SPARK PLUG

Abstract

A spark plug (10) comprises: a metal shell (15) having a cylindrical shape extending along an axial line (O), the metal shell (15) comprising a front end (16) that is open; and a cover (18) having one or more injection holes (19) passing through the cover (18), the cover (18) closing the front end (16) of the metal shell (15). In at least one of the one or more injection openings (19), in a section which includes a center line (22) of the at least one injection opening (19) and is parallel to the axial line (O), a distance (D) in the axial direction between two intersection points (24, 25) at which an outer surface (23) of the cover (18) and the at least one injection opening (19) intersect, a first intersection point (25) on a front end side and a front end (28) of the cover (18) is 1 mm or more.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to a spark plug including a cover closing an opening at a front end of a metal shell. 2. Description of the relevant technology

[0002] A known spark plug includes a metal shell having a cylindrical shape and open at a front end, and a cover closing such an opening of the front end of the metal shell. The cover has an injection port extending through the cover (JP 2020 - 159 355 A). This type of spark plug ignites the fuel that has entered the cover through the injection port, injects a gas stream including the generated flame through the injection port into a combustion chamber, and burns the fuel gas in the combustion chamber. SUMMARY OF THE INVENTION

[0003] If the amount of fuel entering the cover through the injection port is small, a large amount of burned gas remains in the cover, and the fuel concentration is not increased. Therefore, the fuel may not ignite and no flame is generated. In this case, the gas flow is not injected into the combustion chamber through the injection port, and the gas flow loses momentum, resulting in unstable fuel combustion in the combustion chamber.

[0004] The present invention has been made to solve the above-mentioned problem, and it is an object of the present invention to provide a spark plug capable of improving the stability of combustion.

[0005] A first aspect for achieving the object is a spark plug comprising: a metal shell having a cylindrical shape extending along an axial line, the metal shell having a front end that is open; and a cover having one or more injection holes extending through the cover, the cover closing the front end of the metal shell. In at least one of the one or more injection holes, in a section including a center line of the at least one injection hole and parallel to the axial line, of two intersection points where an outer surface of the cover and the at least one injection hole intersect, a distance in the axial direction between a first intersection point on a front end side and a front end of the cover is 1 mm or more.

[0006] According to a second aspect in the first aspect, the distance is 5 mm or less.

[0007] According to a third aspect, in the first or second aspect, in the section, an angle formed by the at least one injection port including the first intersection point and the outer surface of the cover is 57° or more and 123° or less.

[0008] According to a fourth aspect, in any one of the first to third aspects, the front end of the cover is present on the cut (ie, the cut extends through the front end of the cover), and in the cut, a length of a shortest line segment of line segments connecting the first intersection point and the front end of the cover is greater than a maximum value, except infinity, of a radius of curvature of a line of the outer surface of the cover connecting the first intersection point and the front end of the cover.

[0009] According to a fifth aspect, in any one of the first to fourth aspects, the front end of the cover is present on the section, and in the section, a line of the outer surface of the cover connecting the first intersection point and the front end of the cover includes a line segment extending from the first intersection point.

[0010] According to a sixth aspect, in any one of the first to fifth aspects, at least a portion of the outer surface of the cover on the front end side with respect to the at least one injection port includes a conical surface whose diameter decreases toward the front end of the cover.

[0011] According to the present invention, since the distance in the axial direction between the first intersection point where the outer surface of the cover and the injection port intersect and the front end of the cover is 1 mm or more, a large flow of fuel moving along the front end of the cover and a small flow of fuel about to enter the cover through the axial hole are prevented from converging, and the amount of fuel entering the cover through the injection port can be ensured. The concentration of fuel in the cover at the time of ignition can be increased, and the stability of fuel ignition and flame generation is thereby improved. The stability of gas flow injection is increased, and the combustion stability can thereby be improved. BRIEF DESCRIPTION OF THE CHARACTERS

