spark plug

Abstract

A spark plug (100) comprising: a rod-shaped center electrode (10); a ground electrode (13) having a facing section (14) which is directed towards a front end section (11) of the center electrode (10) and forms a discharge gap (SG) between the facing section (14) and the front end section (11) of the center electrode (10); a cylindrical insulator (20) in which the center electrode (10) is housed, wherein the front end section (11) of the center electrode (10) is exposed from a front end of the insulator (20); a cylindrical metal housing (40) in which the insulator (20) is housed; and a cover part (50) which, from a front end side of the spark plug (100), covers the front end section (11) of the center electrode (10) and the facing section (14) of the ground electrode (13) to form a pre-chamber (51), wherein the cover part (50) includes at least one injection opening (55) which is a through-hole,wherein the facing section (14) has a near section (19) which is closest to the front end section (11) of the center electrode (10), wherein the near section (19) is positioned in a virtual space (S1) which is an interior of a cylindrical shape formed by extending an outer circumference of the front end section (11) of the center electrode (10) in a direction of an axial line (X1) of the center electrode (10), wherein a midpoint (M1) of a shortest line segment connecting the front end section (11) of the center electrode (10) and the near section (19) of the facing section (14) is arranged offset from the axial line (X1) of the center electrode (10), wherein the prechamber (51) has an opening section (45) in an inner circumferential surface (43) of the metal housing (40) into which a base end section (15) of the ground electrode (13) is inserted, and wherein,when the pre-chamber (51) is divided into a first part (51A) and a second part (51B), along a plane that includes a center of the opening section (45) and the axial line (X1) of the center electrode (10), which has at least one injection port (55) in both the first part (51A) and the second part (51B).

Description

BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a spark plug. 2. Description of the state of the art Spark plugs are known, such as those disclosed in JP 2007-234511A, JP 2011-187437A, and JP 2016-184558A. JP 2007-234511A discloses a spark plug with a columnar center electrode and a ground electrode, the end section of which is curved towards an inner circumferential side and forms a spark discharge gap between a leading end section of the center electrode and the end section. The spark plug is configured such that the axial line of the end section of the ground electrode and the axial line of the center electrode are arranged at an angle to each other. JP 2007 - 234 511 A describes that such a configuration improves the ignition capability of the spark plug, since a flame nucleus formed in the spark discharge gap can quickly spread over the entirety of a combustion chamber without being hindered by the end section of the ground electrode in a direction towards the center of the combustion chamber.DE 101 44 976 A1 discloses a spark plug with a center electrode and pre-chamber. SUMMARY OF THE INVENTION In recent years, high efficiency has been increasingly demanded of internal combustion engines. Improving the combustion rate is known to be effective in improving the efficiency of internal combustion engines. A pre-chamber spark plug (hereinafter also referred to as a pre-chamber spark plug) has attracted attention in recent years as an effective means of improving combustion rate (see JP 2015-130302A and JP 2018-006304A). The pre-chamber spark plug has been used in generators and racing engines, and an improvement in combustion efficiency has been confirmed. The pre-chamber spark plug is also effective in improving combustion efficiency when used in internal combustion engines other than generators and racing engines. In pre-chamber spark plugs, after a spark ignites between the electrodes, combustion initially takes place in a pre-chamber. The combustion flame in the pre-chamber is then directed outwards through through-holes (injection ports) that connect to the outside, and the injected high-temperature gas, acting as an ignition source, causes explosive combustion in a main combustion chamber. The injection velocity of the high-temperature gas from the pre-chamber is higher than the combustion velocity caused by ignition in a spark plug without a pre-chamber. Furthermore, the entire path of the injected high-temperature gas serves as an ignition source. Therefore, it is possible to bring a larger quantity of fuel into contact with the high-temperature gas.Therefore, the combustion speed of the pre-chamber spark plug is higher than the combustion speed of the spark plug without a pre-chamber, and therefore the effect of improved combustion efficiency can be expected. Incidentally, it is known that a constant flow exists in the main combustion chamber, and the state of this flow varies considerably depending on the position within the chamber, such as the intake and exhaust