SPARK PLUG

Abstract

Containing a spark plug (10; 50; 80): a central electrode (20) extending along an axis (O) from a front end to a rear end; an insulator (11) with a front end section (14) designed as a tube with a bottom, surrounding a tip (21) of the central electrode (20); a metal casing (30) which is tubular in shape and structured to support the insulator (11) such that the front end section (14) of the insulator (11) projects from a front end section (31) of the metal casing (30) to the front end side; and a plurality of ground electrodes (40; 60; 90), each of which has a first end (43, 46; 65, 68, 71, 74; 95, 98, 101, 104) which forms a discharge gap (45, 48; 67, 70, 73, 76; 97, 100, 103, 106) with the front end section (14) of the insulator (11), and a second end (44, 47; 66, 69, 72, 75; 96, 99, 102, 105) which is connected to the front end section (31) of the metal shell (30), wherein the majority of ground electrodes (40; 60; 90) comprise a pair of ground electrodes whose associated discharge gaps (45, 48; 67, 70, 73, 76; 97, 100, 103, 106) differ in size from each other.

Description

Technical area

[0001] The present invention relates to a spark plug, in particular to a spark plug with barrier discharge. Technical background

[0002] The patent document discloses prior art for a spark plug that uses a barrier discharge, in which a tubular metal casing with a tubular front end (namely, a metal tube) serving as an electrode is arranged radially outside an insulator. The insulator has a front end configured as a tube with a base, surrounding a central electrode, such that the metal tube of the casing faces the front end of the insulator with a gap between them. The prior art according to patent document 1 includes arranging a central axis of the metal tube and a central axis of the front end of the insulator parallel to and spaced apart from each other to generate an electric field between the metal tube and the front end of the insulator, the intensity of which is inhomogeneous in the circumferential direction.This serves to activate the plasma at a point where the electric field is intense and facilitates the generation of the initial flame. Further relevant prior art is disclosed in the following documents: WO 2017 / 110 209 A1, JP 2017-152 143 A, JP 2010-37 949 A. State of the art document(s) Patent document(s)

[0003] Patent Document 1: JP 2017-152143 A Summary of the invention: Problem(s) to be solved by the invention

[0004] However, according to the conventional state of the art described above, the metal tube of the metal casing completely encloses the front end of the insulator. This can lead to the problem that the initial flame disappears due to the flame-extinguishing effect, where the metal tube extracts energy from the initial flame generated between the metal tube and the front end of the insulator, or to the problem that the metal tube interferes with the growth of the flame.

[0005] To solve the problems, it is desirable to provide a spark plug that is structured in such a way as to facilitate the generation of the initial flame and the growth of the flame according to the present invention. Means to solve the problem(s)

[0006] To solve the problem described above, a spark plug with the features of claim 1 is specified. Further advantageous embodiments of the spark plug are described in the dependent claims.

[0007] In light of the foregoing, according to a first aspect of the present invention, a spark plug is specified comprising: a central electrode extending along an axis from a front end to a rear end; an insulator having a front end section configured as a tube with a bottom surrounding a tip of the central electrode; a metal casing shaped like a tube and structured to support the insulator such that the front end section of the insulator projects from a front end section of the metal casing to the front end;and a plurality of ground electrodes, each having a first end forming a discharge gap with the front end section of the insulator and a second end connected to the front end section of the metal shell, the plurality of ground electrodes including a pair of ground electrodes differing in the size of their discharge gaps. Effect(s) of the invention

[0008] According to the first aspect of the spark plug design, each of the multiple ground electrodes comprises the first end, which is connected to the front end of the metal casing, and the second end, which forms the discharge gap with the front end of the insulator. A spark plug configured in this way creates a gap between two adjacent ground electrodes, thereby reducing the flame-suppressing effect of the ground electrodes and facilitating the generation of an initial flame and promoting flame growth.

[0009] The majority of the spark plug electrodes consist of a pair of electrodes that differ in the size of their discharge gaps. Accordingly, if, for example, it is difficult to induce a discharge due to high pressure in a combustion chamber, a smaller discharge gap facilitates discharge induction. This helps to produce an initial flame. Conversely, if it is easy to induce a discharge due to low pressure in the combustion chamber, a larger discharge gap allows for discharge induction. This facilitates flame growth. Thus, the spark plug adjusts the discharge gaps according to the pressure conditions in the combustion chamber. This simultaneously stabilizes the initial flame formation and improves ignition performance.

[0010] According to a second aspect of the spark plug, the ground electrode pairs of the majority of the spark plugs differ in the size of their discharge gaps. This facilitates the induction of discharge in one of the discharge gaps, independent of the pressure in the combustion chamber. Conversely, if the majority of the spark plugs contain a pair of ground electrodes with the same discharge gap size, such a pair does not contribute to improved ignition performance and actually draws energy away from the flame, potentially leading to increased losses. Thus, according to this second aspect, the spark plug serves to reduce the energy loss caused by the ground electrodes while simultaneously improving ignition performance, in addition to the effects caused by the first aspect of the spark plug.

