Prechamber spark plug with surface discharge spark gap and method for operating a prechamber spark plug

Abstract

Pre-chamber spark plug (110), comprising: a metal shell (116) with an axial bore, an end section (150) which has a distal end surface (152), and an inner protrusion (154); an insulator (114) which is at least partially arranged in the axial bore of the metal shell (116) and has an axial bore (146) and an end tip section (140) with a distal end surface (142); a central electrode (112) which is at least partially arranged in the axial bore (146) of the insulator (114) and has an end section (130) with a distal end surface (132); and a pre-chamber cap (120) attached to the metal casing (116) and having a pre-chamber wall (174) with one or more opening(s) (172) that allows a gas flow between a pre-chamber (122) and a main combustion chamber when the pre-chamber spark plug (110) is installed in an engine; wherein the pre-chamber spark plug (110) has a surface discharge spark gap (G2) extending between the distal end face (132) of the center electrode (112) and the inner protrusion (154) of the metal shell (116) along the distal end face (142) of the insulator (114), and wherein the surface discharge spark gap (G2) has both a radial spark formation section (190) in which spark formation - with respect to a central axis (A) of the spark plug - takes place in a generally radial direction, and an axial spark formation section (192) in which spark formation - with respect to a central axis of the spark plug - takes place in a generally axial direction.

Description

AREA

[0001] This invention relates generally to spark plugs and in particular to pre-chamber spark plugs, as well as to a method for operating a pre-chamber spark plug. BACKGROUND

[0002] The successful operation of pre-chamber spark plugs, particularly when used in internal combustion engines of motor vehicles, can depend on the unimpeded flow of unburned or fresh gas into a pre-chamber and of burnt or residual gas from the pre-chamber. If, for example, the burnt or residual gas is not sufficiently purged or otherwise expelled after the combustion process, irregular and undesirable combustion processes can occur. Such processes can cause pre-ignition in the main combustion chamber, increase the temperature of certain spark plug components, and lead to other undesirable consequences.

[0003] Pre-chamber spark plugs are known from documents US 2 687 713 A and DE 10 2013 223 721 A1.

[0004] It may therefore be desirable to provide a pre-chamber spark plug with improved gas flow into and out of the pre-chamber, as well as an improved method for operating a pre-chamber spark plug. SUMMARY

[0005] According to the invention, a pre-chamber spark plug with the features of claim 1 is provided.

[0006] According to various embodiments, the pre-chamber spark plug may further include one of the following features or any technically possible combination of some or all of these features: • The inner protrusion of the metal shell has a radial surface and an inner shoulder on opposite axial sides of the inner protrusion; the radial surface faces the antechamber, and the inner shoulder holds the insulator; • the distal end surface of the center electrode is in an axial position which, viewed in the forward direction, extends beyond the distal end surface of the insulator by an axial distance (DA1, DA2) ranging from 0 mm to 5 mm; • the radial surface of the inner protrusion is in an axial position which, viewed in the forward direction, extends an axial distance (DC1, DC2) ranging from 0 mm to 5 mm beyond the distal end surface of the insulator; • the distal end face of the metal sheath is in an axial position which, viewed in the forward direction, extends beyond the radial surface of the inner protrusion, the distal end face of the insulator and the distal end face of the center electrode; • the radial surface of the inner protrusion is in an axial position extending beyond the distal end surface of the insulator when viewed in the forward direction, the distal end surface of the insulator is in an axial position extending beyond the radial surface of the inner protrusion when viewed in the forward direction, and the distal end surface of the metal sheath is in an axial position extending beyond the distal end surface of the insulator when viewed in the forward direction; • the distal end face of the center electrode is in an axial position which, viewed in the forward direction, extends beyond the distal end face of the insulator, the radial surface of the inner protrusion is located at an axial position which, viewed in the forward direction, extends beyond the distal end face of the insulator, and the distal end face of the metal sheath is in an axial position which, viewed in the forward direction, extends beyond the radial surface of the inner protrusion; • The surface discharge spark gap has a radial spark formation section where a large part of the spark formation takes place in a direction that is generally radial to the central axis, as opposed to an axial direction; • The spark formation section extends between the center electrode and the inner protrusion along the distal end face of the insulator over a radial distance (GFS1) of 0.5 mm to 3.0 mm; • The surface discharge spark gap also has a surface spark gap where a large part of the spark formation takes place along the distal end face of the insulator, in contrast to an air spark gap; • the radial sparking section extends between the center electrode and the inner protrusion along the distal end face of the insulator over a radial distance (GSF2) of 0.5 mm to 3.0 mm, and the axial sparking section extends between the insulator and the inner protrusion over an air spark gap (LSF2) of 0.5 mm to and including 2.0 mm; • The distal end surface of the insulator is a largely flat, annular surface extending radially with respect to a central axis of the spark plug, such that the distal end surface of the center electrode, the distal end surface of the insulator and a radial surface of the inner protrusion all meet with each other of equal or nearly equal length; • The atrium is defined by the atrium wall of the atrium cap at an anterior end of the atrium, by the atrium wall of the atrium cap and an atrium wall of the metal shell on sides of the atrium, and by a combination of the distal end face of the center electrode, the distal end face of the insulator and a radial surface of the inner protrusion at a posterior end of the atrium; • The surface discharge spark gap is located at the rear end of the prechamber and is configured so that spark-generating components minimally block the electrode and promote unimpeded gas exchange between the prechamber and the main combustion chamber; and • The pre-chamber spark plug is a passive pre-chamber spark plug and is configured only to receive air and fuel from the main combustion chamber through the one or more openings in the pre-chamber cap and not from additional injectors that direct air or fuel directly into the pre-chamber.

