GB2147362A - Igniters for gas turbine engines - Google Patents

Abstract

The electrical igniter (40) has a cavity (46) formed by a tubular semiconductive pellet (51) which is contacted by an electrode (53) at one end, and by the outer shell of the igniter at its other end, forming the other electrode. The cavity (46) opens into combustion chamber (30) through a bore (50) in a tubular nose portion (42) of reduced diameter. When the electrodes are energised, discharge is produced in the cavity (46), and a plasma jet (60) is projected through the bore (50) into the engine combustion chamber (30). The narrow nose portion (42) enables a smaller aperture (25) to be used in the inner wall (21) of the combustion chamber. The semiconductive pellet may be omitted and a high voltage used to produce the plasma jet. <IMAGE>

Description

SPECIFICATION Electrical igniters This invention relates to electrical igniters and engines The invention is more particularly but not exclusively, concerned with igniters for use in gas-turbine engines.

Conventional igniters, such as are described, for example, in UK patent specifications Nos. 1,450,825 and 1,570,125, have an outer electrode that is separated from a concentric inner electrode by an annular semiconductive surface over which discharge occurs when voltage is applied across the two electrodes. In such igniters, the outer electrode, inner electrode and semiconductive surface form an operative tip of the igniter which is located as close as possible to the combustion region of the engine. The thickness of the electrodes at the operative tip must be sufficient to withstand the erosion and heating effects at the igniter tip. For this reason, such igniters are usually at least about 1 2mm in diameter at their tip, and therefore a similar size aperture must be provided in the wall of the combustion chamber to receive the igniter.The size of these apertures is not inconsiderable with many engines, especially those having relatively small combustion chambers.

Providing apertures of this size can present problems to the engine designer since they can disrupt flow of gases over the wall of the combustion chamber and alter the performance of the engine.

It is an object of the present invention to provide an igniter that requires a smaller aperture than previous igniters, and an engine including such an igniter.

According to one aspect of the present invention there is provided an electrical igniter having a cavity, first and second electrode arranged, when energised, to cause discharge within said cavity, and a bore through which a plasma jet produced by discharge within said cavity is projected from the operative tip of the igniter, wherein said cavity is located within a first portion of the igniter, wherein said bore opens through a second portion of the igniter located forwardly of said first portion, and wherein said second portion has a smaller external cross-sectional area than said first portion.

Whilst the size of the cavity must be sufficient to withstand the erosion produced by the discharge, there is no such limitation on the size of the operative tip of the igniter. With this form of igniter it is therefore possible to have a reduced diameter operative tip, so that the size of the aperture in the inner, combustion wall can be correspondingly reduced, whilst that part of the igniter housing the cavity can be displaced from the operative tip.

The igniter preferably has an outer shell forming one electrode of the igniter, the second portion being formed as a part of the outer shell. The second portion may be of cylindrical shape. The igniter preferably includes a semiconductive surface within the cavity over which the discharge occurs, and the semiconductive surface may be provided on the inner surface of a tubular member which may be of a semiconductive material.

According to another aspect of the present invention there is provided a gas-turbine engine including an igniter according to the said one aspect of the present invention, the second portion of the igniter being arranged to extend within an aperture in an inner wall of the engine.

The second portion may be longer than the thickness of the inner wall, or the aperture in the inner wall may be stepped such that one end of the aperture receives the first portion of the igniter, and the second portion projects into the other end of the said aperture.

An igniter, and a gas-turbine engine including an igniter, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically the engine and the igniter; Figure 2 is a partly sectional side elevation of the igniter to a larger scale; and Figure 3 shows a part of an alternative igniter.

With reference first to Fig. 1, the gasturbine engine has a compressor 1 at its inlet end 2 which produces a flow of air and of a fuel-air mixture along the engine to its outlet end 3. The engine has an outer casing 20 within which there is supported an inner wall 21 spaced from the casing so as to define an annular passageway 22. The region immediately downstream of the compressor 1, within the inner wall 21, forms the combustion chamber 30 of the engine where ignition and combustion of the fuel-air mixture occur. Cooling air is directed (as shown by the arrows 31) around the annular passageway 22, to cool the outer surface 23 of the wall 21.

Some of the cooling air also passes through openings 32 in the wall 21 along its inner surface 24.

