Spark plug

Abstract: In a spark plug for an internal combustion engine, a center electrode has a firing tip at its front end. The firing tip is made from iridium powder compact which is degreased, and sintered in a condition such as a vacuum, or inert atmosphere, so that the density of the powder compact is more than 90 percent to impart a spark-corrosion resistant property.

Description: This invention relates to spark plugs, in particular the improvements of their resistance to spark erosion. Due to a demand for high power outputs from internal combustion engines, the need has arisen for the spark erosion resistance of spark plugs to be improved. To meet this demand, firing tips have been manufactured individually from precious metals such as platinum or platinum-iridium based alloys. These firing tips may be columnar or laminar, and respectively bonded to a center electrode and an outer electrode by welding. The columnar tip, however, requires a great amount of precious metal thus raising manufacturing costs. On the other hand, the use of a laminar tip leads to a waste of precious metal because the tip is punched from a sheet of the metal thus reducing the yield by 30 percent. Further, a tip which is made of a Pt-Ir alloy has a melting point of more than 2300 degrees Celsius, so that it is difficult to machine the tip unless it is at an extremely high temperature. The Pt-Ir tip, which is alloyed by means of a metallurgical process, is ductile but fragile, and cracks readily occur on the tip at the time of machining, thus making it very difficult to machine the tip into a complex shape. The tip may also be shaped by means of electric discharge machining, but this takes a long time to finish because of its high melting point. US-A-3 466 158 discloses a method of manufacturing a spark plug for an internal combustion engine, comprising the steps of providing a cylindrical metallic shell and placing a tubular ceramic insulator concentrically therein, placing a centre electrode concentrically within the tubular insulator, forming a firing tip from iridium material, and metallurgically bonding the firing tip to the front end of the centre electrode to form a spark gap between the firing tip and an outer electrode which extends from the metallic shell. FR-A-1,490,828 discloses a method of making a spark plug firing tip by sintering a compact of ruthenium powder. FR-A-1 490 828 further discloses a method of manufacturing a spark plug for an internal combustion engine, comprising the steps of preparing a ruthenium-iridium powder compact, forming a firing tip from the ruthenium-iridium powder compact, and sintering the ruthenium-iridium powder in a reducing atmosphere at a temperature of 1500°C to impart a density of 89% of theoretical to the ruthenium-iridium powder compact so as to provide spark-erosion resistance. DE-A-3,038,649 discloses a method of making a spark plug firing tip but using a compact of platinum powder. Therefore, it is an object of the present invention to reduce the above drawbacks, and provide a firing tip structure which has improved spark-erosion resistance for a long service life at a reduced cost. According to the present invention, there is provided a method of manufacturing a spark plug for an internal combustion engine, the method comprising the steps of: providing a cylindrical metallic shell and placing a tubular ceramic insulator concentrically therein; and placing a centre electrode concentrically within the tubular insulator; characterised by the further steps of: preparing an iridium powder compact; forming a firing tip from the iridium powder compact; sintering the iridium powder compact in a vacuum, or a non-oxidizing or reducing atmosphere at a temperature in the range of from 2000 to 2200°C to impart a density of at least 90% of theoretical to the iridium powder compact so as to provide spark-erosion resistance; and metallurgically bonding the firing tip to the front end of the centre electrode to form a spark gap between the firing tip and an outer electrode which extends from the metallic shell. The present invention also provides a spark plug comprising: a cylindrical metallic shell; a tubular ceramic insulator positioned concentrically within the shell; and a centre electrode concentrically placed in the insulator; characterised by a firing tip metallurgically bonded to the centre electrode, the firing tip being formed from a sintered iridium powder compact and having a density greater than 90% of theoretical. The firing tip is made from an iridium powder compact which may be compressed by means of metallic die press, thus lending itself to simple mass production, at reduced costs, with minimal waste of precious metal. Further, the use of powder metallurgy makes it possible to form firing