Patent Application
20170018909
The present application claims priority to Japanese Patent Application No. 2015-141103, filed on Jul. 15, 2015, and Japanese Patent Application No. 2016-085620, filed on Apr. 21, 2016, the disclosures of which are herein incorporated by reference.
Field of the Invention
The present invention relates to spark plugs used for ignition of fuel gas in internal combustion engines and the like.
Description of Related Art
In a spark plug used for ignition in an internal combustion engine or the like, when a voltage is applied to a center electrode and a ground electrode which are insulated from each other by an insulator, a spark occurs in a spark gap formed between a front end portion of the center electrode and a front end portion of the ground electrode (e.g., Patent Document 1).
Patent Document 1 is Japanese Patent Application Laid-Open (kokai) No. 2013-206740.
However, with reduction in diameter and size of a spark plug, an insulator tends to be thinner and the outer diameter of a metal terminal tends to be smaller. As a result, cracking of the insulator tends to occur easily due to vibration or the like of the metal terminal. Therefore, it may become difficult to ensure resistance to cracking of the insulator.
The present specification discloses a technique that is able to improve resistance to cracking of an insulator of a spark plug.
The technique disclosed in the present specification can be embodied in the following application examples.
A spark plug comprising:
an insulator having an axial bore extending along an axis from a rear end of the insulator toward a front end of the insulator;
a center electrode extending along the axis, and having a rear end located inside the axial bore;
a metal terminal including a trunk portion and a head portion, the trunk portion being located inside the axial bore and having a front end located rearward of the rear end of the center electrode (i.e., at a rear side with respect to the rear end of the center electrode), the head portion being located rearward of the trunk portion (i.e., at the rear side with respect to the trunk portion) and being exposed to the outside at the rear end of the insulator (i.e., the rear side with respect to the insulator); and
a conductive seal member that is in contact with the front end of the trunk portion of the metal terminal in the axial bore,
wherein the insulator includes:
a cylindrical first portion having a first inner diameter, at which the front end of the trunk portion of the metal terminal is disposed (i.e., the front end of the trunk portion of the metal terminal is disposed within the cylindrical first portion of the axial bore);
a cylindrical second portion having a second inner diameter larger than the first inner diameter, and including a portion (of the insulator) 1 mm or more forward of the rear end of the insulator (i.e., distant from a rear end of the insulator toward the front side); and
a cylindrical third portion disposed between the first portion and the second portion, and having a rear end and a third inner diameter larger than the first inner diameter and smaller than the second inner diameter, and
wherein the trunk portion of the metal terminal includes:
a cylindrical front trunk portion including a front end, and
a cylindrical rear trunk portion located rearward of the front trunk portion (i.e., at the rear side with respect to the front trunk portion), and having an outer diameter larger than an outer diameter of the front trunk portion, and
the third portion has a rear end located at the rear side with respect to a front end of the rear trunk portion (i.e., the cylindrical rear trunk portion has a front end positioned forward of the rear end of the cylindrical third portion of the insulator).
According to the above configuration, for example, when the metal terminal vibrates, the trunk portion is more likely to come into contact with the third portion of the insulator which is relatively distant from the rear end of the insulator, and is less likely to come into contact with the second portion of the insulator. As a result, impact applied from the metal terminal to the insulator can be reduced, whereby cracking of the insulator can be suppressed.
The spark plug according to Application Example 1, wherein
the third portion of the insulator has a thickness, in a radial direction, equal to or smaller than 6.1 mm.
According to the above configuration, cracking of the third portion of the insulator having a relatively small thickness in the radial direction can be effectively suppressed.
The spark plug according to Application Example 1 or 2, wherein
the first inner diameter is equal to or smaller than 2.9 mm.
According to the above-configuration, cracking of the insulator can be effectively suppressed although vibration is likely to occur because of the relatively small outer diameter of the trunk portion of the metal terminal.
The present invention can be implemented in various forms. For example, the present invention may be implemented as a spark plug, an insulator for the spark plug, an internal combustion engine equipped with the spark plug, an ignition system using the spark plug, and an internal combustion engine equipped with the ignition system.
