SPARK PLUG FOR AIRCRAFT TURBOMACHINE AND METHOD FOR MANUFACTURED SUCH A SPARK PLUG

Abstract

A longitudinally oriented (L22) (22) spark plug for an aircraft turbomachine, comprising a nose (30) including an outer electrode (32), an inner electrode (31), a semiconductor body (35) arranged radially between the inner electrode (31) and the outer electrode (32) and positioned longitudinally behind a free end (34) of the outer electrode (32) and a free end (33) of the inner electrode (31) so as to define an annular cavity for arc formation, and an insulating body (36) arranged radially between the inner electrode (31) and the outer electrode (32), and positioned longitudinally opposite the semiconductor body (35), the insulating body (36) and the semiconductor body (35) being sealed together by an interlayer of fused glass (50). (See Figure 2 for more details.)

Description

Title of the invention: SPARK PLUG FOR AIRCRAFT TURBOMACHINE AND METHOD OF MANUFACTURING SUCH A CANDLE

[0001] The present disclosure relates to a spark plug for an aircraft turbomachine and a method of manufacturing such a plug. The disclosure also relates to a combustion chamber of a turbomachine comprising a spark plug as described. Prior art

[0002] A turbomachine combustion chamber generally comprises an annular chamber bottom wall connected to two substantially cylindrical and coaxial shells extending downstream, and a fairing or cowling fixed to the chamber bottom and extending upstream.

[0003] The fairing guides the air flow supplied by the turbomachine compressor and divides it into a central stream which feeds the combustion chamber and into two peripheral streams which bypass the combustion chamber.

[0004] In a known manner, a spark plug comprises a nose comprising an outer electrode, an inner electrode, a semiconductor body arranged between the inner electrode and the outer electrode and set back relative to a free end of the outer electrode and a free end of the inner electrode, so as to define an electric arc formation cavity, allowing the formation of a spark outside the spark plug, and an insulating body arranged between the semiconductor body, the inner electrode and the outer electrode. The spark plug is mounted on an outer casing and passes through an opening in the outer shell, to initiate the combustion of the air from the compressor of the turbomachine, mixed with fuel.

[0005] In order to mount the spark plug on the casing, it is known to mount in this opening a device for fixing the spark plug in the form of a tubular device, this device receiving and fixing the spark plug on the casing. This device thus ensures the guidance and fixing of the spark plug on the casing as well as the sealing of the combustion chamber at this opening.

[0006] However, it has been observed that the operation of the turbomachine can cause damage to the spark plug, for example by cracking. This is due in particular to the vibrations and forces generated by the operation of the turbomachine. These different forces are in fact transmitted to the spark plug via the external skin of the casing and are diffused at the level of the fixing device and its interface with the spark plug, which stresses the spark plug by lateral forces, and in particular shearing.

[0007] It has in particular been observed that the spark plug may have an internal crack, between the external electrode and the internal electrode, at the junction between the insulating body and the semi-conductor body. This crack attests to the presence of a discharge channel, that is to say the formation of several electric arcs, inside the spark plug, and therefore of a short circuit which prevents the proper functioning of the spark plug. It has further been observed that this internal crack originates on the external electrode, at the interface with the fixing device. Lateral forces are thus detrimental to the proper functioning of the spark plug.

[0008] The present disclosure aims in particular to provide a solution to the problem of damage which may be encountered by the spark plug, and thus to provide a solution for preserving and increasing the service life of a spark plug for a combustion chamber. Summary

[0009] For this purpose, a longitudinal axis spark plug for an aircraft turbomachine is proposed, comprising a nose comprising an outer electrode, an inner electrode, a semiconductor body arranged radially between the inner electrode and the outer electrode and arranged longitudinally set back from a free end of the outer electrode and from a free end of the inner electrode so as to define an annular cavity for forming an electric arc, and an insulating body arranged radially between the inner electrode and the outer electrode, and longitudinally facing the semiconductor body, the insulating body and the semiconductor body being sealed together by an intermediate thickness of molten glass. Along the longitudinal axis, the thickness of molten glass can be defined according to a ratio relative to the thickness of the outer electrode, between 0.9 and 1.5, preferably 1.25.

