Low tension ignition system
US 2733369 A
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Description (OCR text may contain errors)
w. B. SMITS LOW TENSION IGNITION SYSTEM Filed Oct. 19, 1954 INVENTOR ATTORNEY United States Patent Ofiice 2,733,369 Patented Jan. 31, 1956 2,733,369 LOW TENSION IGNITION SYSTEM Wytze Beye Smits, Voorburg, Netherlands, assignor to Smitsvonk N. V., Leidschendam, Netherlands, a research laboratory Application October 19, 1954, Serial No. 463,249 8 Claims. (Cl. 313-131) This invention relates to a low tension surface discharge spark plug and more particularly to the construction of the sparking element for such a plug. The low tension ignition system with which the spark plug according to the invention is to be used is described in my Patent No. 2,506,472; and is the type in which a condenser is periodically charged from a voltage source and then discharged across the electrodes of a surface discharge spark plug with a hot intense spark. A known low tension surface discharge spark plug is described in my Patent No. 2,625,922 and consists primarily ofv two concentric circular electrodes, between which is disposed a separating body of insulating or semi-conducting material. The hot spark from the condenser discharge burns across the surface of the separating body at the point of least resistance between the electrodes. This is a fundamental feature of spark plugs of this type and one which must be given primary consideration in the design of such spark plugs.
A second fundamental aspect of these spark plugs is that their life depends upon a uniform burning away of the separating body. If the spark across the separating body was always at one point, and a separating body was burned away at that point deep in between the electrodes, the life of the plug would be extremely short. On the other hand, if the spark occurred at varying locations around the circle between the electrodes, the separating body would be burned uniformly and gradually away, and the life of theplug would be greatly extended. v
The construction shown in Patent No. 2,625,922 is designed to permit the uniform burning away of the separating body. Whilethe arrangement shown therein has proved highly satisfactory, it has'the disadvantage that the electrodes must be precisely positioned with respect to each other since any eccentricity in alignment will result in a sparking occurring only at one point (the point of least resistance). The manufacture of these spark plugs is precision work and calls for the utmost accuracy because of the requirement for precise alignment of the electrodes. The spark plug as shown in that patent also has the further disadvantage that the plug has a restricted life since the insulating material between the electrodes may be burned away inwardly only to a given depth. If burning takes place beyond that depth, it will become difficult for the spark to contact a combustion mixture and ignition will be unsatisfactory.
Upon wear, the spark plugs are dependent to a considerable extent on the voltage applied between the electrodes; and with a combustion engine in which a good deal of oil deposits on the plugs there is always a risk of the spark burning inwardly only at one place rather than uniformly around the circle of the separating body.
It is the object of the present invention to eliminate the disadvantages considered above and to provide a spark plug which can be manufactured very inexpensively.
It is a further object of thejinvention to provide a spark plug having a useful life which can, practically speaking, be adjusted to any desired value.
.It is a further object of the invention to provide a for known spark plugs. a
The disadvantages of the known spark plugs are eliminated by constructing a spark plug having a large ing material, the spark willalways be exposedto the sparking surface, as compared with the above described the existing spark plugs.
As will be demonstrated, the spark plug according trodes during its whole life, since the resistance between can be controlled. To understand this, it .must be appreciated that the spark will occur at the point of least resistance as mentioned above. If the thickness of :the insulating layer continually increases, the sparking will continually take place where the insulating layer .is
which the insulating material grows thicker. An0ther advantage in having a very thin outer electrode is that the outer electrode together with the .insulating mass will gradually be burned away while the other electrode remains substantially intact. Thus, good heat discharge from the core of the. spark plug is ensured.
According to one aspect of the invention, the thicker electrode may consist of a cylindrical metal body with a substantially semi-spherical end. The enamel layer is applied to the thicker electrode from above as by pouring or dripping onto the tip of the semi-spherical end. This dripping process will cause the enamel to form a thicker layer at the lower end (at the base of the plug) than at the top of the semi-sphere.
The semi-sphere with its enamel layer is subsequently fired in order to bake the enamel layer.
After firing, a thin metal layer is applied to the enamel Further, since the area of the y These simple steps are all which are required to com-- plete the formation of the active parts of the spark plug. In order to obtain the initial sparking point, a small portion of the thin metal layer (the outer electrode) and of the enamel layer is removed at the top of the semi-sphere where the enamel layer is thinnest. It is at this point that the plug will first start sparking.
