Spark plug

Abstract

To provide a spark plug which can be used for a long period of time without causing degradation in manufacturing efficiency.SOLUTION: A spark plug 10 comprises: a resistor 70 which is disposed, along a center axis CX of a shaft hole 200, at a position between a center electrode 30 and a terminal fitting 40; a conductive seal layer 90 which is disposed, along the center axis CX, at a position between the resistor 70 and the terminal fitting 40; and an intermediate layer 91 which is disposed, along the center axis CX, at a position between the resistor 70 and the conductive seal layer 90. The intermediate layer 91 has a thermal expansion coefficient higher than that of the resistor 70 but lower than that of the conductive seal layer 90.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to spark plugs. [Background technology]

[0002] For example, an internal combustion engine mounted on a vehicle is provided with a spark plug for igniting fuel. The spark plug has a center electrode and a ground electrode that are spaced apart from each other. When igniting fuel, a high voltage is applied between the center electrode and the ground electrode, causing a spark discharge between the two electrodes.

[0003] As described in Patent Document 1 below, a resistor is generally placed inside a spark plug to suppress the generation of electromagnetic noise caused by spark discharge. In a spark plug with such a configuration, the electrical resistance of the resistor gradually increases as spark discharge is repeated, and it may become impossible to obtain sufficient ignition performance within a short period of time.

[0004] Therefore, in the spark plug described in Patent Document 1 below, the length of the resistor is set within a predetermined range to suppress deterioration of the resistor. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-183163 A Summary of the Invention [Problem to be solved by the invention]

[0006] In a spark plug, a terminal metal fitting and a resistor are generally connected via a conductive seal layer. The conductive seal layer is, for example, an electrically conductive layer formed of glass containing copper powder. When manufacturing the spark plug, the resistor, the conductive seal layer, the terminal metal fitting, and the insulator that holds them are all heated to soften the glass of the conductive seal layer, etc. Then, the whole is cooled to solidify the glass of the conductive seal layer, etc. This results in an electrical connection between the resistor and the terminal metal fitting via the conductive seal layer.

[0007] The temperature during the above heating is higher than the temperature when the spark plug is used in an internal combustion engine. Therefore, when the conductive seal layer and the like are cooled after heating, a relatively large tensile stress is applied, for example, near the boundary between the resistor and the conductive seal layer due to the difference in thermal expansion coefficient. If the stress becomes too large, cracks may occur in the material along the electric path from the terminal metal fitting to the center electrode. As a result, the current does not flow uniformly, and the life of the spark plug may be shortened due to localized heat generation. In other words, the spark plug may not be usable for a long period of time.

[0008] One possible method to solve this problem is to control the rate at which the temperature is lowered during the cooling process when manufacturing the spark plugs. However, this method requires a long time for cooling, which reduces production efficiency. In addition, equipment for controlling the rate at which the temperature is lowered is also required.

[0009] An object of the present disclosure is to provide an ignition plug that can be used for a long period of time without reducing production efficiency. [Means for solving the problem]

[0010] The spark plug (10) according to the present disclosure includes an insulator (20) having an axial hole (200), a center electrode (30) held by the insulator at one end along the central axis (CX) of the axial hole, a terminal fitting (40) held by the insulator at the other end along the central axis of the axial hole, a resistor (70) disposed along the central axis of the axial hole between the center electrode and the terminal fitting, a conductive seal layer (90) disposed along the central axis of the axial hole between the resistor and the terminal fitting, and an intermediate layer (91) disposed along the central axis of the axial hole between the resistor and the conductive seal layer. The thermal expansion coefficient of the intermediate layer is greater than that of the resistor and less than that of the conductive seal layer.

[0011] In the spark plug having such a configuration, the resistor and the conductive seal layer are not directly connected to each other, but are connected to each other via an intermediate layer provided between them. The thermal expansion coefficient of the intermediate layer is larger than that of the resistor and smaller than that of the conductive seal layer. By providing such an intermediate layer between the resistor and the conductive seal layer, the stress caused by the difference in thermal expansion coefficient during the manufacture of the spark plug, specifically, during cooling to solidify the conductive seal layer, can be suppressed to a smaller value than in the past. This makes it difficult for cracks to occur in the material of the conductive seal layer or its vicinity, making it possible to use the spark plug for a long period of time. During cooling, there is no need to manage the temperature drop rate, so there is no decrease in production efficiency. Effect of the Invention

