spark plug

Abstract

Spark plug (10), having: a central electrode (15); a metal housing (20) that holds the central electrode (15) in an insulated manner; and a rod-shaped mass electrode (25, 53) which is electrically connected to the metal housing (20) and whose one end (32) is opposite the central electrode (15), wherein the metal housing (20) comprises a tubular distal end section (21) in which one end (32) of the ground electrode (25, 53) is arranged, wherein the distal end section (21) has a hole (24) into which the other end (33) of the ground electrode (25, 53) is inserted, wherein a fused section (37, 59) connects the ground electrode (25, 53) to the distal end section (21), wherein the fused section (37, 59) is part of the ground electrode (25, 53), where a value S / E obtained by dividing a total area S of a section of the ground electrode (25, 53), wherein the section is located on an inner circumferential side of the distal end section (21), by a side area E of a section of the ground electrode (25, 53), wherein the side area E includes an area of ​​an interface (39, 61) between the fused section (37, 59) and the distal end section (21), wherein the section is located inside the hole (24), is greater than 0.5 and less than or equal to 13.1.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The present invention relates to a spark plug in which a ground electrode is connected to a metal housing. 2. Description of the relevant technology

[0002] PTL 1 discloses a spark plug comprising a center electrode, a metal housing that insulates the center electrode, and a ground electrode connected to the metal housing. In the spark plug, the ground electrode is arranged in a hole provided in a tubular distal end section of the metal housing. CITATION LIST PATENT LITERATURE

[0003] PTL 1: JP 2020 - 145 018 A DE 10 2022 214 073 A1; DE 10 2020 211 356 A1 SUMMARY OF THE INVENTION

[0004] In the spark plug described in PTL 1, the ground electrode, which is surrounded by the distal end section, is hardly cooled by the fuel or the intake mixture. Therefore, the overheated ground electrode can act as an ignition source and cause pre-ignition.

[0005] The present invention was produced to solve this problem and aims to provide a spark plug that is able to reduce the occurrence of pre-ignition.

[0006] To achieve the objective, according to one aspect of the present invention, a spark plug is provided comprising: a center electrode; a metal housing that holds the center electrode in an insulated manner; and a rod-shaped ground electrode that is electrically connected to the metal housing and has an end opposite the center electrode. The metal housing comprises a tubular distal end section in which one end of the ground electrode is located. The distal end section has a hole into which the other end of the ground electrode is inserted. A value obtained by dividing the area of ​​a portion of the ground electrode located on an inner circumferential side of the distal end section by the area of ​​a lateral surface of a portion of the ground electrode located inside the hole is greater than 0.5 and less than or equal to 13.1.Preferably the value is less than or equal to 6.0 and even more preferably it is less than or equal to 5.5.

[0007] The present invention provides that the value obtained by dividing the surface area of ​​the ground electrode section located on the inner circumferential side of the distal end section (i.e., the section exposed to the heat of the combustion gas) by the lateral surface area of ​​the ground electrode section located inside the hole (i.e., the section through which heat is transferred to the metal housing) is greater than 0.5 and less than or equal to 13.1. In this way, overheating of the ground electrode can be prevented and the occurrence of pre-ignition reduced. BRIEF DESCRIPTION OF THE FIGURES

[0008] The invention is described with reference to the accompanying drawings, without being limited thereto. Fig.Figure 1 is a partial sectional view of a spark plug according to a first embodiment. Fig. 2 is an enlarged sectional view of section II of the spark plug made of Fig. 1. Fig. Figure 3 is a sectional view of a spark plug according to a second embodiment. DESCRIPTION OF PREFERRED EXECUTION FORMS

[0009] Preferred embodiments of the present invention are described below with reference to the accompanying drawings. Fig. Figure 1 is a partial sectional view of a spark plug 10 according to a first embodiment. Fig. Figure 1 shows a cross-section encompassing an axial line O of a distal section of the spark plug 10. Fig. 1 corresponds to the lower side of the drawing, the distal side of spark plug 10, and the upper side of the drawing corresponds to the proximal side of spark plug 10 (the same applies to the Fig. 2 and Fig. 3) As in Fig.As shown in Figure 1, the spark plug 10 comprises an insulator 11, a center electrode 15, a metal housing 20 and a ground electrode 25.

