Multielectrode spark plug

Abstract: In order to provide a multielectrode spark plug which is excellent in resistance to fouling and in which the voltage required for producing a spark can be lowered, the tip portion of at least one of plural ground electrodes is positioned in the side of an annular front end face of an insulator to form a semi-creeping spark discharging gap elongating along the surface of the insulator with the tip portion of a center electrode, and the tip portions of the ground other electrodes form aerial spark discharging gaps with the tip portion of the center electrode.

Description: The invention relates to a multielectrode spark plug in accordance with the generic clause of claim 1 which has improved resistance to fouling. Such a multielectrode spark plug is already known from document FR-A-2126686. The spark plug of this document has a structure in which an aerial spark discharge ground electrode is arranged on an extended line of the center electrode. In general, the spark flies to the parallel electrode, but when smolder occurs, the spark flies to a side electrode near the end face of an insulator. From document FR-A-1446036 there is also already known a spark plug having an aerial spark discharge ground electrode and an auxiliary electrode. In this prior art spark plug the insulator does not project from the metal shell. In order to reduce spark wear of a ground electrode and improve ignitability, a multielectrode spark plug is used. In order to conduct burn-cleaning of conductive materials (mainly carbon caused by unburned fuel) deposited on the surface of the front end portion of an insulator and prevent resistance to fouling from being impaired, a creeping spark plug or a semi-creeping spark plug is used. As an example of such a spark plug, Unexamined Japanese Patent Publication (Kokai) No. SHO51-95540 discloses a multigap spark plug having a plurality of ground electrodes 102 which are opposed to a center electrode 101 as shown in Figs. 14A and 14B. The spark plug has two kinds of spark discharging gaps, namely, a semi-creeping spark discharge gap (creeping spark discharge gap 111 + first aerial spark discharging gap 113) which partly elongates along a tip end face of a front end portion of an insulator 104 and a second aerial spark discharging gap 112. U.S. Patent 2,650,583 discloses a spark plug 200 which, as shown in Figs. 15A and 15B, has a plurality of layer-like ground electrodes 203 including electrodes 202 the tips of which oppose a center electrode 201, and a plurality of spark discharging gaps formed between the center electrode 201 and the tips of the ground electrodes 203, and in which the ground electrodes 203 cover a part of a front face 205 of an insulator 204. Furthermore, noble metal spark plugs are popularly used in which a noble metal is fixed to a firing position of an electrode so as to prevent spark wear from occurring, thereby lengthening the life. In a conventional spark plug 300 of the parallel electrode type such as shown in Fig. 16, when it is used in reversed polarity, the discharge voltage is raised, so that, when smolder occurs, a discharge may hardly take place across the normal spark discharge gap. Specifically, when smolder occurs and the insulation resistance between a center electrode 301 and a ground electrode 302 are lowered, the output voltage of a power coil is divided by the output impedance of the power coil and the insulation resistance between the center electrode 301 and the ground electrode 302, and hence the voltage from the power coil which can appear across the normal spark discharge gap is lowered. When smolder occurs and carbon is deposited, therefore, the discharge voltage of the normal spark discharge gap is raised and a discharge hardly takes place. In the spark plug 100 disclosed in Unexamined Japanese Patent Publication (Kokai) No. SHO51-95540 shown in Figs. 14A and 14B, the position where a spark discharge is caused by the aerial spark discharge gap 112 is not largely different from that where a spark discharge is caused by the creeping spark discharge gap (111+113). Specifically, when a spark discharge takes place across the creeping spark discharge gap, the spark is produced along the tip end face of the front end portion 106 of the insulator 104 and the shortest distance between the front end portion 106 of the insulator 104 and the ground electrode 102. By contrast, when a spark discharge takes place across the second aerial spark discharge gap 112, the spark is produced along the shortest distance between the ground electrode 102 and the center electrode 101. Between the positions of the sparks in the air gaps, there is only a difference which substantially corresponds to the thickness