SPARK PLUG

Abstract

Spark plug (10), comprising: a metal sleeve (20) which has a tubular shape and extends along an axis in one direction from front to back; a central electrode (13) which is held in the metal sleeve (20) with insulation; a grounding electrode (30) which is electrically connected to the metal sleeve (20) and arranged such that a spark gap (33) is formed between the central electrode (13) and an end section of the grounding electrode (31); and a spark plug cap (40) connected to a front end section of the metal housing (22), wherein the spark plug cap (40) covers the center electrode (13) and the end section of the grounding electrode (31) from the front and has a plurality of through holes (41) in an area in front of the grounding electrode (30), wherein in a cross-section enclosing the axis line, the candle cap (40) encloses an extension section (53) in at least one section of a region between a first imaginary straight line (49) and a second imaginary straight line (50), the first imaginary straight line (49) passes through a front end of the end section of the grounding electrode (32) and is perpendicular to the axis line, the second imaginary straight line (50) passes through a rear end of one of the inner open ends (47) of the plurality of through holes (41) that is closest to a front end of the spark plug (10) and is perpendicular to the axis line, the extension section (53) is a section in which a minimum distance from an outer surface (43) to an inner surface (44) of the spark plug cap (40) increases with increasing distance in a direction from back to front, and a cross-sectional area of ​​a region (51) surrounded by the extension section (53) along a plane perpendicular to the axis line decreases with increasing distance in the rear-to-front direction, wherein the rear ends (47) of the inner open ends (47) of the plurality of through holes (41) are in contact with the extension section (53), and in the cross-section enclosing the axis line, a line showing an inner surface (44) of the extension section (53) has a substantially constant radius of curvature.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The present invention relates to a spark plug with a pre-chamber for a combustion chamber of an internal combustion engine. 2. Description of the state of the art

[0002] A spark plug with a pre-chamber for the combustion chamber of an internal combustion engine is known (see, for example, PTL 1). This type of spark plug comprises a spark plug cap with through-holes connected to a front end section of a metal housing. The spark plug ignites combustible air-fuel mixture that has flowed from the combustion chamber through the through-holes into the spark plug cap. The air-fuel mixture is burned to generate an expansion pressure that causes a gas stream, including the flame, to be injected through the through-holes into the combustion chamber, so that the combustible air-fuel mixture in the combustion chamber is rapidly combusted due to the injected jet stream. Bibliography Patent Literature

[0003] PTL 1: Publication of the Japanese unexamined patent application JP 2017 - 103 179 A. According to the technology described in PTL 1, the spark plug cap has a substantially constant wall thickness from the rear end to the front end. Therefore, a front end section of the spark plug cap is slightly cooled, with the heat energy of the gas flow, including the flame, being easily transferred to the spark plug cap. This reduces the energy of the gas flow injected into the combustion chamber through the through-holes, leading to a decrease in the combustion velocity in the combustion chamber or to misfires. This reduces combustion stability. Further relevant prior art is disclosed in DE 10 2012 022 872 A1 and DE 10 2010 022 564 A1. OVERVIEW OF THE INVENTION

[0004] The present invention was made to solve the problem described above, wherein one objective of the present invention is to provide a spark plug with which stable combustion can be achieved.

[0005] To solve the problem described above, spark plugs with the features of independent claims 1, 3, 5 and 6 are specified. Further advantageous embodiments are described in the dependent claims.

[0006] The spark plug comprises a tubular metal sleeve extending along an axis in a front-to-rear direction; a center electrode held insulated within the metal sleeve; a grounding electrode electrically connected to the metal sleeve and arranged such that a spark gap is formed between the center electrode and an end section of the grounding electrode; and a spark plug cap connected to a front end section of the metal sleeve, the spark plug cap covering the center electrode and the end section of the grounding electrode from the front and having a plurality of through-holes in an area in front of the grounding electrode.In a cross-section enclosing the axis line, the spark plug cap comprises an extension section in at least one section of a region between a first imaginary straight line and a second imaginary straight line, wherein the first imaginary straight line extends through a front end of the end section of the grounding electrode and is perpendicular to the axis line, the second imaginary straight line extends through a rear end of one of the inner open ends of the through-holes, which is closest to a front end of the spark plug and is perpendicular to the axis line, and the extension section is a section in which a minimum distance from an outer surface to an inner surface of the spark plug cap increases with increasing distance in a rear-to-front direction.The cross-sectional area of ​​a region surrounded by the extension section along a plane perpendicular to the axis line decreases with increasing distance in a direction from back to front.