[0011] According to a third aspect of the spark plug, the two ground electrodes, which differ in the size of their discharge gaps, differ in the position of their first ends along the axis. If a pair of first ends is positioned such that they have the same position along the axis but differ in the size of the discharge gaps formed between the respective first end and the front end of the insulator, discharge will tend to be induced more frequently in the smaller of the discharge gaps. To avoid this, discharge induction is facilitated not only in the smaller of the discharge gaps but also in the larger of the discharge gaps by arranging the pair of ground electrodes, which differ in the size of their discharge gaps, such that their first ends differ along the axis.This serves to further facilitate flame growth, in addition to the effects of the first or second aspect of the spark plug.

[0012] According to a fourth aspect of the spark plug, the discharge slots comprise a pair of slots shaped such that the front side of the pair is larger than the rear side. This facilitates flame growth more in the front-side slot than in the rear-side slot, with the front-side slot being closer to the center of the combustion chamber. This facilitates the combustion of the air-fuel mixture in the combustion chamber due to the improved flame growth, in addition to the effects of the third aspect of the spark plug.

[0013] According to a fifth aspect of the spark plug, the discharge slots comprise a pair of slots shaped such that the front slot is smaller than the rear slot. This facilitates the generation of the initial flame in the front slot rather than the rear slot, with the front slot being closer to the center of the combustion chamber. This facilitates the combustion of the air-fuel mixture in the combustion chamber due to the easier generation of the initial flame, in addition to the effects of the third aspect of the spark plug.

[0014] According to a sixth aspect of the spark plug, the pair of ground electrodes, which differ in the size of their discharge gaps, are arranged such that the first ends of the pair of ground electrodes are separated by an angle of 80° or more around the axis. This facilitates discharge induction not only in a smaller but also in a larger discharge gap. This, in turn, facilitates flame growth due to discharge in the larger discharge gap, in addition to the effects of the first through fifth aspects of the spark plug.

[0015] According to a seventh aspect of the spark plug, the majority of the ground electrodes consist of two electrodes. This minimizes energy loss due to the ground electrodes and simultaneously improves ignition performance, in addition to the effects of the first through sixth aspects of the spark plug. Brief description of the drawings Fig. Figure 1 is a half-section of a spark plug according to a first embodiment. Fig. 2A is a diagram showing the relationship of a number of discharges to a distance between the ground electrodes in the direction of an axis. Fig. 2B is a diagram showing the relationship of a number of discharges to an angle between the ground electrodes measured around the axis. Fig. Figure 3A is a side view of a spark plug according to a second embodiment. Fig. 3B is a side view of the spark plug in the direction of arrow IIIb, which points in Fig. 3A is shown. Fig. Figure 4A is a side view of a spark plug according to a third embodiment. Fig. 4B is a side view of the spark plug, seen in the direction of arrow IVb according to Fig. 4A. Mode(s) for carrying out the invention

[0016] Exemplary embodiments of the present invention are described below with reference to the accompanying drawings. Fig. Figure 1 is a half-section of a spark plug 10 according to a first embodiment, wherein the spark plug 10 has an axis O which serves as the boundary of the half-section. Fig. In the drawing, a lower side is designated as the front end of spark plug 10 and an upper side as the rear end of spark plug 10. This also applies to Fig. 3A, Fig. 3B, Fig. 4A and Fig. 4B. As in Fig. As shown in Figure 1, the spark plug 10 contains an insulator 11, a center electrode 20, a metal casing 30 and ground electrodes 40.

[0017] The insulator 11 is a tubular element with an aluminum oxide base, possessing sufficient insulating performance and adequate mechanical properties at high temperatures. The insulator 11 contains a hole 12 extending along axis O and having a circular cross-section. The hole 12 is open at a rear end of the insulator 11 and closed at a front end. The insulator 11 has a rearward-facing surface 13 on its inner circumference, located in a front end section of the hole 12 and extending around its entire circumference. The rearward-facing surface 13 is structured to engage with a rear section of the central electrode 20 to retain it.

[0018] The center electrode 20 is a cylindrical electrode made of conductive metal, such as a nickel-based alloy, extending along axis O in the hole 12. The spark plug 10 also includes a metal terminal 22, which is a rod element structured to receive an alternating voltage or pulse voltage and made of conductive metal, such as low-carbon steel. The metal terminal 22 is attached to the rear end of the insulator 11 such that a front-end section of the metal terminal 22 is positioned in the hole 12 and is electrically connected to the center electrode 20 via a connecting element 23 made of a material such as conductive glass.

[0019] The insulator 11 comprises a front end section 14, a small-diameter section 15, a large-diameter section 16, a rear end section 17, and an overhang 18. The front end section 14 is designed as a tube with a base, surrounding a tip 21 of the central electrode 20. The small-diameter section 15 is tubular and adjoins the front end section 14 from its rear end. The large-diameter section 16 is tubular and adjoins the small-diameter section 15 from its rear end. The rear end section 17 is tubular and adjoins the large-diameter section 16 from its rear end.The overhang 18 is formed between the large diameter section 16 and the rear end section 17 over its entire circumference, so that it overhangs radially outwards from the outer circumferences of the large diameter section 16 and the rear end section 17.