[0007] According to the invention, a method for operating a pre-chamber spark plug with the features of claim 13 is also provided.

[0008] According to various embodiments, the method for operating the pre-chamber spark plug may further include and / or incorporate one of the following features or a technically possible combination of some or all of these features: • The generation step may further include the generation of an ignition spark originating from the central electrode and flamering along the distal end face of the insulator over a radial distance (GFS1) to the inner protrusion of the metal shell; and • The generation step may further include the generation of an ignition spark originating from the central electrode, igniting along the distal end face of the insulator over a radial distance (GFS2) and jumping over an air spark gap with an axial distance (LFS2) to the inner protrusion of the metal shell. DRAWINGS

[0009] The following describes one or more embodiments in conjunction with the accompanying drawings, in which the same designations mean the same elements, and wherein the drawings show the following: Fig. 1. A view of a pre-chamber spark plug; Fig. 2 an enlarged cross-sectional view of the pre-chamber spark plug Fig. 1, wherein the spark plug has a surface discharge spark gap according to an example; and Fig. 3 an enlarged cross-sectional view of the pre-chamber spark plug Fig. 1, wherein the spark plug has a surface discharge spark gap according to an embodiment of the invention. DESCRIPTION

[0010] The present application relates to a pre-chamber spark plug for an internal combustion engine, designed to improve the flow of unburned or fresh gas into the pre-chamber and the flow of burnt or residual gas out of the pre-chamber. The flow of such gases, in particular the scavenging of burnt gas and associated residues, can have a significant impact on the spark plug's performance. If, for example, the burnt gas is not completely scavenged or removed from the pre-chamber, the spark plug may be subjected to irregular combustion processes within the pre-chamber, which can lead not only to an increase in the temperature of various spark plug components but also to irregular combustion processes in the main combustion chamber.Therefore, the pre-chamber spark plug of the present invention uses a pre-chamber design with minimal electrode obstruction, so that optimal gas exchange between the main combustion chamber and the pre-chamber can be achieved for various engine loads and operating conditions, while maintaining a thermally robust pre-chamber spark plug.

[0011] Fig. Figure 1 shows an example of a pre-chamber spark plug 10, 110, which generally has a surface discharge spark gap located in a pre-chamber cap 20, 120. The pre-chamber spark plug 10, 110 is not limited to a specific insulator base arrangement and can, for example, have the surface discharge spark gap configurations in Fig. 2 or Fig. Include 3. Other types of surface discharge spark gaps are also usable.

[0012] According to an example, Fig. 2 a section of a pre-chamber spark plug 10, which includes a center electrode 12, an insulator 14, a metal casing 16, a sealing ring 18, a pre-chamber forming cap 20, a pre-chamber 22, a surface discharge spark gap G1, and a central axis A. Other components not shown here may include a connecting bolt, central wiring components such as resistors and suppressors, and various seals, all of which are known to those skilled in the art and may be part of the spark plug 10. The pre-chamber spark plug 10 is preferably a "passive pre-chamber spark plug," meaning that the pre-chamber receives the gases necessary for combustion (e.g., a fuel / air mixture) only from the main combustion chamber, in contrast to an "active pre-chamber spark plug," which includes one or more additional air or fuel injector(s) directly in the pre-chamber.

