US3088921A

US3088921A - Resistor compositions and spark plugs having integral resistors

Info

Publication number: US3088921A
Application number: US32237A
Authority: US
Inventors: Frederick E Heischman
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1960-05-27
Filing date: 1960-05-27
Publication date: 1963-05-07
Anticipated expiration: 1980-05-07

Description

May 7, 1963 F. E. Hll-:iscHMAN 3,088,921

REsIsToR coNPosITIoNs AND SPARK muss HAVING INTEGRAL REsIsToRs Filed May 27, 1960 Sig. 5

FREDERICK E. HEISCHMAN United States Patent Filed May 27, 196i), Ser. No. 32,237 4 Claims. (Cl. 252-516) This invention relates to resistors and resistor type spark plugs and more particularly to spark plug insulators havmg an integral resistance element as a part of a composite center electrode.

The invention contemplates a method of providing a composite center electrode fabricated from a vitreous resistance material positioned Within the bore of a spark plug insulator body that will, after the final assembly process, become an integral part of the insulator body. The invention also contemplates the provision of an improved resistance material combined with a vitreous body having improved sealing characteristics.

The invention includes the provision of a spark plug insulator assembly having a composite center electrode in the center bore of the insulator that has the required electrical and physical characteristics Ito operate successfully in a spark plug for ignition purposes and also to suppress radio emanations.

ln the prior art, resistor spark plugs have been fabricated which include an integral resistor element as a part of a composite center electrode. In the fabrication of the resistance element, organic carbon was used as one of the constituents which, as is well known, is subject to oxidation, particularly at the operating vtemperatures found in a spark plug in an internal combustion engine. It was, therefore, found necessary to protect the resistance element from oxidation by sealing it in the bore of a spark plug insulator by providing seals at both ends of the resistor element. This complicated the problem of manufacturing spark plugs and materially increased their expense during manufacture. The present invention contemplates the provision of a spark plug having an integral resistance element which is composed of inorganic materials which are relatively stable at the operating temperatures found in the core of a spark plug when used in internal combustion engines. The seal and the resistance element are, therefore, capable of bein(y combined in a single mixture of materials which obviates the use of separate sealing bodies to protect the resistance element from oxidation.

it is, therefore, a principal object of this invention to provide a spark plug having a composite center electrode including an integral resistor.

it is another object of this invention to provide a vitreous resistance material including inorganic substances, the resistal value of which is capable of being controlled in a mass production line within predetermined limits.

It is another object of this invention to provide a vitreous resistance body including inorganic compounds which are stable at temperatures of about l,000 F.

it is another object to provide a vitreous sealing material mixed with inorganic resistance material to form a center electrode seal for a spark plug and also to provide a resistance to reduce radio emanations.

lt is another object of this invention -to provide a spark plug with an improved vitreous sealing element for a composite center electrode having a resistance value within predetermined limits which is stable under varying operating conditions of temperature and pressure.

-lt is still another object to provide a center electrode assembly including a metal component for the firing tip having an upset head with ilutes which cooperate with the improved vitreous resistor composition in a manner to provide an improved contactual relation.

ICC

Other objects and advantages of this invention relating to the arrangement, operation and function of the related elements of the structure, to various details of construction, to combinations of parts and to economies of manufacture, will be apparent to those skilled in the art upon consideration of the following description and appended claims, reference being had to the accompany-ing drawings forming a part of this specication wherein like reference characters designate corresponding parts in the several views.

Referring to the drawings:

FIG. l is a vertical section through a spark plug insulator body showing the components of a composite center eelctrode in assembled position;

FIG. 2 is an enlarged view showing the headed portion of the ring tip element of the center electrode, partially in section; and

FIG. 3 is a triaxial diagram indicating areas of ratios of composition of the resistor materials.

In the drawings, FIG. l show a section through a conventional spark plug insulator body lil formed of a ceramic insulating material such as aluminum oxide. The bore of the insulator body is provided with a shoulder between the smaller bore 12 at the lower end 14 of the body 10 which is itted with a metal center electrode portion 16 and the larger bore 18 which extends upwardly terminating in a funnel-shaped opening 20 at the top 22 of the alumina body 10. The center electrode portion 16 is provided with a 'Fluted head 24 which rests on shoulder 25, already described, to hold it in position in the bore as shown.

