Ceramic insulator, its manufacture and spark plug incorporating it

Abstract: An insulator (2) for a spark plug (A) for an i.c. engine has an outer glaze (3) on its head (21) and trunk (22) for preventing reduction in the flashover voltage during use, the glaze having a Pb content calculated as PbO of less than 10 weight %.

Description: This invention relates to a spark plug for an internal combustion engine, incorporating a glazed ceramic insulator. Conventional spark plugs have an insulator surface coated with glaze in an area extending from the head portion to the trunk portion of the insulator in order to prevent flashover between the terminal electrode and the metallic shell. Since glost firing is carried out at a temperature between 900°C and 1100°C, the glaze has a Pb content equal to or greater than 20 weight % in terms of PbO (PbO contributes toward a decrease in the glost firing temperature). When a corona discharge occurs on the exposed portion of the insulator (the surface of the head portion not covered with the plug cap), the glaze discolors to red or reddish-yellow and the flashover voltage of the spark plug reduces. The cause of this discoloration and the accompanying reduction in flashover voltage has now been identified and attempts to remedy these problems have resulted in the present invention. Under ordinary operating conditions (city driving or open highway driving), there arises no problem from a spark plug having an insulator coated with glaze whose Pb content is rather high at 20 to 40 weight % in terms of PbO. However, during operation for long periods under extraordinary conditions, such as along a road having many uphill and downhill sections, a strong electric field is induced, mainly on the trunk portion of the insulator in the vicinity of the caulked portion of the metallic shell, resulting in the occurrence of a corona discharge. The energy of the corona discharge causes Pb contained in the glaze to convert into Pb 3 O 4 (red) or Pb 2 O 3 (reddish-yellow) and causes the insulation resistance of the insulator to decrease drastically. As a result, there is a reduction in the flashover voltage between the terminal electrode and the metallic shell, resulting in a higher likelihood of the occurrence of flashover. US-A-4120733 describes lead-free glazes for alumina bodies such as spark plug insulators, such glazes consisting essentially of 48 to 54% SiO 2 , from 7 to 11% Al 2 O 3 , from 16.5 to 20% B 2 O 3 , from 11 to 14% BaO, from 2 to 3% CaO, from 2 to 2.5% ZnO, from 4.5 to 5.5% Na 2 O, and from 0.4 to 1% K 2 O. Li 2 O and MgO to the extent of up to about 0.25% and 1%, respectively may also be present, and minor amounts of other impurities. The present invention seeks to solve the above flashover voltage problem involved in the conventional spark plug, and to provide a spark plug which prevents a reduction in the flashover voltage between the terminal connected to the terminal electrode and the metallic shell so as to attain a lower likelihood of the occurrence of flashover and which exhibits an improved insulation resistance at elevated temperature. The present invention therefore provides a spark plug comprising a cylindrical metallic shell having a ground electrode; an insulator having the outer surface of its head and trunk coated with glaze, fixed within the metallic shell; and a center electrode fixed in an axial bore formed in the insulator, and having a flashover voltage at 150°C ≥ 35kV, wherein the Pb content of the glaze is not more than 10 wt. % in terms of PbO and the glaze contains 49.1 - 64.5 wt. % of SiO 2 , 5.0 - 25.0 wt. % of B 2 O 3 and 10.5 - 45.9 wt. % of two or more oxides selected from Al 2 O 3 , Na 2 O, CaO, ZnO, Li 2 O and Bi 2 O 3 with the exception of glazes consisting essentially of 48 - 54 % SiO 2 , 16.5 - 20 % B 2 O 3 , 7 - 11 % Al 2 O 3 , 4.25 - 5.25 % Na 2 O, 2 - 3 % CaO, 2 - 2.5 % ZnO, 11 - 14 % BaO, 0.4 - 1% K 2 O, up to about 0.25 % Li 2 O and up to about 1 % MgO. Preferably, the glaze contains 49.1 - 64.5 weight % SiO 2 , 5.0 - 25.0 weight % B 2 O 3 , 0 - 10.1 weight % Al 2 O 3 , 0 - 7.4 weight % Na 2 O, 0 - 5.5 weight % CaO and 0 - 10.2 weight % ZnO. In the spark plug of the invention, the surface of the insulator is coated with glaze in the area extending from the head portion to the trunk portion of the insulator, and the Pb content of the glaze is equal to or less than 10 weight % in terms of PbO. Accordingly, even