JP7713981B2 - Spark plug - Google Patents

Description

The present invention relates to a spark plug.

Patent document 1 discloses a prior art technique in which a center electrode placed in an axial hole of an insulator is fixed to the insulator by a sealing material placed around the head of the center electrode.

JP 2017-199455 A

Further improvements in impact resistance are desired in the prior art.

The present invention was made to meet this demand, and aims to provide a spark plug with improved impact resistance.

To achieve this objective, the first aspect of the present invention comprises an insulator having an axial hole that connects a first hole and a second hole having a smaller diameter than the first hole via a connection part, a central electrode having a head that is disposed in the first hole and contacts the connection part, and a leg that extends from the head toward the tip side and is disposed in the second hole, and a sealant that is filled between the outer periphery of the head and the first hole, and the outer periphery of the head includes an inclined surface whose diameter increases at a constant rate toward the tip side, and a side that is connected to the tip side of the inclined surface and contacts the connection part, and in a cross section including the central axis of the central electrode, the angle between the first hole and the inclined surface is 20° or more and 40° or less.

In the second aspect, the shortest distance between the side of the head and the first hole in the first aspect is 0.4 mm or less.

In the third aspect, in the first or second aspect, the shortest distance between the side of the head and the first hole is 0.05 mm or more.

In the fourth aspect, in any of the first to third aspects, the shortest distance between the rear surface of the head circumference that is rearward of the inclined surface and the first hole is 0.5 mm or more.

The outer periphery of the head of the center electrode includes an inclined surface whose diameter increases at a constant rate toward the tip side, and a side surface that is connected to the tip side of the insulator and contacts the connection part of the insulator, and in a cross section including the central axis of the center electrode, the angle between the first hole of the insulator and the inclined surface is 20° or more and 40° or less. When manufacturing the spark plug, the molding pressure of the sealant filled between the outer periphery of the head and the first hole is easily transmitted along the inclined surface to the sealant between the side surface and the first hole, improving the filling rate and density of the sealant. The mechanical strength of the sealant is improved, improving impact resistance.

1 is a cross-sectional view of a spark plug according to a first embodiment. FIG. FIG. 2 is a cross-sectional view of a portion indicated by III in FIG. FIG. 11 is a cross-sectional view of a spark plug according to a second embodiment.

Below, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a cross-sectional view of a spark plug 10 in a first embodiment. The lower side of the paper in Fig. 1 is the leading end side of the spark plug 10, and the upper side of the paper is the trailing end side of the spark plug 10 (the same applies to Figs. 2 to 4). The spark plug 10 includes an insulator 11 and a center electrode 16.

The insulator 11 is a cylindrical member having an axial hole 12 extending along the axis O, and is made of ceramic such as alumina, which has excellent insulating properties and mechanical properties at high temperatures. The axial hole 12 is connected in the order of the first hole 13, the connection part 14, and the second hole 15 from the rear end to the tip of the insulator 11. In this embodiment, the first hole 13 and the second hole 15 are cylindrical surfaces with the same diameter over their entire length, and the connection part 14 is a conical surface whose diameter decreases toward the tip. The connection part 14 may be an annular plane perpendicular to the center line of the first hole 13 and the second hole 15. The diameter of the second hole 15 is smaller than the diameter of the first hole 13.

A rod-shaped metal center electrode 16 is disposed in the axial hole 12 of the insulator 11. The center electrode 16 has a core material with excellent thermal conductivity embedded in a base material. The base material is made of a Ni-based alloy or a metal material made of Ni. The core material is made of copper or an alloy mainly made of copper. The core material can be omitted. The center electrode 16 has a head 17 disposed in the first hole 13 and in contact with the connection part 14, and a leg 18 extending from the head 17 toward the tip side and disposed in the second hole 15. The head 17 and the leg 18 are molded integrally.

The terminal fitting 19 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is made of a conductive metal material (such as low carbon steel). The tip side of the terminal fitting 19 is disposed in the first hole 13 of the insulator 11, and the rear end side of the terminal fitting 19 protrudes from the insulator 11. In the first hole 13 of the insulator 11, a seal material 20, a resistor 21, and a seal material 22 are disposed between the center electrode 16 and the terminal fitting 19, in that order from the tip side.

