Spark plug

Abstract

Provided is a spark plug capable of improving combustion stability. A spark plug is provided with: a cylindrical main fitting having an open tip and extending along an axis; and a cover that includes one or more nozzles penetrating the cover and closes the tip of the main fitting, and in the at least one nozzle, the distance in the axial direction between the tip of the cover and a first point of intersection on the tip side of two points of intersection at which the outer surface of the cover intersects the nozzle in a cross section that includes the center line of the nozzle and is parallel to the axis is 1 mm or more.

Description

Technical Field

[0001] The present invention relates to a spark plug having a cover that closes an opening at the front end of a closed main body fitting. Background Art

[0002] There is known a spark plug that includes a cylindrical main body fitting having an opening at the front end and a cover that closes the opening at the front end of the main body fitting, and a nozzle that penetrates the cover is provided in the cover (Patent Document 1). In this spark plug, fuel gas that enters the cover through the nozzle is ignited, and an air flow containing the generated flame is ejected from the nozzle into the combustion chamber to burn the fuel gas in the combustion chamber. Prior Art Documents Patent Documents

[0003] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2020-159355. Summary of the Invention Problems to be Solved by the Invention

[0004] If the amount of fuel gas that enters the cover through the nozzle is small, a large amount of the burned gas stays in the cover, and the concentration of the fuel gas does not increase, so that the fuel gas cannot be ignited and a flame cannot be generated. As a result, the air flow cannot be ejected from the nozzle into the fuel chamber and the momentum of the air flow weakens, so that the combustion of the fuel gas in the combustion chamber becomes unstable.

[0005] The present invention has been made to solve this problem, and an object thereof is to provide a spark plug capable of improving combustion stability. Means for Solving the Problems

[0006] A first aspect for achieving this object is a spark plug including: a cylindrical main body fitting having an opening at the front end and extending along an axis; and a cover including one or more nozzles that penetrate the cover and closing the front end of the main body fitting. In at least one nozzle, the distance in the axial direction between the front-end side first intersection point, at which the outer surface of the cover intersects the nozzle in a cross section including the center line of the nozzle and parallel to the axis, and the front end of the cover is 1 mm or more.

[0007] In a second aspect, in the first aspect, the distance is 5 mm or less.

[0008] In a third aspect, based on the first or second aspect, in this cross section, the angle formed by the nozzle including the first intersection point and the outer surface of the cover is 57° or more and 123° or less.

[0009] In the fourth mode, based on any one of the first to third modes, the front end of the cover is on a cross section in which the length of the shortest line segment among the line segments connecting the first intersection point and the front end of the cover is longer than the maximum value of the radius of curvature of the line connecting the first intersection point and the front end of the cover on the outer surface of the cover, and this maximum value is a value other than infinity.

[0010] In the fifth mode, based on any one of the first to fourth modes, the front end of the cover is on a cross section in which the line connecting the first intersection point and the front end of the cover on the outer surface of the cover includes a line segment starting from the first intersection point.

[0011] In the sixth mode, based on any one of the first to fifth modes, at least a part of the outer surface of the cover on the front end side of the nozzle includes a conical surface whose diameter becomes smaller as it approaches the front end of the cover. Advantages of the Invention

[0012] According to the present invention, the distance in the axial direction between the first intersection point where the outer surface of the cover intersects the nozzle and the front end of the cover is 1 mm or more. Therefore, it is possible to reduce the confluence of the large air flow of the fuel gas flowing along the front end of the cover and the small air flow of the fuel gas to enter the cover from the nozzle, and it is possible to ensure the flow rate of the fuel gas entering the cover from the nozzle. Since the concentration of the fuel gas at the time of ignition in the cover can be increased, the stability of the ignition of the fuel gas and the generation of the flame is increased. Since the stability of the jet of the air flow is increased, the combustion stability can be improved. Description of the Drawings

[0013] Figure 1 is a partial cross-sectional view of the spark plug of the first embodiment.

