US1958580A - Spark plug - Google Patents

Description

May 15, 1934. A. A. KASARJIAN SPARK PLUG Filed July 17, 1951 1 orgklzszzga m, or e fie.

INVENTOR.

Armen A kasa/y/an A TTORNEY.

Patented May 15, 1934 UNITED STATES PATENT OFFICE SPARK PLUG Application July 17, 1931, Serial No. 551,318 5 Claims. (Cl. 123-169) This invention relates to spark plugs, and the object of the invention is to improve the spark at the proper gap through the elimination of parasitic discharges and leakages of the ignition current.

Exhaustive experimentation and research, particularly with aviation plugs, has convinced me that the most serious difficulty encountered in plug operation is the direct result of parasitic discharges, due, primarily, to the presence of air pockets in the inner plug structure. Such pockets will of course vary in size according to the design of the plug or their location therein. In some cases, they may be relatively large, while in other cases, they are a mere film of air between adjacent parts of the plug. I have found that the greatest difiiculty arises when these pockets are substantially sealed by the plug structure during assembly of the parts and where such pockets occur between metallic parts of opposite electrical polarity.

Air under ordinary conditions is a fairly good insulator, but it is by no means a perfect insulator particularly with respect to high tension current, such as is used for ignition purposes. My experiments have definitely shown that when high tension current is impressed upon metallic parts separated by an air gap, there occurs a silent discharge or corona discharge. If the air is confined within the substantially closed pocket, such as is commonly found in conventional plug assembly, ionization of the air results, and such ionization proceeds rapidly until a point is reached at which the di-electric property of air is so lowered that electric current will pass through it in the form of a jump spark. When this occurs within a plug structure, short circuiting of the proper gap of the plug results.

In the design of a spark plug, the resistance between the electrodes of the proper sparking gap of the plug is made very small in comparison to the resistance to any other path which the current may take between the conductive parts of opposite polarity. These other paths, I will, for the sake of brevity, hereinafter refer to as parallel paths. This is purposely done so that the gap will be ionized as rapidly as possible and broken through by the spark. Electrical discharges in the other parallel paths are parasitic and harmful to the proper operation of the plug and thus the higher the resistance of these parasitics the more efiicient the discharge at the proper gap.

It is characteristic, however, of spark plug oper ation that as the spark jumps between the electrodes at the gap proper, the electrodes are eroded or burned away and the gap is gradually increased in dimensions with consequent increase in resistance. This action of erosion is particularly noticed where the high test fuels are used or where tetra ethyl lead is used in the fuel. Eventually the gradual increase of the resistance at the gap proper reaches a value above the resistance of one or more of the so-called parallel paths, so that the current-instead of jumping the gap proper to form the igniting spark, jumps across the parallel path of less resistance and the plug is said to be short circuited.

Where air pockets are sealed in the plug during assembly of the plug structure, the ionization of the air in such pockets materially decreases the di-electric property of the air as stated, and, consequently, the di-electric properties of the air in such pocket are rapidly decreasing at this point while the resistance at the proper gap is increasing. In other words, the resistance to the proper path of current is increasing, while the resistance to the improper path of current is decreasing. With such conditions prevalent, the plug does not long remain operative, but becomes short circuited and ceases to function. This condition is particularly noticeable in plugs used for aviation purposes, and which plugs generally employ, because of the high temperatures encountered, mica insulation. Thepurpose of the present invention is to eliminate the so-called parallel paths, within the plug structure which, in the great, majority of'cases, I have found to be directly attributable to the presence of air pockets, and consequent setting up of a so-called silent or corona discharge as hereinbefore described. I have discovered that by eliminating these air pockets, the silent or corona discharge within the plug structure is entirely eliminated and the resistance within the plug structure can at no time become less than the resistance at the gap. Accordingly, I thus eliminate parasitic discharges and am able to produce a plug that is absolutely dependable for a much longer time.

The present invention may be practically carried out in various ways without departing from my broad inventive concept. However, I find it essential to the carrying out of this invention that the presence of air pockets within the plug structure be eliminated and thus according to this invention, the presence of air within the plug structure is eliminated by the actual filling of all spaces within the plug structure with a suitable substance of high (ii-electric value under the temperatures prevailing in the plug during operation.

Various substances may be employed for the carrying out of this invention, but I may mention, for example, silicate or ceramic compounds or mixtures, plastic mica or the like which may be applied in a plastic or semiliquid condition under pressure to fill all the pockets and crevices inside the plug. This filling is accomplished during the assembly of the parts of the plug and all air is thereby expelled from the interior of the plug structure. I

By preference, the substance or compound referred to is applied in liquid or semiliquid form, and after assembly may be left in liquid form or baked into a solid, insulating substance.

