US2391455A - Spark plug and electrode therefor - Google Patents

Description

Dec. 25, 1945. F. R. HENS EL 2,391,455

SPARK PLUG AND ELECTRODE THEREFOR Filed June 22, 1943 7&5

NICKEL, IE 02 COPPfR ALL Il 1 II I llll TUA/GSTFNOE 1 v Tun/657E TUA/GS mv BASE METAL COMPOSITION oz 325f mvomvsrs Patented Dec. 25, 1945 SPARK PLUG ELECTRODE THEREFOR Franz R. Hensel, Indianapolis, Ind., asslgnor to P. R. Mallory & 00., Inc.,- Indianapolis, Ind., a corporation of-Delawarc Application June 22, i943, Serial No. 491,748

4 Claims.

This invention relates to spark plugs and th electrodes therefor.

An object of the invention is tc-improve spark plugs and the metal compositions used forthe electrodes thereof. v

Other objects of the invention'will be apparent from the description and claims.

In the drawing: v

Figure 1 shows a portion of a spark plug, partly in section, illustrating the spark plug electrodes:

Figure 2 is an end view of the 'spark'plug;

Figure 3 is a section through a portion of a modified plug; and

Figure 4 is anend view of a further modification. I

The invention contemplates a spark plug having a center electrode of tungsten or a tungsten base metal composition and a side or ground electrode formed of platinum base alloy.

Referring to Figures 1 and 2, the center electrode may comprise a cylindrical section II of tungsten or tungsten base metal rod welded to the end of a nickel, nickel alloy, iron alloy or copper alloy rod l2 which is embedded in the ceramic insulator I3 of th spark plug. The rod I2 mayconsist of an upper portion of nickel alloy to be welded to the tungsten and a copper backing to extract the heat. The side electrodes l may be formed of platinum alloy-wire of rectangular 'or circular cross section spot-welded to themetal shell I 4 of the plug.

Figure 3 shows a modification wherein the side electrode is formed of a short section 30 of platinum alloy wire welded to the end of a supporting wire 3| of base metal such as nickel, steel, or a high heat conductivity metal such as copper sheathed with nickel.

Inthe modification of Figure 4 the side electrodes comprise bimetallic strips having a layer 40 of platinum alloy facin the center electrode welded to a'layer 4] of bas metal behind it.

An alloy of platinum and molybdenum containing about 4% molybdenum is the preferred material. This alloy has a hardness of about 104 Rockwell B, and an ultimate tensile strength of 117,000 1bs. per square inch. This material may be used in the form of .030 inch square wire cut into pieces about A inch long which are welded onto opposite sides of the spark plug shell. This construction provides two sparkgaps at opposite sides of the center electrode N. This construction is economical and permits easy adjustment .of the spark gap. The platinum-molybdenum alloy can be readily resistance welded tothe steel shell l4. The range of compositions between 0.5 and 7.5% molybdenum, balance platinum, produces 5 the most satisfactory results. However, molybdenum will alloy with platinum. in all pr9D tions up to above 40% molybdenum and forms" solid solutions of molybdenum in the platinumthroughout this range. Compositions in. this 10 permissible range of proportions, but outside the preferred range, may in some cases be used.

Platinum molybdenum alloys. when used for spark plug electrodes hav the advantages of withstanding the erosive action of the spark rel5 sulting in gap growth. In a number of alloys usedat present frequent adjustment of the 'electrodes i necessary due tosuch gap growth. long range flying it becomes necessary to have spark plugs which will not need overhaulingfor '20 long periods of time. Also in high altitude flying the spark gap must remain properly adjusted since otherwise the sparking voltage increases resulting in misfiring.

Another platinum base alloy suitable for the side electrode is a platinum beryllium alloy containing 0.025 to 5% beryllium, balance platinum. The preferred. alloy .contains 0.20% beryllium. The beryllium has an extraordinary hardening effect on the platinum, as little as 0.05% beryllium giving a hardness equal to that of the 10% iridium-platinum alloy and 0.25% berylliumgiving a hardness equal to a 25% iridium-platinum alloy.

Beryllium also improves'the resistance of platinum to heat. Platinum-beryllium alloys undergo a much smaller loss in weight on prolonged heat-' ving than does pure platinum.

In a spark plug the electrodes are subjected to severe conditions of electric and chemical ero- 40 sion. In order for the electrodes to have a lon life, it is necessary that the alloys used have a high degree of cohesion. The beryllium imparts a very fine grain structure to the platinum and yields an alloy of high tensile strength. Furthermore the high octane fuels will deposit lead oxide which-at the sparking temperatures acts as a catalyst for oxygen promoting erosion by oxidation.

Oth'er platinum alloys which can be used are alloys of platinum with rhenium, columbium or tin. All of these materials reduc the volatility of platinum and increases its strength at high temperatures.

PATENT OFF C j In the case or platinum-rhenium alloys suitable percentages of rhenium range. from 2 to 8% the preferred range extending from 3 to 6%. Rhenium has a similar'eflect to molybdenum namely increasing the stiflness and erosion resistance of the platinum alloy.