[0014] Figure 2 is a cross-sectional view of the spark plug with a part of the cover enlarged.

[0015] Figure 3 is a schematic diagram of the air flow in the combustion chamber.

[0016] Figure 4 is a cross-sectional view of the spark plug of the second embodiment.

[0017] Figure 5 is a cross-sectional view of the spark plug of the third embodiment. Detailed Embodiments

[0018] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Figure 1 is a partial cross-sectional view of the spark plug 10 of the first embodiment. In Figure 1 a cross section including the axis O of the front end side portion of the spark plug 10 is shown. Figure 1The lower side of the paper surface is referred to as the front end side of the spark plug 10, and the upper side of the paper surface is referred to as the rear end side of the spark plug 10 (the same applies in Figures 2 to 4 ).

[0019] As Figure 1 shown, the spark plug 10 includes an insulator 11, a main body fitting 15, and a cover 18. The insulator 11 is a substantially cylindrical member having an axial hole 12 extending along the axis O, and is formed of a ceramic such as alumina having excellent mechanical properties and insulation properties at high temperatures. A center electrode 13 is disposed on the front end side of the axial hole 12 of the insulator 11. The front end of the center electrode 13 projects from the insulator 11 toward the front end side.

[0020] The terminal fitting 14 is electrically connected to the center electrode 13 within the axial hole 12. The terminal fitting 14 is a rod-shaped member that connects to a power source (not shown), and is formed of a conductive metal material (such as low-carbon steel, etc.). The terminal fitting 14 is fixed to the rear end of the insulator 11.

[0021] The main body fitting 15 is a substantially cylindrical member extending along the axis O and formed of a conductive metal material (such as low-carbon steel, etc.). The main body fitting 15 has an external thread 15a on its outer periphery. The main body fitting 15 is disposed on the outer periphery of the insulator 11. The front end 16 of the main body fitting 15 is located on the front end side of the center electrode 13, and the front end 16 is open.

[0022] A ground electrode 17 is disposed in the main body fitting 15. The ground electrode 17 is, for example, a rod-shaped member made of a metal mainly composed of one or more of Pt, Ni, Ir, etc. A part of the ground electrode 17 faces the center electrode 13, and a spark gap is provided between the center electrode 13 and the ground electrode 17.

[0023] The cover 18 that closes the opening of the front end 16 of the main body fitting 15 is connected to the main body fitting 15. Examples of the material of the cover 18 include metal materials mainly composed of one or more of Fe, Ni, Cu, etc. In the present embodiment, the cover 18 is welded to the main body fitting 15. The cover 18 is provided with nozzles 19, 20, 21 that penetrate the cover 18 in the thickness direction. The nozzles 19, 20, 21 extend radially from the axis O.

[0024] In the present embodiment, three nozzles 19, 20, 21 are illustrated, but at least one nozzle is sufficient. The number of nozzles can be set appropriately. The cross-sectional shape of the nozzles 19, 20, 21 can also be set appropriately. Examples of the cross-sectional shape of the nozzles 19, 20, 21 include a circle, an ellipse, a polygon, and a polygon with rounded corners.

[0025] In the present embodiment, it is illustrated that the outer surface 23 of the cover 18 includes a cylindrical surface, but the shape of the outer surface 23 of the cover 18 can be appropriately set. Examples of the shape of the outer surface 23 of the cover 18 include one or more shapes among a conical surface, a spherical zone, and a spherical crown, and shapes formed by combining two or more of them.

[0026] The spark plug 10 installed in an engine (not shown) allows fuel gas (a mixture generated in the combustion chamber) to enter the inside of the cover 18 through the nozzles 19, 20, and 21 by the upward movement of the piston and valve operation during the compression stroke. The spark plug 10 generates a flame kernel by discharge between the center electrode 13 and the ground electrode 17. When the flame kernel grows, it ignites the fuel gas, and the fuel gas burns. Due to the expansion pressure generated by the combustion of the fuel gas, an air flow containing the flame is generated, and the gas containing the flame is ejected from the nozzles 19, 20, and 21 into the combustion chamber. Through the jet of this flame, the fuel gas in the combustion chamber burns (deflagration), and the piston is pushed down.