sides. In other words, the ignitability of each section differs depending on the flow conditions in that section. In contrast, in a pre-chamber spark plug, the injection strength from multiple injection ports is generally uniform. Such a configuration is unable to respond to an arrangement that corresponds to the ignitability of each section within the main combustion chamber and, from an ignitability standpoint, still has room for improvement. The present invention was developed taking into account the above-mentioned circumstances, and one object of the present invention is the provision of a spark plug with excellent ignition properties. The present invention can be implemented in the following embodiments. A spark plug according to an embodiment of the present invention comprises: a rod-shaped center electrode; a ground electrode having a facing section which is directed towards a front end section of the center electrode and forms a discharge gap between the facing section and the front end section of the center electrode; a cylindrical insulator with a center electrode housed therein, wherein the front end section of the center electrode is exposed from a front end of the insulator; a cylindrical metal housing with an insulator housed therein;and a cover part which, from a front end of the spark plug, covers the front end section of the center electrode and the facing section of the ground electrode to form a pre-chamber, the cover part comprising at least one injection port which is a through-hole, wherein the facing section has a near section which is closest to the front end section of the center electrode, the near section being positioned in a virtual space which is an interior of a cylindrical shape formed by extending an outer circumference of the front end section of the center electrode in the direction of an axial line of the center electrode, and wherein a midpoint of a shortest line segment connecting the front end section of the center electrode and the near section of the facing section is arranged offset from the axial line of the center electrode. According to this configuration, it is possible to provide a spark plug with excellent ignition capability by placing the midpoint of a shortest straight line connecting the front end section of the center electrode and the near section of the facing section offset from the axial line of the center electrode, thereby adjusting the injection strength from the injection port. The aforementioned spark plug is configured according to the embodiment such that the pre-chamber contains, on an inner wall surface thereof, an opening section into which a base end section of the ground electrode is inserted, and if the pre-chamber is divided into a first part and a second part along a plane which contains a center of the opening section and the axial line of the center electrode, the at least one injection opening is present in both the first part and the second part. According to this configuration, it is possible to vary the injection strength between the injection port in the first part and the injection port in the second part by adjusting the position of the midpoint of the shortest straight line connecting the front end section of the center electrode and the near end of the facing section, and by adjusting the position of the ground electrode. Therefore, it is possible to improve ignition capability by arranging the first and second parts, for example, according to the arrangement in the main combustion chamber. In the spark plug mentioned above, the facing section can be located on the axial line of the center electrode according to one embodiment. With this configuration, it is possible to ensure a sufficient size of the facing section, thus improving the wear resistance of the facing section. BRIEF DESCRIPTION OF THE FIGURES The following describes embodiments of the invention with reference to the drawings, without being limited thereto. Fig. 1 is a sectional view illustrating the structure of a spark plug according to a first embodiment. Fig. 2 is a partially enlarged sectional view of the spark plug. Fig. 3 is a partially enlarged sectional view of the spark plug, taken along line III-III from Fig. 2. Fig. 4 is a sectional view of the spark plug, taken along line IV-IV from Fig. 2. Fig. 5 is a schematic view of a front end section of a center electrode and a near section of an adjacent section. Fig. 6 is a sectional view illustrating a state in which the spark plug is arranged in an internal combustion engine. Fig. 7 is a schematic view of the front end section of the center electrode and the near section of the adjacent section in a further embodiment. DESCRIPTION OF PREFERRED EXECUTION FORMS <Erste Ausführungsform> A first embodiment of a spark plug 100 is described in detail below with reference to the drawings. In the following description, the lower side of Fig. 1 is a front end (front side) of the spark plug 100, the upper side of Fig. 1 is a rear end, and an up-down direction is a Z-axis direction. The left-right direction of Fig. 2 is a Y-axis direction of the spark plug 100, and