[0012] The embodiments are described with reference to the figures, without being limited thereto. Fig. 1 is a partial sectional view of a spark plug according to a first embodiment. Fig. 2 is a sectional view of the spark plug with the portion of a cover enlarged. Fig. Figure 3 is a schematic representation of gas flows in a combustion chamber. Fig. 4 is a sectional view of a spark plug according to a second embodiment. Fig. 5 is a sectional view of a spark plug according to a third embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is a partial sectional view of a spark plug 10 according to a first embodiment. Fig.1 shows a section of a portion of the spark plug 10 on the front end side, which includes an axial line O. The lower side on the paper sheet of Fig. 1 is called the front end side of the spark plug 10, and the upper side on the paper sheet of Fig. 1 is called the rear end side of the spark plug 10 (the same applies to the Fig. 2 to 4).

[0014] How Fig.As shown in FIG. 1, the spark plug 10 includes an insulator 11, a metal shell 15, and a cover 18. The insulator 11 is a substantially cylindrical member having an axial hole 12 extending along the axial line O and made of ceramics such as alumina, which exhibits excellent mechanical properties and excellent high-temperature insulation performance. A center electrode 13 is disposed in a front-end region of the axial hole 12 of the insulator 11. A tip of the center electrode 13 protrudes from the insulator 11 toward the front end.

[0015] In the axial hole 12, a metal terminal 14 is electrically connected to the center electrode 13. The metal terminal 14 is a rod-shaped member to which a power source (not shown) is connected, and the metal terminal 14 is made of a conductive metallic material (such as low-carbon steel). The metal terminal 14 is attached to a rear end of the insulator 11.

[0016] The metal shell 15 is a substantially cylindrical member made of a conductive metallic material (e.g., low-carbon steel) and extending along the axial line O. An external thread 15a is provided in an outer periphery of the metal shell 15. The metal shell 15 is disposed on an outer periphery of the insulator 11. A front end 16 of the metal shell 15 is disposed on the front end side relative to the center electrode 13, and the front end 16 is open.

[0017] A ground electrode 17 is arranged on the metal shell 15. The ground electrode 17 is a rod-shaped member made of a metal containing one or more types of metal, such as Pt, Ni, and Ir, as its main component. A portion of the ground electrode 17 is located opposite the center electrode 13, and an ignition gap is provided between the center electrode 13 and the ground electrode 17.

[0018] The cover 18, which closes the opening of the front end 16 of the metal shell 15, is bonded to the metal shell 15. Examples of the material of the cover 18 include a metallic material containing one or more types of, for example, Fe, Ni, and Cu as a main component. In the present embodiment, the cover 18 is welded to the metal shell 15. The cover 18 has injection holes 19, 20, and 21 penetrating the cover 18 in the thickness direction. The injection holes 19, 20, and 21 extend radially from the axial line O.

[0019] Although three such injection ports 19, 20, and 21 are illustrated in the present embodiment, the provision of at least one injection port is sufficient. The number of injection ports is appropriately determined. The shape of the cross section of each of the injection ports 19, 20, and 21 perpendicular to a center line 22 of the respective injection port is also appropriately determined. Examples of the cross-sectional shape of each of the injection ports 19, 20, and 21 include a circle, an ellipse, a polygon, and a polygon with a rounded corner.

[0020] Although the cover 18 in the present embodiment has an outer surface 23 with a cylindrical surface, the shape of the outer surface 23 of the cover 18 is appropriately determined. Examples of the shape of the outer surface 23 of the cover 18 include a shape including one or more of the following elements: a conical surface, a spherical zone, and a spherical cap, and a shape combining two or more of the above-described three parts.

[0021] When the spark plug 10 is installed in an engine (not shown), fuel (an air-fuel mixture generated in a combustion chamber) enters the interior of the cover 18 through the injection ports 19, 20, and 21 by a piston stroke and valve operation in a compression stroke. The spark plug 10 generates a flame kernel through a discharge between the center electrode 13 and the ground electrode 17. As such a flame kernel grows, it ignites and burns the fuel. The expansion pressure generated by the combustion of the fuel creates a gas flow containing a flame and injects the gas including the flame into the combustion chamber through the injection ports 19, 20, and 21. Due to the jet of the flame, the fuel in the combustion chamber is burned (exploded) and pushes the piston downward.

