JP2005203349A - Spark plug - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spark plug in which ignitability is improved by thinning a noble metal tip and increasing a protrusion amount, and the fluctuation of a discharge voltage can be suppressed so that the ignitability is maintained. <P>SOLUTION: The spark plug 100 comprises a second ground electrode 5 having the other end face disposed so as to oppose at least the side peripheral face of an insulator 2. An area S of the opposing face 41a of the noble metal tip 41, a distance t between the opposing face 41a and the inner peripheral face of the center electrode side of the first ground electrode 4, a distance of an discharge gap g1 M, a distance F between the side peripheral face of the center electrode 3 and the other end face of the second ground electrode 5, a distance A between the other end face of the second ground electrode 5 and the side peripheral face of the insulator 2, a minimum distance H in the axial direction between the tip end face of the insulator 2 and a middle point of the discharge gap g1, and a minimum distance L in the axial direction between the tip end face of the insulator 2 and the tip end side edge of the other end face of the second ground electrode 5 satisfy following relationships: 0.12≤S≤1.15, 0.3≤t≤1.5, A+0.7(F-A)≤1.8M, and -0.3≤L/H. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a spark plug used for ignition of an internal combustion engine.

  Many spark plugs used for ignition of an internal combustion engine such as an automobile engine have a columnar noble metal tip not only at the tip of the center electrode but also at the tip of the ground electrode. This is because the center electrode and the ground electrode forming the air gap are consumed by the spark discharge, and the durability is lowered. Therefore, durability is maintained by forming a noble metal tip at the position of the air gap of the ground electrode.

  More recently, there has been an increasing demand for higher performance of engines, and spark plugs are required to further improve ignitability. In order to improve the ignitability, it is effective to make the columnar noble metal tip thinner. This is because the flame kernel formed in the air gap tries to grow by swirl or the like. At that time, the noble metal tip comes into contact with the grown flame kernel and hinders the growth (hereinafter also referred to as a flame extinguishing action). At this time, if the noble metal tip is thick, the flame kernel tends to come into contact with the noble metal tip and is easily affected by the flame extinguishing action. Therefore, many structures are adopted in which the noble metal tip is thinned to suppress the influence of the flame extinguishing action caused by the contact of the flame kernel with the noble metal tip, and the growth of the flame kernel is promoted.

Further, the axial distance (hereinafter also referred to as the protrusion amount) between the facing surface facing the center electrode of the noble metal tip and the center electrode side inner peripheral surface of the ground electrode (specifically, the ground electrode main body) is increased. Is also effective. This is because, when the amount of protrusion is small in the process of growth of the flame kernel, the flame kernel easily comes into contact with the ground electrode body at an early stage, and the flame extinguishing action is likely to occur. Therefore, many structures that promote the growth of flame nuclei by increasing the protruding amount are employed. (See Patent Document 1)
JP 2001-345162 A

  However, as a result of investigations by the inventors, it has been found that the discharge voltage varies in the spark plug as in Patent Document 1. When the discharge voltage varies, there is a risk that when the discharge voltage becomes high, flame nuclei are hardly formed in the air gap, the ignitability is lowered, and misfires occur. In addition, when carbon or the like adheres to the surface of the insulator, resulting in a so-called “smoldering” state, when the discharge voltage increases, the spark discharge does not fly between the center electrode and the ground electrode, and the insulator There was a risk of fire with the metal shell along the surface.

  Accordingly, an object of the present invention is to provide a spark plug in which the precious metal tip formed on the ground electrode is thinned and the ignitability is improved by increasing the protruding amount, thereby suppressing variations in discharge voltage and improving the ignitability. It is to provide a spark plug that can be maintained.

