Spark plug

Abstract

To provide a spark plug that can improve impact resistance.SOLUTION: A spark plug includes an insulator having an axial hole connecting a first hole and a second hole having a smaller diameter than the first hole via a connection portion, a central electrode having a head disposed in the first hole and in contact with the connection portion, and a leg extending from the head toward the tip side and disposed in the second hole, and a sealant filled between the outer periphery of the head and the first hole. The outer periphery of the head 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 connection portion. 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.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a spark plug.

Background Art

[0002] The prior art of fixing a center electrode disposed in an axial hole of an insulator to the insulator by a sealing material disposed around the head of the center electrode is disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, further improvement in impact resistance is desired.

[0005] The present invention has been made to meet this requirement, and an object thereof is to provide a spark plug capable of improving impact resistance.

Means for Solving the Problems

[0006] To achieve this object, a first aspect of the present invention includes an insulator provided with an axial hole in which a first hole and a second hole having a diameter smaller than that of the first hole are connected via a connecting portion, a head disposed in the first hole and contacting the connecting portion, and a leg portion extending from the head toward the tip side and disposed in the second hole. The center electrode includes a sealing material filled between the outer periphery of the head and the first hole. The outer periphery of the head includes an inclined surface whose diameter increases at a constant rate toward the tip side, and a side surface connected to the tip side of the inclined surface and contacting the connecting portion. In a cross section including the central axis of the center electrode, the angle formed by the first hole and the inclined surface is 20° or more and 40° or less.

[0007] In the second aspect, in the first aspect, the shortest distance among the distances between the side surface of the head and the first hole is 0.4 mm or less.

[0008] In the third aspect, in the first or second aspect, the shortest distance among the distances between the side surface of the head and the first hole is 0.05 mm or more.

[0009] In the fourth aspect, in any one of the first to third aspects, the shortest distance among the distances between the rear surface on the rear end side of the outer periphery of the head than the inclined surface and the first hole is 0.5 mm or more.

Advantages of the Invention

[0010] 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 inclined surface and contacts the connection portion of the insulator. In a cross section including the central axis of the center electrode, the angle formed by the first hole of the insulator and the inclined surface is 20° or more and 40° or less. When manufacturing the spark plug, the pressure for forming the sealing material filled between the outer periphery of the head and the first hole is easily transmitted to the sealing material between the side surface and the first hole along the inclined surface, so that the filling rate and density of the sealing material can be improved. Since the mechanical strength of the sealing material is improved, the impact resistance can be improved.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a cross-sectional view of the spark plug 10 in the first embodiment. The lower side of the paper surface of FIG. 1 is the tip side of the spark plug 10, and the upper side of the paper surface is the rear 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.

[0013] The insulator 11 is a cylindrical member provided with an axial hole 12 extending along the axis O, and is made of a ceramic such as alumina, which is excellent in insulation properties and mechanical properties at high temperatures. The axial hole 12 is connected in order from the rear end to the front end of the insulator 11 as the first hole 13, the connecting portion 14, and the second hole 15. In this embodiment, the first hole 13 and the second hole 15 are cylindrical surfaces having the same diameter over the entire length, and the connecting portion 14 is a conical surface whose diameter decreases toward the tip side. The connecting portion 14 may be an annular plane perpendicular to the center lines 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.

[0014] 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 excellent in thermal conductivity embedded in the base material. The base material is formed of a metal material mainly composed of Ni or Ni. The core material is formed of copper or an alloy mainly composed of copper. The core material can be omitted. The center electrode 16 includes a head 17 disposed in the first hole 13 and in contact with the connecting portion 14, and a leg portion 18 extending from the head 17 toward the tip side and disposed in the second hole 15. The head 17 and the leg portion 18 are integrally formed.

[0015] The terminal fitting 19 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is formed of a conductive metal material (for example, low-carbon steel, etc.). 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 in order from the tip side between the center electrode 16 and the terminal fitting 19.