[0020] The front end section 14 surrounds a front end section of the center electrode 20 and is closed at a tip 14a. According to the present embodiment, it is shaped as a cylindrical tube with a constant outer diameter along its entire length in the direction of axis O. The small-diameter section 15 surrounds a central section of the center electrode 20 and is shaped as a cylindrical tube with a constant outer diameter along its entire length in the direction of axis O, according to the present embodiment. The small-diameter section 15 has a larger outer diameter than the front end section 14. The large-diameter section 16 surrounds a rear end section of the center electrode 20 and is shaped as a cylindrical tube with a constant outer diameter along its entire length in the direction of axis O, according to the present embodiment.The large diameter section 16 has a larger outer diameter than the small diameter section 15.

[0021] The metal sheath 30 is an essentially cylindrical tubular element made of conductive metal, such as low-carbon steel. The metal sheath 30 comprises: a front end section 31 that surrounds the insulator 11 in a region extending from the small-diameter section 15 to a portion of the large-diameter section 16; a seat section 32 that is adjacent to the front end section 31 from the rear end; a connecting section 33 that is adjacent to the seat section 32 from the rear end; a tool engagement section 34 that is adjacent to the connecting section 33 from the rear end; and a rear end section 35 that is adjacent to the tool engagement section 34 from the rear end.

[0022] The front end section 31 has an external thread 36 on its outer circumference, designed to be screwed into a threaded hole of a motor (not shown), and a rearward-facing surface 37 on its inner circumference, extending over the entire circumference of the section. The rearward-facing surface 37 of the front end section 31 faces forward with respect to the large-diameter section 16 of the insulator 11. The seat section 32 is designed to close a gap between the threaded hole of the motor and the external thread 36 and has a larger outer diameter than the front end section 31. The connecting section 33 is a section that is plastically deformed to be bent when the metal sleeve 30 is installed in the insulator 11. The tool engagement section 34 is a section with which a tool, e.g.,A wrench engages when the external thread 36 is screwed into the threaded hole of the motor. The rear end section 35 is bent radially inwards and positioned rearward with respect to the overhang 18 of the insulator 11.

[0023] A powder-filled seal 38, such as talc, is formed between the rear end section 35 and the overhang 18, extending over the entire circumference of the outer circumference of the rear end section 17 of the insulator 11. A section of the metal sleeve 30, extending from the rearward-facing surface 37 to the rear end section 35, exerts a compressive load in the direction of axis O on a section of the insulator 11 from the large-diameter section 16 to the overhang 18 via the seal 38. Thus, the metal sleeve 30 supports the insulator 11. The rearward-facing surface 37 of the inner circumference of the metal sleeve 30 secures the large-diameter section 16 of the insulator 11 from the front end, and the front end section 14 and part of the small-diameter section 15 of the insulator 11 project from the front end section 31 of the metal sleeve 30 to the front end.

[0024] The ground electrodes 40 are rod elements made of metal, such as a nickel-based alloy, which are connected to the front end section 31 of the metal shell 30. According to the present embodiment, the ground electrodes 40 consist of a first electrode 41 and a second electrode 42, each having a rectangular cross-section and extending in the direction of axis O.

[0025] The first electrode 41 comprises a first end 43 and a second end 44, the first end 43 being positioned forward with respect to the second end 44, which is connected to the metal sheath 30. In the present embodiment, the first electrode 41, with the exception of the first end 43, is arranged parallel to the axis O. The first end 43 is bent toward the front end section 14 of the insulator 11 and is located closer to the front end section 14 than the other parts of the first electrode 41. The first end 43 of the first electrode 41 and the front end section 14 of the insulator 11 form a discharge gap 45 between them.

[0026] The second electrode 42 comprises a first end 46 and a second end 47, the first end 46 being positioned forward with respect to the second end 47, which is connected to the metal sheath 30. In the present embodiment, the second electrode 42 is arranged parallel to the axis O, except for the first end 46. The first end 46 is bent towards the front end section 14 of the insulator 11 and is located closer to the front end section 14 than the other parts of the second electrode 42. The first end 46 of the second electrode 42 and the front end section 14 of the insulator 11 form a discharge gap 45 between them.

[0027] The first end 43 of the first electrode 41 and the first end 46 of the second electrode 42 are positioned 180° apart at an angle around the axis O. The first end 46 of the second electrode 42 is positioned in front of the first end 43 of the first electrode 41. The discharge gap 48 formed by the second electrode 42 is larger than the discharge gap 45 formed by the first electrode 41, where the size of the discharge gap 45 or 48 represents the shortest distance from the front end section 14 to the first end 43 or 46, respectively.

