Spark plug

Abstract

To provide a spark plug that can increase the resistance of the central electrode to external forces. [Solution] The spark plug comprises an insulator having an axial hole extending from the tip to the rear end, a central electrode positioned in the axial hole with a portion protruding from the tip of the insulator, a main body fitting positioned on the outer circumference of the insulator, a ground electrode connected to the main body fitting and forming a spark gap with the central electrode, and cement fixing the outer surface of the rear end of the central electrode to the inner circumference of the insulator. The cement contains Al2O3, CaO, and Fe2O3, with Al2O3 accounting for 39 wt% or more of the cement, and the ratio of Al2O3 to CaO being 1.0 or more and 1.3 or less.

Description

Technical Field

[0001] The present invention relates to a spark plug in which a center electrode is fixed to an insulator with cement.

Background Art

[0002] A spark plug in which the outer peripheral surface of a center electrode is fixed to the inner periphery of an insulator with cement 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] For this type of spark plug, in order to prevent the axial position of the center electrode fixed to the insulator from changing, it is preferable that the resistance of the center electrode to an external force is as large as possible.

[0005] The present invention has been made to meet this requirement, and an object thereof is to provide a spark plug capable of increasing the resistance of the center electrode to an external force.

Means for Solving the Problems

[0006] A first aspect for achieving this object includes an insulator provided with a shaft hole extending from the tip toward the rear end side, a center electrode disposed in the shaft hole and partially protruding from the tip of the insulator, a main metal fitting disposed on the outer periphery of the insulator, a ground electrode connected to the main metal fitting and forming a spark gap between the center electrode, and cement for fixing the outer peripheral surface of the rear end side of the center electrode to the inner periphery of the insulator. The cement contains Al2O3, CaO, and Fe2O3, the ratio of Al2O3 in the cement is 39 wt% or more, and the ratio obtained by dividing the ratio of Al2O3 in the cement by the ratio of CaO is 1.0 or more and 1.3 or less.

[0007] In the second embodiment, the ratio of the proportion of Al2O3 in the cement to the proportion of CaO is 1.0 or more and 1.2 or less. [Effects of the Invention]

[0008] According to the present invention, the cement used to fix the outer surface of the rear end of the central electrode to the inner circumference of the insulator contains Al2O3, CaO, and Fe2O3, with Al2O3 accounting for 39 wt% or more of the cement, and the ratio of Al2O3 to CaO being between 1.0 and 1.3, thereby increasing the bonding strength of the cement. This increases the resistance of the central electrode to external forces. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional view of one side of a spark plug in one embodiment. [Figure 2] This is a magnified partial cross-sectional view of a spark plug. [Modes for carrying out the invention]

[0010] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Figure 1 is a one-sided cross-sectional view combining an outline view and a full cross-sectional view of a spark plug 10 in one embodiment, with the axis X as the boundary. The lower side of the paper in Figure 1 is referred to as the tip side of the spark plug 10, and the upper side of the paper is referred to as the rear end side of the spark plug 10 (the same applies to Figure 2). The spark plug 10 comprises an insulator 11, a center electrode 20, a main body fitting 34, and a ground electrode 35.

[0011] The insulator 11 is a cylindrical member provided with an axial hole 14 extending along the axis X, and is made of a ceramic such as alumina, which has excellent insulating and mechanical properties at high temperatures. The axial hole 14 extends from the tip 12 to the rear end 13 of the insulator 11 and includes a first part 15 that opens at the tip 12 of the insulator 11 and a second part 16 adjacent to the rear end of the first part 15. The second part 16 opens at the rear end 13 of the insulator 11, and the diameter of the second part 16 is larger than the diameter of the first part 15. The insulator 11 is provided with an internal thread near the rear end 13 of the second part 16.

[0012] A central electrode 20 is positioned at the tip end of the axial hole 14 of the insulator 11. The central electrode 20 is a rod-shaped conductor, with a core material having excellent thermal conductivity embedded in the base material. Examples of the base material include an alloy mainly composed of Ni or a metal made of Ni. Examples of the core material include copper or an alloy mainly composed of copper. The core material can be omitted.

[0013] The central electrode 20 includes a shaft portion 21 and a head portion 22 adjacent to the rear end of the shaft portion 21. The head portion 22 is thicker than the shaft portion 21. The shaft portion 21 is located in the first portion 15 of the shaft hole 14, and the head portion 22 is located in the second portion 16 of the shaft hole 14. Since the head portion 22, located in the second portion 16, cannot enter the first portion 15, the movement of the central electrode 20 toward the tip is restricted.

[0014] A portion of the shaft portion 21 protrudes from the tip 12 of the insulator 11. A tip containing a precious metal such as Pt, Ir, or Ru may be provided at the tip of the shaft portion 21, or the tip may be omitted.

[0015] A connecting portion 23 is joined to the rear end of the head 22 of the central electrode 20. The connecting portion 23 is a rod-shaped conductor shorter than the length of the second portion 16 of the axial hole 14 and is positioned within the second portion 16. There are no restrictions on the form of the joint between the connecting portion 23 and the head 22. Examples of joint forms include mechanical joints such as screws and press-fits, and metallurgical joints such as welding and pressure welding.