[0016] The sealing material 20 has the function of fixing the head 17 of the central electrode 16 to the insulator 11 and closing the first hole 13, and is arranged around the head 17. The sealing material 22 has the function of fixing the terminal fitting 19 to the insulator 11. The sealing materials 20 and 22 have conductivity and contain, for example, glass particles and metal particles (such as Cu, Fe, etc.) in a ratio of about 1:1. As the glass particles, materials such as B2O3-SiO2-based, BaO-B2O3-based, SiO2-B2O3-CaO-BaO-based, etc. can be adopted. The thermal expansion coefficients of the sealing materials 20 and 22 are located between the thermal expansion coefficient of the ceramic insulator 11 and the thermal expansion coefficient of the metal central electrode 16.

[0017] The resistor 21 is a member for suppressing the radio wave noise generated during sparking. The resistor 21 is a mixture containing glass particles as the main component, ceramic particles other than glass, and a conductive material. As the material of the glass particles, materials such as B2O3-SiO2-based, BaO-B2O3-based, SiO2-B2O3-CaO-BaO-based, etc. can be adopted. As the material of the ceramic particles, for example, TiO2, ZrO2, etc. can be adopted. As the conductive material, for example, non-metal conductive materials such as carbon particles (carbon black, etc.), TiC particles, TiN particles, and metals such as Al, Mg, Ti, Zr, and Zn can be adopted. Since the sealing materials 20 and 22 are in contact with the resistor 21, the central electrode 16 and the terminal fitting 19 are electrically connected through the sealing materials 20 and 22 and the resistor 21.

[0018] The main body fitting 23 is a substantially cylindrical member formed of a conductive metal material (such as low carbon steel, etc.). The main body fitting 23 is arranged on the outer periphery of the insulator 11. The ground electrode 24 is a rod-shaped metal (such as nickel-based alloy) member connected to the main body fitting 23. A spark gap is formed between the ground electrode 24 and the leg 18 of the central electrode 16. A plurality of ground electrodes 24 may be connected to the main body fitting 23.

[0019] 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 center electrode 16 is disposed such that the head 17 is positioned within the first hole 13 in contact with the connection portion 14 of the insulator 11, and the leg portion 18 is positioned within the second hole 15.

[0020] Next, the raw material powder of the sealing material 20 is introduced through the first hole 13, and after filling the periphery of the head 17 and the rear end side of the first hole 13 behind the head 17 with the raw material powder, the raw material powder is pre-compressed using a compression rod (not shown). Next, after filling the raw material powder of the resistor 21 on top of the raw material powder of the sealing material 20, the raw material powder is pre-compressed using a compression rod. Finally, after filling the raw material powder of the sealing material 22 on top of the raw material powder of the resistor 21, the raw material powder is pre-compressed using a compression rod.

[0021] The terminal fitting 19 is inserted from the rear end side of the first hole 13 such that the tip of the terminal fitting 19 contacts the raw material powder of the sealing material 22, and then, while heating to a temperature higher than the softening point of the glass component contained in each raw material powder, for example, the terminal fitting 19 is press-fitted to apply an axial load to the raw material powder by the terminal fitting 19. Each raw material powder is compressed and sintered to form the sealing materials 20 and 22 and the resistor 21 within the first hole 13. Next, after assembling the main body fitting 23 to which the ground electrode 24 is connected to the outer periphery of the insulator 11, 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.

[0022] The center electrode 16 will be described with reference to FIGS. 2 and 3. FIG. 2 is a perspective view of the center electrode 16. In FIG. 2, the illustration of the tip side of the leg portion 18 is omitted. The head 17 of the center electrode 16 includes an outer periphery 26 that connects to an end face 25 facing the rear end side. The outer periphery 26 of the head 17 includes a rear surface 27, an inclined surface 28, and a side surface 29 in order from the rear end side toward the tip side.

[0023] Figure 3 is a cross-sectional view of the vicinity of the head 17 with the portion shown by III in Figure 1 enlarged. Figure 3 is a cross-sectional view including the central axis O of the central electrode 16, and the illustration of one side of the central electrode 16 is omitted with the central axis O as the boundary. The inclined surface 28 is a surface whose diameter increases at a constant rate toward the tip side, and is shown by a straight line in Figure 3. The rear surface 27 is the surface of the outer periphery 26 on the rear end side of the inclined surface 28. In the present embodiment, the diameter of the rear surface 27 is constant over the entire axial length of the rear surface 27.

