Flame igniter The present invention relates to a flame igniter with sliding spark discharge for igniting a combustion air- combustion gas mixture passing along an external surface of the flame igniter via an external sliding spark, com- prising a first central electrode with a forward end having an external surface forming part of the external surface of the flame igniter, an insulator, or an assembly of insulators coaxially surrounding the central electrode and having an exposed external surface which is part of the external surface of the flame igniter, and a metal housing coaxially surrounding the insulator or assembly of insulators, in which a forward end of the housing has an external surface forming a second electrode and a part of the external surface of the flame igniter, which second electrode is spaced from the forward end of the first electrode for defining a spark-gap where the surface of the insulator is exposed.

In a flame igniter of this known sort a high-powered external sliding spark ignition occurs in the spark-gap on the external surface of the flame igniter, between the two electrodes, the spark passing over the exposed, external part of the surface of the insulator thus igniting the combustion air-combustion gas mixture passing around the flame igniter, in which an ignition gas, hereafter called the flame gas, is contained for obtaining a correct ig- nition. The spark discharge of the high-powered type is generated by an ignition system containing a capacitor, which is charged to a voltage that is sufficient to cause the spark ignition.

One of the disadvantages of a suchlike flame ignition is the erosion of the electrode, as a result of which the voltages needed for generating the spark discharge would have to be larger and larger in the course of the life span of the flame igniter. The flame igniter therefore has a relatively short live span. A further disadvantage is that the diameters of suchlike flame igniters, especially when used as industrial gas igniters for high output, are relatively large and that for various combustion air- combustion gas mixtures different igniters are needed.

It is an object of the invention to provide a flame ig- niter with a reliable ignition, a long life-span, which has relatively small dimensions and can be used with different combustion air-combustion gas mixtures.

For this purpose a flame igniter of the kind described above according to the invention is characterized in that the flame igniter is provided with a passage connectable to a flame gas source, which passage discharges near the spark-gap. The invention is based on the insight that the problems with known flame igniters with external sliding spark discharge occur substantially because the sliding spark ignition should ignite the combustion air-combus- tion gas mixture flowing past the igniter. By now making a passage in the flame igniter, in accordance with the invention, the flame gas to be ignited can be partially or entirely led through this passage and flow to the spark- gap. This flame gas portion can apparently be reproducibly ignited in a simple manner at a low output by the sliding spark discharge whilst forming a primary flame. Because of the relatively low output used, the electrode erosion is also relatively low, thus extending the life-span of the flame igniter. The primary flame is mixed with air from the combustion air-combustion gas mixture which flows past the flame igniter, so that a stable combustion oc- curs. The external sliding spark can reliably ignite the

primary flame under extremely changeable gas/air con- ditions, as the sliding spark ignition always takes place on the boundary surface of flame gas and combustion air- combustion gas mixture. In addition, with the inventive flame igniter with external sliding spark discharge it is not necessary to mix the flame gas and the combustion air-combustion gas mixture before leading it to or about the flame igniter.

In an embodiment of the flame igniter with external sli- ding spark ignition according to the invention the passage is arranged in the metal housing. Alternatively, the passage is a through-going longitudinal central bore arranged in the first electrode, in which a backward end of the first electrode is connectable to the flame gas source. These inventive flame igniters can be applied among other things in lighting and pilot burners and industrial gas igniters for small output, smaller than 50 kW. For a larger output up to 500kW it is preferable if the flame igniter with external sliding spark discharge according to the invention is characterized in that the first electrode is provided with a through-going longitu- dinal central bore, and that the forward end of the first electrode is provided with at least one throughgoing radial auxiliary bore which discharges on the one hand in the central bore and on the other hand near the spark-gap, in which a backward end of the first electrode is connec- table to the flame gas source. In this way a portion of the flame gas carried in the central bore flows via the auxiliary bore to the spark-gap. This flame gas portion can be reproducibly ignited in a simple manner at a rela- tively low output by the external sliding spark discharge whilst forming a primary flame. The primary flame is mixed with air from the combustion air-combustion gas mixture which flows past the flame igniter, as a result of which a stable combustion occurs, ensuring a reproducible ignition of the main flame gas flow emitting from the central bore.

The required combustion air is drawn in by the main flame gas flow, which creates underpressure on account of high exit speed. Because the main flame gas stream draws in the combustion air, the nozzle can be described as self- priming.

In a preferred embodiment of a flame igniter according to the invention, the flame gas portion flowing through the auxiliary bore is controllable in that the forward end of the central bore is internally provided with a thread, which extends from the forward end to beyond the exit of the auxiliary bore, and that a nozzle is screwed into the forward end of the central bore. By screwing the nozzle farther or less far past the exit of the auxiliary bore in the central bore, the flame gas flowing to the auxiliary bore should cover a longer or shorter route through the thread, whereby control of the quantity of flame gas that flows to the auxiliary bore is acquired.