[0012] According to the present disclosure, there is provided an ignition plug that can be used for a long period of time without reducing production efficiency. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the internal configuration of an ignition plug according to an embodiment. [Diagram 2] FIG. 2 is an enlarged view showing a partial configuration of the ignition plug according to the embodiment. [Diagram 3] FIG. 3 is a diagram showing a method for manufacturing the spark plug according to the embodiment. [Figure 4] FIG. 4 is a diagram showing a method for manufacturing the ignition plug according to the embodiment. [Diagram 5] FIG. 5 is a diagram for explaining the configuration of an ignition plug according to a comparative example and its problems. [Figure 6] FIG. 6 is a graph showing the thermal expansion coefficients of the resistor and the conductive seal layer at each temperature. [Figure 7] FIG. 7 is a graph showing the relationship between the thickness of the intermediate layer and the failure rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the present embodiment will be described with reference to the accompanying drawings. In order to facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals as much as possible, and duplicated description will be omitted.

[0015] The configuration of an ignition plug 10 according to this embodiment will be described with reference to Fig. 1. In Fig. 1, a cross section of the ignition plug 10 taken along a plane including a central axis CX (described later) is shown on the left side. However, of the components constituting the ignition plug 10, the center electrode 30 and the terminal fitting 40 are shown not in cross section but in their respective external appearances.

[0016] The ignition plug 10 is a device provided in each cylinder of an internal combustion engine (not shown) for igniting fuel in the combustion chamber of the cylinder. The ignition plug 10 includes an insulator 20, a center electrode 30, a terminal metal fitting 40, a metal shell 50, and a ground electrode 60.

[0017] The insulator 20 is a cylindrical member made of an insulating material such as alumina. An axial hole 200 is formed in the insulator 20. The axial hole 200 is a through hole formed to pass through the insulator 20 along its central axis. The central axis of the axial hole 200 coincides with the central axis of the insulator 20. The central axis of the axial hole 200 is hereinafter also referred to as the "central axis CX." In a cross section of the insulator 20 cut perpendicularly to the central axis CX, the shape of the axial hole 200 is circular.

[0018] The center electrode 30 is a metal member held by the insulator 20 at a position that is one end of the axial hole 200 along the central axis CX (the lower end in FIG. 1). The center electrode 30 is a rod-shaped member, most of which is disposed inside the axial hole 200. A part of the center electrode 30 protrudes from the axial hole 200 to the outside of the insulator 20, and a discharge tip 31 is attached to the tip of the protruding part. The insulator 20 described above can be said to be a member that holds the center electrode 30 from the outer periphery.

[0019] The terminal fitting 40 is a metal member held by the insulator 20 at a position that is the other end of the axial hole 200 along the central axis CX (the upper end in FIG. 1). The terminal fitting 40 is a rod-shaped member, most of which is disposed inside the axial hole 200. A part of the terminal fitting 40 protrudes from the axial hole 200 to the outside of the insulator 20. This protruding part serves as an electrode terminal to which a voltage is applied from an external power source (not shown).

[0020] The side of the insulator 20 where the center electrode 30 is attached along the central axis CX is also referred to as the "front end side" below. Also, the side of the insulator 20 where the terminal fitting 40 is attached along the central axis CX is also referred to as the "rear end side" below.

[0021] A resistor 70 is disposed in the axial hole 200 at a position between the center electrode 30 and the terminal fitting 40 along the central axis CX. The resistor 70 is a member disposed to adjust the electrical resistance of the electric path from the terminal fitting 40 to the center electrode 30. The configuration of the resistor 70 and its vicinity will be described later.

[0022] The metal shell 50 is a tubular member that holds a part of the insulator 20 from the outer periphery. The metal shell 50 is entirely made of metal. The metal shell 50 is fixed to the insulator 20 by crimping, and holds the insulator 20 in this state. The metal shell 50 has a fitting portion 52, a flange portion 55, and an insertion portion 56.

[0023] The fitting portion 52 is a portion that is fitted with a tool such as a plug wrench when the spark plug 10 is attached to an internal combustion engine. The shape of the fitting portion 52 when viewed along the central axis CX is, for example, hexagonal.

[0024] The flange portion 55 is a portion that comes into contact with the outer surface of the internal combustion engine via the gasket GK when the spark plug 10 is attached to the internal combustion engine. The flange portion 55 is provided at a position that is closer to the tip side than the fitting portion 52, and protrudes toward the outer periphery.