[0027] Figure 2 It is a cross-sectional view of the spark plug 10 with a part of the cover 18 magnified, illustrating the cross-section of the spark plug 10 cut by a plane including the center line 22 of the nozzle 19 and parallel to the axis O. The plane parallel to the axis O also includes the plane including the axis O. Therefore, as an example of the plane parallel to the axis O, Figure 2 a cross-section including the center line 22 of the nozzle 19 and the axis O is illustrated.

[0028] The outer surface 23 of the cover 18 (the surface that appears in the combustion chamber of the engine) sequentially includes a first intersection point 25, a point 26, and a point 27 starting from the second intersection point 24 where the outer surface 23 intersects the nozzle 19 toward the front end side. The point 27 is included in the front end 28 of the cover 18. The front end 28 is a substantially circular plane perpendicular to the axis O. The first intersection point 25 is the intersection point on the front end side among the two intersection points where the outer surface 23 intersects the nozzle 19. The point 26 is the end point of a line segment 29 starting from the first intersection point 25. The point 27 is the end point of an arc 30 starting from the point 26. The distance D in the axial direction between the first intersection point 25 of the nozzle 19 and the front end 28 of the cover 18 is 1 mm or more.

[0029] In the cross-section including the center line (not shown) of the nozzle 21 and the axis O, the distance in the axial direction between the intersection point on the front end side among the intersection points where the nozzle 21 intersects the outer surface 23 and the front end 28 can be less than 1 mm. Similarly, in the cross-section including the center line (not shown) of the nozzle 20 and the axis O, the distance in the axial direction between the intersection point on the front end side among the intersection points where the nozzle 20 intersects the outer surface 23 and the front end 28 can also be less than 1 mm. It is sufficient for there to be one nozzle 19 with a distance D of 1 mm or more on the cover 18.

[0030] Figure 3It is a schematic diagram of the gas flow in the combustion chamber of an engine (not shown). Through the upward movement of the piston and valve operation during the compression stroke, the fuel gas flows in the combustion chamber. The gas flow of the fuel gas includes a small gas flow 34 that enters the inside of the cover 18 through the nozzle 19 and a large gas flow 35 that flows along the front end 28 of the cover 18. If the axial distance D between the first intersection point 25 (refer to Figure 2 ) of the nozzle 19 and the front end 28 of the cover 18 is 1 mm or more, the gas flow 34 that merges with the large gas flow 35 can be reduced, so that the flow rate of the gas flow 34 entering the cover 18 from the nozzle 19 can be ensured. Thus, the burned gas 36 can be exhausted from the nozzles 20 and 21 to the outside of the cover 18 along with the gas flow 34, and the concentration of the fuel gas in the cover 18 can be increased by the gas flow 34. Therefore, the stability of ignition of the fuel gas and the generation of flames in the cover 18 increases. The stability of the generation of the gas flow containing flames injected from the nozzles 19, 20, and 21 into the combustion chamber increases, so that the combustion stability can be improved.

[0031] Return Figure 2 Explanation is as follows. The distance D is preferably 1 mm or more and 5 mm or less. When there are multiple nozzles with a distance D of 1 mm or more, the longest distance D is preferably 5 mm or less. This is because when the distance D is longer than 5 mm, the heat capacity of the portion of the cover 18 including the front end 28 becomes larger, and the front end 28 is difficult to cool, so the front end 28 may become a source of pre-ignition.