tips of complex shapes which metallurgical processes may be unable to achieve. These and other aspects of the present invention will be more readily understood from the following description, when taken with the attached drawings, which are given by way of example only, and in which: Fig. 1 is a view of a spark plug according to a first embodiment of the invention, with the lower half of the spark plug in section; Fig. 2 is an enlarged cross sectional view of the firing tip of Fig. 1; Fig. 3 is a schematic view showing a dimensional relationship of the firing tip; Fig. 4 is a view similar to Fig. 1, of a second embodiment of the invention; Fig. 5 is an enlarged cross sectional view of a firing tip of Fig. 4; and Fig. 6 is a graph showing the comparison between the erosion of a sintered compact body and an equivalent conventionally produced iridium tip at different extents of sintering. Referring to Fig. 1, there is shown a spark plug 1 which is to be incorporated into an internal combustion engine. The spark plug 1 has a cylindrical metallic shell 3 having a nut portion 3a and a thread portion 5 at its outer surface. Within the metallic shell 3, a tubular insulator 2 is concentrically enclosed which is made of alumina and nitride based ceramics such as aluminum nitride (AlN) and silicon nitride (Si 3 N 4 ). The interior of the insulator 2 has as an axial bore 8 in which a terminal electrode 10 and a center electrode 7 are concentrically enclosed. These electrodes 10, 7 are linearly aligned by way of an electrically conductive glass sealant 9 which is encapsulated into the axial bore 8. The center electrode 7 is made of a metallic material, but may be made of an electrically conductive ceramic powder or a metal-coated ceramic body. A front end of the center electrode 7 is terminated somewhat short of the front end of the insulator 2. Into the front end of the insulator 2, a firing tip 6 is inserted and sintered simultaneously at the insulator, and metallurgically bonded to the front end of the centre electrode 7 by means of an electrical resistant welding (Wd) as shown in Fig. 2. In this instance, a front end of the firing tip 6 extends somewhat beyond the front end of the insulator 2 so as to provide a spark gap (Ga) between the tip 6 and an outer electrode 4 mounted on the metallic shell 5. The firing tip 6 is made from an iridium powder compact body by means of a metallic die press, C.I.P., extrusion or injection. Then, the compact body is degreased, and sintered at a temperature of more than 2000 degrees Celsius in a vacuum, or an inert reductive atmosphere, so that the density of the powder compact is more than 90 percent, thus improving the spark-erosion resistance. The firing tip 6 has a frusto-conically shaped stem 11 at a side in which the tip 6 is metallurgically bonded to the insulator 2. As shown in Fig. 3, the firing tip 6 measures 2.0 mm in length (C), and measures 0.8 mm, and 1.2 mm at its diameters (a), (b) at its stem 11. The stem 11 measures 0.5 mm in height (d) with a 45 degree taper of its stem 11. Now, Table 1 shows a relationship between relative press forming density, sintering atmosphere, sintering conditions and density of a sintered compact body. If the sintered compact body is placed in air at the time of sintering, the iridium of the compact body is oxidized to produce a black iridium oxide leading to volatilization. In order to protect the compact body against over-volatilization, it is necessary to sinter the compact body in a vacuum, or an inert atmosphere. The compact body is then sintered at a temperature ranging from 1700 to 2200 degrees Celsius, and preferably sintered at more than 2000 degrees Celsius. When the density of the sintered compact body is low, the number of pores in the compact body is high, which causes a temperature rise, quickly eroding the firing tip due to the high energy released when a spark discharge occurs. In order to reduce erosion, it is necessary to ensure that the density of the sintered compact body is more than 90 percent, as shown in Fig. 6 which is a graph showing the comparative erosion between a sintered compact body and a metallurgically processed ingot iridium tip at different densities of sintered compact. The value 100 is taken to be the erosion of a metallurgically processed iridium tip. As shown in Fig. 5, the comparative erosion of the firing tip decreases with the increase of the density of the sintered compact body. In the meantime, it is desirable to sinter the compact body a second time to increase its density again as shown at Table 2. A spark erosion experiment is carried out with the sample (K) at Table 2 employed. The sample is an iridium powder compact which is compressed by means of a die press, so that the press forming density ratio