[0025] The insertion portion 56 is a portion further toward the tip side than the flange portion 55, and is a portion to be inserted into an insertion hole (not shown) formed in an internal combustion engine. A male screw 561 is formed on the outer circumferential surface of the insertion portion 56. When the spark plug 10 is attached to the internal combustion engine, the fitting portion 52 rotates about the central axis CX due to the force received from a tool. As a result, a female screw formed on the inner circumferential surface of the insertion hole and the male screw 561 of the insertion portion 56 are screwed together. As a result, the spark plug 10 is fastened and fixed to the internal combustion engine. When the spark plug 10 is attached to the internal combustion engine, the potential of the metallic shell 50 becomes the same ground potential as that of the internal combustion engine.

[0026] The ground electrode 60 is a metal member formed so as to extend from the tip surface S on the tip side of the metallic shell 50 toward the tip side. The tip surface S is a surface perpendicular to the central axis CX. The ground electrode 60 is bent, and a part of it faces the discharge tip 31 of the center electrode 30 along the central axis CX. In other words, one end of the ground electrode 60 is connected to the tip surface S of the metallic shell 50, and the other end faces the center electrode 30. The discharge tip 61 is attached to a part of the ground electrode 60 facing the center electrode 30. As a result, the discharge tip 61 and the discharge tip 31 are lined up along the central axis CX and face each other. The gap formed between the discharge tip 61 and the discharge tip 31 facing each other is a discharge gap GP where spark discharge occurs.

[0027] When the internal combustion engine is in operation, a pulsed high voltage is applied between the terminal metal fitting 40 of the spark plug 10 and the body of the internal combustion engine. This high voltage is applied between the opposing discharge tips 31 and 61, causing a spark discharge in the discharge gap GP.

[0028] The discharge tip 31 is joined to the base material, the center electrode 30, by welding. Similarly, the discharge tip 61 is joined to the base material, the ground electrode 60, by welding. The discharge tip 31 is a precious metal tip formed from a material containing the precious metal iridium or platinum as a main component.

[0029] The resistor 70 and the structure in the vicinity thereof will be described with reference to Fig. 2. Fig. 2 shows a cross section of the spark plug 10 taken along a plane including the central axis CX. As in Fig. 1, Fig. 2 also shows the external appearance of the center electrode 30 and the terminal metal fitting 40 rather than a cross section. Note that Fig. 2 omits the illustration of the metal shell 50 that holds the insulator 20 from the outside.

[0030] As described above, the resistor 70 is disposed as a member for adjusting the electrical resistance of the electrical path from the terminal fitting 40 to the center electrode 30. The resistor 70 is formed from a material in which a predetermined amount of carbon powder is added to powdered glass and zirconia. The electrical resistance of the resistor 70 is adjusted by the amount of carbon added. By disposing the resistor 70 in the electrical path from the terminal fitting 40 to the center electrode 30, the generation of electromagnetic noise accompanying the spark discharge of the ignition plug 10 is suppressed.

[0031] A conductive seal layer 80 is provided in the axial hole 200 at a position between the center electrode 30 and the resistor 70 along the central axis CX. That is, the center electrode 30 and the resistor 70 are connected to each other via the conductive seal layer 80. The conductive seal layer 80 is a conductive layer formed of a material in which copper powder is added to powdered glass. With this configuration, the center electrode 30 and the resistor 70 are electrically connected to each other.

[0032] A conductive seal layer 90 is provided in a portion of the axial hole 200 that is between the resistor 70 and the terminal fitting 40 along the central axis CX. An intermediate layer 91 is provided in a portion of the axial hole 200 that is between the resistor 70 and the conductive seal layer 90 along the central axis CX. In other words, the resistor 70 and the terminal fitting 40 are connected to each other via the intermediate layer 91 and the conductive seal layer 90 in this order from the tip side.

[0033] The conductive seal layer 90 is formed of the same material as the conductive seal layer 80, that is, a material in which copper powder is added to powdered glass. The intermediate layer 91 is formed of a material in which the material of the resistor 70 and the material of the conductive seal layer 90 are mixed. The mixture ratio is 30:70 by weight. That is, the proportion of "the material of the resistor 70" contained in the intermediate layer 91 is 30% by weight in this embodiment. With this configuration, the resistor 70 and the terminal fitting 40 are electrically connected to each other.

