US3548239A - Spark plug electrode construction - Google Patents

Description

Dec. 15, 1970 c. J. EATON 3,548,239

SPARK PLUG ELECTRODE CONSTRUCTION Filed Sept. 5, 1968 EYE-3- ll. ll "5 I NVENTOR.

P: \W EARL JEATUN.

TIE-E- W A TTYE.

3 548 239 SPARK PLUG nrnrnbnn coNsrnUorroN Carl J. Eaton, Toledo, Ohio, assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation of Delaware Filed Sept. 3, 1968, Ser. No. 756,743 Int. Cl. Hillt 13/20 US. Cl. 313-136 8 Claims ABSTRACT OF THE DISCLOSURE An improved center electrode construction for spark plugs used in high performance internal combustion engines. The body portion of the electrode comprises a corrosion resistant metallic sheath which has a sealed end and which is filled with a heat conductive metal. A headed, fine wire firing tip of a non-weldable, corrosive nesistant metal is inserted into a cap which is then welded to the sealed end of the metallic sheath.

BACKGROUND OF THE INVENTION This invention relates broadly to spark plugs, and, more particularly, to a fine wire tip electrode which can be incorporated into a spark plug assembly for high performance internal combustion engines.

It is desirable to construct the center electrodes of spark plugs used in highly corrosive conditions of a material having a high degree of corrosion resistance. Spark plugs made of expensive corrosion resistant materials generally employ small sized electrodes and are often referred to as fine wire spark plugs. The center electrode of a typical fine wire spark plug terminates at its firing end in a tip constructed of a highly corrosion resistant material. Platinum and its alloys have been used for this purpose in the prior art. Such materials can be cold worked, and in prior art electrodes having a platinum or alloy tip, a head or shoulder portion is formed on a platinum rod by cold upsetting.

It has been discovered that other elements, for examples, iridium, tungsten, molybdenum, ruthenium, rhodium, and alloys thereof are often superior to platinum for use in spark plug electrodes which are subjected to arcing in the corrosive atmosphere of an internal combustion engine. However, these elements are much more diflic'ult to work than is platinum because of their fibrous characteristics which render them very brittle. It is virtually impossible to form heads, on rods made of materials such as iridium, by a cold upsetting operation.

A portion of the center electrode immediately above the tip electrode often consists of a conductor such as silver. When platinum tips are used, a chemical surface bond is formed between the platinum tip and the silver. However, this is not true when, for example, an iridium tip is used. Because no appreciable chemical or metallurgical bonding occurs, the iridium tip has a tendency to move or rotate in its seat.

Attempts have been made to place a spherical head on an iridium rod, which head is then embedded in the silver portion of the center electrode. However, when the iridium rod is heated to the melting point and then molded into the desired shape, recrystallization occurs. Recrystallized fibrous metals are extremely brittle and, as such, are very susceptible to tensile or vibrational failures. One solution to this problem is described in US. Pat. No. 3,315,113, which discloses a method for forming a shoulder on fine pieces of iridium rods.

When using a shouldered tip, the electrode is normally constructed by inserting the tip into acorrosive resistant United States Patent 0 sleeve until the shoulder abuts a flange at the end of the sleeve. A silver core is then cast in the hollow sleeve to hold the tip in place and to provide good heat conduction along the electrode. However, since no chemical bonding occurs between the silver core and the iridium tip, the tip may move or rotate. Such movement may eventually cause spark plug failure when the tip loosens and falls out of the sleeve. Even when a chemical bond is formed, for example between silver and platinum, the silver is subject to corrosion and the tip may eventually fall out of the sleeve.

SUMMARY OF THE INVENTION According to the present invention, the center electrode for a spark plug comprises a body portion having a heat conductive core and a line wire firing tip. The wire tip is attached to the cored body portion by heading one end of the wire, inserting the headed end into a cap such that the other end projects through the cap, and then welding the cap to a sealed end of the cored body portion.

The cored body portion of the electrode generally comprises a cylindrical sleeve of nickel, nickel alloy, or some other alloy having similar corrosion resistant properties. An axial hole is bored partially through the cylindrical sleeve, leaving a thin wall sealing one end, and the bore is filled with a copper or silver heat conductive slug.

The tip is constructed from a short piece of iridium wire or some other suitable corrosion resistant wire. One end of the wire, the firing end, is inserted into a recess in a die. A head is formed on the end of the wire which projects from the die by moving a second die against the projecting end and simultaneously applying an electric current between the two dies. The current must be sufficient to heat the wire to a temperature at which it can be hot headed, but must be applied for an extremely short period of time so that only a minor amount of recrystallization occurs.

A small cap is used to attach the tip to the body portion of the electrode. The cap must be made of a corrosive resistant material which is easily welded to the plugged end of the sleeve, preferably of the same material as the sleeve. The cap is constructed with a small hole through it which will pass the firing end of the tip but will not pass the headed end of the tip. In assembly, the firing end of the tip is inserted through the hole in the cap such that the headed end of the tip lies within the cap. The cap is then welded to the plugged end of the cored body portion of the electrode and the remainder of the spark plug is assembled.