[0013] Although the pre-chamber spark plug 10 is suitable for various motor vehicle internal combustion engines such as lean-running gasoline engines with direct injection, the spark plug of the present invention can also be used with any number of other engines.

[0014] The center electrode 12 is at least partially located in an axial bore of the insulator 14 and has an end section 30 with a distal end face 32, which may or may not be exposed beyond the insulator. The center electrode 12 is designed to conduct or otherwise convey the high-voltage ignition pulse from a terminal end of the spark plug to the surface discharge spark gap G1. In one example, the center electrode 12 simply comprises a nickel-based material (e.g., pure nickel or a nickel alloy such as Inconnel 600, 601) as the entire center electrode. In another example, not shown here, the center electrode 12 comprises the aforementioned nickel-based material as the outer or cladding section of the electrode, as well as a copper-based material (e.g., pure copper or a copper alloy) serving as the inner, thermally conductive core.According to a multi-part example not shown here, the center electrode 12 comprises a base electrode component made of the aforementioned nickel-based material (with or without the copper core) and one or more additional insulator base components welded or otherwise attached to end section 30. Insulator base components may be made of precious metals such as platinum, iridium, ruthenium, or palladium-based materials, which can improve the corrosion or erosion resistance of the electrode, or they may be made of intermediate materials that improve the adhesion of the precious metals to the base electrode components. In the multi-part example, the distal end face 32 is part of the insulator base component welded to the base electrode component.

[0015] The insulator 14 is at least partially arranged in an axial bore of the metal shell 16 and has an end tip section 40 with a distal end surface 42, an outer shoulder 44, and an axial bore 46. The insulator 14 is made of a material such as a ceramic material like aluminum oxide, which insulates the central electrode 12 from the metal shell 16. It is obvious to those skilled in the art that a number of different types of insulators with various configurations and compositions can be used, including the embodiments shown in the drawings.

[0016] According to the example from Fig. 2 The end tip section 40 extends to an axial position that ends slightly before the center electrode end section 40, such that an axial distance (DA1) separates the corresponding distal end surfaces 32, 42. In this particular example, the distal end surface 42 is a flat, annular surface which, as shown in the cross-section (as Fig. 2) visible, extending in a radial direction between the central electrode 12 and the metal shell 16 (i.e., at least a section of the distal end surface 42 is perpendicular to the central axis A). In other examples, however, the distal end surface 42 may be inclined, convex, concave, or have another configuration. The outer shoulder 44 is inclined and extends outward from end tip section 40 at such an angle that the insulator 14 can be held on a corresponding shoulder of the metal shell 16 by the sealing ring or the flat gasket 18. The axial bore 46 preferably extends over the entire length of the insulator 14 and accommodates one or more components of the central electrode.

[0017] The metal casing 16 has an end section 50 with a distal end surface 52, an inner protrusion 54 with a radial surface 56 and an inner shoulder 58, external threads 60, and pre-chamber walls 62. The metal casing 16 is preferably made of a suitable type of steel and is designed to accommodate many of the other spark plug components and can be screwed into a cylinder head of the internal combustion engine by means of the external thread 60. According to the Fig. In the example shown, the end section 50 of the metal casing extends beyond the end sections 30, 40 of the center electrode and the insulator, respectively, so that the corresponding distal end surfaces 52, 42 are separated by an axial distance (DB1). However, the exact extent of the end section 50 can vary depending on the embodiment. The inner bulge 54 plays a crucial role in the pre-chamber spark plug 10, as it contributes to spark generation and also helps to hold the insulator 14. The diameter of the external thread 60 depends on the specific engine in which the spark plug is to be used, and according to some possible embodiments, the external thread ranges from approximately M10 to M18 (the external thread 60 can, for example, correspond to an M12 threaded bolt).