The seal material 20 has a function of fixing the head 17 of the center electrode 16 to the insulator 11 and closing the first hole 13, and is disposed around the head 17. The seal material 22 has a function of fixing the terminal fitting 19 to the insulator 11. The seal materials 20 and 22 are conductive and contain, for example, glass particles and metal particles (Cu, Fe, etc.) in a ratio of about 1:1. As the glass particles, for example, B ₂ O ₃ -SiO ₂ -based, BaO-B ₂ O _3- based, SiO ₂ -B ₂ O ₃ -CaO-BaO-based materials can be used. The thermal expansion coefficient of the seal materials 20 and 22 is between the thermal expansion coefficient of the ceramic insulator 11 and the thermal expansion coefficient of the metal center electrode 16.

The resistor 21 is a member for suppressing radio noise generated at the time of sparking. The resistor 21 is a mixture containing glass particles as a main component, ceramic particles other than glass, and a conductive material. As the material of the glass particles, for example, B ₂ O ₃ -SiO ₂ system, BaO-B ₂ O ₃ system, SiO ₂ -B ₂ O ₃ -CaO-BaO system, etc. can be adopted. As the material of the ceramic particles, for example, TiO ₂ , ZrO ₂ , etc. can be adopted. As the conductive material, for example, non-metallic conductive materials such as carbon particles (carbon black, etc.), TiC particles, TiN particles, etc., and metals such as Al, Mg, Ti, Zr, and Zn can be adopted. Since the seal materials 20 and 22 are in contact with the resistor 21, the center electrode 16 and the terminal metal fitting 19 are electrically connected via the seal materials 20 and 22 and the resistor 21.

The metal shell 23 is a substantially cylindrical member made of a conductive metal material (e.g., low carbon steel, etc.). The metal shell 23 is disposed on the outer periphery of the insulator 11. The ground electrode 24 is a rod-shaped metal (e.g., nickel-based alloy) member connected to the metal shell 23. A spark gap is formed between the ground electrode 24 and the leg 18 of the center electrode 16. Multiple ground electrodes 24 may be connected to the metal shell 23.

The spark plug 10 is manufactured, for example, by the following method. First, the center electrode 16 is inserted through the first hole 13 of the insulator 11. The head 17 of the center electrode 16 is placed in the first hole 13 in contact with the connection portion 14 of the insulator 11, and the leg 18 is placed in the second hole 15.

Next, the raw powder of the sealing material 20 is poured in through the first hole 13 to fill the area around the head 17 and the rear end side of the head 17 of the first hole 13, and the raw powder is then pre-compressed using a compression rod (not shown). Next, the raw powder of the resistor 21 is filled on top of the raw powder of the sealing material 20, and the raw powder is pre-compressed using a compression rod. Finally, the raw powder of the sealing material 22 is filled on top of the raw powder of the resistor 21, and the raw powder is pre-compressed using a compression rod.

The terminal metal fitting 19 is inserted from the rear end side of the first hole 13 so that the tip of the terminal metal fitting 19 comes into contact with the raw powder of the sealing material 22, and then the terminal metal fitting 19 is pressed into the raw powder while being heated, for example, to a temperature higher than the softening point of the glass component contained in each raw powder, and the terminal metal fitting 19 applies an axial load to the raw powder. Each raw powder is compressed and sintered, and the sealing materials 20, 22 and resistor 21 are created in the first hole 13. Next, the main metal fitting 23 to which the ground electrode 24 is connected is assembled to the outer periphery of the insulator 11, and the ground electrode 24 is bent to set a spark gap between the ground electrode 24 and the center electrode 16, thereby obtaining the spark plug 10.

The center electrode 16 will be described with reference to Figures 2 and 3. Figure 2 is a perspective view of the center electrode 16. The tip side of the leg 18 is not shown in Figure 2. The head 17 of the center electrode 16 includes an outer periphery 26 that is connected to an end face 25 facing the rear end. The outer periphery 26 of the head 17 includes, in order from the rear end side to the tip end side, a rear face 27, an inclined face 28, and a side face 29.

Figure 3 is a cross-sectional view of the head 17 and its vicinity, enlarging the portion indicated by III in Figure 1. Figure 3 is a cross-sectional view including the central axis O of the center electrode 16, and one side of the center electrode 16 across the central axis O is omitted. The inclined surface 28 is a surface whose diameter increases at a constant rate toward the tip side, and is shown as a straight line in Figure 3. The rear surface 27 is the surface of the outer periphery 26 that is rearward of the inclined surface 28. In this embodiment, the diameter of the rear surface 27 is constant over the entire axial length of the rear surface 27.