[0032] Similarly, the thickness T (the length of the axis O cut by the bottom 33 and the front end 28) between the bottom 33 on the front end side of the cover 18 and the front end 28 is preferably 5 mm or less. This is because if the thickness T is thicker than 5 mm, the heat capacity of the portion of the cover 18 including the front end 28 becomes larger, and the front end 28 and the bottom 33 are difficult to cool, so it may cause pre-ignition.

[0033] Regarding the line segment 37 connecting the first intersection point 25 and the second intersection point 24 as the outer surface 23 of the cover 18, the angle θ formed by the nozzle 19 including the first intersection point 25 and the line segment 37 (outer surface 23) is determined. The angle θ formed by the nozzle 19 and the outer surface 23 is preferably 57° or more and 123° or less. This is because compared with the case where the angle θ is outside this range, the length of the nozzle 19 can be shortened, so that the loss caused by the friction of the nozzle 19 in the energy of the gas flow 34 (refer to Figure 3 ) and the loss (cooling loss) of the energy of the burned gas that passes through the nozzle 19 and is discarded as heat through the cover 18, the main body fitting 15, and the engine can be reduced. Thus, the combustion stability can be further improved.

[0034] The angle α formed by the center line 22 of the nozzle 19 and the axis O is preferably less than 90°. This is to inject the air flow from the nozzle 19 toward the front end side of the shroud 18 into the combustion chamber, causing the fuel gas in the combustion chamber to burn instantaneously.

[0035] The length of the nozzle 19 starting from the first intersection point 25 is preferably shorter than the thickness T. This is to reduce the energy loss caused by the friction of the nozzle 19 and the cooling loss caused by the nozzle 19, and improve the combustion stability.

[0036] A line (hereinafter referred to as "line 29-30") formed by the line segment 29 and the arc 30 in the outer surface 23 of the shroud 18 connects the first intersection point 25 and the front end 28. The curvature radius of the line segment 29 is infinite, and the arc 30 has a curvature radius R. Therefore, the maximum value of the curvature radius (except for infinity) of the line 29-30 is R.

[0037] Since there is an arc 30 between the line segment 29 and the front end 28, the line segment (hereinafter referred to as "line segment 29-32") formed by the line segment 32 and the line segment 29 is defined as the shortest line segment connecting the first intersection point 25 and the front end 28. This line segment 32 connects the intersection point 31 where the straight line containing the line segment 29 intersects the straight line containing the front end 28 and the point 26. This is to reduce the influence of the arc 30 on the length of the line segment (line segment 29-32) connecting the first intersection point 25 and the front end 28.

[0038] If the length of the line segment 29-32 (equal to the distance D in this embodiment) is compared with the maximum value R of the curvature radius (except for infinity) of the line 29-31, it is preferable that the line segment 29-32 is longer. This is because when the line segment 29-32 is longer than the maximum value R of the curvature radius, the air flow 34 that merges with the air flow 35 (refer to Figure 3 ) can be reduced. Since the flow rate of the air flow 34 can be ensured, the burned gas 36 can be exhausted from the nozzles 20 and 21 to the outside of the shroud 18, and the concentration of the fuel gas in the shroud 18 can be increased by the air flow 34. Since the stability of the ignition of the fuel gas and the generation of the flame in the shroud 18 is increased, the stability of the air flows from the nozzles 19, 20, and 21 is increased, and the combustion stability can be improved.

[0039] The line 29-30 includes the line segment 29 starting from the first intersection point 25. Therefore, compared with the case where the whole line 29-30 is a continuous arc and there is no line segment 29, the air flow 34 that merges with the air flow 35 (refer to Figure 3 ) can be reduced. The flow rate of the air flow 34 can be ensured, and the concentration of the fuel gas in the shroud 18 at the time of ignition can be increased. Therefore, the stability of the ignition and the generation of the flame is increased, and the stability of the air flows from the nozzles 19, 20, and 21 is increased. Therefore, the combustion stability can be improved.