Therefore, as a result of intensive studies by the present inventors, the spark plug of the present invention has an insulator having an axial hole extending in the axial direction, a center electrode disposed on the tip side of the axial hole of the insulator, and the insulating plug. A metal shell surrounding the body, a first ground electrode body having one end coupled to the metal shell, and a central electrode side inner peripheral surface of the other end of the first ground electrode body; A first ground electrode having a tip surface and a noble metal tip facing with a discharge gap therebetween, one end coupled to the metal shell, and the other end facing at least a side peripheral surface of the insulator, A second ground electrode that forms a creeping gap between the side peripheral surface of the center electrode and the other end surface of the center electrode;
The area of the facing surface of the noble metal tip facing the tip surface of the center electrode is S (unit: mm ² ), and the axial direction of the facing surface of the noble metal tip and the inner peripheral surface on the center electrode side of the first ground electrode body T (unit is mm), the air gap distance is M (unit is mm), and the shortest radial distance between the side surface of the center electrode and the other end surface of the second ground electrode is F ( The unit is mm), the shortest radial distance between the other end surface of the second ground electrode and the side peripheral surface of the insulator is A (unit is mm), and the middle between the tip surface of the insulator and the air gap. The shortest distance H (unit: mm) from the point in the axial direction, and the shortest distance L (unit: mm) in the axial direction between the front end surface of the insulator and the front end side edge of the other end surface of the second ground electrode. , When the tip side edge of the other end face of the second ground electrode protrudes from the tip face of the insulator is defined as +. When,
0.12 ≦ S ≦ 1.15
0.3 ≦ t ≦ 1.5
A + 0.7 (F−A) ≦ 1.8M
-0.3 ≦ L / H
It is characterized by being.

According to the spark plug of the present invention, the area S of the opposing surface of the noble metal tip which is formed in a columnar shape, and a 0.12 mm ² or more 1.15 mm ² or less. In this way, when the flame kernel formed in the air gap by thinning the noble metal tip grows by swirl etc., the influence of the flame extinguishing action due to the contact of the flame nucleus with the noble metal tip can be suppressed, and the growth of the flame nucleus Can be promoted. That is, the ignitability is improved. If the area of the facing surface of the noble metal tip is less than 0.12 mm ² , the noble metal tip itself becomes too thin, and the durability of the spark plug decreases. On the other hand, if the area of the facing surface of the noble metal tip exceeds 1.15 mm ² , the noble metal tip becomes too thick and it is difficult to obtain the effect of improving the ignitability. The noble metal tip may be a columnar shape and may be a polygonal column such as a cylinder, a triangular column, or a quadrangular column.

  According to the spark plug of the present invention, the distance in the axial direction between the facing surface of the noble metal tip and the inner peripheral surface on the center electrode side of the first ground electrode body is t (unit: mm), 0.3 mm or more. 5 mm or less. As described above, when the amount of protrusion of the noble metal tip is increased, when the flame nucleus formed in the air gap formed by the center electrode and the first ground electrode grows due to swirl or the like, the first ground is quickly brought forward. The possibility of coming into contact with the electrode body is reduced, and the ignitability is improved in combination with the area of the facing surface of the noble metal tip. In addition, when the axial distance t between the facing surface of the noble metal tip and the inner peripheral surface of the first ground electrode body on the center electrode side is less than 0.3 mm, as described above, the flame core is quickly brought into contact with the first ground electrode body. It is difficult to obtain an effect of effectively suppressing contact with the surface. On the other hand, if the axial distance t between the facing surface of the noble metal tip and the central electrode side inner peripheral surface of the first ground electrode body exceeds 1.5 mm, the heat capacity of the noble metal tip increases and the durability of the noble metal tip itself is increased. May decrease. The “center electrode side inner peripheral surface” in the present invention is a surface on the side facing the center electrode in the first ground electrode main body.

  By the way, in the spark plug in which the noble metal tip is thinned and the protruding amount of the noble metal tip is increased as described above, there is a possibility that the discharge voltage varies greatly and the ignitability cannot be maintained. Therefore, the spark plug of the present invention is disposed so that one end is coupled to the metal shell and the other end face is opposed to the side peripheral surface of the center electrode and / or the side peripheral surface of the insulator. And a second ground electrode that forms a creeping gap between the surface and the other end surface of the second ground electrode. By providing the second ground electrode in this way, the electric field strength near the air gap can be concentrated, and variations in the discharge voltage can be suppressed. In addition, the concentration of the electric field in the vicinity of the air gap concentrates, so that the discharge voltage is reduced and misfire is less likely to occur.