[0028] In response to the application of an alternating voltage or a pulse voltage between the metal terminal 22 and the metal sleeve 30 of the spark plug 10 mounted on the engine (not shown), a non-equilibrium plasma (streamer discharge) is generated between the front end section 14 of the insulator 11 and the ground electrodes 40. The non-equilibrium plasma results in only a small conversion to thermal energy and therefore causes only a slight temperature increase of the combustible air-fuel mixture in a combustion chamber (not shown), but it generates high-energy electrons. The collision between these high-energy electrons produces large quantities of radicals of O, N, OH, etc. The temperature increase due to the exothermic reaction and the chain reaction of the radicals then leads to ignition, generating an initial flame in a reaction zone of a specific volume that includes the discharge gaps 45 and 48.The initial flame grows to ignite the combustible air-fuel mixture.

[0029] The discharge gaps 45 and 48 of the first electrode 41 and the second electrode 42 of the ground electrodes 40 differ in size. Accordingly, the small discharge gap 45, for example, allows the induction of a discharge in the combustion chamber when it is difficult to induce a discharge due to high pressure in the combustion chamber. When it is easy to induce a discharge due to low pressure in the combustion chamber, the large discharge gap 48 enables the induction of a discharge in the combustion chamber. The small discharge gap 45 facilitates the induction of discharge and serves to stabilize the generation of an initial flame via the pressure increase in the combustion chamber. The large discharge gap 48 has a large reaction area and thus facilitates flame growth and improves ignition performance.This simultaneously serves to ensure the stable generation of the initial flame and to improve the ignition behavior.

[0030] The ground electrodes 40 consist of the first electrode 41 and the second electrode 42, which are connected to the metal shell 30 and arranged at a distance along the circumference of the metal shell 30. Thus, the first electrode 41 and the second electrode 42 have a gap between them along the circumference. This creates specific spaces around the first electrode 41 and the second electrode 42, respectively, which serve as reaction zones and enclose the discharge gaps 45 and 48. This serves to suppress the flame-extinguishing effect of the ground electrodes 40 and to facilitate the generation of an initial flame and flame growth.

[0031] The different sizes of the discharge gaps 45 and 48 of the first electrode 41 and the second electrode 42 of the ground electrodes 40 serve to facilitate the induction of discharge in one of the discharge gaps 45 and 48, independent of the pressure in the combustion chamber. The ground electrodes 40 do not contain a pair of ground electrodes with the same discharge gap size. This eliminates a ground electrode that does not contribute to improving ignition performance and instead draws energy from the flame. This reduces energy loss through the ground electrodes 40 while simultaneously improving ignition performance. Since the ground electrodes 40 contain two ground electrodes, one for the first electrode 41 and one for the second electrode 42, energy loss through the ground electrodes 40 can be minimized while simultaneously improving ignition performance.

[0032] The first end 43 of the first electrode 41 and the first end 46 of the second electrode 42 are each shaped such that they are bent towards the front end section 14 of the insulator 11. This serves to concentrate the electric field on the first ends 43 and 46, and thereby facilitates the induction of discharge between the first end 43 and the front end section 14 and between the first end 46 and the front end section 14.

[0033] The following describes how the position of the first end 43 of the first electrode 41 relative to the first end 46 of the second electrode 42 influences the discharge of the second electrode 42 in the discharge gap 48, with reference to Fig. 2A. Fig. 2A is a diagram showing the ratio of a number of discharges in the discharge gap 48 to a distance D (see Fig. 1) between the first ends 43 and 46 of the ground electrodes 40 in the direction of the axis O of the spark plug 10.

[0034] Fig. Figure 2A shows the result of an experiment comprising: fixing the position of the first end 46 of the second electrode 42; shifting the position of the first end 43 of the first electrode 41 relative to the first end 46 by 1 mm increments towards the rear end; and preparing eight samples of the spark plug 10 in which the axial distance D between the first ends 43 and 46 varied from 0 mm to 7 mm. Each of the samples was shaped such that: the first end 43 of the first electrode 41 and the first end 46 of the second electrode 42 were separated at an angle of 180° about the axis O; the size (i.e., the shortest distance in the radial direction) of the discharge gap 48 at the second electrode 42 was 2 mm; and the size of the discharge gap 45 at the first electrode 41 was 0.5 mm. The samples were identical in dimensions, shape, material, etc., with the exception of the distance D.

[0035] Each sample was mounted in a chamber (not shown) with an observation window. The chamber was filled with test gas, which in this embodiment was air. While the pressure in the chamber was maintained at 0.4 MPa, a pulse voltage was applied between the metal terminal 22 and the metal sheath 30. The pulse voltage had a repetition frequency of 40 kHz and a voltage amplitude of 20 kV. The discharge was observed with a high-speed camera to count the number of discharges in the discharge gap 48 out of a total of 100 discharges induced in the discharge gap 45 or 48. Fig. 2A has a horizontal axis representing the distance D (mm) between the first end 43 of the first electrode 41 and the first end 46 of the second electrode 42 in the axial direction, and a vertical axis representing the number of discharges (≤ 100 times) in the discharge gap 48 of the second electrode 42.

