JP2007234435A - Spark plug - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spark plug capable of improving ignitability while efficiently arranging it in a small range by specifying an optimum positional relationship between a spark discharge gap and a ground electrode. <P>SOLUTION: It is preferable that the distance (d) between a central point C of the spark discharge gap composed of a center electrode 20 of this spark plug 100 and the ground electrode 30 thereof, and the ground electrode 30 is not smaller than 2.6 mm. In the circumference of the spark plug gap, when a flame kernel formed in the spark discharge gap grows, contact to the ground electrode 30 in its growth initial stage can be avoided, and an anti-flammatory action can be reduced. By specifying the optimum positional relationship between the spark discharge space and the ground electrode 30, respective members arranged around the spark discharge gap can be efficiently arranged in a small range. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spark plug that is assembled in an internal combustion engine and ignites an air-fuel mixture.

  Conventionally, spark plugs for ignition are used in internal combustion engines. A typical spark plug has a center electrode that forms an electrode for spark discharge on its tip side, an insulator that holds the center electrode in the shaft hole, and surrounds the periphery of the insulator in the radial direction. And a metal shell. One end of the ground electrode is joined to the front end surface of the metal shell, and the other end is bent so as to face the center electrode to form a spark discharge gap between the front end of the center electrode. . A spark discharge is performed in the spark discharge gap, whereby the mixture is ignited.

  By the way, one end of the ground electrode is usually arranged forward in the axial direction with respect to the tip of the center electrode, and has the shortest distance between the two in the axial direction, and discharge in the spark discharge gap is performed in this direction. On the other hand, since the other end of the ground electrode is joined to the front end surface of the metal shell, the ground electrode is bent so as to draw a gentle curve from one end to the other end. That is, in the direction orthogonal to the discharge direction, the distance between the ground electrode and the spark discharge gap becomes shorter as it is closer to one end. In such a spark plug, the flame core formed in the spark discharge gap is likely to come into contact with the bent portion of the ground electrode in the process of growing (burning out) in the direction perpendicular to the discharge direction, and may be extinguished. .

Therefore, the ground electrode is extended in parallel with the center electrode, bent forward in the axial direction from the tip surface of the center electrode, and the other end is configured to face the tip surface of the center electrode from the front in the axial direction (for example, , See Patent Document 1). In this way, the space around the spark discharge gap can be widened, and the flame extinguishing action by the ground electrode can be made difficult during the growth process of the flame kernel.
Japanese Unexamined Patent Publication No. 6-68951

  However, in Patent Document 1, since the ground electrode protrudes greatly forward in the axial direction from the front end surface of the center electrode, the ground electrode protrudes in the combustion chamber when the spark plug is assembled to the internal combustion engine, and the internal combustion engine There was a possibility of interfering with the constituent members. On the other hand, if the ground electrode is simply brought close to the spark discharge gap, there is a problem that the flame nucleus formed in the spark discharge gap is easily subjected to the extinguishing action by the ground electrode and the ignitability is lowered.

  The present invention has been made to solve the above-mentioned problems, and it is possible to improve the ignitability while efficiently arranging the spark discharge gap and the ground electrode within a small range by defining the optimum positional relationship between the spark discharge gap and the ground electrode. An object is to provide a spark plug that can be used.

In order to achieve the above object, a spark plug according to a first aspect of the present invention has a center electrode and an axial hole extending in the axial direction, with the tip of the center electrode protruding from its tip. An insulator that holds the center electrode inside the shaft hole, and a metal shell that surrounds and holds the periphery of the insulator in the radial direction in a state where the tip of the insulator protrudes from the tip surface of the insulator. One end is joined to the front end surface of the metal shell, one side of the other end is bent so as to face the center electrode, and a spark is formed between one side of the other end and the front end surface of the center electrode. A ground electrode that forms a discharge gap, wherein the size of the spark discharge gap is 2.0 mm or less, and between a midpoint of the spark discharge gap and one side surface of the other end of the ground electrode. For spark plugs with a distance of 1.0 mm or less When the width of the ground electrode in the direction perpendicular to the axial direction and perpendicular to the extending direction of the ground electrode is t and the size of the spark discharge gap is G, d−0.5t (G−2 ) ² ≧ 1.0 is satisfied.