[0016] Cement 25 is filled into the gap between the head 22 of the central electrode 20 and the tip of the connecting portion 23 and the second portion 16 (the inner circumference of the insulator 11), and sealant 26 is filled into the gap between the connecting portion 23 and the second portion 16. The sealant 26 is adjacent to the rear end of the cement 25. The cement 25 fixes the head 22 of the central electrode 20 to the insulator 11. The sealant 26 is a component that prevents gas leakage from between the connecting portion 23 and the inner circumference of the insulator 11. The volume of the sealant 26 is larger than the volume of the cement 25.

[0017] The sealing material 26 is exemplified by a powder made of minerals or artificial inorganic materials. The powder preferably contains talc. This is because talc has low hardness and, when filled between the connecting portion 23 and the insulator 11, is crushed and broken into flakes, thus allowing the sealing material 26 made of powder to be made dense.

[0018] Figure 2 is an enlarged partial cross-sectional view of a part of the spark plug 10 (near the head 22). The cement 25 fixes the outer surface 24 of the rear end (head 22) of the center electrode 20 to the inner circumference of the insulator 11. The material of the cement 25 is alumina cement. Alumina cement is fast-setting, hardening in a short time, and is a refractory material with excellent heat resistance after hardening, making it suitable for fixing the center electrode 20 to the insulator 11.

[0019] Let's return to Figure 1 for explanation. A terminal fitting 30 is located on the rear end side of the second part 16 of the insulator 11. The terminal fitting 30 is a metal component to which an ignition device (not shown) is connected. Low carbon steel is an example of a material for the terminal fitting 30. The terminal fitting 30 includes a rod-shaped terminal 31, a flange 32 extending around the terminal 31, and a bottomed cylindrical connecting portion 33 protruding coaxially from the flange 32 to the opposite side of the terminal 31.

[0020] A screw (male thread) is provided on the outer periphery of the connecting portion 33, and the male thread fits into the female thread provided on the second portion 16 of the insulator 11. Since the extension of the flange 32 is larger than the diameter of the second portion 16, the flange 32 is abutted against the rear end 13 of the insulator 11, and the male thread of the connecting portion 33 is tightened against the female thread of the insulator 11 to couple the connecting portion 33 to the insulator 11. It is preferable to adhere between the male thread of the connecting portion 33 and the female thread of the insulator 11 using an inorganic adhesive such as cement, as it can reduce loosening of the screw.

[0021] The inner peripheral surface of the connecting portion 33 of the terminal fitting 30 disposed on the insulator 11 contacts the outer peripheral surface of the rear end side of the connecting portion 23, and electrical connection is established between the central electrode 20 to which the connecting portion 23 is joined and the terminal fitting 30. Since there is a gap between the bottom of the connecting portion 33 and the rear end of the connecting portion 23, elongation due to thermal expansion of the central electrode 20 and the connecting portion 23 is allowed.

[0022] The main body fitting 34 is a substantially cylindrical member formed of a conductive metal material (e.g., low carbon steel, etc.). The main body fitting 34 is disposed on the outer periphery of the insulator 11. The ground electrode 35 is a rod-shaped conductor connected to the main body fitting 34. A spark gap 36 is formed between the ground electrode 35 and the central electrode 20. A spark gap 36 is provided between the tip of the shaft portion 21 of the central electrode 20 and the ground electrode 35, but it is not limited thereto. A spark gap 36 may be provided between the side surface of the shaft portion 21 of the central electrode 20 and the ground electrode 35. A plurality of ground electrodes 35 may be connected to the main body fitting 34.

[0023] The spark plug 10 is manufactured, for example, by the following method. First, the central electrode 20 to which the connecting portion 23 is connected is inserted into the shaft hole 14 from the rear end 13 of the insulator 11. Next, water is added to the powder of the cement 25 before coagulation and kneaded to form a cement paste, which is put into the shaft hole 14 and tamped with a push rod (not shown) to fill the periphery of the head portion 22 of the central electrode 20 with the cement paste. After curing under a wet condition and at a constant temperature, the powder of the sealing material 26 is put into the shaft hole 14 and tamped with a push rod to fill the powder around the connecting portion 23.

[0024] Next, the connecting portion 23 is brought into contact with the connection portion 33 of the terminal fitting 30, and the terminal fitting 30 is attached to the rear end 13 of the insulator 11. Then, the main fitting 34 to which the ground electrode 35 is connected is assembled to the outer circumference of the insulator 11, the ground electrode 35 is bent, and a spark gap 36 is set between the ground electrode 35 and the center electrode 20 to obtain the spark plug 10.

[0025] Cement 25 contains Al2O3, CaO, and Fe2O3. Cement 25 also contains SiO2. Cement 25 may also contain FeO, MgO, TiO2, and Mn2O3. Chemical analysis of cement 25 shall be performed in accordance with the method specified in JIS R2522:1995.