[0024] The side surface 29 is the surface of the outer periphery 26 on the tip side of the inclined surface 28, and a part of it is in contact with the connecting portion 14. In the present embodiment, the side surface 29 has a shape in which a conical surface is connected to the tip side of the cylindrical surface, and a part of the conical surface is in contact with the connecting portion 14. The corner where the cylindrical surface and the conical surface of the side surface 29 intersect is rounded. The diameter of the portion of the side surface 29 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.

[0025] In the cross-section (Figure 3) including the central axis O of the central electrode 16, the angle θ (acute angle) formed by the first hole 13 and the inclined surface 28 is 20° or more and 40° or less. When the angle θ is 20° or more and 40° or less, when manufacturing the spark plug 10, the pressure for molding 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 to the raw material powder between the side surface 29 and the first hole 13 along the inclined surface 28. Therefore, the filling rate and density of the sealing material 20 can be improved. Since the mechanical strength of the sealing material 20 is improved, the impact resistance of the spark plug 10 can be improved.

[0026] On the one hand, when the angle θ is less than 20°, the gap between the inclined surface 28 and the first hole 13 becomes small. As a result, when manufacturing the spark plug 10, the pressure for forming 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 less likely to be transmitted to the raw material powder filled between the side surface 29 and the first hole 13. Also, even when the angle θ exceeds 40°, the pressure for forming 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 less likely to be 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 can be solved, the mechanical strength of the sealing material 20 can be improved, and the impact resistance of the spark plug 10 can be improved.

[0027] The sealing material 20 whose coefficient of thermal expansion is between the coefficient of thermal expansion of the insulator 11 and the coefficient of thermal expansion of the center electrode 16 has a function of buffering the dimensional change in the radial direction between the first hole 13 and the head 17 due to the temperature change when heating the raw material powder compressed in the first hole 13 to form the sealing material 20 when manufacturing the spark plug 10. Since the diameter of the portion of the side surface 29 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, among the dimensional changes in the radial direction of the head 17, the dimensional change in the radial direction of the side surface 29 in contact with the sealing material 20 is the largest. Therefore, the portion of the sealing material 20 in contact with the side surface 29 needs to buffer the largest dimensional change.

[0028] Therefore, it is preferable that the shortest distance D1 among the distances between the side surface 29 and the first hole 13 is 0.4 mm or less. This is because when the distance D1 exceeds 0.4 mm, the sealing material 20 cannot buffer the dimensional change between the side surface 29 and the first hole 13, and there is a risk that the inside and interface of the sealing material 20 will break. When the inside and interface of the sealing material 20 are broken, the holding force of the head 17 by the sealing material 20 decreases, and the impact resistance decreases.

[0029] In order for the inclined surface 28 to exhibit the function of transmitting the forming 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, the length L of the inclined surface 28 is preferably 1.5 times or more and 6 times or less of the distance D1.

[0030] When manufacturing the spark plug 10, after filling the raw material powder of the sealing material 20 between the side surface 29 of the head 17 and the first hole 13 and then heating to form the sealing material 20, the distance D1 is preferably 0.05 mm or more. If the distance D1 is less than 0.05 mm, it is likely that the filling of the raw material powder between the side surface 29 and the first hole 13 will be insufficient. As a result, the holding force of the head 17 by the sealing material 20 will decrease, and the impact resistance will tend to decrease.

[0031] The shortest distance D2 among the distances between the rear surface 27 and the first hole 13 is preferably 0.5 mm or more. If the distance D2 is less than 0.5 mm, when manufacturing the spark plug 10, the frictional resistance until the raw material powder of the sealing material 20 reaches between the side surface 29 and the first hole 13 will increase, so that the filling of the raw material powder between the outer periphery 26 of the head 17 and the first hole 13 is likely to be insufficient.

[0032] Since the rear surface 27 is a cylindrical surface with the same diameter over the entire axial length, when manufacturing the spark plug 10, the raw material powder of the sealing material 20 can easily reach the inclined surface 28 through the rear surface 27. Since the raw material powder can easily reach between the side surface 29 and the first hole 13, it is less likely that the filling of the raw material powder between the outer periphery 26 of the head 17 and the first hole 13 will be insufficient. As a result, it is easier to ensure the holding force of the head 17 by the sealing material 20.