  In addition to the configuration of the invention of claim 1, a spark plug of the invention according to claim 2 includes a midpoint of the spark discharge gap and a cross section of the spark plug perpendicular to the discharge direction in the spark discharge gap. When the distance between the midpoint position of the spark discharge gap and the position of the ground electrode is d, d ≧ 2.6 (mm).

The flame nuclei formed in the spark discharge gap affect the magnitude of the flame extinguishing action received from the ground electrode depending on the volume of the ground electrode in the vicinity of the spark discharge gap. The volume of the ground electrode is proportional to the size of the width t of the ground electrode in the direction perpendicular to the axial direction and perpendicular to the extending direction of the ground electrode, and the extinguishing action that the ground electrode gives to the flame kernel as t decreases. Becomes smaller. On the other hand, the ignitability improves as the distance d between the midpoint of the spark discharge gap and the ground electrode increases. Therefore, as in the invention according to claim 1, it is preferable to satisfy the relational expression d-0.5t (G-2) ² ≧ 1.0 found by the inventors of the present application. In this relational expression, as the value of the distance d between the midpoint of the spark discharge gap and the ground electrode is larger, the value of the width t of the ground electrode is smaller, and the value G of the spark discharge gap is 2. As it approaches, d−0.5t (G−2) ² tends to be a value of 1.0 or more.

  As the distance d between the midpoint of the spark discharge gap and the ground electrode is larger, as described above, it is possible to avoid contact with the ground electrode in the early stage of the growth of the flame kernel, so that the extinguishing action is reduced. This makes it difficult to misfire and improves the ignitability of the spark plug. In addition, the smaller the width t of the ground electrode, the smaller the volume of the ground electrode. Therefore, when the growing flame nucleus contacts the ground electrode, the amount of heat taken away by the ground electrode is reduced, and the flame extinguishing action can be reduced. . Further, if the width t of the ground electrode is reduced, if the flow of the air-fuel mixture in the combustion chamber is from the ground electrode side toward the spark discharge gap, the air-fuel mixture bypasses the periphery of the ground electrode and enters the spark discharge gap. The ignitability is unlikely to decrease because it tends to flow toward the vehicle. In the range of 0 ≦ G ≦ 2.0, the size of the spark discharge gap increases as the spark discharge gap size G approaches 2.0 mm. Therefore, when a flame nucleus formed in the spark discharge gap grows. It is possible to reduce the flame extinguishing effect received by the ground electrode. The values of d, t, and G are respectively determined by the above relational expressions so that the spark plug has an effect of extinguishing action on the flame nucleus formed in the spark discharge gap around the spark discharge gap. It is possible to obtain a sufficient space to reduce the amount of spark plugs and improve the ignitability of the spark plug. Furthermore, each member arrange | positioned around a spark discharge gap can be efficiently arrange | positioned in a small range, and size reduction of a spark plug can be achieved.

  In the invention according to claim 2, since the distance d between the midpoint of the spark discharge gap and the ground electrode is secured in the direction orthogonal to the discharge direction in the spark discharge gap, a size of 2.6 mm or more is secured. In the process of growing the flame nuclei formed in the spark discharge gap, the flame nuclei grow sufficiently in the direction perpendicular to the discharge direction so that there is no risk of misfire before contacting the ground electrode. be able to. That is, since it is possible to avoid contact with the ground electrode in the initial stage of the growth of the flame kernel, the flame extinguishing action is reduced and misfire is difficult to occur, and the ignitability of the spark plug is improved. In this way, by defining the optimum positional relationship between the spark discharge gap and the ground electrode, the ignitability of the spark plug is improved, and each member disposed around the spark discharge gap is within a small range. Therefore, the spark plug can be efficiently arranged and the spark plug can be downsized.

