Spark plug for internal combustion engine

Abstract: A spark plug for an internal combustion engine is disclosed. The spark plug (1) has a metal shell (4) and an insulator (2). The shell defines a through hole and a shoulder seat (18) and has threads (6) for mount­ ing the spark plug (1) on the engine. The insulator is disposed inside the through hole, fixed on the shoulder seat and holds a center electrode (3) therein. The in­ sulator also has a leg portion (7) extending from the shoulder seat into a combustion chamber of the engine when the spark plug is mounted on the engine. A semi­ conductor material having a resistance of 5 x 10² - 5 x 10⁴ MΩ/mm is coated or baked in the form of a band (8) on the peripheral surface of a basal part (14) of the leg portion (7) of the insulator (2). Preferably, at least the basal part (14) of the leg portion (7), including the band (8), is covered with a water-­ repellant insulating coating (9′). An inner wall (15) of the metal shell (4) is desirably coated with a water-repellant material (9) at an area facing at least the peripheral surface of the basal part (14) of the leg portion (7) of the insulator (2).

Description: The present invention relates to a spark plug for an internal combustion engine according to the first part of claim 1, especially to a spark plug assuring fail-free ignition over a long period of time. Since an air-fuel mixture is enriched from the starting of an internal combustion engine until the completion of warming-up, carbon formed as a result of combustion of the fuel may deposit in a large quantity together with the liquid fuel on a leg portion of an insulator of a conventional spark plug. The carbon thus deposited on the leg portion of the insulator may then be oriented under electric forces (impressed voltages), whereby a path of carbon may extend from a basal part of the leg portion, i.e., a ground side toward a free end of the leg portion and the insulation resistance of the insulator may hence be lowered. This may cause engine troubles. With a view toward removing carbon deposited as described above and also promoting the self-cleaning action of the insulator itself, the leg portion of the insulator is made longer to prevent the insulation resistance from being lowered, and/or a highly water-repellant material such as silicone oil is coated to the surface of the insulator and/or the inner wall of the metal shell so as to avoid the formation of water which induces the deposition of carbon. When the leg portion of the insulator is made longer to facilitate the rise of the surface temperature of the insulator, the insulator can maintain insulation resistance against the deposition of carbon due to combustion of the fuel. The thus-lengthened leg portion is therefore effective for improving the smear resistance. However, the leg portion thus lengthened tends to induce pre-ignition. A limitation is also imposed on the length of the leg portion from the standpoint of heat resistance. In the case of a spark plug in which the surface of an insulator and the inner wall of a metal shell are coated with a highly water-repellant material such as silicone oil, the highly-water repellant material such as silicone oil is caused to gradually evaporate and its effect for the elimination of water and the like is lost in a short time when repeatedly exposed to hot combustion gas. JP-A-53 43142 discloses a sintered film of semi-conductor substance provided on the end face and circumference of the ignition part of a porcelain insulating body to eliminate uneveness of discharging voltage. DE-C-3 152 877 discloses a spark plug the porcelain insulator of which is provided with a coating of a solution of fluor oil comprising elemental fluor and/or elemental silicon for avoiding a contamination of the insulator. An object of the present invention is to improve the above-described drawbacks of the conventional spark plugs, and specifically to prevent carbon, which is formed upon combustion, from depositing on a leg portion of an insulator of a spark plug, thereby avoiding the reduction of insulation resistance and hence maintaining fail-free ignition of the spar plug. This object is achieved by the features in claim 1. The spark plug has a metal shell and an insulator. The metal shell defines a through hole and a shoulder seat, and has threads for mounting the spark plug on the internal combustion engine. The insulator is disposed inside the through hole of the metal shell and is fixed on the shoulder seat, and holds a center electrode therein. The insulator has a leg portion extending from the shoulder seat into a combustion chamber of the internal combustion engine when the spark plug is mounted on the internal combustion engine. A semiconductor material having a resistance