Some embodiments of a flame igniter with external sliding spark discharge according to the invention will, by way of example, be described on the basis of the accompanying drawing, in which a flame igniter according to the inven- tion is schematically shown in cross section.

In the figure a flame igniter with external sliding spark discharge for igniting a combustion air-combustion gas mixture passing along an external surface of the flame igniter via an external sliding spark is shown in cross section and placed in a housing tube 15. The flame igniter contains a first central electrode 1 with a forward end 4 that has an external surface forming part of the external surface of the flame igniter. An insulator 3, or an as- sembly of insulators, which coaxially surrounds the central electrode 1, has an exposed external surface 31 that is part of the external surface of the flame igniter.

A metal housing 6 coaxially surrounds the insulator 3, or

the assembly of insulators, a forward end of the housing 6 having an external surface that forms a second electrode 2 and a part of the external surface of the flame igniter.

This second electrode 2 is spaced from a forward end 4 of the first electrode 1 for defining a spark-gap 5, where the external surface of the insulator 3 is exposed. Flame igniters with this general structure are known and it is therefore not considered necessary here to discuss details of the parts of the flame igniter which are not essential to the invention. Thus an expert knows, for example, how to arrange electrical connections on the flame igniter.

In this flame igniter a high-powered external spark dis- charge takes place in the spark-gap 5 between the two electrodes 1 and 2, the spark passing over the surface of the insulator 3, a so-called sliding spark discharge.

The central electrode 1 is for instance provided with a through-going longitudinal bore 7. The forward end 4 of the first electrode 1 is preferably provided with at least one through-going radial auxiliary bore 8. This auxiliary bore 8 discharges on the one hand in central bore 7 and on the other hand near spark-gap 5. A backward end 9 of the first electrode 1 is connectable to a flame gas source, so that flame gas 12 can flow through the central bore 7 and the auxiliary bore 8 through the flame igniter. The flame gas portion that flows out through the auxiliary bore 8 near the spark-gap 5 is ignited by the sliding spark discharge and thus forms a primary flame, which is located approximately in the area 13. A combustion air current 11 (possibly but not necessarily containing a gas mixture which can differ from the flame gas) is led through the casing tube 15 in which the flame igniter is positioned, this current flowing past and around the flame igniter, whereby the primary flame is mixed with air. The combus- tion air is drawn in by the main flame gas flow that flows at high speed out of the forward end 10, thus creating an

underpressure. In this way a stable combustion of the primary flame is obtained, which reproducibly ignites the main flame gas portion flowing out of the forward end 10 of the central bore 7, whilst forming a main flame which is located approximately in the area 14.

It has been shown that the ignition of the primary flame is very reproducible and only requires a low output, the main flame being reliably ignited. With a small diameter the inventive flame igniter also appears to provide a relatively large ignition capacity and is applicable in various densities, differing pressures and different combustion air-combustion gas mixtures and kinds of flame gas, without the structure having to be adapted.

The number of auxiliary bores 8, their diameter, and the diameter of the central bore can be determined and ad- justed to one another with the help of simple experiments so that a specified ratio of flame gas flowing through the auxiliary bores 8 and from the forward end 10 is obtained, providing a flame igniter with a specified capacity or with a particular igniting characteristic.

In a preferred embodiment of the flame igniter according to the invention the flame gas portion flowing through the auxiliary bore 8 is adjustable by providing the forward end 10 of the central bore 7 with an internal thread which extends to beyond the exit of the auxiliary bore 8. By screwing a nozzle further or less far in the forward end 10 of the central bore 7, beyond the exit of the auxiliary bore, the length of the path which the flame gas has to cover through the thread in order to reach into the auxil- iary bore 8 is adjusted, thus providing control of the quantity of flame gas flowing into the auxiliary bore.

Because the main flame gas flow draws in the combustion air-combustion gas mixture, the nozzle can be described as self-priming.

Contrary to the known flame igniters with external sliding spark discharge, in the flame igniter with external sli- ding spark discharge according to the present invention the f lame gas supply is integrated in the flame igniter, thus forming a reproducible ignition. It is therefore not necessary to mix flame gas with the combustion air combus- tion gas mixture prior to leading the latter along the flame igniter. As opposed to the invention, combustion air -combustion gas mixture is first of all mixed with a flame gas in flame igniters with internal spark discharge, after which this mixture is added to the internal spark discharge. This prior mixing requires a considerably more complex flame igniter structure.

The inventive igniter can, among other things, be applied in lighting and pilot burners and industrial gas igniters with a capacity of 5 to 500 kW. Should a larger capacity be desired, a further gas can be introduced directly into the main flame. With smaller capacities, smaller that 50 kw, the main flame gas flow and the primary flame gas flow can be combined, it being possible to arrange a passage which discharges near the spark-gap into the metal housing or in the first electrode, which passage is connectable to a flame gas source. Other embodiments, in which the flame gas supply is integrated in another way in the flame igniter, have the same advantages, as long as at least part of the flame gas is led past the sliding spark-gap.