During continued operation, the whole of the spherical thin metal surface will be burned away gradually. The spark will always be well exposed to the combustion mixture, because the burning away of the outer electrode always prevents a burning in between the two electrodes. 7
In order to provide an even more effective control of the burning away of the electrode, before the application of the enamel layer, the spherical electrode may be provided with a number of shallow grooves. In these grooves, the insulating layer will be slightly thicker than at other parts of the electrode assembly so that the burning away will be controlled in a given direction in segments.
Further advantages of the invention as well as those described above will become more readily apparent from the following detailed description of the invention taken in conjunction with the drawings in which:
Fig. l is an elevational view, partly in section, of the spark plug according to the invention,
Figs. 2, 3, 4 and 5 are cross-sectional views through the end portion of the electrode assembly and showing alternative embodiments of said assembly, and
Fig. 6 is an end elevation of still a further embodiment of the electrode assembly.
In Fig. 1 the numeral 1 indicates the plug body with screwthread 2, insulator 3, and central terminal 4. Secured in the screwthreadedpart with the aid of an insulating mass 6 is the central pin 5, and this pin is connected with the central terminal 4. Screwed onto the pin 5 with the aid of screwthread 7 is a substantially semispherical part 8 of metal, which serves as the tension electrode of the sparking plug. This electrode is covered with an enamel layer 9, which from the point 10 downwards gradually becomes slightly thicker, and from'the point 11 becomes thickest. Down to this point the sparking surface may be consumed, since the spark could no longer come into contact with the gas, if the spark could burn in deeper down. On this enamel layer 9 has been applied a metal layer 12, which is connected with the plug body and serves as the second electrode. At this point 10 a small portion of the thin metal layer 12 and of the enamel layer 9 has been removed, so that there the first sparking point is formed.
Figs. 2 and 3 illustrate how this removal can be effected. According to Fig. 2 the semi-sphere 8 has a projecting tip 13, which is broken 01f upon completion of the sparking element.
It is also possible to make a notch 14 in the semisphere 8 before the application of the enamel layer (Fig. 3). After completion of the sparking element the outer electrode and the enamel layer are then removed in this place by a stroke on a pointed object. From the top downwards the metal layer 12 and the enamel layer 9 will be gradually burned away, and finally all the material will have been burned away down to just above the point 11, upon which the electrode assembly 8, 9, 12 has to be renewed. Since the area of said element is very large and can be increased according'to requirement, this sparking plug has a very long life. The enamel layer at the point 11 being so thick that the sparking plug can no longer be caused to spark without considerably increasing the tension, it cannot burn it, which would have the detrimental efiect referred to above.
In order to increase the area of the sparking element even further, it may also be given the vaulted form of Fig. 4 or the corrugated form of Fig. 5.
In the construction according to Fig. 6, the semispherical electrode 8 is formed with shallow grooves 15. When the enamel is formed on the semispherical electrode, that insulating layer will be thicker at grooves 15 than on the remainder of the surface of electrode 8. Thus, the outer electrode, which is subsequently applied, and the insulating material will burn away more readily at the areas other than the grooves 15. In this manner, it is possible to regulate the direction in which the materials will be burned away from the beginning to the end of the life of the electrode assembly.
Finally, one more stripor point-shaped layers 12a of conductive material may be provided in the insulating layer, as a result of which on the one hand, better sparking action'is obtained while on the other hand, the burning away will become very uniform.
In a general manner, while there has been disclosed in the above description what is deemed to be the most practical and efiicient embodiments of the invention, it should be well understood that the invention is not limited to such embodiments as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.
1. In a low-tension surface discharge spark plug for internal combustion engines, an electrode assembly comprising, an inner electrode, a coating of insulating material on said inner electrode, and a thin outer electrode disposed on said insulating material.
2. A spark plug according to claim 1, in which said outer electrode surrounds substantially the entire inner electrode. a
3. A spark plug according to claim 1, in which said inner electrode comprises a cylindrical metal body terminating in a semi-sphere.
4. A spark plug according to claim 3, in which a small portion of the thin metal layer and the insulating material is removed in one place.
5. A spark plug according to claim 1, in which said insulating material is enamel.
6. A spark plug according to claim 1, in which the surface of the inner electrode extending beyond the plug body is at least partly corrugated so as to increase the "active surface.
7. A spark plug according to claim 3, in which the semi-sphere is provided longitudinally with a number of shallow grooves filled with insulating material.
8. A spark plug according to claim 3 in which the insulating material is thinnest at the tip of the semisphere and gradually increases in thickness toward the end of the electrode remote from the semi-sphere.
No references cited.