[0010] In a known manner, the insulating body and the semiconductor body are arranged in contact with each other in a bonding zone, in which several points of glue are placed between the two surfaces, this bonding zone itself being arranged opposite the contact zone of the outer electrode of the spark plug with the spark plug guide through which the spark plug passes axially. This contact zone is subjected to lateral forces, in particular shear forces, due to the operation of the turbomachine, which can lead to a break in the contact between the insulating body and the semiconductor body and promote the creation of a discharge channel from the outer electrode to the inner electrode, between the insulating body and the semiconductor body, and therefore a short circuit. The sealing element thus makes it possible to strongly secure the insulating body to the semiconductor, so as to increase the resistance of the spark plug to lateral forces induced by the combustion chamber and thus avoid the appearance of a discharge channel. More generally, sealing makes it possible to increase the durability of the spark plug. Glass perfectly meets this expectation, given its high mechanical resistance, and also makes it possible to avoid the creation of electric arcs, thanks to its high electrical insulating power. In addition, glass has the advantage of being malleable in the molten state, by obtaining the necessary viscosity allowing a sealing time of between 2 and 3 minutes and good finishing for good distribution of the material in the bonding zone for homogeneous adhesion between the insulating body and the semiconductor body.

[0011] Alternatively, the thickness of molten glass has, between the insulating body and the semiconducting body, a longitudinal dimension of between 4 and 8 mm.

[0012] In the bonding zone, the insulating body and the semiconductor body are thus spaced from each other by a gap, the sealing element filling the gap. The gap is sized so as to provide sufficient space between the elements making up the candle to allow the glass to penetrate in the molten state and allow good distribution of the glass in the bonding zone.

[0013] The present description also relates to a combustion chamber of a turbomachine, comprising at least one spark plug as described previously, carried by an external casing and extending in guide means carried by a wall of revolution of the chamber and comprising a tubular guide through which the spark plug passes axially and mounted on a chimney fixed to the wall of revolution of the chamber and opening into the latter.

[0014] The present description also relates to a method of manufacturing a spark plug as described previously, the method comprising: - place a glass bead between the insulating body and the semiconductor body, - melt the glass bead to seal the insulating body and the semiconductor together.

[0015] Furthermore, the glass bead may be melted by an induction device. The induction device may be an induction ring, arranged radially around the outer electrode, at an interstitial space longitudinally separating the insulating body from the semiconductor body. Brief description of the drawings

[0016] Other characteristics, details and advantages will appear on reading the detailed description below, and on analyzing the attached drawings, in which: Fig.l

[0017] [Fig.l] [Fig.l] is a schematic sectional view of a combustion chamber comprising a tubular fixing device and a spark plug according to the known technique; Fig. 2

[0018] [Fig.2] [Fig.2] is a schematic sectional view of part of the spark plug ignition according to the present description. Description of the embodiments

[0019] [Fig.l] illustrates a combustion chamber 10 of a turbomachine. The combustion chamber 10 comprises a radially external casing 11, comprising an opening 12. The combustion chamber 10 also comprises a radially external shell 13 of revolution. The opening 12 is provided up to an opening 131 of the external shell 13 which comprises a downstream annular flange for fixing to the casing 11. The combustion chamber 10 further comprises a radially internal shell 14 of revolution comprising a downstream annular flange for mounting on an internal casing, and a chamber bottom 15 on which are mounted fairings 16 extending upstream.

[0020] Fuel injection rods 17, distributed around the axis of the turbomachine, open into the chamber bottom 15, via injection heads 18 around which mixers 19 are arranged.

[0021] The air flow supplied by the compressor of the turbomachine is guided by the fairings 16 and divided into a central vein intended to supply the combustion chamber 10 and into two peripheral veins 21 intended to bypass the combustion chamber.

