Spark plug

Abstract

To provide a spark plug that can improve combustion stability.SOLUTION: A spark plug includes a cylindrical metal shell that is open at its tip and extends along an axis, and a cover that includes one or more nozzles that penetrate the metal shell and closes the tip of the metal shell, and at least one of the nozzles has an axial distance of 1 mm or more between the tip of the cover and a first of two intersection points where the outer surface of the cover and the nozzle intersect in a cross section that includes the center line of the nozzle and is parallel to the axis.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a spark plug provided with a cover for closing an opening at the tip of a main fitting.

Background Art

[0002] There is known a spark plug including a cylindrical main fitting having an opening at its tip, and a cover for closing the opening at the tip of the main fitting, and provided with a nozzle passing through the cover (Patent Document 1). This type of spark plug ignites fuel gas that has entered the cover through the nozzle, and injects a gas flow including the generated flame 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

Summary of the Invention

Problems to be Solved by the Invention

[0004] If the amount of fuel gas entering the cover through the nozzle is small, a large amount of burned gas will remain in the cover and the concentration of the fuel gas will not increase, so the fuel gas cannot be ignited or a flame cannot be generated. Then, the gas flow is not injected from the nozzle into the fuel chamber or the momentum of the gas flow becomes weak, so 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, which includes a cylindrical main fitting with an open tip and extending along an axis, and a cover that includes one or more nozzles passing through itself and closes the tip of the main fitting. At least one of the nozzles has an axial distance between a first intersection on the tip side among two intersections where the outer surface of the cover and the nozzle intersect in a cross-section including the center line of the nozzle and parallel to the axis, and the tip of the cover, which is 1 mm or more.

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

[0008] A third aspect is that in the first or second aspect, in the cross-section, the angle formed by the nozzle including the first intersection and the outer surface of the cover is 57° or more and 123° or less.

[0009] A fourth aspect is that in any of the first to third aspects, the tip of the cover exists on the cross-section, and in the cross-section, the length of the shortest line segment among the line segments connecting the first intersection and the tip of the cover is longer than the maximum value (excluding infinity) of the radius of curvature of the line connecting the first intersection and the tip of the cover on the outer surface of the cover.

[0010] A fifth aspect is that in any of the first to fourth aspects, the tip of the cover exists on the cross-section, and in the cross-section, the line connecting the first intersection and the tip of the cover on the outer surface of the cover includes a line segment starting from the first intersection.

[0011] A sixth aspect is that in any of the first to fifth aspects, at least the tip side of the outer surface of the cover relative to the nozzle includes a conical surface whose diameter decreases toward the tip of the cover.

Advantages of the Invention

[0012] According to the present invention, since the axial distance between the first intersection where the outer surface of the cover and the nozzle intersect and the tip of the cover is 1 mm or more, the large flow of the fuel gas flowing along the tip of the cover and the small flow of the fuel gas trying to enter the cover from the nozzle are reduced, and the flow rate of the fuel gas entering the cover from the nozzle can be ensured. Since the concentration of the fuel gas at the time of ignition in the cover can be increased, the ignition of the fuel gas and the stability of the flame generation are enhanced. Since the stability of the injection of the gas flow is increased, the combustion stability can be improved.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0014] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a partial cross-sectional view of a spark plug 10 in the first embodiment. FIG. 1 shows a cross-section including the axis O of the tip side portion of the spark plug 10. The lower side of the paper surface of FIG. 1 is the tip side of the spark plug 10, and the upper side of the paper surface is the rear end side of the spark plug 10 (the same applies to FIGS. 2 to 4).

[0015] As shown in FIG. 1, 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 ceramics such as alumina that is excellent in mechanical properties and insulation properties at high temperatures. A center electrode 13 is disposed on the tip side of the axial hole 12 of the insulator 11. The tip of the center electrode 13 protrudes from the insulator 11 toward the tip side.

[0016] 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 to which a power source (not shown) is connected, and is formed of a metallic material having conductivity (for example, low-carbon steel or the like). The terminal fitting 14 is fixed to the rear end of the insulator 11.

[0017] The main body fitting 15 is a substantially cylindrical member extending along the axis O, formed of a metallic material having conductivity (for example, low-carbon steel or the like). An external thread 15a is provided on the outer periphery of the main body fitting 15. The main body fitting 15 is disposed on the outer periphery of the insulator 11. The tip 16 of the main body fitting 15 is located on the tip side of the center electrode 13, and the tip 16 is open.