the left-right direction of Fig. 3 is an X-axis direction of the spark plug 100. Fig. 1 is a sectional view showing an outline of a configuration of the spark plug 100 in the first embodiment. In Fig. 1, a mean axial line CX of the spark plug 100 is represented by a single-dot dashed line. In Fig. 6, a top surface and a side wall surface of a combustion chamber 105, when the spark plug 100 is mounted on an internal combustion engine, are represented by a two-dot dashed line. A piston 107 is arranged in the combustion chamber 105. The spark plug 100 is mounted on an internal combustion engine and serves to ignite it. When mounted on the internal combustion engine, the front end of the spark plug 100 (lower side in Fig. 6) is located inside the combustion chamber 105 of the internal combustion engine, and the rear end (upper side in Fig. 6) is located outside the combustion chamber 105. As shown in Fig. 1, the spark plug 100 comprises a center electrode 10, a ground electrode 13, an insulator 20, a terminal electrode 30, a metal housing 40, and a cover 50. The center electrode 10 is formed from a rod-shaped electrode element and is arranged such that an axial line X1 of it coincides with the central axial line CX of the spark plug 100. The center electrode 10 is held by the metal housing 40 with the insulator 20 arranged between them, such that a front end section 11 of the center electrode 10 is positioned in a front-end opening section 40A of the metal housing 40. The center electrode 10 is electrically connected to an external power source via the terminal electrode 30 located on its rear end. The ground electrode 13 is a rod-shaped electrode extending towards the front end section 11 of the center electrode 10. The ground electrode 13 extends from an inner circumferential surface 43 to the inside of the front end opening section 40A of the metal housing 40. The ground electrode 13 extends to the front face of the front end section 11 of the center electrode 10. The ground electrode 13 includes a facing section 14 that faces the front end section 11 of the center electrode 10. A discharge gap SG is formed between the facing section 14 of the ground electrode 13 and the front end section 11 of the center electrode 10. A configuration of the arrangement of the ground electrode 13 is described later. The insulator 20 is a cylindrical element containing an axial hole 21 passing through its center. The insulator 20 is formed, for example, from a ceramic sintered body of aluminum oxide or aluminum nitride. The center electrode 10 is located at the front end of the axial hole 21 of the insulator 20, with its front end section 11 exposed. The terminal electrode 30, which is a shaft-shaped electrode element, is held at the rear end of the axial hole 21. A rear end section 31 of the terminal electrode 30 projects from a rear opening section 22 of the insulator 20 to connect to the external power source. The center electrode 10 and the terminal electrode 30 are electrically connected to each other via a resistor 35, which is held between glass sealing materials to suppress the generation of radio interference noise when a spark discharge occurs.The central axis of the insulator 20 coincides with the central axial line CX of the spark plug 100. The metal housing 40 is an essentially cylindrical metal element containing a cylindrical hole 41 in its center, in which the insulator 20 is housed. The metal housing 40 is, for example, made of carbon steel. The central axis of the metal housing 40 coincides with the central axial line CX of the spark plug 100. As described above, the ground electrode 13 is mounted in the front-end opening section 40A of the metal housing 40. As shown in Figs. 2 and 3, the inner circumferential surface 43 of the metal housing 40 forms part of an inner wall surface of a prechamber 51. The metal housing 40 has an opening section 45 on its inner circumferential surface 43, into which a base end section 15 of the ground electrode 13 is inserted. The opening section 45 is an inner circumferential opening of a through-hole that penetrates the metal housing 40 in a direction from the inside to the outside. The through-hole is configured such that the ground electrode 13 can be inserted from the outer circumferential side towards the inner circumferential side of the metal housing 40. The cover part 50 is dome-shaped. The rear end of the cover part 50 is attached to the front end of the metal housing 40. From its front end, the cover part 50 covers the front end section 11 of the center electrode 10 and the adjacent section 14 of the ground electrode 13, forming the prechamber 51. In other words, the prechamber 51 is a space enclosed by the inner wall surface of the cover part 50 and the inner circumferential surface 43 of the metal housing 40. The cover part 50 has injection ports 55 as through-holes. The prechamber 51 (ignition chamber), which is a space covered by the cover part 50, communicates with the combustion chamber 105 via the injection ports 55. A section of the cover part 50 on the front end side in relation to the injection ports 55 is