[0034] A method for manufacturing the spark plug 10 will be described with reference to Fig. 3 and Fig. 4. First, as shown in Fig. 3(A), the center electrode 30 is attached to the insulator 20. At this time, the insulator 20 is held in advance so that its rear end is on the upper side and its front end is on the lower side. The center electrode 30 is inserted into the axial hole 200 of the insulator 20 from its rear end side, and falls due to gravity and moves to the front end side. At this time, the enlarged diameter portion 32 (see Fig. 2) provided on the center electrode 30 abuts against the reduced diameter portion 201 provided midway through the axial hole 200, and the center electrode 30 is held in place.

[0035] Next, as shown in Fig. 3(B), the material of the conductive seal layer 80 is filled into the inside of the axial hole 200 from the rear end side. This material is a material in which copper powder is added to powdered glass, and is still in powder form at the time of Fig. 3(B). Thereafter, as shown in Fig. 3(C), a rod-shaped member 100 is inserted into the inside of the axial hole 200 from the rear end side, and the material of the conductive seal layer 80 is compressed by the tip of the rod-shaped member 100. This increases the density of the powdered material.

[0036] Next, as shown in Fig. 3(D), the material of resistor 70 is filled into axial hole 200 from the rear end side. This material is a material in which a predetermined amount of carbon powder is added to powdered glass and zirconia, and is still in powder form at the time of Fig. 3(D). After that, rod-shaped member 100 is inserted again from the rear end side into axial hole 200, and the material of resistor 70 is compressed by the tip of rod-shaped member 100. This increases the density of the powdered material.

[0037] The filling and pressurization of the resistor 70 material are carried out in two separate steps. FIG. 3(E) shows a state in which the second filling of the material is carried out and the material is being pressurized. The material of the resistor 70 filled in the first step and the material of the resistor 70 filled in the second step may be different from each other, or may be the same as each other. Also, the filling and pressurization of the resistor 70 material may be carried out in one step instead of being carried out in two separate steps.

[0038] Next, as shown in FIG. 3(F), the material of the intermediate layer 91 is filled into the inside of the axial hole 200 from the rear end side. The material is formed by mixing the powder material of the resistor 70 and the powder material of the conductive seal layer 90, which is filled next. As described above, the mixing ratio is 30:70 by weight. The material of the intermediate layer 91 is also in a powder state at the time of FIG. 3(F). After that, the rod-shaped member 100 is inserted into the inside of the axial hole 200 from the rear end side, and the material of the intermediate layer 91 is pressurized by the tip of the rod-shaped member 100. This increases the density of the powder material.

[0039] Next, as shown in Fig. 3(G), the material of the conductive seal layer 90 is filled into the inside of the axial hole 200 from the rear end side. This material is the same as the powder material of the conductive seal layer 80 filled in Fig. 3(B). After that, a rod-shaped member 100 is inserted into the inside of the axial hole 200 from the rear end side, and the material of the conductive seal layer 90 is pressurized by the tip of the rod-shaped member 100. This increases the density of the powder material.

[0040] 3(H), the terminal fitting 40 is inserted into the axial hole 200 from the rear end side. The terminal fitting 40 is formed with a flange 41 that abuts against the rear end side end of the insulator 20. However, when the front end side end of the terminal fitting 40 abuts against the conductive seal layer 90 as shown in FIG. 3(H), the flange 41 and the rear end side end of the insulator 20 do not abut against each other, and a gap is formed between them.

[0041] The insulator 20 with the terminal fitting 40 inserted as described above is held by the holding member 120 as shown in Fig. 4. The holding member 120 has a through hole 121 that passes through it vertically, and the insulator 20 is inserted into this through hole 121 from above.

[0042] A protruding portion 21 that protrudes outward is formed on the outer peripheral surface of the insulator 20. A protruding portion 122 that protrudes inward is formed on the inner peripheral surface of the through hole 121. The protruding portion 21 of the insulator 20 abuts against the protruding portion 122 of the holding member 120 from above, so that the insulator 20 is supported from below.

[0043] A downward force is applied to the rear end of the terminal fitting 40 by the pressure member 110. The powder material filled inside the axial hole 200 is compressed by the terminal fitting 40.