[0018] The inner protrusion 54 aids in the formation of the surface discharge spark gap G1 and also provides an inner shoulder 58 for holding the insulator 14; therefore, the inner protrusion 54 is a seal that supports the ignition spark projection. According to the Fig. In the non-restrictive embodiment shown in Figure 2, the inner protrusion 54 is an integral part of the metal shell 16 and projects, as shown in the cross-section (as shown in Figure 2). Fig. 2) visible, extending radially inward from an inner surface of the metal shell. In other embodiments, the inner protrusion 54 may not be an integral part of the metal shell 16, but may include separate, independent components welded to an inner surface of the shell (the inner protrusion 54 could, for example, include precious metal or non-precious metal parts welded to an inner surface of the shell). Viewed from the direction of the central axis A (this view is not shown), the inner protrusion 54 may have a continuous, uninterrupted ring shape (i.e., it completely surrounds the end-tip section 40 of the insulator 14) or it may have a discontinuous ring shape, forming a number of individual or discrete inner protrusions (i.e., it does not completely surround the end-tip section 40 of the insulator 14).On one side facing the pre-chamber 22, the inner protrusion 54 can enclose a radial surface 56 which is flat and extends as shown in cross-section (as shown in . Fig. 2) visible, generally extending in a radial direction (i.e., at least one section of the radial surface 56 is perpendicular to the central axis A). It is also possible for the radial surface 56 to be slightly inclined, curved, concave, or convex in the radial direction, rather than being perfectly flat, and it may be capped, pointed, or otherwise provided with precious metal parts to enhance its corrosion and / or electrical erosion resistance. According to the present example, the radial surface 56 of the inner protrusion 54 projects slightly beyond the end section 40 of the insulator, such that an axial distance (DC1) separates the corresponding end surfaces 56, 42. In non-limiting examples, the axial distances DA1 and DC1 may be between 0 mm and 5 mm, or preferably between 1 mm and 3 mm, and DA1 may be greater than DC1.Although the axial positions of surfaces 32, 42, and 56 preferably correspond to those described above, it is possible that the distal end surface 42 of an insulator instead projects axially beyond surfaces 32 and 56, respectively. As shown in . Fig. As shown in Figure 2, the inner shoulder 58 of the inner protrusion 54 is inclined so that it supports the inner shoulder 44 of the insulator 14 by means of a sealing ring or a flat gasket 18, such that the insulator is held by a gas-tight seal formed for the pre-chamber 22. The size, inclination, or other properties of the inner shoulder 56 may vary depending on the embodiment.

[0019] The prechamber cap 20 is attached to the end section 50 of the metal shell such that it forms an internal space or volume (i.e., the prechamber 22) and has a mounting surface 70, one or more openings 72, and the prechamber walls 74. According to one possible embodiment, the mounting surface 70 of the prechamber cap is welded to the distal end surface 52 of the metal shell such that the prechamber walls 62 and 74 of the metal shell and the prechamber cap, respectively, meet and help define or limit the volume of the prechamber 22. The prechamber cap 20 may include one or more openings 72 that allow gas exchange between the main combustion chamber and the prechamber 22 (i.e., either the flow of unburned gas from the main combustion chamber into the prechamber or the flow or expulsion of burnt gas from the prechamber to the main combustion chamber).According to a non-limiting example, the pre-chamber 20 includes four to seven openings 72, each with a diameter of approximately 0.6 mm to 1.4 mm. It is obvious to those skilled in the art that any number of openings of suitable size, position, and orientation are possible, and that the present application is not limited to this embodiment.

[0020] The prechamber 22 is a volume or space surrounding the surface discharge spark gap G1 such that a gas (e.g., a fuel / air mixture) can first be ignited in the prechamber 22 before propagating through the opening(s) 72 into the main combustion chamber. It is obvious to those skilled in the art that the prechamber 22 can be designed with any number of different configurations, components, volumes, features, etc., to promote or control turbulence, flame front propagation, and other prechamber operating parameters; the prechamber of the present application is not limited to a specific embodiment. According to the Fig. In the non-restrictive example shown in Figure 2, the prechamber 22 is a space or volume defined by the prechamber wall 74 at a front end 80 of the prechamber, by prechamber walls 62 and 74 on the sides of the prechamber, and by the combination of the distal end faces 32, 42 and the radial face 56 at a rear end 82 of the prechamber (i.e., the end of the prechamber opposite the front end 80). Unlike some conventional prechamber spark plugs, in which one or more ground electrode(s) protrude into the center of the prechamber space, the prechamber 22 is designed to minimize electrode obstruction in order to promote unimpeded gas exchange between the main combustion chamber and the prechamber and to improve the thermal properties of the spark plug.