is 66.3 percent. Then, the compact body, thus compressed, is primarily sintered at 2200 degrees Celsius in a hydrogen atmosphere for 60 minutes, so that the density of the compact body is 92.5 percent. The compact body is secondarily sintered at 1400 degrees Celsius in an argon atmosphere for 60 minutes, and shaped as shown in Fig. 3 by means of a hot isostatic hydraulic press. The result shows that the erosion of sample (K) is substantially identical to that of a Pt-Ir tip which is metallurgically processed to impart an increased erosion-resistant property. sample press forming density ratio (%) primary sintering atmosphere sintering condition °C × min. density of compact body (%) A 66.4 vacuum 1300 × 30 75.6 B 66.4 ditto 1700 × 60 86.7 C 67.3 hydrogen 1700 × 60 87.7 D 66.4 ditto 1800 × 60 87.9 E 66.4 ditto 1900 × 60 89.0 F 66.4 ditto 2000 × 60 92.1 G 66.3 ditto 2200 × 60 92.5 sample secondary sintering sintering condition °C × min. density of compact body (%) H corresponding to sample D vacuum 1700 × 60 91.6 I corresponding to sample E ditto 91.8 J corresponding to sample F argon 1400 × 60 under hot isostatic hydraulic press 94.4 K corresponding to sample G ditto 95.6 Referring to Fig. 4 which shows a spark plug (1A) according to a second embodiment of the invention, the firing tip 6a is formed into a columnar shape, and measures 0.5 mm in diameter (e), and 1.5 mm in length (f). In this embodiment, like reference numerals in Fig. 1 are identical to those in Fig. 4. A center electrode 12 is made of a copper core clad by a nickel alloy. The center electrode 12 is encapsulated in the axial bore 8 with the glass sealant 9, and has a flanged head 14 at its upper end to engage against a stepped portion 2a of the inner wall of the insulator 2. The front end face of the center electrode 12 has a recess 13 into which the firing tip 6a is fitted and securely attached at the center electrode 12 by means of a metallurgical bond (L) such as an electron beam welding or a laser welding. It is noted that the firing tip 6a may also be used in igniter plugs in general. Various other modifications and changes may be also made without departing from the scope of the appended claims.

Claims: A method of manufacturing a spark plug for an internal combustion engine, the method comprising the steps of: providing a cylindrical metallic shell and placing a tubular ceramic insulator concentrically therein; and placing a centre electrode concentrically within the tubular insulator; characterised by the further steps of: preparing an iridium powder compact; forming a firing tip from the iridium powder compact; sintering the iridium powder compact in a vacuum, or a non-oxidizing or reducing atmosphere at a temperature in the range of from 2000 to 2200°C to impart a density of at least 90% of theoretical to the iridium powder compact so as to provide spark-erosion resistance; and metallurgically bonding the firing tip to the front end of the centre electrode to form a spark gap between the firing tip and an outer electrode which extends from the metallic shell. A method according to claim 1 comprising the step of: inserting the firing tip into the front end of the tubular insulator, wherein said sintering of the firing tip is simultaneously carried out on the insulator, the firing tip having a stem for placing the firing tip in position when inserting the firing tip into the tubular insulator. A method according to claim 1 wherein the front end face of the centre electrode is provided with a recess into which the firing tip is fitted, and said metallurgically bonding step is performed using electron beam welding or laser welding. A method according to claim 1 wherein said sintering of the iridium powder compact is performed at about 2000°C for 60 minutes, and the method comprises the further sintering step of the iridium powder compact at about 1400°C for 60 minutes in an argon atmosphere by means of a hot isostatic press procedure. A spark plug comprising: a cylindrical metallic shell; a tubular ceramic insulator positioned concentrically within the shell; and a centre electrode concentrically placed in the insulator; characterised by a firing tip metallurgically bonded to the centre electrode, the firing tip being formed from a sintered iridium powder compact and having a density greater than 90% of theoretical. A spark plug according to claim 5 in which a front end face of the centre electrode has a recess into which the firing tip is to be fitted and secured to the centre electrode by means of metallurgical bond such as electron beam welding or laser welding.

Inventors: KOZUKA KAZUHIKO, OSHIMA TAKAFUMI, KOZUKA, KAZUHIKO, OSHIMA, TAKAFUMI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1990
Main classification: H01T21
IPC codes: H01T21, H01T13