[0044] In this state, the entire insulator 20 is heated, for example, by a heater. The materials filled inside the axial hole 200 are heated together with the insulator 20, etc., and the temperature is raised to about 800° C. to 900° C. Even after the temperature of the insulator 20, etc. reaches the target temperature, the state in which the materials such as the intermediate layer 91 are pressed between the pressing member 110 and the holding member 120 is maintained.

[0045] The materials filled inside the axial hole 200 become fluid as the glass melts, and are compressed by the terminal fitting 40. The air contained in the powder materials is pushed out. Also, part of the material of the conductive seal layer 90 that has become fluid enters the gap between the outer surface of the terminal fitting 40 and the inner surface of the axial hole 200, and invades toward the rear end. As a result, the terminal fitting 40 gradually moves toward the front end. Finally, as shown in FIG. 4, the flange 41 abuts against the rear end of the insulator 20.

[0046] The boundary surfaces between adjacent materials among the materials filled inside the axial hole 200 are initially flat surfaces perpendicular to the central axis CX. However, after the terminal fitting 40 moves toward the tip end, the outer peripheral portion becomes inclined toward the rear end as part of the material of the conductive seal layer 90 penetrates toward the rear end as described above. In the central portion through which the central axis CX passes, the boundary surfaces between the materials remain flat surfaces perpendicular to the central axis CX.

[0047] After the flange 41 abuts against the rear end of the insulator 20, heating of the insulator 20 is stopped. The temperatures of the insulator 20 and the materials filled inside the axial hole 200 are gradually lowered. At this time, cooling air is supplied to the insulator 20 by, for example, a fan (not shown). This promotes the temperature drop of the insulator 20 and the like.

[0048] As the glass hardens, the fluidity of each material filled inside the axial hole 200 decreases. Eventually, the materials become one mass, connecting the center electrode 30 and the terminal fitting 40.

[0049] After cooling is completed, the insulator 20 is removed from the holding member 120. Thereafter, the metal shell 50 is crimped and fixed to the insulator 20. In this way, the ignition plug 10 shown in FIG. 1 is completed.

[0050] In the spark plug 10 of this embodiment manufactured through the above-mentioned steps, the resistor 70 and the conductive seal layer 90 are not directly connected to each other, but are connected to each other via an intermediate layer 91 disposed therebetween. In order to explain the advantages of such a configuration, a spark plug 10A according to a comparative example will be described with reference to Fig. 5. Fig. 5 shows the resistor 70 and its surrounding structure of the spark plug 10A in a manner similar to that shown in Fig. 2.

[0051] 5, in this comparative example, the intermediate layer 91 is not provided, and the resistor 70 is directly connected to the conductive seal layer 90. The spark plug 10A having such a configuration is also manufactured by a method similar to that described with reference to FIGS.

[0052] In the process of manufacturing the spark plug 10A, in the step of cooling the insulator 20 in a state similar to that shown in FIG. 4, the temperatures of the resistor 70 and the conductive seal layer 90 inside the insulator 20 also gradually decrease.

[0053] The conductive seal layer 90 containing copper powder has a higher thermal conductivity than the resistor 70 containing zirconia or the like, and therefore heat is easily taken away from the upper terminal fitting 40. Furthermore, in the above cooling process, the conductive seal layer 90 is located closer to the open end of the through hole 121 than the resistor 70 (or a part of the conductive seal layer 90 is outside the through hole 121), and is easily cooled by the surrounding air. Due to the above factors, the conductive seal layer 90 reduces its temperature faster than the resistor 70. Since the amount of contraction of the conductive seal layer 90 due to the temperature reduction is greater than the amount of contraction of the resistor 70, a tensile stress as shown by the arrow AR1 in FIG. 5 is applied near the boundary between the conductive seal layer 90 and the resistor 70.

[0054] 4 enters into the gap on the outer surface side of the terminal fitting 40. This portion of the conductive seal layer 90 is compressed less than the portion on the tip side, and therefore tends to have a larger thermal expansion coefficient and thermal conductivity. Therefore, the conductive seal layer 90 in the gap on the outer surface side of the terminal fitting 40 shrinks significantly, and the tensile stress described above further increases.

[0055] If the tensile stress becomes too large, there is a possibility that cracks will occur in the material along the electrical path from the terminal fitting 40 to the center electrode 30. If such a crack occurs, the current will not flow uniformly, and localized heat will be generated, increasing the resistance at that location, accelerating the deterioration of the material. As a result, the life of the spark plug 10A may be shortened. In other words, there is a possibility that the spark plug 10A cannot be used for a long period of time.