[0010] The insulator 11 is an essentially cylindrical element with an axial hole 12 extending along the axial line O. The insulator 11 is made of ceramic, such as aluminum oxide, which is characterized by excellent mechanical properties and insulating properties at high temperatures. The insulator 11 comprises an engaging section 13 and an end section 14 adjacent to the distal side of the engaging section 13. The outer diameter of the end section 14 is smaller than that of the engaging section 13.

[0011] The central electrode 15 is arranged in the axial hole 12 along the axial line O such that it extends at least from the engaging section 13 to the end section 14 of the insulator 11. The central electrode 15 comprises a rod-shaped base material 16, mainly composed of Ni, a tip 17 located at the distal end of the base material 16 and mainly composed of at least one noble metal such as Pt, Rh, Ru, and Ir, and a fused section 18 connecting the tip 17 and the base material 16. The tip 17 and the fused section 18 can be omitted.

[0012] The distal end of the central electrode 15 protrudes from the insulator 11 towards the distal side. The central electrode 15 is electrically connected to a metal terminal 19 within the axial hole 12. The metal terminal 19 is a rod-shaped element to which an ignition system (not shown) is attached and is made of a conductive metallic material (e.g., low-carbon steel). The metal terminal 19 is attached to the proximal end of the insulator 11.

[0013] The metal housing 20 is a substantially cylindrical element formed from a conductive metallic material (for example, copper, a copper alloy, or low-carbon steel). The metal housing 20 is arranged on the outer circumference of the insulator 11. The metal housing 20 comprises a cylindrical distal end section 21 located on the outer circumferential side of at least the engaging section 13 and the end section 14 of the insulator 11. The distal end section 21 has an external thread 22 on its outer circumference and a stepped section 23 on its inner circumference. The external thread 22 engages an internal thread provided in a spark plug hole of an engine (not shown). The stepped section 23 is located on the distal side of the engaging section 13 of the insulator 11 to engage the engaged section 13.The distal end section 21 extends further towards the distal side than the end section 14 of the insulator 11.

[0014] The distal end section 21 has a hole 24, which is a recess provided in a portion of the inner circumference of the distal end section 21, specifically in a section closer to the distal end than the step section 23. In this embodiment, the hole 24 has a circular cross-section and extends radially through the distal end section 21. The presence of the hole 24 creates a cavity in a portion of the external thread 22. A portion of the ground electrode 25 is located in the hole 24. The ground electrode 25, which is partially located in the hole 24, projects from the distal end section 21 toward the central electrode 15.

[0015] A cap 26, which closes the distal end of the metal housing 20, creates a space 29 inside the cap 26. The cap 26 is, for example, made of a metallic material consisting mainly of at least one of the metals Fe, Ni, Cu, and the like. In this embodiment, the cap 26 is connected to the distal end of the distal end section 21 by means of a fused section 28. The cap 26 has through-holes 27 that provide a connection between the inside and outside of a space 29 enclosed by the cap 26.

[0016] Fig. Figure 2 is an enlarged sectional view of section II of spark plug 10. Fig. 1. The ground electrode 25 is rod-shaped and has an end 32 that connects to the center electrode 15 (see Fig.1) opposite each other to form a gap between them, and the other end 33 is positioned in the hole 24. One end 32 is located within an inner circumference 30 of the distal end section 21. This end 32 is cylindrical and has a circular cross-section. The other end 33 is located within an outer circumference 31 of the distal end section 21 and within a foot 22a of the external thread 22. This other end 33 is cylindrical and has a circular cross-section to fit into the hole 24. The hole 24, which penetrates the distal end section 21, is closed by the other end 33.

[0017] The ground electrode 25 comprises a base material 34, mainly composed of, for example, Ni, a tip 35, mainly composed of at least one of the noble metals such as Pt, Rh, Ru, and Ir, and a fused section 36 connecting the tip 35 and the base material 34. One end 32 of the ground electrode 25 comprises a portion of the tip 35, and the other end 33 comprises a portion of the base material 34. The tip 35 and the fused section 36 can be omitted.

[0018] The ground electrode 25 is connected to the distal end section 21 by means of a fused section 37 in the hole 24. The fused section 37 is formed by irradiating a base surface 38 of the ground electrode 25, which is arranged in the hole 24, with a laser beam. The fused section 37 is a section in which a section comprising a portion of the base surface 38 of the ground electrode 25 and a section comprising a portion of the distal end section 21 are fused together. In this embodiment, the fused section 37 extends over the entire circumference of the base surface 38 of the ground electrode 25, and the radially inner end of the fused section 37 reaches the inner circumference 30 of the distal end section 21.