of the ground electrode 102. Therefore, there arises a problem in that the position where a spark discharge takes place cannot be protruded from the front end of the spark plug and its ignitability cannot be sufficiently improved. In the spark plug 200 of U.S. Patent 2,650,583 shown in Figs. 15A and 15B, a part of each ground electrode 203 partly covers the tip end face of the insulator 204. Therefore, it is impossible to conduct burn-cleaning of carbon deposited on the portions of the insulator 204 covered by the ground electrodes 203, and the ability of burning off carbon adhering to the surface of the insulator 204 is lowered. When the distance between the front end portion of the insulator 204 and the ground electrodes 203 is short, a carbon bridge is easily produced, thereby producing a large possibility that the engine stops. When the distance between the front end portion of the insulator 204 and the ground electrodes 203 is long, the voltage required for producing a spark across the semi-creeping spark discharge gap is raised. Consequently, the possibility that a spark takes place along the front end portion of the insulator 204 is lowered, and the cleaning ability of burning off carbon deposited on the front end portion of the insulator 204 is lowered. It is an object of the invention to provide a multielectrode spark plug in which, even if the plug is used in a reversed-polarity system, the voltage required for producing a spark can be lowered and the plug is excellent in resistance to fouling. This object is met by the characterizing features of claim 1. A multielectrode spark plug according to the present invention comprises a metallic shell; an insulator having an axial bore, the insulator being fitted to the metallic shell in a state where a front end portion of the insulator is protruded from a tip of the metallic shell; a center electrode which is fitted to the axial bore in a state where a tip portion of the center electrode is protruded from the front end portion of the insulator; and a plurality of ground electrodes secured to the tip of the metallic shell, a tip portion of each of the ground electrodes being bent toward the center electrode to form a spark discharge gap with the tip portion of the center electrode. The plurality of ground electrodes includes a semi-creeping spark discharge ground electrode, the tip portion of the semi-creeping spark discharge ground electrode being positioned in a side of the tip portion of the center electrode to form a semi-creeping spark discharging gap with a basal portion of the tip portion of the center electrode, a part of the semi-creeping spark discharging gap elongating along a tip end face of the front end portion of the insulator; and an aerial spark discharge ground electrodes which forms an aerial spark discharging gap with a side face of the tip portion of the center electrode. According to the present invention, the semi-creeping spark discharge ground electrodes are used for burn-cleaning of conductive materials (carbon caused by unburned fuel) deposited on the surface of the insulator. Therefore, high resistance to fouling can be attained and smolder can be surely prevented from occurring. Since the aerial spark discharge ground electrode is disposed aside from the semi-creeping spark discharge ground electrode, it is possible to ensure ignitability when the fouling state is recovered. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a perspective view showing main portions of the multielectrode spark plug of a first embodiment according to the present invention; Fig. 2 is a side view illustrating main portions of the multielectrode spark plug of the first embodiment; Fig. 3 is a perspective view showing main portions of a multielectrode spark plug of a second embodiment according to the present invention; Fig. 4 is a perspective view showing main portions of a multielectrode spark plug of a third embodiment according to the present invention; Fig. 5 is a perspective view showing main portions of a multielectrode spark plug of a fourth embodiment according to the present invention; Fig. 6 is a diagram illustrating the rate of the spark cleaning area and the increased amount of a spark discharge gap according to the invention in the case of four ground electrodes; Fig. 7 is a diagram illustrating the burn-cleaning state according to the invention in the case of two semi-creeping spark discharge ground electrodes; Fig. 8 is a diagram illustrating results of smolder fouling tests of spark plugs of an example of the invention; Fig. 9 is a diagram illustrating the running pattern in the smolder fouling tests in the example of the invention. Fig. 10 is a diagram illustrating results of smolder tests of spark