  By the way, also in the discharge direction of the spark discharge gap, the flame extinguishing action can be reduced as the spark discharge gap is increased because the formed flame nucleus grows and contacts the ground electrode. In this discharge direction, the flame kernel ignites the air-fuel mixture along the discharged sparks and spreads, making it difficult to misfire early in the growth process. Specifically, if the size of the spark discharge gap is larger than 2.0 mm, the ignitability is not improved as compared with 2.0 mm. For this reason, it is difficult to evaluate the ignitability by paying attention to the growth of flame nuclei in the direction orthogonal to the discharge direction when the flame nuclei are sufficiently grown by increasing the spark discharge gap. Specifically, it is important to evaluate the ignitability when the size of the spark discharge gap is 2.0 mm or less.

  Further, if a projection such as a noble metal tip is provided on one side surface of the other end of the ground electrode facing the tip of the center electrode, and a spark discharge gap is formed between the projection and the center electrode, a flame nucleus As the electrode grows in the discharge direction, the distance until it comes into contact with the base material of the ground electrode is increased, and the flame extinguishing action is reduced, thereby improving the ignitability. However, as described above, when the distance between the midpoint of the spark discharge gap and the one side surface of the other end of the ground electrode is increased, the flame nucleus can be sufficiently grown. It is difficult to evaluate the ignitability focusing on the growth of flame kernels. Specifically, it is important to evaluate the ignitability when the distance between the midpoint of the spark discharge gap and one side surface of the other end of the ground electrode is 1.0 mm or less.

  Hereinafter, an embodiment of a spark plug embodying the present invention will be described with reference to the drawings. First, the structure of a spark plug 100 as an example will be described with reference to FIG. FIG. 1 is a partial cross-sectional view of a spark plug 100. In FIG. 1, the axis O direction of the spark plug 100 will be described as the vertical direction in the drawing, the lower side will be described as the front end side (front), and the upper side will be described as the rear end side (rear).

  As shown in FIG. 1, the spark plug 100 generally includes an insulator 10, a metal shell 50 that holds the insulator 10, a center electrode 20 that is held in the insulator 10 in the direction of the axis O, and a metal shell. 50 is provided at the rear end portion of the insulator 10 and the ground electrode 30 which is bent so that the inner end 33 of the front end portion 31 faces the front end portion 22 of the center electrode 20. Terminal metal fitting 40.

  First, the insulator 10 constituting the insulator of the spark plug 100 will be described. As is well known, the insulator 10 is formed by firing alumina or the like, and has a cylindrical shape in which an axial hole 12 extending in the direction of the axis O is formed at the axial center. A flange portion 19 having the largest outer diameter is formed substantially at the center in the direction of the axis O, and a rear end body portion 18 is formed on the rear end side (upper side in FIG. 1). A front end side body portion 17 having an outer diameter smaller than that of the rear end side body portion 18 is formed on the front end side (lower side in FIG. 1) from the flange portion 19, and further, the front end side is closer to the front end side than the front end side body portion 17. A long leg portion 13 having an outer diameter smaller than that of the side body portion 17 is formed. The long leg portion 13 is reduced in diameter toward the tip side, and when the spark plug 100 is attached to the engine head (not shown) of the internal combustion engine, it is exposed to the combustion chamber. A step portion 15 is formed between the long leg portion 13 and the rear end side trunk portion 18.

  Next, the center electrode 20 is formed of a nickel-based alloy such as Inconel (trade name) 600 or 601 and has a metal core 23 made of copper or the like having excellent thermal conductivity. The center electrode 20 is held on the distal end side in the shaft hole 12 of the insulator 10 so that the axis thereof coincides with the axis O of the spark plug 100. The distal end side of the center electrode 20 protrudes from the distal end surface of the distal end portion 11 of the insulator 10, and the protruding portion is formed so that the diameter decreases toward the distal end side. A noble metal tip 91 for improving the spark wear resistance is joined to the tip of the protruding portion, and a small-diameter tip 22 is formed integrally with the center electrode 20 body. In the present embodiment, the noble metal tip 91 integrated with the center electrode 20 is referred to as a “center electrode”.