With the improved electrode construction, the headed end of the fine wire tip may be irregular and define small air spaces between the cap and the adjacent end of the cored body portion. Corrosive gases from the combustion chamber may leak into these small air spaces, but the gases cannot attack the easily corroded copper or silver heat conductive core of the electrode as in prior art electrode constructions.

It is a primary object of the present invention to provide a new and improved fine wire electrode construction for spark plugs.

Another object of the invention is to provide a spark plug having an improved iridium tip electrode construction.

Still another object of the invention is to provide an improved spark plug electrode having a heat conductive core and a fine wire tip and constructed such that the corrosive combustion gases cannot attack the easily corroded heat conductive core.

Further objects and advantages of the invention will become apparent from the following detailed description of the drawings.

3 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of a spark plug assembly embodying the invention;

FIG. 2 is a bottom end view of a spark plug assembly embodying the invention;

FIG. 3 is an exploded view of a headed fine wire tip and a cap constructed according to the instant invention;

FIG. 4 is a fragmentary sectional view of the bottom of the cored body portion of the electrode, the tip and the cap, prior to welding the cap to the bottom end of the body portion;

FIG. 5 is a fragmentary sectional view, similar to FIG. 4, showing a portion of an assembled spark plug electrode constructed according to the instant invention; and

FIG. 6 is a diagrammatic view showing apparatus for heading the fine wire electrode tip.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, a spark plug assembly embodying the invention is generally indicated by the reference numeral 10. The spark plug assembly 10 is of the fine wire variety and is used in high performance internal combustion engines, such as aircraft engines, where highly corrosive conditions are present. The spark plug assembly 10 generally comprises an annular insulator 11 having an axially stepped bore 12 extending therethrough; a center electrode assembly 13 fixed in the stepped bore 12; a metallic sleeve 14 surrounding the insulator 11; and an outer metallic shell 15. Ground electrodes 16 extend inwardly at a lowermost end 17 of the metallic shell 15.

The lower portion of the center electrode assembly 13 includes a ceramic sealing material 18 which is in contact with the upper end 19 of an annular metallic sheath 20. The sheath 20 is constructed of a corrosion resistant material, such as nickel or nickel alloy, and has an enlarged flange 21 at its upper end. The flange 21 seats on a shoulder 22 within the stepped bore 12 of the insulator 11. The upper end 19 of the metallic sheath 20 has an opening 23 which communicates with a central opening portion 24 in the metallic sheath 20. An electrode 25 projects upwardly from the central portion 24 through the opening 23 and through the sealing material 18. The lower part of the electrode 25 is in electrical communication with a core 26 which fills the central portion 24 of the metallic sheath 20. The core 26 may consist of silver, copper, or other suitable materials having high thermal conductivity.

The central portion 24 of the metallic sheath 20 does not extend completely through the metallic sheath 20: a wall 27 remains, sealing or closing the lower end of the sheath 20. The corrosion resistant wall 27 protects the silver or copper core 26 from exposure to the highly corrosive gases present in the combustion chamber of an internal combustion engine. However, the wall 27 should be sufficiently thin such that a good heat transfer path is maintained from an attached firing tip 28, through the Wall 27, and to the core 26.

The firing tip 28, according to the present invention, comprises a short piece of wire coaxially aligned with and attached to the lower end of the metallic sheath 20. This attachment is made with a cap 29. The upper part of the center electrode assembly 13 includes a conductive seal in electrical communication with the electrode 25, a resistor 31, a spring 32, and a terminal stud 33 which threadably engages the insulator 11.

Referring now to FIGS. 3-5, the firing tip 28 and the end cap 29 are shown in detail. The tip 28, which is constructed from a short cylindically shaped piece of wire, has a shank portion 34, a firing end and an enlarged headed end 36. With this design, the firing tip 28 can be constructed of an extremely expensive metal, such as 4 iridium, using only small quantities of the metal and having negligible material Waste.

The cap 29 comprises a short cylindrically shaped piece of metal having a stepped axial bore 37. The cap 29 is of the same material as the annular metallic sheath 20, or of any other corrosion resistant metal which Will permit the cap 29 to be easily welded to the sheath 20. The stepped bore 37 includes an enlarged portion 38 adapted to receive the headed end 36 of the tip 28 and a reduced portion 39 having a diameter just large enough to receive the shank portion 34 of the tip 28. Construction of the lower portion of the center electrode assembly 13 is completed by inserting the firing end 35 of the tip 28 within the stepped bore 37 in the cap 29 such that the headed end 36 of the tip 28 lies within the enlarged bore portion 38 and the firing end 35 projects from the cap 29. The cap is welded at 40 to the lower end of the sheath 20 over the plug 27. The center electrode 13 is then positioned in the axially stepped bore 12 which extends through the insulator 11.

