JP7600839B2 - 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 has a center electrode inserted into a cylindrical insulator, a cylindrical housing provided on the outer periphery of the insulator, and a ground electrode fixed to the lower end surface of the housing. A flange portion having an outer diameter larger than the other portions is formed at the base end of the center electrode. This flange portion contacts a stepped surface formed on the inner periphery of the insulator, thereby positioning the center electrode relative to the insulator.

Patent No. 4353080

Some spark plugs have a seal body and resistor housed inside an insulator. Specifically, inside the insulator, a first seal body, resistor, and second seal body are stacked in this order from the area where the flange of the center electrode is located toward the base end of the insulator. The tip of a terminal fitting is press-fitted into the second seal body. An external circuit that applies high voltage is connected to the base end of the terminal fitting.

In the manufacture of a spark plug having such a structure, after inserting the center electrode into the insulator, the process of adding powder for the first seal body and applying pressure, the process of adding powder for the resistor body and applying pressure, and the process of adding powder for the second seal body and applying pressure are carried out in that order, and then the process of pressing the tip of the terminal metal fitting into the second seal body while heating it is carried out. As the spark plug is manufactured through these processes, a force is applied to the flange of the center electrode in a direction toward the stepped surface of the insulator, in other words, in a direction toward the tip of the spark plug. This force pulls the stepped surface of the insulator in a direction toward the tip of the spark plug, so stress remains around the stepped surface of the insulator. Such residual stress is undesirable as it can cause damage such as cracks in the insulator.

This disclosure was made in light of these circumstances, and its purpose is to provide a spark plug that makes it difficult for the insulator to be damaged.

The spark plug that solves the above problem includes a cylindrical insulator (12) having an axial hole (120) formed therein, a center electrode (13) inserted and held in the axial hole so as to protrude from the tip of the insulator, a ground electrode (14) arranged with a predetermined gap from the center electrode and forming a spark discharge between the center electrode and the ground electrode based on application of a voltage to the center electrode, and a seal body (15) arranged around the base end of the center electrode inside the axial hole. The axial hole has a first hole portion (20) in which the seal body is accommodated, a second hole portion (21) arranged on the tip side of the insulator with respect to the first hole portion and having an inner diameter smaller than that of the first hole portion, and a tapered hole portion (22) located between the first hole portion and the second hole portion and having a tapered inner wall surface formed so that the inner diameter becomes smaller toward the tip of the insulator. The center electrode has a main body (30) that is inserted into the second hole, a flange (31) that is provided at the base end of the main body and has a larger outer diameter than the main body and is disposed inside the first hole, and a tapered portion (32) that is disposed between the main body and the flange and has a tapered outer wall surface that faces the inner wall surface of the tapered hole portion of the axial hole. The tapered portion has a step portion (323, 324) that has an outer diameter larger than the outer diameter of the main body and smaller than the outer diameter of the flange.

By forming a step portion in the tapered portion of the center electrode as in this configuration, the tapered portion can be provided with a contact portion that contacts the inner peripheral surface of the tapered hole portion and a spaced portion that is spaced apart from the inner peripheral surface of the tapered hole portion. In this case, the seal is sandwiched and positioned between the spaced portion of the tapered portion of the center electrode and the inner wall surface of the tapered hole portion of the insulator, so that the force applied from the tapered portion of the center electrode to the inner wall surface of the tapered hole portion of the insulator can be reduced compared to a spark plug that does not have such a structure. As a result, the stress acting on the inner wall surface of the tapered hole portion of the insulator can be reduced, making it less likely that the insulator will be damaged.

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 makes it possible to prevent damage to the insulator.

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 cross-sectional view showing a cross-sectional structure around a base end portion of a center electrode in the spark plug of the embodiment. FIG. 3 is a cross-sectional view showing a part of a manufacturing process of the spark plug of the embodiment. FIG. 4 is a cross-sectional view showing a part of a manufacturing process for the spark plug of the reference example. FIG. 5 is a cross-sectional view showing a cross-sectional structure around a base end portion of a center electrode in a spark plug according to another embodiment. FIG. 6 is a cross-sectional view showing a cross-sectional structure around a base end portion of a center electrode in a spark plug 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 provided, for example, in a cylinder head of an internal combustion engine. The spark plug 10 ignites an air-fuel mixture in a cylinder of the internal combustion 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 a central axis m10 of the spark plug 10. The housing 11 is formed of a metal material such as carbon steel. A lower end of an insulator 12 is inserted coaxially inside the housing 11.
The insulator 12 is formed in a cylindrical shape centered on a central axis m10. The insulator 12 is made of an insulating material such as alumina. 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 to penetrate the insulator 12 from a tip end 121 to a base end 122 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 from the tip end 121 side.

