spark plug

Abstract

Spark plug (10; 70; 90; 110), comprising: an insulator (11) having an axial hole (12) formed therein in the direction of an axis (O) of the spark plug (10; 70; 90; 110;); a center electrode (20) arranged in a front end side of the axial hole (12); a cylindrical metal housing (30) which holds the insulator (11); and a ground electrode (40; 71; 91) comprising a rod-shaped ground electrode base (41; 72; 92) formed of a Ni-based material and coupled to the metal case (30) at a base end portion (43) thereof, and a ground electrode tip (53) made of a noble metal-based material and attached to a distal end portion (42) of the ground electrode base (41; 72; 92), wherein the distal end portion (42) of the ground electrode base (41; 72; 92) includes: a first surface (44; 74; 93) opposite a front end surface (26) of the center electrode (20); and second and third surfaces (47, 50; 75, 77; 95, 96) connected to the first surface (44; 74; 93) and extending from the distal end portion (42) to the base end portion (43) of the ground electrode base (41; 72; 92), wherein the ground electrode tip (53) comprises: a first tip element (54; 79; 99) attached to the first surface (44; 74; 93) and having a first discharge surface (55; 80; 100) to allow spark discharge between the center electrode (20) and the first discharge surface (55; 80; 100); and a second tip element (56; 81; 101) attached to at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) and having a second discharge surface (57; 82; 102) to allow spark discharge between the center electrode (20) and the second discharge surface (57; 82; 102), wherein the first tip element (54; 79; 99) and the second tip element (56; 81; 101) are spaced apart from each other, and wherein a volume of the second tip element (56; 81; 101) is greater than a volume of the first tip element (54; 79; 99).

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a spark plug, particularly of the type having a ground electrode with a noble metal-containing tip attached to an electrode base thereof. BACKGROUND OF THE INVENTION

[0002] A spark plug is known that includes: a metal shell attached to an engine; a ground electrode having an electrode base coupled to the metal shell and a tip attached to the electrode base and containing a precious metal as a main component; and a center electrode insulated and held in the metal shell. This spark plug generates a spark discharge on a discharge gap between the center electrode and the ground electrode upon breaking the insulation between the center electrode and the ground electrode. Due to the flow of air-fuel mixture in a combustion chamber of the engine, the discharge gap is expanded toward a downstream side of the air-fuel mixture flow. Consequently, a spark wear area of ​​the ground electrode increases in area with the expansion of the discharge gap.JP 2004 - 152 682 A discloses a technique for covering the entire electrode base of the ground electrode, except for a portion thereof to which the tip is attached, with a protective film made of a noble metal-based material for the purpose of improving spark wear resistance.

[0003] JP 2017 - 147 086 A discloses an internal combustion and EP 2 922 157 A1 discloses a spark plug arranged to form a gap 33 in cooperation with a front end portion of the center electrode. SUMMARY OF THE INVENTION

[0004] With respect to the above-disclosed technique, there is a demand for further improvement of the spark wear resistance of the ground electrode.

[0005] The present invention was made in consideration of such a demand. It is an object of the present invention to provide a spark plug having a ground electrode with further improved spark wear resistance.

[0006] According to a first aspect of the present invention, there is provided a spark plug comprising: an insulator having an axial hole formed therein in the direction of an axis of the spark plug; a center electrode disposed in a front end side of the axial hole; a cylindrical metal shell holding the insulator therein; and a ground electrode having a rod-shaped ground electrode base made of a Ni-based material and coupled to the metal shell at a base end portion thereof, and a ground electrode tip made of a noble metal-based material and attached to a distal end portion of the ground electrode base, wherein the distal end portion of the ground electrode base has: a first surface opposite to a front end surface of the center electrode; and second and third surfaces connected to the first surface and extending from the distal end portion to the base end portion of the ground electrode base, wherein the ground electrode tip comprises: a first tip member attached to the first surface and having a first discharge surface to allow spark discharge between the center electrode and the first discharge surface; and a second tip member attached to at least one of the second and third surfaces and having a second discharge surface to allow spark discharge between the center electrode and the second discharge surface, and wherein the first and second tip members are spaced apart from each other.

[0007] As mentioned above, the rod-shaped ground electrode base made of Ni-based material has one (distal) end portion to which the ground electrode tip made of noble metal-based material is coupled, and the other (base) end portion is coupled to the metal shell. The ground electrode tip includes two tip members: the first tip member attached to the first surface of the ground electrode base to allow spark discharge between the center electrode and the first discharge surface of the first tip member; and the second tip member attached to at least one of the second and third surfaces of the ground electrode base to allow spark discharge between the center electrode and the second discharge surface of the second tip member.Therefore, the spark wear of at least one of the second and third surfaces of the ground electrode base is suppressed compared to the case where no second tip member is provided.

[0008] Furthermore, the first tip member and the second tip member are spaced apart from each other such that, even though the first and second tip members are heated by the spark discharge, the heat can be prevented from being transferred directly from the first tip member to the second tip member or from the second tip member to the first tip member through contact of these first and second tip members. The higher the temperature of the tip member, the more susceptible the tip member is to spark wear. A temperature rise of the tip member is prevented by preventing direct heat transfer between the first and second tip members, thereby enhancing heat transfer from the first and second tip members to the ground electrode base. Spark wear of the first and second tip members is thus inhibited.

[0009] It is therefore possible to improve the spark wear resistance of the ground electrode.

[0010] According to the first aspect of the present invention, a volume of the second tip element is larger than a volume of the first tip element.

[0011] In this case, it is possible to secure the service life of the second tip element against spark wear due to the discharge gap that expands due to the air-fuel mixture flow.

[0012] According to a second aspect of the present invention, there is provided a spark plug as described above, wherein the first tip member covers at least a portion of a side of the first surface to which the at least one of the second and third surfaces is connected.

[0013] In this case, the first tip element is configured to suppress the occurrence of spark wear over a wide area near the first surface side. Therefore, it is possible to effectively improve the spark wear resistance of the first surface of the ground electrode base.

[0014] According to a third aspect of the present invention, there is provided a spark plug as described above, wherein the second tip member covers at least a portion of a side of the at least one of the second and third surfaces to which the first surface is connected.

[0015] In this case, the second tip member is configured to suppress the occurrence of spark wear over a wide area near the side of at least one of the second and third surfaces. Therefore, it is possible to effectively improve the spark wear resistance of at least one of the second and third surfaces of the ground electrode base.

