US1962669A - Spark plug - Google Patents

Description

June 12, 1934.

J. w. PARKIN, JR

SPARK PLUG Filed Aug. 12, 1931 2 Sheets-Sheet l 3mm .IZParfLin,JE?

J. W. PARKIN, JR

SPARK PLUG Filed Aug. 12, 1931 2 Sheets-Sheet 2 .4% a Wra h ,1121

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June 12; 1934.

Z m 37% 3 w r Patented june 12, 1934 SPARK PLUG Joseph W. Parkin, Jr., Philadelphia, Pa. Application August 12, 1931, Serial No. 556,648

3 Claims.

This invention relates to spark plugs and proposes improvements in the construction and arrangement of the cooperating elements at the sparking end of the plug by which the spark is rendered fatter and more intense, the spark plug self-cleaning, and the optimum temperature value maintained of the insulator and its appurtenant parts.

One of the objects of the invention is to construct a spark plug having an annular gap between the middle electrode and shell, with the end face of the insulator interposed in the path of spark travel across said gap, whereby the insulator is kept clean by the detergent action of the spark itself.

Another object of the invention is the provision of means for increasing the depth of the ionized path which the spark traverses, so as to spread the spark, presenting a greater surface to contact with the combustible mixture.

A further object of the invention is'to provide a metallic shield for the insulator, projecting beyond the threaded zone of the shell and surrounding the insulator, absorbing heat at the peak of the explosion and giving it back to the insulator by radiation as the temperature curve of the exploded mixture declines so that the temperature of the insulator is maintained at a substantially even high value at which the hydrocarbon will not condense upon the surface of the insulator.

Still another object of the invention is to construct a spark plug which normally sparks to a wire ground electrode, but as soon as the end of the insulator starts to foul, transfer the sparking path to the surface of the insulator, functioning thereafter in the manner previously described until the dielectric value of the insula tor surface has been restored, whereupon the sparking circuit shifts back to its normal path between the central electrode and the wire ground electrode.

Other objects of the invention will appear as the following description of a preferred and practical embodiment thereof proceeds.

In the drawings in which the same characters of reference have been used throughout the several figures to designate identical parts:

Figure 1 is a perspective view of a spark plug embodying the features of the present invention;

Figure 2 is a plan view of the inner end of the spark plug;

Figure 3 is a longitudinal section through the inner end;

ionized path of spark travel and the detergent action of the spark upon the peripheral lip surrounding the lower end of the insulator;

Fig. 5 is a similar view showing a contrasting construction in which the insulator projects to fall into the combustion chamber causing the spark to are over the lip of the insulator without contacting with the same;

Fig. 6 is a cross section taken along the line 66 of Figure 3;

Fig. 7 is a modified form of the invention;

Fig. 8 represents still another modification, the insulator being formed of mica; and

Fig. 9 is a plan view of the plug shown in Figure 8.

Referring now in detail to the several figures, the numeral 1 represents in general, a spark plug having the insulator 2, the shell 3 and the central electrode 4 mounted in the insulator. Figure 3 shows that the insulator seats in the shell in the usual manner by means of coopcrating shoulders between which a gasket 5 is compressed. The shell has a preferably frustoconical shield 7 projecting beyond the threaded zone of the shell and the skirt 8 of the insulator is sufficiently long as to bring the end of the insulator just a trifle beyond the peripheral edge of the shield 7. A narrow inert gas space 9 is formed between the skirt of the insulator and the shell.

The insulator is preferably slightly cupped at the end as is indicated at 10 in Figure 3 forming a peripheral lip 11 and the central electrode projects slightly beyond the bottom of the cupped portion as shown at 12 in Figure 4. Y

An annular gap is formed oetween' the central electrode 10 and the peripheral edge of the shield '7. The end surface of the insulator obstructs a portion of this gap and constitutes a path for the passage of the spark which crawls along the surface of the insulator and then leaps the narrow gas chasm 13 remaining between the insulator and the shell. When the spark plug is new and clean, there is no preferred path for the spark and the current therefore, generally divided into a plurality of sparks leaping radially in several directions across the surface of the insulator and across the gas chasm 13 to the shell.

If the plug becomes fouled at any point on the end of the insulator, the resistance of the gap is reduced at this point and it becomes the preferred path of spark passage. The spark passing at this point quickly burns up the fouling deposit and the spark plug is again clean. The provision of the gas chasm 13 is principally to prevent a short circuit in the event that the surface of the insulator becomes fouled. If it were not for this gas chasm, the current would leak across the fouled area of the insulator without producing a disruptive discharge.

The spark during the greater part of its passage across the gap and by the term gap, it will be noted that the end surface of the insulator is regarded as a part thereof, the spark crawls or contacts with the surface of the insulator. This of course presents only the outer surface of the spark to contact with the explosive mixture and some uncertainty of the explosion might result. It is highly desirable to spread the spark depthwise so as to bring a greater ignition surface into contact with the explosive mixture. This is accomplished by increasing the depth dimension of the path of ionization which the spark traverses in its travel across the gap. With the central electrode terminating flush with the surface of the insulator in the manner indicated in Figure 5, the spark will be flat and only the outer surface thereof will be in contact with the combustible mixture. In Figure 4, the central electrode projects a very slight distance beyond the bottom of the cup portion 12 and so produces an ionized path of considerable depth as is indicated by the dotted lines in Figure 4. Experiments have shown that the central electrode extends a considerable distance beyond the cupped portion 12, the path of ionization will not be correspondingly deepened but will emanate very close to the base of the projecting part of the elec trode in the manner indicated by the dotted lines. Wnen the spark traverses this deepened sides are rendered available for ignition contact with the combustible mixture.