[0007] According to a first aspect of a spark plug, the extension section is provided between the front end of the ground electrode's terminal section and the rear end of one of the inner open ends of the through-holes in the spark plug cap, which is closest to the front end of the spark plug. The extension section is a segment in which the minimum distance from the outer surface to the inner surface of the spark plug cap increases with increasing distance from back to front. Heat conduction through the extension section to the rear becomes more difficult with increasing distance from front to back, and the heat capacity of the extension section increases with increasing distance from back to front. Therefore, the temperature in a front region of the extension section is not excessively reduced. As a result, the jet stream injected into the combustion chamber has sufficient thermal energy.Additionally, the cross-sectional area of ​​the region enclosed by the expansion section decreases along a plane perpendicular to the axis with increasing distance from back to front. Therefore, the velocity of the gas flow can be increased in the front region of the expansion section. Consequently, the jet flow injected into the combustion chamber has sufficient kinetic energy. Since the jet flow possesses sufficient thermal and kinetic energy, stable combustion of the combustible air-fuel mixture can be achieved in the combustion chamber. Secondly, in the cross-section enclosing the axis, a line representing an inner surface of the expansion section has a substantially constant radius of curvature. Therefore, protrusions on the inner surface of the expansion section do not readily form.Consequently, overheating of the inner surface of the extension section at the projections can be prevented. In this way, not only can the effects of the first aspect be achieved, but it can also be prevented that combustible air-fuel mixture, which has flowed from the combustion chamber through the through-holes into the spark plug cap, experiences pre-ignition, with projections on the inner surface of the extension section acting as ignition sources. According to a spark plug of a third aspect, in the cross-section enclosing the axis, the minimum distance from the outer surface to the inner surface of the spark plug cap is smallest at a leading edge of the inner surface of the spark plug cap. In this case, the heat capacity of a section including the leading edge of the inner surface of the spark plug cap is lower than the heat capacity of a section including the leading edge of the extension section.Accordingly, the temperature of the section, including the front end of the inner surface of the spark plug cap, can be easily reduced. In this way, not only can the effects of the first and second aspects be achieved, but it can also be prevented that the combustible air-fuel mixture, which has flowed from the combustion chamber through the through-holes into the spark plug cap, experiences pre-ignition, with the front end of the inner surface of the spark plug cap acting as an ignition source.

[0008] In a spark plug of a fourth aspect, the minimum distance between the outer and inner surfaces of the spark plug cap is smallest at the leading edge of the outer surface of the spark plug connector in the cross-section that includes the axis line. In this case, the heat capacity of the section including the leading edge of the inner surface of the spark plug cap can be reduced. Consequently, the temperature of the section including the leading edge of the outer surface of the spark plug cap can be slightly reduced. In this way, not only can the effects of the third aspect be achieved, but it can also be prevented that the combustible air-fuel mixture in the combustion chamber undergoes pre-ignition, with the leading edge of the outer surface of the spark plug cap acting as the ignition source. According to a spark plug of a fifth aspect, the leading edge of the outer surface of the spark plug cap is a flat surface.In this case, the overheating of the front end of the spark plug cap's outer surface can be reduced. Therefore, not only can the effects of aspects one through four be achieved, but it can also prevent the combustible air-fuel mixture in the combustion chamber from pre-ignition, with the front end of the spark plug cap's outer surface acting as an ignition source.

[0009] According to a sixth aspect of a spark plug, the plug cap includes a straight section formed between the expansion section and the rear ends of the multiple through-holes. The cross-sectional area of ​​the region enclosed by the straight section along a plane perpendicular to the axis is constant along the entire length of the straight section in the direction of the axis. Consequently, the gas flow in the straight section can be accelerated with less loss. Since the jet flow injected into the combustion chamber possesses sufficient kinetic energy, not only can the effects of aspects one through five be achieved, but stable combustion of the combustible air-fuel mixture in the combustion chamber can also be attained. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partial sectional view of a spark plug according to a first embodiment; Fig. 2 is an enlarged sectional view of Part II of the in Fig. 1 spark plug shown; Fig. 3 is a sectional view through a spark plug according to a second embodiment; Fig. 4 is a sectional view through a spark plug according to a third embodiment; and Fig. Figure 5 is a sectional view of a spark plug according to a fourth embodiment.

[0010] The spark plug claimed in claim 1 covers in particular the embodiment according to Fig. 3. The spark plug claimed in claim 3 covers in particular the embodiment according to Fig. 2. The spark plug claimed in claim 5 covers in particular the embodiment according to Fig. 2. The spark plug claimed in claim 6 covers in particular the embodiment according to Fig. 2 from. The in Fig. 4 and Fig. The five illustrated embodiments relate to further advantageous variants of a candle cap. However, the scope of protection is defined by the claims. DESCRIPTION OF PREFERRED EXECUTION FORMS

[0011] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. Figure 1 is a partial sectional view of a spark plug 10 according to a first embodiment. The underside of Fig. 1 is the front of the spark plug 10 and the top of Fig. 1 is defined as the back of the spark plug 10. This also applies to Fig. 2, Fig. 3, Fig. 4 to Fig. 5. Fig. Figure 1 shows a cross-section of a front end section of the spark plug 10 including an axis line O. As in Fig. As shown in Figure 1, the spark plug 10 comprises an insulator 11, a center electrode 13, a metal sleeve 20, a grounding electrode 30 and a spark plug cap 40.

[0012] The insulator 11 is an essentially cylindrical element with an axial hole 12 extending along the axis O and is made of a ceramic, such as aluminum oxide, which exhibits good mechanical properties and high insulating properties at high temperatures. The central electrode 13 is located in a front region of the axial hole 12 in the insulator 11. The central electrode 13 is electrically connected to a metal terminal 14 in the axial hole 12. The metal terminal 14 is a rod-shaped element to which a high-voltage cable (not shown) is attached and is made of a conductive metal material (for example, low-carbon steel). The metal terminal 14 is attached to the rear end of the insulator 11.

[0013] The metal sleeve 20 is an essentially cylindrical element made of a conductive metal material (for example, low-carbon steel). The metal sleeve 20 comprises a front end section 22 with an external thread 21 formed on an outer circumferential surface thereof, a seat section 23 adjacent to the rear end of the front end section 22, and a tool engagement section 24 located behind the seat section 23. The external thread 21 is screwed into a threaded bore 2 in an internal combustion engine 1. The seat section 23 is a section that seals a gap between the threaded bore 2 in the internal combustion engine 1 and the external thread 21 and has an outer diameter larger than the outer diameter of the external thread 21.The tool engagement part 24 engages with a tool, such as a wrench, which is used to screw the external thread 21 into the threaded bore 2 in the internal combustion engine 1.

