US 2926275 A
Description (OCR text may contain errors)
L. PRAs SLIDING SPARK IGNITION SPARK PLUG Feb. 23, 1960 2 Sheets-Sheet 1 Filed April 8, 1957 Belleville Washers Bakelite Insulator orcelain lnsulalor CERAMlC MATERIAL coMPosl'rloN Rullle lf v Barium Ttonule Calclum Tlonole Stronlum Tilanote Alkollne'Earth Zrconale Alkaline-Earth Slonnale Feb. 23, 1960 l.. PERAS sLIDING SPARK IGNITION SPARK PLUG 2 Sheets-Sheet 2 OmtN OOO-
BSVDOA BALLdmiS IO United States Patent SLIDING SPARK IGNITION SPARK PLUG Lucien Peras, Billancourt, France, assignor to Regie Nationale des Usines Renault, Billancourt, France Application April 8, 1957, Serial No. 651,264
Claims priority, application France April 17, 1956 '3 Claims. `(Cl. 313-131) This invention relates to ignition devices of the capacity-discharge type, wherein the electric tension and the electrode gap are of such values that the spark cannot occur in the atmosphere in which the ignition is to take place unless it remains in contact with a solid insulator disposed between the electrodes.
vExperience teaches that in the absence of a solid insulator, the spark cannot occur unless either Vthe 'tension value is extremely high (in which case capacitydischarge ignition must be given up), or very closely- Srpaced-electrodes are used, with a gap not greater than 0,03 mm. (0.0012); however, this latter alternative, apart vfrom being hardly feasible under practical conditions, results in a very rapid increase in the spark gap due to the abnormal wear of the spark forming faces which leads to the failure ofthe device.
It is already known, for example in the application of this .type of ignition system to internal combustion engines, to provide an assembly (called hereinafter an insert or cartridge) consisting of two vmetal electrodes spacedabout 0.1 to 0.2mm. (0.004" to 0.008) apart across the spark gap, and of a solid insulator. To form a spark plug, the aforesaid cartridge or insert is mounted in a body adapted to accurately position this insert and also to provide the necessary electrical connections to the electrodes. These electrical connections are preferably of the sealed type.
The useful life of capacity-discharge spark plugs of hitherto known types is generally considered as insulcient or at any rate definitely shorter than that of hightension induction sparkl plugs now commonly utilized in internal combustion engines of all types.
As a rule, the failure of these capacity-discharge spark plugs results from one o-r both of the following different causes of deterioration:
`Premature wear of the electrodes which causes the spark forming faces of the spark gap to `recede fromV each other until their relative distance is too great to permit effective sparking to take place;
Premature wear of the insulator due to the thermal action of the spark, whereby the spark is caused to recede inwards of the insert until it is unable to ignite the fuel mixture.
.However, this latter cause of failure may be retarded or reduced to a substantial degree by utilizing, in the manufacture of insulators, materials such as alumina which are both refractory vand -moderately sensitive to thermal shocks.
Thus, in the present state ofthe art, the useful life of these spark plugs is essentially dependent on the wear of the metal electrode. Many comprehensive tests have shown that tungsten has a definite superiority over any other metal or alloy, it was not deemed possible that the useful life of compact-tungsten electrode spark plugs could be further increased.
Now, it is the principal object of this invention to increase in a very substantial manner, much beyond what was considered up to now as the utmost limit, the
2 useful life of capacity-discharge spark plugs. vThis invention is the consequence of the discovery made by the applicant that the higher the dielectric constant of the insulator, the greater the upper limit or maximum distance of the spark gap, whether in air or other kgaseous media but in contact with the solid insulator.
Whereas up to now the only desired properties of'lowtension Vspark plug insulators were their thermal VVresistance (refractory insulators) andthe ease of assembling, whereby only substances having at the most a dielectric constant of 12 (on the basis of the dielectric constant of a vacuum taken as unity) were utilized, such as mica, special glass compositions, porcelain, etc., it yhas now been discovered that useful lifes of a completely different order of magnitude than those considered as satisfactory up to now `could be obtained by utilizing as insulating materials inorganic substances having la dielectric constant of 50 or more.