While the maintenance of an insulator clean in its entirety is a desideratum in all spark plugs, it suffices if the peripheral lip of the insulator presents a clean non-conductive surface. In the present spark plug the insulator projects to a very slight degree beyond the end of the shell so that the course of the spark in changing its direction as it leaves the cupped portion of the insulator brings it into intimate contact with the insulator at the lip and causes it to perform the necessary detergent action in clearing the lip of all foulness. The extent to which the insulator may project may be decidedly overdone and Figure 5 shows an instance in which the insulator projects to fall through the gap. The spark has a certain inertia and in traversing the steep sides 14 of the cupped portion of the insulator, it flies beyond the lip of the insulator before changing its direction and descending toward the shell. By so doing, it altogether misses making contact with the lip of the insulator and in consequence, has no detergent action upon said lip.

Figures 1 and 3 show that only a very small part of the insulator is exposed direct to the heat of the combustion chamber. While the skirt of the insulator projects Well inwardly beyond the threaded zone of the shell, yet the latter is provided with an inwardly extending and preferably frustro-conical shield '7 likewise extending inwardly beyond the threaded zone of the shell and surrounding with a fair degree of closeness the lower portion of the skirt of the insulator. At the moment of the explosion when the heat of combustion is at its maximum,

the insulator is protected from the intense heat, the heat being in the main absorbed by the shield 7. As the piston of the engine descends on its working stroke and the temperature curve of the expanding mixture quickly falls due to its doing work, the heat previously absorbed by the shield 7 is now given out by radiation to the insulator. The latter in consequence, is not so hot as the combustible mixture at the peak of heat of the latter, nor so, cool as the combustible mixture during the major portion of its working stroke. In other words, the temperature of the insulator remains substantially normal and its value is such at all times that liquid combustible will not condense upon it, nor will the products of the explosion.

Figure 7 shows a form of the invention in which the spark normally leaps the gap between the central electrode 4 and a wire ground electrode 14. The relation of the end of the insulator to the shell and middle electrode is the same in this instance as in the previously described forms of the invention, the gap between the electrodes 4 and 14 being less than the gap value between the central electrode and the shell 15 when the end of the electrode is clean. Should sufficient foulness accumulate on the end of the insulator to reduce the gap value across the insulator in less time than that between the electrodes 4 and 14, the spark path will shift, the spark travelling from the central electrode over the end of the insulator along the path of least resistance, leaping the chasm 13 to the shell 15. The disruptive discharge along the surface of the insulator exerts both a mechanical and burning effect upon the foulness restoring the insulator to clean condition, whereupon, since the di-electric value of the spark path across the insulator has again exceeded that between the electrodes 4 and 14, the spark path will shift back to the gap between the last named electrodes.

Figure 8 shows a form of the invention in which the insulator is built up in known manner of a roll of mica surrounding the central electrode and the pile of mica disks 16 surrounding the roll and compressed together. The mica disks are turned or otherwise shaped so as to give the proper form to the insulator. In the case of mica it is impossible to form a well in the end of the insulator such as that shown in Figure 4. The central electrode is provided with an enlarged head 17 abutting the fiat lower end of the insulator, said end projecting slightly beyond the end of the shell extension 7 so that the peripheral edge of the end of the insulator will be in the path of spark passage from the central electrode to the shell extension. The thickness of the head 17 causes the ionized path of spark passage to be spread depthwise, thus producing a fat spark.

With the construction of the spark plug according to the present invention, it is practicable to build it of very small size. This has a distinct advantage, particularly in the engines of air-craft. A large percentage of accidents in the operation of air-craft occurs during the taking-off period. This is due to the fact that the spark plug is cold, that the combustible mixture condenses upon it and causes the engine to mis-fire and possibly fail before the aircraft has gained altitude. By making the plug of very small size as may be done in the present invention, it heats up much quicker than does the ordinary spark plug so that the engine attains an operating temperature at which hydro-carbon moisture will not condense upon the insulator, much more quickly than when using the ordinary spark plug.

While I have in the above invention disclosed what I believe to be a practical and preferred embodiment of the same, it is to be understood that such changes may be made in the construction of parts as to adapt them to the varying exigencies of use and that the illustrative embodiment is to be considered as exemplary but not as necessarily restricting the claims to the construction as shown.

What I claim is:

1. Spark plug including an annular shell, an annular insulator arranged coaxially within said shell and extending slightly beyond the plane of the end of said shell, and having a central well, a central electrode carried by said insulator and projecting into and terminating within said well, the spark gap thus formed between said electrodes comprising in serial relation an annular peak of insulation surface, separated from the ground electrode by a deep annular chasm, and extending outwardly slightly beyond the ground electrode, the wall of said well forming a spark path from said peak to the central electrode.

2. Spark plug as claimed in claim 1, the peak of insulating material intersecting the otherwise direct path between said electrodes, determining a deflection in the direction of'the'passage of the spark.

3. Spark plug as claimed in claim 1, the extending portion of the central electrode determining a spreading 'of the spark within said well longitudinally of said electrode.

JOSEPH W. PARKIN, JR.

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