[0019] The heat from the mass electrode 25 is transferred to the distal end section 21 via an interface 39 between the fused section 37 and the distal end section 21. As a result, the mass electrode 25 is cooled. The amount of heat transferred depends strongly on the area of ​​the interface 39. The fused section 37 is part of the mass electrode 25. A side surface 40 and a bottom surface 41 of a section of the mass electrode 25 located within the inner circumference 30 of the distal end section 21 (i.e., the section located within space 29) are heat-absorbing surfaces that absorb the heat generated by the combustion of the fuel in space 29.

[0020] In spark plug 10 (see Fig.1) Installed in an engine (not shown), when the engine's valve is actuated, fuel flows from the engine's combustion chamber through the through-holes 27 into chamber 29. The spark plug 10 generates a flame nucleus by a discharge between the center electrode 15 and the ground electrode 25. As the flame nucleus grows, the fuel in chamber 29 is ignited and combusted. The expansion pressure generated by the combustion of the fuel produces a stream of flame-containing gas, which is injected into the combustion chamber through the through-holes 27. The fuel in the combustion chamber is combusted by the jet of flame. That is, chamber 29 within cap 26 acts as an auxiliary combustion chamber provided within the engine's combustion chamber.

[0021] The spark plug 10 is configured to have an S / E ratio of less than or equal to 13.1, where S / E is a value obtained by dividing the total surface area S of the side surface 40 and the bottom surface 41 of the section of the ground electrode 25 located within the inner circumference 30 of the distal end section 21 by the surface area E of the section of the ground electrode 25 located within the hole 24 (in this embodiment, the area of ​​the interface 39). This serves to balance the heat absorbed by the ground electrode 25 and the heat transferred from the ground electrode 25 to the distal end section 21, in order to prevent overheating of the ground electrode 25 and to reduce the occurrence of pre-ignition. The S / E ratio is preferably less than or equal to 6.0 and even more preferably less than or equal to 5.5.

[0022] In the spark plug 10, the section of the ground electrode 25 located on the inner circumferential side 30 of the distal end section 21 is covered by the cap 26. Since the space 29 on the inner circumferential side 30 of the distal end section 21 communicates with the combustion chamber (not shown) via the through-holes 27 in the cap 26, the ground electrode 25 is hardly cooled by the fuel or the intake mixture supplied to the combustion chamber. Therefore, the ground electrode 25 tends to overheat. However, since the S / E value of the spark plug 10 is set to less than or equal to 13.1, overheating of the ground electrode 25 can be prevented and the occurrence of pre-ignition can be reduced. Therefore, the invention is suitable for the spark plug 10 with the cap 26.

[0023] A second embodiment is described with reference to Fig.3 described. In the first embodiment, the case is described in which the fused section 37 extends continuously over the entire circumference of the base surface 38 of the ground electrode 25 and reaches the inner circumference 30 of the distal end section 21. In the second embodiment, a case is described in which an interrupted fused section 59 is provided along the edge of a base surface 60 of a ground electrode 53 and the fused section 59 does not reach the inner circumference 30 of the distal end section 21. In the second embodiment, the same reference numerals denote the same parts as in the first embodiment, and detailed descriptions are omitted.

[0024] Fig. Figure 3 is a sectional view of a spark plug 50 according to the second embodiment. Fig.Figure 3 is an enlarged view of a hole 51 provided in and near the distal end section 21 of the metal housing 20. Fig. Figure 3 shows no further parts. A portion of the inner circumference 30 of the distal end section 21 is recessed by the presence of the hole 51. In this embodiment, the hole 51, which has a circular cross-section, penetrates the distal end section 21 radially. The hole 51 is provided with a countersink 52 in the outer circumference 31 of the distal end section 21. The countersink 52 is located on a further inner surface than the root 22a of the external thread 22.

[0025] The ground electrode 53 comprises a base material 54, mainly composed of, for example, Ni, a tip 55, mainly composed of at least one noble metal such as Pt, Rh, Ru, and Ir, and a fused section 56 connecting the tip 55 and the base material 54. The ground electrode 53 is rod-shaped and has an end 57 opposite the central electrode 15 (see Fig. 1) to form a gap between them, and the other end 58, which is positioned in the hole 51.

[0026] One end 57 is cylindrical and has a circular cross-section. The other end 58 is cylindrical and has a circular cross-section to fit into the hole 51. The hole 51, which penetrates the distal end section 21, is closed by the other end 58. The other end 58 and the hole 51 can be joined by an interference fit, a clearance fit, or a transition fit.