[0021] The surface discharge spark gap G1 extends between the central electrode 12 and the metal casing 16 and serves to ignite the air / fuel mixture in the pre-chamber 22. According to the illustrated example, the surface discharge spark gap G1 includes a radial spark formation section 90 (also referred to as a radial spark formation section), meaning that a large part of the spark formation in this section, viewed with respect to the central axis A, takes place in a generally radial direction. The radial spark formation section 90 is preferably a surface spark gap, in which a large part of the spark formation occurs along the surface, as opposed to an air gap. This is not to say that spark formation in a non-radial direction or over a small air gap (i.e.,, the air gap between the distal end face 42 and the radial face 56) can occur – except that the majority of the sparking is expected to occur radially along one or more faces. It is obvious to those skilled in the art that surface spark gaps sometimes require lower voltages than corresponding air spark gaps and can sustain longer sparking, although this may vary depending on the motor, operating conditions, etc. In the non-restrictive example of . Fig. 2. An ignition spark originates from the end section 30 of the center electrode 12, propagates across the end tip section 40 of the insulator 14 (e.g., along the distal end surface 42), and terminates at the inner protrusion 54. The radial spark formation section 90 has a radial distance GFS1 (i.e., the distance from an edge of the distal end surface 32 to an opposite edge of the radial surface 56), which can be between 0.5 mm and 3.0 mm and is preferably between 1.0 mm and 2.0 mm. The surface discharge spark gap G1 is located at the rear end 82 of the prechamber 22 so that the spark formation components do not obstruct the resulting flame front, which is advantageous for flame propagation and the subsequent pressure build-up in the prechamber 22.Regarding the materials of the spark gap components, the surface discharge spark gap G1 can begin or originate at a central electrode 12 made of a nickel material or an insulator base component made of a precious metal, flame across an insulator 14 made of a ceramic material, and terminate at an inner protrusion 54 that is part of a metal shell 16 made of a type of steel. Therefore, a nickel / ceramic / steel surface discharge spark gap G1 is possible.

[0022] If one now Fig. Figure 3 shows an embodiment of a pre-chamber spark plug 110 according to the invention. It is self-evident that much of the preceding description of a pre-chamber spark plug 10 also applies to this embodiment, unless otherwise stated. Therefore, corresponding reference numbers have been used to describe mutually corresponding components. The pre-chamber spark plug 110 includes a center electrode 112, an insulator 114, a metal casing 116, a sealing ring 118, a pre-chamber cap 120, a pre-chamber 122, a surface discharge spark gap G2, and a central axis A. Like the preceding embodiment, the pre-chamber spark plug 110 is also a passive pre-chamber spark plug suitable for various motor vehicle internal combustion engines, such as lean-running gasoline engines with direct injection.

[0023] The central electrode 112 includes an end section 130 with a distal end surface 132. The insulator 114 includes a tip end section 140, a distal end surface 142, an outer shoulder 144, an axial bore 146, and an axial distance DA2. The axial distance DA2, which lies between the surfaces 132 and 142, can be within the same dimensional ranges as those specified for the axial dimension DA1. The metal sheath 116 has an end section 150 with a distal end surface 152, an inner protrusion 154 with a radial surface 156 and an inner shoulder 158, external threads 160, pre-chamber walls 162, and axial distances DB2 and DC2. The axial distance DB2, which lies between surfaces 152 and 142, can lie within the same dimensional ranges specified for the axial dimension DB1. As in Fig. However, as shown in Figure 3, the axial length or extent of the inner bulge 154 is greater than that of the corresponding component 54 in the embodiment of Fig. 2; however, the distance DC2 can still lie within the same dimensional ranges specified for the axial distance DC1. Therefore, the axial distance DC2, which is the distance between the radial surface 156 and the distal end surface 142, is greater than its counterpart made of Fig. 2, the axial distance DC1. This modification affects the surface discharge spark gap G2, as explained below. The pre-chamber cap 120 encloses the mounting surfaces 170, one or more openings 172, and the pre-chamber walls 174. Finally, the pre-chamber 122 has a front end 180 and a rear end 182.