  Further, the center electrode 20 is electrically connected to the upper terminal fitting 40 via the seal body 4 and the ceramic resistor 3 provided in the shaft hole 12. A high voltage cable (not shown) is connected to the terminal fitting 40 via a plug cap (not shown) so that a high voltage is applied.

  Next, the metal shell 50 will be described. The metal shell 50 is a cylindrical metal fitting for fixing the spark plug 100 to an engine head of an internal combustion engine (not shown), and the leg length is set in a state where the tip portion 11 of the insulator 10 protrudes from its tip surface 57. The insulator 10 is held inside so as to surround a portion from the portion 13 to the rear end side body portion 18. The metal shell 50 is formed of a low carbon steel material, and includes a tool engaging portion 51 into which a spark plug wrench (not shown) is fitted, and a male screw-like screw portion 52 that is screwed into an engine head provided at the upper portion of the internal combustion engine. I have. A flange-like seal portion 54 is formed between the tool engaging portion 51 and the screw portion 52, and a gasket for preventing airtight leakage in the engine at a position between the seal portion 54 and the screw portion 52. 5 is inserted.

  A thin caulking portion 53 is provided on the rear end side of the metal fitting 50 from the tool engaging portion 51, and a thin buckling portion 58 is provided between the seal portion 54 and the tool engaging portion 51. ing. An annular ring is formed between the inner peripheral surface of the metal shell 50 from the tool engaging portion 51 to the caulking portion 53 and the outer peripheral surface of the rear end side body portion 18 of the insulator 10 held inside. Members 6 and 7 are interposed, and a powder of talc (talc) 9 is filled between the ring members 6 and 7. Then, the insulator 10 is pressed toward the front end side in the metal shell 50 through the ring members 6 and 7 and the talc 9 by bending the end portion of the crimp portion 53 inward. As a result, the step portion 15 of the insulator 10 is supported via the annular plate packing 8 on the step portion 56 formed at the position of the screw portion 52 on the inner periphery of the metal shell 50, and the metal shell 50 and the insulator 10 is integrated. At this time, the airtightness between the metal shell 50 and the insulator 10 is maintained by the plate packing 8, and the outflow of combustion gas is prevented. In addition, the buckling portion 58 is configured to bend outwardly and deform as the compression force is applied during caulking, and increases the compression stroke of the talc 9 to improve airtightness.

  Next, the ground electrode 30 will be described. The ground electrode 30 is made of a metal having high corrosion resistance. As an example, a nickel alloy such as Inconel (trade name) 600 or 601 is used. The ground electrode 30 has a substantially rectangular cross section in the longitudinal direction, and the base end portion 32 is joined to the front end surface 57 of the metal shell 50 by welding. The tip 31 of the ground electrode 30 is bent so that the inner surface 33 faces the tip 22 of the center electrode 20, and the inner surface 33 and the tip 21 of the center electrode 20 (in the present embodiment, the noble metal tip 91 of the noble metal tip 91). A spark discharge gap is formed between the tip surface 21). In the present embodiment, since the tip surface 21 of the center electrode 20 and the inner surface 33 of the ground electrode 30 are configured by planes facing each other and orthogonal to the axis O, the shortest distance between them is in the direction of the axis O, With this direction as the discharge direction, discharge in the spark discharge gap is performed. In addition, the front-end | tip part 31 and the base end part 32 of the ground electrode 30 are corresponded to the "other end part" and the "one end part" in this invention, respectively. The inner surface 33 corresponds to “one side surface” in the present invention.

  In the spark plug 100 according to the present embodiment configured as described above, in the process in which the flame kernel formed in the spark discharge gap grows in the direction orthogonal to the discharge direction, it does not misfire until it contacts the ground electrode 30. Based on the result of an evaluation test described later, an optimum positional relationship with the size of the ground electrode 30 and the spark discharge gap is defined so that sufficient growth is achieved. Hereinafter, with reference to FIG. 2 and FIG. 3, the rules regarding the positional relationship and size between the ground electrode 30 and the spark discharge gap will be described. FIG. 2 is a side view of the spark plug 100 in which the joining position of the metal shell 50 and the ground electrode 30 is arranged directly beside and the vicinity of the spark discharge gap of the spark plug 100 is enlarged. FIG. 3 is another side view of the spark plug 100 viewed from the right side of the spark plug 100 in FIG. 2 in an enlarged manner near the spark discharge gap.