[0036] As in Fig. As shown in Figure 2A, the number of discharges in discharge gap 48 increased with increasing distance D. The experiment showed that 10% of the discharge in discharge gap 45 or 48 was induced in discharge gap 48 when the distance D was equal to or greater than 1 mm.

[0037] Accordingly, as in Fig. As shown in Figure 2A, discharge induction is facilitated not only in the small, easily discharged discharge gap 45, but also in the large discharge gap 48, by designing the discharge gaps 45 and 48 in their respective sizes between the first end 43 of the first electrode 41 and the front end section 14, and between the first end 46 of the second electrode 42 and the front end section 14, such that the first ends 43 and 46 are positioned differently in the axial direction: i.e., so that the distance D is not equal to 0. This serves to ensure the stable generation of the initial flame in the discharge gap 45 and to facilitate flame growth through discharge in the large discharge gap 48.

[0038] Furthermore, the spark plug 10 is designed such that the front discharge gap 48 is larger than the rear discharge gap 45. This allows the reaction area, including the discharge gap 48, to be larger than the reaction area, including the discharge gap 45. This facilitates flame growth in the front discharge gap 48, which is closer to the combustion chamber (not shown), than in the rear discharge gap 45, and thus facilitates the combustion of the air-fuel mixture in the combustion chamber due to the increased flame size.

[0039] Next, it is described how an angle θ between the first end 43 of the first electrode 41 and the first end 46 of the second electrode 42, measured around the axis O, affects the discharge in the discharge gap 48 of the second electrode 42. Fig. Figure 4B is a diagram showing a relationship between the number of discharges in the discharge gap 48 and the angle θ (measured as acute angle) between the first ends 43 and 46 of the ground electrodes 40 about the axis O.

[0040] Fig. Figure 2B shows a result of the experiment with five samples of the spark plug 10, which varied in the angle θ between the first end 43 of the first electrode 41 and the first end 46 of the second electrode 42 about the axis O. The five samples each had angles θ (the measured apex) of 40°, 80°, 120°, 160°, and 180°. Each of the samples was shaped such that: the size (i.e., the shortest distance in the radial direction) of the discharge gap 48 at the second electrode 42 was 2 mm; the size of the discharge gap 45 at the first electrode 41 was 0.5 mm; the first end 43 of the first electrode 41 was offset backwards with respect to the first end 46 of the second electrode 42; and the axial distance D between the first end 43 of the first electrode 41 and the first end 46 of the second electrode 42 was 6 mm. The samples were identical in dimensions, shape, material, etc., except for the angle θ.

[0041] Each sample was mounted on a chamber (not shown) with an observation window. While the pressure in the chamber, filled with the test gas (air), was maintained at 0.4 MPa, a pulse voltage was applied between the metal terminal 22 and the metal sleeve 30. The pulse voltage had a repetition frequency of 40 kHz and a voltage amplitude of 20 kV. The discharge was observed by a high-speed camera to count the number of discharges in discharge gap 48 out of a total of 100 discharges induced in discharge gap 45 or 48. Fig. 2B has a horizontal axis representing the angle θ (°) between the first ends 43 and 46 of the mass electrodes 40 about the axis O, and a vertical axis representing the number of discharges (≤ 100 times) in the discharge gap 48 of the second electrode 42.

[0042] As in Fig. As shown in Figure 2B, the number of discharges in discharge gap 48 increased with increasing angle θ. The experiment showed that 10% of the discharge in discharge gap 45 or 48 was induced in discharge gap 48 when the angle θ was equal to or greater than 80°.

[0043] Accordingly, as in Fig. As shown in Figure 2B, the induction of discharge is facilitated not only in the small, easily discharged discharge gap 45, but also in the large discharge gap 48, by making the first end 43 of the first electrode 41 and the first end 46 of the second electrode 42 different in size from each other in the discharge gaps 45 and 48, such that the first ends 43 and 46 are separated by an angle of 180° about the axis O (where the angle θ ≥ 80°). This serves to ensure the stable generation of the initial flame in the discharge gap 45 and to facilitate flame growth through discharge in the large discharge gap 48.

[0044] A second embodiment is described below with reference to the Fig. 3A and Fig. 3B is described. The first embodiment illustrates a case in which the number of ground electrodes 40 is two. As a variation, the second embodiment illustrates a case in which the number of ground electrodes 60 is four. The configurations common to the first embodiment are represented by common reference numerals for the sake of simplicity.

[0045] Fig. Figure 3A is a side view of a spark plug 50 according to the second embodiment. Fig. 3B is a side view of spark plug 50 in the direction of arrow IIIb according to Fig. 3A. Each of the Fig. 3A and Fig. 3B are drawn without their back part and without any of the Fig. 4A and Fig. 4B are drawn similarly.

[0046] As in the Fig. 3A and Fig. As shown in Figure 3B, the spark plug 50 contains the ground electrodes 60, each of which is a rod element made of metal such as a nickel-based alloy and is connected to the front end section 31 of the metal housing 30 (see Figure 3B). Fig. 1) The ground electrodes 60 consist of a first electrode 61, a second electrode 62, a third electrode 63 and a fourth electrode 64, each of which has a rectangular cross-section and is arranged parallel to the axis O.