[0022] Fig.2 is a sectional view of the spark plug 10 in which the area of the cover 18 is enlarged, and shows a section of the spark plug 10 along a plane containing the center line 22 of the injection hole 19 and parallel to the axial line O. Since the planes parallel to the axial line O also contain a plane containing the axial line O, Fig. 2 the section containing the center line 22 of the injection opening 19 and the axial line O, as an example of the plane parallel to the axial line O.

[0023] The outer surface 23 of the cover 18 (the surface that appears in the combustion chamber of the engine) includes a second intersection point 24 where the outer surface 23 and the injection port 19 intersect, a first intersection point 25, a point 26, and a point 27, which are arranged in this order toward the front end side. The point 27 is included in a front end 28 of the cover 18. The front end 28 is a substantially circular surface perpendicular to the axial line O. The first intersection point 25 is an intersection point of two intersection points where the outer surface 23 and the injection port 19 intersect and which are located on the front end side. In this embodiment, the first intersection point 25 is closer to the front end 28 of the cover 18 than the second intersection point 24. The point 26 is an end point of a line segment 29 extending from the first intersection point 25. Point 27 is an endpoint of a circular arc 30 starting from point 26.A distance D in the axial direction between the first intersection point 25 of the injection port 19 and the front end 28 of the cover 18 is 1 mm or more.

[0024] In the section including a center line of the injection port 21 (not shown) and the axial line O, a distance in the axial direction between a front-end intersection point of the intersection points where the injection port 21 and the outer surface 23 intersect and the front end 28 may be less than 1 mm. Similarly, in the section including a center line of the injection port 20 (not shown) and the axial line O, a distance in the axial direction between a front-end intersection point of the intersection points where the injection port 20 and the outer surface 23 intersect and the front end 28 may also be less than 1 mm. It is sufficient if an injection port 19 with a distance D of 1 mm or more is provided in the cover 18.

[0025] Fig. Figure 3 is a schematic representation of the gas flows in the combustion chamber of the engine (not shown). The fuel flows in the combustion chamber through a piston stroke and the operation of the valve in the compression stroke. Such fuel flows include a small stream 34 entering the interior of the cover 18 through the injection port 19 and a large stream 35 moving along the front end 28 of the cover 18. When the distance D in the axial direction between the first intersection point 25 of the injection port 19 (see Fig.2) and the front end 28 of the cover 18 is 1 mm or more, the flow 34 that joins the large flow 35 can be reduced, and the amount of the flow 34 that enters the cover 18 through the injection port 19 can be ensured. Thus, with the flow 34, the burned gas 36 is discharged from the cover 18 through the injection ports 20 and 21, and the flow 34 can increase the concentration of the fuel in the cover 18. Accordingly, the stability of the ignition of the fuel and the generation of a flame that occur in the cover 18 is increased. The stability of the generation of the gas flows containing a flame and injected into the combustion chamber through the injection ports 19, 20, and 21 is increased, and the stability of the combustion can thereby be improved.

[0026] The description is again made with reference to Fig.2. The distance D is preferably 1 mm or more and 5 mm or less. When there are multiple injection ports each with a distance D of 1 mm or more, the longest distance D is preferably 5 mm or less.

[0027] This is because the heat capacity of a portion of the cover 18 including the front end 28 is increased when the distance D is more than 5 mm, and the front end 28 is hardly cooled and may therefore be a source of pre-ignition.

[0028] Likewise, the thickness T between the bottom 33 of the cover 18 on the front end side and the front end 28 (the length of a portion of the axial line O cut out by the bottom 33 and the front end 28) is preferably 5 mm or less. This is because if the thickness T is more than 5 mm, the heat capacity of a portion of the cover 18 including the front end 28 is increased, and the front end 28 and the bottom 33 are hardly cooled, which may be a cause of preignition.