These substances are used in the form `of sintered ceramics prepared'from a pure substance or from Va mixture of substances having `a h igh dielectric constant. It is also possible to employ insulators wherein vone or more high-dielectric constant substances is or are bound by a small amount of vitriiable substance, such as calcium silicate, Silico-berate of alumina, etc., the proportion of vitrilied material remaining low enough to keep the dielectric constant of the assembly at a value higherthan the limit set up by this invention.
According to a more specific object of thiS invention, the ceramics employed consist of one or more of the following inorganic substances which .are well-known on account of their high specific inducting power: rutile, barium titanate, calcium titanate, mixed Crystals 0f barium titanate and of one or more of `the compounds.; strontium titanate, alkaline-earth zvirconate, alkaline-.earth stannate.
The'variation in the dielectric constant as a function of temperature is a determinant factor in the choice of the substance. A high value of this constant at `temperature values close to rthe ,ambient temperature, as in the case of calcium titanate or barium titanate, is advantageous in that spark plugs for internal `combustion engines which incorporate this insulator produce very hot sparks so that the starting of cold engines is greatly facilitated.
The judicious use, in the insulator of Vcapacity-discharge spark plugs, of already known lmixtures of titanates, zirconates, stannates, makes it possible to limit the variation inthe dielectric properties as a functionof temperature and to preserve to a high degree, Vwhen the engine is hot, the advantages vderived from easy spark production. The mixtures obtained by sintering lthe above-mentioned oxides with their limit compositions are particularly suitable and cited hereafter andby AWay-0f example.
Ti02 from 40% to 60% ZrO2 from 2% to 10% BaO from 20% to 40% CaO from 10% Vto130% `In these'known `mixtures the rtotal number `of molecules of the basic oxides BaO, CaO is `equal tothertotalnurnbier of molecules of the acidoxiles' TiQg, YZrQz), so that the sintered mixture contains 4o nly titanates Tand zi-rconates in the forrn'MO2 MO.
The regularity of operation as a function of temperature is further improved by the addition to the abovementioned mixtures of titanium oxide of the rutile mineralogical variety.
it had already been attempted to improve the resistance of sliding-spark ignition plugs by modifying the nature of the insulator and according to one of these attempts spark forming has been made somewhat easier, in that a spark can take place through a wider gap for a given electric tension, by incorporating powdered metals in theinsulator composition. Nevertheless, it has not been possible to extend this improvement to the field of industrial production because the insulator was very rapidly decomposed by small internal sparks produced by relatively high local electrical fields. As a matter of fact, up to now the essential property which had to be Sought for this type of insulating substance was unknown.
Reference will now be made to the attached drawings forming part of this specification and wherein:
Fig. 1 is a sectional view showing a spark plug and its insert made in accordance with the teachings of this invention, that is, with a high-dielectric constant insulator; and
Fig. 2 is a diagram illustrating the properties of a spark plug comprising an insulating susbtance according to this invention.
Referring first to Fig. l, it will be seen that the insert comprises two concentrically disposed electrodes; one outer cylindrical electrode 1 is of annular configuration and the other inner electrode 2 is of disc form. Above this disc-shaped electrode 2 is a ceramic insulator core 3 of refractory composition according to this invention. The component elements of the plug are assembled and held in rigid relationship by a molten-glass or any other insulating cement-like filling shown at 4. The inner electrode 2 is clamped by means of a conducting rod 5 held axially in tension by belleville washers 6 bearing on a Bakelite insulator 7. The conducting rod is also intended for electrically connecting the inner electrode 2 with the current generator and distributor (not shown); the other electrode 1 is grounded through the intermediary of the plug body 8. Insulators 9 and 10 are positioned axially of the plug body as shown.