Illustrative aspects of the invention will be described in detail with reference to the following figures wherein:
Embodiments of the present invention will be described in the following order:
Hereinafter, a technique disclosed in the present specification will be described on the basis of an embodiment.
The insulator (ceramic insulator) 10 is formed by baking alumina or the like. The insulator 10 is a substantially cylindrical member having an axial bore 12 which extends along the axis CO to penetrate the insulator 10. The insulator 10 includes a flange portion 19, a rear trunk portion 18, a front trunk portion 17, a step portion 15, and a leg portion 13. The rear trunk portion 18 is located at the rear side with respect to the flange portion 19 and has an outer diameter smaller than the outer diameter of the flange portion 19. The front trunk portion 17 is located at the front side with respect to the flange portion 19 and has an outer diameter smaller than the outer diameter of the flange portion 19. The leg portion 13 is located at the front side with respect to the front trunk portion 17 and has an outer diameter smaller than the outer diameter of the front trunk portion 17. The leg portion 13 is exposed to a combustion chamber of an internal combustion engine (not shown) when the spark plug 100 is mounted on the internal combustion engine. The step portion 15 is formed between the leg portion 13 and the front trunk portion 17.
The metallic shell 50 is formed from a conductive metal material (e.g., a low-carbon steel material) and is a cylindrical metal member for fixing the spark plug 100 to an engine head (not shown) of the internal combustion engine. The metallic shell 50 has an insertion hole 59 extending along the axis CO and through the metallic shell 50. The metallic shell 50 is disposed on the outer periphery of the insulator 10. That is, the insulator 10 is disposed and held within the insertion hole 59 of the metallic shell 50. The front end of the insulator 10 protrudes to the front side with respect to the front end of the metallic shell 50. The rear end of the insulator 10 protrudes to the rear side with respect to the rear end of the metallic shell 50.
The metallic shell 50 includes: a hexagonal columnar tool engagement portion 51 for engaging a spark plug wrench; a mounting screw portion 52 for mounting the spark plug 100 to the internal combustion engine; and a flange-like seat portion 54 formed between the tool engagement portion 51 and the mounting screw portion 52. A nominal diameter of the mounting screw portion 52 is set to any of M8 (8 mm (millimeters)), M10, M12, M14, and M18.
An annular gasket 5 which is formed by bending a metal plate is inserted between the mounting screw portion 52 and the seat portion 54 of the metallic shell 50. The gasket 5 seals a gap between the spark plug 100 and the internal combustion engine (engine head) when the spark plug 100 is mounted on the internal combustion engine.
The metallic shell 50 further includes: a thin crimp portion 53 provided at the rear side of the tool engagement portion 51; and a thin compressive deformation portion 58 provided between the seat portion 54 and the tool engagement portion 51. Annular packings 6 and 7 are disposed in an annular region formed between the inner peripheral surface of a portion of the metallic shell 50 from the tool engagement portion 51 to the crimp portion 53, and the outer peripheral surface of the rear trunk portion 18 of the insulator 10. The space between the two packings 6 and 7 in this region is filled with powder of a talc 9. The rear end of the crimp portion 53 is bent radially inward and fixed to the outer peripheral surface of the insulator 10. The compressive deformation portion 58 of the metallic shell 50 compressively deforms when the crimp portion 53, which is fixed to the outer peripheral surface of the insulator 10, is pressed toward the front side during manufacturing. The insulator 10 is pressed within the metallic shell 50 toward the front side via the packings 6 and 7 and the talc 9 due to the compressive deformation of the compressive deformation portion 58. The step portion 15 (ceramic insulator side step portion) of the insulator 10 is pressed by a step portion 56 (metallic shell side step portion), which is formed on the inner periphery of the mounting screw portion 52 of the metallic shell 50, via an annular plate packing 8 made of metal. As a result, the plate packing 8 prevents gas in the combustion chamber of the internal combustion engine from leaking to the outside through a gap between the metallic shell 50 and the insulator 10.