[0018] A ground electrode 17 is disposed on the main body fitting 15. The ground electrode 17 is a rod-shaped member made of a metal mainly composed of one or more of, for example, 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.

[0019] A cover 18 that closes the opening of the tip 16 of the main body fitting 15 is connected to the main body fitting 15. Examples of the material of the cover 18 include a metallic material mainly composed of one or more of Fe, Ni, Cu, etc. In this 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.

[0020] In this embodiment, three nozzles 19, 20, and 21 are illustrated, but at least one nozzle is sufficient. The number of nozzles is set as appropriate. The cross-sectional shape of the nozzles 19, 20, and 21 is also set as appropriate. Examples of the cross-sectional shape of the nozzles 19, 20, and 21 include a circle, an ellipse, a polygon, and a polygon with rounded corners.

[0021] In this embodiment, the outer surface 23 of the cover 18 is illustrated as including a cylindrical surface, but the shape of the outer surface 23 of the cover 18 is set as appropriate. Examples of the shape of the outer surface 23 of the cover 18 include a conical surface, a spherical zone, a spherical cap, a combination of one or more of these, and a combination of two or more of these.

[0022] The spark plug 10 attached to an engine (not shown) allows the fuel gas (the air-fuel mixture generated in the combustion chamber) to enter the inside of the cover 18 through the nozzles 19, 20, and 21 due to the upward movement of the piston and valve operation during the compression stroke. The spark plug 10 generates a flame kernel by means of a 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. The expansion pressure generated by the combustion of the fuel gas creates a gas flow containing a flame, and the gas containing the flame is injected from the nozzles 19, 20, and 21 into the combustion chamber. The jet of the flame causes the fuel gas in the combustion chamber to burn (explode) and push down the piston.

[0023] Figure 2 is a cross-sectional view of the spark plug 10 with the portion of the cover 18 enlarged, showing the cross-section of the spark plug 10 cut by a plane parallel to the axis O and including the center line 22 of the nozzle 19. Since the plane parallel to the axis O also includes the plane containing the axis O, as an example of the plane parallel to the axis O, Figure 2 shows a cross-section including the center line 22 of the nozzle 19 and the axis O.

[0024] The outer surface 23 of the cover 18 (the surface that appears in the combustion chamber of the engine) includes, in order from the second intersection point 24 where the outer surface 23 and the nozzle 19 intersect, the first intersection point 25, point 26, and point 27 toward the tip side. Point 27 is included in the tip 28 of the cover 18. The tip 28 is a substantially circular plane perpendicular to the axis O. The first intersection point 25 is the intersection point located on the tip side among the two intersection points where the outer surface 23 and the nozzle 19 intersect. Point 26 is the end point of the line segment 29 starting from the first intersection point 25. Point 27 is the end point of the arc 30 starting from point 26. The axial distance D between the first intersection point 25 of the nozzle 19 and the tip 28 of the cover 18 is 1 mm or more.

[0025] The axial distance between the tip-side intersection point where the nozzle 21 and the outer surface 23 intersect and the tip 28 in a cross section including the center line of the nozzle 21 (not shown) and the axis O may be less than 1 mm. Similarly, the axial distance between the tip-side intersection point where the nozzle 20 and the outer surface 23 intersect and the tip 28 in a cross section including the center line of the nozzle 20 (not shown) and the axis O may also be less than 1 mm. It is sufficient that there is one nozzle 19 for which the distance D is 1 mm or more in the cover 18.

[0026] Figure 3 is a schematic diagram of the gas flow in the combustion chamber of an engine (not shown). The fuel gas flows in the combustion chamber due to the upward movement of the piston and valve operation during the compression stroke. The flow of the fuel gas includes a small flow 34 that enters the inside of the cover 18 through the nozzle 19 and a large flow 35 that flows along the tip 28 of the cover 18. When the axial distance D between the first intersection point 25 of the nozzle 19 (see Figure 2) and the tip 28 of the cover 18 is 1 mm or more, the flow 34 that merges into the large flow 35 can be reduced, so that the flow rate of the flow 34 entering from the nozzle 19 into the cover 18 can be ensured. As a result, the burned gas 36 is exhausted from the nozzles 20 and 21 to the outside of the cover 18 along with the flow 34, and the concentration of the fuel gas in the cover 18 can be increased by the flow 34. Therefore, the stability of ignition and flame generation in the fuel gas occurring in the cover 18 is increased. The stability of the generation of the gas flow including the flame injected from the nozzles 19, 20, and 21 into the combustion chamber is increased, so that the combustion stability can be improved.