[0040] As the nominal diameter of the external thread 15a of the main body fitting 15 decreases, the outer circumference of the cover 18 decreases for the spark plug 10. Therefore, considering the mechanical strength of the cover 18 and the like, the size of the nozzle 19 that can be provided on the cover 18 becomes smaller, and the amount of fuel gas entering the inside of the cover 18 through the nozzle 19 during the compression stroke decreases. Due to such a tendency, the effect of improving combustion stability is particularly large when the nominal diameter of the external thread 15a is 14 mm or less. This is because, even if the nozzle 19 is small, the spark plug 10 can ensure the flow rate of the air flow 34 entering the inside of the cover 18 from the nozzle 19. The diameter of the outer surface 23 of the cover 18 at this time is exemplified as 12.5 mm or less.

[0041] In addition, compared with a gas with good ignitability such as compressed natural gas (CNG) for which the spark plug 10 ignites, the effect of improving combustion stability is greater in the case of gasoline. This is because, if the amount of gasoline in the fuel gas (mixture) entering the inside of the cover 18 from the nozzle 19 cannot be ensured, the flame kernel generated by the discharge is likely to extinguish inside the cover 18.

[0042] Refer to Figure 4 The second embodiment will be described. In the first embodiment, the case where the outer surface 23 of the cover 18 includes the line segment 29 starting from the first intersection point 25 has been described. In contrast, in the second embodiment, the case where the front end 42 of the cover 41 is connected to the first intersection point 25 by an arc 43 in a cross section cut by a plane including the center line 22 of the nozzle 19 and parallel to the axis O will be described. The same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description is omitted.

[0043] Figure 4 It is a cross-sectional view of the spark plug 40 of the second embodiment in which a part of the cover 41 is enlarged, showing a cross section of the spark plug 40 cut by a plane including the center line 22 of the nozzle 19 and parallel to the axis O. The cover 41 closes the opening of the front end 16 of the main body fitting 15.

[0044] The outer surface 23 of the cover 41 includes the first intersection point 25 where the outer surface 23 intersects the nozzle 19 and the arc 43 connecting the first intersection point 25 and the front end 42. The distance D in the axial direction between the first intersection point 25 of the nozzle 19 and the front end 42 is 1 mm or more. Thereby, the air flow 34 that merges with the air flow 35 (refer to Figure 3 ) can be reduced, and the flow rate of the air flow 34 entering the cover 41 from the nozzle 19 can be ensured. Since the stability of ignition of the fuel gas and the generation of the flame in the cover 41 increases, the stability of the generation of the air flow containing the flame injected into the combustion chamber from the nozzles 19, 20, and 21 increases, and the combustion stability can be improved.

[0045] The thickness T between the bottom 44 and the front end 42 on the front end side of the cover 41 (the length of the axis O cut by the bottom 44 and the front end 42) is preferably 5 mm or less. This is because if the thickness T is thicker than 5 mm, the heat capacity of the portion of the cover 41 including the front end 42 becomes large, and it is difficult to cool the front end 42 and the bottom 44, so it may cause pre-ignition.

[0046] The angle θ formed with the line segment 37 (outer surface 23) connecting the first intersection point 25 and the second intersection point 24 is preferably 57° or more and 123° or less. This is to reduce the loss caused by the friction of the nozzle 19 and the cooling loss, and further improve the combustion stability.

[0047] The length of the line segment 45 connecting the first intersection point 25 and the front end 42 of the cover 41 is shorter than the maximum value R of the radius of curvature of the line (circular arc 43) connecting the first intersection point 25 and the front end 42. However, since the distance D is 1 mm or more, the combustion stability can be improved.

[0048] Refer to Figure 5 The third embodiment will be described. In the first embodiment, the case where the outer surface 23 of the cover 18 includes a cylindrical surface has been described. In contrast, in the third embodiment, the case where the outer surface 23 of the cover 51 includes a conical surface will be described. The same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description is omitted.