Stud 26, positioned in the upper larger bore 18, is made of steel and has an enlarged or pilot section 32, which closely fits the bore 18, to maintain concentricity and also to give added strength to prevent bending during the hot pressing operation, to be described hereinafter. At the top of the stud 26 a threaded terminal section 34 is provided for connecting an ignition cable to the spark plug and also a ilange 36 which rests on the upper end surface 38 of :the insulator body 10 to control the length of the stud projecting into the center bore 18, so that the effective length of a resistor body 30 positioned between the bottom end 4b of the stud and the head 24 of the center electrode is controlled within predetermined limits. The lower end of the stud 26 is provided with threaded section 41 which anchors the stud in the resistor body 36 and insures a good electrical contact with the resistor body by mechanically interlocking therewith. Since the thermal eX- pansion characteristics of the metallic stud 26, the vitreous resistor body 30, and the insulator 1t) are different, the stud, after being hot-pressed into posi-tion will, upon cooling, decrease in length to a greater degree, resulting `in a tensile force in the stud arising between the surface 38 at the top of the insulator body 10 and the ilange 36, and the integral resistance body 30 which anchors the lower threaded portion 41 of the stud.

The -center electrode portion 16 is formed of a suitable refractory material, such as a nickel alloy, which will resist electrical spark erosion and corrosion by combustion gases. The head portion 24 of the center electrode is upset in a special manner to insure good electrical contact with the vitreous resistory body 30. After the upsetting operation, a knurling step is performed around the outer annular surface of the head 24 which provides numerous angular-shaped ridges 42, forming a serrated cup-shaped portion, which is a locking means to prevent the center electrode from rotating in the resistor element during the spark plug operation. The angular ridges also provide additional surface area 44 to contact the resistor body element 30.

The knurling operation on the head portion 24 creates annular projecting ridges around the top and bottom of the head 24, creating concave or cup-

shaped portions

48 and 49. These cup-

shaped portions

48 and 49, with their serrated lips, are important because they provide a mechanical interlock which improves the electrical connection between the metal of the electrode portion |16 and the resistor body element 30. Since the metallic center electrode 16 is formed of a material having a high thermal expansion (approximately 13.0 6 inches/ inch/ C.) and the vitreous resistor is formed of materials having a relatively lower thermal expansion (3.2 l06 inches/inch/ C.) the cup-shaped head 24, upon cooling, wili shrink more than the enclosed resistor portion causing the serrated lip of the cup to tighten around the portion of the resistor element Within the cupshaped element 48, thus insuring a good electrical contact and a tight mechanical connection preventing movement of the electrode. A cone-shaped portion 50 projecting from the top of the head 24 centrally of the cup also provides additional electrical contact area with the resistor element 30.

The resistor material can be prepared in several ways suitable for feeding into the center bore 18 of the insulator body 10. A granulated mix may be used, a spraydried powder may also be used, or extruded or dry-pressed pellets or slugs may be used.

The resistor mix comprises a low loss electrical grade borosilicate glass such as Corning Number 7070 glass which has a low thermal expansion (32X l0*6 inches/ inch/ C.). A pigment-grade titanium dioxide is used which is easily reducible to form semi-conductive suboxides of titanium. The pigment grade is very finely divided in particle size which aids in obtaining homogeneity in the mix. A reducing agent, boron carbide (220 mesh), is also used in the mix which is also a semi-conductor. Boron carbide appears to function better than other carbides or other reducing agents, in that it gives better reproducibility of resistance values. A plasticizer and binder called Methocel (8000 centipoises) is also used which is water-soluble cellulose gum. This material is available commercially and is a product of the Dow Chemical Company. It is an excellent plasticizer and binder which aids in the preparation of the mix and facilitates feeding the mix into the center hole 18 of the insulator body 10. Methocel also acts as a reducing agent.

Various compositions of these materials can be prepared depending on the resistance values desired for specilic applications of the resistor element.