[0029] A line segment 37 connecting the first intersection point 25 and the second intersection point 24 is regarded as the outer surface 23 of the cover 18, and an angle θ formed by the injection port 19 including the first intersection point 25 and the line segment 37 (the outer surface 23) is determined. The angle θ formed by the injection port 19 and the outer surface 23 is preferably 57° or more and 123° or less. The reason for this is as follows: The length of the injection port 19 can be shortened compared to the case where the angle θ is outside the above-described range, thereby reducing energy loss of the current 34 (see Fig.3) caused by friction at the injection port 19, and a loss (cooling loss) of the energy possessed by the combusted gas caused by being passed through the injection port 19 via the cover 18, the metal casing 15, and the engine to be wasted as heat are reduced. This can further improve combustion stability.

[0030] An angle α formed by the center line 22 of the injection port 19 and the axial line O is preferably less than 90°. This serves to ensure immediate combustion of the fuel in the combustion chamber by injecting the gas flow through the injection port 19 to the front end side of the cover 18 into the combustion chamber.

[0031] The length of the injection opening 19, starting from the first intersection point 25, is preferably smaller than the thickness T. This serves to improve combustion stability by reducing the energy loss caused by the friction of the injection opening 19 and the cooling loss caused by the injection opening 19.

[0032] A line of the outer surface 23 of the cover 18, formed from the line segment 29 and the circular arc 30 (hereinafter referred to as "line 29-30"), connects the first intersection point 25 and the front end 28. The radius of curvature of the line segment 29 is infinite, and the circular arc 30 has a radius of curvature R; therefore, a maximum value, other than infinity, of the radius of curvature of the line 29-30 is R.

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

[0034] In comparison between the length of the line segment 29-32 (which in the present embodiment corresponds to the distance D) and the maximum value R of the radius of curvature of the line 29-30, which is not infinite, the line segment 29-32 is preferably larger. This is because the current 34, which merges into the current 35 (see Fig. 3), can be reduced when the line segment 29-32 is larger than the maximum value R of the curvature radius. Since the amount of flow 34 can be ensured, the flow 34 can increase the concentration of fuel in the cover 18 by discharging the burned gas 36 to the outside of the cover 18 through the injection ports 20 and 21. The stability of the fuel ignition and flame generation occurring in the cover 18 is increased, and the stability of the gas flows from the injection ports 19, 20, and 21 is thereby increased, which can improve combustion stability.

[0035] Since the line 29-30 contains the line segment 29 starting from the first intersection point 25, compared to the case where the entire line 29-30 is a continuous circular arc without the line segment 29, the current 34, which merges into the current 35 (see Fig. 3), can be reduced. The amount of flow 34 is ensured, and the concentration of fuel in the cover 18 at the time of ignition can be increased. This increases the stability of ignition and flame formation, and improves the stability of the gas flows from the injection ports 19, 20, and 21. Accordingly, combustion stability can be improved.

[0036] In the spark plug 10, since the outer circumference of the cover 18 decreases with the decrease in the nominal diameter of the external thread 15a of the metal shell 15, the size of the injection hole 19 that can be provided in the cover 18 decreases, for example, in consideration of the mechanical strength of the cover 18, and the amount of fuel entering the interior of the cover 18 through the injection hole 19 at the time of the compression stroke is reduced. Due to such a tendency, the effect of improving combustion stability is particularly enhanced when the nominal diameter of the external thread 15a is 14 mm or less. This is because the amount of the current 34 entering the interior of the cover 18 through the injection hole 19 can be ensured even when the spark plug 10 has the small injection hole 19.The diameter of the outer surface 23 of the cover 18 at this time is, for example, 12.5 mm or less.

[0037] For the fuel ignited by the spark plug 10, the effect of improving combustion stability is more pronounced for gasoline than for a gas such as compressed natural gas (CNG) with good ignitability. This is because the flame kernel generated by the discharge easily disappears inside the cover 18 if the amount of gasoline in the fuel (air-fuel mixture) that has entered the interior of the cover 18 through the injection port 19 cannot be ensured.