  First, in the cross section of the spark plug 100 including the midpoint of the spark discharge gap and orthogonal to the discharge direction in the spark discharge gap, d is the distance between the midpoint of the spark discharge gap and the position of the ground electrode 30. It is defined that ≧ 2.6 (mm) is satisfied. In the spark plug 100 of the present embodiment, as shown in FIG. 2, the tip surface 21 of the center electrode 20 having a plane orthogonal to the direction of the axis O and the inner surface 33 of the ground electrode 30 are mutually connected with the axis O direction as the shortest distance. It is the structure which forms a spark discharge gap | interval facing each other. That is, the discharge direction in the spark discharge gap coincides with the axis O direction. Therefore, in the direction of the axis O, the intermediate point between the tip surface 21 of the center electrode 20 and the inner surface 33 of the ground electrode 30 can be said to be the midpoint C of the spark discharge gap in the present embodiment. When viewed from the side, the cross section of the spark plug 100 including the midpoint C of the spark discharge gap and perpendicular to the discharge direction in the spark discharge gap passes through this midpoint C and is perpendicular to the axis O as shown in FIG. Can be seen as a virtual line. For this reason, the distance between the position of the midpoint C of the spark discharge gap and the position of the ground electrode in the cross section of the spark plug 100 is the contour of the virtual line and the ground electrode 30 in the side view shown in FIG. It can be said that it is the distance between the middle point C and the intersection point E, where E is the intersection point on the side close to the spark discharge gap. In the present embodiment, the distance between the midpoint C and the intersection E is d.

  When the flame nucleus formed in the spark discharge gap grows in a direction orthogonal to the axis O (burns out), if the distance d is smaller than 2.6 mm, it contacts the ground electrode 30 before the flame nucleus grows sufficiently. Therefore, the ignitability is reduced. Therefore, it is preferable to adjust the bending position and bending condition of the ground electrode 30 so that the distance d is 2.6 mm or more.

Further, when the size of the spark discharge gap is G, the width of the ground electrode 30 in the direction orthogonal to the axis O direction and the direction orthogonal to the extension direction of the ground electrode 30 is t, d−0.5t (G− 2) It is defined that ² ≧ 1.0 is satisfied. More specifically, with respect to the size G of the spark discharge gap, as shown in FIGS. 2 and 3, the tip surface 21 of the center electrode 20 and the inner surface 33 of the ground electrode 30 facing each other are located at the shortest distance. The size between the two is the size G of the spark discharge gap. The width of the ground electrode 30 in the direction perpendicular to the direction of the axis O and perpendicular to the extending direction of the ground electrode 30, that is, the direction from the base end portion 32 to the tip end portion 31 (the direction indicated by the alternate long and short dash line P in FIG. 2). T.

The relational expression, d-0.5t (G-2) ^2, is an index indicating the quality of ignitability, and is obtained based on the result of an evaluation test described later. It can be seen that in order to make this value 1.0 or more, it is preferable to increase d, decrease t, and approximate G to 2 (mm). That is, increasing d means that the distance from the ground electrode 30 is increased in the direction orthogonal to the discharge direction in the process of growth of the flame kernel formed in the spark discharge gap. Further, reducing t means reducing the volume of the ground electrode 30 disposed at least in the vicinity of the spark discharge gap. Further, if the width t of the ground electrode 30 is reduced in the direction orthogonal to the discharge direction, the air-fuel mixture flows in the combustion chamber from the ground electrode 30 side toward the spark discharge gap. The ignitability does not deteriorate because it tends to flow around the spark discharge gap around the periphery.