[0014] The grounding electrode 30 is a rod-shaped element made of a metallic material, comprising, for example, nickel as its main component. In the present embodiment, the grounding electrode 30 is arranged at the location of the external thread 21 and extends through the front end section 22 to project into the interior of the front end section 22. The grounding electrode 30 comprises an end section 31 that is opposite the center electrode 13. The spark plug cap 40 is connected to the front end section 22 of the metal housing 20.

[0015] The spark plug cap 40 is a part that covers the center electrode 13 and the end section 31 of the ground electrode 30 from the front. The spark plug cap 40 is made of a metallic material, for example, nickel as its main component. The spark plug cap 40 has a number of through-holes 41 in an area in front of the ground electrode 30. When the spark plug 10 is installed by screwing the external thread 21 into the threaded bore 2 in the internal combustion engine 1, the spark plug cap 40 is exposed in the combustion chamber 3 of the internal combustion engine 1. The through-holes 41 connect a pre-chamber 42 in the spark plug cap 40 to the combustion chamber 3.

[0016] Fig. 2 is an enlarged sectional view of Part II of the in Fig. Figure 1 shows the spark plug 10, including the axis line O. The front end section 22 of the metal housing 20 has a recess 25, which extends radially inward in a region where the external thread 21 is provided. The front end section 22 also has a hole 26, which is thinner than the recess 25, in a region radially inside the recess 25. The hole 26 extends radially through the front end section 22. The grounding electrode 30 is inserted through the hole 26 and connected to the front end section 22 by a molten section 27. A spark gap 33 is formed between the end section 31 of the grounding electrode 30 and the center electrode 13. Since the grounding electrode 30 is connected to the metal housing 20 in the area where the external thread 21 is provided, the heat is transferred from the grounding electrode 30 to the internal combustion engine 1 via the external thread 21.

[0017] The candle cap 40 has a spherical outer surface 43 and a conical inner surface 44. The through-holes 41 have outer open ends 45 in the outer surface 43 of the candle cap 40 and inner open ends 46 in the inner surface 44 of the candle cap 40. Each through-hole 41 is inclined forward in the direction from the inner open end 46 to the outer open end 45. In the present embodiment, the rear ends 47 of the inner open ends 46 of the through-holes 41 are all positioned on a plane perpendicular to the axis line O. The candle cap 40 is connected to the front end section 22 of the metal sleeve 20 by a molten section 48.

[0018] In a cross-section enclosing the axis O, the spark plug cap 40 comprises a first extension section 53 and a second extension section 55 in a region 51 between a first imaginary straight line 49 and a second imaginary straight line 50. The first imaginary straight line 49 passes through a front end 32 of the end section 31 of the grounding electrode 30 and is perpendicular to the axis O. The second imaginary straight line 50 passes through the rear end 47 of one of the inner open ends 46 that is closest to the front end of the spark plug 10 and is perpendicular to the axis O. The second extension section 55 is located in front of the first extension section 53. The first extension section 53 and the second extension section 55 are each sections in which the minimum distance from the outer surface 43 to the inner surface 44 of the spark plug cap 40 increases with increasing distance from back to front.The minimum distance from the outer surface 43 to the inner surface 44 of the candle cap 40 is the length of the shortest line segment between a point on the outer surface 43 and a point on the inner surface 44 in the area 51.

[0019] The candle cap 40 comprises a first straight section 52, a second straight section 54, and a third straight section 56, in addition to the first widening section 53 and the second widening section 55 in region 51. The first straight section 52, the first widening section 53, the second straight section 54, the second widening section 55, and the third straight section 56 are arranged in that order from back to front. The front end of the molten section 48 is in contact with the first straight section 52.

[0020] The first extension section 53 lies next to the front end of the first straight section 52, and the second straight section 54 lies next to the front end of the first extension section 53. The second extension section 55 lies next to the front end of the second straight section 54, and the third straight section 56 lies next to the front end of the second extension section 55. The rear ends 47 of the inner open ends 46 are in contact with the third straight section 56.

[0021] The cross-sectional area of ​​a first inner region 57, which is a region enclosed by the first straight section 52, is constant along a plane perpendicular to the axis line O over the entire length of the first straight section 52 in the direction of the axis line. The cross-sectional area of ​​the first inner region 57 along a plane perpendicular to the axis line O is equal to the cross-sectional area of ​​the pre-chamber 42 along a plane perpendicular to the axis line O at the front end 32 of the end section 31 of the grounding electrode 30.

[0022] The cross-sectional area of ​​a second inner region 58, which is a region enclosed by the first extension section 53, decreases along a plane perpendicular to the axis line O with increasing distance in the direction from back to front. The cross-sectional area of ​​a third inner region 59, which is a region enclosed by the second straight section 54, is constant along a plane perpendicular to the axis line O over the entire length of the second straight section 54 in the direction of the axis line. The cross-sectional area of ​​a fourth inner region 60, which is a region enclosed by the second extension section 55, decreases along a plane perpendicular to the axis line O with increasing distance in the direction from back to front.The cross-sectional area of ​​a fifth inner region 61, which is a region surrounded by the third straight section 56, is constant along a plane perpendicular to the axis line O over the entire length of the third straight section 56 in the direction of the axis line.

