JP7475317B2 - Spark plug - Google Patents

Description

This disclosure relates to spark plugs.

A spark plug is known for use in an internal combustion engine to ignite an engine, and is attached to the engine head to generate a spark discharge between the tip of the center electrode and the ground electrode (see, for example, Patent Document 1). In the spark plug described in Patent Document 1, a through hole is formed in the metal shell, penetrating it in the radial direction, and a rod-shaped ground electrode extending along the radial direction is inserted and fixed into the through hole.

JP 2019-046660 A

In the spark plug described in Patent Document 1, a through hole for fixing the ground electrode is formed at one location in the circumferential direction of the metal shell, and a male thread is also formed on the metal shell further forward in the axial direction than the through hole. Therefore, the area where the through hole is formed is separated from the female thread of the engine head because no male thread is formed, and is difficult to cool. As a result, at the axial position where the through hole is formed, a difference in the degree of thermal expansion occurs due to a temperature difference in the circumferential direction, and there is a risk of stress being generated that bends the metal shell. For this reason, there has been a demand for technology that can suppress the generation of stress in the metal shell.

This disclosure can be realized in the following forms:

(1) According to one embodiment of the present disclosure, a spark plug is provided. The spark plug includes an insulator having an axial hole extending in the axial direction, a center electrode disposed in the axial hole and having a tip end protruding toward the tip end of the axial hole, a cylindrical metal shell that holds the insulator on its inner periphery and has a threaded portion formed on its outer periphery, and a ground electrode having one end fixed to a through hole provided in the metal shell and having the other end forming a discharge gap with the tip end of the center electrode, the threaded portion having a first threaded portion located rearward in the axial direction from the through hole and a second threaded portion located forward in the axial direction from the through hole, the metal shell having a non-threaded portion located between the first threaded portion and the second threaded portion and having no thread formed around the entire circumference, and the non-threaded portion having the through hole. According to this form of spark plug, the metal shell has a non-threaded portion located between the first and second threaded portions, where the threads are not formed over the entire circumference, and a through hole is provided in the non-threaded portion, so that the metal shell is separated from the female thread of the engine head over the entire circumference at the axial position where the through hole is formed. As a result, it is possible to suppress the occurrence of temperature differences in the circumferential direction, which in turn suppresses the occurrence of differences in the degree of thermal expansion, and as a result, it is possible to suppress the occurrence of stress in the metal shell.

(2) In the spark plug of the above embodiment, the length of the second threaded portion in the axial direction may be shorter than the length of the first threaded portion. With this spark plug, the length of the second threaded portion in the axial direction is shorter than the length of the first threaded portion. Therefore, in a configuration in which a non-threaded portion is formed on the axial tip side, which is more likely to become hot, the temperature difference in the circumferential direction can be suppressed, and as a result, the generation of stress can be effectively suppressed.

(3) In the spark plug of the above embodiment, the first threaded portion, the second threaded portion, and the non-threaded portion may be integrally formed. With this spark plug, the first threaded portion, the second threaded portion, and the non-threaded portion are integrally formed, so that the phase shift between the first threaded portion and the second threaded portion can be suppressed. As a result, the manufacturing process of the metal shell can be prevented from becoming complicated in order to match the phase of the first threaded portion and the second threaded portion.

(4) In the spark plug of the above embodiment, the outer diameter of the non-threaded portion may be smaller than the root diameter of the threaded portion. With this spark plug, the outer diameter of the non-threaded portion is smaller than the root diameter of the threaded portion, so that the non-threaded portion can be prevented from being scraped by the female threads of the engine head when the spark plug is assembled to the engine head.

(5) In the spark plug of the above embodiment, the average thickness of the non-threaded portion may be 8% or more of the effective diameter of the threaded portion. With this spark plug, the average thickness of the non-threaded portion is 8% or more of the effective diameter of the threaded portion, so that excessive reduction in strength of the non-threaded portion can be suppressed.

The present invention can be realized in various forms, such as a method for manufacturing a spark plug, an engine head with a spark plug attached, etc.