when a strong electric field is induced at the trunk portion of the insulator with the resultant occurrence of a corona discharge, there is little or no conversion of Pb into conductive substances, such as Pb 3 O 4 and Pb 2 O 3 . Accordingly, the flashover voltage between the terminal electrode and the metallic shell is less likely to decrease, thus preventing spark failure of the spark plug. The invention will now be described in greater detail by way of example only with reference to the accompanying drawings, in which Fig. 1 is a partially sectional view of a spark plug according to the present invention; Fig. 2 is a schematic diagram of test equipment for measuring the insulation resistance of a spark plug in a heated state; and Fig. 3 is a schematic diagram showing flashover test equipment. Referring first to Fig. 1, a spark plug A is composed of a cylindrical metallic shell 1 having a ground electrode 11 which projects from a tip end 12, an insulator 2 whose surface is coated with a glaze 3 and which is fixed within the metallic shell 1, and a center electrode 4 fixed within an axial bore 20 formed in the insulator 2. The spark plug A is attached for use to the cylinder head (not shown) of an internal combustion engine. The metallic shell 1 is formed of low-carbon steel and includes a threaded portion 13 on which external threads 131 are formed; a barrel portion 14 having a thin-walled rear end; and a hexagonal-portion 15 which may be accommodated in a plug wrench. A gasket 10 is disposed on the leading end of the barrel portion 14. 16 denotes packing, and 17 denotes an O ring. The substantially L-shaped ground electrode 11 is disposed such that its discharge surface 111 faces the end surface of the center electrode 4. The ground electrode 11 is formed of a nickel alloy and includes a copper core, which has good heat conductivity. The insulator 2 is formed by sintered alumina-based ceramic and includes a corrugated head portion 21, a trunk portion 22 which is located in a space formed by the hexagonal portion 15 and the barrel portion 14 of the metallic shell 1, and a leg portion 23 which is located in a space formed by the threaded portion 13 of the metallic shell 1. The glaze 3, whose Pb content is 0 to 10 weight % in terms of PbO, covers the surface of the insulator 2 in an area extending from the head portion 21 to the trunk portion 22, and in area corresponding to a base portion 231 of leg portion 23. The center electrode 4 is formed of a nickel alloy and includes a copper core, which has good heat conductivity, and is accommodated within the axial bore 20 such that the end portion thereof projects from the front end of the insulator 2 and the other end portion is sealed within the axial bore 20 through use of seal glass (not shown). The center electrode 4 is electrically connected to a terminal electrode 5 via the seal glass. The terminal electrode 5 is formed of low carbon steel. The terminal electrode 5 is glass-sealed within the axial bore 20 of the insulator 2, and a terminal portion 51 of the terminal electrode 5 projects from the end surface of the head portion 21 of the insulator 2. A plug cap (not shown) is capable of fitting onto the terminal portion 51 and the rear end section of the head portion 21. The insulator 2 of the spark plug A may be manufactured as follows: (1) At least two oxides selected from Al 2 O 3 , Na 2 O, CaO, ZnO, Li 2 O, Bi 2 O 3 , BaO, and PbO are added to a B 2 O 3 -SiO 2 base glass in predetermined proportions and then mixed. (2) The resultant mixture is placed into a crucible, and melted at a temperature of 1400°C within a furnace. (3) The molten mixture is water-quenched, and the resultant solid is wet-pulverized within the crucible. An organic binder is added to the resultant powder in an amount of about 2 weight %, to obtain a glaze slurry. (4) The glaze slurry is sprayed onto the surface of the insulator 2 to cover the area extending from the head portion 21 to the trunk portion 22, and the area corresponding the leg base portion 231. (5) The insulator 2 is fired for about 10 minutes at the glost firing temperature corresponding to the applied glaze (see Table 2). Subsequently, the insulator 2 undergoes visual inspection. Table 1 shows the composition of various glazes 3 and their respective glost firing temperatures. The Pb content of glazes Nos 1 to 3 and Nos 7 to 10 is not more than 10 weight % in terms of PbO. (6) The