[0023] A front end region 62, located upstream of the fifth inner region 61 in the pre-chamber 42, comprises a front end 63 of the inner surface 44 of the spark plug cap 40. The cross-sectional area of ​​the front end region 62 along a plane perpendicular to the axis line O at the rear end of the front end region 62 is smaller than the cross-sectional area of ​​the first extension section 53 along a plane perpendicular to the axis line O at the rear end of the first extension section 53. The front end 63 of the inner surface 44 forms part of a spherically curved surface. The front end 63 of the inner surface 44 is spaced apart from the inner open ends 46 of the through holes 41. The front end 63 of the inner surface 44 is located on the axis line O. The minimum distance from the outer surface 43 to the inner surface 44 of the candle cap 40 is smallest at the front end 63 of the inner surface 44.In particular, in the present embodiment, the minimum distance from the outer surface 43 to the inner surface 44 of the candle cap 40 is smallest between a front end 64 of the outer surface 43 of the candle cap 40 and the front end 63 of the inner surface 44 of the candle cap 40. The front end 64 of the outer surface 43 is a flat surface that is perpendicular to the axis line O.

[0024] The spark plug 10 generates a discharge in the spark gap 33 to ignite the combustible air-fuel mixture that has flowed from the combustion chamber 3 through the through-holes 41 into the spark plug cap 40. The air-fuel mixture is burned to generate an expansion pressure that injects a gas stream, including the flame, through the through-holes 41 into the combustion chamber 3, so that the combustible air-fuel mixture in the combustion chamber 3 is burned by the injected jet stream.

[0025] The spark plug 10 is designed such that the first extension section 53 and the second extension section 55 are located in the area 51 between the front end 32 of the end section 31 of the grounding electrode 30 and the rear end 47 of one of the inner open ends 46 of the through holes 41 in the spark plug cap 40, which is closest to the front end of the spark plug 10. The first extension section 53 and the second extension section 55 are each sections in which the minimum distance from the outer surface 43 to the inner surface 44 of the spark plug cap 40 in a cross-section including the axis line O increases with increasing distance in the rear-to-front direction.Heat conduction through the first expansion section 53 and the second expansion section 55 to the rear becomes more difficult with increasing distance from front to rear, while the heat capacities of the first expansion section 53 and the second expansion section 55 increase with increasing distance from rear to front. Therefore, the temperature in a front region of the second expansion section 55 is not excessively reduced, for example, when the load on the internal combustion engine 1 is low. Accordingly, the jet stream injected into the combustion chamber 3 has sufficient thermal energy.

[0026] The cross-sectional area of ​​the second inner region 58, surrounded by the first extension section 53, and of the fourth inner region 60, surrounded by the second extension section 55, decreases in a plane perpendicular to the axis line O with increasing distance from back to front. Therefore, the velocity of the gas flow, including the flame, can be increased in the front region of the second extension section 55. Consequently, the jet stream injected into the combustion chamber 3 has sufficient kinetic energy. As a result of the high-energy jet stream, a reduction in the combustion velocity in the combustion chamber 3 and misfires can be prevented, and the combustible air-fuel mixture in the combustion chamber 3 can be rapidly combusted.

[0027] Since the second straight section 54 is provided between the first extension section 53 and the second extension section 55, the inclination of the inner surface 44 of the spark plug cap 40 relative to the axis O varies in a cross-section that includes the axis O. Therefore, turbulent flow can easily be generated in the pre-chamber 42. Accordingly, the combustion velocity in the pre-chamber 42 can be increased. The spark plug 10 is designed such that, in a cross-section that includes the axis O, the minimum distance from the outer surface 43 to the inner surface 44 of the spark plug cap 40 is smallest at the front end 63 of the inner surface 44 of the spark plug cap 40. Therefore, the heat capacity of a section including the front end 63 of the inner surface 44 of the spark plug cap 40 is lower than the heat capacities of sections including the front ends of the first extension section 53 and the second extension section 55.Accordingly, the temperature of the section including the front end 63 of the inner surface 44 of the spark plug cap 40 can be easily reduced, for example by radiation. Therefore, for example under high load of the internal combustion engine 1, the combustible air-fuel mixture that has flowed from the combustion chamber 3 through the through holes 41 into the spark plug cap 40 can be prevented from pre-ignition, with the front end 63 of the inner surface 44 of the spark plug cap 40 serving as the ignition source.

[0028] The spark plug 10 is configured such that, in a cross-section including the axis O, the minimum distance from the outer surface 43 to the inner surface 44 of the spark plug cap 40 is smallest at the front end 64 of the outer surface 43 of the spark plug cap 40. Therefore, the heat capacity of the section including the front end 63 of the inner surface 44 of the spark plug cap 40 can be reduced in this area. Consequently, the temperature of the section including the front end 64 of the outer surface 43 of the spark plug cap 40 can be easily reduced, for example, by radiation. Therefore, for example, under high load of the internal combustion engine 1, it can be prevented that the combustible air-fuel mixture in the combustion chamber 3 undergoes pre-ignition, with the front end 64 of the outer surface 43 of the spark plug cap 40 acting as the ignition source.