Ignition within the combustion chamber 30 is brought about by means of an igniter 40 which projects through the outer casing 20, and across the annular passageway 22. The operative tip 41 of the igniter 40 is exposed to be the combustion chamber 30 through an opening 25 in the inner wall 21. As so far described, the configuration of the engine is entirely conventional.

The construction of the igniter 40 and its mounting with the engine will now be described in greater detail with reference to Fig.

2. The operative tip 41 of the igniter 40 is located at the forward end of a cylindrical nose portion 42 of a tubular metal shell 43 that provides the external surface of the igniter. Rearwardly of the nose portion 42 the shell 43 is enlarged to provide an intermediate portion 44 within which is located a discharge cavity 46. The rear end of the shell 43 provides a head portion 47 having two screw threads 48 and 49 that are for use, respectively, in mounting the igniter and in the establishment of electrical connection with it.

The cavity 46 opens at the operative tip 41 of the igniter, via a bore 50 through the nose portion 42. A tubular semiconductive pellet 51 is located axially with the cavity 46 and is contacted at its forward end by a shoulder 52 between the nose portion 42 and the intermediate portion 44. At its rear end, the pellet 51 is contacted by the forward end of a metal rod 53 that extends axially of the igniter and is insulated from the shell 43 by a tubular ceramic insert 54. The rod 53 provides an inner electrode of the igniter whilst its shell 43 provides an outer electrode.

In operation, application of a low voltage, high energy source across the outer and inner electrodes causes discharge within the cavity 46, over the exposed, curved surface of the pellet 51. This discharge produces a plasma of hot gases which expands through the bore 50 of the nose portion and is projected from the operative tip as a plasma jet 60 to ignite a fuel-air mixture in the combustion chamber 30.

Because the nose portion 42 of the igniter is smaller than the intermediate portion 44 of the igniter, housing the discharge cavity 46, the aperture 25 in the inner wall 21 can be smaller than is required with conventional igniters. This reduces any disruption to gas flow over the inner surface of the wall 21.

It will be appreciated that the reduced diameter nose portion could be shorter or longer than that shown. For example with reference to Fig. 3, the aperture 25' in the inner wall 21' could be stepped so that it has smaller diameter on the inner surface 24' than on the outer surface 23'. In this way, the length of the nose portion 42' can be reduced to less than the thickness of the wall 21.

The igniter need not have a semiconductive surface over which discharge occurs, instead, a high voltage could be used to produce a plasma jet by discharge within a cavity.

Claims (14)

1. An electrical igniter having a cavity, first and second electrodes arranged, when energised, to cause discharge within said cavity, and a bore through which a plasma jet produced by discharge within said cavity is projected from the operative tip of the igniter, wherein said cavity is located within a first portion of the igniter, wherein said bore opens through a second portion of the igniter located forwardly of said first portion, and wherein said second portion has a smaller external cross-sectional area than said first portion.

2 An igniter according to Claim 1, wherein the igniter has an outer shell form;ng one electrode of the igniter, and wherein said second portion is formed as a part of said outer shell.

3. An igniter according to Claim 1 or 2, wherein said second portion is of cylindrical shape.

4. An igniter according to any one of the preceding claims, including a semiconductive surface within the cavity over which the discharge occurs.

5. An igniter according to Claim 4, wherein the semiconductive surface is provided on the inner surface of a tubular member.

6. An igniter according to Claim 5, wherein the tubular member is of a semiconductive material.

7. A gas-turbine engine including an igniter according to any one of the preceding claims, the second portion of the igniter being arranged to extend within an aperture in an inner wall of the engine.

8. A gas-turbine engine according to Claim 7, wherein the second portion is longer than the thickness of the said inner wail.

9. A gas-turbine engine according to Claim 7, wherein the aperture in the said inner wall is stepped such that one end of said aperture receives the first portion of the igniter, and the second portion projects into the other end of the said aperture.

10. An igniter substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

11. An igniter substantially as hereinbefore described with reference to Figs. 1 and 2, as amended by Fig. 3, of the accompanying drawings.

1 2. A gas-turbine engine substantially as hereinbefore described with reference to Figs.

1 and 2 of the accompanying drawings.

1 3. A gas-turbine engine substantially as hereinbefore described with reference to Figs.

1 and 2, as amended by Fig. 3, of the accompanying drawings.

14. Any novel feature or combination of features as hereinbefore described.