1 is a partial cross-sectional view showing a schematic configuration of a spark plug; FIG. 2 is an enlarged cross-sectional view showing the tip portion of the spark plug;

A. First embodiment:
FIG. 1 is a partial cross-sectional view showing a schematic configuration of a spark plug 100 according to an embodiment of the present disclosure. In FIG. 1, the outer shape of the spark plug 100 is shown on the right side of the drawing, and the cross-sectional shape of the spark plug 100 is shown on the left side of the drawing, with the axis CA being the axis of the spark plug 100 as the boundary. In the following description, the lower side of FIG. 1 along the axis CA (the side where a ground electrode 40 described later is arranged) is called the leading end side, the upper side of FIG. 1 (the side where a terminal metal fitting 50 described later is arranged) is called the rear end side, and the direction along the axis CA is called the axial direction AD. In FIG. 1, for convenience of description, an engine head 90 to which the spark plug 100 is attached is shown by a broken line. The engine head 90 is generally provided with a coolant flow passage (not shown) for circulating a cooling medium. The spark plug 100 is attached to the engine head 90 so that its leading end side is exposed in a combustion chamber 95.

The spark plug 100 comprises an insulator 10, a center electrode 20, a metal shell 30, a ground electrode 40, and a terminal metal fitting 50. The axis CA of the spark plug 100 coincides with the axis of each of the insulator 10, the center electrode 20, the metal shell 30, and the terminal metal fitting 50.

The insulator 10 has a generally cylindrical exterior shape with an axial hole 11 extending in the axial direction AD. The axial hole 11 accommodates a part of the center electrode 20 at the front end and a part of the terminal metal fitting 50 at the rear end. Thus, the insulator 10 holds the center electrode 20 on its inner periphery. The front end portion of the insulator 10 is accommodated in the axial hole 31 of the main metal fitting 30 described below, and the rear end portion is exposed from the axial hole 31. The insulator 10 is made of an insulating porcelain formed by sintering a ceramic material such as alumina.

The center electrode 20 is a rod-shaped electrode extending along the axial direction AD, and is disposed in the axial hole 11. The tip portion 21 of the center electrode 20 protrudes toward the tip side of the axial hole 11. A precious metal tip made of, for example, a platinum or iridium alloy may be joined to the tip portion 21. The center electrode 20 in this embodiment is made of a nickel alloy containing nickel as the main component.

In the axial hole 11 of the insulator 10, a leading end seal material 61, a resistor 62, and a trailing end seal material 63 are arranged between the center electrode 20 and the terminal metal fitting 50 in this order from the leading end side to the trailing end side. Therefore, the center electrode 20 is electrically connected to the terminal metal fitting 50 at the trailing end side via the leading end seal material 61, the resistor 62, and the trailing end seal material 63.

The resistor 62 contains ceramic powder, conductive material, and glass as its materials. The resistor 62 functions as an electrical resistor between the terminal fitting 50 and the center electrode 20, thereby suppressing the generation of noise when spark discharge occurs. The leading end sealing material 61 and the trailing end sealing material 63 each contain conductive glass powder as their materials. In this embodiment, the leading end sealing material 61 and the trailing end sealing material 63 contain a powder mixture of copper powder and calcium borosilicate glass powder as their materials.

The terminal fitting 50 is provided at the rear end of the spark plug 100. The tip side of the terminal fitting 50 is housed in the axial hole 11 of the insulator 10, and the rear end side of the terminal fitting 50 is exposed from the axial hole 11. A high-voltage cable (not shown) is connected to the terminal fitting 50, and high voltage is applied. This application generates a spark discharge in the discharge gap G, which will be described later. The spark generated in the discharge gap G ignites the mixture in the combustion chamber 95.