[0038] A second embodiment is 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 extending from the first intersection point 25 is described. In contrast, in the second embodiment, the case where, in a section along a plane including a center line 22 of an injection port 19 and parallel to an axial line O, a front end 42 of a cover 41 and a first intersection point 25 are connected by a circular arc 43 is described. The same parts as described in the first embodiment are designated by the same reference numerals, and the description of these parts is omitted below.

[0039] Fig.4 is a sectional view of a spark plug 40 in the second embodiment, in which the area of the cover 41 is enlarged and shows the section of the spark plug 40 along the plane containing the center line 22 of the injection hole 19 and parallel to the axial line O. The cover 41 closes an opening at the front end 16 of a metal shell 15.

[0040] An outer surface 23 of the cover 41 includes the first intersection point 25 where the outer surface 23 and the injection port 19 intersect, and the circular arc 43 connecting the first intersection point 25 and the front end 42. A distance D in the axial direction between the first intersection point 25 of the injection port 19 and the front end 42 is 1 mm or more. This creates a current 34 that communicates with a current 35 (see Fig.3) merges, and the amount of flow 34 entering the cover 41 through the injection port 19 can be ensured. The stability of the ignition of the fuel and the generation of a flame that occurs in the cover 41 is increased, and the stability of the generation of the gas flows containing a flame and injected into the combustion chamber through the injection ports 19, 20, and 21 is thereby increased, which can improve combustion stability.

[0041] The thickness T between the bottom 44 of the cover 41 on the front end side and the front end 42 (length of a portion of the axial line O cut out by the bottom 44 and the front end 42) is preferably 5 mm or less. This is because if the thickness T is more than 5 mm, the heat capacity of a portion of the cover 41 including the front end 42 is increased, and the front end 42 and the bottom 44 are hardly cooled, which may be a cause of preignition.

[0042] An angle θ formed by the injection port 19 including the first intersection point 25 and a line segment 37 (the outer surface 23) connecting the first intersection point 25 and a second intersection point 24 is preferably 57° or more and 123° or less. This serves to further improve combustion stability by reducing the loss caused by friction at the injection port 19 and the cooling loss.

[0043] In the cover 41, the length of a line segment 45 connecting the first intersection point 25 and the front end 42 is smaller than a maximum value R of the radius of curvature of a line (the circular arc 43) connecting the first intersection point 25 and the front end 42. However, combustion stability can be improved because the distance D is 1 mm or more.

[0044] A third embodiment will be described with reference to Fig.5. The case where the outer surface 23 of the cover 18 includes the cylindrical surface is described in the first embodiment. In contrast, the third embodiment describes the case where an outer surface 23 of a cover 51 includes a conical surface. The same parts as those described in the first embodiment are denoted by the same reference numerals, and the description of these parts will be omitted below.

[0045] Fig.5 is a sectional view of a spark plug 50 in the third embodiment, in which the area of the cover 51 is enlarged, showing a section of the spark plug 50 taken along a plane including a center line 22 of an injection hole 19 and parallel to an axial line O. The cover 51 closes an opening of a front end 16 of a metal shell 15. A portion of the outer surface 23 of the cover 51 on the front end side relative to the injection hole 19 has a tapered surface whose diameter decreases toward the front end 28 of the cover 51. According to the third embodiment, combustion stability can be improved as in the first embodiment.

[0046] In addition, since the portion of the outer surface 23 of the cover 51 on the front end side with respect to the injection port 19 includes the conical surface, the area of the front end 28 can be reduced compared to the cover 18 (see Fig.2) with the cylindrical surface having a diameter equal to the diameter of a portion of the conical surface at the injection port 19. The amount of a part of the fuel in the combustion chamber that hits the front end 28 to return to the combustion chamber without entering the cover 51 through the injection port 19 can be reduced, and the fuel gas can thereby be easily introduced into the cover 51 through the injection port 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. EXAMPLE

[0047] The present invention will be described in more detail by means of examples, but is not limited to the examples. Preparation of samples

[0048] A tester prepared spark plugs of Sample Nos. 1 to 12 in Table 1 in a similar manner to the spark plug 10 in the first embodiment, and the tester prepared spark plugs of Sample Nos. 13 to 20 in Table 2 in a similar manner to the spark plug 50 formed in the third embodiment.