[0016] According to a fourth aspect of the present invention, there is provided a spark plug as described above, wherein a height of the first tip member from the first surface in a direction perpendicular to the first surface, and a height of the second tip member from the at least one of the second and third surfaces in a direction perpendicular to the at least one of the second and third surfaces is 0.1 mm to 1 mm.

[0017] In this case, the first and second tip elements, each subject to spark wear, are provided with sufficient strength. Therefore, it is possible to ensure the service life of the ground electrode.

[0018] According to a fifth aspect of the present invention, there is provided a spark plug as described above, wherein a first imaginary plane including the first discharge surface and a second imaginary plane including the second discharge surface form an obtuse angle.

[0019] In this case, the discharge distance between the center electrode and the second discharge surface is more likely to expand to the center of the combustion chamber due to the flow of the air-fuel mixture in the combustion chamber compared to the case where the first and second imaginary planes intersect at a right angle. Therefore, it is possible to effectively improve the ignition performance of the spark plug.

[0020] According to a sixth aspect of the present invention, there is provided a spark plug as described above, wherein the obtuse angle is 120° to 170°.

[0021] In this case, the discharge path expands more easily toward the center of the combustion chamber. Therefore, it is possible to more effectively improve the ignition performance of the spark plug.

[0022] According to a seventh aspect of the present invention, there is provided a spark plug as described above, wherein the volume of the second tip member is 1.3 times or more larger than the volume of the first tip member.

[0023] In this case, it is possible to more reliably ensure the spark wear resistance of the second tip element.

[0024] According to an eighth aspect of the present invention, there is provided a spark plug as described above, wherein, when the ground electrode is viewed from a direction perpendicular to the second discharge surface, the entire front end surface of the center electrode lies within an existence range of the second tip member in a direction perpendicular to a minimum line segment connecting the front end surface of the center electrode to the first discharge surface of the first tip member.

[0025] In this case, spark discharge is more likely to generate between the center electrode and the second tip member. Therefore, it is possible to effectively suppress spark wear of at least one of the second and third surfaces of the ground electrode base to which the second electrode tip is attached.

[0026] According to a ninth aspect of the present invention, there is provided a spark plug as described above, wherein, when the ground electrode is viewed from a direction perpendicular to the second discharge surface, the entire first discharge surface of the first tip member lies within an existence range of the second tip member in a direction perpendicular to a minimum line segment connecting the front end surface of the center electrode to the first discharge surface of the first tip member.

[0027] In this case, a ground electrode-side end of the discharge gap is easily shifted from the first tip element to the second tip element by radiating the spark discharge. Therefore, it is possible to effectively prevent spark wear of the ground electrode base. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention is described below using embodiments without being limited thereto. Fig. 1 is a side view, semi-cross-sectionally, of a spark plug according to a first embodiment of the present invention. Fig. 2 is a perspective view of a ground electrode of the spark plug. Fig. 3 is an enlarged side view, half in cross section, of a front end part of the spark plug in the direction of an arrow III of the Fig. 1 is considered. Fig. 4 is an enlarged side view, half in cross section, of the front end part of the spark plug in a direction perpendicular to the paper surface of the Fig. 1 is considered. Fig. 5 is an enlarged side view, half in cross section, of a front end part of a spark plug according to a second embodiment of the present invention. Fig. 6 is an enlarged side view, half in cross section, of a front end part of a spark plug according to a third embodiment of the present invention. Fig. 7 is an enlarged side view, half in cross section, of a front end part of a spark plug according to a fourth embodiment of the present invention. The Fig. 8A and Fig. 8B are schematic views showing experimental tests. DESCRIPTION OF EMBODIMENTS

[0029] Exemplary embodiments of the present invention will be described below with reference to the drawings. FIRST EMBODIMENT

[0030] Fig. 1 is a side view, semi-cross-sectionally, of a spark plug 10 for an engine according to a first embodiment of the present invention. Fig. 1, one side of the spark plug 10 (excluding a ground electrode 40) is shown in cross section with respect to an axis O of the spark plug 10. Further, reference is made to the lower and upper sides of the Fig. 1 are referred to as a front and back of the spark plug 10. (The same applies to the Fig. 2 to 7.)

[0031] As in Fig. 1, the spark plug 10 has an insulator 11, a center electrode 20, a metal shell 30 and a ground electrode 40.

[0032] The insulator 11 is substantially cylindrical, with an axial hole 12 formed therethrough along the axis O, and is made of a ceramic material (such as alumina) with good mechanical properties and high-temperature insulation properties. The insulator 11 has an annular rearward-facing surface 13 formed on a front end portion of an inner circumferential surface of the axial hole 12 and decreasing in diameter toward the front.

[0033] The center electrode 20 is rod-shaped and is arranged in the axial hole 12, wherein a head portion 21 of the center electrode 20 is retained on the rearward facing surface 13 of the insulator 11, and a part of the center electrode 20 other than the head portion 21 is arranged in a front end side of the axial hole 12 which is forward of the rearward facing surface.

[0034] In the first embodiment, the center electrode 20 includes a cylindrical bottomed electrode base 22 (as a center electrode base); a core 23 embedded in the electrode base 22; and a cylindrical columnar tip 25 (as a center electrode tip) attached to a front end of the electrode base 22. The electrode base 22 is made of a Ni-based material. The core 23 is made of a Cu-based material. The tip 25 is made of a noble metal-based material. The term "X-based material" as used herein means a material containing X as a main component in an amount of 50 wt% or more. Specifically, the electrode base 22 has a chemical composition containing 50 wt% or more of Ni. The core 33 has a chemical composition containing 50 wt% or more of Cu.The tip 25 has a chemical composition containing one or two or more types of precious metals, such as Rt, Rh, Ir, and Ru, in an amount of 50 wt% or more. The core 23 may optionally be omitted.

[0035] The center electrode 20 (more precisely the head portion 21) is electrically connected to a metal terminal 27 via a conductive member within the axial hole 12.

[0036] The metal clamp 27 is made of a conductive metal material (such as low-carbon steel) in a rod shape for connecting to a high-voltage cable (not shown). The metal clamp 27 is fixed to a rear end of the insulator 11, with a front end portion of the metal clamp 27 inserted into a rear end side of the axial hole 12.

[0037] The metal casing 30 is substantially cylindrical in shape and made of a conductive metal material (such as low-carbon steel). The metal casing 30 has a body portion 31 located on its front end. An external thread 32 is formed on an outer circumferential surface of the body portion 31 so as to be screwed into a screw hole of the motor. The metal casing 30 also includes a seat portion 33 located rearward of the body portion 31 and a tool engagement portion 34 located behind the seat portion 33. The seat portion 33 is made larger in outer diameter than the body portion 31 and is adapted to seal a gap between the screw hole of the motor and the external thread 32 of the body portion 31. The tool engagement portion 31 can be engaged with a tool, such as a wrench, to screw the external thread 31 into the screw hole of the motor.