[0027] Returning to FIG. 2 for description, the distance D is preferably 1 mm or more and 5 mm or less. When there are a plurality of nozzles having a distance D of 1 mm or more, the longest distance D is preferably 5 mm or less. If the distance D becomes longer than 5 mm, the heat capacity of the portion including the tip 28 of the cover 18 increases, and the tip 28 becomes difficult to cool. Therefore, the tip 28 may become the ignition source of pre-ignition.

[0028] Similarly, the thickness T (the length of the axis O cut off by the bottom 33 and the tip 28) between the bottom 33 on the tip side of the cover 18 and the tip 28 is preferably 5 mm or less. If the thickness T becomes thicker than 5 mm, the heat capacity of the portion including the tip 28 of the cover 18 increases, and the tip 28 and the bottom 33 become difficult to cool. Therefore, there is a possibility of causing pre-ignition.

[0029] Regarding the line segment 37 connecting the first intersection 25 and the second intersection 24 as the outer surface 23 of the cover 18, the angle θ formed by the nozzle 19 including the first intersection 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. Since the length of the nozzle 19 can be shortened compared to the case where the angle θ is outside this range, the loss due to friction of the nozzle 19 among the energy of the flow 34 (see FIG. 3) and the loss (cooling loss) of the energy of the burned gas discarded as heat through the nozzle 19 to the cover 18, the main metal fitting 15, and the engine can be reduced. Thereby, the combustion stability can be further improved.

[0030] 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 gas flow from the nozzle 19 into the combustion chamber toward the tip side of the cover 18 and instantaneously burn the fuel gas in the combustion chamber.

[0031] The length of the nozzle 19 starting from the first intersection 25 is preferably shorter than the thickness T. This is to reduce the energy loss due to friction of the nozzle 19 and the cooling loss due to the nozzle 19 and improve the combustion stability.

[0032] Of the outer surface 23 of the cover 18, a line (hereinafter referred to as "line 29-30") consisting of a line segment 29 and an arc 30 connects the first intersection point 25 and the tip 28. Since the radius of curvature of the line segment 29 is infinite and the arc 30 has a radius of curvature R, the maximum value of the radius of curvature (excluding infinity) of the line 29-30 is R.

[0033] Since the arc 30 exists between the line segment 29 and the tip 28, a line segment 32 connecting the intersection point 31 where the straight line including the line segment 29 and the straight line including the tip 28 intersect and the point 26, and the line segment 29, are defined as the shortest line segment among the line segments connecting the first intersection point 25 and the tip 28 (hereinafter referred to as "line segment 29-32"). This is to reduce the influence of the arc 30 on the length of the line segment connecting the first intersection point 25 and the tip 28 (line segment 29-32).

[0034] When comparing the length of the line segment 29-32 (equal to the distance D in this embodiment) with the maximum value R of the radius of curvature (excluding 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 radius of curvature, the flow 34 that merges into the flow 35 (see FIG. 3) can be reduced. Since the flow rate of the flow 34 can be ensured, the burned gas 36 can be exhausted from the nozzles 20, 21 to the outside of the cover 18, and the concentration of the fuel gas in the cover 18 can be increased by the flow 34. Since the stability of ignition and flame generation in the cover 18 is increased, the stability of the gas flow from the nozzles 19, 20, 21 is increased, and the combustion stability can be improved.

[0035] Since the line 29-30 includes the line segment 29 starting from the first intersection point 25, the entire line 29-30 can reduce the flow 34 that merges into the flow 35 (see FIG. 3) compared to the case where the entire line 29-30 is a continuous arc without the line segment 29. Since the flow rate of the flow 34 can be ensured and the concentration of the fuel gas in the cover 18 at the time of ignition can be increased, the stability of ignition and flame generation is increased, and the stability of the gas flow from the nozzles 19, 20, 21 is increased. Therefore, the combustion stability can be improved.

[0036] As the nominal diameter of the male thread 15a of the main fitting 15 decreases, the outer circumference of the cover 18 decreases for the spark plug 10. Considering the mechanical strength etc. of the cover 18, the size of the nozzle 19 that can be provided in 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 becomes particularly large when the nominal diameter of the male thread 15a is 14 mm or less. This is because the spark plug 10 can ensure the flow rate of the flow 34 entering the inside of the cover 18 from the nozzle 19 even if the nozzle 19 is small. The diameter of the outer surface 23 of the cover 18 at this time is exemplified as 12.5 mm or less.