of 5 x 10² - 5 x 10⁴ MΩ/mm is applied, for example, coated or baked in the form of a band on the peripheral surface of a basal part of the leg portion of the insulator. Preferably, the band of the semiconductor material may be imparted with water repellency. The leg portion of the insulator, including the band, may be covered by a water-repellant insulating coating. The inner wall of the metal shell may be coated with a water-repellant material at an area facing at least the band on the leg portion of the insulator. The band of the semiconductor material is effective for preventing the orientation of carbon even when carbon deposits together with water and the like on the surface of the insulator. The formation of water can be minimized by making the band water-repellant and/or by coating the water-repellant material on the inner wall of the metal shell, so that the resistance of the surface of the insulator to carbon smear can be improved further. It is therefore possible to avoid the reduction of insulation resistance. The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which: FIG. 1 is a partly cross-sectional view of a spark plug according to a first embodiment of the present invention, which is suited for use in an internal combustion engine; FIG. 2 is an enlarged, partly cross-sectional, fragmentary view of the spark plug according to the first embodiment; FIG. 3 is an enlarged, partly cross-sectional, fragmentary view of a spark plug according to a third embodiment of the present invention; FIG. 4 is an enlarged, partly cross-sectional, fragmentary view of a spark plug according to a fifth embodiment of the present invention; FIG. 5 is an enlarged, partly cross-sectional, fragmentary view of a spark plug according to a sixth embodiment of the present invention; FIG. 6 is an enlarged, partly cross-sectional, fragmentary view of a spark plug according to a seventh embodiment of the present invention; FIG. 7 is an enlarged, partly cross-sectional, fragmentary view of a spark plug according to an eighth embodiment of the present invention; FIG. 8 is an enlarged, partly cross-sectional, fragmentary view of a spark plug according to a ninth embodiment of the present invention; and FIGS. 9 and 10 diagrammatically illustrate results of smear tests. In FIG. 1, numeral 1 indicates the spark plug according to the first embodiment of the present invention. This spark plug 1 is composed of an insulator 2 having a center electrode 3 at a free end thereof and a metal shell 4 having a ground electrode 5 located at a position opposite to the center electrode 3 and threads 6 employed upon mounting the spark plug on an unillustrated internal combustion engine. Numeral 11 indicates a terminal electrode, which is sealed together with a resistor 13 within an axial cavity 10 formed in the insulator 2 with a glass sealing interposed between the terminal electrode 11 and the resistor 13. The insulator 2 is fixed on a shoulder seat 18 formed in a through hole of the metal shell 4. As is shown in FIG. 2, a band 7 of a semiconductor material having a resistance of 5 x 10² - 5 x 10⁴ MΩ/mm is coated or baked on the insulator 2 holding the center electrode 3 at the free end thereof, especially on the surface of a basal part 14 of a leg portion 7 of the insulator 2, said leg portion 7 extending from the shoulder seat 18 into a combustion chamber when the spark plug 1 is mounted on the unillustrated internal combustion engine (the first embodiment). This band 8 of the semiconductor material has been formed by mixing alumina or silica as a principal component with 0.1-5% of the semiconductor material [TiO₂, Nb₂O₅, ZrO₂, BaTiO₃, IrO₂, or a ferrite represented by MO·Fe₂O₃ (M: Mn, Mg, Ni, Co, Cu, Zn or the like)], coating the mixture, drying the thus-coated mixture in the air for 1 hour and then baking it at 100-300°C. When carbon is formed by combustion and is about to be deposit together with fuel, water or the like on the surface of the insulator 2, the provision of the band 8 of the semiconductor material having the resistance of 5 x 10² - 5 x 10⁴ MΩ/mm on the insulator 2, in particular, on the surface of the basal part 14 of the leg portion 7 of the insulator 2 has made it possible to prevent the carbon, water and the like from being electrically oriented by impressed voltages. It is hence possible to avoid the reduction of insulation resistance, which would otherwise occur due to deposition of carbon on the surface of the insulator 2. The smear preventing effect of the band 8 of the semiconductor material coated or baked on the surface of the insulator 2 can be brought about when its axial length is not greater than one third of the axial length (ℓ) of the leg portion 7. The band 8 cannot exhibit smear preventing effect if it is