[0038] The ground electrode 40 includes a rod-shaped electrode base 41 (as a ground electrode base) made of a Ni-based material and a cylindrical columnar tip 53 (as a ground electrode tip) made of a noble metal-based material and attached to the electrode base 41. The electrode base 41 has two opposite end portions: a distal end portion 42 to which the tip 53 is attached, and a base end portion 43 coupled to a front end portion of the metal shell 30. As in the case of the electrode base 22, the electrode base 41 has a chemical composition containing 50 wt% or more of Ni. Although not specifically shown, a core of a Cu-based material may be embedded in the electrode base 41.As in the case of the tip 25, the tip 53 has a chemical composition containing one kind or two or more kinds of noble metals such as Rt, Rh, Ir and Ru in an amount of 50 wt% or more.

[0039] The ground electrode 40 and the center electrode 20 are opposite each other via a so-called electrode gap. Fig. 2 is a perspective view of the distal end portion 42 of the ground electrode 40 to which the tip 53 is attached. As in Fig. 2, the distal end portion 42 of the electrode base 41 is rectangular in cross section and has a first surface 44 defined between first and second sides 45 and 46 and opposite a front end surface 26 of the center electrode 20 (see Fig. 1, Fig. 3); a second surface 47 connected to the second side 46 of the first surface 44 and defined between first and second sides 48 and 49; a third surface 50 connected to the first side 45 of the first surface 44; and a fourth surface 51 connecting the second surface 47 and the third surface 50. The first to fourth surfaces 44, 47, 50, and 51 are each connected to a distal end surface 41a of the electrode base 41 and extend from the distal end portion 42 (from the distal end surface 41a) to the base end portion 43 (see Fig. 1).

[0040] In the first embodiment, the tip 53 includes a first tip element 54 joined at a bottom surface thereof to the first surface 44, and a second tip element 56 joined at a bottom surface thereof to the second surface 47. Each of the first and second tip elements 54 and 56 is rectangular parallelepiped (plate)-shaped. The first tip element 54 has a first discharge surface 55 opposite the bottom surface of the first tip element 54 and oriented in the same direction as the first surface 44 (i.e., in a direction perpendicular to the axis O), while the second tip element 56 has a second discharge surface 57 opposite the bottom surface of the second tip element 56 and oriented in the same direction as the second surface 47 (i.e., in a direction parallel to the axis O).

[0041] A height T1 of the first tip element 54 from the first surface 44 in a direction perpendicular to the first surface 44 is 0.1 mm to 1 mm. A height T2 of the second tip element 56 from the second surface 47 in a direction perpendicular to the second surface 47 is also 0.1 mm to 1 mm. With this configuration, the first and second tip elements 56 are provided with sufficient strength so that the ground electrode 40 can ensure durability against spark wear.

[0042] As in Fig. 2, the second surface 47 is connected to the first surface 44 via a chamfered portion 52. The chamfered portion 52 has the second side 46 in common with the first surface 44 and has the first side 48 in common with the second surface 47. In the first embodiment, the chamfered portion 52 is formed as a round curved surface. The first tip member 54 is configured to cover a portion of the second side 46 of the first surface 44 (in the first embodiment, a portion of the second side 46 near the distal end surface 41a). The second tip member 56 is configured to cover a portion of the first side 48 of the second surface 47 (in the first embodiment, a portion of the side 47 near the distal end surface 41a). The first and second tip members 54 and 56 are thus spaced apart from each other by a distance of the chamfered portion 52.

[0043] Fig. 3 is an enlarged side view, half in cross section, of a front end part of the spark plug 10, as viewed in the direction of an arrow III of the Fig. 1 is considered.

[0044] In the center electrode 20, the tip 25 is connected to the front end of the electrode base 22 by a welding zone 24, as in Fig. 3. In the first embodiment, the welding zone 24 is formed by laser welding and is located within the axial hole 12 of the insulator 11. On the other hand, a front end surface 26-side part of the tip 25 is exposed and protrudes to the front side of the axial hole 12.

[0045] In the ground electrode 40, the first tip element 54 is joined to the first surface 44 of the electrode base 41 through a weld zone 58 such that the first discharge surface 55 faces the front end surface 26 of the center electrode 20. In the first embodiment, the weld zone 58 is formed as a "nugget" by resistance welding. Similarly, the second tip element 56 is joined to the second surface 47 of the electrode base 41 through a weld zone (not shown).

[0046] In the tip 53, the second tip element 56 (excluding the weld zone) is made larger in volume than the first tip element 54 (excluding the weld zone 58). Specifically, the volume of the second tip element 56 is made 1.3 times or more larger than the volume of the first tip element 54 in the first embodiment.

[0047] Further, a first imaginary plane 59 including the first discharge surface 55 of the first tip member 54 and a second imaginary plane 60 including the second discharge surface 57 of the second tip member 56 intersect each other at a predetermined angle θ. In the first embodiment, the angle θ formed between the first and second imaginary planes 59 and 60 is set to substantially 90°.

[0048] The spark plug 10 having the above structure can be manufactured, for example, by the following method. The center electrode 20 is first inserted and arranged in the axial hole 12 of the insulator 11 such that the welding zone 24 is located inside the axial hole 12, and the front end surface 26 of the center electrode 20 is exposed outside the axial hole 12. The metal terminal 27 is then inserted into the axial hole 12 and electrically connected to the center electrode 20. Then, the metal shell 30, to which the electrode base 41 has been attached, is mounted on the outer periphery of the insulator 11. After the first and second tip members 54 and 56 are attached to the electrode base 41, the electrode base 41 is bent such that the first tip member 54 faces the center electrode 20. Thus, the spark plug 10 is obtained.

[0049] In a state where the spark plug 10 is mounted to the engine (not shown), the tip 25 of the center electrode 20 and the ground electrode 40 are exposed within a combustion chamber of the engine. In this mounted state, the third surface 50 of the ground electrode 40 faces the upstream side of the air-fuel mixture flow in the combustion chamber; and the second surface 47 of the ground electrode 40 faces the downstream side of the air-fuel mixture flow in the combustion chamber (i.e., the exhaust valve side).