plugs of the example of the invention; Fig. 11 is a diagram illustrating the running pattern in the smolder tests in the example of the invention; Fig. 12 is a diagram showing main portions of a spark plug as a comparative example; Fig. 13 is a diagram showing main portions of a spark plug as another comparative example; Fig. 14A is a section view of main portions of a conventional spark plug; Fig. 14B is a plan view of the conventional spark plug as shown in Fig. 14A; Fig. 15A is a section view of main portions of another conventional spark plug; Fig. 15B is a plan view of the conventional spark plug as shown in Fig. 15A;and Fig. 16 is a diagram showing main portions of a further conventional spark plug. DETAILED DESCRIPTIONS OF THE INVENTION Detailed descriptions of the present invention will be described as follows. First, according to the present invention, in a multielectrode spark plug having a plurality of ground electrodes, at least one of the ground electrodes is a semi-creeping spark discharge ground electrode and remaining ground electrodes are an aerial spark discharge ground electrodes. The tip portion of the semi-creeping spark discharge ground electrode is positioned in a side of the tip portion of the center electrode to form a semi-creeping spark discharge gap with the basal portion of the tip portion of the center electrode, a part of the semi-creeping spark discharging gap elongating along a tip end face of the front end portion of the insulator. The remaining ground electrodes are aerial spark discharge ground electrodes, the tip portion of the aerial spark discharge ground electrode form an aerial spark discharge gap with a side face of the tip portion of the center electrode. Second, in the first configuration of the multielectrode spark plug, it is preferable that a first firing portion of the center electrode in which an aerial spark discharging gap is formed between the aerial spark discharge ground electrode and the side face of the tip portion of the center electrode is configured by fixing a noble metal, a noble metal alloy or a material containing a noble metal such as platinum Pt, platinum-iridium Pt-Ir, platinum-nickel Pt-Ni, platinum-iridium-nickel Pt-Ir-Ni, platinum-rhodium Pt-Rh, or iridium-yttria Ir-Y 2 O 3 . Third, in the second configuration of the multielectrode spark plug, it is preferable that the first firing portion is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material or the noble metal alloy material and an electrode base material. Fourth, in the first to third configurations of the multielectrode spark plug, it is preferable that a second firing portion of the center electrode in which the semi-creeping semi-creeping spark discharging gap is formed between the spark discharge ground electrode and the basal portion of the tip portion of the center electrode is configured by fixing a noble metal or a noble metal alloy such as platinum Pt, platinum-iridium Pt-Ir, platinum-nickel Pt-Ni, platinum-iridium-nickel Pt-Ir-Ni, platinum-rhodium Pt-Rh or iridium-yttria Ir-Y 2 O 3 . Fifth, in the fourth configuration of the multielectrode spark plug, the second firing portion is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material or the noble metal alloy material and an electrode base material. Sixth, in the first to fifth configurations of the multielectrode spark plug, it is preferable that a third firing portion of the aerial spark discharge ground electrode in which an aerial spark discharging gap is formed between the aerial spark discharge ground electrode and the side face of the tip portion of the center electrode is configured by fixing a noble metal or a noble metal alloy such as platinum Pt, platinum-iridium Pt-Ir, platinum-nickel Pt-Ni, platinum-iridium-nickel Pt-Ir-Ni, platinum-rhodium Pt-Rh or iridium-yttria Ir-Y 2 O 3 . Seventh, in the sixth configuration of the multielectrode spark plug, it is preferable that the third firing portion is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material, the noble metal alloy material or a material containing a noble metal and an electrode base material. Eighth, in the first to seventh configurations of the multieletrode spark plug, a fourth firing portion of the semi-creeping spark discharge ground electrode in which a semi-creeping spark discharging gap is formed between the semi-creeping spark discharge ground electrode and the basal portion of the tip portion of the center electrode is configured by fixing a noble metal or a noble metal alloy such as platinum Pt, platinum-iridium Pt-Ir, platinum-nickel Pt-Ni, platinum-iridium-nickel Pt-Ir-Ni, or platinum-rhodium Pt-Rh or iridium-yttria Ir-Y 2 O 3 . Ninth, in the eighth configuration of the multielectrode spark plug, the