longer than the above upper limit (the second embodiment). The band 8 of the semiconductor material can be formed on the basal part 14 other than a root part 17 by coating or baking as shown in FIG. 3 (the third embodiment). This form of band 8 can exhibit still better smear resistance because a path of carbon extended from the root part 17 is interrupted by the band 8 and the band 8 also serves to retard the extension of a path of carbon from the band 8 toward the free end of the leg portion 7 of the insulator 2. The band 8 made of the semiconductor material coated or baked on the insulator 2, especially, on the surface of the basal part 14 of the leg portion 7 of the insulator 2, said leg portion 7 extending from the shoulder seat 18 into a combustion chamber when the spark plug 1 is mounted on an internal combustion engine can be imparted with water repellency by coating a mixture of silicone or a silicone varnish and boron nitride, said mixture containing 2-5% of carbon black, drying the mixture and then baking the thus-dried mixture or by using TEFLON (trade mark) or the like as a base material to improve the heat resistance. The water-repellant band thus formed can prevent fuel or water or the like, which is formed as a result of combustion, from depositing on the surface of the insulator 2, so that more effective prevention of smear due to deposition of carbon and the like is feasible (the fourth embodiment). As is shown in FIG. 4, it is possible to prevent water from depositing on the surface of the insulator 2 and hence to eliminate one of causes for the deposition of carbon by coating a water-repellant material 9 on an inner wall 15 of the metal shell 4 at an area facing the band 8 coated or baked on the surface of the basal part 14 of the insulator 2 (the fifth embodiment). In FIG. 5, the band 8 is formed on the surface of the basal part 14 of the leg portion 7 of the insulator 2 by coating or baking, and the axial length s of a shelf portion 16 of the metal shell, said shelf portion 16 forming at an upper surface thereof the shoulder seat 18 for the insulator 2, is made longer in the axial direction. This can reduce the intrusion of carbon to the basal part 14 of the insulator 2, whereby the smear resistance can be improved further (the sixth embodiment). In this sixth embodiment, the water-repellant material 9 can also be coated on the inner wall 15 of the metal shell 4 as in the fifth embodiment. Still better smear preventing effect can be obtained in this case. As is shown in Table 1, predelivery smear tests in which a drive pattern consisting of a vehicle speed of 35 km/hr x 60 sec, an idling period of 20 sec and a vehicle speed of 15 km/hr x 40 sec was repeated as a single cycle were conducted at a low temperature of 10°C on the spark plugs of the invention examples, those of comparative examples and a conventional example, using a commercial car equipped with a 4-cycle, 2,000 cc internal combustion engine. The effects of the spark plugs of the invention examples were demonstrated as shown in FIG. 9. As a result of the smear tests, the following finding was obtained. The insulation resistance of the conventional spark plug provided with no semiconductor band dropped abruptly from the third cycle, and decreased to 1 MΩ and misfired in the sixth cycle. In contrast, the insulation resistance dropped only slowly in the case of the spark plugs of the first, third, fourth, fifth and sixth embodiments of the present invention, thereby demonstrating good smear resistance. As the resistance of the semiconductor material, the range of 5 x 10² - 5 x 10⁴ MΩ/mm is particularly preferred. As is readily understood from the comparative example, 100 MΩ/mm are too low to exhibit sufficient smear resistance because the insulation resistance gradually drops as more cycles are performed. Further, as is indicated by the comparative product of the second embodiment, the insulation resistance drops sharply and the band 8 is not effective for the prevention of smear if the length t of the band 8 is about 50% of the length ℓ of the leg portion 7. The suitable band length t is therefore not greater than one third of the length ℓ of the leg portion 7. The band 8 is more effective for the prevention of smear when provided in the form of a ring within the above range t on the basal part 14 other than the root part 17 as demonstrated by the third embodiment. In addition, it is more effective to impart water repellency to the band 8 as demonstrated by the fourth embodiment. Still better smear resistance can be obtained when water repellency is imparted to the inner wall of the metal shell 4 as in the fifth embodiment or the axial length s of the shelf portion 16 of the metal shell 4 is made longer as in the sixth embodiment. Especially, the