[0049] Figure 5 It is a cross-sectional view of the spark plug 50 of the third embodiment in which a part of the cover 51 is enlarged, and shows a cross-section of the spark plug 50 cut by a plane including the center line 22 of the nozzle 19 and parallel to the axis O. The cover 51 closes the opening of the front end 16 of the main body fitting 15. The outer surface 43 of the cover 51 includes a conical surface on the front end side of the nozzle 19, and the diameter becomes smaller toward the front end 28 of the cover 51. According to the third embodiment, the combustion stability can be improved in the same manner as in the first embodiment.

[0050] In addition, since the outer surface 43 of the cover 51 on the front end side of the nozzle 19 includes a conical surface, compared with the cover 18 (refer to Figure 2 ) having a cylindrical surface with a diameter equal to the diameter of the conical surface of the portion of the nozzle 19, the area of the front end 28 can be reduced. The amount of gas that does not enter the cover 51 from the nozzle 19 but contacts the front end 28 and returns to the combustion chamber in the fuel gas in the combustion chamber can be reduced, so that the fuel gas can be easily introduced into the cover 51 from the nozzle 19. The shape of the cover 51 including the conical surface is particularly effective for the fuel gas flowing from the front end side of the cover 51. Examples

[0051] The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples. (Fabrication of Samples)

[0052] The tester fabricated the spark plugs of Sample Nos. 1-12 shown in Table 1 in the same manner as the spark plug 10 of the first embodiment, and fabricated the spark plugs of Sample Nos. 13-No. 20 shown in Table 2 in the same manner as the spark plug 50 of the third embodiment.

[0053] For Sample Nos. 1-12 and 13-20, the distance D between the intersection point 25 and the front end 28, the angle θ formed by the nozzle 19 and the outer surface 23, the length of the shortest line segment among the line segments connecting the intersection point 25 and the front end 28 (hereinafter referred to as "line segment length"), the maximum value R of the radius of curvature (except infinity) of the line connecting the intersection point 25 and the front end 28, and the presence or absence of a line segment starting from the intersection point 25 (hereinafter referred to as "straight portion") are different. The line segment length is set by changing the shape of the line connecting the intersection point 25 and the front end 28 and the angle between the line connecting the intersection point 25 and the front end 28 and the axis O.

[0054] The dimensions and shapes of the other parts of Sample Nos. 1-12 are constant, and four nozzles are provided on each of the covers 18. The dimensions and shapes of the other parts of Sample Nos. 13-20 are constant, and four nozzles are provided on each of the covers 51.

[0055]

[0056] (Test 1)

[0057] Test 1 is a test on combustion stability. The tester installed the samples on each cylinder of a supercharged 4-cylinder direct injection gasoline engine with a displacement of 1.6 liters, operated the engine, and calculated the COV (variation rate of the indicated mean effective pressure) between cycles for 3000 cycles under the conditions of a rotational speed of 2000 rpm, a pressure of 1200 kPa, and an air-fuel ratio of 14.5.

[0058] The smaller the COV, the higher the combustion stability. Samples with a COV less than 1.5% are judged as A (excellent), samples with a COV of 1.5% or more and less than 2.0% are judged as B (very good), samples with a COV of 2.0% or more and less than 2.5% are judged as C (good), samples with a COV of 2.5% or more and less than 3.0% are judged as D (fairly good), and samples with a COV of 3.0% or more are judged as E (poor). The results are recorded in the stability columns of Table 1 and Table 2. (Test 2)

[0059] Test 2 is a test regarding heat resistance. The tester installs the samples on each cylinder of a naturally aspirated 4-cylinder gasoline engine with a displacement of 1.3 liters, operates the engine, and keeps the intake throttle valve fully open. The engine is operated for 1 minute in such a way as to reach a certain ignition timing, and it is investigated whether pre-ignition occurs. The operation of operating the engine for 1 minute with a 2° advance if pre-ignition does not occur is repeated until pre-ignition occurs.