[0029] The spark plug 10 is configured such that the front end 64 of the outer surface 43 of the spark plug cap 40 is a flat surface. Accordingly, the overheating of the front end 64 of the outer surface 43 of the spark plug cap 40 can be reduced compared to a thinned front end, such as a spherical front end. Thus, for example, under high load of the internal combustion engine 1, the combustible air-fuel mixture in the combustion chamber 3 can be prevented from pre-ignition, with the front end 64 of the outer surface 43 of the spark plug cap 40 serving as the ignition source.

[0030] The spark plug cap 40 comprises the third straight section 56, which is formed between the second expansion section 55 and the rear ends 47 of the through holes 41. Since the cross-sectional area of ​​the fifth inner region 61, which is surrounded by the third straight section 56 along a plane perpendicular to the axis line O, is constant over the entire length of the third straight section 56 in the direction of the axis line, the gas flow in the third straight section 56 can be accelerated with less loss. As a result, the jet stream injected into the combustion chamber 3 has sufficient kinetic energy to achieve stable combustion of the combustible air-fuel mixture in the combustion chamber 3.

[0031] A second embodiment is described using the following Fig. 3 described. In the first embodiment, the spark plug cap 40 comprises a plurality of extension sections (first extension section 53 and second extension section 55). In the second embodiment, a spark plug cap 71 comprises one extension section 73. Components identical to those described in the first embodiment are designated with the same reference numerals, therefore a description of these components is omitted. Fig. Figure 3 is a sectional view of a spark plug 70 according to the second embodiment, including the axis line O. Similar to the first embodiment, Fig. 3 an enlarged view of Part II (see Fig. 1) the spark plug 70. This also applies to Fig. 4 and Fig. 5.

[0032] The spark plug cap 71 of the spark plug 70 is connected to the front end section 22 of the metal housing 20 by the molten section 48. The spark plug cap 71 has the outer open ends 45 of the through holes 41 in its outer surface 43 and the inner open ends 46 of the through holes 41 in an inner surface 72.

[0033] The candle cap 71 includes the extension section 73 in the region 51. The extension section 73 is configured such that the minimum distance from the outer surface 43 to the inner surface 72 of the candle cap 71 increases with increasing distance from back to front in a cross-section including the axis line O. The front end of the molten section 48 is in contact with the extension section 73. The rear ends 47 of the inner open ends 46 are in contact with the extension section 73. The cross-sectional area of ​​an inner region 74, which is a region surrounded by the extension section 73, decreases along a plane perpendicular to the axis line O with increasing distance from back to front. A front end region 75, which is a region in front of the inner region 74 in the pre-chamber 42, includes the front end 63 of the inner surface 72 of the candle cap 71.

[0034] In a cross-section enclosing the axis O, a line showing the inner surface 72 of the extension section 73 has a substantially constant radius of curvature. Since the inner surface 72 of the extension section 73 has no significant inflection points, projections on the inner surface 72 of the extension section 73 do not readily form. Therefore, overheating of the inner surface 72 of the extension section 73 at projections can be prevented, thus preventing the combustible air-fuel mixture, which has flowed from the combustion chamber 3 through the through-holes 41 into the spark plug cap 71, from pre-ignition by projections on the inner surface 72 of the extension section 73 acting as ignition sources.Preferably, the line defining the inner surface 72 of the extension section 73 in the cross-section enclosing the axis line O has an exactly constant radius of curvature over the entire length of the extension section 73 in the direction of the axis line.

[0035] A third embodiment is described with reference to Figure 4. In the first embodiment, the candle cap 40 comprises the second straight section 54 between the first extension section 53 and the second extension section 55. In the third embodiment, however, a candle cap 81 comprises an extension section 84 with a vertical surface 83 that is perpendicular to the axis line O. Components that correspond to those of the first embodiment are designated with the same reference numerals, which is why their description is omitted. Fig. Figure 4 is a section through a spark plug 80 according to the third embodiment including the axis line O.

[0036] The spark plug cap 81 of the spark plug 80 is connected to the front end section 22 of the metal housing 20 by the molten section 48. The spark plug cap 81 has an inner surface 82 including the perpendicular surface 83, which is annular in shape and perpendicular to the axis O. The spark plug cap 81 has the outer open ends 45 of the through holes 41 in its outer surface 43 and the inner open ends 46 of the through holes 41 in its inner surface 82.

[0037] The candle cap 81 includes the extension section 84 in the region 51. The front end of the molten section 48 is in contact with the extension section 84. The rear ends 47 of the inner open ends 46 are in contact with the extension section 84. The cross-sectional area of ​​an inner region 85, which is a region surrounded by the extension section 84, decreases along a plane perpendicular to the axis line O with increasing distance in the rear-to-front direction. A front end region 86, which is a region in front of the inner region 85 in the pre-chamber 42, includes the front end 63 of the inner surface 82 of the candle cap 81.

[0038] The extension section 84 has the vertical surface 83. In a cross-section enclosing the axis line O, the minimum distance from an intersection point 43a between a straight line 83a enclosing the vertical surface 83 and the outer surface 43 to a point on the vertical surface 83 (part of the inner surface 82) increases with increasing distance in a radially inward direction of the vertical surface 83. The extension section 84 is designed such that the minimum distance between a point on the outer surface 43 in a region in front of the intersection point 43a and the inner surface 82 is greater than the minimum distance between a point on the outer surface 43 in a region behind the intersection point 43a and the inner surface 82. The extension section 84 is therefore designed such that the minimum distance from the outer surface 43 to the inner surface 82 of the candle cap 81 increases with increasing distance in the rear-to-front direction.