  Further, bringing G closer to 2 (mm) means that the flame nuclei formed in the spark discharge gap grow, and in the discharge direction, the extinction by the ground electrode 30 becomes closer to 2.0 mm in the discharge direction. It means that it becomes difficult to receive the action. In the discharge direction in the spark discharge gap, the flame nuclei ignite the air-fuel mixture along the discharged sparks and spread, so that the growth is fast. For this reason, even if the size G of the spark discharge gap is larger than 2.0 mm, the ignitability is not improved as compared with 2.0 mm. Therefore, in order to evaluate the ignitability with respect to the growth of flame nuclei in the direction orthogonal to the discharge direction, it is important to evaluate the case where the spark discharge gap size G is 2.0 mm or less.

  Further, if a protrusion such as a noble metal tip is provided on the inner surface 33 of the ground electrode 30, flame nuclei generated in the spark discharge gap formed between the protrusion and the center electrode 20 grow in the discharge direction. Above, the distance until it contacts with the base material of the ground electrode 30 can be increased, and the flame extinguishing action can be reduced. However, as described above, in evaluating the ignitability to the growth of flame nuclei in the direction orthogonal to the discharge direction, the flame nuclei are increased by increasing the distance between the midpoint C of the spark discharge gap and the inner surface 33 of the ground electrode 30. It is difficult to get enough growth. Therefore, when the above-described protrusions of the ground electrode 30 are not formed, that is, the distance between the center C of the spark discharge gap and the inner surface 33 of the tip 31 of the ground electrode 30 is 1.0 mm or less. In some cases, it is important to evaluate ignitability.

  Thus, in order to confirm that the ignitability can be improved by defining the size of the ground electrode 30 and the positional relationship with the spark discharge gap, the following evaluation test was performed.

[Example 1]
First, an evaluation test was performed on the relationship between the spark discharge gap size G and the width t of the ground electrode 30 in terms of the ignition limit air-fuel ratio. In this evaluation test, three types of 1.0, 1.4, and 2.8 (mm) as the width t of the ground electrode, and 0.6, 0.8, 1.1, as the size G of the spark discharge gap, Samples of 21 types of spark plugs having a configuration combining 7 types of 1.3, 1.5, 2.0, and 2.5 (mm) were produced. Each sample was assembled into a 6-cylinder 2000 cc engine. The engine was driven at 2000 rpm, and the air-fuel ratio at which the frequency of misfire was 1% during 1000 spark discharges was measured, and this was determined as the ignition limit air-fuel ratio.

  As a result of this evaluation test, the ignition limit air-fuel ratio is improved as the spark discharge gap size G is increased. When the spark discharge gap size G is 2.0 mm or more, the ignition limit air-fuel ratio is not improved. It was. Therefore, with reference to the ignition limit air-fuel ratio when the spark discharge gap size G is 2.0 mm, the ignition limit sky of each sample having a different spark discharge gap size G for each size of the width t of the ground electrode. The difference between the fuel ratio and the reference ignition limit air-fuel ratio was determined as the ignition limit air-fuel ratio difference.

  The ignition limit air-fuel ratio difference of the seven types of samples in which the width t of the ground electrode is 1.0 mm and the spark discharge gap size G is varied between 0.6 and 2.5 mm as described above is as follows. It was -1.01, -0.74, -0.40, -0.23, -0.12, 0,0. In addition, the ignition limit air-fuel ratio difference of the seven types of samples in which the width t of the ground electrode is 1.4 mm and the spark discharge gap size G is varied between 0.6 and 2.5 mm as described above is They were -1.38, -1.20, -0.58, -0.32, -0.18, 0, 0, respectively. And the ignition limit air-fuel ratio difference of the seven types of samples in which the width t of the ground electrode is 2.8 mm and the size G of the spark discharge gap is varied between 0.6 and 2.5 mm as described above. It was -2.68, -2.01, -1.21, -0.71, -0.35, 0, 0, respectively. The results of this evaluation test are shown in Table 1. Further, FIG. 4 shows a graph in which the relationship between the ignition limit air-fuel ratio difference and the spark discharge gap size G is plotted for each difference in the width t of the ground electrode from the result of this evaluation test.