[0024] The surface discharge spark gap G2 extends between the center electrode 112 and the metal casing 116 and includes both a radial spark formation section 190 and an axial spark formation section 192. A difference between this embodiment and the previous example is that significant sparking occurs in both the radial and axial directions, relative to the central axis A of the spark plug. The surface discharge spark gap G2 includes both a radial spark formation section 190, which serves as a surface spark gap where sparking occurs along a surface, and an air spark gap where sparking occurs across an air gap. In the non-restrictive example of Fig.3. A spark originates from the end section 130 of the center electrode 112, jumps to the end-tip section 140 of the insulator 114 (i.e., along the distal end face 142), jumps or strikes an arc across the axial spark-generating section 192, and terminates at the inner protrusion 154. The radial distance GFS2 of the surface discharge spark gap G2 can be within the same dimensions as those specified for surface discharge spark gap GFS1, and the dimensions of the axial distance LFS2 can be approximately two-thirds of GFS2 (i.e., the radial distance GFS2 can be larger than the axial distance LFS2). For example, the surface discharge spark gap G2 can have a radial distance of 0.5 mm to 3 mm inclusive, and even 1 mm to 2 mm inclusive, and even 0.6 mm to 1.4 mm inclusive. Other dimensions are certainly possible.The surface discharge spark gap G2 is located at the rear end 182 of the pre-chamber 122, and the surface discharge spark gap G2 can begin at a central electrode 112, which consists of a nickel-based material or an insulator base component made of a precious metal, jump over an insulator 114 made of a ceramic material, form an arc across an air gap, and terminate at an inner protrusion 154, which is part of a metal shell 116 made of a type of steel. Therefore, a nickel / ceramic / steel surface discharge spark gap G2 is possible.

[0025] It is understood that the foregoing is a description of one or more preferred embodiments of the invention. The invention is not limited to the specific embodiment(s) disclosed herein, but is defined solely by the following claims. Furthermore, the statements contained in the preceding description relate to specific embodiments and are not to be understood as limitations on the scope of the invention or on the definition of the terms used in the claims, unless a term or phrase is expressly defined in the foregoing.

[0026] As used in this patent specification and these claims, the terms "for example", "e.g.", "such as", "as for example", and "like", and the verbs "comprise", "have", "include", and their other verb forms, when used in connection with the listing of one or more components or other items, are each to be understood as unlimited; that is, the listing is not to be considered as excluding other, additional components or items. Other terms are to be interpreted using their broadest meaningful sense, unless they are used in a context that requires a different interpretation.

