Spark plug with creepage spark gap

Details Spark plug with creepage spark gap Spark plug with creepage spark gap

1988-01-13

1989-09-26

Weldon, Ulysses

Electric lamp and discharge devices

Spark plugs

Plural part insulating means

313130, 313143, 123169CA, H01T 1320

Patent

active

048703192

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a spark plug with creepage spark gap for internal combustion engines.
Such spark plugs with creepage spark gap, which is formed on the surface of the insulator, on the combustion chamber side, between the central electrode and the side electrode, are distinguished by a much lower ignition voltage compared with a spark plug with spark gap in air. It has been found in this respect that the firing voltage is all the lower the greater the di-electric constant of the material of the insulator. However, such an insulator of high-dielectric material results in a spark plug of a relatively high capacitance, which causes a disruptive discharge at the creepage spark gap. Due to the very hot spark, of several ten thousand degrees, produced upon disruptive discharge, the surfaces of the electrodes and, in particular, the slideway of the creepage spark gap are greatly eroded, as a result of which the proper function of the spark plug and its service life are in turn considerably impaired.
In the case of a known spark plug of the type stated at the start (German Patent Application No. P 35 33 123.2), to avoid the breakdown phase with high energy, the insulator is divided transversly in the end region on the combustion chamber side and the larger-volume upper part on the connection side is made of a material having a low-dielectric constant and the smaller-volume lower part on the combustion chamber side is made of a material having a very much higher dielectric constant. However, the total capacitance of the spark plug cannot be kept small enough, so that the energy in occasionally occurring breakdown phases does after all cause notable erosion in the insulator surface.


SUMMARY OF THE INVENTION

It is an object of the present invention to provide a
The spark plug according which has the advantage that, due to the coaxial laminar structure of the insulator or plug insulating body, a coaxial capacitor with layered dielectric is produced, the total capacitance of which is determined by the smaller capacitance of the low-dielectric layer, which is the layer having a very much smaller dielectric constant. Therefore, ceramics having a relative dielectric constant of up to 10,000 can be used in the high-dielectric layer without increasing the plug capacitance, if only the low-dielectric layer has a relatively small dielectric constant of approximately 10 to 50.
With the provision of an advantageously high-dielectric slideway, the spark plug according to the invention has an only small plug capacitance and consequently exhibits only slight erosion effects on the slideway. The high dielectric constant of the insulator surface, on which the slideway is formed, makes the ignition voltage requirement of the spark plug low, so that a considerable part of the energy made available by the ignition system is transferred to the fuel mixture. As a result, good inflammation conditions are created even for lean fuel mixtures. The low ignition voltage requirement gives rise to all the advantages of a low-voltage ignition, such as smaller ignition coil, good suppression effect, low expenditure on high-voltage insulation.
Using materials having a very high relative dielectric constant, for example between 500 and 10,000, a low ignition voltage is achieved even with relatively thin layers, the layer thickness of which is between 0.1 and 1 mm. Such layers may be applied by plasma spraying to a plug insulating body of, for example, aluminum oxide. Using this technique, several layers of material having high dielectric constants can be applied one on top of the other, it being possible for the dielectric constant of the individual layers to increase in stages or continuously. It is sufficient in this respect to apply the high-dielectric layer or layers only in the slideway region and otherwise to produce the plug insulating body or insulator completely from aluminum oxide ceramic, without isolating points.
The overlapping of the layers in axial direction may be provided, which achieves a hig

REFERENCES:
patent: 2592754 (1952-04-01), Smits
patent: 3295005 (1966-12-01), Poellet et al.
patent: 4439707 (1984-03-01), Hattori et al.
patent: 4476412 (1984-10-01), Nishida et al.
patent: 4631451 (1986-12-01), Anderson et al.
patent: 4695758 (1987-09-01), Nishida et al.

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