The side surface 29 is the surface of the outer periphery 26 that is closer to the tip side than the inclined surface 28, and a portion of the side surface 29 is in contact with the connection portion 14. In this embodiment, the side surface 29 has a shape in which a conical surface is connected to the tip side of a cylindrical surface, and a portion of the conical surface is in contact with the connection portion 14. The corner where the cylindrical surface of the side surface 29 intersects with the conical surface is rounded. The diameter of the portion of the side surface 29 that is in contact with the sealing material 20 is larger than the diameter of the inclined surface 28 (excluding the boundary between the inclined surface 28 and the side surface 29) and the diameter of the rear surface 27.

In a cross section (FIG. 3) including the central axis O of the center electrode 16, the angle θ (acute angle) between the first hole 13 and the inclined surface 28 is 20° or more and 40° or less. If the angle θ is 20° or more and 40° or less, when manufacturing the spark plug 10, the molding pressure of the raw material powder of the sealing material 20 filled between the outer periphery 26 of the head 17 and the first hole 13 is smoothly transmitted along the inclined surface 28 to the raw material powder between the side surface 29 and the first hole 13, thereby improving the filling rate and density of the sealing material 20. Since the mechanical strength of the sealing material 20 is improved, the impact resistance of the spark plug 10 can be improved.

On the other hand, when the angle θ is less than 20°, the gap between the inclined surface 28 and the first hole 13 becomes small, so that when manufacturing the spark plug 10, the molding pressure of the raw material powder of the sealing material 20 filled between the outer periphery 26 of the head 17 and the first hole 13 is not easily transmitted to the raw material powder filled between the side surface 29 and the first hole 13. Also, even if the angle θ exceeds 40°, the molding pressure of the raw material powder of the sealing material 20 filled between the outer periphery 26 of the head 17 and the first hole 13 is not easily transmitted to the raw material powder between the side surface 29 and the first hole 13. As a result, there is a risk that the filling rate and density of the sealing material 20 between the side surface 29 and the first hole 13 will decrease. According to the present embodiment, this problem is solved, the mechanical strength of the sealing material 20 is improved, and the impact resistance of the spark plug 10 can be improved.

The sealing material 20, whose thermal expansion coefficient is between that of the insulator 11 and that of the center electrode 16, functions to buffer the radial dimensional change between the first hole 13 and the head 17 caused by the temperature change when the raw material powder compressed in the first hole 13 is heated to form the sealing material 20 during the manufacture of the spark plug 10. The diameter of the portion of the side surface 29 with which the sealing material 20 contacts is larger than the diameter of the inclined surface 28 (excluding the boundary between the inclined surface 28 and the side surface 29) and the diameter of the rear surface 27, so that the radial dimensional change of the side surface 29 with which the sealing material 20 contacts is the largest among the radial dimensional changes of the head 17. Therefore, the portion of the sealing material 20 that contacts the side surface 29 needs to buffer the largest dimensional change.

For this reason, it is preferable that the shortest distance D1 between the side surface 29 and the first hole 13 is 0.4 mm or less. If the distance D1 exceeds 0.4 mm, the seal material 20 will not be able to buffer the dimensional change between the side surface 29 and the first hole 13, and there is a risk that the inside or interface of the seal material 20 will break. If the inside or interface of the seal material 20 breaks, the holding force of the seal material 20 on the head 17 will decrease, and impact resistance will decrease.

In order for the inclined surface 28 to transmit the molding pressure of the raw material powder of the sealing material 20 filled between the outer periphery 26 of the head 17 and the first hole 13 to the raw material powder between the side surface 29 and the first hole 13, it is preferable that the length L of the inclined surface 28 is 1.5 to 6 times the distance D1.

When manufacturing the spark plug 10, the raw material powder of the sealing material 20 is filled between the side surface 29 of the head 17 and the first hole 13, and then the sealing material 20 is formed by heating, so it is preferable that the distance D1 is 0.05 mm or more. If the distance D1 is less than 0.05 mm, the raw material powder is likely to be insufficiently filled between the side surface 29 and the first hole 13, which reduces the holding force of the sealing material 20 to hold the head 17 and tends to reduce impact resistance.

It is preferable that the shortest distance D2 between the rear surface 27 and the first hole 13 is 0.5 mm or more. If the distance D2 is less than 0.5 mm, the frictional resistance until the raw material powder of the sealing material 20 reaches the gap between the side surface 29 and the first hole 13 during the manufacture of the spark plug 10 becomes large, and it becomes easy for insufficient raw material powder to be filled between the outer periphery 26 of the head 17 and the first hole 13.