[0037] Also, the effect of improving combustion stability becomes greater in the case of gasoline rather than a gas with good ignitability such as compressed natural gas (CNG) for the fuel ignited by the spark plug 10. This is because if the amount of gasoline in the fuel gas (air-fuel 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 disappear inside the cover 18.

[0038] The second embodiment will be described with reference to FIG. 4. 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 was described. In contrast, in the second embodiment, the case where the tip 42 of the cover 41 and the first intersection point 25 are connected by an arc 43 in the 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 will be omitted.

[0039] FIG. 4 is a cross-sectional view of the spark plug 40 in the second embodiment in which the portion of the cover 41 is enlarged, and the 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 is shown. The cover 41 closes the opening at the tip 16 of the main fitting 15.

[0040] The outer surface 23 of the cover 41 includes a first intersection point 25 where the outer surface 23 intersects with the nozzle 19, and an arc 43 connecting the first intersection point 25 and the tip 42. The axial distance D between the first intersection point 25 of the nozzle 19 and the tip 42 is 1 mm or more. Thereby, the flow 34 that would otherwise merge into the flow 35 (see FIG. 3) is reduced, and the flow rate of the flow 34 entering from the nozzle 19 into the cover 41 can be ensured. Since the ignition of the fuel gas and the stability of the flame generation occurring in the cover 41 are increased, the stability of the generation of the gas flow including the flame injected from the nozzles 19, 20, 21 into the combustion chamber is increased, and the combustion stability can be improved.

[0041] The thickness T (the length of the axis O cut off by the bottom 44 and the tip 42) between the bottom 44 on the tip side of the cover 41 and the tip 42 is preferably 5 mm or less. If the thickness T becomes greater than 5 mm, the heat capacity of the portion including the tip 42 of the cover 41 increases, and the tip 42 and the bottom 44 are less likely to cool, which may cause pre-ignition.

[0042] The angle θ formed by 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 due to friction and the cooling loss of the nozzle 19 and further improve the combustion stability.

[0043] The length of the line segment 45 connecting the first intersection point 25 and the tip 42 of the cover 41 is shorter than the maximum value R of the radius of curvature of the line (arc 43) connecting the first intersection point 25 and the tip 42. However, since the distance D is 1 mm or more, the combustion stability can be improved.

[0044] The third embodiment will be described with reference to FIG. 5. In the first embodiment, the case where the outer surface 23 of the cover 18 includes a cylindrical surface was 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 thereof will be omitted.

[0045] FIG. 5 is a cross-sectional view of the spark plug 50 in the third embodiment with the portion of the cover 51 enlarged, and shows a cross-section of the spark plug 50 cut along a plane including the center line 22 of the nozzle 19 and parallel to the axis O. The cover 51 closes the opening at the tip 16 of the main fitting 15. The tip side of the outer surface 43 of the cover 51 that is farther from the nozzle 19 than the nozzle 19 includes a conical surface whose diameter decreases toward the tip 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.

[0046] Also, since the outer surface 43 of the cover 51 on the tip side of the nozzle 19 includes a conical surface, the area of the tip 28 can be made smaller than that of the cover 18 (see FIG. 2) including a cylindrical surface having a diameter equal to the diameter of the conical surface at the portion of the nozzle 19. Since the amount of gas that hits the tip 28 and returns to the combustion chamber without entering the cover 51 from the nozzle 19 among the fuel gas in the combustion chamber can be reduced, it becomes easier to introduce the fuel gas from the nozzle 19 into the cover 51. The form of the cover 51 including the conical surface is particularly effective for the fuel gas flowing from the tip side of the cover 51.

Example

[0047] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0048] (Preparation of Samples) The tester prepared the spark plugs in Sample Nos. 1-12 shown in Table 1 in the same manner as the spark plug 10 in the first embodiment, and prepared the spark plugs in Sample Nos. 13-20 shown in Table 2 in the same manner as the spark plug 50 in the third embodiment.