The center electrode 20 includes: a bar-shaped center electrode body 21 extending along the axis CO; and a columnar center electrode tip 29 joined to the front end of the center electrode body 21. The center electrode body 21 is disposed within the axial bore 12 and at a front portion of the insulator 10. The center electrode body 21 is formed from, for example, nickel or an alloy containing nickel as a principal component. The center electrode body 21 includes: a flange portion 24 provided at a predetermined position in the axial direction; a head portion 23 which is a portion at the rear side with respect to the flange portion 24; and a leg portion 25 which is a portion at the front side with respect to the flange portion 24. The flange portion 24 is supported by a step portion 12S formed in the axial bore 12 of the insulator 10. The front end of the leg portion 25, that is, the front end of the center electrode body 21 protrudes to the front side with respect to the front end of the insulator 10. The rear end of the head portion 23, that is, the rear end of the center electrode body 21 is located in the axial bore 12 of the insulator 10. The center electrode tip 29 is formed from, for example, a noble metal material having a high melting point, and is joined to the front end of the center electrode body 21.
The ground electrode 30 includes: a ground electrode body 31 joined to the front end of the metallic shell 50; and a columnar ground electrode tip 39. The rear end of the ground electrode body 31 is joined to the front end surface of the metallic shell 50. The ground electrode body 31 is formed by using a metal having high corrosion resistance, for example, a nickel alloy. The ground electrode tip 39 is formed from a noble metal material having a high melting point, and is joined to a surface of a front end portion of the ground electrode body 31, which surface faces the center electrode 20.
The rear end surface of the ground electrode tip 39 and the front end surface of the center electrode tip 29 form a gap in which spark discharge occurs. The vicinity of the gap is also referred to a firing end of the spark plug 100.
The metal terminal 40 is a bar-shaped member extending along the axis CO. The metal terminal 40 is formed from a conductive metal material (e.g., low-carbon steel). The metal terminal 40 includes: a trunk portion 43 having a front end located at the rear side with respect to the rear end of the center electrode 20; and a head portion 45 located at the rear side with respect to the trunk portion 43. The trunk portion 43 is disposed inside the axial bore 12 of the insulator 10, and the head portion 45 is exposed to the outside at the rear side with respect to the insulator 10. The head portion 45 includes: a flange portion 42 (terminal jaw portion); and a cap attachment portion 41 located at the rear side with respect to the flange portion 42.
A resistor 70 for reducing noise generated when spark occurs is disposed inside the axial bore 12 of the insulator 10 and between the front end of the metal terminal 40 and the rear end of the center electrode 20. In the axial bore 12, a gap between the resistor 70 and the center electrode 20 is filled with a conductive seal member 60. In addition, a gap between the resistor 70 and the trunk portion 43 of the metal terminal 40 is filled with a conductive seal member 80. Accordingly, the front end of the trunk portion 43 (i.e., the front end of the metal terminal 40) is in contact with the conductive seal member 80 in the axial bore 12.
The spark plug 100 is mounted to an internal combustion engine of an automobile or the like and used. Specifically, when a DC voltage of about 20 kV, for example, is applied between the metal terminal 40 and the metallic shell 50, spark discharge occurs in a gap between the center electrode 20 and the ground electrode 30. The energy of the spark discharge causes ignition of fuel gas in the internal combustion engine.
Hereinafter, the structure around the metal terminal 40 will be described in more detail.
The trunk portion 43 of the metal terminal 40 includes: a cylindrical front trunk portion 43A including the front end of the trunk portion 43; a cylindrical rear trunk portion 43C located at the rear side with respect to the front trunk portion 43A; and a step portion 43B located between the front trunk portion 43A and the rear trunk portion 43C. An outer diameter Re of the rear trunk portion 43C is larger than an outer diameter Rd of the front trunk portion 43A. The outer peripheral surface of the step portion 43B has a diameter increasing from the front side toward the rear side.
The front end of the front trunk portion 43A (i.e., the front end of the trunk portion 43) is disposed inside the first bore 12A of the first portion 10A.
A rear end P1 of the third portion 10C is located at the rear side with respect to a front end P2 of the rear trunk portion 43C. Therefore, the rear trunk portion 43C is located inside the second portion 10B of the insulator 10 and inside a portion, at the rear side, of the third portion 10C.