[0039] Since the spark plug 80 according to the third embodiment includes the extension section 84, similar effects can be achieved as with the spark plug 10 according to the first embodiment, which includes the first extension section 53 and the second extension section 55. Furthermore, since the extension section 84 has the vertical surface 83, turbulent flow can easily be generated in the pre-chamber 42. This allows the combustion velocity in the pre-chamber 42 to be increased.

[0040] A fourth embodiment is described with reference to Fig. 5 described. In the first to third embodiments, the through holes 41 in the candle caps 40, 71, and 81 are located at the same position in the axial direction. In the fourth embodiment, however, several through holes 93, 97, and 99 are provided at different positions in the axial direction. Components identical to those described in the first embodiment are designated with the same reference numerals, therefore their description is omitted. Fig.Figure 5 is a section through a spark plug 90 according to the fourth embodiment. The spark plug 90 comprises a spark plug cap 91, which is connected to the front end section 22 of the metal housing 20 by the molten section 48. The spark plug cap 91 has a plurality of through holes 93, 97, and 99. Through hole 93 has an outer open end 94 in the outer surface 43 of the spark plug cap 91 and an inner open end 95 in an inner surface 92 of the spark plug cap 91. Through hole 97 has an inner open end 98 in the inner surface 92. Through hole 99 has an outer open end 100 in the outer surface 43 and an inner open end 101 in the inner surface 92. The inner open end 95 is closest to the front end of the spark plug 90, and the inner open end 101 is closest to the rear end of the spark plug 90. Through holes 93, 97, and 99 are inclined forward in the direction from the inner surface 92 to the outer surface 43 of the spark plug cap 91.

[0041] In a cross-section enclosing the axis O, the spark plug cap 91 comprises an extension section 105 in a region 104 between a first imaginary straight line 49 and a second imaginary straight line 103. The first imaginary straight line 49 passes through the front end 32 of the end section 31 of the grounding electrode 30 and is perpendicular to the axis O. The second imaginary straight line 103 extends through a rear end 96 of the inner open end 95, which is closest to the front end of the spark plug 90, and is perpendicular to the axis O. The extension section 105 is configured such that the minimum distance from the outer surface 43 to the inner surface 92 of the spark plug cap 91 increases with increasing distance in the rear-to-front direction.The cross-sectional area of ​​a first inner region 107, which is a region surrounded by the extension section 105, along a plane perpendicular to the axis line O decreases with increasing distance in the direction from back to front.

[0042] A straight section 106 is provided adjacent to the front end of the extension section 105. The straight section 106 is in contact with a rear end 102 of the inner open end 101 of the through-hole 99, which is one of the through-holes 93, 97, and 99 nearest to the rear end of the spark plug 90. The cross-sectional area of ​​a second inner region 108, which is a region surrounded by the straight section 106, is constant along a plane perpendicular to the axis line O over the entire length of the straight section 106 in the direction of the axis line. A front end region 109, which is a region in front of the second inner region 108 in the pre-chamber 42, includes the front end 63 of the inner surface 92 of the spark plug cap 91.

[0043] Since the spark plug 90 according to the fourth embodiment comprises the expanded section 105 and the straight section 106, similar effects can be achieved as with the spark plug 10 according to the first embodiment, which comprises the second expanded section 55 and the third straight section 56. Furthermore, since the inner surface 92 of the expanded section 105 has radially inwardly convex inflection points near the straight section 106 in a cross-section that includes the axis line O, turbulent flow can easily be generated in the pre-chamber 42. This allows the combustion velocity in the pre-chamber 42 to be increased.

[0044] Although the present invention has been described based on embodiments, it is in no way limited to the embodiments described above, and it is readily understood that various improvements and modifications are possible within the scope of the present invention. For example, the shapes of the spark plug caps 40, 71, 81, and 91, as well as the number, shapes, sizes, etc., of the through-holes 41, 93, 97, and 99, can be adjusted accordingly. Although the spark plug caps 40, 71, 81, and 91 are welded to the metal housing 20 in the embodiments described, the spark plug caps are not necessarily limited to this. For example, a spark plug cap can, of course, be a front end section of a tubular element that has a closed front end and is connected to the front end section 22 of the metal housing 20.The tubular element is arranged such that it surrounds the outer circumference of the front end section 22 of the metal sleeve 20. An external thread formed on the outer circumferential surface of the tubular element is screwed into the threaded hole 2 in the internal combustion engine 1.

[0045] The tubular element (candle cap) can be connected to the front end section 22 of the metal housing 20, for example, by forming an internal thread on an inner circumferential surface of the tubular element and screwing the internal thread onto the external thread 21 formed on the front end section 22. Alternatively, a rear end section of the tubular element and the seat section 23 of the metal sleeve 20 can be joined together, for example, by welding. Alternatively, a flange can be formed on the rear end section of the tubular element, with the seat section 23 of the metal sleeve 20 and the flange being joined together, for example, by welding. The tubular element can be made, for example, of a metallic material such as a nickel-based alloy or a ceramic such as silicon nitride.

[0046] Although the grounding electrode 30, which extends through the front end section 22 of the metal sleeve 20, is located at the position where the external thread 21 is provided in the embodiments, the position of the grounding electrode is not necessarily limited to this. For example, the spark plug cap may be arranged such that the front end face of the front end section 22 of the metal sleeve 20 is exposed, and the grounding electrode may, of course, be connected to the front end face of the front end section 22. The grounding electrode may be either straight or curved. The grounding electrode may be connected to the spark plug cap.