[0049] Samples Nos. 1 to 12 and 13 to 20 differ in the distance D between the intersection point 25 and the front end 28, the angle θ formed by the injection port 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, other than infinity, of the radius of curvature of the line connecting the intersection point 25 and the front end 28, and the presence or absence of a line segment extending from the intersection point 25 (hereinafter referred to as "linear section"). Each line segment length is determined 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 axial line O.

[0050] Samples Nos. 1 to 12 have uniform dimensions and shapes of sections that differ from the above list and each contain four injection openings in the cover 18. Samples Nos. 13 to 20 have uniform dimensions and shapes of sections that differ from the above list and each contain four injection openings in the cover 51. Table 1 Nr. D (mm) θ (°) Line segment length (mm) R (mm) linear section stability Pre-ignition inhibition 1 1,5 90 1,6 0,5 Available A A 2 1,5 90 2,5 2,0 Miss B A 3 1,5 90 1,6 2,0 Available B A 4 1,0 57 2,0 2,0 Miss C A 5 1,0 90 2,0 2,0 Miss C A 6 1,0 123 2,0 2,0 Miss C A 7 1,0 45 2,0 2,0 Miss D A 8 1,0 50 2,0 2,0 Miss D A 9 1,0 130 2,0 2,0 Miss D A 10 5,0 45 6,0 6,0 Miss D B 11 5,5 45 6,0 6,0 Miss D C 12 0,5 45 1,4 2,0 Miss E A Table 2 Nr. D (mm) θ (°) Line segment length (mm) R (mm) linear section stability Pre-ignition inhibition 13 1,5 90 1,6 0,5 Available A A 14 1,2 90 2,3 2,0 Miss B A 15 3,0 90 1,6 2,0 Available B A 16 1,0 57 2,0 2,0 Miss C A 17 1,0 123 2,0 2,0 Miss C A 18 1,0 45 2,0 2,0 Miss D A 19 1,0 130 2,0 2,0 Miss D A 20 5,0 45 6,0 10,0 Miss D B Test 1

[0051] Test 1 relates to combustion stability. The tester installed a sample in each cylinder of a 1.6-liter turbocharged, four-cylinder, direct-injection gasoline engine, operated the engine, and calculated the coefficient of variance (COV) of the indicated inter-cycle mean effective pressure when operating the engine for 3,000 cycles under conditions of an engine speed of 2,000 rpm, a pressure of 1,200 kPa, and an air-fuel ratio of 14.5.

[0052] A smaller COV indicates higher combustion stability. Samples with a COV of less than 1.5% are rated A (excellent); samples with a COV of 1.5% or more and less than 2.0% are rated B (very good); samples with a COV of 2.0% or more and less than 2.5% are rated C (good); samples with a COV of 2.5% or more and less than 3.0% are rated D (fair); and samples with a COV of 3.0% or more are rated E (poor). The results are shown in the stability columns of Table 1 and Table 2. Test 2

[0053] Test 2 relates to ignition advance inhibition. The tester installed a sample in each cylinder of a 1.3-liter naturally aspirated four-cylinder engine, operated the engine, and brought an intake throttle valve to a wide-open throttle state. Whether ignition advance occurred was investigated by operating the engine for one minute to achieve a specific ignition timing. If no ignition advance occurred, the engine was operated for one minute with a 2° advance, and this operation was repeated until ignition advance occurred.