[0027] The ground electrode 53 is connected to the distal end section 21 in the hole 51 by means of the fused section 59. The base surface 60 of the ground electrode 53 lies substantially on the same plane as the countersink 52. The fused section 59 is formed by irradiating the base surface 60 of the ground electrode 53, which is located in the hole 51, with a laser beam. The fused section 59 is a section in which a section comprising a portion of the base surface 60 of the ground electrode 53 and a section comprising a portion of the countersink 52 of the distal end section 21 are fused together. In this embodiment, the intermittently fused section 59 is provided along the edge of the base surface 60 of the ground electrode 53. The radially inner end of the fused section 59 is not in contact with the inner circumference 30 of the distal end section 21.

[0028] The heat from the ground electrode 53 is transferred to the distal end section 21 via an interface 61 between the fused section 59 and the distal end section 21, and via a side surface 62 of a section of the ground electrode 53 located within the hole 51. As a result, the ground electrode 53 is cooled. The amount of heat transferred depends strongly on the areas of the interface 61 and the side surface 62. The fused section 59 is part of the ground electrode 53. The side surface 63 and the bottom surface 64 of a section of the ground electrode 53 located within the inner circumference 30 of the distal end section 21 are heat-absorbing surfaces that absorb the heat generated by the combustion of the fuel.

[0029] The spark plug 50 is configured to have an S / E ratio of less than or equal to 13.1, where S / E is a value obtained by dividing the total surface area S of the side surface 63 and the base surface 64 of the ground electrode 53, located within the inner circumference 30 of the distal end section 21, by the surface area E of the section of the ground electrode 53 located within the hole 51 (in this embodiment, the total surface area of ​​the interface 61 and the side surface 62). This serves to balance the heat absorbed by the ground electrode 53 and the heat transferred by the ground electrode 53 to the distal end section 21, in order to prevent overheating of the ground electrode 53 and to reduce the occurrence of pre-ignition. [Example]

[0030] The present invention is described in more detail with reference to an example, but the present invention is not limited to the example.

[0031] The inspector prepared parts, such as metal housings and ground electrodes, for the manufacture of spark plugs. The radial thicknesses of the distal end sections of the prepared metal housings ranged from 0.3 mm to 1.8 mm, and the diameters of the circular holes radially penetrating the distal end sections ranged from 0.8 mm to 2.5 mm. The inspector inserted ground electrodes of varying lengths, whose diameters fit into the holes of the metal housings, into the holes and irradiated the base surfaces of the ground electrodes with a laser beam to weld the entire circumference of the base surfaces of the ground electrodes to the metal housings, thus producing specimens No. 1-12, which have the same shape as the spark plug according to the first embodiment.

[0032] Three-dimensional images were created from tomographic scans of samples 1 to 12, obtained with an X-ray CT scanner. Then, for each sample, an area S (mm²) was defined. 2 ) a section of the ground electrode located on the inner circumferential side of the distal end section, and a lateral surface area E (mm²) 2The area of ​​a section of the ground electrode located within the hole (i.e., the surface area of ​​the fused section) was used to calculate an S / E value, which is the area S divided by the lateral surface area E. The S / E value was obtained by dividing the area S by the lateral surface area E and rounding the quotient to one decimal place. Samples 1 through 12 differed in the area S and the lateral surface area E, but were identical in other dimensions, such as the gap size, the size of the base area at one end of the ground electrode, and the material. Table 1 shows the area S, the lateral surface areas E, and the S / E values ​​for samples 1 through 12. Table 1 Nr. Area area S(mm 2 ) Side surface area E (mm) 2 ) S / E(-) Pre-ignition resistance Wear resistance 1 31.6 2.4 13.4 D A 2 25.9 2.0 13.1 C A 3 6.8 1.1 6.0 B A 4 10.8 2.0 5.5 A A 5 8.4 1.6 5.2 A A 6 3.0 0.8 4.0 A A 7 25.9 6.6 3.9 A A 8 6.8 3.8 1.8 A A 9 10.8 6.6 1.6 A A 10 8.4 5.3 1.6 A A 11 3.0 2.5 1.2 A A 12 3.6 7.3 0.5 A D Pre-ignition test