The resistance of platinum to gaseous attack.

is increased substantially by additions of 0.025 to 10% columbium. The volatility of platinum and its strength at elevated temperatures are improved by the presence of 0.01 to 5% of tin.

It is further contemplated that platinumchromium alloys containing up to chromium may be utilized since they impart i tance to erosion and strength at elevated temperatures.

It is also considered to use ferromagnetic platinum base alloys characterized by high coercive forces such as 1500-4000 oersteds. A typical composition consists of 23% cobalt or iron,'balance platinum. By using permanently magnetized side electrodes the magnetic field will help to stabilize the arc and thereby give more consistent operation.

Further platinum alloys suitable for the purpose of the present invention are manganese platinum alloys with 1 to 15% manganese; platinum palladium alloys with 2 to palladium.

The addition of other elements to platinum up to 15% is permissible provided the performance is not affected. Such other elements may include iridium, rhodium, ruthenium, osmium, tungsten and tantalum.

The center electrode Il may be formed of fine grained tungsten or a metal composition containing a high percentage of tungsten.

One metal composition which is suitable is formed of The composition can be formed by mixing the metal powders together, pressing the mixture into a compact and heating the compact to a temperature slightly above the melting point ofthe nickel. Above this temperature the molybdenum dissolves in the nickel and the tungsten particles are rapidly dissolved by the liquid phase but are reprecipitated on certain nuclei developing large round grains. This process continues until all the originally fine tungsten particles have been converted into fairly uniform grains having a diameter about 100 times those of the original particles. All voids are also eliminated in this process and the resulting product, after cooling, is a strong non-porous body of high cohesive strength.

Other tungsten compositions which can be produced by a similar process and which are suitable are:

Generally the tungsten content may be between 80 and 98% in compositions of this type.

It is often desirable to decrease the workfunction of tungsten materials used for spark plug electrodes. For this purpose it is contemplated iii) ' the periodic system.

percentage of such ingredients is preferably kept quite small as theyhave some tendency to promote erosion. The range between .01 and 3% is preferred. Among the alkaline earth compoimds which are suitable are calcium-oxide, bariumoxide, calcium siliclde, magnesium silicide, strontium oxide. v

Another tungsten base material which is suitable for the center electrode is tungsten containing small pr rtions of boron, alloyed or combined therewith. Instead of boron it is also contemplated that grades of tungsten may be employed containing traces up to several percent of the borides and nitrides of. such metals as are contained in the fourth, fifth and sixth group of As an example a tungsten composition containing of zirconium boride may be used. It is further contemplated to use tungsten composition containing tungsten-zirconium compounds in percentages ranging from .01 to about 4 percent.

For certain applications tungsten-uranium compositions are preferred.

Thecombination of a platinum base side electrode with a tungsten or tungsten base center electrode affords advantages over prior electrode combinations such as nickel versus nickel, nickel versus tungsten; nickel (cobalt, iron) chromium alloys versus itself or nickel. because it provides less gap growth, ease of gap adjustment, more consistent sparking voltage and longer electrode life. When tungsten is used as a center electrode the spark plug construction permits a fairly heavy section usually of a diameter of A; inch or longer. By a suitable backing to the tungsten it is possible to' withdraw the heat and keep the tungsten cool enough so that it will not oxidize. Tungsten operated under such conditions excells all other metals as regards resistance to chemical attack and spark erosion. If tungsten is used however as a side electrode .the conditions are quite different. In side electrodes fairly thin sections are used and proper heat transfer cannot readily be provided. The tungsten therefore overheats, oxidizes and wears away very quickly.

Furthermore tungsten is very brittle and does not permit gap adjustment. Tests carried out with various tungsten side electrode constructions resulted in breakage of the .side electrodes when adjustment was attempted. The diiliculties of tungsten side electrodes'were overcome by providing a combination as suggested in the present invention. The side electrodes made in accordance with the teaching of this invention can be operated at elevated temperatures without oxidation. The electrodes may be used in thin sections in order to retain a low cost and flexibility of construction to permit gap adjustment. The mathat small proportions of oxides and silicides of alkaline earth metals may be added to the metal compositions during manufacture. However, the

terials disclosed for side electrodes have excellent resistance to spark erosion; The electrode combination therefore is superior to all prior constructions.

. While specific embodiments of the invention have been described, it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A spark plug for use in aviation engines comprising a pair of spaced electrodes, one of said electrodes being formed predominantly of platinum and the other of said electrodes being formed predominantly of tungsten.

2. A spark plug for use in aviation engines comprising a pair of spaced electrodes, one of said electrodes being formed of a platinum base alloy and the other of saidelectrodes being formed of a material selected from the group consisting of g tungsten and tungsten base inetai compositions containing at least 80% tungsten.

3. A spark plug for use in aviation engines comprising an insulating body, an electrode terminal extending therethrough and terminating in an exposed electrode tip formed predominantly of tungsten, a, metal shell encircling said insulating body and a second electrode tip secured thereto and disposed adjacent said first-mentioned electrode tip, the active sparking surface'ot said second tip being formed predominantly of platinum.

4. A spark plug electrode combination for use in aviation engines comprising a center electrode

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