thinner than a section of the cover part 50 on the rear end side in relation to the injection ports 55. The cover part 50 contains, in the pre-chamber 51, a plurality of injection ports 55, which are formed on the front end face with respect to the discharge gap SG. The plurality of injection ports 55 are positioned on a virtual circumference centered on the axial line X1 of the center electrode 10 (see Fig. 4). Specifically, four injection ports 55 are arranged at equal intervals on the circumference of a virtual circle centered on the axial line X1 of the center electrode 10. In Fig. 4, assuming that a center point C1 of the opening section 45 corresponds to 0°, with a counterclockwise rotation being positive, the injection ports 55 are arranged at positions 0°, 90°, 180°, and 270°, respectively.In other words, the injection ports 55 are arranged symmetrically to each other in the left-right direction with respect to a plane P1 that contains the center point C1 of the opening section 45 and the axial line X1 of the center electrode 10. In Fig. 4, the injection port 55 at position 0° is not shown. In the following description, the injection port 55 located at position 90°, on the left side of Fig. 4, is referred to as injection port 55A, and the injection port 55 located at position 270°, on the right side, is referred to as injection port 55B. Next, a configuration of the arrangement of the ground electrode 13 is described. As shown in Figs. 2 and 3, the spark plug 100 has only one ground electrode 13. The ground electrode 13 has a circular cross-section and extends linearly. The base end section 15 of the ground electrode 13 is inserted into the opening section 45 of the metal housing 40. The ground electrode 13 is held so that it extends cantilevered from the inner circumferential surface 43 of the metal housing 40, with the base end section 15 being inserted into the opening section 45. The ground electrode 13 projects inwards from a section of the inner wall surface of the prechamber 51, which is located on the top side with respect to the plurality of injection ports 55, and occupies a section of a space in the prechamber 51 on the top side with respect to the plurality of injection ports 55.The ground electrode 13 projects into the pre-chamber 51 such that an axial line X2 of it is positioned obliquely to the axial line X1 of the center electrode 10. As shown in Fig. 4, the ground electrode 13 is, in a cross-section along a plane passing through the axial line X2 of the ground electrode 13 and perpendicular to the axial line X1 of the center electrode 10, offset by an angle θ1 in the direction of the X-axis relative to a reference line, i.e., inclined, which passes through the center C1 of the opening section 45 and the axial line X1 of the center electrode 10. The ground electrode 13 is located between the injection port 55A and the discharge gap SG. In other words, the ground electrode 13 is positioned in the prechamber 51 such that it covers the discharge gap SG from the side of the injection port 55A. The ground electrode 13 can obstruct the path of a flame propagating from the discharge gap SG towards the injection port 55A. The ground electrode 13 can also obstruct the path of a flame propagating from the discharge gap SG towards the injection port 55B; however, it is configured such that the degree of obstruction is less than when the flame propagates towards the injection port 55A. For example, the ground electrode 13 is configured so that it is not located between the injection port 55B and the discharge gap SG. As shown in Fig. 5, a near section 19 of the facing section 14, which is closest to the front end section 11 of the center electrode 10, is positioned in a virtual space S1. This space is the interior of a cylindrical shape formed by extending the outer circumference of the front end section 11 of the center electrode 10 in the direction of the axial line X1 of the center electrode 10. The near section 19 is a section that is determined in the facing section 14 according to the shape and position of both the front end section 11 of the center electrode 10 and the facing section 14. Since, in the present embodiment, the front end section 11 of the center electrode 10 is a flat surface perpendicular to the axial line X1 and the facing section 14 is a side face of a circular column, the near section 19 is uniquely determined.A midpoint M1 of the shortest line segment connecting the front end section 11 of the center electrode 10 and the near section 19 of the facing section 14 is positioned offset from the axial line X1 of the center electrode 10. The midpoint M1 is a point where a flame core is formed in the discharge gap SG. As in the case where the front end section 11 of the center electrode 10 and the facing section have parallel surfaces, if a plurality of shortest line segments can be defined connecting the front end section 11 of the center electrode 10 and the near section 19 of the facing section 14, the midpoints of all defined line segments can satisfy the above requirements. The facing section 14 is located on the axial