1. Field of the Invention
The invention relates to ignition by electric spark plugs of gaseous carburated mixtures used in internal combustion engines, evaporated fuel burners of industrial boilers, gas turbines and ramjets, and more particularly to creeping discharge spark plugs of the flash-spark type in which the spark creeps over the surface of an insulator to ignite a mixture of gases in a combustion chamber.
One method of electrical ignition involves initiating an electric spark at an appropriate moment in a gaseous medium under certain conditions to achieve ignition and to maintain combustion.
As a general rule, the spark is provoked between two electrodes separated by a gap of substantially 0.6 mm, depending on the sparking conditions. Ignition of the mixture is initiated by a cylindrical plasma forming a length equal to that of the gap between the electrodes. The plasma liberates energy by thermal conductivity and radiation and diffuses activated particles. These two actions are conductive to ignition of the carburated gas mixture and propagation of the flame.
Because of the relatively small dimensions of the plasma, the mixture volume affected by the ignition is small compared to the total volume of gas to be ignited. This results in the well-known difficulties of ignition that occur because (1) the richness of the mixture to be ignited is very different from the stoichiometric mixture and (2) the gaseous mixture is not sufficiently locally homogeneous and (3) the plasma created has small dimensions, whereby the richness of mixture zones is very different from stoichiometric proportions.
It is known that elongation of the spark leads to higher amounts of liberated energy that is transmitted by heat conduction of the gaseous medium; spark elongation also causes higher radiation intensity and higher ionized particle density.
2. Description of the Prior Art
Attempts have been made for a considerable time to extend the electrical ignition spark to increase the probability of finding spark or plasma mixture zones that approach the stoichiometric ratio, thus enabling a high performance and reliable ignition procedure to be obtained. In certain spark elongation methods the gap between the electrodes is increased and an element inserted between electrodes acts as a relay for the spark.
French Pat. No. 1 540 265 describes such a device in which an intermediate metallic element, acting as relay for the spark, is located half-way along the gap between the electrodes. The element is electrically insulated from the electrodes and carried by an insulator endowed with good thermal conductivity. The total length of the spark produced attains 1.2 mm in this way.
In French Pat. No. 2 323 253, which concerns a plasma igniter for a gas turbine engine, the element inserted between the two electrodes outer surface of the bar except for a longitudinal strip connecting the two spark establishing electrodes, is insulated from the gaseous mixture by an enamel coating. It is believed that the semiconductor bar causes local preheating of the mixture to be ignited, whereby the density of the gas drops locally, thus making it easier for the spark to jump.
In U.S. Pat. No. 3,974,412 is disclosed another method for creating an unusually long arc discharge having an electronically controlled length and disposition.
According to this Patent, a spark plug comprises, in combination, a high voltage electrode that extends axially outward from the base of the spark plug. An insulating jacket covering the high voltage electrode extends continuously from the base of the plug to the region of the free end of the high voltage electrode. The free end of the high voltage electrode is exposed and formed to have a sparking surface oriented at an angle to the axially extending part of the high voltage axial electrode. A electrode extending from the body of the spark plug to the vicinity of the insulating jacket on the high voltage electrode has a sparking surface oriented at an angle to the axis. A separation between the ground electrode and the high voltage electrode through the insulating jacket is much less than the gap between the sparking surface of the ground electrode and the sparking surface of the high voltage electrode. The sparking surface of the electrodes is properly tapered such that an arc formed in an operating spark plug follows a curved path meeting each tapered sparking surface along a direction such that the discharge bends away from the axially extending part. The arc extends in an outward radial direction away from the axially extending part to distribute the arc energy and temperature. According to the teaching of this Patent non-creeping spark arcs having a maximum strength of 1 cm long are obtainable.
The object of the present invention is to provide relatively long flash sparks of say several cms. or tens of cms. long.
A further object of the invention is to provide an arc having a path complying with any arbitrarily defined pattern.
A further object of the invention is to ensure electrical ignition of a carburated gas mixture, even if this mixture is very lean and heterogeneous in space and time.
The invention provides means for obtaining a spark considerably greater in length then sparks previously obtained, rendering it possible by causing this spark to slide on a high resistivity dielectric surface. The spark can have any shape conductive to ready ignition of the mixture, regardless of the configuration and type of the combustion chamber.