[0060] The larger the crankshaft angle at which pre-ignition occurs, the more difficult it is for pre-ignition to occur. Regarding the original spark plugs of the engine used in Test 2 (spark plugs without the cover 18), samples with a pre-ignition advance angle of the crankshaft angle at which pre-ignition occurs of 10° or more are judged as A (excellent), samples of 5° or more and less than 10° are judged as B (good), and samples of less than 5° are judged as C (fairly good). The results are recorded in the heat resistance columns of Table 1 and Table 2. (Evaluation)

[0061] In Test 1 (combustion stability), the judgments for No.1 - 11, 13 - 20 are A - D, while the judgment for No.12 is E. The distance D of the samples of No.1 - 11, 13 - 20 is 1 mm or more. In contrast, the distance D of the sample of No.12 is less than 1 mm. It can be presumed that the flow rate of the fuel gas entering the cover through the nozzle of the sample of No.12 with a distance D of less than 1 mm is small, so the combustion stability is low. On the other hand, it can be presumed that the samples of No.1 - 11, 13 - 20 with a distance D of 1 mm or more can ensure the flow rate of the fuel gas entering the cover through the nozzle, so the combustion stability is high.

[0062] In Test 2 (heat resistance), the judgments for No.1 - 9, 13 - 19 are A, while the judgments for No.10, 20 are B, and the judgment for No.11 is C. The distance D of the samples of No.1 - 10, 13 - 20 is 5 mm or less. In contrast, the distance D of the sample of No.11 exceeds 5 mm. For the sample of No.11 with a distance D exceeding 5 mm, it can be presumed that because the heat capacity of the part of the cover including the front end becomes larger and it is difficult to cool, the heat resistance is low. For the samples of No.10, 20 with a distance D of 5 mm, it can be presumed that compared with the sample of No.11, due to the improvement of the heat dissipation of the part of the cover including the front end, the heat resistance is high. For the samples of No.1 - 9, 13 - 19, it can be presumed that because the distance D is less than 5 mm, the heat resistance is higher than that of the samples of No.10, 20.

[0063] The determination of Test 1 for the samples of No.1-6, 13-17 was A-C, but the determination of Test 1 for the samples of No.7-11, 18-20 was D. The θ of the samples of No.1-6, 13-17 was 57° or more and 123° or less. In contrast, the θ of the samples of No.7-11, 18-20 was less than 57° or greater than 123°. It can be presumed that, compared with the samples of No.7-11, 18-20 whose θ was less than 57° or greater than 123°, the energy loss caused by friction at the nozzle and the cooling loss caused by the nozzle of the samples of No.1-6 with θ of 57° or more and 123° or less were reduced, so the combustion stability was high.

[0064] The determination of Test 1 for the samples of No.1-3, 13-15 was A-B, but the determination of Test 1 for the samples of No.4-6, 16, 17 was C. The line segment length of the samples of No.1-3, 13-15 was longer than the radius of curvature R or had a straight part. In contrast, the line segment length of the samples of No.4-6, 16, 17 was equal to the radius of curvature R or had no straight part. It can be presumed that, compared with the samples of No.4-6, 16, 17 whose line segment length was equal to the radius of curvature R or had no straight part, the samples of No.1-3, 13-15 whose line segment length was longer than the radius of curvature R or had a straight part could increase the flow rate of the fuel gas entering the hood through the nozzle, so the combustion stability was high.

[0065] The combustion stability of the samples of No.1, 13 whose line segment length was longer than the radius of curvature R and had a straight part was excellent compared with No.2, 14 whose line segment length was longer than the radius of curvature R but had no straight part, and No.3, 15 whose line segment length was shorter than the radius of curvature R but had a straight part. It was clarified that a line segment length longer than the radius of curvature R and having a straight part was beneficial to improving the combustion stability.