[0033] The second embodiment will be described with reference to FIG. 4. In the first embodiment, the case where the side surface 29 of the center electrode 16 has a shape in which a conical surface is connected to the tip side of the cylindrical surface was described. In contrast, in the second embodiment, the case where the shape of the side surface 36 of the center electrode 30 is spherical belt-shaped will be described. The spark plug 10 in the second embodiment has a center electrode 30 disposed in the insulator 11 instead of the center electrode 16 of the spark plug 10 in the first embodiment. 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.

[0034] FIG. 4 is a cross-sectional view of the spark plug 10 in the second embodiment. FIG. 4 is a cross-sectional view of the vicinity of the head 31 including the central axis O of the center electrode 30, and the illustration of one side of the center electrode 30 is omitted with the central axis O as a boundary.

[0035] The head 31 of the center electrode 30 includes an outer periphery 33 that connects to an end face 32 facing the rear end side. The outer periphery 33 includes a rear surface 34, an inclined surface 35, and a side surface 36 in order from the rear end side to the front end side. The inclined surface 35 is a surface whose diameter increases at a constant rate toward the front end side, and is shown as a straight line in FIG. 4.

[0036] The rear surface 27 is a surface of the outer periphery 33 on the rear end side of the inclined surface 35. The middle part of the rear surface 27 in the axial direction bulges. The side surface 36 is a surface of the outer periphery 26 on the front end side of the inclined surface 35 and is in contact with a part of the connection portion 14. In the present embodiment, the side surface 36 is a spherical belt-shaped curved surface. The diameter of the thickest part of the side surface 36 is larger than the diameter of the bulged part of the rear surface 27.

[0037] In a cross-section (FIG. 4) including the central axis O of the center electrode 30, the angle θ (acute angle) formed by the first hole 13 and the inclined surface 35 is 20° or more and 40° or less. Thereby, when manufacturing the spark plug 10, the pressure for molding the raw material powder of the sealing material 20 filled between the outer periphery 33 of the head 31 and the first hole 13 is smoothly transmitted to the raw material powder along the inclined surface 35, so that the filling rate and density of the sealing material 20 can be improved.

[0038] Of the distances between the side surface 36 and the first hole 13, the shortest distance D1 is preferably 0.4 mm or less. This is to buffer, by means of the sealing material 20, the dimensional change between the side surface 36 and the first hole 13 due to thermal expansion and contraction when the sealing material 20 is formed in the first hole 13.

[0039] The length L of the inclined surface 35 is preferably 1.5 times or more and 6 times or less the distance D1. This is to facilitate transmission of the pressure for molding 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 the raw material powder between the side surface 36 and the first hole 13 along the inclined surface 35.

[0040] The distance D1 is preferably 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.

[0041] Of the distances between the rear surface 34 and the first hole 13, the shortest distance D2 is preferably 0.5 mm or more. This is to reduce the frictional resistance until the raw material powder of the sealing material 20 reaches 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.

Example

[0042] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0043] (Example 1) Regarding the spark plug 10 in the first embodiment, 25 types of samples were prepared, which differed in the magnitude of the angle θ formed between the inclined surface 28 of the center electrode 16 and the first hole 13 of the insulator 11, and the shortest distance D1 among the distances between the side surface 29 of the center electrode 16 and the first hole 13. The samples were mainly made 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 3.0 mm (tolerance ±0.05 mm) for the first hole 13, and 30 samples of each type were prepared. The length L of the inclined surface 28 was set in the range from 1.5 times to 6.0 times the distance D1 such that the length L became shorter as the angle θ increased and the length L became longer as the angle θ decreased.

[0044] An impact resistance test conforming to JIS B8031:2006 was conducted on the prepared samples. After applying an impact 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 examined. Those in which none of the 30 samples had the center electrode 16 loosened were judged as A, those in which some of the 30 samples had the center electrode 16 loosened were judged as B, and those in which all 30 samples had the center electrode 16 loosened were judged as C. The results are shown in Table 1.

[0045]

Table 1

[0046] As shown in Table 1, all those with the angle θ formed between the inclined surface 28 and the first hole 13 less than 20° and those with the angle θ exceeding 40° were judged as C. On the other hand, those with the angle θ between 20° and 40° were judged as A or B. In particular, those with the angle θ between 20° and 40° and the distance D1 between 0.05 mm and 0.40 mm were judged as A.