[0022] The mixers 19 form a flow of air entering in a swirling manner into a primary combustion zone 20 of the combustion chamber 10. This air is mixed with the fuel sprayed by the injection heads 18, the mixture being ignited by at least one spark plug 22.

[0023] The spark plug 22 is generally cylindrical in shape with a circular cross-section and extends longitudinally along a longitudinal axis L22. The spark plug 22 is mounted, by its outer end, in a tubular fixing device 23, fixed to the outer casing 11.

[0024] As this is better seen in [Fig.2], the internal end of the spark plug 22, also called nose 30, passes through the external shell opening 131, until it is more or less flush with the internal surface 132 of the external shell 13. The spark plug 22 further extends in a tubular guide 70 mounted on a chimney 71 fixed to the external surface 133 of the external shell 13.

[0025] The nose 30 of the spark plug 22 comprises an inner electrode 31 centered along the longitudinal axis L22 and an outer electrode 32 radially external to the longitudinal axis L22. The free end 34 of the outer electrode 32 has a stop surface 38 internal to the spark plug and radially inclined which extends from the free end 34 towards the opposite end of the outer electrode 32, moving away radially from the longitudinal axis L22 between the free end and the opposite end of the outer electrode. A semiconductor body 35 is furthermore arranged radially between the inner electrode 31 and the outer electrode 32 and arranged longitudinally set back relative to the free end 34 of the outer electrode and to the free end 33 of the inner electrode 31 so as to define an annular cavity for forming an electric arc.

[0026] The semiconductor body 35 is generally cylindrical in shape and extends longitudinally between a first end 41 of the semiconductor body and a second end 42 of the semiconductor body, the first end 41 being the end arranged closest to the free end 34 of the outer electrode 32. The first end 41 of the semiconductor body further has a stop surface 37 by which the semiconductor body 35 bears on the stop surface 38 of the outer electrode 32.

[0027] An insulating body 36 is arranged radially between the inner electrode 31 and the outer electrode 32, and longitudinally facing the semiconductor body 35, i.e. behind the semiconductor body along the longitudinal axis L22. The insulating body 36 is of generally cylindrical shape and extends longitudinally between a first end 43 of the insulating body and a second end (not visible in [Fig. 2]) of the insulating body, the first end 43 being the end arranged closest to the semiconductor body 35. More particularly, the second end 42 of the semiconductor body 35 and the first end 43 of the insulating body 36 are arranged facing each other, defining an interstitial space 60.

[0028] The insulating body 36 and the semi-conductor body 35 are sealed together by an interlayer thickness of molten glass 50. There is therefore a sealing over the entire surface of the interstitial space 60 formed by the second end 42 of the semi-conductor body 35 and the first end 43 of the insulating body 36 is thus in contact with the sealing element, thus providing better mechanical strength of the assembly, as opposed to point bonding on the surface. The thickness of glass is dimensioned so as to provide sufficient space between the elements making up the candle to allow the glass to penetrate in the molten state and allow good distribution of the glass in the bonding zone.

[0029] Along the longitudinal axis L22, the glass 50 has a thickness E50, between the first end 43 of the insulating body 36 and the second end 42 of the body semiconductor 35 between 4 and 8 mm. This thickness E50 depends on the dimensional parameters of the candle. For example, the thickness E50 can be defined according to a ratio relative to the thickness of the external electrode 32, between 0.9 and 1.5, preferably 1.25. The glass being adherent to the insulating body 36 and to the semiconductor body 35, the thickness E50 also corresponds to the distance separating the first end 43 of the insulating body 36 and the second end 42 of the semiconductor body 35.

[0030] The glass, when melted, fills the interstices present between the semiconductor body and the insulating body, which makes the internal part of the spark plug perfectly sealed. This prevents in particular any particles suspended in the polluted fuel present in the spark plug from moving towards the external ground electrode by passing over the second end 42 of the semiconductor body 35, which would place the spark plug in more favorable breakdown conditions inside than outside.