[0033] The tester measured the lead angles relative to the ignition timing of a genuine spark plug from a test engine using the ignition advance test procedure according to the Japanese Industrial Standard (JIS) D1606:2020. The larger the crank angle at which advance occurs, the less likely it is that advance will occur.Samples exhibiting pre-ignition at an angle 7° or more ahead of the crankshaft angle of the actual spark plug were rated A; samples exhibiting pre-ignition at an angle 5° or more and less than 7° ahead of the crankshaft angle of the actual spark plug were rated B; samples exhibiting pre-ignition at an angle 2° or more and less than 5° ahead of the crankshaft angle of the actual spark plug were rated C; and samples exhibiting pre-ignition at an angle less than 2° ahead of the crankshaft angle of the actual spark plug were rated D. The results are listed in the Pre-Ignition Resistance column in Table 1. Durability testing on the test bench

[0034] The inspector examined the service life distance of specimens No. 1 to 12 according to the durability test procedure on the test bench as per JIS D1606:2020. The service life distance is the distance corresponding to an increase of 0.2 mm in the gap between the center electrode and the ground electrode compared to the gap before the test. Specimens with a service life distance of 100,000 km or more were rated A, and those with a service life distance of 50,000 km or more and less than 100,000 km were rated D. The results are listed in the Wear Resistance column of Table 1.

[0035] As shown in Table 1, sample No. 1, where the S / E value is above 13.1, was rated D for pre-ignition resistance, while samples No. 2 to No. 12, where the S / E value is less than or equal to 13.1, were rated A to C for pre-ignition resistance. This demonstrates that a spark plug with an S / E value of 13.1 or less can reduce pre-ignition.

[0036] Sample No. 2, where the S / E value is greater than 6.0 and less than or equal to 13.1, was rated C for pre-ignition resistance, while samples No. 3 to No. 12, where the S / E value is less than or equal to 6.0, were rated A or B for pre-ignition resistance. This suggests that a spark plug with an S / E value of less than or equal to 6.0 can further reduce the pre-ignition.

[0037] Sample No. 3, where the S / E value is greater than 5.5 and less than or equal to 6.0, was rated B with respect to pre-ignition resistance, while samples No. 4 to No. 12, where the S / E value is less than or equal to 5.5, were rated A with respect to pre-ignition resistance. This suggests that a spark plug with an S / E value of less than or equal to 5.5 can further reduce the ignition advance.

[0038] Specimen No. 12, with an S / E value of 0.5, was rated A for pre-ignition resistance but D for wear resistance. This is because, with an S / E value of 0.5 or less, the volume of the section acting as the ground electrode, between which a discharge occurs and the center electrode, is small, and therefore the time until the section ceases to act as the ground electrode is short. It is therefore desirable for the S / E value to be greater than 0.5, taking wear resistance into account.

[0039] Although the present invention has been described with reference to the embodiments, the present invention is not limited to these embodiments, and it can easily be deduced that various improvements and modifications can be made without departing from the spirit of the present invention.

[0040] The embodiments described include cases where the hole 24, 51 is circular, but the present invention is not necessarily limited to such cases. Other examples of the shape of the hole 24, 51 include an ellipse, a semicircle, a polygon such as a triangle, a square, or a hexagon, and a polygon with rounded corners. The cross-section of the ground electrode 25, 53, which is to be arranged in the hole 24, 51, is determined according to the shape of the hole 24, 51 so that it fits into the hole 24, 51.

[0041] The first embodiment described the case in which the continuous fused section 37 is provided around the centerline of the hole 24, and the second embodiment described the case in which the intermittent fused section 59 is provided around the centerline of the hole 51. However, the present invention is not necessarily limited to these two embodiments. It is, of course, possible to provide an intermittent fused section 37 around the centerline of the hole 24 in the first embodiment and a continuous fused section 59 around the centerline of the hole 51 in the second embodiment.

[0042] The embodiments described include cases in which the ground electrode 25, 53 is connected to the distal end section 21 by means of the fused section 37, 59, but the present invention is not necessarily limited to such cases. It is also possible, of course, to secure the ground electrode 25, 53 not by welding, but by fitting it into the hole 24, 51. The fit between the hole 24, 51 and the ground electrode 25, 53 is an interference fit or transition fit. In this case, there is no interface between the fused section and the distal end section, and the side surface area E is the area of ​​the side surface of the section of the ground electrode 25, 53 that is located inside the hole 24, 51.