[0054] A larger crankshaft angle at which pre-ignition occurs indicates a lower probability of pre-ignition. For the advanced crankshaft angle at which pre-ignition occurred, based on an original equipment manufacturer (OES) spark plug without cover 18 of the engine used in Test 2, the sample with an angle of 10° or more is rated A (excellent), the sample with an angle of 5° or more and less than 10° is rated B (good), and the sample with an angle of less than 5° is rated C (fair). The results are shown in the pre-ignition inhibition protection columns in Table 1 and Table 2. Evaluation

[0055] In Test 1 (combustion stability), the evaluation results of Sample Nos. 1 to 11 and 13 to 20 are A to D, while the evaluation result of Sample No. 12 is E. The distance D of each of Sample Nos. 1 to 11 and 13 to 20 is 1 mm or more, while the distance D of Sample No. 12 is less than 1 mm. It is presumed that the combustion stability of Sample No. 12 with a distance D of less than 1 mm was low because the amount of fuel entering the cover through the injection port was small. On the other hand, it is presumed that the combustion stability of Sample Nos. 1 to 11 and 13 to 20 with a distance D of 1 mm or more was high because the amount of fuel flow entering the cover through the injection port could be ensured.

[0056] In Test 2 (preignition inhibition), the evaluation results of Samples Nos. 1 to 9 and 13 to 19 are A, while the evaluation results of Samples Nos. 10 and 20 are B, and the evaluation result of Sample No. 11 is C. The distance D of each of Samples Nos. 1 to 10 and 13 to 20 is 5 mm or less, while the distance D of Sample No. 11 exceeds 5 mm. It is presumed that the preignition inhibition was low in Sample No. 11 with a distance D of more than 5 mm because a portion of the cover including the front end had increased heat capacity and was hardly cooled. It is presumed that the preignition inhibition was high in Samples Nos. 10 and 20 with a distance D of 5 mm because the heat conduction of a portion of the cover including the front end was improved compared to Sample No. 11. It is assumed that the pre-ignition inhibition in samples Nos. 1 to 9 and 13 to 19 is higher than in samples Nos.10 and 20 was even higher because the distance D is less than 5 mm.

[0057] In Test 1, the evaluation results of Sample Nos. 1 to 6 and 13 to 17 are A to C, while the evaluation results of Sample Nos. 7 to 11 and 18 to 20 are D. The angle θ of each of Sample Nos. 1 to 6 and 13 to 17 is 57° or more and 123° or less, while the angle θ of each of Sample Nos. 7 to 11 and 18 to 20 is less than 57° or more than 123°. It is presumed that the combustion stability was high for Samples Nos. 1 to 6 and 13 to 17 with an angle θ of 57° or more and 123° or less because the energy loss caused by friction at the injection port and the cooling loss caused by the injection port were smaller than those for Samples Nos. 7 to 11 and 18 to 20 with an angle θ of less than 57° or more than 123°.

[0058] In Test 1, the evaluation results of Samples Nos. 1 to 3 and 13 to 15 are A and B, while the evaluation results of Samples Nos. 4 to 6, 16, and 17 are C. Samples Nos. 1 to 3 and 13 to 15 have a line segment length greater than the curvature radius R or include the linear section, while Samples Nos. 4 to 6, 16, and 17 have a line segment length equal to the curvature radius R or do not include a linear section. It is presumed that the combustion stability was high in Samples Nos. 1 to 3 and 13 to 15 having a line segment length greater than the curvature radius R or containing the linear section because the amount of fuel flow entering the cover through the injection port could be increased, compared with Samples Nos. 4 to 6, 16 and 17 having a line segment length equal to the curvature radius R or not containing a linear section.

[0059] The combustion stability was excellent for samples Nos. 1 and 13, whose line segment length was greater than the radius of curvature R and which included the linear section, compared to samples Nos. 2 and 14, whose line segment length was greater than the radius of curvature R but did not include a linear section, and to samples Nos. 3 and 15, which included the linear section but whose line segment length was smaller than the radius of curvature R. It was found that it is beneficial for improving combustion stability if the line segment length is greater than the radius of curvature R and if the linear section is included.

[0060] Although the present invention has been described so far with reference to the embodiments, the present invention is not limited to the above-described embodiments, and it goes without saying that various improvements and changes can be made without departing from the spirit of the present invention.