sixth embodiment is easy to manufacture and is hence useful because it is only necessary to change the machining dimensions of the shelf portion 16. In the seventh embodiment illustrated in FIG. 6, the band 8 made of the semiconductor material whose resistance is 5 x 10² - 5 x 10⁴ MΩ/mm is provided on an upper peripheral surface of the leg portion 7 of the insulator 2 having the center electrode 3 at the free end thereof and the band 8 and basal part 17 are both covered by a water-repellant insulating coating 9′. When carbon is formed and is about to deposit together with raw gas, water and/or the like on the surface of the insulator 2, the deposition of water is hard to occur because of the water-repellant insulating coating 9′. Even if water deposits together with carbon, the carbon deposited together with the water is prevented from being electrically oriented by impressed voltages because the band 8 of the semiconductor material having the resistance of 5 x 10² - 5 x 10⁴ MΩ/mm is provided on the upper peripheral surface of the insulator 2. As a result, it is possible to prevent the reduction of the insulation resistance which would be caused by the deposition of carbon (carbon smear) on the surface of the insulator 2. This band (8) of the semiconductor material has been formed by mixing alumina or silica as a principal component with 0.1-5% of the semiconductor material [TiO₂, Nb₂O₅, ZrO₂, BaTiO₃, IrO₂, or a ferrite represented by MO·Fe₂O₃ (M: Mn, Mg, Ni, Co, Cu, Zn or the like)], coating the mixture, drying the thus-coated mixture in the air for 1 hour and then baking it at 100-300°C or by coating a mixture of at least two oxides selected from lanthanum oxide, chromium oxide, cupric oxide, ferrous oxide and ferric oxide and then baking the thus-coated mixture at 1,250-1,370°C for 10 minutes. The insulating coating 9′ covering the surface of the band 8 has been formed by applying a coating formulation of silicone or a silicone varnish and fine particles of boron nitride dispersed therein and then drying and solidifying the thus-applied coating formulation at ambient temperature and moisture for 1-12 hours. It is also possible to prevent the intrusion of carbon, water and the like to the basal part 14 of the leg portion 7 of the insulator 2 and hence to improve the smear resistance by making the axial length s of the shelf portion 16 of the metal shell 4, said shelf portion 16 supporting the insulator 2 thereon, longer than the axial length of the band 8 made of the semiconductor material as shown in FIG. 7 (the eighth embodiment). It is possible to completely eliminate water and the like, which promote the deposition of carbon, by coating a water-repellant material to the surface of the inner wall 15 of the metal shell 4 as shown in FIG. 4 (the ninth embodiment). As is shown in Table 2, predelivery smear tests in which a drive pattern consisting of a vehicle speed of 35 km/hr x 60 sec, an idling period of 20 sec and a vehicle speed of 15 km/hr x 40 sec was repeated as a single cycle were conducted at a low temperature of 10°C on the spark plugs of the invention examples, that of the comparative example and a conventional example, using a commercial car equipped with a 4-cycle, 2,000 cc internal combustion engine. The effects of the spark plugs of the invention examples were demonstrated as shown in FIG. 10. As a result of the smear tests, the following finding was obtained. The insulation resistance of the conventional spark plug provided with neither a semiconductor band nor a water-repellant insulating coating dropped abruptly from the third cycle, and decreased to 1 MΩ and misfired in the sixth cycle. In contrast, the insulation resistance dropped only slowly in the case of the spark plugs of the seventh, eighth and ninth embodiments of the present invention, thereby demonstrating good smear resistance. As the resistance of the semiconductor material, the range of 5 x 10² - 5 x 10⁴ MΩ/mm is particularly preferred. As is readily understood from the comparative example, the insulation resistance of the 100 MΩ/mm band imparted with water repellency dropped as more cycles were performed. It cannot therefore exhibit sufficient smear resistance. When the axial dimension s of the shelf portion 16 of the metal shell 4 is made longer than the axial length of the band 8, the intrusion of carbon to the basal part 14 of the leg portion 7 of the insulator 2 is prevented, whereby the deposit of carbon is minimized and the smear resistance can be improved further. Further, the coating of the inner wall 15 of the metal shell 4 with the water-repellant material is effective for preventing water and the like from intruding to the basal part 14 of the leg portion 7, so that the smear resistance can be improved further.