  Clearly from the graphs shown in Table 1 and FIG. 4, in the evaluation test of Example 1, the spark discharge gap size G increases and approaches to 2.0 mm regardless of the size of the width t of the ground electrode. It has been found that the limit air-fuel ratio difference is reduced, that is, the ignition limit air-fuel ratio is improved. On the other hand, it was confirmed that when the spark discharge gap size G is constant, the ignition limit air-fuel ratio difference becomes smaller and the ignition limit air-fuel ratio improves as the width t of the ground electrode decreases.

By the way, based on the graph shown in FIG. 4, it can be seen that there is a certain correlation among the spark discharge gap size G, the ground electrode width t, and the ignition limit air-fuel ratio difference. This correlation can be expressed by an approximate expression obtained by a known statistical method shown below.
Ignition limit air-fuel ratio difference = −0.5t (G−2) ² (where 0 ≦ G ≦ 2.0) (1)

[Example 2]
Next, the influence of the relationship between the distance d between the above-described midpoint C of the spark discharge gap and the intersection E (see FIG. 2) on the contour line of the ground electrode 30 on the ignition limit air-fuel ratio affects the ignitability. An evaluation test was conducted. In this evaluation test, a spark plug with a thread diameter of M14 and a spark plug with M10 were targeted. In the M14 spark plug, four types of samples with a ground electrode width t of 2.8 mm and spark discharge gap sizes G of 0.9, 1.1, 1.5, and 2.0 (mm), respectively. 16 pieces were produced, 4 pieces each. Each sample was arbitrarily adjusted so that the distance d was different when the tip of the ground electrode was bent during the manufacturing process. Similarly, for the M10 spark plug, the width t of the ground electrode is set to 2.2 mm, and the spark discharge gap size G is set to 0.7, 0.9, 1.1, 1.5 (mm) 4. Twelve samples of three types were prepared, each of which was arbitrarily adjusted so that the distances d were different.

  And the desktop evaluation test was done with respect to each sample. A spark plug to be evaluated and assembled in an ignition device is placed in a test chamber, and the test chamber is filled with an evaluation mixture obtained by mixing air and propane, and the spark plug is ignited. The area of the flame kernel ignited by ignition was measured when 3 ms passed after ignition, and the areas were compared. The area was measured using an image obtained by imaging the flame kernel from the same direction as in FIG.

  In this comparison, the sample having the longest distance d among the samples has good ignitability because the growth of the flame kernel is not inhibited. Based on the area of the flame kernel ignited by this sample, for the sample in which the area of the flame nucleus did not change even when the distance d was reduced, similarly, it was determined that the growth of the flame nucleus was not inhibited by the ground electrode, It evaluated as "(circle)" that ignitability was favorable. On the other hand, the sample in which the area of the flame kernel was reduced was determined to be that the growth of the flame nucleus was inhibited by the ground electrode, and was evaluated as “x” because it was poor in ignitability.

By the way, as described above, the larger the distance d, the more difficult it is to prevent the growth of flame nuclei, and the ignitability improves as the ignition limit air-fuel ratio difference approaches zero. Therefore, as an index for determining whether the ignitability is good or bad, a value obtained by adding the ignition limit air-fuel ratio difference based on the equation (1) to the distance d is represented by the following equation.
f = d−0.5t (G−2) ² (2)
And the value of f was calculated | required about each sample, and the relationship with the quality of ignitability was confirmed. The results are shown in Table 2. In addition, the graph which divided and plotted the relationship between f calculated | required by Formula (2) and the distance d according to the quality of ignitability was shown in FIG.

As apparent from the graphs shown in Table 2 and FIG. 5, it can be seen that all the sample groups in which the value of f is smaller than 1.0 are inferior in ignitability. In other words, it was shown that whether or not the ignitability is good can be determined by using the expression (2). Specifically, it was found that if d−0.5t (G-2) ² ≧ 1.0 is satisfied, the ignitability is improved.