Claims

[1] Pre-chamber spark plug (110), comprising: a metal shell (116) having an axial bore, an end portion (150) having a distal end surface (152), and an inner protrusion (154); an insulator (114) disposed at least partially within the axial bore of the metal shell (116) and having an axial bore (146) and an end tip portion (140) having a distal end surface (142); a center electrode (112) disposed at least partially within the axial bore (146) of the insulator (114) and having an end portion (130) with a distal end surface (132); and a pre-chamber cap (120) attached to the metal shell (116) and having a pre-chamber wall (174) with one or more openings (172) that allow gas flow between a pre-chamber (122) and a main combustion chamber when the pre-chamber spark plug (110) is installed in an engine; wherein the pre-chamber spark plug (110) has a surface discharge spark gap (G2) extending between the distal end surface (132) of the center electrode (112) and the inner protrusion (154) of the metal shell (116) along the distal end surface (142) of the insulator (114), and wherein the surface discharge spark gap (G2) has both a radial sparking section (190) in which sparking takes place in a generally radial direction with respect to a central axis (A) of the spark plug, and an axial sparking section (192) in which sparking takes place in a generally axial direction with respect to a central axis of the spark plug. [2] Pre-chamber spark plug according to claim 1, wherein the inner protrusion (154) of the metal shell (116) has a radial surface (156) and an inner shoulder (158) on opposite axial sides of the inner protrusion (154), the radial surface (156) facing the pre-chamber (122), and the inner shoulder (158) holding the insulator (114). [3] Pre-chamber spark plug according to claim 2, wherein the distal end surface (132) of the center electrode (112) is in an axial position which, viewed in a forward direction, is beyond or behind the distal end surface (142) of the insulator (114) by an axial distance (DA1, DA2) of 0 mm to 5 mm, inclusive. [4] A pre-chamber spark plug according to claim 2 or 3, wherein the radial surface (156) of the inner protrusion (154) is in an axial position which, viewed in a forward direction, extends beyond the distal end surface (142) of the insulator (114) by an axial distance (DC1, DC2) of 0 mm to 5 mm, inclusive. [5] A pre-chamber spark plug according to any one of claims 2 to 4, wherein the radial surface (156) of the inner protrusion (154) is in an axial position which, viewed in the forward direction, extends beyond the distal end surface (142) of the insulator (114), wherein the distal end surface (132) of the center electrode (112) is in an axial position which, viewed in the forward direction, extends beyond the radial surface (156) of the inner protrusion (154), and wherein the distal end surface (152) of the metal shell (116) is in an axial position which, viewed in the forward direction, extends beyond the distal end surface (132) of the center electrode (112). [6] A pre-chamber spark plug according to any one of claims 2 to 5, wherein the distal end surface (132) of the center electrode (112) is in an axial position which, viewed in the forward direction, extends beyond the distal end surface (142) of the insulator (114), the radial surface (156) of the inner protrusion (154) is in an axial position which, viewed in the forward direction, extends beyond the distal end surface (132) of the center electrode (112), and the distal end surface (152) of the metal shell (116) is in an axial position which, viewed in the forward direction, extends beyond the radial surface (156) of the inner protrusion (154). [7] A pre-chamber spark plug according to any one of claims 1 to 6, wherein the surface discharge spark gap (G2) has the radial sparking portion (190) in which a major part of the sparking takes place in the generally radial direction with respect to the central axis (A) of the spark plug. [8] A pre-chamber spark plug according to claim 7, wherein the radial sparking portion extends between the center electrode (112) and the inner protrusion (154) along the distal end surface (142) of the insulator (114) over a radial distance (GFS1) of 0.5 mm to 3.0 mm, inclusive. [9] A pre-chamber spark plug according to any one of claims 7 to 8, wherein the surface discharge spark gap (G2) comprises a surface spark gap (190) in which a majority of the sparking takes place along the distal end surface (142) of the insulator (114), as opposed to an air spark gap. [10] A pre-chamber spark plug according to any one of claims 1 to 9, wherein the radial sparking section (190) extends between the center electrode (112) and the inner protrusion (154) along the distal end surface (142) of the insulator (114) over a radial distance (GFS2) of 0.5 to 3.0 mm, inclusive, and wherein the axial sparking section (192) extends between the insulator (114) and the inner protrusion (154) over an air gap with an axial distance (LFS2) of 0.3 to 2.0 mm, inclusive. [11] Prechamber spark plug according to any one of claims 1 to 10, wherein the prechamber (122) is defined by the prechamber wall (174) of the prechamber cap (120) at a front end (180) of the prechamber, by the prechamber wall (174) of the prechamber cap (120) and a prechamber wall of the metal shell (116) on the prechamber (122) side, and by a combination of the distal end surface (132) of the center electrode (112), the distal end surface (142) of the insulator (114) and a radial surface (156) of the inner protrusion (154) at a rear end (182) of the prechamber (122), and wherein the surface discharge spark gap (G2) is located at the rear end (182) of the prechamber (122) and is configured such that sparking components have minimal electrode interference and promote unhindered gas exchange between the pre-chamber (122) and the main combustion chamber. [12] The pre-chamber spark plug of any one of claims 1 to 11, wherein the pre-chamber spark plug (110) is a passive pre-chamber spark plug and is configured to receive air and fuel only from a main combustion chamber through the one or more openings (174) in the pre-chamber cap (120) - and not from additional injectors that direct air or fuel directly into the pre-chamber. [13] Method for operating a pre-chamber spark plug, wherein the pre-chamber spark plug comprises: a metal shell (116) having an axial bore and an inner protrusion; an insulator (114) disposed at least partially within the axial bore of the metal shell (116) and having an axial bore and a distal end surface; a center electrode (112) disposed at least partially within the axial bore of the insulator and having a distal end surface; and a pre-chamber cap;wherein the pre-chamber spark plug has a surface discharge spark gap (G2) extending between the distal end surface of the center electrode (112) and the inner protrusion of the metal shell (116) along the distal end surface of the insulator, the surface discharge spark gap (G2) having both a radial sparking section (190) in which sparking takes place in a generally radial direction with respect to a center axis (A) of the spark plug, and an axial sparking section (192) in which sparking takes place in a generally axial direction with respect to a center axis of the spark plug; the method comprising the steps of: applying a voltage to the center electrode (112) and generating an ignition spark which extends across the surface discharge spark gap from the center electrode (112) to the inner protrusion of the metal shell (116).

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