A rear end P3 of the first portion 10A is located at the front side with respect to the front end P2 of the rear trunk portion 43C. Therefore, the front trunk portion 43A is located inside the first portion 10A of the insulator 10.
Further, as shown in
The trunk portion 43 of the metal terminal 40 and the second portion 10B of the insulator 10 are not in contact with each other over the entire periphery thereof in the circumferential direction. That is, the outer peripheral surface of the rear trunk portion 43C of the trunk portion 43 and the inner peripheral surface of the second portion 10B are separated from each other. In addition, as shown in
The thickness (wall thickness) of the third portion 10C in the radial direction is denoted by T. In addition, the outer diameter of a portion, at the rear side, of the insulator 10, that is, the outer diameter of the third portion 10C and the second portion 10B is denoted by Rf. The thickness T of the third portion 10C can be expressed by T={(Rf−Rc)/2} by using the outer diameter Rf of the third portion 10C, and the inner diameter Rc of the third portion 10C.
The length in the axial direction from a rear end Pe of the insulator 10 to the rear end P1 of the third portion 10C is denoted by Ld. In addition, the length in the axial direction from the rear end Pe of the insulator 10 to the rear end P3 of the first portion 10A is denoted by Lb. The length in the axial direction from the rear end Pe of the insulator 10 (i.e., the rear end of the trunk portion 43) to a front end Ps of the trunk portion 43 is denoted by La. The length in the axial direction from the rear end Pe of the insulator 10 (i.e., the rear end of the trunk portion 43) to the front end P2 of the rear trunk portion 43C is denoted by Lc.
An impact resistance test for evaluating resistance to impact was executed using samples of a spark plug. In the first evaluation test, as shown in Table 1, five types of samples A1 to A5 of the spark plug 100 were produced. The dimensions common to each sample are as follows:
the length La from the rear end Pe of the insulator 10 to the front end Ps of the trunk portion 43: 41 mm;
the length Lb from the rear end Pe of the insulator 10 to the rear end P3 of the first portion 10A: 19.2 mm;
the length Lc from the rear end Pe of the insulator 10 to the front end P2 of the rear trunk portion 43C: 7.0 mm;
the inner diameter Ra of the first portion 10A: 3 mm;
the inner diameter Rb of the second portion 10B: 3.9 mm;
the inner diameter Rc of the third portion 10C: 3.4 mm;
the outer diameter Rd of the front trunk portion 43A: 2.85 mm;
the outer diameter Re of the rear trunk portion 43C: 3.2 mm; and
the outer diameter Rf of the third portion 10C: 9.0 mm.
The five types of samples A1 to A5 have different lengths Ld in the axial direction from the rear end Pe of the insulator 10 to the rear end P1 of the third portion 10C, which are 0.5 mm, 0.9 mm, 1 mm, 3 mm, and 5 mm, respectively. In the samples A1 to A5, the insulator 10 was formed by using alumina, and the metal terminal 40 was formed by using low-carbon steel.
In the impact resistance test, impact was applied to each sample under the conditions specified in section 7.4 of JIS B 8031:2006 (Internal combustion engine—Spark plugs). The insulator 10 of each sample after the test was visually checked to confirm whether crack occurred in the insulator 10. In the test, for each type of sample, ten pieces of the sample were tested.
Then, a sample for which cracking occurrence was not observed in any of the 10 pieces of the sample was evaluated as “A”. A sample for which cracking occurrence was observed in more than or equal to 1 and less than or equal to 3 pieces out of the 10 pieces of the sample was evaluated as “B”. A sample for which cracking occurrence was observed in more than or equal to 4 and less than or equal to 6 pieces out of the 10 pieces of the sample was evaluated as “C”. A sample for which cracking occurrence was observed in more than or equal to 7 pieces out of the 10 pieces of the sample was evaluated as “D”.