[0050] When the insulation between the center electrode 20 and the ground electrode 40 is broken by increasing the secondary voltage of an ignition coil in an ignition device of the engine, a so-called capacitive spark is generated between the center electrode 20 and the ground electrode 40 by electrical energy accumulated in the secondary circuit. Subsequently, a so-called inductive spark is generated by electromagnetic energy of the ignition coil. The inductive spark is lower in current and shorter in duration than the capacitive spark. This spark discharge is likely to be generated between the center electrode 20 and a part of the ground electrode 40 close to or projecting toward the center electrode 20. In other words, the spark discharge path is likely to be formed between the front end surface 26 of the center electrode 20 and the first discharge surface 55 of the first tip member 54.

[0051] In the case where the air-fuel mixture flow rate in the combustion chamber is low, the discharge gap is formed between the front end surface 26 of the center electrode 20 and the first discharge surface 55 of the first tip member 54. In the case where the air-fuel mixture flow rate in the combustion chamber is high, such as in a lean-burn engine, etc., the discharge gap is expanded to the downstream side by the air-fuel mixture flow so that it extends from the front end surface 26 of the center electrode 20 to the second surface 47 of the electrode base 41 or the second tip member 56.

[0052] In the first embodiment, the second tip member 56 is attached to the second surface 47 of the electrode base 41 as mentioned above, so that the discharge gap (mainly the discharge gap of the inductive spark), when expanded by the air-fuel mixture flow to the downstream side, between the front end surface 26 of the center electrode 20 and the second discharge surface 57 of the second tip member 56, as indicated by the reference numeral 61 in Fig. 3. Spark wear of the electrode base 41 (second surface 47) is thus suppressed in the first embodiment compared to the case where no second tip member 56 is attached to the second surface 47. Furthermore, the extended discharge gap 61 results in a larger number of fuel particles being activated by the spark discharge and a larger flame kernel being developed in the combustion chamber. The extended discharge gap 61 also results in a larger distance from the center and ground electrodes 20 and 40 to the flame kernel, so that flame-extinguishing actions of the center and ground electrodes 20 and 40 can be reduced with increasing distance from the flame kernel. It is therefore possible to improve the ignition performance of the spark plug 10.

[0053] In the spark plug 10, the higher the temperature of the tip 53, the more susceptible it is to spark wear. The first tip member 54 and the second tip member 56 are spaced apart from each other in the first embodiment such that, even though the first and second tip members 54 and 56 are heated by the spark discharge, heat can be prevented from being directly transferred from the first tip member 54 to the second tip member 56 or from the second tip member 56 to the first tip member 54 by contacting these tip members 54 and 56. A temperature rise of the tip member 53 is suppressed by preventing direct heat transfer between the first and second tip members 54 and 56, thereby improving heat transfer from the first and second tip members 54 and 56 to the ground electrode base 41. Spark wear of the tip 53 is thus suppressed.The ground electrode 40 is therefore improved in terms of spark wear resistance.

[0054] Since the weld zone 58 is formed by melting and fusion welding the tip 53 (first and second tip members 54 and 56) made of noble metal-based material and the electrode base 41 made of Ni-based material, the thermal conductivity of the weld zone 58 is lower than that of the tip 53. In the presence of such a weld zone 58, separating the tip members 54 and 56 is effective in suppressing a temperature rise of the tip members 54, 56 caused by heat transfer. For this reason, it is preferable that the first and second tip members 54 and 56 be spaced apart from each other by a distance in the case where the weld zone 58 is formed between the tip 53 and the electrode base 41.

[0055] The first tip member 54 is configured to cover a portion of the second side 46 of the first surface 44 of the electrode base 41, to which the second surface 47 is connected to the second tip member 56. With this arrangement, the occurrence of spark wear near the second side 46 of the first surface 44 is suppressed over a wider area by the first tip member 54. The first surface 44 of the electrode base 41 is therefore improved in terms of spark wear resistance. Similarly, the second tip member 56 is configured to cover a portion of the first side 48 of the second surface of the electrode base 41, to which the first surface 44 is connected to the first tip member 54. With this arrangement, the occurrence of spark wear near the first side 48 of the second surface 47 is suppressed over a wider area by the second tip member 56.The second surface 47 of the electrode base 41 is therefore improved in terms of spark wear resistance.

[0056] Furthermore, the volume of the second tip member 56 (except for the welding zone) is made larger than that of the first tip member 54 (except for the welding zone 58), so that the second tip member 56 can ensure durability against spark wear due to the discharge gap 61 (mainly the discharge gap of the inductive spark) being widened by the air-fuel mixture flow.

[0057] The welding zone 24 of the center electrode 20, which is more susceptible to spark wear than the tip 25, is located within the axial hole 12 of the insulator 10, so that spark wear of the welding zone 24 can be prevented.

[0058] Fig. Fig. 4 is an enlarged side view, half in cross section, of the front end part of the spark plug 10 as seen in a direction perpendicular to the paper surface of the Fig. 1 is considered.

[0059] The ground electrode 40 is designed such that when the ground electrode 40 is in a direction perpendicular to the second discharge surface 57 of the first tip element 56 as shown in Fig. 4 (that is, from a direction perpendicular to the paper surface of the Fig. 4), the entire front surface 26 of the center electrode 20 extends within an existence range 62 of the second tip element 56 in a direction perpendicular to a minimum line segment connecting the first front end surface 26 of the center electrode 20 with the first discharge surface 55 of the first tip element 54 at the shortest distance (i.e., in a direction parallel to the paper surface of the Fig. 4). With this configuration, spark discharge is more likely to be generated between the front end surface 26 of the center electrode 20 and the second tip element 56. Spark wear of the second surface 47 is therefore effectively prevented (see also Fig. 3).

[0060] Further, the ground electrode 40 is formed such that when the ground electrode 40 is tilted from the direction perpendicular to the second discharge surface 57 as shown in Fig. 4 (that is, from the direction perpendicular to the paper surface of the Fig. 4), the entire first discharge surface 55 of the first tip member 54 within the existence area 62 of the second tip member 56 in the direction perpendicular to the minimum line segment connecting the front end surface 26 of the center electrode 20 to the first discharge surface 55 of the first tip member 54 in the shortest direction (that is, in the direction parallel to the paper surface of the Fig. 4). In this configuration, a ground electrode 40-side end of the discharge gap is readily connected from the first tip element 54 to the second tip element 56, as designated by reference numeral 61 (see also Fig. 3) when the spark discharge is radiated between the center electrode 20 and the first tip element 54 by the air-fuel mixture flow. Spark wear of the electrode base 41 is therefore effectively prevented.