[0049] Samples No. 1 - 12 and 13 - 20 differed in the distance D between the intersection point 25 and the tip 28, the angle θ formed between 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 tip 28 (hereinafter referred to as "line segment length"), the maximum value R of the radius of curvature of the line connecting the intersection point 25 and the tip 28 (excluding infinity), and the presence or absence of a line segment starting from the intersection point 25 (hereinafter referred to as "straight line part"). The line segment length was set by changing the shape of the line connecting the intersection point 25 and the tip 28 and the angle between the line connecting the intersection point 25 and the tip 28 and the axis O.

[0050] For Samples No. 1 - 12, the dimensions and shapes of the other parts were kept constant, and four nozzles were provided on the cover 18. For Samples No. 13 - 20, the dimensions and shapes of the other parts were kept constant, and four nozzles were provided on the cover 51.

[0051]

Table 1

[0052]

Table 2

[0053] (Test 1) Test 1 is a test regarding combustion stability. The tester attached samples to 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 (coefficient of variation of the indicated mean effective pressure) between 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.

[0054] The smaller the COV, the higher the combustion stability. Samples with a COV of less than 1.5% were judged as A (excellent), samples with a COV of 1.5% or more and less than 2.0% were judged as B (very good), samples with a COV of 2.0% or more and less than 2.5% were judged as C (good), samples with a COV of 2.5% or more and less than 3.0% were judged as D (fairly good), and samples with a COV of 3.0% or more were judged as E (inferior). The results were recorded in the stability columns of Table 1 and Table 2.

[0055] (Test 2) Test 2 is a test regarding heat resistance. The tester attached samples to each cylinder of a naturally aspirated 4-cylinder gasoline engine with a displacement of 1.3 liters, operated the engine, and set the intake throttle valve to the fully open state. The engine was operated for 1 minute at a certain ignition timing to check whether pre-ignition occurred. If pre-ignition did not occur, the engine was operated for 1 minute with a 2° advance, and this operation was repeated until pre-ignition occurred.

[0056] The larger the crank angle at which pre-ignition occurred, the more difficult it was for pre-ignition to occur. Samples with an advance of the crank angle at which pre-ignition occurred of 10° or more with respect to the genuine spark plug (spark plug without cover 18) of the engine used in Test 2 were judged as A (excellent), samples with 5° or more and less than 10° were judged as B (good), and samples with less than 5° were judged as C (fair). The results were recorded in the heat resistance columns of Table 1 and Table 2.

[0057] (Evaluation) For Test 1 (combustion stability), the judgments for No.1 - 11, 13 - 20 were A - D, but the judgment for No.12 was E. The distance D of the samples of No.1 - 11, 13 - 20 was 1 mm or more, while the distance D of the sample of No.12 was less than 1 mm. It is presumed that the sample of No.12 with a distance D of less than 1 mm had low combustion stability because the flow rate of the fuel gas entering the cover through the nozzle was small. On the other hand, it is presumed that the samples of No.1 - 11, 13 - 20 with a distance D of 1 mm or more had high combustion stability because they could ensure the flow rate of the fuel gas entering the cover through the nozzle.

[0058] In Test 2 (heat resistance), the judgments for No. 1-9, 13-19 were A, but the judgments for No. 10, 20 were B, and the judgment for No. 11 was C. The samples of No. 1-10, 13-20 had a distance D of 5 mm or less, while the sample of No. 11 had a distance D exceeding 5 mm. It is presumed that the sample of No. 11 with a distance D exceeding 5 mm had a low heat resistance because the heat capacity of the part including the tip of the cover was large and it was difficult to cool. The samples of No. 10, 20 with a distance D of 5 mm are presumed to have had a high heat resistance because the heat dissipation of the part including the tip of the cover was improved compared to the sample of No. 11. The samples of No. 1-9, 13-19 are presumed to have had an even higher heat resistance than the samples of No. 10, 20 because the distance D was less than 5 mm.

[0059] For the samples of No. 1-6, 13-17, the judgments in Test 1 were A - C, but for the samples of No. 7-11, 18-20, the judgments in Test 1 were D. The samples of No. 1-6, 13-17 had a θ of 57° or more and 123° or less, while the samples of No. 7-11, 18-20 had a θ less than 57° or greater than 123°. The samples of No. 1-6 with a θ of 57° or more and 123° or less are presumed to have had high combustion stability because the energy loss due to friction at the nozzle and the cooling loss due to the nozzle were reduced compared to the samples of No. 7-11, 18-20 with a θ less than 57° or greater than 123°.