The central electrode 13 has an electrode base material 130 and an electrode tip 131. The electrode base material 130 is formed in a cylindrical shape centered on the central axis m10. The electrode base material 130 is formed of a nickel (Ni) alloy or the like having excellent heat resistance. Specifically, the inner material of the electrode base material 130 is formed of copper, and the outer material is formed of a nickel alloy. The electrode tip 131 is joined to the tip of the electrode base material 130. The electrode tip 131 is formed in a cylindrical shape centered on the central axis m10. The electrode tip 131 is formed of an iridium alloy or the like containing iridium (Ir), which has a high melting point and excellent wear resistance, and rhodium (Rh) to suppress the high-temperature volatility of iridium.

The first seal body 15 is disposed between the center electrode 13 and the resistor 16. The first seal body 15 is disposed around the base end of the center electrode 13. The first seal body 15 is made of a sintered body of conductive glass powder formed by mixing metal powder such as copper into glass. The first seal body 15 ensures airtightness within the axial hole 120 by being in close contact with the inner wall surface of the axial hole 120. The tip end of the first seal body 15 is in contact with the center electrode 13, and the base end of the first seal body 15 is in contact with the resistor 16. The center electrode 13 and the resistor 16 are electrically connected via the first seal body 15.

The resistor 16 is disposed between the first seal body 15 and the second seal body 17. The resistor 16 is made of a sintered body obtained by sintering resistor composition powder in which resistive material powder such as carbon is mixed into glass. The resistor 16 suppresses the transmission of radio noise caused by the spark discharge of the spark plug 10 to the outside of the spark plug 10. The resistivity of the resistor 16 in the direction along the central axis m10 is greater than the resistivity of each of the first seal body 15 and the second seal body 17.

The second seal body 17 is disposed between the resistor 16 and the terminal metal fitting 18. Like the first seal body 15, the second seal body 17 is also made of a sintered body of conductive glass powder, and ensures the airtightness of the axial hole 120 by closely contacting the inner wall surface of the axial hole 120. The tip end of the second seal body 17 is in contact with the resistor 16, and the base end of the second seal body 17 is in contact with the terminal metal fitting 18. The terminal metal fitting 18 and the resistor 16 are electrically connected via the second seal body 17.

The terminal fitting 18 is formed in a generally cylindrical shape centered on the central axis m10. The terminal fitting 18 is made of, for example, a steel material. A terminal portion 180 is formed at the base end of the terminal fitting 18. The terminal fitting 18 is inserted into the axial hole 120 so that the terminal portion 180 protrudes from the base end of the insulator 12.

The ground electrode 14 has an electrode base material 140 and an electrode tip 141. The electrode base material 140 is made of a nickel alloy or the like. The electrode base material 140 is fixed to the lower end surface of the housing 11 and is formed so as to extend from the lower end surface to a position facing the electrode tip 131 of the center electrode 13. The electrode tip 141 is joined to the tip of the electrode base material 140. The electrode tip 141 is made of a precious metal alloy such as an iridium alloy or a platinum alloy, for example, a Pt-20Ir material. The electrode tip 141 is arranged to face the electrode tip 131 of the center electrode 13 with a predetermined gap 19. Hereinafter, the gap 19 formed between the electrode tip 131 of the center electrode 13 and the electrode tip 141 of the ground electrode 14 is referred to as the "spark gap 19".

In this spark plug 10, an external circuit that applies 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 131 of the center electrode 13 and the electrode tip 141 of the ground electrode 14. This spark discharge ignites the air-fuel mixture in the cylinder of the internal combustion engine, forming a flame, and the mixture is burned.

Next, the structure of the contact portion between the base end of the center electrode 13 and the insulator 12 will be described in detail.
As shown in Fig. 2, the axial hole 120 of the insulator 12 has a first hole portion 20, a second hole portion 21, and a tapered hole portion 22. The first hole portion 20 is a portion of the axial hole 120 in which the first seal body 15 is housed. The second hole portion 21 is disposed on the tip portion 121 side of the insulator 12 with respect to the first hole portion 20. The second hole portion 21 has an inner diameter smaller than that of the first hole portion 20. The tapered hole portion 22 is a portion of the axial hole 120 disposed between the first hole portion 20 and the second hole portion 21. The tapered hole portion 22 has a tapered inner wall surface formed such that the inner diameter becomes smaller toward the tip portion 121 of the insulator 12.