The rear surface 27 is a cylindrical surface with the same diameter over the entire axial length, so that when manufacturing the spark plug 10, the raw material powder of the sealing material 20 can easily pass through the rear surface 27 and reach the inclined surface 28. Because the raw material powder can easily reach the area between the side surface 29 and the first hole 13, insufficient filling of the raw material powder between the outer periphery 26 of the head 17 and the first hole 13 is unlikely to occur. As a result, it is easier to ensure the holding force of the sealing material 20 on the head 17.

The second embodiment will be described with reference to FIG. 4. In the first embodiment, the side surface 29 of the center electrode 16 is a cylindrical surface connected to a conical surface at the tip side. In contrast, in the second embodiment, the side surface 36 of the center electrode 30 is a spherical shape. In the spark plug 10 of the second embodiment, instead of the center electrode 16 of the spark plug 10 of the first embodiment, the center electrode 30 is disposed in the insulator 11. Therefore, in the second embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description will be omitted.

Figure 4 is a cross-sectional view of a spark plug 10 in a second embodiment. Figure 4 is a cross-sectional view of the head 31 including the central axis O of the center electrode 30, and one side of the center electrode 30 across the central axis O is not shown.

The head 31 of the center electrode 30 includes an outer periphery 33 that is connected to an end face 32 that faces the rear end. The outer periphery 33 includes, in order from the rear end to the tip end, a rear face 34, an inclined face 35, and a side face 36. The inclined face 35 is a face whose diameter increases at a constant rate as it approaches the tip end, and is shown as a straight line in FIG. 4.

The rear surface 27 is the surface of the outer periphery 33 that is closer to the rear end than the inclined surface 35. The axially middle portion of the rear surface 27 is bulged. The side surface 36 is the surface of the outer periphery 26 that is closer to the tip end than the inclined surface 35, and a portion of the side surface 36 is in contact with the connection portion 14. In this embodiment, the side surface 36 is a spherically curved surface. The diameter of the thickest portion of the side surface 36 is larger than the diameter of the bulged portion of the rear surface 27.

In a cross section (FIG. 4) including the central axis O of the center electrode 30, the angle θ (acute angle) between the first hole 13 and the inclined surface 35 is 20° or more and 40° or less. This allows the molding pressure of the raw material powder of the sealing material 20 filled between the outer periphery 33 of the head 31 and the first hole 13 to be smoothly transmitted along the inclined surface 35 to the raw material powder when manufacturing the spark plug 10, improving the filling rate and density of the sealing material 20.

It is preferable that the shortest distance D1 between the side surface 36 and the first hole 13 is 0.4 mm or less. This is because the sealant 20 buffers the dimensional changes between the side surface 36 and the first hole 13 caused by thermal expansion and contraction when the sealant 20 is made in the first hole 13.

The length L of the inclined surface 35 is preferably 1.5 to 6 times the distance D1. This is to make it easier to transmit the molding pressure of the raw material powder of the sealing material 20 filled between the outer periphery 33 of the head 31 and the first hole 13 along the inclined surface 35 to the raw material powder between the side surface 36 and the first hole 13.

It is preferable that the distance D1 is 0.05 mm or more. This is to reduce the occurrence of insufficient filling of the raw material powder between the side surface 36 and the first hole 13.

It is preferable that the shortest distance D2 between the rear surface 34 and the first hole 13 is 0.5 mm or more. This is to reduce the frictional resistance until the raw material powder of the sealing material 20 reaches the gap between the side surface 36 and the first hole 13, and to reduce the occurrence of insufficient filling of the raw material powder between the outer periphery 33 of the head 31 and the first hole 13.

The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1
For the spark plug 10 of the first embodiment, 25 types of samples were prepared, which differed in the size of the angle θ between the inclined surface 28 of the center electrode 16 and the first hole 13 of the insulator 11, and in the shortest distance D1 between the side surface 29 of the center electrode 16 and the first hole 13. The samples were prepared mainly using 25 types of center electrodes 16 with different diameters (tolerance ±0.1 mm) of the side surface 29 of the head 17 and different angles θ of the inclined surface 28, and insulators 11 with a diameter of the first hole 13 of 3.0 mm (tolerance ±0.05 mm), with 30 samples per type. The length L of the inclined surface 28 was set in the range of 1.5 to 6.0 times the distance D1 so that the length L becomes shorter as the angle θ increases and the length L becomes longer as the angle θ decreases.

An impact resistance test was conducted on the prepared samples in accordance with JIS B8031:2006. After applying impacts to the samples at a rate of 400 times per minute for 10 minutes, the presence or absence of loosening of the center electrode 16 was checked. Samples in which none of the 30 samples had loose center electrodes 16 were rated as A, samples in which some of the 30 samples had loose center electrodes 16 were rated as B, and samples in which all of the 30 samples had loose center electrodes 16 were rated as C. The results are shown in Table 1.