  The spark plug of the present invention has an air gap distance of M (unit: mm), and a radial distance between the side surface of the center electrode and the other end surface of the second ground electrode is F (unit: mm). The shortest radial distance between the other end surface of the second ground electrode and the side peripheral surface of the insulator is A (unit: mm) The shortest distance H in the axial direction between the front end surface of the insulator and the midpoint of the air gap ( The unit is mm), and the shortest distance L (unit: mm) between the front end surface of the insulator and the front end side edge of the other end surface of the second ground electrode is the front end side edge of the other end surface of the second ground electrode. A + 0.7 (FA) ≦ 1.8M and −0.3 ≦ L / H when the case of protruding from the front end surface of the insulator is +. If A + 0.7 (FA) exceeds 1.8M, it becomes difficult to concentrate the electric field strength in the vicinity of the air gap, and the effect of suppressing variation in the discharge voltage cannot be obtained. Further, even if L / H is less than −0.3, the electric field strength in the vicinity of the air gap cannot be concentrated, and variations in discharge voltage cannot be suppressed. In the present invention, the “front end side edge of the other end face of the second ground electrode” refers to a peripheral edge farthest from the center electrode in the axial direction among the other end faces of the second ground electrode.

  Furthermore, A is preferably 0.2 mm or more. When A is less than 0.2 mm, the distance between the insulator and the second ground electrode becomes narrow, and a bridge may be generated between the insulator and the second ground electrode.

  Furthermore, the spark plug of the present invention preferably satisfies M ≦ A + 0.7 (FA) and L / H ≦ 0.7. When A + 0.7 (FA) is less than M, spark discharge is likely to occur in the creeping gap between the other end surface of the second ground electrode and the side peripheral surface of the center electrode, and spark discharge in the air gap. May be less likely to occur and ignitability may be reduced. On the other hand, if L / H is greater than 0.7, the second ground electrode may be too close to the air gap and the flame extinguishing action by the second ground electrode may easily occur. That is, in the process of the growth of flame nuclei, the flame nuclei are likely to come into contact with the second ground electrode body at an early stage, and the ignitability may be reduced.

  In the spark plug of the present invention, it is preferable that the noble metal tip is mainly composed of either Ir or Pt. Thereby, the durability of the noble metal tip is improved. Furthermore, when the main component is Ir, the noble metal tip is preferably an alloy containing at least one of Rh, Pt, Ni, W, Pd, Ru, and Os. In addition, when the main component is Pt, the noble metal tip is preferably an alloy containing at least one of Rh, Ir, Ni, W, Pd, Ru, and Os. In the present invention, the “main component” is a component having the largest amount of components.

  Furthermore, the spark plug of the present invention preferably has M> 0.6 mm. In such a spark plug having an air gap distance M larger than 0.6 mm, when the noble metal tip is made thinner and the protrusion amount is increased, the variation in the discharge voltage tends to increase. Therefore, in the spark plug in which the distance M of the air gap is set to be larger than 0.6 mm, it is possible to effectively suppress the variation in the discharge voltage by applying the present invention.

  Furthermore, the spark plug of the present invention is preferably provided with a plurality of second ground electrodes. Thus, by providing a plurality of second ground electrodes, the electric field strength near the air gap can be further concentrated, and variations in the discharge voltage can be suppressed. On the other hand, the number of second ground electrodes is preferably at most three. When the number is four or more, the number of second ground electrodes arranged in the vicinity of the air gap increases, and the flame extinguishing action by the second ground electrode is likely to occur, and the ignitability may be reduced.

  Hereinafter, some embodiments of the present invention will be described with reference to the drawings. 1 and FIG. 2, a spark plug 100 with a resistor according to the first embodiment of the present invention includes a cylindrical metal shell 1, an insulator 2 fitted into the metal shell 1 so that a tip portion protrudes, and a tip side. The first noble metal tip 31 bonded to the center electrode 3 provided inside the insulator 2 with the first noble metal tip 31 protruding, and the first noble metal tip 31 (center electrode 3) disposed opposite to the front end surface. One ground electrode 4, a center electrode 3, and two second ground electrodes 5 disposed so as to face the insulator 2 are provided. The second ground electrodes 5 are respectively disposed at positions shifted by 90 ° from the first ground electrode 4, and the second ground electrodes 5 are disposed at positions shifted by 180 °. The other end of the first ground electrode 4 is bent so as to face the tip surface of the first noble metal tip 31 in a substantially parallel manner, and a second noble metal tip 41 is formed at a position facing the first noble metal tip 31. ing. An air gap g1 is formed between the first noble metal tip 31 and the second noble metal tip 41. Further, the second ground electrode 5 has a creeping gap g2 between the other end surface and the side peripheral surface of the center electrode. In this creepage gap, the spark discharge includes a creeping discharge over the surface of the insulator and an air discharge flying in the air.