[0056] The type of oxide contained in the glass of the resistor 70, the conductive seal layer 90, etc. housed inside the insulator 20 is adjusted so that the thermal expansion coefficient of the resistor 70 and the conductive seal layer 90 roughly matches the thermal expansion coefficient of the insulator 20. Line L1 in Fig. 6 is a graph showing the relationship between the temperature and the thermal expansion coefficient of the resistor 70, and line L2 in Fig. 6 is a graph showing the relationship between the temperature and the thermal expansion coefficient of the conductive seal layer 90.

[0057] As shown in each graph, in the temperature range of 470°C or less, which is the operating temperature range when an ignition plug is normally used, the thermal expansion coefficient of resistor 70 and the thermal expansion coefficient of conductive seal layer 90 are roughly the same. However, in the temperature range higher than 470°C, the respective thermal expansion coefficients begin to diverge, and in the temperature range (approximately 800°C to 900°C) when heated in the state shown in Fig. 4, the thermal expansion coefficient (L2) of conductive seal layer 90 becomes larger. Therefore, in the above cooling process, the amount of contraction of conductive seal layer 90 becomes even smaller, making the cracks in the material described above even more likely to occur.

[0058] The glass selected for the resistor 70 must have low reactivity with zirconia, etc., and the glass selected for the conductive seal layer 90 must have low reactivity with copper powder. As a result, it is necessary to use different glasses for the resistor 70 and the conductive seal layer 90, and it is difficult to make the thermal expansion coefficients of the two the same over the entire temperature range.

[0059] As described above, the spark plug 10A according to the comparative example is configured such that tensile stress is likely to occur near the boundary between the resistor 70 and the conductive seal layer 90 during the manufacturing process in which the insulator 20 is cooled in the same state as in FIG. 4. For this reason, in order to prevent cracks in the material caused by tensile stress, the temperature drop rate must be controlled during the cooling process, for example by adjusting the number of rotations of a fan. However, controlling the temperature drop rate increases the time required for cooling, reducing production efficiency. In addition, additional equipment, such as a control device for controlling the temperature drop rate, is required.

[0060] 4, the cooling of the multiple insulators 20 is performed simultaneously while the multiple insulators 20 are lined up. Therefore, the temperature drop rate needs to be controlled simultaneously for each of the multiple insulators 20, but the appropriate cooling conditions differ depending on the position of the insulator 20. Therefore, there is also the problem that it is difficult to control the temperature drop rate itself.

[0061] In the spark plug 10 according to the present embodiment, the above problem is solved by interposing an intermediate layer 91 between the resistor 70 and the conductive seal layer 90. As described above, the intermediate layer 91 is formed of a material obtained by mixing the material of the resistor 70 and the material of the conductive seal layer 90. Therefore, when the insulator 20 and its inside are at a high temperature of about 800° C., that is, when the insulator 20 is cooled in the state shown in FIG. 4, the thermal expansion coefficient of the intermediate layer 91 is larger than that of the resistor 70 and smaller than that of the conductive seal layer 90. Moreover, the thermal conductivity of the intermediate layer 91 at this time is larger than that of the resistor 70 and smaller than that of the conductive seal layer 90.

[0062] Since such intermediate layer 91 is interposed between resistor 70 and conductive seal layer 90, in the manufacturing process of spark plug 10 according to this embodiment, in the step of cooling insulator 20 in the state shown in Fig. 4, intermediate layer 91 functions to mitigate differences in thermal expansion and thermal conduction between the upper and lower members. As a result, even without controlling the temperature drop rate in the cooling process, it is possible to prevent cracks in the material, and it becomes possible to use spark plug 10 for a long period of time.

[0063] The above-mentioned magnitude relationship between the thermal expansion coefficients of the resistor 70, the intermediate layer 91, and the conductive seal layer 90 and the above-mentioned magnitude relationship between the thermal conductivities need only be satisfied in a temperature range of at least 800° C. to 900° C. The intermediate layer 91 having the above-mentioned characteristics may be made of a material obtained by mixing the material of the resistor 70 and the material of the conductive seal layer 90 as in this embodiment, but may also be made of a different material.