The electrode base material 130 of the center electrode 13 has a main body portion 30 , a flange portion 31 , and a tapered portion 32 .
The main body portion 30 is a portion of the electrode base material 130 that is inserted into the second hole portion 21 .

The flange 31 is a portion of the electrode base material 130 that is provided at the base end of the main body 30. The flange 31 has an outer diameter larger than the main body 30 and is disposed inside the first hole 20. The first seal body 15 is disposed between the outer wall surface of the flange 31 and the inner wall surface of the first hole 20.

The tapered portion 32 is a portion of the electrode base material 130 that is disposed between the main body portion 30 and the flange portion 31. The tapered portion 32 has a tapered outer wall surface that faces the inner wall surface of the tapered hole portion 22 of the axial hole 120. As shown enlarged in the figure, the tapered portion 32 has a step portion 323 that is larger than the outer diameter of the main body portion 30 and smaller than the outer diameter of the flange portion 31. Hereinafter, in the tapered portion 32, the portion that is disposed closer to the main body portion 30 than the step portion 323 is referred to as the contact portion 321, and the portion that is disposed closer to the flange portion 31 than the step portion 323 is referred to as the separation portion 322. The first seal body 15 is disposed so as to be sandwiched between the separation portion 322 and the inner wall surface of the tapered hole portion 22. The corner portion provided between the contact portion 321 and the separation portion 322 in the tapered portion 32 is formed in an R shape. Similarly, the corners between the contact portion 321 of the tapered portion 32 and the main body portion 30, and the corners between the separation portion 322 of the tapered portion 32 and the flange portion 31 are also formed in an R shape.

Next, a part of the manufacturing process of the spark plug 10 of this embodiment, specifically, the process of assembling the center electrode 13 and the first seal body 15 into the axial hole 120 of the insulator 12 will be described.
When assembling the center electrode 13 and the first seal body 15 to the insulator 12, first, the center electrode 13 is inserted into the axial hole 120 from the opening of the base end portion 122 of the insulator 12. As a result, the contact portion 321 of the tapered portion 32 of the center electrode 13 comes into contact with the inner wall surface of the tapered hole portion 22 of the insulator 12, and in this state, the center electrode 13 is held inside the axial hole 120 of the insulator 12.

Next, as shown in FIG. 3 , powder corresponding to the first seal body 15 is poured into the inside of the axial hole 120 from the opening portion of the base end portion 122 of the insulator 12, and then the powder corresponding to the first seal body 15 is pressurized using a pressure pin 50.
As a result of the first seal body 15 being pressurized by the pressure pin 50 in this manner, a force is applied to the flange 31 of the center electrode 13 in a direction toward the tip 121 of the insulator 12. This force presses the tapered portion 32 of the center electrode 13 against the inner wall surface of the tapered hole 22 of the insulator 12, generating stress on the inner wall surface of the tapered hole 22 of the insulator 12. At this time, in the spark plug 10 of this embodiment, since the tapered portion 32 of the center electrode 13 is formed in a stepped shape, the stress applied to the insulator 12 can be reduced compared to the spark plug 100 of the reference example in which the tapered portion 32 of the center electrode 13 is not formed in a stepped shape as shown in FIG.

Specifically, when the tapered portion 32 of the center electrode 13 is not formed in a stepped shape as in the spark plug 100 of the reference example shown in Figure 4, the outer wall surface of the tapered portion 32 of the center electrode 13 comes into contact with the inner wall surface of the tapered hole 22 of the insulator 12. In this case, when the first seal body 15 is pressurized by the pressure pin 50, a force F10 shown in the figure is applied from the tapered portion 32 of the center electrode 13 to the inner wall surface of the tapered hole 22 of the insulator 12 due to the force applied to the flange portion 31 of the center electrode 13. As a result, the inner wall surface of the tapered hole portion 22 of the insulator 12 is pulled downward with the corner 23 formed at the boundary between the tapered hole portion 22 and the first hole portion 20 as a fulcrum, so that a tensile stress F21 is generated in the area around the corner 23 on the inner wall surface of the tapered hole portion 22, and a tensile stress F22 is generated in the area around the corner 23 in the first hole portion 20. Such tensile stresses F21 and F22 can cause damage to the insulator 12.

In this regard, in the spark plug 10 of this embodiment, the tapered portion 32 is formed with a contact portion 321 and a separation portion 322. Therefore, the force F10 applied from the tapered portion 32 of the center electrode 13 to the inner wall surface of the tapered hole portion 22 of the insulator 12 is divided into a force F11 applied from the contact portion 321 of the tapered portion 32 to the inner wall surface of the tapered hole portion 22, and a force F12 applied from the separation portion 322 of the tapered portion 32 to the first seal body 15. In other words, "F10 = F11 + F12".