[0047] The first electrode 61 comprises a first end 65 and a second end 66, with the first end 65 positioned forward of the second end 66, which is connected to the metal sheath 30. The first electrode 61 includes an axially central section bent toward the front end section 14 of the insulator 11, with the first end 65 and its surroundings extending parallel to the axis O. The first end 65 and its surroundings are located closer to the front end section 14 of the insulator 11 than the other sections of the first electrode 61, thus forming a discharge gap 67 with the front end section 14. This arrangement of the first end 65 and its surroundings parallel to the axis O serves to induce a discharge at distributed locations within the discharge gap 67, thereby expanding a reaction area for generating an initial flame.

[0048] The second electrode 62 comprises a first end 68 and a second end 69, with the first end 68 positioned forward of the second end 69, which is connected to the metal sheath 30. The second electrode 62 extends parallel to the axis O, except for the first end 68. The first end 68 is bent toward the front end section 14 of the insulator 11 and is located closer to the front end section 14 than the other parts of the second electrode 62. The first end 68 of the second electrode 62 forms a discharge gap 70 with the front end section 14 of the insulator 11.

[0049] The third electrode 63 comprises a first end 71 and a second end 72, with the first end 71 being located forward of the second end 72, which is connected to the metal sheath 30. The third electrode 63 extends parallel to the axis O, except for the first end 71. The first end 71 is bent toward the front end section 14 of the insulator 11 and is located closer to the front end section 14 than the other parts of the third electrode 63. The first end 71 of the third electrode 63 forms a discharge gap 73 with the front end section 14 of the insulator 11.

[0050] The fourth electrode 64 comprises a first end 74 and a second end 75, with the first end 74 positioned forward of the second end 75, which is connected to the metal shell 30. The fourth electrode 64 runs parallel to axis O. The first end 74 of the fourth electrode 64 is intended to be a point of electric field concentration. This causes a discharge to be induced between the first end 74 and the front end section 14. Thus, the first end 74 of the fourth electrode 64 forms a discharge gap 76 with the front end section 14 of the insulator 11. The first end 74 of the fourth electrode 64 is positioned forward of the tip 14a of the front end section 14.

[0051] The first end 65 of the first electrode 61, the first end 68 of the second electrode 62, the first end 71 of the third electrode 63, and the first end 74 of the fourth electrode 64 are arranged at 90° intervals around the axis O. Furthermore, the first end 65 of the first electrode 61, the first end 68 of the second electrode 62, the first end 71 of the third electrode 63, and the first end 74 of the fourth electrode 64 are arranged in this order in a direction from the rear end to the front end of the spark plug 50. The first ends 65, 68, 71, and 74 are also arranged in this order with respect to the size of their discharge gaps 67, 70, 73, and 76, from smallest to largest. The size of the discharge gap 76 represents the shortest distance from the first end 74 and its surroundings to the front end section 14.

[0052] The spark plug 50 comprises the first end 65 of the first electrode 61, the first end 68 of the second electrode 62, the first end 71 of the third electrode 63, and the first end 74 of the fourth electrode 64, which are arranged at 90° intervals at an angle around the axis O (where the angle θ ≥ 80°), their discharge gaps 67, 70, 73, and 76 differing in size. This facilitates discharge not only in the small, easily discharged discharge gap 67, but also in the discharge gaps 70, 73, and 76, which are larger than the discharge gap 67, and thus facilitates flame growth due to the discharge induced in the discharge gaps 70, 73, and 76.

[0053] Furthermore, the spark plug 50 is shaped such that the discharge gap 76 is larger than the discharge gap 67, which is set further back relative to the discharge gap 76. This facilitates flame growth in the discharge gap 76 more than in the discharge gap 67, with the discharge gap 76 being closer to a combustion chamber center point (not shown) than the discharge gap 67. This serves to facilitate the combustion of the air-fuel mixture in the combustion chamber due to a larger flame.

[0054] A third embodiment is described below with reference to Fig. 4A and Fig. 4B is described. The first embodiment and the second embodiment each represent a case in which one of the discharge gaps is larger than another discharge gap and is set back relative to the first. In contrast, the third embodiment is an example of a case in which one of the discharge gaps is smaller than another discharge gap and is located behind the first. The configurations common to the first embodiment are represented by common reference numerals for the sake of simplicity. Fig. Figure 4A is a side view of a spark plug 80 according to the third embodiment. Fig. 4B is a side view of spark plug 80 in the direction of arrow IVb according to Fig. 4A.

[0055] As in Fig. 4A and Fig. As shown in Figure 4B, the spark plug contains 80 ground electrodes 90, each of which is a rod element made of metal such as a nickel-based alloy and which is connected to the front end section 31 of the metal housing 30 (see Figure 4B). Fig. 1) The ground electrodes 90 consist of a first electrode 91, a second electrode 92, a third electrode 93 and a fourth electrode 94, each of which has a rectangular cross-section and is arranged parallel to the axis O.