fourth firing portion is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material or the noble metal alloy material and an electrode base material. Tenth, in the first to ninth configurations of the multielectrode spark plug, it is preferable that, when a thickness of the semi-creeping spark discharge ground electrode is indicated by T and a direction separating from the tip of the metallic shell is +, a distance A in an axial direction between a tip of the fourth firing portion of the semi-creeping spark discharge ground electrode and a tip of the front end portion of the insulator is -1.5 mm ≤ A ≤ T + 0.5 mm, and the aerial spark discharging gap G1, the semi-creeping spark discharging gap G2, and a shortest distance G3 between the fourth firing portion of the semi-creeping spark discharge ground electrode and the front end portion of the insulator satisfies a relation of G2 > G1 > G3. Eleventh, in the tenth configurations of the multielectrode spark plug, it is preferable that the shortest distance G3 between the fourth firing portion of the semi-creeping spark discharge ground electrode and the front end portion of the insulator is G3 ≤ 0.7 mm. Twelfth, in the first to eleventh configurations of the multielectrode spark plug, the aerial spark discharge ground electrode and/or the semi-creeping spark discharge ground electrode consists of four electrodes which are disposed at intervals of equal angles, two opposing electrodes of the four electrodes are the aerial spark discharge ground electrodes, and the other two opposing electrodes are the semi-creeping spark discharge ground electrodes. In the first to tenth configurations, among the plural ground electrodes, the semi-creeping spark discharge ground electrodes are used for burn-cleaning of conductive materials (carbon caused by unburned fuel) deposited on the surface of the insulator. Therefore, high resistance to fouling can be attained and smolder can be surely prevented from occurring. Since the aerial spark discharge ground electrode is disposed aside from the semi-creeping spark discharge ground electrode, it is possible to ensure ignitability when the fouling state is recovered. In the second, fourth, sixth and eighth configurations, the use of a noble metal or a noble metal alloy which has a high melting point can reduce spark wear of the electrodes, and the durability is improved. As a method of fixing such a noble metal or a noble metal alloy, for example, a method using resistance welding, or a method in which only the boundary between an electrode base material and a member of a noble metal or a noble metal alloy placed on the base material is irradiated with a laser beam may be employed. In the third, fifth, seventh and ninth configurations, since the firing portion is formed by an alloy layer which is obtained by melting and then solidifying a noble metal material or a noble metal alloy material and an electrode base material, the alloy layer can be firmly fixed and hence the durability can be enhanced. In order to realize this configuration, for example, it is preferable to employ a method in which a member of a noble metal or a noble metal alloy is placed on an electrode base material, a part of the electrode base material is melted at the same time when the member of a noble metal, a noble metal alloy or a material containing a noble metal is completely melted by irradiation of a laser beam, whereby the member and the material are mixed with each other, and then the molten solidifies. In the tenth configuration, an aerial spark discharge and a semi-creeping spark discharge are adequately produced and ignitability and the function of burn-cleaning are optimized. The semi-creeping spark discharging gap G2 is larger than the aerial spark discharging gap G1. Therefore, under the state where fouling materials such as carbon are not deposited on the front end portion of the insulator, a spark is easily produced across the aerial spark discharging gap G1. The aerial spark discharging gap G1 is larger than the shortest distance G3 between the fourth firing portion of the semi-creeping spark discharge ground electrode and the front end portion of the insulator. Therefore, under the state where fouling materials such as carbon are deposited on the front end portion of the insulator, a spark is easily produced across the semi-creeping spark discharging gap G2. On the other hand, as the tip of the firing portion of the semi-creeping spark discharge ground electrode is located at a position nearer the tip of the metallic shell, burn-cleaning more hardly occurs, because a spark is prevented from being produced across the semi-creeping spark discharging gap G2, until a smolder state of a high degree in which deposited