[0026] The proportion of Al2O3 in cement 25 is 39 wt% or more. The ratio R, obtained by dividing the proportion of Al2O3 in cement 25 by the proportion of CaO, is between 1.0 and 1.3, with a ratio of 1.0 and 1.2 being more preferable. This is because setting the ratio R to an appropriate value increases the bonding strength of cement 25.

[0027] The proportion of Fe2O3 in cement 25 is preferably 11 wt% or more, and more preferably 13 wt% or more. This is because the presence of iron salt crystals such as 4CaO·Al2O3·Fe2O3 and 2CaO·Fe2O3, along with the calcium aluminate formed in cement 25 by hydration, can greatly increase the bonding strength of cement 25.

[0028] As the force of the cement 25 fixing the center electrode 20 to the insulator 11 increases, the resistance of the center electrode 20 to external forces increases. Since the center electrode 20 fixed to the insulator 11 becomes less likely to be displaced toward the rear end, the expansion of the spark gap 36 due to the displacement of the center electrode 20 toward the rear end can be reduced. This prevents an increase in the required voltage to generate a discharge in the spark gap 36. [Examples]

[0029] The present invention will be described in more detail by reference to examples, but the present invention is not limited to these examples.

[0030] Samples No. 1-5 were prepared by varying the proportion of Al2O3 in alumina cement containing Al2O3, CaO, Fe2O3, and SiO2, and the ratio of Al2O3 to CaO. For example, the chemical composition of sample No. 2 was SiO2: 4.5%, Al2O3: 42.0%, CaO: 36.6%, Fe2O3: 15.0%, with the remainder being TiO2 and MgO. Samples No. 1-5 were kept nearly constant in terms of chemical composition and particle size distribution, except for the proportions of Al2O3 and CaO.

[0031] Table 1 shows the proportion of Al2O3 in cement samples No. 1-5, and the ratio of Al2O3 to CaO. The ratios were rounded to two decimal places.

[0032] [Table 1]

[0033] In the embodiment, the central electrode 20, to which the connecting portion 23 is attached, was inserted into the axial hole 14 of the insulator 11. Then, cement paste, made by mixing cement powder with water, was placed in the axial hole 14 and compacted with a tamping rod to fill the area around the head 22 of the central electrode 20 with cement paste. In samples No. 1-5, the amount of water added to the cement and the amount of cement paste filled into the axial hole 14 were kept constant. After that, the cement paste was cured and the cement was allowed to harden, and samples were obtained in which the central electrode 20 was fixed to the insulator 11 with the cement in sample No. 1-5.

[0034] The rear end 13 of the insulator 11 was placed on the test stand, and the tip of the central electrode 20 was pushed at a speed of 2 mm / min in the direction of axis X (towards the rear end 13 of the insulator 11). The load (N) at which the cement fixing the central electrode 20 to the insulator 11 broke was measured. The results are shown in Table 1. Samples with a load of 1400 N or more were judged as A, samples with a load of 1350 N or more but less than 1400 N were judged as B, and samples with a load of less than 1350 N were judged as C.

[0035] According to Table 1, samples No. 1-4, in which the proportion of Al2O3 is 39 wt% or more and the ratio of Al2O3 to CaO is between 1.0 and 1.3, were found to be able to achieve a greater load (adhesion force of the cement) compared to sample No. 5, in which the ratio of Al2O3 to CaO was 1.4.

[0036] In particular, samples No. 1-3, where the ratio of Al2O3 to CaO was between 1.0 and 1.2, showed that they could achieve even greater cement bonding strength compared to sample No. 4, where the ratio was 1.3.

[0037] Although the present invention has been described above based on embodiments, it can be easily inferred that the present invention is not limited in any way to the above embodiments, and that various improvements and modifications are possible without departing from the spirit of the present invention.

[0038] In the embodiment, a spark plug 10 in which the ground electrode 35 is exposed to the combustion chamber when the spark plug 10 is attached to an engine (not shown) has been described, but it is not necessarily limited to this. It is certainly possible to apply the configuration of the embodiment to a spark plug in which the ground electrode 35 is covered with a cap that has a through hole (a spark plug that has a sub-chamber in the combustion chamber). [Explanation of Symbols]

[0039] 10 Spark plugs 11 Insulator 12 Tip 13 Rear end 14 Shaft holes 20 center electrode 24 Outer surface 25 Cement 34 Main fittings 35 Ground electrode 36 Spark Gap

Claims

1. An insulator having an axial hole extending from the tip towards the rear end, A central electrode is disposed in the axial hole and a portion of it protrudes from the tip, The main metal fitting arranged on the outer circumference of the insulator, A ground electrode connected to the main body fitting and forming a spark gap between itself and the central electrode, A spark plug comprising cement that fixes the outer surface of the rear end of the central electrode to the inner circumference of the insulator, The aforementioned cement is Al ２ O ３ CaO and Fe ２ O ３ Includes, Al in the aforementioned cement ２ O ３ The proportion is 39 wt% or more. Al in the aforementioned cement ２ O ３ A spark plug in which the ratio of the amount of to the amount of CaO is between 1.0 and 1.

3.

2. The spark plug according to claim 1, wherein the ratio is 1.0 or more and 1.2 or less.

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