A spark plug structure

Abstract: A spark plug has a metallic shell which is made of material having a tensile strength of more than 40 kg/mm² with a thermal conductivity of more than 60 W/m.k. The ground electrode may be connected to the metallic shell through a metallic ring which is made of different metal from the metallic shell, such as steel, stainless steel or nickel alloy.

Description: The invention relates to spark plugs for use in internal combustion engines, and particularly concerns spark plugs having improved heat and fouling resistance. A spark plug generally used in internal combustion engines has a metallic shell with a male thread on its outer surface and an insulator into which a centre electrode is placed. The metallic shell is usually made of steel carbide, while the insulator is normally made of alumina porcelain. The physical properties, such as thermal conductivity, of these materials play important roles in determining the thermal characteristics of the spark plug. These characteristics include the heat-resistance of the plug on which preignition resistance at high temperature is dependent and fouling resistance on which carbon formation at low temperature atmosphere is dependent. GB-A-2195398 shows a spark plug comprising a cylindrical metallic shell, a tubular insulator having a centre bore, and a centre electrode in the centre bore of the insulator forming a spark gap with a ground electrode connected to the metallic shell. The precharacterising portion of claim 1 is based on this disclosure. It is desirable to provide a spark plug of enhanced performance which is capable of complying with the variable demands accruing from the high output of modern engines and the desire for low fuel consumption. According to the present invention, there is provided a spark plug comprising: a cylindrical metallic shell; a tubular insulator having a centre bore, and a centre electrode in the centre bore of the insulator forming a spark gap with a ground electrode connected to the metallic shell; characterised in that the insulator includes a front portion made of aluminium nitride (AlN), and the metallic shell is made of material having a tensile strength of greater than or equal to 40 kg/mm² and a thermal conductivity of greater than or equal to 60 W/m.K. In preferred embodiments of the present invention, the spark plug further comprises: a terminal positioned in the centre bore of the insulator in alignment with the centre electrode; electrically conductive glass sealant provided in an annular space between the insulator and the terminal, and between the insulator and the centre electrode; the ground electrode being made of nickel or a nickel alloy, the ground electrode being connected to the metallic shell through a metallic ring which is made of a different metal than the metallic shell. The present invention will be further described hereinafter with reference to the following description of exemplary embodiments and the accompanying drawings, in which: Figure 1 is a partly sectioned side view of a spark plug; Figure 2 is a graph showing the heat resistance of a spark plug with an alumina insulator and various metallic shells; Figure 3 is a graph showing the heat resistance of a spark plug with an insulator of AlN and BeO; Figure 4 is a graph showing the relationship between the length of the insulator and the amount of fouling; Figure 5 is an enlarged cross-section of the main part of a modified spark plug; Figure 6 is a longitudinal view in partial cross-section of a spark plug; Figure 7 is a graph showing the relationship between the temperature and the thermal conductivity of an alloy used in the construction of a spark plug; Figure 8 is a graph showing the relationship between the temperature and the hardness of various alloys; Figure 9 is a graph showing the relationship between the cold working rate and the mechanical strength of various alloys; Figure 10 is a graph showing the relationship between the cold working rate and the mechanical strength with a cold working rate of 14% after 1 hour at each temperature; Figure 11 is a longitudinal cross-section of a spark plug body according to another embodiment of the invention; Figure 12 is a partially sectioned view of a part of a spark plug according to another embodiment of the invention; and Figure 13 is a partially sectioned view of a prior art counterpart. Referring to Figure 1 which shows a spark plug according to the present invention. The spark plug has a centre electrode 301 which has a copper core 301a clad in nickel. The centre electrode 301 is placed in a tubular insulator 302 which has an axial bore 302a. The flanged head 301b of the centre electrode engages against a step 302b in the insulator. The flanged head 301b is connected to a terminal electrode 305 via a resistor 304 and electrically conducting glass sealant 303. A metallic shell 306 has a male thread 306a at its outer surface and into it the insulator 302 is placed against a spacer 307 seated on a step 306b. The rear part 306c of the metallic shell 306 is turned inward for fixing