[0060] For the samples of No. 1-3, 13-15, the judgments in Test 1 were A - B, but for the samples of No. 4-6, 16, 17, the judgments in Test 1 were C. The samples of No. 1-3, 13-15 had a line segment length longer than the radius of curvature R or had a straight part, while the samples of No. 4-6, 16, 17 had a line segment length equal to the radius of curvature R or had no straight part. The samples of No. 1-3, 13-15 with a line segment length longer than the radius of curvature R or having a straight part are presumed to have had high combustion stability because they could increase the flow rate of the fuel gas entering the cover through the nozzle compared to the samples of No. 4-6, 16, 17 with a line segment length equal to the radius of curvature R or having no straight part.

[0061] Samples No.1 and 13, in which the line segment length was longer than the radius of curvature R and which had straight portions, had better combustion stability than Samples No.2 and 14, in which the line segment length was longer than the radius of curvature R but which had no straight portions, and Samples No.3 and 15, which had straight portions but in which the line segment length was shorter than the radius of curvature R. It was revealed that the fact that the line segment length was longer than the radius of curvature R and that there were straight portions was advantageous for improving combustion stability.

[0062] Although the present invention has been described based on the embodiments, it can be easily inferred that the present invention is not limited to the above embodiments at all, and various improvements and modifications can be made without departing from the spirit of the present invention.

[0063] In the embodiment, the case where the linear grounding electrode 17 is arranged at the position of the male screw of the main fitting 15 has been described, but it is not necessarily limited to this. The grounding electrode 17 may be arranged on the main fitting 15 or on the covers 18, 41, 51. The grounding electrode 17 is not limited to being linear. The grounding electrode 17 may be bent. The present invention is not limited to providing a spark gap between the tip side of the center electrode 13 and the grounding electrode 17. A spark gap may be provided between the outer side in the radial direction of the center electrode 13 and the grounding electrode 17.

[0064] In the embodiment, the case where the covers 18, 41, 51 are welded to the main fitting 15 has been described, but it is not necessarily limited to this. It is of course possible to prepare a cylindrical member provided with a cover at the tip and connect this to the main fitting 15. The cylindrical member is a cylindrical member whose tip is closed by a cover, and a female screw for coupling to the male screw of the main fitting 15 is formed on the inner peripheral surface. A male screw for coupling to the screw hole of the engine is provided on the outer peripheral surface of the cylindrical member. By coupling the female screw of the cylindrical member to the male screw of the main fitting 15, a cover is arranged on the tip side of the main fitting 15. A nozzle 19 is provided in this cover.

[0065] The means of connecting the cylindrical member to the main fitting 15 and arranging the cover on the tip side of the main fitting 15 is not limited to coupling the internal thread on the inner peripheral surface of the cylindrical member to the external thread of the main fitting 15. It is of course possible to connect the cylindrical member to the main fitting by other means. Examples of other means include joining the cylindrical member and the main fitting by welding or the like. 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 Signs

[0066] 10, 40, 50 Spark plug 15 Main fitting 16 Tip 18, 41, 51 Cover 19 Nozzle 22 Center line 23 Outer surface 25 First intersection point 28, 42 Tip 29 Line segment D Distance R Radius of curvature θ Angle

Claims

1. A spark plug comprising a cylindrical main fitting with an open tip extending along an axis, and a cover including one or more nozzles penetrating therethrough and closing the tip of the main fitting, wherein at least one of the nozzles has an axial distance between a first intersection on the tip side of two intersections where the outer surface of the cover and the nozzle intersect in a cross-section including the center line of the nozzle and parallel to the axis, and the tip of the cover, which is 1 mm or more.

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

3. The spark plug according to claim 1 or 2, wherein in the cross-section, the angle formed by the nozzle including the first intersection and the outer surface of the cover is 57° or more and 123° or less.

4. The tip of the cover is present on the cross-section, wherein in the cross-section, the length of the shortest line segment among the line segments connecting the first intersection and the tip of the cover is longer than the maximum value (excluding infinity) of the radius of curvature of the line connecting the first intersection and the tip of the cover on the outer surface of the cover. The spark plug according to claim 1 or 2.

5. The tip of the cover is present on the cross-section, wherein in the cross-section, the line connecting the first intersection and the tip of the cover on the outer surface of the cover includes a line segment starting from the first intersection. The spark plug according to claim 1 or 2.

6. The spark plug according to claim 1 or 2, wherein at least the tip side of the outer surface of the cover relative to the nozzle includes a conical surface whose diameter decreases toward the tip of the cover. ​

Images