[0043] The embodiments described include cases in which a portion of the base surface 38, 60 of the ground electrode 25, 53 remains unmelted, but the present invention is not necessarily limited to such cases. The entire base surface 38, 60 of the ground electrode 25, 53 can be melted into the fused section 37, 59 until no base surface 38, 60 remains. Even when the entire base surface 38, 60 is melted into the fused section 37, 59, the area of ​​the interface 39, 61 between the fused section 37, 59 and the distal end section 21 in the side surface region E is included.

[0044] In the first embodiment, the case was described in which the size of the hole 24 is constant in the radial direction of the distal end section 21. However, the present invention is not necessarily limited to this. For example, it is possible to provide a hole in the distal end section 21 that decreases in the radial direction of the distal end section 21 from the outside to the inside (i.e., a tapered hole), and to arrange the ground electrode 25 in this hole.

[0045] The embodiments described include cases where the hole 24, 51 is provided in the external thread 22 in the distal end section 21. However, the present invention is not necessarily limited to this. For example, it is of course possible to provide the distal end section 21 with a tubular section without an external thread 22, to make a hole in the tubular section, and to install the ground electrode 25, 53 therein.

[0046] The embodiments described include cases in which the hole 24, 51 penetrates the distal end section 21. However, the present invention is not necessarily limited to such cases. Even if the hole does not penetrate the distal end section 21, the other end 33, 58 of the ground electrode 25, 53 can be arranged in the hole, provided that a portion of the inner circumference 30 of the distal end section 21 is recessed.

[0047] The embodiments described include cases in which the other end 33, 58 of the ground electrode 25, 53 has essentially the same diameter as one end 32, 57 of the ground electrode 25, 53. However, the present invention is not necessarily limited to this. One end 32, 57 and the other end 33, 58 of the ground electrode 25, 53 can, of course, have different diameters.

[0048] The embodiments described include cases in which one end 32, 57 and the other end 33, 58 of the ground electrode 25, 53 have the same cross-sectional shape. However, the present invention is not necessarily limited to this. One end 32, 57 and the other end 33, 58 can, of course, also have different cross-sectional shapes.

[0049] The embodiments described include cases where the gap is provided between the side surface of the center electrode 15 and one end 32, 57 of the ground electrode 25, 53. However, the present invention is not necessarily limited to this. The gap can be provided between the distal end of the center electrode 15 and the side surface of the ground electrode 25, 53 by shifting the position of the hole 24, 51 in the distal end section 21 towards the distal side and slightly increasing the length of the ground electrode 25, 53.

[0050] In the embodiment described, the cap 26 is arranged at the distal end of the distal end section 21 of the metal housing 20. However, the present invention is not necessarily limited to this. The cap 26 can, of course, also be omitted. Even without the cap 26, if one end 32, 57 of the ground electrode 25, 53 is surrounded by the distal end section 21 of the metal housing 20, the cooling by the fuel or the intake air mixture is insufficient compared to a spark plug in which the ground electrode is arranged outside the metal housing.

[0051] In the embodiment described, the hemispherical cap 26 is arranged on the metal housing 20. However, the present invention is not necessarily limited to this. The shape of the cap 26 can be determined as desired. Other examples of the shape of the cap 26 include a cylindrical shape with a base and a disc shape.

[0052] In the embodiment described, the cap 26 is welded to the metal housing 20. However, the present invention is not necessarily limited to this. It is also possible to produce a tubular element with a cap at its distal end and to connect the tubular element to the metal housing 20 to form the space 29. The tubular element is closed by the cap at its distal end and has an internal thread on its inner circumferential surface that engages with the external thread 22 of the metal housing 20. The circumferential surface of the tubular element is provided with an external thread that engages with an internal thread in the spark plug hole of the engine. By engaging the internal thread in the tubular element with the external thread 22 on the metal housing 20, the cap is positioned at the distal end of the metal housing 20. The cap has the through holes 27.

[0053] The means for connecting the tubular element to the metal housing 20, in order to attach the cap to the distal end of the metal housing 20, is not limited to the means by which the internal thread in the inner circumferential surface of the tubular element is engaged with the external thread 22 of the metal housing 20. The tubular element can, of course, also be connected to the metal housing in other ways. Examples of other means include joining the tubular element and the metal housing by welding or similar processes. The tubular element is made, for example, of a metallic material, such as a nickel-based alloy or stainless steel, or of a ceramic, such as silicon nitride. REFERENCE MARK LIST 10.50 Spark plug 15 Center electrode 20 metal housings 21 distal end section 24, 51 holes 25, 53 Ground electrode 32, 57 an end 33, 58 the other end

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