The metal shell 30 has a generally cylindrical external shape with an axial hole 31 formed along the axial direction AD, and holds the insulator 10 within the axial hole 31. In other words, the metal shell 30 holds the insulator 10 on the inner periphery. The metal shell 30 is made of, for example, low carbon steel, and is entirely plated with nickel plating, zinc plating, or the like. The outer periphery of the metal shell 30 is formed with a tool engagement portion 32, a threaded portion 33, and a non-threaded portion 37. The tool engagement portion 32 engages with a tool (not shown) when the spark plug 100 is attached to the engine head 90. The threaded portion 33 has a thread formed on the outer periphery in the tip region of the metal shell 30, and is screwed into the female threaded portion 93 of the engine head 90.

Figure 2 is an enlarged cross-sectional view showing the tip of the spark plug 100. The threaded portion 33 formed on the outer peripheral surface of the metal shell 30 has a first threaded portion 34 and a second threaded portion 35. In the axial direction AD, the first threaded portion 34 is located on the rear end side of the second threaded portion 35. The non-threaded portion 37 of the metal shell 30 is located between the first threaded portion 34 and the second threaded portion 35. Therefore, the first threaded portion 34, the non-threaded portion 37, and the second threaded portion 35 are formed on the outer peripheral surface of the metal shell 30 in this order from the rear end side to the tip end side in the axial direction AD.

The non-threaded portion 37 does not have a thread formed around its entire circumference. In this embodiment, the non-threaded portion 37 is recessed radially inward of the metal shell 30 around the entire circumference of the outer circumferential surface of the metal shell 30. The non-threaded portion 37 has a through hole 39 penetrating the thickness of the metal shell 30. The through hole 39 is formed on the tip side of the tip portion 21 of the center electrode 20 in the axial direction AD, and connects the outer circumferential surface and the inner circumferential surface of the metal shell 30. The through hole 39 is provided at one location in the circumferential direction of the metal shell 30. A ground electrode 40, which will be described later, is fixed to the through hole 39.

The first threaded portion 34 is located on the rear end side of the axial direction AD from the through hole 39, and the second threaded portion 35 is located on the tip side of the axial direction AD from the through hole 39. In this embodiment, the length of the second threaded portion 35 is shorter than the length of the first threaded portion 34 in the axial direction AD. In this embodiment, the outer diameter of the non-threaded portion 37 is smaller than the root diameter of the threaded portion 33. The root diameter of the threaded portion 33 can be obtained by calculating the average value of the values measured for each thread root of the first threaded portion 34 and the second threaded portion 35. In this embodiment, the average thickness of the non-threaded portion 37 is 8% or more of the effective diameter of the threaded portion 33. In this specification, the "average thickness of the non-threaded portion 37" means the average value of the thickness of the non-threaded portion 37 in the circumferential direction, and more specifically, means the average value of the thickness values measured at three points at equal intervals in the circumferential direction at a position in the axial direction AD passing through the center of the through hole 39. In this specification, "effective diameter" refers to the value specified in JIS B 0205 2001. The effective diameter of the threaded portion 33 can be determined by calculating the average value of the values measured for each thread of the first threaded portion 34 and the second threaded portion 35. The average thickness of the non-threaded portion 37 is preferably 17% or less of the effective diameter of the threaded portion 33.

The threaded portion 33 and the non-threaded portion 37 can be formed, for example, by rolling or cutting. When forming by rolling, for example, two dies may be used to form the first threaded portion 34 and the second threaded portion 35, respectively. Also, for example, the first threaded portion 34 and the second threaded portion 35 may be rolled using one die, and then the non-threaded portion 37 may be formed by cutting off the outer peripheral surface over the entire circumference. Also, for example, before threading, the portion corresponding to the non-threaded portion 37 may be formed in advance by being recessed toward the radial inside. In the spark plug 100 of this embodiment, the first threaded portion 34, the second threaded portion 35, and the non-threaded portion 37 are formed integrally. Note that the through hole 39 may be formed before the threaded portion 33 and the non-threaded portion 37 are formed, or may be formed after the threaded portion 33 and the non-threaded portion 37 are formed.

