Patent Publication Number: US-2023151770-A1

Title: Spark plug for a single-piece combustion chamber

Description:
DISCLOSURE OF THE INVENTION 
     The present invention relates to a combustion chamber for a gas turbomachine, such as an aircraft turbojet or turboprop engine, in which fluids (such as air and at least one fuel) flow generally from upstream to downstream to operate it. In the text, upstream and downstream are to be considered along what is hereinafter referred to as “the first axis” or “axis X”, which is the general axis of the turbomachine. 
     Upstream is the side from which the air and fuel mixture enters the combustion chamber. “Outer” and “inner” are to be understood as radially relative to said first axis (X). Outer is what is radially further away from this first axis than what is inner. 
     This being said, such aircraft gas turbomachine parts are already known, having a first axis (X) and comprising:
         an outer casing ( 12 ),   an inner casing ( 14 ), both annular, coaxial with said first axis (X), and   a space ( 9 ) surrounding an annular combustion chamber ( 10 ) about the first axis (X),
 
the space ( 9 ) being delimited between the outer casing ( 12 ) and the inner casing ( 14 ),
 
the annular combustion chamber comprising:
   respectively inner and outer annular walls radially to the first axis (X),   an annular chamber bottom (wall) extending between said inner and outer annular walls and having first openings for the passage of devices for injecting an air and fuel mixture at least partly attached to the annular chamber bottom,   a deflector, disposed downstream of the chamber bottom, to thermally protect the chamber bottom, and having second openings for aforementioned inner and outer annular walls delimit the combustion area of the combustion chamber.       

     Furthermore, the chamber bottom overall actually includes ports for the passage of the air (A)/fuel (C) mixture towards the combustion area of the combustion chamber, air (A) coming from a compressor of the turbomachine and fuel (C) being supplied by injectors. 
     By definition, the chamber bottom (CB) is the member:
         to which said devices for injecting the air/fuel mixture are attached, and   with which said respectively inner and outer annular walls are attached. It is a structural bottom.       

     The chamber bottom undergoes high thermal stresses which can deform it. It is therefore common to protect it thermally (from the flame in the combustion area) by means of a deflector (or a circle of deflectors) mounted just downstream of the CB. 
     In spite of this, clearances may occur (especially with said respectively inner and outer annular walls), which generate problems of pollution, fuel consumption, and re-ignition of the chamber in case of flameout. 
     Moreover, it is necessary to be able to initiate the combustion of the air/fuel mixture in the combustion area of the combustion chamber, or even to feed it with an additional amount of fuel, downstream of the CB. 
     To this end, it is known that one of said respectively inner and outer annular walls (typically the outer wall) includes at least one port for the passage towards the combustion chamber of one end of (at least) one energy feed element (spark plug for said mixture and/or fuel injector), the other end of which may be attached to an outer casing of the chamber. 
     During operation of the turbomachine, the walls of the combustion chamber expand thermally, which causes relative displacements between the pieces. 
     To compensate for and allow these displacements, EP-A-1770332 suggests using at least one guiding device which comprises, for guiding the energy feed element in question into the port of the annular wall through which it passes:
         a metal flange, and   a floating bushing, floatingly mounted in the flange, the floating bushing and the flange having the energy feed element passing coaxially therethrough, the flange being shrink fit or welded to said annular wall, which is typically a metal wall.       

     The term welding encompasses both soldering and actual welding. 
     In this context, the purpose of the invention is to provide an effective and economical solution to at least some of the following problems and drawbacks:
         improved service life of the combustion chamber,   reduction of parasitic gas leakage in the zone of the equipped CB,   reduction of pollution,   reduction of fuel consumption,   improved chamber ignition and re-ignition conditions,   control of the overall mass of the combustion chamber,   control of the way the combustion chamber is manufactured,   good mechanical strength,   improved resistance to thermal stresses,   management of the contacts between metal and refractory material (such as a ceramic); indeed, these contacts make it complex to attach the guiding device of the energy feed element (a device often referred to as a spark plug guide) to a refractory material wall. US2013055716 certainly teaches a turbomachine part as set forth above and in which the combustion chamber comprises:   respectively inner and outer annular refractory material walls, radially to the first axis (X),   an annular chamber bottom extending between said inner and outer refractory material walls and having first openings for the passage of devices for injecting an air and fuel mixture partly attached to the annular chamber bottom,   first respectively inner and outer annular metal connecting walls, radially to the first axis (X), to which are attached:
           said refractory material walls, respectively, and   the chamber bottom,
 
the combustion chamber furthermore comprising:
   
           two ports passing coaxially through, respectively (along an axis which may intersect the axis X):
           one of the first metal connecting walls, and   one of said refractory material walls which it covers, and   
           a guiding device, for guiding an energy feed element into said two ports.       