line X1 of the center electrode 10. Fig. 5 is a sectional view along a plane perpendicular to the axial line X2 of the ground electrode 13 and containing the central axis CX. In Fig. 5, a section 14A of the facing section 14 is positioned on the axial line X1 of the center electrode 10. Section 14A is positioned at the front end with respect to the near section 19. As shown in Figs. 3 and 4, in the spark plug 100, when the pre-chamber 51 is divided into a first part 51A and a second part 51B along the plane P1, which contains the center point C1 of the opening section 45 and the axial line X1 of the center electrode 10, at least one of the injection ports 55 is present in both the first part 51A and the second part 51B. The injection port 55 present in the first part 51A is injection port 55A, and the injection port 55 present in the second part 51B is injection port 55B. In other words, if the plane P1 is specified as the YZ plane, injection port 55A and injection port 55B are present on both sides in the direction of the X-axis. The number of injection ports 55 present in the first part 51A and in the second part 51B is counted by ignoring the injection ports 55 that are located in the first part 51A and the second part 51B. Next, the effects of the present embodiment will be described with reference to Fig. 6. As shown in Fig. 6, a flow is generated in the combustion chamber 105, as indicated, for example, by the solid-line arrows. Near the spark plug 100, a flow is generated from the right side to the left side of Fig. 6. In other words, the combustion chamber 105 has a configuration in which, with respect to the spark plug 100, a flow is generated on the left side of Fig. 6 in a direction identical to the direction of flame injection from the injection port 55A, and a flow is generated on the right side of Fig. 6 in a direction opposite to the direction of flame injection from the injection port 55B. In the spark plug 100, the midpoint M1 of the shortest line segment connecting the leading end section 11 of the center electrode 10 and the near section 19 of the facing section 14 is offset from the axial line X1 of the center electrode 10 to the left of Fig. 6. When an electrical discharge occurs between the center electrode 10 and the ground electrode 13, and a flame nucleus forms at midpoint M1, combustion takes place in the prechamber 51. The combustion flame in the prechamber 51 is injected into the combustion chamber 105 through the plurality of injection ports 55. At this point, the ground electrode 13 acts as a structure that causes a pressure drop as the flame propagates in the prechamber 51. As the flame propagates in the first part 51A of the prechamber 51, a pressure drop is generated by the ground electrode 13, which is located between midpoint M1 and injection port 55A.However, if the flame propagates in the second part 51B of the pre-chamber 51, the generation of a pressure loss is less likely because the ground electrode 13 is not located between the center point M1 and the injection port 55B, or only a small section of it is located between them. Therefore, in the spark plug 100, the injection intensity of the flame injected from injection port 55A is small, and the injection intensity of the flame injected from injection port 55B is large. In Fig. 6, the flame injected from injection port 55A is schematically represented by a small outlined arrow, and the flame injected from injection port 55B is schematically represented by a large outlined arrow. The flame injected from injection port 55A reaches the vicinity of the left side wall surface of the combustion chamber 105 by moving along the flow.The flame injected from injection port 55B reaches the vicinity of the right side wall of combustion chamber 105 by moving against the flow. In combustion chamber 105, the entire path of the flame injected from injection ports 55A and 55B serves as an ignition source, ensuring efficient combustion. As described above, according to the present embodiment it is possible to provide a spark plug with excellent ignition capability by arranging the midpoint M1 of the shortest straight line connecting the front end section 11 of the center electrode 10 and the near section 19 of the facing section 14 such that it is offset from the axial line X1 of the center electrode 10, thereby adjusting the strength of the injection from the injection ports 55A and 55B. Furthermore, according to the present embodiment, it is possible to vary the injection strength between the injection port 55A located in the first part 51A and the injection port 55B located in the second part 51B, according to the position of the midpoint M1 of the shortest straight line connecting the front end section 11 of the center electrode 10 and the near section 19 of the facing section 14. Therefore, it is possible to improve the ignition capability by designing the arrangement of the first part 51A and the second part 51B in accordance with, for example, the arrangement in the combustion chamber 105. In the present embodiment, the facing section 14 is located on the axial line X1 of the central electrode 10. This makes it possible to ensure a sufficiently large size for the facing section 14, thus improving its wear resistance. <Verschiedene Ausführungsformen (Modifikation)> The present invention is not limited to the embodiment mentioned above and can be implemented in various forms within a core area thereof.