[0061] It should be noted that a width of the existence region 62 of the second electrode tip 56 (i.e., a dimension of the region 62 in the lateral direction of the Fig. 4) is equal to a width of the widest portion of the second electrode tip 56. SECOND EMBODIMENT

[0062] Fig. 5 is a side view, semi-cross-sectional, of a front end portion of a spark plug 70 according to the second embodiment of the present invention. The spark plug 70 according to the second embodiment is structurally the same as the spark plug 10 according to the first embodiment, except for the configuration of a ground electrode 71. Herein, the same parts or portions of the second embodiment as those of the first embodiment are denoted by the same reference numerals to omit their explanations.

[0063] As in Fig. 5, the ground electrode 71 of the spark plug 70 includes: an electrode base 72 (as a ground electrode base) made of a Ni-based material; and first and second tip members 79 and 80 (as a ground electrode tip), both made of a noble metal-based material and attached to the electrode base 72.

[0064] The electrode base 72 is pentagonal in cross section and includes: a first surface 73 opposite the front end surface 26 of the center electrode 20; a second surface 75 connected to one side of the first surface 73 via a chamfered portion 74; a third surface 77 connected to the other opposite side of the first surface 73; a fourth surface 76 opposite the third surface 77 and connected to the second surface 75; and a fifth surface 78 connecting the third surface 70 and the fourth surface 76. In the second embodiment, the first surface 73 and the second surface 75 are aligned to form an obtuse angle. The chamfered portion 74 is formed as a tapered surface by cutting (chamfering) an edge between the first surface 73 and the second surface 75.

[0065] The first tip element 79 is rectangular-plate-shaped and is joined to the first surface 73 of the electrode base 72 by a weld zone 83. The weld zone 83 is formed by melting and fusing the first tip element 79 and the electrode base 72. The first tip element 79 has a first discharge surface 80 that faces the front end surface 26 of the center electrode 20 and is oriented in the same direction as the first surface 73 (i.e., in a direction perpendicular to the axis O).

[0066] The second tip element 80 is rectangular plate-shaped and is joined to the second surface 75 of the electrode base 72 by a weld zone (not shown). The weld zone is also formed by melting and fusing the second tip element 80 and the electrode base 72. The second tip element 80 has a second discharge surface 82 oriented in the same direction as the second surface 75 (i.e., in a direction inclined to the axis O).

[0067] In contrast to the first embodiment, in which the first imaginary plane 59 and the second imaginary plane 60 intersect each other substantially at 90°, in the second embodiment, a first imaginary plane 84 including the first discharge surface 80 and a second imaginary plane 85 including the second discharge surface 82 intersect each other at a predetermined obtuse angle θ (>90°). The obtuse angle θ formed between the first and second imaginary planes 84 and 85 is preferably in the range of 120° to 170°.

[0068] In general, spark discharge is more likely to occur between the center electrode 20 and a protruding part or the center electrode 20 side part of the second discharge surface 82 than between the center electrode 20 and the other part of the second discharge surface 82. In the case where the first and second imaginary planes 84 and 85 form an obtuse angle θ, the second discharge surface 82 is more oriented and aligned to the center electrode 20 side (i.e., the back side; the upper side of the Fig. 5) compared to the case where the first and second imaginary planes 84 and 85 intersect each other at a right angle. Therefore, the spark discharge is more likely to be generated along a discharge gap 86 between the center electrode 20 and the second discharge surface 82. The discharge gap 86 is more easily extended to the center of the combustion chamber by the air-fuel mixture flow in the combustion chamber (not shown), so that fuel particles near the center of the combustion chamber are activated, thereby achieving improvement in ignition performance. THIRD EMBODIMENT

[0069] Fig. 6 is a side view, semi-cross-sectionally, of a front end portion of a spark plug 90 according to the third embodiment of the present invention. The spark plug 90 according to the third embodiment is structurally the same as the spark plug 10 according to the first embodiment, except for the configuration of a ground electrode 91. Herein, the same parts or portions of the second embodiment as those of the first embodiment are denoted by the same reference numerals to omit their explanations.

[0070] As in Fig. 6, the ground electrode 91 of the spark plug 90 includes: an electrode base 92 (as a ground electrode base) made of a Ni-based material; and first and second tip members 99 and 101 (as a ground electrode tip), both made of a noble metal-based material and attached to the electrode base 92.

[0071] The electrode base 92 is approximately rectangular in shape and includes: a first surface 93 opposite the front end surface 26 of the center electrode 20; a second surface 95 connected to one side of the first surface 93 via an intermediate surface 94; a third surface 96 connected to the other opposite side of the first surface 93; and a fourth surface 97 connecting the second surface 95 and the third surface 96. The second surface 95 is oriented substantially perpendicular to the first surface 93. The intermediate surface 94 is formed as a flat surface connecting the first surface 93 and the second surface 95.

[0072] The first tip element 99 is rectangular plate-shaped and is joined to the first surface 93 of the electrode base 92 by a weld zone 103. The weld zone 103 is formed by melting and fusing the first tip element 99 and the electrode base 92. The first tip element 99 has a first discharge surface 100 that faces the front end surface 26 of the center electrode 20 and is oriented in the same direction as the first surface 93 (i.e., in a direction perpendicular to the axis O).

[0073] The second tip element 101 is substantially triangular-prism-like in shape and is joined to the second surface 95 of the electrode base 92 by a weld zone (not shown). The weld zone is formed by melting and fusing the second tip element 101 and the electrode base 92. The second tip element 101 has a second discharge surface 102 oriented in a direction inclined to the second surface (i.e., inclined to the axis O).

[0074] In contrast to the second embodiment in which the rectangular plate-shaped first and second tip elements 79 and 81 are attached to the obtusely oriented first and second surfaces 73 and 75 of the electrode base 72, whereby the first imaginary plane 84 and the second imaginary plane 85 intersect each other at an obtuse angle θ, the rectangular plate-shaped first tip element 99 and the triangular prism-like shaped second tip element 101 are attached to the perpendicularly oriented first and second surfaces 93 and 95 of the electrode base 92, whereby a first imaginary plane 104 including the first discharge surface 100 and a second imaginary plane 105 including the second discharge surface 102 intersect each other at a predetermined obtuse angle θ (> 90°) in the third embodiment.The obtuse angle θ formed between the first and second imaginary planes 104 and 105 is preferably in the range of 120° to 170°.

[0075] Since the first imaginary surface 104 and the second imaginary surface 105 in the third embodiment form an obtuse angle θ as in the second embodiment, it is possible to obtain the same effects as in the second embodiment in the third embodiment. FOURTH EMBODIMENT

[0076] Fig. 7 is a side view, semi-cross-sectional, of a front end portion of a spark plug 110 according to the fourth embodiment of the present invention. The spark plug 110 according to the fourth embodiment is structurally the same as the spark plugs 10, 70 according to the first and second embodiments, except for the configuration of a ground electrode 111. Herein, the same parts or portions of the second embodiment as those of the first embodiment are denoted by the same reference numerals to omit their explanations.