As shown in Table 1, all of the angles θ between the inclined surface 28 and the first hole 13 that were less than 20° and all of the angles θ that were more than 40° were rated C. In contrast, the angles θ that were between 20° and 40° were rated A or B. In particular, the angles θ that were between 20° and 40° and the distance D1 was between 0.05 mm and 0.40 mm were rated A.

According to Example 1, it was revealed that impact resistance can be improved when the angle θ is 20° or more and 40° or less. In particular, it was revealed that impact resistance can be further improved when the distance D1 is 0.05 mm or more and 0.40 mm or less.

Example 2
For the spark plug 10 of the first embodiment, the angle θ between the inclined surface 28 of the center electrode 16 and the first hole 13 of the insulator 11 was fixed at 30°, and 20 types of samples were prepared in which the shortest distance D1 between the side surface 29 of the center electrode 16 and the first hole 13 and the shortest distance D2 between the rear surface 27 of the center electrode 16 and the first hole 13 were different. The samples were prepared mainly using 20 types of center electrodes 16 with different diameters (tolerance ±0.1 mm) of the side surface 29 and the diameter (tolerance ±0.1 mm) of the rear surface 27 of the head 17, and insulators 11 with a diameter of the first hole 13 of 3.0 mm (tolerance ±0.05 mm), with 30 samples per type.

An impact resistance test was conducted on the prepared samples in accordance with JIS B8031:2006. The samples were subjected to impacts at a rate of 400 times per minute for 10 minutes, after which the presence or absence of loosening of the center electrode 16 was checked. Samples in which none of the 30 samples had loose center electrodes 16 were rated as A, samples in which some of the 30 samples had loose center electrodes 16 were rated as B, and samples in which all of the 30 samples had loose center electrodes 16 were rated as C. The results are shown in Table 2.

As shown in Table 2, all of the tested items were rated A or B, and none were rated C. In particular, items with distance D1 between 0.05 mm and 0.40 mm and distance D2 of 0.50 mm or more were rated A. Example 2 revealed that impact resistance can be further improved when distance D1 is between 0.05 mm and 0.40 mm and distance D2 is 0.50 mm or more.

When tests similar to those in Examples 1 and 2 were conducted using an insulator 11 with a first hole 13 diameter of 3.9 mm and an insulator 11 with a first hole 13 diameter of 2.7 mm, the same results as in Examples 1 and 2 were obtained.

The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily imagined that various improvements and modifications are possible within the scope of the present invention. For example, the shape and size of the heads 17, 31 of the center electrodes 16, 30 are only examples and can be set as appropriate.

In the embodiment, the spark plug 10 in which the resistor 21 is disposed inside the insulator 11 has been described, but this is not necessarily limited to this. It is of course possible to apply each embodiment to a spark plug that does not incorporate the resistor 21. This is because, as long as the heads 17, 31 of the center electrodes 16, 30 are fixed by the sealing material 20, the same effects as those of the above-mentioned embodiments can be achieved.

REFERENCE SIGNS LIST 10 Spark plug 11 Insulator 12 Shaft hole 13 First hole 14 Connection portion 15 Second hole 16, 30 Center electrode 17, 31 Head portion 18 Leg portion 20 Sealing material 26, 33 Outer periphery 27, 34 Rear surface 28, 35 Inclined surface 29, 36 Side surface O Center axis

Claims (4)

an insulator having an axial hole that connects a first hole and a second hole having a smaller diameter than the first hole via a connection part;
a center electrode including a head portion disposed in the first hole and in contact with the connection portion, and a leg portion extending from the head portion toward a tip side and disposed in the second hole;
a sealant filled between an outer periphery of the head and the first hole,
The outer periphery of the head portion includes an inclined surface whose diameter increases at a constant rate toward the tip side, and a side surface that is connected to the tip side of the inclined surface and in contact with the connecting portion,
a first hole having an inclined surface and a second hole having an inclined surface, the first hole being inclined toward the inclined surface and having a first hole extending in a direction perpendicular to the inclined surface; The spark plug according to claim 1, wherein the shortest distance between the side surface of the head and the first hole is 0.4 mm or less. The spark plug according to claim 1 or 2, wherein the shortest distance between the side surface of the head and the first hole is 0.05 mm or more. The spark plug according to claim 1 or 2, wherein the shortest distance between the rear surface of the outer periphery of the head that is rearward of the inclined surface and the first hole is 0.5 mm or more.