The workload applied to the first seal body 15 by the force F12 is consumed as energy that generates heat due to friction between the particles that make up the first seal body 15 and plastic deformation of the particles. The force actually applied to the inner wall surface of the tapered hole portion 22 of the insulator 12 based on the force F12 is reduced by the amount of this energy consumption. In other words, if the coefficient α is defined as "α<1", the force applied from the tapered portion 32 of the center electrode 13 to the inner wall surface of the tapered hole portion 22 of the insulator 12 is "F11 + α × F12", which is lower than the above force F10. This reduces the tensile stress generated on the inner circumferential surfaces of the second hole portion 21 and the tapered hole portion 22 starting from the corner portion 23, thereby improving the robustness of the insulator 12.

In this manner, in the spark plug 10 of the present embodiment, the tapered portion 32 of the center electrode 13 is formed with a step portion 323 having an outer diameter larger than the outer diameter of the body portion 30 and smaller than the outer diameter of the flange portion 31. This allows the tapered portion 32 to be provided with a contact portion 321 that contacts the inner wall surface of the tapered hole portion 22 and a separation portion 322 that is disposed apart from the inner wall surface of the tapered hole portion 22. With this configuration, the first seal body 15 is sandwiched and disposed between the separation portion 322 of the tapered portion 32 of the center electrode 13 and the inner wall surface of the tapered hole portion 22. Therefore, compared to the spark plug 100 of the reference example that does not have such a structure, the force applied from the tapered portion 32 of the center electrode 13 to the inner wall surface of the tapered hole portion 22 of the insulator 12 can be reduced. As a result, the stress acting from the center electrode 13 on the inner wall surface of the tapered hole portion 22 of the insulator 12 can be reduced, making the insulator 12 less likely to be damaged.

The above embodiment can also be implemented in the following manner.
The shape of the tapered portion 32 of the center electrode 13 may be changed as appropriate. For example, the tapered portion 32 may be formed in a multi-step shape having a plurality of step portions 323, 324 with different outer diameters as shown in FIG.

Instead of forming the tapered portion 32 of the center electrode 13 in multiple steps, an expanded diameter portion 310 may be formed in the flange portion 31 of the center electrode 13, the outer diameter of which is partially increased, as shown in FIG. 6. The outer peripheral surface of the expanded diameter portion 310 is curved. Even with this configuration, it is possible to achieve the same or similar actions and effects as the spark plug 10 shown in FIG. 2.

- This disclosure is not limited to the specific examples above. Any design modifications made by a person skilled in the art to the specific examples above are also included within the scope of this disclosure as long as they have the features of this disclosure. The elements of each of the specific examples described above, as well as their arrangement, conditions, shape, etc., are not limited to those exemplified and can be modified as appropriate. The elements of each of the specific examples described above can be combined in different ways as appropriate, as long as no technical contradictions arise.

10: Spark plug 12: Insulator 13: Center electrode 14: Ground electrode 15: Seal body 20: First hole portion 21: Second hole portion 22: Tapered hole portion 30: Main body portion 31: Flange portion 32: Tapered portion 120: Shaft hole 323, 324: Step portion

Claims (1)

A cylindrical insulator (12) having an axial hole (120) formed therein;
a center electrode (13) inserted and held in the axial hole so as to protrude from the tip end of the insulator;
a ground electrode (14) disposed with a predetermined gap between it and the center electrode and configured to generate a spark discharge between the ground electrode and the center electrode upon application of a voltage to the center electrode;
a seal body (15) disposed inside the axial hole around a base end of the center electrode,
The shaft hole is
A first hole portion (20) in which the seal body is accommodated;
a second hole portion (21) disposed on a tip end side of the insulator with respect to the first hole portion and having an inner diameter smaller than that of the first hole portion;
a tapered hole portion (22) located between the first hole portion and the second hole portion and having a tapered inner wall surface formed so that the inner diameter becomes smaller toward the tip portion of the insulator,
The center electrode is
A main body portion (30) to be inserted into the second hole portion;
a flange portion (31) provided at a base end of the main body portion, having an outer diameter larger than that of the main body portion, and disposed inside the first hole portion;
a tapered portion (32) disposed between the main body portion and the flange portion and having a tapered outer wall surface facing an inner wall surface of the tapered hole portion of the axial hole,
The tapered portion has an outer diameter larger than the outer diameter of the main body portion and smaller than the outer diameter of the flange portion, and a plurality of stepped portions (323, 324) having different outer diameters are formed.

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