  The metal shell 1 is made of carbon steel or the like, and as shown in FIG. 1, a threaded portion 12 for attaching the spark plug 100 to an engine block (not shown) is formed on the outer peripheral surface thereof. The insulator 2 is made of a ceramic sintered body such as alumina or aluminum nitride, and has a through hole 6 for fitting the center electrode 3 along its own axial direction. A terminal fitting 13 is inserted and fixed on one end side of the through hole 6, and the center electrode 3 is inserted and fixed on the other end side. A resistor 15 is disposed between the terminal fitting 13 and the center electrode 3 in the through hole 6. Conductive glass sealing layers 16 and 17 are provided at both ends of the resistor 15, and are electrically connected to the center electrode 3 and the terminal fitting 13 through the conductive glass sealing layers 16 and 17, respectively. .

  The center electrode 3 has an electrode base material 3a formed on the surface and a metal core 3b inserted therein. The electrode base material 3a of the center electrode 3 is a Ni alloy such as INCONEL600 (registered trademark of INCO). On the other hand, the metal core 3b is made of an alloy mainly composed of Cu, Ag, or the like. The metal core 3b has a higher thermal conductivity than the electrode base material 3a. The electrode base material 3a of the center electrode 3 is reduced in diameter on the tip side and has a flat tip surface. A disc-shaped noble metal tip is superimposed on the tip surface, and laser welding is performed along the outer edge of the joint surface. The first noble metal tip 31 is formed by forming a welded portion by electron beam welding, resistance welding or the like and fixing it. The first noble metal tip 31 is made of a metal having Pt and Ir as main components. Specifically, Pt alloys such as Pt-20 wt% Ir and Pt-20 wt% Rh, Ir-5 wt% Pt, Ir-20 wt% Rh, Ir-5 wt% Pt-1 wt% Rh-1 wt% Ni, Ir- An Ir alloy such as 10 wt% Rh-5 wt% Ni can be used.

The first ground electrode 4 includes a first ground electrode body 4 a and a second noble metal tip 41. Among them, the first ground electrode body 4a has one end 42 fixed and integrated with the front end surface of the metal shell 1 by welding or the like. On the other hand, a second noble metal tip 41 is provided at the other end 43 of the first ground electrode body 4a. The second noble metal tip 41 is formed by providing a cylindrical noble metal tip at a predetermined position of the first ground electrode body 4a and fixing it by laser welding, electron beam welding, resistance welding or the like. . The second noble metal tip 41 is made of a metal containing Pt, Ir, and W as main components. Specifically, Pt alloys such as Pt-20 wt% Ni, Pt-20 wt% Rh, Pt-20 wt% Rh-5 wt% Ni, Ir-5 wt% Pt, Ir-20 wt% Rh, Ir-11 wt% Ru- Ir alloys such as 8 wt% Rh-1 wt% Ni can be used. The facing surface 41a of the second noble metal tip 41 faces the center electrode tip surface (specifically, the tip surface 31a of the first noble metal tip 31). Further, the first ground electrode body 4a is formed of a Ni alloy such as INCONEL600. In the present embodiment, the distance M of the discharge gap g1 between the tip surface 31a of the first noble metal tip 31 of the center electrode 3 and the opposing surface 41a of the second noble metal tip 41 of the balanced ground electrode 4 is 1.1 mm. It has become.

In the present embodiment, the diameter B of the opposed surface 41a of the center electrode 3 of the second noble metal tip 41 facing the noble metal tip 31 is 0.7 mm (area S is 0.38 mm ² ), and it protrudes from the first ground electrode body 4a. The amount t is 0.8 mm. Thus, the area S of the opposing surfaces of the second noble metal tip 41 has a 0.12 mm ² or more 1.15 mm ² or less, it is possible to suppress the influence of the flame quenching by contact with the noble metal tip of the flame kernel, the flame Nuclear growth can be promoted and ignitability is improved. In addition, since the protruding amount t of the second noble metal tip 41 from the first ground electrode body 4a is 0.3 mm or more and 1.5 mm or less, the possibility of coming into contact with the first ground electrode body early is reduced. The ignitability is improved by promoting the growth of the flame kernel.