Practical tests have been made with spark plugs incorporating insulators according to this invention and the advantageous results obtained therefrom are set forth hereafter with reference to the diagram of Fig. 2.
This diagram illustrates the values of the disruptive tension for various electrode gaps; the insulators disposed between the electrodes diiferring by their dielectric constant. The values were taken from two different crystallographic forms of mica, one being of the muscovite variety and the other of the biotite variety, both having a specific inducting power close to 8, and also from two dielectric substances A and B for which this coe'icient was 300 and 3,300 respectively.
The curves for the different insulators contemplated are designated as follows:
Composition ceramic: TiOz 40%; BaO 59%; and CaO Dielectric constant 3,000;
Composition ceramic: T102 45%; BaO 50%; and CaO Dielectric constant 300;
Muscovite mica 6 to 8;
Biotite mica 6 to 8.
The values cited by way of example were taken at 20 C. in the air at atmospheric pressure, but the inferences that can be drawn from these measures regarding the influence of the dielectric constant are valid in all cases. Consequently, it appears that for a given electrode gap, for example 1 millimeter, the tension required for producing the spark, which of 2,300 in the case of mica, may be as low as 1,000 volts with the high-dielecf '4 tric constant insulator and that, for a given tension, for example 2,000 volts, the minimum electrode gaps to be maintained for producing the ignition sparks are 0.75, 1.50 and 2.6 millimeters respectively.
From the electrode arrangement illustrated diagrammatically in Fig. 1, it will be seen that for a given initial gap between the electrodes 1 and 2, the nal gap may be even three times the initial gap if a high-dielectric constant insulator is susbtituted for mica, and this accounts for the considerable useful life characterizing the spark plugs according to this invention.
One may be led to try to find the maximum dielectric constant of the insulator of the low-tension plug not for the temperature normally attained by the insulator during its actual service but for a slightly lower temperature, so that the local heating caused by the passage of a spark will cause the next spark to be displaced somewhat and to occur at another point where the dielectric constant is higher.
Thus, the electrode wear is not localized and becomes quite irregular, so that it is no longer a cause of premature failure of the plug on account of pitting or the formation of cavities at only one and the same point of the electrodes and insulator.
In certain cases, it is also possible to facilitate the starting of engines when cold by using a high dielectric constant at a temperature slightly different from the ambient temperature.
The invention is applicable to any spark-producing devices, that is, not only to ignition spark plugs for internal combustion engines. Thus, for example, an electrodeinsulator sparking system according to this invention may be used in vacuum spectrographs of the type employed for the analysis of metalloidic elements in metals.
1. An insert for a low-tension surface discharge spark plug comprising a disc inner electrode having a spark forming face, an outer cylindrical electrode disposed radially of the inner electrode having a spark forming face spaced from said face of the inner electrode, a ceramic insulator core in the outer electrode holding the electrodes in fixed spaced relationship providing a discharge surface between the two electrode faces, and the insulator discharge surface comprising a composition having a dielectric constant greater than 50 where unity is the dielectric constant of a vacuum, and comprising a ceramic material from the group consisting of rutile, barium titanate and calcium titanate.
2. An insert for a low-tension surface discharge spark plug according to claim 1, in which said composition further includes at least one compound from the group consisting of strontium titanate, alkaline-earth zirconate, and alkaline-earth stannate.
3. An insert for a low tension surface discharge spark plug according to claim 1, in which said composition comprises titanium oxide of the rutile mineralogical variety.
References Cited in the file of this patent UNITED STATES PATENTS 2,017,364 Anderson Oct. 15, 1935 2,069,951 Hastings Feb. 9, 1937 2,115,666 Dorn Apr. 26, 1938 2,527,489 Smits Oct. 24, 1950 2,786,158 Tognola Mar. 19, 1957 FOREIGN PATENTS 574,577 Great Britain `Ian. l1, 1946 719,628 Great Britain Dec. 8, 1954