The samples A1 and A2 having the length Ld smaller than 1 mm were evaluated as “D”, and the samples A3 to A5 having the length Ld larger than or equal to 1 mm were evaluated as “B”. The reason for this is considered as follows. When impact is applied to the spark plug 100, the metal terminal 40 vibrates, with the front end Ps of the front trunk portion 43A fixed in the insulator 10 by the seal member 80 (refer to
As is seen from the above description, in the spark plug 100 according to the embodiment, the distance (La−Ld) from the fulcrum Ps of vibration to the point of action P1 of impact is smaller than the distance La from the fulcrum Ps of vibration to the point of action Pe of impact in the spark plug 100b according to the comparative embodiment. Thus, impact (moment) applied to the insulator 10 due to the metal terminal 40 can be reduced. As a result, impact resistance of the insulator 10 can be improved.
However, when the length Ld is excessively small, the distance (La−Ld) from the fulcrum Ps of vibration to the point of action P1 of impact cannot be sufficiently reduced, which may cause insufficient impact resistance. It is found from the result of the first evaluation test that if the length Ld is 1 mm or larger, impact resistance can be improved by reducing the distance (La−Ld) from the fulcrum Ps of vibration to the point of action P1 of impact. In the spark plug 100 shown in
Further, a second evaluation test was executed in order to verify the structure that can improve impact resistance. In the second evaluation test, as shown in Table 2, five types of samples B1 to B5 were produced. The dimensions common to each sample are as follows:
the length La from the rear end Pe of the insulator 10 to the front end Ps of the trunk portion 43: 41 mm;
the length Lb from the rear end Pe of the insulator 10 to the rear end P3 of the first portion 10A: 19.2 mm;
the length Lc from the rear end Pe of the insulator 10 to the front end P2 of the rear trunk portion 43C: 10 mm;
the inner diameter Ra of the first portion 10A: 3 mm;
the inner diameter Rb of the second portion 10B: 3.9 mm;
the inner diameter Rc of the third portion 10C: 3.4 mm;
the outer diameter Rd of the front trunk portion 43A: 2.85 mm;
the outer diameter Re of the rear trunk portion 43C: 3.2 mm; and
the outer diameter Rf of the third portion 10C: 9.0 mm.
The materials of the respective components such as the insulator 10 and the metal terminal 40 are the same as those of the first evaluation test. Further, details of the impact resistance test for each sample and ratings for evaluation are the same as those of the first evaluation test.
The five types of samples B1 to B5 have different lengths Ld in the axial direction from the rear end Pe of the insulator 10 to the rear end P1 of the third portion 10C, which are 5 mm, 9 mm, 10 mm, 11 mm, and 15 mm, respectively. The length Lc from the rear end Pe of the insulator 10 to the front end P2 of the rear trunk portion 43C is fixed to 10 mm. As a result, in the two samples B1 and B2, (Lc−Ld) has a value larger than 0, and the rear end P1 of the third portion 10C of the insulator 10 is located at the rear side with respect to the front end P2 of the rear trunk portion 43C, like in the spark plug 100 shown in
Meanwhile, in the samples B4 and B5, (Lc−Ld) has a value smaller than 0. In this case, in contrast to the spark plug 100 shown in
As seen from the above description, the two samples B1 and B2 are samples of the spark plug according to the embodiment shown in
The two samples B1 and B2 having (Lc−Ld) larger than 0 were evaluated as “B”. On the other hand, the sample B3 having (Lc−Ld)=0 was evaluated as “C”, and the two samples B4 and B5 having (Lc−Ld) smaller than 0 were evaluated as “D”. The reason for this is considered as follows. When (Lc−Ld) is larger than 0, that is, when the rear end P1 of the third portion 10C of the insulator 10 is located at the rear side with respect to the front end P2 of the rear trunk portion 43C, the rear end P1 of the third portion 10C faces, in the radial direction, the rear trunk portion 43C having the relatively large outer diameter Re. As a result, it is assured that the rear end P1 of the third portion 10C becomes a contact point that comes into contact with the trunk portion 43 of the metal terminal 40 when vibration occurs. As a result, impact resistance of the insulator 10 can be sufficiently improved.