insulator 2 is held so that the leg portion 23 points downwards. The center electrode 4 is inserted into the axial bore 20 from the head portion 21. Then, a conductive powder glass for the glass seal and resistance powder are placed into the axial bore 20. Finally, the terminal electrode 5 is inserted into the axial bore 20. (7) The insulator 2 is then heated to a temperature of 800 to 950°C to melt the powder glass, and a downward force is applied to the terminal portion 51 of the terminal electrode 5. (8) The insulator 2 is cooled naturally so as to solidify the molten glass, thereby completing the glass seal. (9) The glass-sealed insulator 2 is fitted into the metallic portion shell 1. A thin-walled portion 18 of the metallic shell 1 is caulked, thereby fixedly attaching the insulator 2 into the metallic shell 1. The spark plug A is thereby completed. Table 2 shows test results for spark plugs which are respectively coated with glazes Nos 1 to 10 listed in Table 1. The test results include color observed immediately after glost firing, insulation resistance under heated conditions, color observed after engine test, flashover voltage, and evualation. The insulation resistance of the spark plug A under heated condition appearing Table 2 was measured in the following manner: As shown in Fig. 2, the spark plug A coated with each of the glazes listed in Table 1 was hung within a constant-temperature oven regulated to a temperature of 500 ± 10°C. After the spark plug A had been heated for 30 minutes, the resistance between the terminal portion 51 and the metallic shell 1 was measured using a megohmmeter which applied 1000 VDC. As the alkali component (Na 2 O, Li 2 O, etc.) content in the glaze 3 increases, the insulation resistance under heated condition reduces. However, measurements in the test above revealed that the alkali component content in the glaze 3 did not cause a reduction in flashover voltage. For example, for the spark plugs A which are respectively coated with glazes Nos 1, 2, 7, 8, and 10 whose Na 2 O or Li 2 O content is relatively high, the insulation resistance under heated condition is in a relatively low range of 130 to 200 MΩ, but the flashover voltage exceeds 35 kV. On the other hand, for the spark plugs A which are respectively coated with glazes Nos 4 and 5, the insulation resistance under heated condition is relatively high, i.e., 800 MΩ and 700 MΩ, respectively, but the flashover voltage is relatively low, i.e., 33 and 30 kV, respectively, because the insulation resistance under heated condition reduces to tens of megohms after an engine test, described below. The engine test referred to in Table 2 was conducted in the following manner: The spark plug A coated with each glaze listed in Table 1 was installed in a 250 cc single-cylinder 4 stroke engine. The engine was continuously run for 100 hours at 6500 rpm in the full-throttle state. The temperature of the insulator 2 (in an area extending from the head portion 21 to the trunk portion 22) was 100 to 150°C. In the spark plugs whose head portion 21 and the area between the head portion 21 and the trunk portion 22, are coated with glazes Nos 4 to 6 whose Pb content exceeds 10 weight % in terms of PbO, a strong electric field was induced at that section of the trunk portion 22 located in the vicinity of the caulked portion 18 of the metallic shell 1 due to a high voltage applied during the engine test, resulting in a frequent occurrence of corona discharge. The energy of this corona discharge caused Pb contained in the glaze 3 to change into Pb 3 O 4 (red) or Pb 2 O 3 (reddish yellow), indicating a reduction in insulation resistance. Consequently, as described below, the flashover voltage between the terminal 51 of the terminal electrode 5 and the metallic shell 1 fell to 35 kV or less. The flashover voltage referred to in Table 2 was measured using the following test equipment: Each of the spark plugs which had undergone the engine test was set in a test apparatus B having the structure shown in Fig. 3. After the spark plug A was maintained at a temperature of 150°C for 1 hour, a direct-current impulse voltage was applied to it in the following manner: initially a voltage of 20 kV was applied, and then the applied voltage was increased at one-minute intervals by 1 kV each time. When a flashover occurred 3 times or more within one minute at a certain applied voltage, the voltage was taken as the flashover voltage. In