carbon caused by unburned fuel reaches the basal portion of the front portion of the insulator arises. In other words, when the tip of the firing portion of the semi-creeping spark discharge ground electrode is positioned at a higher position, a spark is produced across the semi-creeping spark discharging gap G2 even in an initial stage of carbon deposition (i.e., slight smolder), and hence the carbon is easily subjected to burn-cleaning. Consequently, it is preferable to position the tip of the firing portion of the semi-creeping spark discharge ground electrode in such a manner that, when the direction separating from the tip portion of the metallic shell is +, the distance in an axial direction between the tip of the firing portion and that of the front end portion of the insulator is -1.5 mm or larger. As the tip of the firing portion of the semi-creeping spark discharge ground electrode is positioned more remote from the tip of the metallic shell than the tip of the front end portion of the insulator, the shortest distance G3 between the firing portion of the semi-creeping spark discharge ground electrode and the front end portion of the insulator is larger. In this case, as the distance G3 is made larger, the voltage required for a spark discharge is higher. In the eleventh configuration, the voltage required for a spark discharge across the semi-creeping spark discharge gap can be prevented from being raised. When the shortest distance G3 between the firing portion of the semi-creeping spark discharge ground electrode and the front end portion of the insulator is larger than 0.7 mm, the discharge machining (hereinafter, also referred to as channeling) of the front end portion of the insulator may proceed with the result that the insulator is easily broken. Therefore, it is preferable to set the shortest distance G3 between the firing portion of the semi-creeping spark discharge ground electrode and the front end portion of the insulator, to be not larger than 0.7 mm. However, if G3 the distance G3 of the aerial discharge of the semi-creeping spark discharge for cleaning-up carbon is made too small, thereby causing the difficulty of ignition of the engine. In the view point of the durability and burn-cleaning, as the twelfth configuration, it is preferable to configure the aerial spark discharge ground electrode as two opposing electrodes, and the semi-creeping spark discharge ground electrode as two opposing electrodes. As shown in Fig. 6, as the number of the semi-creeping spark discharge ground electrodes is increased, the area where carbon caused by unburned fuel is removed away by a semi-creeping spark discharge is widened, but the number of the aerial spark discharge ground electrodes is reduced, so that the occurrence rate of the aerial discharges in each electrode is increased, thereby impairing the durability. In order to balance the burn-cleaning of unburned fuel with the durability of the ground electrodes, it is preferable to configure the ground electrodes by two aerial spark discharge ground electrodes and two semi-creeping spark discharge ground electrodes. As shown in Fig. 7, on the other hand, with respect to carbon caused by unburned fuel and deposited on the tip 23 of the front end portion 21 of the insulator 2, the total area of a burn-cleaning zone (1) 24 and a burn-cleaning zone (2) 25 which are due to spark discharges is widest when semi-creeping spark discharge ground electrodes 42 and 44 are opposed to each other. This is because, when the semi-creeping spark discharge ground electrodes 42 and 44 have a mutual positional relationship of 180°, the burn-cleaning zone (1) 24 of the tip 23 of the front end portion 21 of the insulator 2 which is formed by the semi-creeping spark discharge ground electrode 42 and the center electrode, and the burn-cleaning zone (2) 25 of the tip 23 of the front end portion 21 of the insulator 2 which is formed by the semi-creeping spark discharge ground electrode 44 and the center electrode 3 overlap each other in a wide range. Therefore, the configuration in which the semi-creeping spark discharge ground electrodes are two electrodes opposing via the center electrode is most excellent in ignitability and fouling recovery property, and has high practicality. Next, preferred embodiments according to the present invention will be described as follows referring to the accompanying drawings. Figs. 1 and 2 show the multielectrode spark plug according to the present invention. The multielectrode spark plug includes a metallic shell 1, and an insulator 2 having an axial bore 22. The insulator 2 is fitted to the metallic shell 1 in a state where the front end portion of the insulator 2 is protruded from the tip 11 of the