[0077] As in Fig. As shown in FIG. 7, the ground electrode 111 of the spark plug 110 includes a third tip member 112 made of a noble metal-based material and joined to the fourth surface 76 of the electrode base 72, in addition to the first and second tip members 79 and 81, which are respectively joined to the first and second surfaces 73 and 75 of the electrode base 72. The third tip member 112 is rectangular plate-shaped and joined to the fourth surface 79 of the electrode base 72 through a weld zone (not shown). The weld zone is also formed by melting and fusing the third tip member 112 and the electrode base 72.The third tip element 112 is spaced apart from each of the first and second tip elements 79 and 81 and has a flat third discharge surface 113 oriented such that the third imaginary plane 114, which includes the third discharge surface 113, intersects the second imaginary plane 85 at a predetermined obtuse angle θ (>90°). The obtuse angle θ formed between the second and third imaginary planes 85 and 114 is preferably in the range of 120° to 170°.

[0078] In the fourth embodiment, since the third tip member 112 is joined to the fourth surface 76 of the electrode base 72, spark wear of the fourth surface 76 is effectively suppressed. When spark discharge is generated along a discharge gap 86 between the center electrode 20 and the third tip member 112, the discharge gap 86 is readily extended to the center of the combustion chamber to activate fuel particles near the center of the combustion chamber, thereby achieving improvement in ignition performance. EXAMPLES

[0079] The present invention is described in more detail below with reference to the following experimental examples. It should be noted that the following experimental examples are intended only to enhance the understanding of the present invention and are not intended to limit the present invention thereto. EXPERIMENTAL TEST 1

[0080] A spark plug was produced as a test sample, with both the center and ground electrodes having a tip attached to an electrode base thereof. The electrode bases of the center and ground electrodes used were each made of a Ni-based alloy. The tip of the center electrode used (hereinafter referred to as the "center electrode tip") was made of Ir. The tip of the ground electrode used (hereinafter referred to as the "ground electrode tip") was made of a Pt-based alloy containing 10 wt% Ni and was circular disk-shaped with a diameter of 1.6 mm and a thickness of 0.5 mm.

[0081] The resulting spark plug was mounted on a test engine and connected to an ignition coil in an ignition device of the test engine. While air-fuel mixture was supplied to a combustion chamber of the test engine, the pressure inside the combustion chamber was controlled at 1.5 MPa. In this condition, the spark plug test was conducted by charging the ignition coil with 50 mJ of electromagnetic energy and causing a spark discharge between the center and ground electrodes of the spark plug at a charging / discharging frequency of 60 Hz for 250 hours. This charging / discharging frequency (60 Hz) corresponds to 720 rpm of a four-stroke engine. Furthermore, 100 hours of testing corresponds to approximately 50,000 km of driving a vehicle by an ordinary driver.

[0082] The spark plug was removed from the test engine every 50 hours until 250 hours had elapsed from the initiation of the test. Each time the spark plug was removed, the distance between the center electrode tip and the ground electrode tip was measured with a gauge. The amount of wear (thickness reduction) of the ground electrode tip was determined by subtracting the amount of wear (thickness reduction) of the center electrode tip from the measured distance. The test results are shown below in TABLE 1. TABLE 1 Zeit (Std.) Abnutzungsmenge (mm) 50 0,03 100 0,10 150 0,13 200 0,17 250 0,20

[0083] As shown in Table 1, the wear amount (thickness reduction) of the ground electrode tip after 100 hours from the initiation of the test was 0.1 mm. Therefore, it can be seen that the ground electrode tip, when formed with a thickness of 0.1 mm or greater, ensures sufficient durability against spark wear.

[0084] The amount of wear of the ground electrode tip increases with the test duration. Therefore, the greater the strength of the ground electrode tip, the longer the service life of the ground electrode tip. On the other hand, the mass of the ground electrode tip increases proportionally to the strength of the ground electrode tip. The load applied to the electrode base on which the ground electrode tip is supported increases with the increase in the strength of the ground electrode tip. Applying a high load may result in deformation or fracture of the ground electrode base under a high-temperature environment. Given these facts, the upper limit of the ground electrode tip thickness is preferably set to 1.0 mm. EXPERIMENTAL TEST 2

[0085] Fig. Fig. 8A is a schematic view showing a spark plug 120 produced and used as a test sample in this experimental test. As shown in Fig. As shown in Figure 8A, the spark plug 120 was provided with a center electrode 121 and a ground electrode 122 such that a first surface 123 of the ground electrode 122 faces the center electrode 121 across an electrode gap. The ground electrode 122 used consisted of a rectangular cross-section Ni-based alloy electrode base without an attached tip.

[0086] The spark plug 120 was mounted on a flow channel that was uniform in cross-sectional area from its upstream end to its downstream end, with a third surface 125 of the ground electrode 122 facing the upstream side of the flow F of the test gas, and with a second surface 124 of the ground electrode 122 facing the downstream side of the flow F of the test gas. The position of the spark plug 120 was adjusted such that the electrode gap of the spark plug 120 was in the center of the cross-sectional area of ​​the flow channel. While the test gas was fed through the flow channel at a flow rate of 10 m / s, the electric spark was caused by inducing discharge between the center electrode 121 and the ground electrode 122. The test gas used was air. The discharge energy for each electric spark was 100 mJ.

[0087] In the test, 100 discharges were photographed with a high-speed camera to measure the discharge time (A) between the center electrode 121 and the first surface 123 of the ground electrode 122 and the discharge time (B) between the center electrode 121 and the second surface 124 of the ground electrode 120. The ratio of the discharge time B to the total discharge time A + B was determined to be 57%.

[0088] Given the above result that the discharge time B at the second surface is longer than the discharge time A at the first surface, it is found that the second tip element ensures its durability against spark wear due to the discharge gap expanded by the flow of air-fuel mixture when the second tip element connected to the second surface has a larger volume than the first tip element attached to the first surface. In particular, given the ratio of the discharge time B to the discharge time A (B / A = 57% / 43%), it is found that the second tip element ensures its durability against spark wear when the volume of the second tip element is 1.3 times or more larger than that of the first tip element. EXPERIMENTAL TEST 3

[0089] Fig. Fig. 8B is a schematic view showing a spark plug 130 produced and used as a test sample in this experimental test. As shown in Fig. As shown in Figure 8B, the spark plug 130 was provided with a center electrode 131 and a first surface 133 of the ground electrode 132 such that a front end surface of the center electrode 131 and a first surface 133 of the ground electrode 132 were opposed to each other in parallel across an electrode gap 136. The ground electrode 132 used was composed of a Ni-based alloy electrode base without an attached tip.