Claims: A spark plug for an internal combustion engine, said spark plug having a metal shell (4) and an insulator (2), said metal shell defining a through hole and a shoulder seat (18) and having threads (6) for mounting the spark plug on the internal combustion engine, said insulator being disposed inside the through hole of the metal shell, fixed on the shoulder seat and holding a center electrode (3) therein, and said insulator having a leg portion (7) extending from the shoulder seat into a combustion chamber of the internal combustion engine when the spark plug is mounted on the internal combustion engine, characterized in that a semiconductor material having a resistance of 5 x 10² - 5 x 10⁴ MΩ/mm is applied in the form of a band (8) on the peripheral surface of a basal part (14) of the leg portion (7) of the insulator (2). The spark plug according to claim 1, wherein the basal part (14) is not longer than one third of the overall length of the leg portion (7). The spark plug according to claim 1, wherein the band (8) has water repellency. The spark plug according to any one of the preceding claims, wherein an inner wall (15) of the metal shell (4) is coated with a water-repellant material (9) at an area facing at least the peripheral surface of the basal part (14) of the leg portion (7) of the insulator (2). The spark plug according to any of the preceding claims, wherein the shoulder seat (18) is formed on an upper surface of a shelf portion (16) of the metal shell (4) and the shelf portion (16) has a substantial axial length. A spark plug according to claim 1, wherein at least the basal part (14) of the leg portion (7), including the band (8), is covered with a water-repellant insulating coating (9'). The spark plug according to any one of the preceeding claims, wherein the semiconductor material is coated or baked. The spark plug according to claim 6 or 7, wherein the shoulder seat (18) is formed on an upper surface of a shelf portion (16) of the metal shell (4) and the shelf portion (16) has an axial length longer than that of the band (8). The spark plug according to any one of claims 6-8, wherein an inner wall (15) of the metal shell (4) is coated with a water-repellant material (9).

Inventors: GOTO EIGO C O NGK SPARK PLUG C, HARADA AKIHISA C O NGK SPARK P, MORIYA TORU C O NGK SPARK PLUG, SUGIMOTO MAKOTO C O NGK SPARK, TOYA AKIHIRO C O NGK SPARK PLU, YOSHIDA MITSUTAKA C O NGK SPAR, GOTO, EIGO, HARADA, AKIHISA, MORIYA, TORU, SUGIMOTO, MAKOTO, TOYA, AKIHIRO, YOSHIDA, MITSUTAKA, GOTO, EIGO, C/O NGK SPARK PLUG CO., LTD., HARADA, AKIHISA, C/O NGK SPARK PLUG CO., LTD., MORIYA, TORU, C/O NGK SPARK PLUG CO., LTD., SUGIMOTO, MAKOTO, C/O NGK SPARK PLUG CO., LTD., TOYA, AKIHIRO, C/O NGK SPARK PLUG CO., LTD., YOSHIDA, MITSUTAKA, C/O NGK SPARK PLUG CO., LTD.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO. LTD.
Application year: 1990
Main classification: H01T13
IPC codes: H01T13, F02B75