metallic shell 1. A center electrode 3 is fitted to the axial bore 22 of the insulator 2 in a state where the tip portion 31 of the center electrode 3 is protruded from the tip 23 of the front end portion 21 of the insulator 2. Four ground electrodes 41 to 44 are welded at intervals of equal angles to the tip 11 of the metallic shell 1. The tip portion 4A of each of the ground electrodes is bent toward the center electrode 3, and the front end face 4B and the tip portion 31 of the center electrode 3 forms a spark discharging gap. Among the ground electrodes 41 to 44, two opposing electrodes are aerial spark discharge ground electrodes 41 and 43 which form aerial spark discharging gaps G1 with the side face of the end face 32 of the tip portion 31 of the center electrode 3. The other two electrodes are positioned in the side of the front end portion 21 of the insulator 2, and configure semi-creeping spark discharge ground electrodes 42 and 44. The semi-creeping spark discharge ground electrodes 42 and 44 and the basal portion 33 of the tip portion 31 of the center electrode 3 form semi-creeping spark discharge gaps G2 each of which consists of: a creeping face extending along the front end portion 21 of the insulator 2; and the shortest distance G3 between the front end portion 21 and the front end face 4B. In the multielectrode spark plug shown in Fig. 1, the side face (firing portion) of the tip portion 31 of the center electrode 3 forming the aerial spark discharging gaps G1 is configured by an alloy layer 5 which is obtained by melting and solidifying a noble metal, a noble metal alloy or a material containing a noble metal such as platinum Pt, platinum-iridium Pt-Ir, platinum-nickel Pt-Ni, platinum-iridium-nickel iridium-nickel Pt-Ir-Ni, platinum-rhodium Pt-Rh or Ir-Y 2 O 3 . Specifically, a noble metal or a noble metal alloy is irradiated with a laser beam to melt the noble metal or the noble metal alloy and an electrode base material, and the molten materials are solidified, thereby forming the alloy layer 5. According to this configuration, spark wear of the firing portion of the center electrode is reduced, so that the life of the spark plug is prolonged. In view of the workability, preferably, the noble metal to be used is platinum Pt. As shown in Fig. 3, the firing portion of the tip portion 31 of the center electrode 3 may be configured by resistance-welding a noble metal 51 such as platinum Pt, platinum-iridium Pt-Ir, platinum-nickel Pt-Ni, platinum-iridium-nickel iridium-nickel Pt-Ir-Ni platinum-rhodium Pt-Rh or Ir-Y 2 O 3 . Alternatively, the firing portion may be configured by forming the noble metal 51 partly or only in the side faces of the center electrode 3 which oppose the aerial spark discharge ground electrodes 41 and 43, respectively. In view of the workability, preferably, platinum Pt is used as the noble metal 51. Hereinafter, the function of the spark plug will be described. The spark plug is attached to an internal combustion engine such as a gasoline engine by means of a threaded portion formed on the metallic shell 1 so that the center electrode 3 and the ground electrodes 41 to 44 are located in a combustion chamber, and used as a source for igniting a fuel-air mixture supplied to the combustion chamber. When the engine is operated intermittently or continuously for a long term under a light load condition, for example, materials such as carbon are easily deposited on the front end portion of the insulator of the spark plug. Deposition of a conductive material such as carbon on the insulator 2 lowers the surface electrical resistance of the insulator. When the discharge voltage of the semi-creeping spark discharging gap G2 is higher than that of the aerial spark discharging gap G1, a spark takes place across the semi-creeping spark discharging gap G2 and carbon caused by unburned fuel is burned off. Alternatively, as shown in Fig. 4, a three-electrode spark plug may be configured in which one electrode is an aerial spark discharge ground electrode 45 which forms an aerial spark discharge gap with the outer periphery of the tip of the center electrode 3, and the other two electrodes are semi-creeping spark discharge ground electrodes 46 and 47. The ground electrodes are disposed at equal intervals of about 120° A part of the firing face of each semi-creeping spark discharge ground electrode is positioned on the extension of the front end face of the tip portion of the insulator. The firing portion of the center electrode 3 which forms semi-creeping spark discharge gaps with the semi-creeping spark discharge ground electrodes 46 and 47 may be configured by conducting laser-beam welding of a noble metal or a noble metal alloy such as platinum Pt, platinum-iridium