[0090] The spark plug 130 was mounted on a flow channel that was uniform in cross-sectional area from its upstream end to its downstream end, with a third surface 135 of the ground electrode 132 facing the upstream side of the flow F of the test gas, and with a second surface 134 of the ground electrode 132 facing the downstream side of the flow F of the test gas. The position of the spark plug 130 was adjusted such that the electrode gap 136 of the spark plug 130 was at the center of the cross-sectional area of ​​the flow channel. While the test gas was fed through the flow channel at a flow rate of 10 m / s, an electric spark was caused by inducing discharge along a discharge path 137 between the center electrode 131 and the second surface 134 of the ground electrode 132. The test gas used was air. The discharge energy for each electric spark was 100 mJ.

[0091] In the test, 100 discharge images were photographed with a high-speed camera. The thus obtained image data were subjected to image processing. Assuming a flat figure surrounded by the discharge gap 137 and a line segment connecting both ends of the discharge gap 137, the center of gravity of the flat figure was determined as the center of gravity 138 of the discharge gap. Here, the center of gravity 138 was determined by a known method using an arithmetic mean of coordinates of the flat figure, a moment of the flat figure, etc. Based on the determination results, the average position of the center of gravity 138 was determined.

[0092] The positional relationship of the angle θ to the center of gravity 138 was tested using a plurality of test samples, wherein the angle θ was changed between the first and second surfaces 133 and 134 without changing the sizes of the first and second surfaces 133 and 134. The test sample was classified as "A" if the average position of the center of gravity 138 was in front of the first surface 133 (i.e., on the lower side of the Fig. 8B in relation to the first surface 133). On the other hand, the test sample was classified as “B” if the mean position of the center of gravity 138 was rearward of the first surface 133 (i.e., on the upper side of the Fig. 8B relative to the first surface 133). The test results are shown below in TABLE 2. TABLE 2 Winkel θ (°) Einstufung 90 B 100 B 110 B 120 A 130 A 140 A 150 A 160 A 170 A 180 B

[0093] As shown in TABLE 2, the "A" rating was in the range of 120° ≤ θ ≤ 170°. It is assumed that the center of gravity 138 of the discharge gap 137 would be brought closer to the center of the engine combustion chamber in the "A" rating spark plug than in the "B" rating spark plug. The closer the center of gravity of the discharge gap is to the center of the combustion chamber, the easier the fuel particles near the center of the combustion chamber are activated by spark discharge. Consequently, the "A" rating spark plug would have better ignition performance than the "B" rating spark plug. Therefore, it can be seen that the spark plug is more improved in ignition performance in the range of 120° ≤ θ ≤ 170°.

[0094] Although the present invention has been described with reference to the above embodiments, the above embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention thereto. Various changes and modifications can be made to the above embodiments without departing from the scope of the present invention.

[0095] In the above embodiments, the electrode base 41, 71, 92 of the ground electrode 40, 71, 91, 111 is rectangular or pentagonal in cross section. However, the cross-sectional shape of the electrode base of the ground electrode is not necessarily limited to a rectangular or pentagonal shape and can be determined as appropriate. For example, the electrode base of the ground electrode may alternatively be semicircular in cross section.

[0096] In the above embodiments, the first tip members 54, 79, 99 and the second tip members 56, 81, as well as the third tip member 112 of the ground electrode 40, 41, 91, 111, are rectangular plate (parallelpiped)-shaped. However, the cross-sectional shape of the electrode base of the first, second, and third tip members of the ground electrode is not necessarily limited to such a rectangular plate shape and can be determined as appropriate. For example, each of the first, second, and third tip members of the ground electrode can be formed into any disk shape other than the rectangular plate shape as in the case of the above Experimental Test 1.

[0097] In the above first embodiment, the first tip member 54 is configured to cover the part of the second side 46 of the first surface 44 adjacent to the distal end surface 41a of the electrode base 41; and the second tip member 56 is configured to cover the part of the first side 48 of the second surface 47 adjacent to the distal end surface 41a of the electrode base 41. However, the installation positions of the first and second tip members 54 and 56 are not necessarily limited to such positions. Since the size of the tip 53 and the length of the distal end portion 42 of the electrode base 41 can be adjusted as appropriate, it is feasible to arbitrarily adjust the distance between the tip 53 and the distal end surface 41a of the electrode base 41. This also applies to the above second to fourth embodiments.

[0098] In the above embodiments, the second tip member 56, 81, 101 is connected to the second surface 47, 75, 95 of the electrode base 41, 71, 72 without attaching the second tip member to the third surface 50, 77, 96 of the electrode base 41, 71, 92. However, the tip of the ground electrode is not necessarily limited to such a configuration. In addition to or instead of the second tip member 56, 81, 101 being attached to the second surface 47, 75, 5, and 90, a second electrode tip may be attached to the third surface 50, 77, 96. By connecting the second electrode tip to the third surface 50, 77, 96, spark wear of the third surface 50, 77, 96 is effectively prevented.

[0099] Furthermore, a third tip member may be attached to the distal end surface 41a of the electrode base 41. By connecting the third tip member to the distal end surface 41a, spark wear of the distal end surface 41 is effectively prevented.

[0100] Each of the above-mentioned embodiments can be modified by adding one or more of the features of the other embodiments or by replacing one or more of the features of the embodiment with those of the other embodiments. For example, the spark plug 10 of the first embodiment can be modified by attaching the second tip member 101 of the third embodiment to the second surface 47 of the electrode base 41 instead of the second tip member 56. In this modified embodiment, the second imaginary plane 105, which includes the second discharge surface 102 of the first tip member 101, forms an obtuse angle with the first imaginary surface 59.

[0101] Of course, the chamfered portion 52 of the electrode base 41 may alternatively be formed as a tapered surface in the above first embodiment. In the above second or fourth embodiment, the chamfered portion 74 of the electrode base 72 may alternatively be formed as a round curved surface. In the above third embodiment, the intermediate surface 94 of the electrode base 92 may alternatively be omitted or formed as a curved surface.