The ground electrode 40 is made of a rod-shaped metal member and is arranged to extend in the radial direction. One end 41 of the ground electrode 40 is inserted into the through hole 39 and fixed. Therefore, the one end 41 can also be said to be a fixed part. The other end 42 of the ground electrode 40 faces the tip 21 of the center electrode 20. The other end 42 forms a discharge gap G for spark discharge between the tip 21 of the center electrode 20. The ground electrode 40 of this embodiment is made of a nickel alloy containing nickel as a main component, like the center electrode 20. The ground electrode 40 of this embodiment is fixed by welding in the through hole 39, but it may be fixed by pressing instead of welding.

Here, the ground electrode 40 is generally exposed to the combustion of the air-fuel mixture and reaches a high temperature. Therefore, in the spark plug 100 in which the ground electrode 40 is fixed to the through hole 39 formed in the metal shell 30, the one end 41 of the ground electrode 40 may be oxidized due to overheating. However, in the spark plug 100 of this embodiment, the second screw portion 35 is formed on the tip side of the through hole 39 in the axial direction AD, so that the second screw portion 35 of the metal shell 30 and the female screw portion 93 of the engine head 90 can be screwed together even on the tip side of the through hole 39. Since the engine head 90 generally has a refrigerant flow path, by screwing the second screw portion 35 and the female screw portion 93, a heat transfer path for the ground electrode 40 can be secured even on the tip side, which is more likely to become hot. Therefore, the one end 41 of the ground electrode 40 can be prevented from excessively increasing in temperature, and therefore the one end 41 of the ground electrode 40 can be prevented from being oxidized. As a result, the ground electrode 40 is prevented from falling out of the through hole 39 during the cooling and heating cycles of the combustion chamber 95, improving the durability of the spark plug 100.

Unlike the spark plug 100 of this embodiment, in a spark plug in which no male thread is formed only at the position where the through hole is formed in the circumferential direction, the male thread is separated from the female thread portion 93 of the engine head 90 only at the position where the through hole is formed in the circumferential direction. For this reason, the area where the through hole is formed in the circumferential direction is difficult to cool, while the area where the through hole is not formed in the circumferential direction is easily cooled. As a result, at the axial position where the through hole is formed, a difference occurs in the degree of thermal expansion due to the temperature difference in the circumferential direction, and stress that bends the metal shell occurs.

In contrast, in the spark plug 100 of this embodiment, the metal shell 30 has a non-threaded portion 37 that is located between the first threaded portion 34 and the second threaded portion 35 and does not have a thread formed all around, and a through hole 39 is provided in the non-threaded portion 37. Therefore, since the metal shell 30 does not have a thread formed all around at the position in the axial direction AD where the through hole 39 is formed, the metal shell 30 is separated from the female threaded portion 93 of the engine head 90 all around. As a result, it is possible to suppress the occurrence of temperature differences in the circumferential direction, and therefore it is possible to suppress the occurrence of differences in the degree of thermal expansion, and as a result, it is possible to suppress the occurrence of stress in the metal shell 30.

In addition, since the length of the second threaded portion 35 in the axial direction AD is shorter than the length of the first threaded portion 34, in a configuration in which the non-threaded portion 37 is formed on the tip side in the axial direction AD, which is more likely to become hot, the temperature difference in the circumferential direction can be suppressed, and as a result, the occurrence of stress in the metal shell 30 can be effectively suppressed. In addition, since the first threaded portion 34, the second threaded portion 35, and the non-threaded portion 37 are integrally formed, the phase shift between the first threaded portion 34 and the second threaded portion 35 can be suppressed compared to a configuration in which the first threaded portion 34 and the second threaded portion 35 are formed separately and fixed via the non-threaded portion 37. As a result, the manufacturing process of the metal shell 30 can be suppressed from becoming complicated in order to match the phase of the first threaded portion 34 and the second threaded portion 35.

In addition, since the outer diameter of the non-threaded portion 37 is smaller than the root diameter of the threaded portion 33, the non-threaded portion 37 is prevented from being scraped by the female threaded portion 93 of the engine head 90 when the spark plug 100 is assembled to the engine head 90. In addition, since the average thickness of the non-threaded portion 37 is 8% or more of the effective diameter of the threaded portion 33, the thickness of the non-threaded portion 37 is prevented from becoming excessively thin. As a result, the strength of the non-threaded portion 37 is prevented from decreasing excessively, so that the strength suitable for the tightening torque can be maintained, and damage to the non-threaded portion 37 is prevented when the spark plug 100 is assembled to or removed from the engine head 90.