On the other hand, when (Lc−Ld) is equal to or smaller than 0, that is, when the rear end P1 of the third portion 10C of the insulator 10 is located at the front side with respect to the front end P2 of the rear trunk portion 43C, the rear end P1 of the third portion 10C faces, in the radial direction, the front trunk portion 43A having the relatively small outer diameter Rd. As a result, it is not assured that the rear end P1 of the third portion 10C becomes a contact point that comes into contact with the trunk portion 43 of the metal terminal 40 when vibration occurs, and the rear end Pe of the second portion 10B is highly likely to become a contact point that comes into contact with the trunk portion 43. As a result, impact resistance of the insulator 10 cannot be sufficiently improved.
On the other hand, when (Lc−Ld) is 0, the rear end P1 of the third portion 10C of the insulator 10 faces, in the radial direction, the rear trunk portion 43C having the relatively large outer diameter Re, and also faces the step portion 43B having an outer diameter smaller than the outer diameter Re. Therefore, as compared to the case where (Lc−Ld) is larger than 0, it is not sufficiently assured that the rear end P1 of the third portion 10C becomes a contact point that comes into contact with the trunk portion 43 of the metal terminal 40 when vibration occurs, and the rear end Pe of the second portion 10B might become a contact point that comes into contact with the trunk portion 43. As a result, impact resistance of the insulator 10 cannot be sufficiently improved.
As seen from the above description, it is found from the result of the second evaluation test that when (Lc−Ld) is larger than 0, that is, when the rear end P1 of the third portion 10C of the insulator 10 is located at the rear side with respect to the front end P2 of the rear trunk portion 43C, impact resistance of the insulator 10 can be improved.
As described above, the results of the first evaluation test and the second evaluation test indicate that it is preferable that the second portion 10B includes a portion, of the insulator 10, 1 mm or more distant from the rear end of the insulator 10 toward the front side, and the rear end P1 of the third portion 10C is located at the rear side with respect to the front end P2 of the rear trunk portion 43C. By so doing, when the metal terminal 40 vibrates, the trunk portion 43 is more likely to come into contact with the third portion 10C relatively distant from the rear end of the insulator 10, and is less likely to come into contact with the second portion 10B. As a result, impact applied from the metal terminal 40 to the insulator 10 can be reduced, whereby cracking of the insulator 10 can be suppressed.
Further, a third evaluation test was executed in order to verify the structure that can improve impact resistance. In the third evaluation test, as shown in Table 3, eight types of samples C1 to C8 of the spark plug 100 were produced. The dimensions common to each sample are as follows:
the length La from the rear end Pe of the insulator 10 to the front end Ps of the trunk portion 43: 41 mm;
the length Lb from the rear end Pe of the insulator 10 to the rear end P3 of the first portion 10A: 19.2 mm;
the length Lc from the rear end Pe of the insulator 10 to the front end P2 of the rear trunk portion 43C: 10 mm;
the length Ld from the rear end Pe of the insulator 10 to the rear end P1 of the third portion 10C: 5.0 mm;
the inner diameter Ra of the first portion 10A: 3 mm;
the inner diameter Rb of the second portion 10B: 4.1 mm;
the inner diameter Rc of the third portion 10C: 4.0 mm;
the outer diameter Rd of the front trunk portion 43A: 2.85 mm; and
the outer diameter Re of the rear trunk portion 43C: 3.8 mm.
The materials of the respective components such as the insulator 10 and the metal terminal 40 are the same as those of the first evaluation test. Further, details of the impact resistance test for each sample and ratings for evaluation are the same as those of the first evaluation test.
The eight types of samples C1 to C8 have different outer diameters Rf (
The sample C1 in which the wall thickness T of the third portion 10C was 8 mm was evaluated as “A”, and the samples C2 to C8 in which the wall thickness T of the third portion 10C was equal to or smaller than 7 mm was evaluated as “B”. In the samples C2 to C8 in which the wall thickness T of the third portion 10C was equal to or smaller than 7 mm, the reason why reduction in impact resistance was not observed even when the wall thickness T was reduced is considered to be that the insulator 10 having the third portion 10C and the second portion 10B assures that the rear end P1 of the third portion 10C becomes a contact point that comes into contact with the trunk portion 43 of the metal terminal 40 as described above.