[0066] As described above, the present invention has been described based on the embodiments, but the present invention is not limited to any of the above embodiments, and it can be easily inferred that various improvements and deformations can be made without departing from the gist of the present invention.

[0067] In the embodiment, the case where the linear grounding electrode 17 was arranged at the position of the external thread of the main body fitting 15 was described, but it is not necessarily limited thereto. The grounding electrode 17 can be arranged on the main body fitting 15, or can be arranged on the hoods 18, 41, 51. The grounding electrode 17 is not limited to being linear. The grounding electrode 17 can also be bent. It is not limited to setting a spark gap between the front end side of the center electrode 13 and the grounding electrode 17. A spark gap can also be set between the outer side in the radial direction of the center electrode 13 and the grounding electrode 17.

[0068] In the embodiment, the case where the covers 18, 41, and 51 are welded to the main body fitting 15 has been described, but it is not necessarily limited thereto. Of course, a cylindrical member having a cover at the front end can be prepared and connected to the main body fitting 15. The cylindrical member is a cylindrical member whose front end is closed by a cover, and an internal thread that engages with the external thread of the main body fitting 15 is formed on the inner peripheral surface. An external thread that engages with the threaded hole of the engine is provided on the outer peripheral surface of the cylindrical member. By engaging the internal thread of the cylindrical member with the external thread of the main body fitting 15, the cover is disposed on the front end side of the main body fitting 15. A spout 19 is provided on the cover.

[0069] The means of connecting the cylindrical member to the main body fitting 15 and disposing the cover on the front end side of the main body fitting 15 is not limited to engaging the internal thread on the inner peripheral surface of the cylindrical member with the external thread of the main body fitting 15. Of course, the cylindrical member can also be connected to the main body fitting by other means. As other means, for example, joining the cylindrical member and the main body fitting by welding or the like can be cited. Examples of the material of the cylindrical member include metal materials such as nickel-based alloys and stainless steels, and ceramics such as silicon nitride. Explanation of reference numerals

[0070] 10, 40, 50 Spark plugs

[0071] 15 Main body fitting

[0072] 16 Front end

[0073] 18, 41, 51 Covers

[0074] 19 Spout

[0075] 22 Center line

[0076] 23 Outer surface

[0077] 25 First intersection point

[0078] 28, 42 Front end

[0079] 29 Line segment

[0080] D Distance

[0081] R Radius of curvature

[0082] θ Angle.

Claims

1. A spark plug comprising: A cylindrical body fitting having an open front end and extending along an axis; and The cover includes one or more nozzles penetrating the cover and closes the front end of the main body accessory. In at least one of the nozzles, the distance in the axial direction between the first intersection on the front end side of the two intersections where the outer surface of the cover intersects the nozzle in a cross section that includes the center line of the nozzle and is parallel to the axis and the front end of the cover is greater than 1 mm.

2. The spark plug according to claim 1, wherein: The distance is less than 5 mm.

3. The spark plug according to claim 1 or 2, wherein: In the cross section, an angle formed by the nozzle including the first intersection and the outer surface of the cover is greater than or equal to 57° and less than or equal to 123°.

4. The spark plug according to claim 1 or 2, wherein: The front end of the cover is located on the cross section, In the cross section, the length of the shortest line segment among the line segments connecting the first intersection and the front end of the cover is longer than the maximum value of the curvature radius of the line connecting the first intersection and the front end of the cover in the outer surface of the cover, and the maximum value is a value other than infinity.

5. The spark plug according to claim 1 or 2, wherein: The front end of the cover is located on the cross section, In the cross section, a line connecting the first intersection point and the front end of the cover in the outer surface of the cover includes a line segment having the first intersection point as a start point.

6. The spark plug according to claim 1 or 2, wherein: At least a portion of the outer surface of the cover closer to the front end than the nozzle includes a conical surface whose diameter decreases toward the front end of the cover.

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