(1) In the aforementioned embodiment, a configuration is presented as an example in which the ground electrode has a circular column shape; however, it is not limited thereto. For example, a configuration can be used in which, as shown in ground electrode 113 in Fig. 7, the ground electrode has a substantially rectangular column shape. In the aforementioned embodiment, a configuration is presented as an example in which the facing section lies on the axial line of the central electrode; however, as in ground electrode 113, the facing section 14 can lie on the axial line X1 of the central electrode 10. The ground electrode is not limited to extending linearly but can be curved and elongated.(2) In the embodiment described above, an example configuration is presented in which the ground electrode 13 is not located between the injection port 55B and the discharge gap SG; however, it is not limited to such a configuration. For example, the ground electrode 13 may be arranged such that the area located between the injection port 55B and the discharge gap SG is smaller than the area located between the injection port 55A and the discharge gap SG. (3) In the embodiment described above, an example configuration is presented in which the metal housing includes an opening section into which the base end section of the ground electrode is inserted; however, it is not limited to such a configuration. For example, a configuration may be used in which the cover part has an opening section.(4) Apart from the aforementioned embodiment, the number, arrangement, and direction of penetration of the injection ports may be modified as appropriate. For example, each of the injection ports present in the first part and the second part may be positioned at any angle, provided the center of the port section is at 0°. In the aforementioned embodiment, a configuration in which the injection ports are arranged symmetrically in a left-right direction with respect to a plane containing the center of the port section and the axial line of the center electrode is presented as an example; however, it is not limited to this configuration. (5) In the above-mentioned embodiment, the shape of the cover part is a specific shape; however, the shape may be modified as appropriate. The shape of the cover part may, for example, be a circular cylindrical shape, a rectangular box shape, or a conical shape.(6) The arrangement of the spark plug in the combustion chamber may, if necessary, be modified differently than in the aforementioned embodiment.

Claims

A spark plug (100) comprising: a rod-shaped center electrode (10); a ground electrode (13) having a facing section (14) which is directed towards a front end section (11) of the center electrode (10) and forms a discharge gap (SG) between the facing section (14) and the front end section (11) of the center electrode (10); a cylindrical insulator (20) in which the center electrode (10) is housed, wherein the front end section (11) of the center electrode (10) is exposed from a front end of the insulator (20); a cylindrical metal housing (40) in which the insulator (20) is housed; and a cover part (50) which, from a front end side of the spark plug (100), covers the front end section (11) of the center electrode (10) and the facing section (14) of the ground electrode (13) to form a pre-chamber (51), wherein the cover part (50) includes at least one injection opening (55) which is a through-hole,wherein the facing section (14) has a near section (19) which is closest to the front end section (11) of the center electrode (10), wherein the near section (19) is positioned in a virtual space (S1) which is an interior of a cylindrical shape formed by extending an outer circumference of the front end section (11) of the center electrode (10) in a direction of an axial line (X1) of the center electrode (10), wherein a midpoint (M1) of a shortest line segment connecting the front end section (11) of the center electrode (10) and the near section (19) of the facing section (14) is arranged offset from the axial line (X1) of the center electrode (10), wherein the prechamber (51) has an opening section (45) in an inner circumferential surface (43) of the metal housing (40) into which a base end section (15) of the ground electrode (13) is inserted, and wherein,when the pre-chamber (51) is divided into a first part (51A) and a second part (51B), along a plane that includes a center of the opening section (45) and the axial line (X1) of the center electrode (10), which has at least one injection port (55) in both the first part (51A) and the second part (51B). Spark plug according to claim 1, wherein the facing section (14) is located on the axial line (X1) of the center electrode (10).

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