B. Other embodiments:
The configuration of the threaded portion 33 in the above embodiment is merely an example, and can be modified in various ways. For example, in the axial direction AD, the length of the second threaded portion 35 may be the same as the length of the first threaded portion 34, or may be longer than the length of the first threaded portion 34. Also, for example, the first threaded portion 34, the second threaded portion 35, and the non-threaded portion 37 are not limited to being formed integrally, and at least a part of them may be formed separately and fixed. Also, for example, the average thickness of the non-threaded portion 37 may be less than 8% of the effective diameter of the threaded portion 33. Also, for example, the outer diameter of the non-threaded portion 37 may be the same as the root diameter of the threaded portion 33, or may be larger than the root diameter of the threaded portion 33. Note that, in an embodiment in which the outer diameter of the non-threaded portion 37 is larger than the root diameter of the threaded portion 33, it is preferable that the outer diameter of the non-threaded portion 37 is smaller than the standard minimum value of the thread diameter of the female threaded portion 93. Even with this configuration, by providing a through hole 39 in the non-threaded portion 37 located between the first threaded portion 34 and the second threaded portion 35 and not having a thread formed around the entire circumference, the thermal expansion difference in the circumferential direction can be suppressed, and therefore the generation of stress in the main metal shell 30 can be suppressed.

The configuration of the spark plug 100 in the above embodiment is merely an example and can be modified in various ways. For example, the spark plug 100 may be a pre-chamber plug that has a cover at the tip of the metal shell 30 and forms a sub-combustion chamber.

The present invention is not limited to the above-described embodiments, and can be realized in various configurations without departing from the spirit of the present invention. For example, the technical features in the embodiments corresponding to the technical features in each form described in the Summary of the Invention column can be replaced or combined as appropriate to solve some or all of the above-described problems or to achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be deleted as appropriate.

10...insulator, 11...shaft hole, 20...center electrode, 21...tip portion, 30...metal shell, 31...shaft hole, 32...tool engagement portion, 33...threaded portion, 34...first threaded portion, 35...second threaded portion, 37...non-threaded portion, 39...through hole, 40...ground electrode, 41...one end portion, 42...other end portion, 50...terminal metal fitting, 61...tip side seal material, 62...resistor, 63...rear end side seal material, 90...engine head, 93...female thread portion, 95...combustion chamber, 100...spark plug, AD...axial direction, CA...axis, G...discharge gap

Claims (5)

an insulator having an axial hole extending in an axial direction;
a center electrode disposed in the axial hole and having a tip end portion protruding toward a tip end of the axial hole;
a cylindrical metal shell having an inner periphery on which the insulator is held and an outer periphery on which a thread portion is formed;
a ground electrode having one end fixed in a through hole formed in the metallic shell and having the other end forming a discharge gap between itself and the tip end of the center electrode;
A spark plug comprising:
The threaded portion has a first threaded portion located on a rear end side of the through hole in the axial direction, and a second threaded portion located on a front end side of the through hole in the axial direction,
the metallic shell has a non-threaded portion located between the first threaded portion and the second threaded portion, the non-threaded portion having no thread formed over the entire periphery,
A spark plug, wherein the through hole is provided in the non-threaded portion. 2. The spark plug according to claim 1,
A spark plug, wherein a length of the second threaded portion in the axial direction is shorter than a length of the first threaded portion. The spark plug according to claim 1 or 2,
A spark plug, wherein the first threaded portion, the second threaded portion and the non-threaded portion are integrally formed. The spark plug according to any one of claims 1 to 3,
A spark plug, wherein an outer diameter of the non-threaded portion is smaller than a root diameter of the threaded portion. The spark plug according to any one of claims 1 to 4,
13. A spark plug comprising: a non-threaded portion having an average thickness that is 8% or more of an effective diameter of the threaded portion.

Images