  One end 52 of the second ground electrode 5 is fixed and integrated with the front end surface of the metal shell 1 by welding or the like. On the other hand, the other end 53 of the second ground electrode 5 faces the central electrode side peripheral surface and the side peripheral surface of the insulator 2. The second ground electrode 5 is made of a Ni alloy containing 90 wt% or more of Ni.

  In this embodiment, the shortest distance F in the radial direction between the side peripheral surface of the center electrode 3 and the other end surface 53 of the second ground electrode 5 is 1.6 mm, and the other end surface 53 of the second ground electrode 5 and the insulator 2 The shortest distance A in the radial direction from the side peripheral surface is 0.6 mm. That is, A + 0.7 (F−A) = 1.3, which is 1.18M. Thus, by satisfying A + 0.7 (FA) ≦ 1.8M, the electric field strength near the air gap g1 can be concentrated, and variations in the discharge voltage can be suppressed. Therefore, ignitability can be maintained.

  Furthermore, the axial distance H between the front end surface 2a of the insulator 2 and the midpoint P of the air gap g1 in the present embodiment is 2.05 mm, the front end surface 2a of the insulator 2 and the other end surface of the second ground electrode 5 The distance L in the axial direction from the front end side edge 53a of 53 is 1 mm. Thus, even when L / H is −0.3 or more, variation in discharge voltage can be effectively suppressed. If the value protruding from the front end surface of the insulator is + and is negative, the front end side edge of the other end surface 53 of the second ground electrode 5 is more than the front end surface 2a of the insulator 2 as shown in FIG. 53a is recessed.

  Such a spark plug 100 is manufactured as follows. However, the manufacturing method of the main part of the spark plug 100 will be mainly described, and the description of the known part will be omitted or simplified.

  First, the insulator 2 is formed by using alumina as a main raw material and firing it into a predetermined shape at a high temperature. Moreover, the metal shell 1 is formed by using a steel material and plastic processing into a predetermined shape. At this time, the threaded portion 12 is formed on the outer peripheral surface of the distal end portion of the metal shell 1. Next, a rod-shaped center electrode 3 made of a Ni heat-resistant alloy, a first ground electrode body 4a, and a second ground electrode 5 are formed. Note that when the center electrode 3 is formed, the metal core 3b is inserted. Then, the first ground electrode body 4 a and the second ground electrode 5 are electrically resistance-welded to the front end surface of the metal shell 1. Thereafter, the second ground electrode 5 is bent in the axial direction by a known method. On the other hand, the center electrode 3 is reduced in diameter at the tip, and the first noble metal tip 31 is fixed to the tip surface by electrical resistance welding, laser welding, or the like.

  Then, the center electrode 3 is inserted into the shaft hole 6 of the insulator 2 so that the tip side protrudes from the insulator 2, and then the conductive seal layer 16, the resistor 15, and the conductive seal layer 17 are sequentially arranged on the rear end side. Further, the terminal fitting 8 is inserted on the rear end side of the insulator 2 so that the rear end side of the terminal fitting 5 protrudes from the rear end of the insulator 2 and fixed using a known method. . Then, the insulator 2 to which the center electrode 3, the terminal fitting 8 and the like are fixed is connected to the metal shell 1 to which the first ground electrode body 4a and the second ground electrode 5 are fixed, and the second ground electrode 5 is connected to the second ground electrode 5. Assembling is performed by a known method while adjusting the discharge gap g2. Then, the second noble metal tip 41 is fixed to the other end of the first ground electrode body 4a by electric resistance welding, laser welding, or the like. The first ground electrode 4 is arranged so that the second noble metal tip 41 of the first ground electrode 4 faces the tip surface 31a of the first noble metal tip 31 of the center electrode 3 via the first discharge gap g1. 4 (first ground electrode body 4a) is bent to complete the spark plug 100 for the internal combustion engine as shown in FIG.