[0061] Although the embodiments describe the case where the ground electrode 17 is arranged in a linear shape at the position of the external thread of the metal shell 15, this is not the only possibility. The ground electrode 17 may be arranged on the metal shell 15 or on one of the respective covers 18, 41, and 51. The ground electrode 17 is not limited to having a linear shape. The ground electrode 17 may be curved. The case where the ignition gap is provided between the front end side of the center electrode 13 and the ground electrode 17 is not the only possibility. The ignition gap may also be provided between the radially outer side of the center electrode 13 and the ground electrode 17.

[0062] Although the embodiments describe the case where one of the respective covers 18, 41, and 51 is welded to the metal shell 15, this is not the only possibility. A cylindrical member having a cover at a front end may be prepared, and the cylindrical member may be connected to the metal shell 15. The cylindrical member is a cylindrical member whose front end is closed by the cover, and an inner peripheral surface of the cylindrical member has an internal thread connected to the external thread of the metal shell 15. An outer peripheral surface of the cylindrical member is provided with an external thread connected to a cylindrical hole of the motor. By connecting the internal thread of the cylindrical member to the external thread of the metal shell 15, the cover is arranged on the front end side of the metal shell 15.The cover has an injection opening 19.

[0063] The means for disposing the cover on the front end side of the metal shell 15 by connecting the cylindrical member to the metal shell 15 is not limited to the means for connecting the internal thread in the inner peripheral surface of the cylindrical member to the external thread of the metal shell 15. Other means may also be used to connect the cylindrical member to the metal shell. Examples of such other means include means for joining the cylindrical member and the metal shell by welding. Examples of the material of the cylindrical member include metallic materials such as nickel-based alloy and stainless steel, and ceramic materials such as silicon nitride. LIST OF REFERENCE SYMBOLS 10, 40, 50 spark plug 15 metal housings 16 Front end 18, 41, 51 cover 19 Injection port 22 Center line 23 Exterior area 25 first intersection point 28, 42 front end 29 line segment D Distance R radius of curvature θ angle QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] JP 2020 - 159 355 A

[0002]

Claims

[1] Spark plug (10), comprising: a metal casing (15) having a cylindrical shape extending along an axial line (O), the metal casing (15) having a front end (16) which is open; and a cover (18) having one or more injection openings (19) extending through the cover (18), the cover (18) closing the front end (16) of the metal housing (15), in at least one of the one or more injection openings (19) in a section which includes a center line (22) of the at least one injection opening (19) and is parallel to the axial line (O), a distance (D) in an axial direction between two intersection points (24, 25) at which an outer surface (23) of the cover (18) and the at least one injection opening (19) intersect, a first intersection point (25) on a front end side and a front end (28) of the cover (18) is 1 mm or more. [2] Spark plug (10) according to claim 1, wherein the distance (D) is 5 mm or less. [3] Spark plug (10) according to claim 1 or 2, wherein in the section, an angle (θ) formed by the at least one injection hole (19) including the first intersection point (25) and the outer surface (23) of the cover (18) is 57° or more and 123° or less. [4] Spark plug (10) according to one of claims 1 to 3, wherein the front end (28) of the cover (18) is present on the cut, and in the section, a length of a shortest line segment (29-32) of line segments connecting the first intersection point (25) and the front end (28) of the cover (18) is greater than a maximum value, except infinity, of a radius of curvature (R) of a line of the outer surface (23) of the cover (18) connecting the first intersection point (25) and the front end (28) of the cover (18). [5] Spark plug (10) according to one of claims 1 to 4, wherein the front end (28) of the cover (18) is present on the cut and in the section, a line (29-30) of the outer surface (23) of the cover (18) connecting the first intersection point (25) and the front end (28) of the cover (18) contains a line segment (29) starting from the first intersection point (25). [6] Spark plug (50) according to one of claims 1 to 5, wherein at least a part of the outer surface (23) of the cover (51) on the front end side with respect to the at least one injection opening (19) comprises a conical surface whose diameter decreases towards the front end (28) of the cover (51).

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