the structure together by caulking. A spark gap 309 is formed between the centre electrode 301 and an outer electrode 308 attached to the front end 306d of the metallic shell 306. In this embodiment of the present invention, the metallic shell 306 has a tensile strength of greater than or equal to 40 kg/mm² and a thermal conductivity greater than or equal to 60 W/m.k. The insulator has a breakdown voltage of greater than or equal to 10 KV/mm, a bending strength of greater than or equal to 15 kg/mm² and a thermal conductivity of greater than or equal to 60 W/m.k. The metallic shell is made of a copper alloy selected from specimens A - G of Table 1, or an aluminium alloy selected from specimens H - K of Table 2. Of the specimens, copper alloys A - F and aluminium alloy specimens I and K are acceptable for this invention. A heat resistance experiment was conducted on three conventional spark plugs (BPR5ES) and spark plugs having metallic shells made of specimens F and K and alumina insulators. The test was carried out by incrementing the ignition advance angle of a 4-cylinder 2000cc engine. From the results, it is seen that the heat resistance is improved by an angle of 2.5 to 7.5°, see Figure 2. Of the insulator specimens I - V of Table 3, (AlN) have acceptable thermal conductivity, breakdown voltage and bending strength. Experiments were carried out using an insulator of specimen II of Table 3 and metallic shells of copper alloy and (S10C) steel. The combination of the (AlN)-insulator and the copper metallic shell enables significant improvements in the heat resistance as shown in Figure 3. The improved heat resistance allows the leg of the insulator to be lengthened from (1₁) to (1₂) as seen in Figure 4, and at the same time enhances the fouling resistance of the plug. In this experiment, each cycle is formed from periods of racing, idling, 15 (km/h) and 35 (km/h) at a temperature of -10°C. The cycle is repeated and fouling is deemed to have occurred when the engine inadvertently stops or fails to restart. In a modification of this invention, a tubular insulator 212 is made of (AlN) as seen in Figure 5. The insulator 212 is sintered with a platinum (Pt) alloy wire placed into a small hole 212c to form a centre electrode 211. The small hole 212c is provided in the leg 212a of the insulator. The platinum (Pt) alloy of the centre electrode 211 is made of (Pt-Ir), (Pt-Rh) or the like. The centre electrode 211 is connected to a middle electrode 213 and a terminal 205, and rigidly secured by an electrically conductive adhesive 203. The insulator 212 is placed inside a metallic shell 206 which is made from a copper or aluminium alloy from Tables 1 and 2. A spark plug with the insulator 212 integrally sintered with the centre electrode 211, has a somewhat reduced heat resistance. However, the combination of the insulator 212 and the metallic shell according to the invention, makes it possible to compensate for the reduction of the heat resistance. The insulator 212 of this type is particularly useful for small scale spark plugs (10 mm - 8 mm diameter male screw) since it is possible to make the centre electrode 211 thin and reduce the diameter of the insulator 212 while still maintaining a high heat resistance. Numerals 208 and 209 respectively designate a ground electrode and a spark gap. Referring now to Figures 6 to 10, a spark plug body (A) according to a further embodiment of the invention, has a cylindrical metallic shell 1 and an insulator 2 which has an axial centre bore 21. Into the centre bore 21 of the insulator 2, a centre electrode 3 is concentrically inserted. The metallic shell 1 is made from pure copper which initially has a hardness of HRB 58 at normal temperature, and a hardness of HRB 15 at a temperature of 350°C and is treated as described below. It also has an electrical conductivity of IACS 100% (at 20°C), a thermal conductivity of 390 W/m.k. and tensile strength of 35 kg/mm². After melting the copper 0.85% by weight of alumina (Al₂O₃) powder of average diameter 1 micron (µm), is evenly dispersed in melted copper to form an alumina-dispersed copper. The alumina-dispersed copper thus made, is manufactured by plastic working in which 60% of all the manufacturing process is by means of cold deforming processes. The properties of the alumina-dispersed copper are shown in Table 4. The metallic shell 1 has a threaded surface 11 at its rear end to enable the plug to be screwed to the cylinder head of an internal combustion engine and has a middle barrel and a rear caulking pad 16a. A J-shaped ground electrode 12 is welded to the front of the metallic shell 1 with the front end of the centre electrode 3. The inner surface of the metallic shell 1 has a shoulder portion 13 on which an annular spacer 17 is positioned. A hexagonal ring nut 14 is provided near the caulking pad 16a. The caulking pad is turned inward to retain the tubular insulator 2 and spacers 16. The annular space remaining is filled with powdered talc 15. The insulator 2 is a sintered ceramic body of aluminium nitride (AlN) which has a thermal conductivity