In order to confirm the effect of the present invention, the following various experiments were conducted.
First, the variation in the discharge voltage in the spark plug in which the ignitability was improved by thinning the noble metal tip joined to the ground electrode and increasing the protrusion amount was measured. First, in the spark plug of the embodiment, sintered alumina ceramic is used as the material of the insulator 2, INCONEL 600 is used as the material of the electrode body 3a of the center electrode 3, copper core is used as the core material 3b, and INCONEL 600 is used as the material of the first ground electrode body 4a. The second ground electrode 5 is made of a heat-resistant Ni alloy (90 wt% Ni alloy), the first noble metal tip 31 is made of Ir-20 wt% Rh, and the second noble metal tip 41 is made of Pt-20 wt% Ni. did. The first noble metal tip 31 has a cylindrical shape with a diameter of φ0.6, the protruding amount t of the second noble metal tip 41 has a cylindrical shape with a diameter of 0.8 mm, and a diameter of φ0.7 mm. Further, the width of the first ground electrode body 4a is 2.5 mm and the height is 1.4 mm. On the other hand, in the spark plug of the comparative example, the protruding amount t of the second noble metal tip 41 was 0.1 mm, and other materials and dimensions were the same as in the example.

  Then, voltage variations were compared in the spark plugs of the examples and comparative examples when M was set to 0.4, 0.6, 0.8, 1.0, and 1.2 (units were mm). . The results are shown in Table 1. As an evaluation test, one spark plug of each of the example and the comparative example was prepared, and a desktop spark test was performed. Specifically, spark discharge was caused 500 times in the spark plug in an atmosphere of 0.6 MPa. Then, the standard deviation of the discharge voltage of 500 times was measured, and the standard deviation of the discharge voltage in the spark plug of the example in the same gap and the standard deviation of the discharge voltage in the spark plug of the comparative example were compared.

  According to Table 1, when M was 0.6 mm or less, there was almost no difference in the standard deviation of the discharge voltage between the comparative example and the example. However, when M exceeded 0.6 mm, this example became a comparative example. In comparison, the standard deviation of the discharge voltage was increased. In other words, spark plugs with improved ignitability by thinning the noble metal tip bonded to the ground electrode and increasing the amount of protrusion tend to cause variations in discharge voltage when M exceeds 0.6 mm. I understand.

  Next, various test pieces of the spark plug having the shape shown in FIGS. 1 to 3 were prepared as follows. Similar to the first embodiment, sintered alumina ceramic is used as the material of the insulator 2, INCONEL 600 is used as the material of the electrode body 3 a of the center electrode 3, copper core is used as the core material 3 b, and INCONEL 600 is used as the material of the first ground electrode body 4 a. A heat resistant Ni alloy (90 wt% Ni alloy) was selected as the material of the ground electrode 5, Ir-20 wt% Rh was selected as the material of the first noble metal tip 31, and Pt-20 wt% Ni was selected as the material of the second noble metal tip 41. The first noble metal tip 31 has a cylindrical shape with a diameter of φ0.6, the protruding amount t of the second noble metal tip 41 has a cylindrical shape with a diameter of 0.8 mm, and a diameter of φ0.7 mm. Furthermore, the width of the first ground electrode body 4a was 2.5 mm and the height was 1.4 mm, and the width of the second ground electrode 5 was 2.2 mm and the height was 1.2 mm. On the other hand, as a comparative example, a spark plug without the second ground electrode 5 was prepared. The other materials and dimensions of the spark plug of the comparative example are the same as those of the test product.

2 and FIG. 3, regarding the relationship between M and electric field strength when F is 1.5 mm and A is 0.5 mm, M is 0.4, 0.6, 0.8, 0.9, 1 .1, 1.3, 1.5, 1.7, 1.9 (each unit is mm), the electric field strength was calculated by FEM analysis (finite element analysis). The results are shown in Table 2.
2 and FIG. 3, regarding the relationship between M and electric field strength when F is 1.8 mm and A is 0.8 mm, M is 0.4, 0.6, 0.8, 0.9, 1 .1, 1.3, 1.5, 1.7, 1.9 (each unit is mm), the electric field strength was calculated by FEM analysis (finite element analysis). The results are shown in Table 3.
2 and FIG. 3, regarding the relationship between M and electric field strength when F is 2.1 mm and A is 1.1 mm, M is 0.4, 0.6, 0.8, 0.9, 1 .1, 1.3, 1.5, 1.7, 1.9 (each unit is mm), the electric field strength was calculated by FEM analysis (finite element analysis). The results are shown in Table 4.