[0056] The first electrode 91 comprises a first end 95 and a second end 96, with the first end 95 positioned forward of the second end 96, which is connected to the metal shell 30. The first electrode 91 runs parallel to axis O. The first end 95 of the first electrode 91 is intended to be a point of electric field concentration. This causes a discharge to be induced between the first end 95 and the front end section 14 of the insulator 11. Thus, the first end 95 of the first electrode 91 forms a discharge gap 97 with the front end section 14 of the insulator 11.

[0057] The second electrode 92 comprises a first end 98 and a second end 99, with the first end 98 positioned forward of the second end 99, which is connected to the metal sheath 30. The second electrode 92 includes an axially central section bent toward the front end section 14 of the insulator 11, with the first end 98 and its surroundings extending parallel to axis O. The first end 98 and its surroundings are closer to the front end section 14 of the insulator 11 than the other sections of the second electrode 92, thus forming a discharge gap 100 with the front end section 14. This arrangement of the first end 98 and its surroundings parallel to axis O serves to induce a discharge at distributed locations within the discharge gap 100, thereby expanding a reaction area for generating an initial flame.

[0058] The third electrode 93 comprises a first end 101 and a second end 102, with the first end 101 being located forward of the second end 102, which is connected to the metal sheath 30. The third electrode 93 extends parallel to the axis O, except for the first end 101. The first end 101 is bent toward the front end section 14 of the insulator 11 and is located closer to the front end section 14 than the other parts of the third electrode 93. The first end 101 of the third electrode 93 forms a discharge gap 103 with the front end section 14 of the insulator 11.

[0059] The fourth electrode 94 comprises a first end 104 and a second end 105, with the first end 104 positioned forward of the second end 105, which is connected to the metal sheath 30. The fourth electrode 94 extends parallel to the axis O, except for the first end 104. The first end 104 is bent toward the front end section 14 of the insulator 11 and is located closer to the front end section 14 than the other parts of the fourth electrode 94. The first end 104 of the fourth electrode 94 forms a discharge gap 106 with the front end section 14 of the insulator 11.

[0060] The first end 95 of the first electrode 91, the first end 98 of the second electrode 92, the first end 101 of the third electrode 93, and the first end 104 of the fourth electrode 94 are arranged at 90° intervals around the axis O. Furthermore, the first end 95 of the first electrode 91, the first end 98 of the second electrode 92, the first end 101 of the third electrode 93, and the first end 104 of the fourth electrode 94 are arranged in this order in a direction from the rear end to the front end. The first ends 95, 98, 101, and 104 are also arranged in this order with respect to the size of their discharge gaps 97, 100, 103, and 106, from largest to smallest. The size of the discharge gap 97 represents the shortest distance from the first end 95 and its surroundings to the front end section 14. The size of the discharge gap 100 represents the shortest distance from the first end 98 and its proximity to the front end section 14.

[0061] The spark plug 80 is shaped such that the discharge gap 106 is smaller than the discharge gap 97, which is set further back relative to the discharge gap 106. This facilitates the generation of the initial flame more in the discharge gap 106 than in the discharge gap 97, with the discharge gap 106 being located closer to the center of the combustion chamber (not shown) than the discharge gap 97. This serves to facilitate the ignition of the air-fuel mixture in the combustion chamber due to the easier generation of the initial flame.

[0062] While the above describes the present invention based on the embodiments described above, the present invention is not limited to the embodiment described above. The present invention naturally includes any improvement and / or modification within the scope of the present invention.

[0063] Although the number of ground electrodes is two or four according to the embodiments, the present invention is not limited thereto. The number of ground electrodes can be suitably modified and can of course be three, five, or a larger number.

[0064] Although the discharge gaps between the front section 14 of the insulator 11 and the ground electrodes 40, 60, or 90 are of different sizes according to the embodiments, the present invention is not limited thereto. The discharge gaps can comprise a pair of discharge gaps of the same size. Provided that the ground electrodes comprise a pair of ground electrodes that differ in the size of their discharge gaps, the induction of discharge in one of these differently sized discharge gaps is facilitated, independent of the pressure in the combustion chamber.

[0065] Although, according to the embodiments, the front end section 14 of the insulator 11 has a constant outer diameter over its entire axial length, the present invention is not limited thereto. For example, the outer circumference of the front end section 14 can be shaped such that the diameter decreases towards the front end, so that the front end of the front end section 14 has a smaller outer diameter than the rear end of the front end section 14. Alternatively, the outer circumference of the front end section 14 can be shaped such that the diameter increases towards the front end, so that the front end of the front end section 14 has a larger outer diameter than the rear end of the front end section 14.Alternatively, the outer circumference of the front end section 14 can be shaped as a cylindrical tube with a bulging central section, such that the front end section 14 has an axial central section with a larger outer diameter than the front end or the rear end of the front end section 14. It is possible to adjust the intensity of the electric field between the central electrode and the ground electrodes by changing the shape of the outer circumference of the front end section 14 of the insulator 11, depending on the radial thickness of the front end section 14, and so on.