of 180 W/m.k. (at 20°C). The insulator 2 has a leg portion 22 at its front end, the upper end of which has a tapered outer surface, and is supported by the metallic shell 1 with the tapered surface engaging the shoulder portion 13 via the spacer 17. The diameter of the centre bore 21 is somewhat smaller at the leg portion 22 having a step portion 24 above the tapered surface 23. The centre electrode 3 is made of a copper core 32 clad by heat-resistant nickel alloy 31. The rear end of the centre electrode 3 has a flanged head 33 which engages the step portion 24, while the front end of the centre electrode makes a spark gap (34) with the ground electrode 12. The flanged head 33 is connected to a terminal 35 via a resistor 36 and electrically conductive glass sealants 37 and 38. The metallic shell 1 is made of an alumina-dispersed copper alloy having the following properties: (a) The alumina-dispersed copper alloy has an electrical conductivity of IACS 80% (20°C), and a thermal conductivity of 320 W/m.k. as seen at Table 4 and at curve (4) in Figure 7. The high electrical and thermal conductivity of copper are generally retained. (b) Figure 8 shows the hardness of various samples, numerals 50, 51, 52 and 53 refer to pure copper, (CdCu), (CrCu) and (BeCu) respectively. From the curve 4 of Figure 8, alumina-dispersed copper has a hardness of HRB 84.5 at normal temperature, and hardness of HRB 80 at 800°C which shows that the hardness of the alumina-dispersed copper is significantly improved as compared with the hardness of pure copper (see curve 50). In the alumina-dispersed copper, the dispersed alumina powder acts as a dislocation barrier increasing recrystallisation of the pure copper and avoiding the dispersed alumina powder from being dissolved into the pure copper. Of the other metallic alloys, (BeCu) has a hardness of HRB 95 below 400°C, however, its hardness rapidly deteriorates at temperatures of 200-400°C. (c) Figure 9 shows the relationship between the percentage cold working and the mechanical strength of the alumina-dispersed copper alloy. In Figure 9, the numerals 41, 42, 43 and 44 in turn represent elongation rate (%), breaking strength, hardness HRB and tensile stress resistance (kg/mm²). From Figure 9, in which broken line 40 indicates a cold working rate of 14%, i.e. a reduction in the thickness of the sample of 14% by cold working, it is seen that the higher the percentage of cold working, the less the mechanical strength deteriorates. Figure 10 shows the mechanical strength with a cold working rate of 14%, numerals 45, 46, 47 and 48 in turn represent elongation rate (%), breaking strength, a hardness HRB and tensile stress resistance (kg/mm²) after one hour at high temperature. From Figure 10, it is seen that high mechanical strength is maintained even after considerable exposure to high temperature conditions. Some experiments were conducted as follows to compare the metallic shell 1 with a corresponding metallic shell made of (S10C) steel. Preignition resistance test It is found that the ignition advance angle can be advanced by an angle of 5 - 7.5° in a 4-cylinder 2000 cc engine. Fouling resistance test A cycle is formed by combining periods of racing, idling, 15 (km/h) and 35 (km/h) at a temperature of -10°C using a 4-cylinder 2000 cc engine. The cycle is repeated, and fouling is deemed to have occurred when the engine inadvertently stops, or fails to restart. It is found that the appropriate ignition is ensured with a plug according to the present invention which continues to spark in the cycle at which the engine stops or fails to restart when using the prior art plug. Zirconium oxide (Zr0₂) or aluminium nitride (AlN) powder may be used instead of alumina powder. A combination of ceramic powders may be used as long as the percentage by weight is within the range of 0.3 to 3.0. The average diameter of the particles of ceramic may be less than 1 micron. Preferably only the leg portion of the insulator is made of aluminium nitride (AlN) although other kinds of ceramics may be added as long as the thermal conductivity remains at 60 W/m.k. (0.1435 cal.sec°C). Referring to Figures 11 to 13, another embodiment of the invention is described hereinafter. A spark plug body 100 has a cylindrical metallic shell 190, the main part 191 of which is made of an aluminium or a copper alloy which has a thermal conductivity of more than 60 W/m.k. An annular ring 192 is connected to the front end of the metallic shell 190. The ring 192 is made of a heat-resistance metal such as steel, stainless steel or nickel alloy. The inner surface of the metallic shell 190 has a step portion 193, while the outer surface of the ring 192 has a step portion 194. The two step portions 193 and 194 mate and are rigidly joined at 195 by a known welding technique such as laser welding, electron-welding, TIG (tungsten inert gas welding) or soldering. A J-shaped ground electrode 196, made of a heat resistance nickel alloy, is attached to the annular ring 192 forming a spark plug gap with a centre electrode 150 described hereinafter. A tubular