Pt-Ir, platinum-nickel Pt-Ni, platinum-iridium-nickel Pt-Ir-Ni, or platinum-rhodium Pt-Rh and then melting and solidification, thereby forming an alloy layer 5. Also the outer periphery of the tip portion of the center electrode 3 is subjected to laser-beam welding of a noble metal and then melting and solidification, thereby forming another alloy layer 5. The alloy layers 5 have functions of reducing spark wear of the respective firing faces so that the life of the spark plug is prolonged, and lowering the quenching action so that ignitability is improved. Alternatively, as shown in Fig. 5, a four-electrode spark plug may be configured in which one electrode is an aerial spark discharge ground electrode 45 which forms an aerial spark discharge gap with the outer periphery of the tip of the center electrode 3, and the other three electrodes are semi-creeping spark discharge ground electrodes 46, 47, and 48. The ground electrodes are disposed at equal intervals of about 90°. Examples As an Example (1) according to the present invention, in the spark plug of Fig. 1 having four ground electrodes, the distance of the aerial spark discharging gaps G1 was set to be 1.0 mm, the distance of the shortest distance G3 between the semi-creeping spark discharge ground electrodes and the front end portion of the insulator was set to be 0.7 mm, the diameter of the front end of the insulator was set to be 4.7 mm, the thickness T of the semi-creeping spark discharge ground electrodes was set to be 1.6 mm, and a distance A in an axial direction between the tips of the firing portions of the semi-creeping spark discharge ground electrodes and the tip of the front end portion of the insulator was set to be 0.5 mm. For a comparison, also the comparative examples of the spark plugs were produced. As the comparative examples, a spark plug 400 as a comparative example (2) having three ground electrodes 402 (see Fig. 12) was produced so that the distance of the spark discharging gaps between a center electrode 401 and each of ground electrodes 402 was set to be 1.0 mm, the diameter of the front end of the insulator 403 was set to be 4.7 mm, the thickness T of the ground electrodes 402 was set to be 1.6 mm, and a distance A in an axial direction between the tips of the firing portions of the ground electrodes 402 and the tip of the front end portion of the insulator 403 was set to be 3.8 mm, and a spark plug 500 as a comparative example (3) having three ground electrodes (see Fig. 13) was produced so that the distance of the spark discharging gaps between a center electrode 501 and each of ground electrodes 502 was set to be 1.0 mm, the diameter of the front end of the insulator 503 was set to be 4.7 mm, the thickness T of the ground electrodes 502 was set to be 1.6 mm, the shortest distance between an auxiliary electrode 504 formed by bending the tip of the front end of the insulator 503 was set to be 0.5 mm, and a distance A in an axial direction between the tips of the firing portions of the ground electrodes 502 and the tip of the front end portion of the insulator 503 was set to be 1.5 mm. These spark plugs were mounted on a test car and smolder fouling tests were conducted with performing the running pattern (according to JIS D1606) shown in Fig. 9 as one cycle. Fig. 8 shows results of the tests. It will be seen that, as compared with the spark plugs of the comparative examples, the spark plugs according to the present invention are lower in reduction of the insulation resistance and superior in resistance to fouling. Spark plugs of Fig. 1 having four ground electrodes were produced, in which the distance of the aerial spark discharging gaps G1 was set to be 1.0 mm, that of the shortest distance G3 between the semi-creeping spark discharge ground electrodes and the front end portion of the insulator was set to be 0.5 mm, the diameter of the front end of the insulator was set to be 4.7 mm, the thickness T of the semi-creeping spark discharge ground electrodes was set to be 1.6 mm, and the distance A in an axial direction between the tips of the firing portions of the semi-creeping spark discharge ground electrodes and the tip of the front end portion of the insulator was variously changed. These spark plugs were subjected to smolder tests with using a four-cycle single-cylinder engine of 270 cc, and their performances were evaluated. In the smolder tests, one cycle consists of 1,800 rpm × 3 minutes and engine stop × 1 minute (see Fig. 11). Fig. 10 shows results of the tests. It will be seen that, when the distance A is within the range of -1.5 mm ≤ A ≤ T + 0.5 mm, the insulation resistance of 1 MΩ or higher can be attained in 20 cycles or more and burn-cleaning can be efficiently conducted.