Claims

[1] Spark plug (10; 70; 90; 110), comprising: an insulator (11) having an axial hole (12) formed therein in the direction of an axis (O) of the spark plug (10; 70; 90; 110;); a center electrode (20) arranged in a front end side of the axial hole (12); a cylindrical metal housing (30) which holds the insulator (11); and a ground electrode (40; 71; 91) comprising a rod-shaped ground electrode base (41; 72; 92) formed of a Ni-based material and coupled to the metal case (30) at a base end portion (43) thereof, and a ground electrode tip (53) made of a noble metal-based material and attached to a distal end portion (42) of the ground electrode base (41; 72; 92), wherein the distal end portion (42) of the ground electrode base (41; 72; 92) includes: a first surface (44; 74; 93) opposite a front end surface (26) of the center electrode (20); and second and third surfaces (47, 50; 75, 77; 95, 96) connected to the first surface (44; 74; 93) and extending from the distal end portion (42) to the base end portion (43) of the ground electrode base (41; 72; 92), wherein the ground electrode tip (53) comprises: a first tip element (54; 79; 99) attached to the first surface (44; 74; 93) and having a first discharge surface (55; 80; 100) to allow spark discharge between the center electrode (20) and the first discharge surface (55; 80; 100); and a second tip element (56; 81; 101) attached to at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) and having a second discharge surface (57; 82; 102) to allow spark discharge between the center electrode (20) and the second discharge surface (57; 82; 102), wherein the first tip element (54; 79; 99) and the second tip element (56; 81; 101) are spaced apart from each other, and wherein a volume of the second tip element (56; 81; 101) is greater than a volume of the first tip element (54; 79; 99). [2] Spark plug (10; 70; 90; 110), comprising: an insulator (11) having an axial hole (12) formed therein in the direction of an axis (O) of the spark plug (10; 70; 90; 110;); a center electrode (20) arranged in a front end side of the axial hole (12); a cylindrical metal housing (30) which holds the insulator (11); and a ground electrode (40; 71; 91) comprising a rod-shaped ground electrode base (41; 72; 92) formed of a Ni-based material and coupled to the metal case (30) at a base end portion (43) thereof, and a ground electrode tip (53) made of a noble metal-based material and attached to a distal end portion (42) of the ground electrode base (41; 72; 92), wherein the distal end portion (42) of the ground electrode base (41; 72; 92) includes: a first surface (44; 74; 93) opposite a front end surface (26) of the center electrode (20); and second and third surfaces (47, 50; 75, 77; 95, 96) connected to the first surface (44; 74; 93) and extending from the distal end portion (42) to the base end portion (43) of the ground electrode base (41; 72; 92), wherein the ground electrode tip (53) comprises: a first tip element (54; 79; 99) attached to the first surface (44; 74; 93) and having a first discharge surface (55; 80; 100) to allow spark discharge between the center electrode (20) and the first discharge surface (55; 80; 100); and a second tip element (56; 81; 101) attached to at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) and having a second discharge surface (57; 82; 102) to allow spark discharge between the center electrode (20) and the second discharge surface (57; 82; 102), wherein the first tip element (54; 79; 99) and the second tip element (56; 81; 101) are spaced apart from each other, wherein, when the ground electrode (40) is viewed from a direction perpendicular to the second discharge surface (57), the entire front end surface (26) of the center electrode (20) lies within an existence range (62) of the second tip element (56) in a direction perpendicular to a minimum line segment connecting the front end surface (26) of the center electrode (20) to the first discharge surface (55) of the first tip element (54). [3] Spark plug (10; 70; 90; 110), comprising: an insulator (11) having an axial hole (12) formed therein in the direction of an axis (O) of the spark plug (10; 70; 90; 110;); a center electrode (20) arranged in a front end side of the axial hole (12); a cylindrical metal housing (30) which holds the insulator (11); and a ground electrode (40; 71; 91) comprising a rod-shaped ground electrode base (41; 72; 92) formed of a Ni-based material and coupled to the metal case (30) at a base end portion (43) thereof, and a ground electrode tip (53) made of a noble metal-based material and attached to a distal end portion (42) of the ground electrode base (41; 72; 92), wherein the distal end portion (42) of the ground electrode base (41; 72; 92) includes: a first surface (44; 74; 93) opposite a front end surface (26) of the center electrode (20); and second and third surfaces (47, 50; 75, 77; 95, 96) connected to the first surface (44; 74; 93) and extending from the distal end portion (42) to the base end portion (43) of the ground electrode base (41; 72; 92), wherein the ground electrode tip (53) comprises: a first tip element (54; 79; 99) attached to the first surface (44; 74; 93) and having a first discharge surface (55; 80; 100) to allow spark discharge between the center electrode (20) and the first discharge surface (55; 80; 100); and a second tip element (56; 81; 101) attached to at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) and having a second discharge surface (57; 82; 102) to allow spark discharge between the center electrode (20) and the second discharge surface (57; 82; 102), wherein the first tip element (54; 79; 99) and the second tip element (56; 81; 101) are spaced apart from each other, wherein, when the ground electrode (40) is viewed from a direction perpendicular to the second discharge surface (57), the entire first discharge surface (55) of the first tip element (54) lies within an existence range (62) of the second tip element (56) in a direction perpendicular to a minimum line segment connecting the front end surface (26) of the center electrode (20) to the first discharge surface (59) of the first tip element (54). [4] Spark plug (10; 70; 90; 110) according to one of claims 1 to 3, wherein the first tip element (54; 79; 99) covers at least a portion of a side (45, 46) of the first surface (44; 74; 93) to which at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) is connected. [5] Spark plug (10; 70; 90; 110) according to one of claims 1 to 4, wherein the second tip member (56; 81; 101) covers at least a portion of a side (48) of the at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) to which the first surface (44; 74; 93) is connected. [6] Spark plug (10; 70; 90; 110) according to one of claims 1 to 5, wherein a height (T1) of the first tip element (54; 79; 99) from the first surface (44; 74; 93) in a direction perpendicular to the first surface (44; 74; 93) and a height (T2) of the second tip element (56; 81; 101) from the at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) in a direction perpendicular to the at least one of the second and third surfaces (47, 50; 75, 77; 95, 96) is 0.1 mm to 1 mm. [7] Spark plug (70; 90; 110) according to one of claims 1 to 6, wherein a first imaginary plane (84; 104) including the first discharge surface (80; 100) and a second imaginary plane (85; 105) including the second discharge surface (82; 102) form an obtuse angle (θ). [8] Spark plug (70; 90; 110) according to claim 7, wherein the obtuse angle (θ) is 120° to 170°. [9] Spark plug (10; 70; 90; 110) according to claim 1, wherein the volume of the second tip member (56; 81; 101) is 1.3 times or more larger than the volume of the first tip member (54; 79; 99).

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