  According to Table 2, when M was 0.6 mm or less, there was no difference in electric field strength between the comparative example and the present example, but at 0.8 mm or more, this example had a higher electric field strength than the comparative example. became. That is, it can be seen that the electric field strength is increased by the second ground electrode. Next, according to Table 3, when M was 0.8 mm or less, there was no difference in electric field strength between the comparative example and the present example, but in the case of 0.9 mm or more, this example was compared with the comparative example. Strength increased. That is, it can be seen that the electric field strength is increased by the second ground electrode. According to Table 4, when M is 0.9 mm or less, there was no difference in electric field strength between the comparative example and the present example, but when 1.1 mm was exceeded, the electric field of this example was higher than that of the comparative example. Strength increased. That is, it can be seen that the electric field strength is increased by the second ground electrode. From the above, it can be seen that when A + 0.7 (FA) ≦ 1.8M, the electric field strength increases and the discharge voltage decreases.

  Next, the distance M of the discharge gap g1 of the spark plug 100 of the present invention, the shortest distance F in the radial direction between the side peripheral surface of the center electrode 3 and the other end surface 53 of the second ground electrode 5, the other of the second ground electrode 5 The shortest radial distance A between the end surface 53 and the side peripheral surface of the insulator 2 is fixed, and the axial distance H between the tip surface 2a of the insulator 2 and the midpoint P of the air gap g1 is determined. The relationship between the electric field intensity and the distance L in the axial direction between the distal end surface 2a of No. 2 and the distal end side edge 53a of the other end surface 53 of the second ground electrode 5 was confirmed. First, a spark plug similar to that of Example 1 was produced. The distance M of the discharge gap g1 is 1.1 mm, the shortest distance F in the radial direction between the side peripheral surface of the center electrode 3 and the other end surface 53 of the second ground electrode 5 is 1.8 mm, and the second ground electrode 5 The shortest distance A in the radial direction between the other end surface 53 and the side peripheral surface of the insulator 2 was fixed to 0.8 mm.

  The electric field strength of this spark plug 100 was calculated by FEM analysis (finite element analysis) in the same manner as in Example 2. The results are shown in Table 5.

  According to Table 5, when L / H is -1, 31, L / H is -0.8, 31, L / H is -0.6, 30.8, and L / H is -0.4. 31, L / H is -0.3, 32, L / H is -0.2, 34, L / H is 0, 37, L / H is 0.2, 37, L / H Is 0.4, 37, L / H is 0.6, 37, L / H is 0.7, 37.5, and L / H is 0.8, 37.2. Thus, when L / H is less than −0.3, the electric field strength is 32 or less. However, when L / H is −0.3 or more, the electric field strength exceeds 32. Thereby, electric field strength improves because L / H shall be more than -0.3.

  Next, 100 spark plugs of Example 2 were prepared, and a desktop spark discharge test was performed. When the atmosphere was pressurized at 0.6 MPa and spark discharge occurred in the spark plug, the number of times of spark discharge at the second ground electrode is shown. The results are shown in Table 6, Table 7, and Table 8.

  According to Table 6, when M is 1.1 mm or less, the number of sparks generated at the second ground electrode was 0, but M was 1.3 mm, 3 times, M was 1.5 mm, 8 times, M Increases 20 times at 1.7 mm and 30 times at M of 1.9 mm. Next, according to Table 7, when M is 1.5 mm or less, the number of sparks generated at the second ground electrode was 0, but M was 1.7 mm and 4 times, and M was 1.9 mm and 15 Increase with times. According to Table 8, when M is 1.7 mm or less, the number of sparks generated at the second ground electrode was 0, but M increased to 15 times at 1.9 mm. From the above, by setting M ≦ A + 0.7 (FA), spark discharge is likely to occur at the creeping gap between the other end surface of the second ground electrode and the side peripheral surface of the center electrode. I understand.

  Next, using the same spark plug as in Example 3, the axial distance H between the tip surface 2a of the insulator 2 and the midpoint P of the air gap g1, the tip surface 2a of the insulator 2 and the second The ignitability relationship with the distance L in the axial direction from the front end side edge 53a of the other end surface 53 of the ground electrode 5 was confirmed.