In order to verify this, as shown in Table 4, eight types of samples D1 to D8 of the spark plug 100b according to the comparative embodiment shown in
The sample D1 in which the wall thickness t of the second portion 10Bb was 7.95 mm was evaluated as “A”, and the samples D2 to D4 in which the wall thickness t of the second portion 10Bb was equal to or larger than 6.15 mm and not larger than 6.95 mm were evaluated as “B”. The sample D5 in which the wall thickness t of the second portion 10Bb was 6.05 mm was evaluated as “C”, and the samples D6 to D8 in which the wall thickness t of the second portion 10Bb was equal to or smaller than 4.95 mm was evaluated as “D”.
As described above, in the samples D5 to D8 in which the wall thickness t of the second portion 10Bb was equal to or smaller than 6.1 mm, reduction in impact resistance was observed with reduction in the wall thickness t. Thus, it is found that, in the spark plug 100 shown in
The reason for this is considered as follows. As the wall thickness of the rear end portion of the insulator 10 becomes smaller, resistance to impact applied to the insulator 10 in the radial direction decreases, and resistance to vibration of the metal terminal 40 also decreases. As a result, as the wall thickness of the rear end portion of the insulator 10 becomes smaller, cracking of the insulator 10 occurs mainly due to vibration of the metal terminal 40. At this time, in the spark plug 100 according to the embodiment shown in
From the results described above, it is found that, in the spark plug 100 according to the embodiment, it is beneficial that the wall thickness T of the third portion 10C of the insulator 10, that is, the wall thickness T in the radial direction is equal to or smaller than 6.1 mm. In this case, cracking of the insulator 10 having a relatively thin wall thickness T of the third portion 10C in the radial direction can be effectively suppressed.
Further, a fourth evaluation test was executed in order to verify the structure that can improve impact resistance. In the fourth evaluation test, as shown in Table 5, nine types of samples E1 to E9 of the spark plug 100 were produced. The dimensions common to each sample are as follows:
the length La from the rear end Pe of the insulator 10 to the front end Ps of the trunk portion 43: 41 mm;
the length Lb from the rear end Pe of the insulator 10 to the rear end P3 of the first portion 10A: 19.2 mm;
the length Lc from the rear end Pe of the insulator 10 to the front end P2 of the rear trunk portion 43C: 10 mm;
the length Ld from the rear end Pe of the insulator 10 to the rear end P1 of the third portion 10C; 5.0 mm;
the inner diameter Rb of the second portion 10B: 4.1 mm;
the inner diameter Rc of the third portion 10C: 4.0 mm; and
the outer diameter Re of the rear trunk portion 43C: 3.8 mm.
The materials of the respective components such as the insulator 10 and the metal terminal 40 are the same as those of the first evaluation test. Further, details of the impact resistance test for each sample and ratings for evaluation are the same as those of the first evaluation test.
The nine types of samples E1 to E9 have different inner diameters Ra (
Specifically, the outer diameter Rd of the front trunk portion 43A is set to a value 0.2 mm smaller than the inner diameter Ra of the first portion 10A (i.e., Rd=Ra−0.2 mm).
Further, the samples E1 to E9 have different outer diameters Rf (
The sample E1 in which the wall thickness T of the third portion 10C was 8 mm and the inner diameter Ra of the first portion 10A was 3 mm was evaluated as “A”, and the other samples E2 to E9 were evaluated as “B”. The reason why reduction in impact resistance was not observed even when the inner diameter Ra of the first portion 10A was reduced is considered to be that the insulator 10 having the third portion 10C and the second portion 10B assures that the rear end P1 of the third portion 10C becomes a contact point that comes into contact with the trunk portion 43 of the metal terminal 40, as described above.