Some preferred resistor mixes to obtain the indicated resistance values are:

150 ohms:

72.5% Corning #7070 glass 25.0% TiO2 2.0% boron carbide (220 mesh) .5% Methocel (8000 cps.) 1,000 ohms:

77.5% Corning #7070 glass 20.0% TiOZ 2.0% boron carbide (220 mesh) .5% Methocel (8000 cps.) '10,000 ohms:

'88.0% Corning #7070 glass 9.5% TiO2 2.0% boron carbide (220 mesh) .5% Methocel (8000 cps.)

As can be noted in the above preferred mixes, the percentage of Methocel and boron carbide is held constant, Iand the ratio of the titanium dioxide to Corning #7070 glass is Vvaried to change resistance values. This gives the'most stable and reproducible resistance values.

In the preparation of the resistor mix, the Corning #7070 glass is dry ball milled for two hours, starting with a -20 mesh material. After two hours, the titanium dioxide, boron carbide, and Methocel are added and the ball milling is continued for an additional ten minutes to insure a good mix. The materials are then removed from the ball mill, water is added and the batch is thoroughly mixed. The amount of water to be added is dependent upon `what final form is desired. For granulating or extrusion into pellets, 25% water should be added to the mix, while 40% should be added if the material is to be spray dried into a powder consisting of tiny spheres.

The procedure for assembling the components in the insulator body begins with dropping the center electrode 16 in the center bore 18, so that the head 24 will rest on the shoulder 25 in the center bore of the insulator body. The resistor mix is then deposited in hole 18 above the head 24 in any one of the forms described hereinabove, after which the stud 26 is dropped in. Since the mix is in a dry state, the stud will not penetrate the mix but will rest on its upper surface. The assembly, as described, is then placed in a suitable furnace and heated to a temperature between l600 and 1800o F., preferably l650 F. As soon as the resistor mix melts, the assembly is removed from the furnace and the stud 26 is pressed into the center bore 18 until the flange 36 of the stud contacts the upper surface 38 of the insulator body 10. This pressing operation aids in forcing the molten vitreous resistor 4material in and around the elements of the assembly and also into the more around the threads 41 provided at the lower end of the stud. The pressure is continuously applied to the stud 25 while the insulator body and assembly is being cooled to below red heat. At this point, the pressure is removed, and the insulator assembly is allowed to cool to room temperature. After the assembly is cooled, the composite center electrode, including the integral resistor, is subjected to a high voltage, such as 35,000 volts, to stabilize the resistance value.

There are various factors that affect the resistance values of the resistor element of the composite center electrode, among which is the resistor compositions outlined hereinabove. Other factors, Well known to those experienced in the art, are length or diameter of the resistor element, the temperature to which the sealing material is subject in the furnace, and the furnace atmosphere while being heated, whether oxidizing or reducing. Ample contact surface between the resistor element and the metal conducting members is also important, such as providing threads 41 on stud 26 and the configuration of head 24 on the center electrode 16, already described.

Referring now to FIG. 3, which shows a tri-axial diagram indicating the ratios of compositions of the three different materials forming the resistor mix, it will be noted that two different areas have been indicated. The 'smaller area, which is double cross-hatched, indicates the constituent ratios of the resistor mix which are suitable for use in connection with resistor spark plugs, while the area, which is singly cross-hatched, indicates the constituent ratios of the three elements of the resistor mix which are suitable for use in resistors which have general application. The tri-axial diagram has been truncated in order to allow the use of a larger scale for purposes of clarity.

In a resistor spark plug, resistors having a relatively low resistance, for example, below 1,000 ohms, are used for `the purpose of reducing erosion at the spark gap arising from the action of the spark. When the resistance value of the resistor element is increased to a range from approximately 10,000 to 30,000 ohms, the resistor not only reduces the erosion at the spark gap due to the action of the spark but also quenches high frequency oscillations in the ignition circuit, which gives rise to radio emanations and causes interference in radios and televisions in the immediate vicinity. The area, which is double crosshatched in the tri-axial diagram shown in FIG. 3, indicates the ratios of the constituents forming the resistor mix which is suitable for use in connection with spark plugs for the purpose outlined above.