  Moreover, as shown in FIG. 5, it turns out that all the samples determined to be inferior in ignitability have a distance d of less than 2.6 mm. In other words, if the positional relationship between the midpoint of the spark discharge gap and the ground electrode or the bending condition of the ground electrode is adjusted so that at least a distance d of 2.6 mm or more can be secured, the ignitability can be improved. I understood.

  If a noble metal tip is provided on the inner surface 33 of the ground electrode 30 and the distance between the spark discharge gap and the base material of the ground electrode 30 in the direction of the axis O is widened, the growth of the flame kernel in that direction will not be hindered and ignition will occur. Although it is possible to improve the property, in Examples 1 and 2, the case where no noble metal tip was provided was verified. As described above, the verification of the case where the noble metal tip is not provided on the inner surface 33 of the ground electrode 30 makes it possible to define the optimum positional relationship between the spark discharge gap and the ground electrode 30 efficiently within a small range. It is important to arrange. Distance to the spark plug 100 in which the spark discharge gap size G is 2.0 mm or less and the distance between the center C of the spark discharge gap and the inner surface 33 of the ground electrode 30 is 1.0 mm or less. If d is configured to be 2.6 mm or more, the ignitability can be improved.

  Needless to say, the present invention can be modified in various ways. For example, in the present embodiment, the distance d between the midpoint of the spark discharge gap and the ground electrode 30 is ensured to be 2.6 mm or more by bending the ground electrode 30 substantially perpendicular to the direction of the axis O. However, in the case of a spark plug having a large screw diameter, the entire ground electrode may be gently curved as long as the distance d is 2.6 mm or more. Even when the discharge direction in the spark discharge gap does not follow the direction of the axis O, the distance d between the midpoint of the spark discharge gap and the ground electrode is 2.6 mm or more in the direction orthogonal to the discharge direction. Should be secured.

1 is a partial cross-sectional view of a spark plug 100. FIG. FIG. 3 is a side view of the spark plug 100 in which the joining position of the metal shell 50 and the ground electrode 30 is arranged directly beside and the vicinity of the spark discharge gap of the spark plug 100 is enlarged. FIG. 3 is another side view of the spark plug 100 in which the vicinity of the spark discharge gap is enlarged from the right side surface side of the spark plug 100 in FIG. 2. 5 is a graph in which the relationship between the ignition limit air-fuel ratio difference and the spark discharge gap size G is plotted for each difference in the width t of the ground electrode. It is the graph which divided and plotted the relationship between f and the distance d according to the quality of ignitability.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Insulator 11 Tip part 12 Shaft hole 20 Center electrode 21 Tip surface 22 Tip part 30 Ground electrode 31 Tip part 32 Base end part 33 Inner surface 50 Metallic body 57 Tip surface 100 Spark plug

Claims (2)

A center electrode;
An insulator that has an axial hole extending in the axial direction, and that holds the center electrode inside the axial hole in a state where the distal end of the central electrode protrudes from the distal end of the axial hole;
A metal shell that surrounds and holds the periphery of the insulator in the radial direction in a state in which the tip of the insulator protrudes from its tip surface,
One end is joined to the front end surface of the metal shell, one side of the other end is bent so as to face the center electrode, and a spark is formed between one side of the other end and the front end surface of the center electrode. A ground electrode that forms a discharge gap, and
In the spark plug in which the size of the spark discharge gap is 2.0 mm or less and the distance between the midpoint of the spark discharge gap and one side surface of the other end of the ground electrode is 1.0 mm or less ,
When the width of the ground electrode in a direction orthogonal to the axial direction and orthogonal to the extending direction of the ground electrode is t, and the size of the spark discharge gap is G,
d-0.5t (G-2) ² ≧ 1.0
A spark plug characterized by satisfying. In the cross-section of the spark plug that includes the midpoint of the spark discharge gap and is orthogonal to the discharge direction in the spark discharge gap, the distance between the position of the midpoint of the spark discharge gap and the position of the ground electrode is d. When
d ≧ 2.6 (mm)
The spark plug according to claim 1, wherein:

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