Experience has shown that this invention materially improves the performance of plugs by eliminating parasitic paths within the plug structure resulting from the elimination of air and thereby reducing the corona discharge. The proper gap at the plug may become larger than heretofore which will give longer life to the spark plug between periods of overhaul. A better and more efficient spark alsoresults through the elimination of the parasitic discharges in the parallel paths to which I have referred, by concentrating all of the energy of the spark generator at the proper gap.

Features of the invention, other than those specified, will be apparent from the hereinafter detailed description and claims, when read in conjunction with the accompanying drawing.

The accompanying drawing illustrates different practical embodiments of the invention, but the constructions therein shown are to be understood as illustrative, only, and not as defining the limits of the invention.

Figure 1 is a central section through a plug of high compression type and with which the present invention is shown as associated.

Figure 2 is a similar view of a plug of the low compression type embodying the present in vention.

Inasmuch as the present invention is not restricted to any particular kind or type of plug, I do not consider it necessary to describe in great detail all the parts of the plug structure shown in the drawing as this is, in the main, conventional and illustrative. It is sufiicient to specifically describe only those parts of the plug as will be necessary for a full understanding of this invention.

In Figure 1, 1 designates the central electrode of the plug to which the, ignition lead is adapted to be secured in the usual manner. The central electrode carriers the sparking point 2, while the grounded electrode 3 is carried by the shell 4 of the plug. In the particular type of plug shown, the central electrode is surrounded for a greater portion of its length with a conductive sleeve 5, preferably of brass or copper or some other highly .conductive material. In fabricating the plug, there is wrapped about this sleeve sufficient thicknesses or turns of cigarette mica 6. In the wrapping of this cigarette mica, one end of each piece as it is applied is dipped into the insulating substance, so as to apply thereto a body of the substance and. as the mica is wrapped about the sleeve 5, there is sufficient insulating substance thereon to spread itself over both surfaces of the mica and over the surface of the sleeve 5, so that each successive turn ofthe mica becomes embedded in the substance. As a result, when the cigarette mica has been wrapped about the sleeve 5 as stated, the

. wrapped body becomes in effect a solid body from which all air has been excluded by the flow of the insulating substance. In other words, no air will be entrained in the pockets or in film form between consecutive turns of the cigarette mica. Of course the same result can be obtained by coating one or both surfaces of the mica before application, but this has not been found necessary as the substance which I have described will fiow freely.

An important advantage in filling the space between the laminations of the cigarette mica is that this mica is apt to crack during winding. These cracks are, of course, very fine, but if the insulating substance is used as stated, all of these minute cracks are filled and possibility of short circuiting between the parts 11 and 12 is avoided.

In assembling the insulating body '7, which is composed, as shown in Figure l, of laminated disks of mica, these disks are coated-with the insulating substance in liquid or plastic form, as they are put on, and, consequently, all spaces between consecutive laminations, as well as the space 8 between the inner periphery of these disks and the cigarette mica body 6 is fiowed full of the insulating substance, as clearly indicated in Figure 1.

In assembling the insulating body 9, which is also built up of mica disks, these disks are also coated with the insulating substance in liquid or plastic form, so that all spaces between consecutive laminations or plies, as well as the space 10 between the inner circumference of this body and the cigarette mica body 6 is flowed full of the insulating substance and thus all air is expelled during this operation. It will of course be understood that the parts of the plug structure are assembled. under pressure, so that the insulating substance is caused to fill up all spaces in the interior of the plug structure and thereby exclude air therefrom through displacements of such air.

After the plug is fully assembled, the substance may be left in liquid or plastic form, or it may be baked into a solid insulating substance. I have referred to the assembling of the cigarette mica about the sleeve 5 by dipping it into the insulating substance before wrapping the mica about the sleeve. The operation is somewhat similar from a mechanical standpoint to the practice which has been heretofore employed of dipping cigarette mica into some lubricant, such as lubricating oil or linseed oil, the purpose heretofore being to facilitate the tightening of the plug against gas leakage.