[0064] The present inventors conducted the following experiment to confirm the effect of providing the intermediate layer 91. First, a plurality of spark plugs 10 were produced, each having a different material for the intermediate layer 91. The material for the intermediate layer 91 in each of the spark plugs 10 was a mixture of the material for the resistor 70 and the material for the conductive seal layer 90, as in the present embodiment, but the mixture ratios were different from each other. In each of the spark plugs 10, the thickness of the conductive seal layer 80 at the central axis CX was 1 mm, the thickness of the resistor 70 at the central axis CX was 18 mm, the thickness of the intermediate layer 91 at the central axis CX was 1 mm, and the thickness of the conductive seal layer 90 at the central axis CX was 2 mm.

[0065] The above-mentioned thickness "at the central axis CX" refers to the length along the central axis CX of the portion of each layer through which the central axis CX passes.

[0066] Thereafter, an operational test was conducted on each of the spark plugs 10 to check for the presence or absence of cracks in the material between the terminal fitting 40 and the center electrode 30. The operational test was conducted by placing the spark plugs 10 in a furnace at 300°C and repeatedly applying a voltage of 35 V between the discharge tip 31 and the discharge tip 61 to generate a spark discharge.

[0067] The results of the above experiments are shown in Table 1 below. [Table 1] The "Mixing Ratio" column on the left side of Table 1 indicates the weight ratio between the material of the resistor 70 contained in the intermediate layer 91 and the material of the conductive seal layer 90. In this experiment, ten spark plugs 10 each having the same mixing ratio in the intermediate layer 91 were produced, and the above-mentioned operation test was performed on all of the spark plugs 10.

[0068] The "Cracks" column in the center of Table 1 indicates whether or not any of the spark plugs 10 developed cracks after the performance test. For each mixture ratio, if at least one of the ten spark plugs 10 produced had a crack, it is marked "Yes," and if none of the ten spark plugs 10 had a crack, it is marked "No."

[0069] The "Crack Occurrence Rate" column on the right side of Table 1 shows, in "%", the percentage of spark plugs 10 that developed cracks after operational testing out of the 10 spark plugs 10 created for each mixture ratio.

[0070] As shown in Table 1, when the above-mentioned mixture ratio was 1:9 or 2:8, i.e., when the ratio of the material of the resistor 70 contained in the intermediate layer 91 was 20% by weight or less, cracks occurred in some of the ignition plugs 10. Similarly, when the above-mentioned mixture ratio was 8:2 or 9:1, i.e., when the ratio of the material of the resistor 70 contained in the intermediate layer 91 was 80% by weight or more, cracks occurred in some of the ignition plugs 10. On the other hand, in other cases, i.e., when the ratio of the material of the resistor 70 contained in the intermediate layer 91 was within the range of 30% by weight to 70% by weight, no cracks occurred even after the operation test. For this reason, it was confirmed that the ratio of the material of the resistor 70 contained in the intermediate layer 91 is preferably within the range of 30% by weight to 70% by weight.

[0071] The inventors further conducted the following experiment to confirm an appropriate thickness of intermediate layer 91. The "thickness of intermediate layer 91" here refers to the thickness of intermediate layer 91 at the central axis CX, that is, the length along the central axis CX of the portion of intermediate layer 91 through which the central axis CX passes, which is "L" shown in FIG.

[0072] First, a number of spark plugs 10 were produced, each differing in the thickness of the intermediate layer 91. The material of the intermediate layer 91 in each of the spark plugs 10 was a mixture of the material of the resistor 70 and the material of the conductive seal layer 90, as in the present embodiment, and the mixture ratio was always 1:1.

[0073] In each of the spark plugs 10, the conductive seal layer 80 had a thickness of 1 mm at the center axis CX, and the resistor 70 had a thickness of 18 mm at the center axis CX. The combined thickness of the intermediate layer 91 and the conductive seal layer 90 at the center axis CX was 3 mm. The initial electrical resistance between the terminal fitting 40 and the center electrode 30 was standardized to 2 kΩ. When manufacturing each of the spark plugs 10, the insulator 20 in the state shown in FIG. 4 was cooled at a temperature drop rate of 300° C. / min for 2 minutes.

[0074] Thereafter, the same operational test as in the previous experiment was performed on each of the spark plugs 10 to check for the presence or absence of cracks in the material between the terminal fitting 40 and the center electrode 30. In this experiment, ten spark plugs 10 each having the same thickness of the intermediate layer 91 were produced, and the above operational test was performed on all of the spark plugs 10.