It will be appreciated that the elements indicated in the tri-axial diagram of FIG. 3 may also be used to fabricate iresistances suitable for use in electronic circuits of various kinds. The fohniic resistance of these resistors require a much wider range and may even approach several megohms in their resistance values. It will be appreciated that these resistors may be manufactured 'in a similar manner as described with reference to the resistors suitable for use with spark plugs shown in FIGS. 1 and 2 of the drawings. The resistor mix may be positioned in the bore of a ceramic tube with suitable terminals being applied thereto in any manner well known in the prior art. It is intended that the disclosure above be interpreted suticiently broad, :so that a man skilled in the art may obtain the necessary information to fabricate resistors of this kind.

It is understood that the above-detailed description of the present invention is intended to disclose an embodiment thereof to those skilled in the art, but that the invention is not to be construed as limited in its application of the details of construction and arrangement of parts illustrated in the accompanying drawings since the invention is capable of being practiced and carried out in various ways Without departing from the spirit of the invention. The language used in the specication relating to the operation and function of the elements` of the invention is employed for purposes of description and not of limitation, and it is not intended to limit the scope of the following claims beyond the requirement of the prior art.

What is claimed:

1. A resistor consisting essentially of borosilicate glass, titanium dioxide and the products of reduction of titanium dioxide, and boron carbide and the products of oxidation of boron carbide, said resistor having a composidation of boron carbide, said resistor having a composition produced by heating a mixture consisting essentially of borosilicate glass, titanium dioxide and boron carbide at a temperature within the range of 1600o F. to 1800 F., the proportions of borosilicate glass, titanium dioxide and boron carbide in the mixture falling within the bounds of the double hatched area in FIGURE 3.

3. A spark plug incorporating a lresistor consisting essentially of borosilicate glass, titanium dioxide and the products of reduction of titanium dioxide, and boron carbide and the products of oxidation of boron carbide, said resistor having a composition produced by heating a mixture consisting essentially of borosilicate glass, titanium dioxide and boron carbide at a temperature within the range of 1600o F. and 1800 F., the proportions of borosilicate glass, titanium dioxide and boron carbide in the mixture falling within the bounds of the single hatched area in FIGURE 3.

4. A spark plug incorporating a resistor consisting essentially of borosilicate glass, titanium dioxide and the products of reduction of titanium dioxide, and boron carbide and the products of oxidation of Iboron carbide, said resistor having a composition produced by heating a mixture consisting essentially of borosilicate glass, titanium dioxide and boron carbide at a temperature within the tion produced by heating a mixture consisting essentially of borosilicate glass, titanium dioxide and boron carbide at a temperature within the range of 1600" F. to 1800" F., the proportions of borosilicate glass, titanium dioxide and boron carbide in the mixture falling within the bounds of the single hatched area in FIGURE 3.

2. A resistor consisting essentially of borosilicate glass, titanium dioxide and the products of reduction of titanium dioxide, and boron carbide and the products of oxirange of 1600 F. to 1800 F., the proportions of borosilicate glass, titanium dioxide and boron carbide in the mixture falling within the bounds of the double hatched area in FIGURE 3.

References Cited in the file of this patent UNITED STATES PATENTS 2,001,297 Boyles May 14, 1935 2,106,578 Schwartzwalder et al. Ian. 25, 1938 2,267,571 McDougal Dec. 23, 1941 2,340,963 Kapp Feb. 8, 1944 2,684,665 Tognola July 27, 1954

Claims

1. A RESISTOR CONSISTING ESSENTIALLY OF BOROSILICATE GLASS, TITANIUM DIOXIDE AND THE PRODUCTS OF REDUCTION OF TITANIUM DIOXIDE, AND BORON CARBIDE AND THE PRODUCTS OF OXIDATION OF BORON CARBIDE, SAID RESISTOR HAVING A COMPOSITION PRODUCED BY HEATING A MIXTURE CONSISTING ESSENTIALLY OF BOROSILICATE GLASS, TITANIUM DIOXIDE AND BORON CARBIDE AT A TEMPERATURE WITHIN THE RANGE OF 1600* F. TO 1800* F., THE PROPORTIONS OF BOROSILICATE GLASS, TITANIUM DIOXIDE AND BORON CARBIDE IN THE MIXTURE FALLING WITHIN THE BOUNDS OF THE SINGLE HATCHED AREA IN FIGURE 3.