My experience, however, with the use of oil, as stated, is that the oil which under such conditions works into the spaces 8 and 10, quickly carbonizes, so that the spaces 8' and 10 are divided graphically speaking into a series of relatively small gaps interposed between bodies of carbon, whereby the resistance to the passage of current is quickly reduced below the resistance of the gap between the electrodes 2 and 3 which describes the usefulness of the plug.- The insulating substance of the present invention is necessarily a refractory di-electric substance, which will not carbonize, but I have found nevertheless that when using such a substance I obtain all the advantages inherent in the use of lubricating or linseed oil, so far as their function of facilitating the tightening of the plug is concerned, and at the same time obtain a permanent di-electric with a permanent exclusion of air. This can never be acccomplished by the oils whichhave neaaaeo heretofore been used to facilitate tightening of the plug parts during assembly.

The advantage of this invention as applied to a plug, such as shown in Fig. 1, may be made clearly apparent by a brief description of what happens in a plug of this type when this invention is not incorporated. If it be assumed that the space 10 is free from the insulating substance of this invention, it will he noted that there is a rather long, narrow annular air space interposed between the conductive material 11 of one polarity and the conductive material 12 which by virtue of its elec rical connection with the shell- 4 is of oppositr' polarity. In practice, air will be entrained in this space 10 during the assembly of the plug. As a result, a so-called silent or corona'discharge will in due course result through this body of air between the parts 11 and 12. 1 lhe air will shortly be ionized and if the gap between the electrodes 2 and 3 increases in resistance materially, it will not be long before the spark will be jumping between the parts 10 and 12, instead of between the parts 2 and 3. r In a similar manner, if the space 8 is in the form of an air space between the part 14 which is grounded with respect to the shell 4 and the sleeve 5 which is of opposite polarity, it will not belong before current will be jumping from the part 5 beneath the lower edge of the cigarette mica body 6, and thence upwardly to the part 14, due to the ionization of the air as hereinbefore described.

When these spaces 8 and 10 are, however, so

filled that the cigarette mica body 6 and the will nevertheless still be the point of least resistance to the passage of current between the central electrode and the shell. Consequently, a plug embodying this invention, will operate longer and with a larger gap than any other similar plug now known.

The structure of Figure 2 will'be perfectly clear from the description already advanced in connection with Fig. 1. It will be noted'that in Fig. 2, 1 is the central electrode, 6 the body of cigarette mica, wound in the manner described,

' directly about the central electrode without the interposition of the sleeve 5 and the body of mica 9 corresponds to a similar body of the plug of Figure 1. The space 10 is filled with the insulating substance in the same manner as described with reference to Fig. 1, and the various laminations of the body 9 are assembled in like manner, so that the cigarette mica body, as well as the insulating body 9, together with the in sulating substance in the space 10 becomes one solid block of insulation, excluding all air from the inner plug structure.

It is important to note that the incorporation of the present invention in the plug construction does not increase the cost of production other than the mere cost of the insulating substance employed which is negligible, and, nevertheless, through the employment of the present inven- Having thus fully described the invention, what I claim as new and desire to secure by Letters Patent is:

1. In a spark plug, a central electrode, cigarette mica wrapped about said electrode in close fitting relation thereto to form a laminated sleeve of insulating material, a refractory insulating substance interposed between said laminations and between the laminated sleeve and said electrode to preclude the formation of air pockets between consecutive laminations, an insulating member surrounding the laminated mica sleeve, and a refractory insulating substance filling the space between said insulating member and the sleeve to preclude the formation of air pockets between the sleeve and said member.

2. A spark plug embodying a shell provided thereon with a grounded electrode, an insulating element mounted within said shell, a central electrode extending through said insulating element and in relatively close fitting relationthereto, and means for precluding corona discharges through the plug comprising an insulating refractory substance filling the air pockets within the insulating element to exclude residual air from said element.

3. A spark plug embodying a central electrode, a, spaced grounded electrode, an insulating element for spacing said electrodes apart which insulating element embraces the central electrode in close fitting relation, and means for preventing corona discharges in the plug comprising an insulating refractory substance impregnating the insulating element to an all pockets therein for the purpose of excluding air from said insulating element.

. 4. In a spark plug, a central electrode, cigarette mica wrapped about said central electrode in close fitting proximity to form a laminated sleeve, and an insulating refractory substance filling all pockets between consecutive laminations of the sleeve and between the sleeve and electrode to exclude air from the sleeve and from between the sleeve and the electrode and to thereby prevent corona discharges through said sleeve.

5. In a spark plug, an insulating element provided with a longitudinal passage therethrough for the insulating electrode of the plug, an insulated electrode extending through said pas- I sage in close fitting relation to the wall thereof,

and an insulating refractory substance impregnating said element to fill all pockets therein for the purpose of excluding residual air to prevent corona discharges.

ARMEN A. KASARJIAN.

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