Claims: A multielectrode spark plug comprising: a metallic shell (1); an insulator (2) having an axial bore (22), said insulator (2) being fitted to said metallic shell (1) in a state where a front end portion of said insulator (2) is protruded from a tip (11) of said metallic shell (1); a center electrode (3) which is fitted to said axial bore (22) in a state where a tip portion (31) of said center electrode (3) is protruded from said front end portion of said insulator (2); and a plurality of ground electrodes (41,42,43,44) secured to said tip (11) of said metallic shell (1), a tip portion (4A) of each of said ground electrodes (41,42,43,44) being bent toward said center electrode (3) to form a spark discharge gap (G) with said tip portion of said center electrode (3), said plurality of ground electrodes including: a semi-creeping spark discharge ground electrode (42,44) said tip portion (4A) of said semi-creeping spark discharge ground electrode being positioned in a side of said tip portion (31) of said center electrode to form a semi-creeping spark discharging gap (G2) with a basal portion (33) of said tip portion of said center electrode, a part of said semi-creeping spark discharging gap (G3) elongating along a tip end face of said front end portion of said insulator; and an aerial spark discharge ground electrode (41,43) characterized in that said aerial spark discharge ground electrode (41,43) forms an aerial spark discharging gap with a side face of said tip portion of said center electrode. A multielectrode spark plug according to claim 1, wherein said center electrode (3) includes a first firing portion (5) for forming said aerial spark discharging gap between said aerial spark discharge ground electrode (41,43) and said side face of said tip portion of said center electrode (3), said first firing (5) portion is comprised of a noble metal. A multielectrode spark plug according to claim 2, wherein said first firing portion (5) is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material, the noble metal alloy material or the material containing a noble metal and an electrode base material. A multielectrode spark plug according to claim 1, wherein said center electrode (3) includes a second firing portion for forming said semi-creeping spark discharging gap (62) between said semi-creeping spark discharge ground electrode (42,44) and said basal portion (33) of said tip portion of said center electrode (3), said second firing portion is comprised of a noble metal, a noble metal alloy or a material containing a noble metal. A multielectrode spark plug according to claim 4, wherein said second firing portion is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material, the noble metal alloy material or the material containing a noble metal and an electrode base material. A multielectrode spark plug according to claim 1, wherein said aerial spark discharge ground electrode includes a third firing portion for forming an aerial spark discharging gap between said aerial spark discharge ground electrode (42,22) and said side facing of said tip portion of said center electrode (3) said third firing portion of said center electrode (3), said third firing portion is comprised of a noble metal, a noble metal alloy or a material containing a noble metal. A multielectrode spark plug according to claim 6, wherein said third firing portion is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material, the noble metal alloy material or the material containing a noble metal and an electrode base material. A multielectrode spark plug according to claim 1, wherein said semi-creeping spark discharge ground electrode (42,44) includes a fourth firing portion for forming a semi-creeping spark discharging gap between said semi-creeping spark discharge ground electrode (42,44) and said basal portion (33) of said tip portion of said center electrode, said fourth firing portion is comprised of a noble metal, a noble metal alloy or a material containing a noble metal. A multielectrode spark plug according to claim 8, wherein said fourth firing portion is formed by an alloy layer which is obtained by melting and then solidifying the noble metal material, the noble metal alloy material or the material containing a noble metal and an electrode base material. A multielectrode spark plug according to claim 8, wherein, when a thickness of said semi-creeping spark discharge ground electrode (42,44) is indicated by T and a direction separating from said tip of said metallic shell (1) is +, a distance A in an axial direction between a tip of said fourth firing portion of said semi-creeping spark discharge ground electrode (42,44) and a tip of said front end portion of said insulator (23) is -1.5 mm ≤ A ≤ T + 0.5 mm, and said aerial spark discharging gap G1, said semi-creeping spark discharging gap G2, and a shortest distance G3 between said fourth firing portion of said semi-creeping spark discharge ground electrode and said front end portion of said insulator satisfies a relation of G2 > G1 > G3. A multielectrode spark plug according to claim 10, wherein said shortest distance G3 between said fourth firing portion of said semi-creeping spark discharge ground electrode (42,44) and said front end portion of said insulator is G3 ≤ 0.7 mm. A multielectrode spark plug according to claim 1, wherein said plurality of ground electrodes are disposed at intervals of equal angles. A multielectrode spark plug according to claim 1 to 12, comprising four ground electrodes (41,42,43,44) which are disposed at intervals of equal angles, wherein two opposing electrodes of said four ground electrodes are said aerial spark discharge ground electrodes (42,44), and the other two opposing electrodes are said semi-creeping spark discharge ground electrodes (41,43).

Inventors: KAGAWA JUNICHI, KAGAWA, JUNICHI, KAGAWA, JUNICHI, C/O NGK SPARK PLUG CO., LTD.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1996
Main classification: H01T13
IPC codes: H01T13