[0031] [Fig.2] also illustrates that the tubular guide 70 has a surface of contact 121 with the spark plug 22, and in particular with the external electrode 32. This contact surface 121 is subjected to lateral forces, in particular shear forces, which can be transmitted to the various components of the spark plug and thus risk creating cracks leading to the appearance of an electric arc within the spark plug promoting the appearance of a discharge channel.

[0032] Within the spark plug, the transmission of lateral forces from the contact surface 121 is greatly limited or even non-existent, thanks to the glass, which has the advantage of strongly securing the insulating body to the semiconductor, so as to increase the resistance of the spark plug to the lateral forces induced by the combustion chamber and thus avoid the appearance of a discharge channel. For example, the glass sealing makes it possible to have an increase in the robustness of the spark plug of nearly 1.8 kN or 25% compared to a spark plug without glass sealing. Glass is also a very good electrical insulator, which makes it possible to avoid the creation of an electric arc in the spark plug.

[0033] In an example of a method for manufacturing the spark plug 22, a glass bead is first placed in the interstitial space 60 existing between the semiconductor body 35 and the insulating body 36, that is to say between the first end 43 of the insulating body 36 and the second end 42 of the semiconductor body 35. For example, the semiconductor body 35 is placed in the spark plug to abut against the abutment surface 38 of the external electrode 32, then the glass bead is inserted into the spark plug before inserting the insulating body 35 therein along the longitudinal axis L22. The glass is then melted, for example between 750°C and 800°C by an induction device such as an induction ring arranged radially around the outer electrode 32, and at the interstitial space 60. The volume of the glass bead provided for sealing of the semiconductor body with the insulating body corresponds to the interstitial volume to be filled, and is thus between 150 and 500 mm3.

Claims

Claims

1. Spark plug (22) with a longitudinal axis (L22) for an aircraft turbomachine, comprising a nose (30) comprising an outer electrode (32), an inner electrode (31), a semiconductor body (35) arranged radially between the inner electrode (31) and the outer electrode (32) and arranged longitudinally set back relative to a free end (34) of the outer electrode (32) and to a free end (33) of the inner electrode (31) so as to define an annular cavity for forming an electric arc, and an insulating body (36) arranged radially between the inner electrode (31) and the outer electrode (32), and longitudinally facing the semiconductor body (35), the insulating body (36) and the semiconductor body (35) being spaced apart from each other by a gap, the insulating body (36) and the semiconductor body (35) being sealed together by an interlayer of molten glass (50) filling the gap,the molten glass (50) being in direct contact with the external electrode (32).,

2. Spark plug (22) according to claim 1, wherein the thickness of molten glass (50) has, between the insulating body (36) and the semiconductor body (35), a longitudinal dimension (E50) of between 4 and 8 mm.

3. Spark plug (22) according to claim 1 or 2, wherein, along the longitudinal axis (L22), the thickness of molten glass (50) is defined according to a ratio relative to the thickness of the outer electrode (32), between 0.9 and 1.

5.

4. Combustion chamber (10) of a turbomachine, comprising at least one spark plug (22) according to any one of the preceding claims, carried by an external casing (11) and extending in guide means carried by a wall of revolution (13) of the chamber and comprising a tubular guide (70) axially traversed by the spark plug (22) and mounted on a chimney (71) fixed on the wall of revolution (13) of the chamber and opening into the latter.

5. A method of manufacturing a spark plug (22) according to one of claims 1 to 3, the method comprising: - placing a glass bead between the insulating body (36) and the semiconductor body (35), - melting the glass bead to seal together the insulating body (36) and the semiconductor (35).

6. The method of claim 5, wherein the glass bead is melted by an induction device.

7. A method according to claim 6, wherein the induction device is an induction ring, arranged radially around the outer electrode (32), at an interstitial space (60) longitudinally separating the insulating body (36) from the semiconductor body (35).

8. A method according to any one of claims 5 to 7, wherein the volume of the glass bead is between 150 and 500 mm3.

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