  A spark plug produced in the same manner as in Example 3 was attached to an engine with a displacement of 2000 cc and a 4-cylinder engine. A test was conducted. In this test, the A / F value when the HC spike occurs 10 times per 3 minutes under the above engine conditions is judged as the ignition limit, and the relationship between the A / F value and L / H at this time is investigated. did. The results are shown in Table 9.

  According to Table 9, when L / H is -1, 17.65, when L / H is -0.8, 17.7, when L / H is -0.6, 17.65 and L / H are-. At 0.4, 17.7, when L / H is -0.2, 17.7, when L / H is 0, 17.7, when L / H is 0.2, 17.65, L / H Is 0.46, 17.62, L / H is 0.6, 17.55, L / H is 0.7, 17.5, L / H is 0.8, 17.1, L When / H is 1, it is 17.06. Thus, when L / H is 0.7 or less, the ignition limit is a high value of 17.5 or more, but when L / H exceeds 0.7, it becomes less than 17.5. Thus, ignitability can be maintained by setting L / H to 0.7 or less.

  The present invention is not limited to the specific embodiments described above, and various modifications can be made within the scope of the present invention depending on the purpose and application. For example, in the spark plug 100 of the present invention, the metal core 3 b is inserted only into the center electrode 3, but the present invention is not limited thereto, and the metal core is inserted into either the first ground electrode body 4 a or the second ground electrode 5. May be. And the material of the metal core in this case is formed from simple substance or alloy, such as Cu and Ag.

  Further, the spark plug 100 of the present invention has two second ground electrodes 5, but is not limited thereto, and may be one. However, the electric field strength can be effectively concentrated by providing a plurality. Therefore, it may be three or more.

The front fragmentary sectional view which shows the spark plug 100 of this invention. The front fragmentary sectional view which shows the principal part of FIG. The side view which shows the principal part of FIG. FIG. 6 is a front partial cross-sectional view showing another example of the spark plug 100 of the first embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Metal shell 2 Insulator 3 Center electrode 4 1st ground electrode 5 2nd ground electrode 6 Through-hole 31 1st noble metal tip 41 2nd noble metal tip 100 Spark plug

Claims (5)

An insulator having an axial hole extending in the axial direction;
A center electrode disposed on the tip side of the shaft hole of the insulator;
A metal shell surrounding the periphery of the insulator;
A first ground electrode body having one end coupled to the metal shell and a center electrode side inner peripheral surface of the other end of the first ground electrode body, and facing the tip surface of the center electrode with a discharge gap therebetween A first ground electrode having a noble metal tip to
One end is coupled to the metal shell, and the other end surface is disposed so as to face at least the side peripheral surface of the insulator, and a creeping gap is formed between the side peripheral surface of the center electrode and the other end surface of itself. A second ground electrode;
With
The area of the facing surface of the noble metal tip facing the tip surface of the center electrode is S (unit: mm ² ), and the axial direction of the facing surface of the noble metal tip and the inner peripheral surface of the first ground electrode body on the center electrode side T (unit is mm), the air gap distance is M (unit is mm), and the shortest radial distance between the side surface of the center electrode and the other end surface of the second ground electrode is F ( The unit is mm), the shortest radial distance between the other end surface of the second ground electrode and the side peripheral surface of the insulator is A (unit is mm), the tip surface of the insulator and the air gap The shortest distance H (unit: mm) in the axial direction from the intermediate point, and the shortest distance L (unit: mm) in the axial direction between the front end surface of the insulator and the front end side edge of the other end surface of the second ground electrode However, when the tip side edge of the other end surface of the second ground electrode protrudes from the tip surface of the insulator, When you,
0.12 ≦ S ≦ 1.15
0.3 ≦ t ≦ 1.5
A + 0.7 (F−A) ≦ 1.8M
-0.3 ≦ L / H
Spark plug characterized by being. The spark plug according to claim 1, wherein
M ≦ A + 0.7 (F−A)
L / H ≦ 0.7
Spark plug characterized by being. The spark plug according to claim 1 or 2,
The noble metal tip is a spark plug mainly composed of one of Ir and Pt. In the spark plug according to any one of claims 1 to 3,
A spark plug characterized by M> 0.6 mm. The spark plug according to any one of claims 1 to 4, wherein a plurality of second ground electrodes are provided.