When the firing tip 28 is constructed from metal such as iridium, tungsten, molybdenum, ruthenium, rhodium, and alloys thereof, difficulties arise in constructing the headed end 36. Such metals are nonmalleable and cannot be cold worked. If, on the other hand, such metals are heated, recrystallization may occur, causing the metals to become brittle. The temperature at which one of this group of metals recrystallizes varies with the period of time at which the metal remains at an elevated temperature. The recrystallization temperature decreases as the time increment increases. Therefore, it is possible to raise the temperature of, for example, iridium to approximately 2,000? P. without substantial recrystallization if the increment of elevated temperature time is in the order of a fraction of a second, even though iridium, heated over a longer period of time, would normally recrystallize at 1,800 F.

It has been found that the fine wire tip 28 can be headed by a process similar to that disclosed in US. Pat. No. 3,315,113. Apparatus for practicing this process is shown in FIG. 6. The apparatus generally comprises a press 41 having a movable platen 42 and a fixed bed 43. The movable platen 42 may be moved toward and away from the fixed bed 43 by a connecting rod 44 and a conventional hydraulic cylinder (not shown). An upper die 45 is secured to the movable platen 42 and a lower die 46 is secured to the fixed bed 43. The lower die 46 has a cylindrical recess 47 adapted to receive and hold the shank portion 34 and the firing end 35 of the tip 28. A step-down transformer 48 supplies current to the upper and lower dies 45 and 46 by lead-in wires 49 and 50.

An electrical circuit is established between the upper die 45 and the lower die 46 through a tip blank 51. A current is applied to the transformer 48 to heat the portion of the tip blank 51 which extends between the upper and lower dies 45 and 46. Thereafter the platen 42 is moved downward to form the headed end 36 on the firing tip 28. The time increment during which the current is applied to the transformer 48 is sufliciently short that only minor amounts of recrystallization will occur in the headed end 36. The lower die 46 acts as a heat sink, keeping the shank portion 34 and the firing end 35 of the tip 28 below the recrystallization temperature.

Even where recrystallization is not a problem as with platinum and platinum alloys, the above-described proc ess may be used to form headed ends on fine wire spark plug electrode tips.

While the invention has been disclosed in conjunction with a specific form and disposition of the parts, it should be expressly understood that modifications and changes may be made without departing from the spirit and scope of the appended claims.

What I claim is:

1. In a spark plug having an insulator with an axial bore therethrough, a center electrode mounted in such bore, said center electrode having a tubular metal sheath encompassing a heat conductive metal core adjacent its lower end, an improved center electrode comprising, a metallic cap attached to the lower end of said tubular metal sheath, said cap having a receiving opening and a co-axial shank opening, and a corrosion resistant metal firing tip having a cylindrical shank portion and a headed end, said headed end of said firing tip being snugly positioned within such cap receiving opening, and said cylindrical shank portion extending through such shank opening of said cap and projecting outwardly from said cap.

2. In a spark plug having an insulator with an axial bore therethrough, a center electrode mounted in such bore, an improved center electrode comprising, a metallic sheath encompassing a heat conductive metal core mounted adjacent the lower end of said insulator, said metallic sheath having a tubular side wall and a continuous integral bottom Wall, a corrosion resistant metal firing tip, said firing tip having a shank portion and a headed end, and means for attaching the headed end of said tip to said continuous bottom wall of said metallic sheath.

3. An improved spark plug center electrode as claimed in claim 2, wherein said means for attaching the headed end of said tip to said bottom wall includes a cap, said cap having an axially stepped bore therethrough, the headed end of said tip being located in the larger portion of said stepped bore, the shank portion of said tip being located in the smaller portion of said stepped bore and projecting from said cap, and means for attaching said cap to said sheath.

4. An improved spark plug center electrode as claimed in claim 3, wherein said means for attaching said cap to said sheath is a welding material.

5. An improved spark plug center electrode as claimed in claim 4, wherein said tip is constructed of a metal selected from the group consisting of iridium, platinum, tungsten, molybdenum, ruthenium, rhodium, and alloys thereof.

6. An improved spark plug center electrode as claimed in claim 4, wherein said tip is constructed from iridium.

7. In a spark plug having an insulating core with an axial bore therethrough, a center electrode mounted in such bore, an outer shell surrounding said insulating core, and at least one ground electrode in electrical communication vvith said outer shell, an improved center electrode comprising: a heat conductive metallic core, a corrosion resistant metallic sheath having an end completely surrounding at least one end of said core, a corrosion resistant metallic cap welded to said end of said metallic sheath, said cap forming a chamber with said sheath, a corrosive resistant firing tip, said firing tip having a shank portion and a headed end, the headed end of said tip being located in said chamber, the shank portion of said tip projecting through an opening in said cap and into spark gap relationship with such ground electrode.

8. The improved spark plug center electrode of claim 7 wherein said tip is constructed from iridium.

References Cited UNITED STATES PATENTS 1/1959 Adair 3l3143 X 4/1967 Lever 313-136 US. 01. XR, 313 141

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