[0066] Although in the embodiments the tip 14a of the front end section 14 of the insulator 11 is flat, the present invention is not limited thereto. For example, the tip 14a can be shaped as a spherical crown.

[0067] Although according to the embodiments each electrode of the ground electrodes 40, 60, and 90 is linearly shaped and the second end and its surroundings are arranged to extend parallel to the axis O, the present invention is not limited thereto. Each electrode of the ground electrodes 40, 60, and 90 can have its first end and its surroundings inclined such that each electrode is arranged along a plane that includes the axis O, and such that the discharge gap is formed between the first end of each electrode and the front end section 14. Alternatively, in contrast to such a configuration in which each electrode of the ground electrodes 40, 60, and 90 is arranged to be enclosed by the plane including the axis O, each electrode of the ground electrodes 40, 60, and 90 can be arranged obliquely with respect to the axis O.

[0068] Although in the embodiments each electrode of the ground electrodes 40, 60 and 90 is linear, the present invention is not limited thereto. Each electrode of the ground electrodes 40, 60 and 90 can be curved. The curved shape of each electrode allows for greater flexibility in its arrangement.

[0069] According to the second embodiment, the size of the discharge gap increases in the sequence of discharge gaps 67, 70, 73, and 76, i.e., in the sequence from the rear to the front face of the spark plug 50. According to the third embodiment, the size of the discharge gap decreases in the sequence of discharge gaps 97, 100, 103, and 106, i.e., in the sequence from the rear to the front face of the spark plug 80. However, the present invention is not limited to these. It is not necessary to arrange the discharge gaps axially in order of their size. For example, it is possible to facilitate flame propagation in the front discharge gap, which is closer to the center of the combustion chamber than the rear discharge gap, by forming at least one front discharge gap that is offset forward relative to a rear discharge gap and is larger than the rear discharge gap.Alternatively, by forming at least one front discharge gap which is directed forward with respect to a rear discharge gap and is smaller than the rear discharge gap, the generation of an initial flame in the front discharge gap which is closer to the center of the combustion chamber than the rear discharge gap can be facilitated.

[0070] Although in the second or third embodiment the four ground electrodes 60 or 90 are arranged at equal intervals in the circumferential direction, the present invention is not limited thereto. The circumferential spacing of the ground electrodes can be modified accordingly.

Claims

[1] A spark plug containing (10; 50; 80): a central electrode (20) extending along an axis (O) from a front end to a rear end; an insulator (11) with a front end section (14) designed as a tube with a bottom, surrounding a tip (21) of the central electrode (20); a metal casing (30) which is tubular in shape and structured to support the insulator (11) such that the front end section (14) of the insulator (11) projects from a front end section (31) of the metal casing (30) to the front end side; and a plurality of ground electrodes (40; 60; 90), each of which has a first end (43, 46; 65, 68, 71, 74; 95, 98, 101, 104) which forms a discharge gap (45, 48; 67, 70, 73, 76; 97, 100, 103, 106) with the front end section (14) of the insulator (11), and a second end (44, 47; 66, 69, 72, 75; 96, 99, 102, 105) which is connected to the front end section (31) of the metal shell (30), wherein the majority of ground electrodes (40; 60; 90) comprise a pair of ground electrodes whose associated discharge gaps (45, 48; 67, 70, 73, 76; 97, 100, 103, 106) differ in size from each other. [2] Spark plug (10; 50; 80) according to claim 1, wherein each pair of ground electrodes from the plurality of ground electrodes (40; 60; 90) is such that the associated discharge gaps of the pair differ from each other in size. [3] Spark plug (10; 50; 80) according to claim 1 or 2, wherein the pair of ground electrodes, whose associated discharge gaps (45, 48; 67, 70, 73, 76; 97, 100, 103, 106) differ from each other in size, differ from each other in the position of their first ends (43, 46; 65, 68, 71, 74; 95, 98, 101, 104) in the direction of the axis (O). [4] Spark plug (10; 50) according to claim 3, wherein the discharge gap (45, 48; 67, 70, 73, 76) comprises a pair of discharge gaps configured such that a front-end discharge gap of the pair is larger than a rear-end discharge gap of the pair. [5] Spark plug (80) according to claim 3, wherein the discharge gaps comprise a pair of discharge gaps (97, 100, 103, 106) which is configured such that a front-end discharge gap of the pair is smaller than a rear-end discharge gap of the pair. [6] Spark plug (10; 50; 80) according to one of claims 1 to 5, wherein the pair of ground electrodes, whose associated discharge gaps (45, 48; 67, 70, 73, 76; 97, 100, 103, 106) differ in size from each other, is arranged such that the first ends (43, 46; 65, 68, 71, 74; 95, 98, 101, 104) of the pair of ground electrodes are spaced apart from each other by an angle of 80° or more around the axis (O) in the circumferential direction. [7] The spark plug (10) according to any one of claims 1 to 6, wherein the majority of the ground electrodes (40) are composed of two electrodes (41, 42).

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