[0047] Although the inner open ends 46, 95, 98, and 101 of the through holes 41, 93, 97, and 99 appear in cross-sections of the candle caps 40, 71, 81, and 91 along a plane that includes the axis line O in the embodiments, the through holes are not necessarily restricted to this. The through holes can, of course, be configured in the candle caps 40, 71, 81, and 91 such that the positions of their inner open ends are displaced relative to the axis line O, so that the inner open ends do not appear in cross-sections along a plane that includes the axis line O. In such a case, the positions of the inner open ends of the through holes can be determined based on the inner open ends that appear in cross-sections of the candle caps 40, 71, 81, and 91 along a plane parallel to the axis line O.Areas 51 and 104 in cross-sections along a plane including axis O are determined based on the established positions of the inner open ends of the through holes. The minimum distances from the outer surfaces 43 to the inner surfaces 44, 72, 82, and 92 of the candle caps 40, 71, 81, and 91 are measured in cross-sections along a plane including axis O.

[0048] Although the front end 64 of the outer surface 43 of each of the candle caps 40, 71, 81 and 91 is a flat surface in the embodiments, the front end 64 is not necessarily limited to this. The front end 64 of the outer surface 43 of each of the candle caps 40, 71, 81 and 91 can of course also be spherical or conical.

[0049] Although the front end 63 of each of the inner surfaces 44, 72, 82 and 92 of the candle caps 40, 71, 81 and 91 is a section of a curved surface in the embodiments, the front end 63 is not necessarily limited to this. The front end 63 of each of the inner surfaces 44, 72, 82 and 92 of the candle caps 40, 71, 81 and 91 can, of course, also be a flat surface.

[0050] Although the end faces 63 of the inner surfaces 44, 72, 82 and 92 of the candle caps 40, 71, 81 and 91 are spaced apart from the end faces of the inner open ends 46, 95, 98 and 101 of the through holes 41, 93, 97 and 99 in the embodiments, the candle caps are not necessarily limited to this. The front ends of the inner open ends 46, 95, 98 and 101 of the through holes 41, 93, 97 and 99 can instead be in contact with the front ends 63 of the inner surfaces 44, 72, 82 and 92.

[0051] Although the leading ends of the inner open ends 46 and 95 of the through holes 41 and 93 formed in the spark plug caps 40, 71, 81, and 91 are positioned behind the leading ends 63 of the inner surfaces 44, 72, 82, and 92 of the spark plug caps 40, 71, 81, and 91 in the embodiments, the positions of the leading ends of the inner open ends 46 and 95 are not necessarily limited to this. The positions of the end faces of the inner open ends 46 and 95 can, of course, be the same as the positions of the end faces 63 of the inner surfaces 44, 72, 82, and 92 in the axial direction. In this case, the gas flow in the pre-chamber 42 is introduced uniformly into the through holes 41 and 93.

[0052] In the fourth embodiment, the lines showing the inner surface 92 of the extension section 105 in a cross-section including the axis line O contain radially inwardly convex inflection points. However, the extension section 105 is not necessarily limited to this. For example, the lines showing the inner surface 92 of the extension section 105 can, of course, have radially outwardly convex inflection points at positions behind the radially inwardly convex inflection points. Similarly, in other embodiments, the inner surfaces of the extension sections can, of course, have radially outwardly convex inflection points.

[0053] Each embodiment can be modified by providing one or more parts of the structures of other embodiments in addition to their structure, in place of one or more parts of their structure, or by omitting a part of their structure. For example, in the first embodiment, the first straight section 52 can of course be omitted, and the first extension section 53 can be extended rearward so that the first extension section 53 is in contact with the molten section 48. In the first embodiment, the second straight section 54 can of course also be omitted so that the first extension section 53 and the second extension section 55 are connected to each other.Furthermore, in the first embodiment, the third straight section 56 can of course be omitted, and the second extension section 55 can be extended forward so that the second extension section 55 is in contact with the rear ends 47 of the inner open ends 46. In addition, in the first embodiment, one of the first extension section 53 and one of the second extension section 55 can of course be omitted, or an additional extension section can of course be provided.

[0054] Additionally, in the third embodiment, a straight section can of course be provided between the extension section 84 and the rear ends 47 of the inner open ends 46. Furthermore, in the third embodiment, a plurality of perpendicular surfaces 83 can of course be provided at different positions in the axial direction.