[0075] The results of the above experiment are shown in the graph of Fig. 7. The horizontal axis of the graph represents the thickness of the intermediate layer 91 in each spark plug 10. The "failure rate" shown on the vertical axis of the graph refers to the percentage of 10 spark plugs 10 produced for each thickness of the intermediate layer 91 in which the electrical resistance between the terminal fitting 40 and the center electrode 30 increased by a specified amount from the initial value. Note that Fig. 7 plots separately those in which the electrical resistance increased to more than twice the initial value and those in which the electrical resistance increased to an unmeasurable level (i.e., to infinity).

[0076] As shown in Fig. 7, when the thickness of the intermediate layer 91 was less than 0.5 mm, an increase in electrical resistance was observed in some or all of the spark plugs 10. On the other hand, when the thickness of the intermediate layer 91 was 0.5 mm or more, there were no spark plugs 10 whose electrical resistance increased by more than twice the original value, even after the performance test. For this reason, it was confirmed that the thickness of the intermediate layer 91 is preferably 0.5 mm or more.

[0077] The present embodiment has been described above with reference to specific examples. However, the present disclosure is not limited to these specific examples. Any design modifications made by a person skilled in the art to these specific examples are also included within the scope of the present disclosure as long as they have the features of the present disclosure. The elements of each of the above-mentioned specific examples and their arrangements, conditions, shapes, etc. are not limited to those exemplified and can be changed as appropriate. The combination of each of the elements of each of the above-mentioned specific examples can be changed as appropriate as long as no technical contradiction occurs. [Explanation of symbols]

[0078] 10: Spark plug 20: Insulator 30: Center electrode 40: Terminal fitting 70: Resistor 90: Conductive seal layer 91: Middle class 200: Shaft hole CX: Central axis

Claims

1. An insulator (20) having an axial hole (200) formed therein; a center electrode (30) held by the insulator at a position that is one end along a center axis (CX) of the axial hole; a terminal fitting (40) held by the insulator at a position that is the other end along the central axis of the axial hole; a resistor (70) disposed along the central axis of the axial hole at a position between the central electrode and the terminal metal fitting; a conductive seal layer (90) disposed along a central axis of the axial hole at a position between the resistor and the terminal metal fitting; an intermediate layer (91) disposed along a central axis of the axial hole at a position between the resistor and the conductive seal layer; a thermal expansion coefficient of the intermediate layer is larger than a thermal expansion coefficient of the resistor and smaller than a thermal expansion coefficient of the conductive seal layer; A spark plug, wherein the thermal conductivity of the intermediate layer is greater than the thermal conductivity of the resistor and less than the thermal conductivity of the conductive seal layer.

2. 2. The spark plug according to claim 1, wherein the intermediate layer is formed from a mixture of a material of the resistor and a material of the conductive seal layer.

3. 3. The spark plug according to claim 2, wherein said intermediate layer includes at least two types of glass layers having different compositions.

4. 4. The spark plug according to claim 2, wherein the ratio of the material of said resistor contained in said intermediate layer is within a range of 30% by weight to 70% by weight.

5. An insulator (20) having an axial hole (200) formed therein; a center electrode (30) held by the insulator at a position that is one end along a center axis (CX) of the axial hole; a terminal fitting (40) held by the insulator at a position that is the other end along the central axis of the axial hole; a resistor (70) disposed along the central axis of the axial hole at a position between the central electrode and the terminal metal fitting; a conductive seal layer (90) disposed along a central axis of the axial hole at a position between the resistor and the terminal metal fitting; an intermediate layer (91) disposed along a central axis of the axial hole at a position between the resistor and the conductive seal layer; a thermal expansion coefficient of the intermediate layer is larger than a thermal expansion coefficient of the resistor and smaller than a thermal expansion coefficient of the conductive seal layer; the intermediate layer is formed of a material obtained by mixing a material of the resistor and a material of the conductive seal layer, A spark plug, wherein the ratio of the material of the resistor contained in the intermediate layer is within a range of 30% by weight to 70% by weight.

6. 6. The spark plug according to claim 5, wherein said intermediate layer includes at least two types of glass layers having different compositions.

7. 7. The spark plug according to claim 1, wherein the thickness of said intermediate layer at the central axis of said axial hole is 0.5 mm or more.

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