[0047] According to Example 1, it was clarified that the impact resistance can be improved when the angle θ is between 20° and 40°. In particular, it was clarified that the impact resistance can be further improved when the distance D1 is between 0.05 mm and 0.40 mm.

[0048] (Example 2) Regarding the spark plug 10 in the first embodiment, the angle θ formed by the inclined surface 28 of the center electrode 16 and the first hole 13 of the insulator 11 was fixed at 30°, and the shortest distance D1 among the distances between the side surface 29 of the center electrode 16 and the first hole 13, and the shortest distance D2 among the distances between the rear surface 27 of the center electrode 16 and the first hole 13 were different. Twenty types of samples were prepared. The samples were mainly made of twenty types of center electrodes 16 with different diameters of the side surface 29 of the head 17 (tolerance ±0.1 mm) and the rear surface 27 (tolerance ±0.1 mm), and insulators 11 with a diameter of the first hole 13 of 3.0 mm (tolerance ±0.05 mm). Thirty samples of each type were prepared.

[0049] An impact resistance test conforming to JIS B8031:2006 was performed on the prepared samples. After applying an impact to the samples at a rate of 400 times per minute for 10 minutes, the looseness of the center electrode 16 was examined. Those with no loosening of the center electrode 16 among the 30 samples were judged as A, those with some loosening of the center electrode 16 among the 30 samples were judged as B, and those with all 30 samples having the center electrode 16 loosened were judged as C. The results are shown in Table 2.

[0050]

Table 2

[0051] As shown in Table 2, all the tested ones were judged as A or B, and there was none judged as C. In particular, those with D2 being 0.50 mm or more among the distances where D1 was 0.05 mm or more and 0.40 mm or less were judged as A. According to Example 2, it was revealed that the impact resistance can be further improved when D1 is 0.05 mm or more and 0.40 mm or less, and D2 is 0.50 mm or more.

[0052] When the same tests as in Examples 1 and 2 were performed using an insulator 11 with a diameter of the first hole 13 of 3.9 mm or an insulator 11 with a diameter of the first hole 13 of 2.7 mm, the same results as in Examples 1 and 2 were obtained.

[0053] The present invention has been described based on the embodiments. However, the present invention is not limited to the above embodiments at all, and it can be easily inferred that various improvements and modifications are possible without departing from the spirit of the present invention. For example, the shapes and sizes of the heads 17 and 31 of the center electrodes 16 and 30 are merely examples and can be set as appropriate.

[0054] In the embodiment, the spark plug 10 in which the resistor 21 is disposed in the insulator 11 has been described. However, it is not necessarily limited thereto. It is naturally possible to apply each of the embodiments to a spark plug that does not incorporate the resistor 21. This is because, if the heads 17 and 31 of the center electrodes 16 and 30 are fixed by the sealing material 20, the same operational effects as those of the above embodiments can be achieved.

Explanation of Reference Numerals

[0055] 10 Spark plug 11 Insulator 12 Axial hole 13 First hole 14 Connection part 15 Second hole 16, 30 Center electrode 17, 31 Head 18 Leg part 20 Sealing material 26, 33 Outer periphery 27, 34 Rear surface 28, 35 Inclined surface 29, 36 Side surface O Central axis

Claims

1. An insulator provided with a shaft hole in which a first hole and a second hole having a diameter smaller than that of the first hole are connected via a connecting portion; A center electrode including a head portion disposed in the first hole and in contact with the connecting portion, and a leg portion extending from the head portion toward the tip side and disposed in the second hole; A spark plug comprising a sealing material filled between the outer periphery of the head portion 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 connected to the tip side of the inclined surface and in contact with the connecting portion; A spark plug in which, in a cross section including the central axis of the center electrode, the angle formed by the first hole and the inclined surface is 20° or more and 40° or less.

2. The spark plug according to claim 1, wherein the shortest of the distances between the side surface of the head portion and the first hole is 0.4 mm or less.

3. The spark plug according to claim 1 or 2, wherein the shortest of the distances between the side surface of the head portion and the first hole is 0.05 mm or more.

4. The spark plug according to claim 1 or 2, wherein the shortest of the distances between the rear surface of the outer periphery of the head portion on the rear end side of the inclined surface and the first hole is 0.5 mm or more.

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