Fig. 3, 61 denotes a direct-current impulse power source, 62 denotes a heating coil for heating the atmosphere of the spark plug A to a temperature of 150°C, 63 denotes a heating chamber, 64 denotes grounding, 65 denotes a terminal fixture, 66 denotes a chamber having a water-cooling jacket, 67 denotes an insulation oil (silicone oil), and 68 denotes an insulating protection tube. In practical use, the maximum voltage to induce a spark across the spark gap is 35 kV. Accordingly, if the flashover voltage is not more than 35 kV, a spark may fail to occur across the spark gap. This is why glazes Nos 4 to 6 are evaluated as "fail" (X) in Table 2. The present invention provides the following advantages: (a) Spark plugs which are coated at the head portion 21 and the area between the head portion 21 and the trunk portion 22 with glazes Nos 1 to 3 and Nos 7 to 10 whose Pb content is not more than 10 weight % in terms of PbO, have flashover voltages, as measured between the terminal 51 of the terminal electrode and the metallic shell 1, exceeding 35 kV, as shown in Table 2. Thus, those spark plugs provide a sufficiently high flashover voltage to prevent the occurrence of spark failure stemming from flashover. (b) The method of manufacturing the glaze slurry, the method of applying the glaze slurry to the insulator surface, and the glost firing temperature are substantially similar to conventional procedures and values. Accordingly, the present invention does not require any modification of tools or manufacturing apparatus. (c) When the Pb content of the glaze 3 is not more than 10 weight % in terms of PbO, the glost firing temperature must be increased. However, it can be adjusted to not more than 1150°C through selection of SiO 2 , B 2 O 3 , Al 2 O 3 , Na 2 O, CaO, Li 2 O, Bi 2 O 3 , ZnO, and BaO contents, thereby avoiding adverse effects (reduction of strength of the insulator 2) which could otherwise arise. In the embodiments described above, the leg base portion 231 is coated with glaze 3 in order to prevent the occurrence of flashover on the side of the leg portion 23. However, the leg base portion 231 does not need to be glazed. The glaze 3 applied to the leg base portion 231 does, however, improve conformability with the packing 16 placed between it and the stepped portion of the metallic shell 1, thereby improving airtightness. Further, if the Pb content of the glaze 3 is not more than 10 weight % in terms of PbO and the glost firing temperature is not more than 1150°C, a fluoride, such as NaF and AlF 3 , may be added to the glass material whose composition is shown in Table 1.

Claims: A spark plug comprising: a cylindrical metallic shell (1) having a ground electrode (11); an insulator (2) having the outer surface of its head (21) and trunk (22) coated with glaze (3), fixed within the metallic shell; and a center electrode (4) fixed in an axial bore (20) formed in the insulator, and having a flashover voltage at 150°C ≥ 35kV, wherein the Pb content of the glaze is not more than 10 wt. % in terms of PbO and the glaze contains 49.1 - 64.5 wt. % of SiO 2 , 5.0 - 25.0 wt. % of B 2 O 3 and 10.5 - 45.9 wt. % of two or more oxides selected from Al 2 O 3 , Na 2 O, CaO, ZnO, Li 2 O and Bi 2 O 3 with the exception of glazes consisting essentially of 48 - 54 % SiO 2 , 16.5 - 20 % B 2 O 3 , 7 - 11 % Al 2 O 3 , 4.25 - 5.25 % Na 2 O, 2 - 3 % CaO, 2 - 2.5 % ZnO, 11 - 14 % BaO, 0.4 - 1% K 2 O, up to about 0.25 % Li 2 O and up to about 1 % MgO. A spark plug according to claim 1, wherein the glaze contains 49.1 - 64.5 wt. % of SiO 2 , 5.0 - 25.0 wt. % of B 2 O 3 , 0 - 10.1 wt. % Al 2 O 3 , 0 - 7.4 wt. % Na 2 O, 0 - 5.5 wt. % CaO and 0 - 10.2 wt. % ZnO. A spark plug according to claim 1, wherein the glaze contains 49.1 - 64.5 weight % SiO 2 , 5.0 - 25 weight % B 2 O 3 , 0 - 10.1 weight % Al 2 O 3 , 0 - 7.4 weight % Na 2 O, 0 - 5.5 weight % CaO, 0 -10.2 weight % ZnO, 0 - 4.0 weight % BaO, 0 - 5.1 weight % Li 2 O and 0-1.0 weight % Bi 2 O 3 . A spark plug according to any one of claims 1 to 3, wherein the glaze also coats the outer surface of the base (231) of the leg (23) thereof.

Inventors: KENICHI NISHIKAWA, MAKOTO SUGIMOTO, KENICHI, NISHIKAWA, MAKOTO, SUGIMOTO, KENICHI, NISHIKAWA, C/O NGK SPARK PLUG CO., LTD, MAKOTO, SUGIMOTO, C/O NGK SPARK PLUG CO., LTD
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO. LTD.
Application year: 1997
Main classification: H01T13
IPC codes: F23Q3, H01T13