In order to verify this, as shown in Table 6, nine types of samples F1 to F9 of the spark plug 100b according to the comparative embodiment shown in
Further, in each of the samples F1 to F9, the outer diameter Rf (
The samples F1 to F3 in which the inner diameter Ra of the first portion 10A was 3 mm was evaluated as “A” or “B”. That is, the sample F1 in which the wall thickness t of the second portion 10Bb was 7.95 mm was evaluated as “A”, and the samples F2 and F3 in which the wall thickness t of the second portion 10Bb was 6.95 mm and 6.45 mm, respectively, were evaluated as “B”. The samples F4 to F6 in which the inner diameter Ra of the first portion 10A was 2.9 mm were evaluated as “C” regardless of the wall thickness t of the second portion 10Bb. The samples F7 to F9 in which the inner diameter Ra of the first portion 10A was 2.7 mm were evaluated as “D” regardless of the wall thickness t of the second portion 10Bb.
As described above, in the samples F4 to F9 in which the inner diameter Ra of the first portion 10A was equal to or smaller than 2.9 mm, reduction in impact resistance was observed with reduction in the inner diameter Ra of the first portion 10A. From the above results, it is found that, in the spark plug 100 shown in
The reason for this is considered as follows. As the inner diameter Ra of the first portion 10A becomes smaller, the outer diameter Rd of the front trunk portion 43A of the metal terminal 40 located inside the first portion 10A has to be made smaller. When the outer diameter Rd of the front trunk portion 43A becomes smaller, rigidity of the front trunk portion 43A is reduced, whereby amplitude of vibration is increased. As a result, when impact is applied to the spark plug 100, the frequency of the trunk portion 43 of the metal terminal 40 coming into contact with the insulator 10 in the radial direction is increased. Accordingly, the insulator 10 becomes easy to crack, leading to reduction in impact resistance. Thus, as the inner diameter Ra of the first portion 10A becomes smaller, cracking of the insulator 10 occurs mainly due to vibration of the metal terminal 40. At this time, in the spark plug 100 according to the embodiment shown in
From the results described above, it is found that the inner diameter Ra of the first portion 10A being equal to or smaller than 2.9 mm is beneficial in the spark plug 100 according to the embodiment. In this case, cracking of the insulator 10 can be effectively suppressed although vibration is likely to occur because of the relatively small outer diameter Rd of the front trunk portion 43A of the metal terminal 40.
(1)
(2)
As described above, due to either or both of deformation and inclination of the metal terminal 40 during manufacturing, the outer peripheral surface of the rear trunk portion 43C of the metal terminal 40 may be, at a part thereof in the circumferential direction, in contact with the second portion 10B of the insulator 10 or the inner peripheral surface of the third portion 10C. That is, the trunk portion 43 of the metal terminal 40 and the inner peripheral surface of the insulator 10 may be in non-contact with each other at a part in the circumferential direction, and may be in contact with each other at another part in the circumferential direction. Also in this case, when impact is applied to the spark plug, the trunk portion 43 of the metal terminal 40 can be prevented from coming into contact with a part of the rear end of the second portion 10B of the insulator 10 (e.g., the rear end Pe shown in
Generally speaking, at least a part of the trunk portion 43 of the metal terminal 40 is preferably in non-contact with the inner peripheral surface of the insulator 10. For example, the rear trunk portion 43C of the metal terminal 40 and the inner peripheral surface of the second portion 10B of the insulator 10 are preferably in non-contact with each other at at least a part of the entire periphery in the circumferential direction. Likewise, the rear trunk portion 43C of the metal terminal 40 and the inner peripheral surface of the third portion 10C of the insulator 10 are preferably in non-contact with each other at at least a part of the entire periphery in the circumferential direction.
(3) The materials of the insulator 10 and the metal terminal 40 are merely examples, and are not limited to the above-described materials. For example, although the insulator 10 is formed by using the ceramic material containing alumina (A12O3) as a principal component, the insulator 10 may be formed by using a ceramic material containing another compound (e.g., AlN, ZrO2, SiC, TiO2, Y2O3 or the like) as a principal component instead.
(4) The specific structure of the spark plug 100 shown in
Although the present invention has been described above based on the embodiments and the modified embodiments, the above-described embodiments of the invention are intended to facilitate understanding of the present invention, but not as limiting the present invention. The present invention can be changed and modified without departing from the gist thereof and the scope of the claims and equivalents thereof are encompassed in the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-141103 | Jul 2015 | JP | national |
| 2016-085620 | Apr 2016 | JP | national |