insulator 101 includes a front piece 101a, and is concentrically placed within the front portion of the metallic shell 190. The front half piece 101a of the insulator 101 acts as a leg portion, and is made of aluminium nitride (AlN) having a thermal conductivity of more than 60 W/m.k. The rear half piece 102 is made of relatively inexpensive alumina (Al₂O₃). The rear half piece 120 may, however, be made of aluminium nitride (AlN). The rear end of the front half piece 101a of the insulator 101 has a concentric projection 111 which fits into a recess 121 provided in the front end of the rear half piece 120 to form a joint-type insulator 130. The two pieces 120 and 101a are, as seen in Figure 11, fitted together in the manner of mortise-tenon joint by a glass sealant 140 which is a mixture of ceramic components such as (CaO), (BaO), (Al₂O₃), (SiO₂) and the like. The front half piece 101a has an axial centre bore 115 consisting of a reduced diameter hole 113 and larger diameter hole 114. The rear half piece 120 has a bore 122 axially communicating with the larger diameter hole 114. The centre electrode 150 is concentrically placed in the bores 113 and 114 with its front end extending out of the front half piece 101a. The centre electrode 150 is made of a copper core clad by a heat-resistant nickel alloy, and has a flanged head 151 at its rear end. At the assembly process, the centre electrode 150 is inserted from the rear end of the bores 115, 122 with the flanged head 151 received by the shoulder of the larger diameter hole 114, and is secured by a heat-resistant inorganic adhesive 152 in the diameter-reduced hole 113. An electrically conductive glass sealant 160 is provided in the bores 115, 122 to connect a noise-suppression resistor 161 between a terminal 180 and the centre electrode 150. The terminal 180 is inserted into the bore 122, and secured by the conductive glass sealant 160. In this embodiment of the invention, the annular ring 192 is welded to the metallic shell 190 at the step portions 193 and 194, thus strengthening the connection and avoiding oxidation of the connection. The nickel-alloy ground electrode 196 is welded directly to the annular ring 192 which is made of metal similar to the ground electrode 196, thus strengthening the weld. By contrast, in the prior art, where a nickel alloy ground electrode 192A is welded to a copper alloy metallic shell 190A, shown at arrow (B) in Figure 13, the mechanical strength of the connection 193A is lower than the desired level. In addition, the copper alloy component at 191A corrodes by oxidation, thus further deteriorating the weld strength.

Claims: A spark plug comprising: a cylindrical metallic shell (306); a tubular insulator (302) having a centre bore (302a), and a centre electrode (301) in the centre bore of the insulator forming a spark gap (309) with a ground electrode (308) connected to the metallic shell; characterised in that: the insulator (302) includes a front portion made of aluminium nitride (AlN), and the metallic shell is made of material having a tensile strength of greater than or equal to 40 kg/mm² and a thermal conductivity of greater than or equal to 60 W/m.K. A spark plug according to claim 1, wherein the insulator has a thermal conductivity of greater than or equal to 60 W/m.K, a breakdown voltage of greater than or equal to 10 KV/mm, and a bending stress of greater than or equal to 15 kg/mm². A spark plug according to claim 1 or 2, wherein the insulator (302) is sintered integrally with the centre electrode (301). A spark plug according to claim 1, 2 or 3, in which the metallic shell (306) is made of a ceramic-dispersed copper alloy comprising copper into which from 0.3 to 3.0% by weight of a ceramic powder is dispersed. A spark plug according to claim 4, in which the ceramic powder comprises at least one of alumina (Al₂O₃), zirconium oxide (ZnO₂) and aluminium nitride (AlN). A spark plug according to any preceding claim, further comprising: a terminal (180) positioned in the centre bore of the insulator in alignment with the centre electrode; electrically conductive glass sealant (160) provided in an annular space between the insulator and the terminal, and between the insulator and the centre electrode; the ground electrode (196) being made of nickel or a nickel alloy, the ground electrode being connected to the metallic shell (190) through a metallic ring (192) which is made of a different metal than the metallic shell (190). A spark plug according to claim 6, wherein the different metal is steel, stainless steel or nickel alloy. A spark plug according to claim 6 or 7, in which the inner surface of the metallic shell (190) has a step portion (193) and the outer surface of the metallic ring (192) having a step portion (194) the two step portions being joined by laser beam welding, electron-beam welding, TIG welding (tungsten inert gas arc) or by soldering.

Inventors: KOZUKA KAZUHIKO, OSHIMA TAKAFUMI, KOZUKA, KAZUHIKO, OSHIMA, TAKAFUMI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1989
Main classification: H01T13
IPC codes: H01T13