Claims

Spark plug (10) comprising: a metal sleeve (20) having a tubular shape and extending along an axis in a front-to-rear direction; a center electrode (13) held insulated within the metal sleeve (20); a grounding electrode (30) electrically connected to the metal sleeve (20) and arranged such that a spark gap (33) is formed between the center electrode (13) and an end section of the grounding electrode (31); and a spark plug cap (40) connected to a front end section of the metal housing (22), the spark plug cap (40) covering the center electrode (13) and the end section of the grounding electrode (31) from the front and having a plurality of through holes (41) in a region in front of the grounding electrode (30), wherein in a cross-section enclosing the axis,the spark plug cap (40) encloses an extension section (53) in at least one section of a region between a first imaginary straight line (49) and a second imaginary straight line (50), the first imaginary straight line (49) passes through a front end of the end section of the grounding electrode (32) and is perpendicular to the axis line, the second imaginary straight line (50) passes through a rear end of one of the inner open ends (47) of the plurality of through holes (41) that is closest to a front end of the spark plug (10) and is perpendicular to the axis line, the extension section (53) is a section in which a minimum distance from an outer surface (43) to an inner surface (44) of the spark plug cap (40) increases with increasing distance in a rear-to-front direction, and a cross-sectional area of ​​a region (51) surrounded by the extension section (53) along a plane perpendicular to the axis line,decreases with increasing distance in the direction from back to front, wherein the rear ends (47) of the inner open ends (47) of the plurality of through holes (41) are in contact with the extension section (53), and in the cross-section enclosing the axis line, a line showing an inner surface (44) of the extension section (53) has a substantially constant radius of curvature. Spark plug (10) according to claim 1, wherein in the cross-section enclosing the axis line the minimum distance is smallest at a front end of the outer surface (64) of the spark plug cap (40). Spark plug (10) comprising: a metal sleeve (20) having a tubular shape and extending along an axis in a front-to-rear direction; a center electrode (13) held insulated within the metal sleeve (20); a grounding electrode (30) electrically connected to the metal sleeve (20) and arranged such that a spark gap (33) is formed between the center electrode (13) and an end section of the grounding electrode (31); and a spark plug cap (40) connected to a front end section of the metal housing (22), the spark plug cap (40) covering the center electrode (13) and the end section of the grounding electrode (31) from the front and having a plurality of through holes (41) in a region in front of the grounding electrode (30), wherein in a cross-section enclosing the axis,the spark plug cap (40) encloses an extension section (53) in at least one section of a region between a first imaginary straight line (49) and a second imaginary straight line (50), the first imaginary straight line (49) passes through a front end of the end section of the grounding electrode (32) and is perpendicular to the axis line, the second imaginary straight line (50) passes through a rear end of one of the inner open ends (47) of the plurality of through holes (41) that is closest to a front end of the spark plug (10) and is perpendicular to the axis line, the extension section (53) is a section in which a minimum distance from an outer surface (43) to an inner surface (44) of the spark plug cap (40) increases with increasing distance in a rear-to-front direction, and a cross-sectional area of ​​a region (51) surrounded by the extension section (53) along a plane perpendicular to the axis line,with increasing distance in the direction from back to front, and wherein in the cross-section enclosing the axis line the minimum distance is smallest at a front end of the inner surface (63) of the candle cap (40). Spark plug (10) according to claim 3, wherein in the cross-section enclosing the axis line the minimum distance is smallest at a front end of the outer surface (64) of the spark plug cap (40). Spark plug (10) comprising: a metal sleeve (20) having a tubular shape and extending along an axis in a front-to-rear direction; a center electrode (13) held insulated within the metal sleeve (20); a grounding electrode (30) electrically connected to the metal sleeve (20) and arranged such that a spark gap (33) is formed between the center electrode (13) and an end section of the grounding electrode (31); and a spark plug cap (40) connected to a front end section of the metal housing (22), the spark plug cap (40) covering the center electrode (13) and the end section of the grounding electrode (31) from the front and having a plurality of through holes (41) in a region in front of the grounding electrode (30), wherein in a cross-section enclosing the axis,the spark plug cap (40) encloses an extension section (53) in at least one section of a region between a first imaginary straight line (49) and a second imaginary straight line (50), the first imaginary straight line (49) passes through a front end of the end section of the grounding electrode (32) and is perpendicular to the axis line, the second imaginary straight line (50) passes through a rear end of one of the inner open ends (47) of the plurality of through holes (41) that is closest to a front end of the spark plug (10) and is perpendicular to the axis line, the extension section (53) is a section in which a minimum distance from an outer surface (43) to an inner surface (44) of the spark plug cap (40) increases with increasing distance in a rear-to-front direction, and a cross-sectional area of ​​a region (51) surrounded by the extension section (53) along a plane perpendicular to the axis line,decreases with increasing distance in the direction from back to front, and wherein a front end of the outer surface (64) of the candle cap (40) is a flat surface. Spark plug (10) comprising: a metal sleeve (20) having a tubular shape and extending along an axis in a front-to-rear direction; a center electrode (13) held insulated within the metal sleeve (20); a grounding electrode (30) electrically connected to the metal sleeve (20) and arranged such that a spark gap (33) is formed between the center electrode (13) and an end section of the grounding electrode (31); and a spark plug cap (40) connected to a front end section of the metal housing (22), the spark plug cap (40) covering the center electrode (13) and the end section of the grounding electrode (31) from the front and having a plurality of through holes (41) in a region in front of the grounding electrode (30), wherein in a cross-section enclosing the axis,the spark plug cap (40) encloses an extension section (53) in at least one section of a region between a first imaginary straight line (49) and a second imaginary straight line (50), the first imaginary straight line (49) passes through a front end of the end section of the grounding electrode (32) and is perpendicular to the axis line, the second imaginary straight line (50) passes through a rear end of one of the inner open ends (47) of the plurality of through holes (41) that is closest to a front end of the spark plug (10) and is perpendicular to the axis line, the extension section (53) is a section in which a minimum distance from an outer surface (43) to an inner surface (44) of the spark plug cap (40) increases with increasing distance in a rear-to-front direction, and a cross-sectional area of ​​a region (51) surrounded by the extension section (53) along a plane perpendicular to the axis line,with increasing distance in the direction from back to front, wherein the candle cap (40) has a straight section (52) formed between the extension section (53) and the rear ends (47) of the plurality of through holes (41), and wherein a cross-sectional area of ​​a region (51) surrounded by the straight section (52) along a plane perpendicular to the axis line is constant over the entire length of the straight section (52) in one direction of the axis line.

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