrode base material 50 on the inner surface 513 side of the base portion 51. If creep occurs in the electrode base material 50 due to this residual stress, the ground electrode 14 may deform so as to tilt in the direction approaching the center electrode 13, as shown in FIG. 12, which may narrow the spark gap 19.

In this regard, in the spark plug 10 of this embodiment, as shown in Figures 3 and 4, the side wall portions 514, 515 of the root portion 51 are formed in a trapezoidal shape when viewed from the plug circumferential direction Dc. With this configuration, when the tip portion of the ground electrode 14 is bent toward the inner surface 513 through the bending process and the gap adjustment process, the inner surface 513 of the root portion 51 functions like a tension rod, making the inner surface 513 of the root portion 51 less likely to deform in the compression direction. As a result, deformation that generates stress is less likely to occur in the inner surface 513 of the root portion 51, and deformation of the ground electrode 14 due to creep can be suppressed.

As shown in FIG. 8, when the secondary molded product 70b of the electrode base material 50 is manufactured, the inner surface 713 of the root portion 71 of the secondary molded product 70b is pressed into a concave shape, so that the hardness of the inner surface 713 of the root portion 71 is increased by work hardening. That is, the hardness of the inner surface 713 of the root portion 51 of the ground electrode 14 shown in FIG. 5 can be increased. With this configuration, when the tip portion of the ground electrode 14 is bent toward the inner surface 513 through the bending process and the gap adjustment process, deformation that generates stress is less likely to occur in the inner surface 513 of the root portion 51. Therefore, deformation of the ground electrode 14 caused by creep can be more accurately suppressed.

On the other hand, in the spark plug 10 of this embodiment, as shown in FIG. 5, the inner surface 513 of the root portion 51 is formed in a concave shape. With this configuration, the section modulus of the root portion 51 against deformation toward the inner surface 513 can be increased, that is, the rigidity of the root portion 51 can be increased. As a result, deformation that generates residual stress is less likely to occur in the inner surface 513 of the root portion 51, and deformation of the ground electrode 14 due to creep can be more accurately suppressed.

In addition, in the spark plug 10 of this embodiment, the width L1 of the inner surface 513 of the root portion 51 is longer than the width L2 of the back surface 512 of the root portion 51. With this configuration, compared to when the width L1 of the inner surface 513 of the root portion 51 and the width L2 of the back surface 512 of the root portion 51 are the same, the length of the portion of the inner surface 513 of the root portion 51 to which force is applied when bending the ground electrode 14 toward the inner surface 513 in the bending process and gap adjustment process can be increased by the amount of the longer width L1 of the inner surface 513. As a result, it is possible to reduce the surface pressure acting on the inner surface 513 of the root portion 51, and therefore the residual stress on the inner surface 513 of the root portion 51 can be reduced.

The above embodiment can also be implemented in the following manner.
The shape of the root portion 51 of the ground electrode 14 can be changed as appropriate. For example, the inner surface 513 of the root portion 51 is not limited to a V-shaped recess, and may have a U-shaped recess as shown in Fig. 13. As shown in Fig. 14, the inner surface 513 of the root portion 51 may have a W-shaped recess. Alternatively, as shown in Fig. 15, the inner surface 513 of the root portion 51 may have a W-shaped recess with recesses in both side walls 514, 515.

- As shown in Figures 16 and 17, the root portion 51 of the ground electrode 14 may have a shape that does not have a "V"-shaped recess on its inner surface 513. In this case, as shown in Figure 18, the width L1 of the inner surface 513 of the root portion 51 and the width L2 of the back surface 512 may be set to the same length.

As shown in FIG. 19 , the inner surface 513 of the base portion 51 of the ground electrode 14 may have a U-shaped recess, and the width L1 of the inner surface 513 and the width L2 of the back surface 512 may be set to the same length.
The present disclosure is not limited to the above specific examples. Any design changes made by a person skilled in the art to the above specific examples are also included in the scope of the present disclosure as long as they have the features of the present disclosure. The elements of each of the above specific examples, as well as their arrangements, conditions, shapes, etc., are not limited to those exemplified and can be changed as appropriate. The elements of each of the above specific examples can be combined as appropriate as long as no technical contradictions arise.

10: Spark plug 11: Housing 12: Insulator (insulator)
13: Center electrode 14: Ground electrode 51: Root portion 53: Opposing portion 510: Base end 511: Tip 512: Back surface 513: Inner surface 514: Right side wall portion 515: Left side wall portion 516: Center portion

Claims (2)

An insulator (12) formed in a cylindrical shape around a predetermined central axis;
A central electrode (13) inserted into the insulator;
A cylindrical housing (11) provided on the outer periphery of the insulator;
a ground electrode (14) joined to a tip surface of the housing;
The ground electrode is
a root portion (51) that is joined to a tip surface of the housing and is formed so as to extend from the tip surface of the housing in a direction along the central axis;
a facing portion (53) formed so as to extend from a tip (511) of the base portion to a position facing the center electrode,
When an end portion of the base portion joined to a tip end surface of the housing is defined as a base end, an outer wall portion of the base portion located in a circumferential direction of the central axis is defined as a side wall portion (514, 515), and a width of the base portion in a radial direction of the central axis is defined as a plate thickness,
When viewed in the circumferential direction of the central axis, the side wall portion of the base portion is formed in a trapezoidal shape such that a portion of the back surface (512) side of the base portion located on the outer side in the radial direction of the central axis extends upright from the tip surface of the housing, and the plate thickness becomes thinner from the base end (510) of the base portion toward the tip,
The inner surface (513) of the base portion located on the inside in the radial direction of the central axis is formed in a concave shape such that the plate thickness of the central portion (516) of the base portion is thinner than the plate thickness of both side wall portions of the base portion located at both ends in the circumferential direction of the central axis.
Spark plug. 2. The spark plug according to claim 1 , wherein a width of an inner surface of the base portion located radially inwardly of the central axis is greater than a width of a back surface of the base portion.

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