     However, US2013055716 does not disclose that, as taught by the invention, the guiding device is attached with one of said first metal connecting walls. 
     By means of the invention, especially excessive mechanical load on the annular refractory material wall through which the port for the passage of the energy feed element in question passes will be avoided and a solution will have been provided to the problem of attachment of the guiding device of the energy feed element to a refractory material wall. 
     It should also be noted that the single-piece aspect between said deflector and said respectively inner and outer annular refractory material walls:
         should make it possible to obtain control of geometric tolerances of the combustion area, especially: elimination of welding operations, maintenance of the volume of the chamber to respect favourable re-ignition limits, in case of in-flight flameout,   can avoid the need to use a thermal protection barrier coating (made especially of yttrium zirconate),   implies that said refractory material “deflector”, by becoming the annular “bottom” of the combustion area of the combustion chamber (denoted as  21  hereafter), continues to act as a deflector, protecting the metal, structural “chamber bottom” (or CB, denoted as  20  hereafter), which is itself annular, and which is thus not directly exposed to thermal radiation. The term “deflector” is therefore appropriate. The term “bottom of the single-piece assembly” has also been used hereafter to avoid confusion with the “chamber bottom, or CB,  20 ”.       

     Favourably, such a single-piece assembly will be made of (that is, based on) refractory material, which may be (may comprise) a ceramic matrix composite (CMC). The wall thickness could be between 0.9 mm and 1.6 mm. 
     To secure the attachment of the guiding device, it is advisable that said guiding device comprises a metal flange and a floating bushing floatingly mounted in the flange, the floating bushing and the flange being adapted to have the energy feed element passing coaxially therethrough, the flange being attached with one of said first metal connecting walls. 
     Still to secure this attachment, it is favourably by shrink fit or by welding that the guiding device—the flange if provided—will be attached with one of said first metal connecting walls. 
     Moreover, favourably:
         the combustion chamber will also comprise a refractory material deflector, disposed (axially) downstream of the chamber bottom, to protect it thermally, and having second openings for the passage of the fuel injection devices,   said deflector and said refractory material walls may form a single-piece assembly.       

     Again, in order to further secure the attachment of the guiding device without significantly stressing the refractory material wall, it is also provided that an intermediate attachment bushing is provided:
         attached, preferably shrink fit or attached by welding, to the flange and to said first annular metal connecting wall through which the energy feed element in question passes, and   which itself will have the energy feed element passing therethrough.       

     By interposing such an intermediate bushing, a priori a metal bushing like the flange, between this flange and said first connecting wall, also a metal wall, mounting the guiding device as a whole will be facilitated. 
     Also, for the same purpose as above, it is even provided that:
         the flange has a shank (also called a chimney) parallel to the axis of said two coaxial ports and which is engaged in the port of said first metal connecting wall through which the energy feed element passes, and   the attachment bushing is shrink fit or welded to said wall and said shank, by being thus interposed between them.       

     It is also provided that, in parallel to the axis of said two ports, the shank of the flange is interrupted at a distance from said annular refractory material wall through which it passes and which faces it. 
     In addition, to facilitate mounting of the flange and to limit mechanical weakening of the refractory material wall as much as possible, it is also provided that, of said two ports, that which passes through said first metal connecting wall in question has a larger diameter than that which passes coaxially through the refractory material wall located opposite. 
     In other words, the port present in one of the first metal connecting walls will advantageously have a larger diameter (D 2 ) than the diameter (D 1 ) of the port present in one of said annular refractory material walls. 
     In order to limit interference between the attachment bushing and this refractory material wall, it is also provided:
         that the attachment bushing has an internal diameter (D 1 ), an external diameter (D 2 ) and a thickness (e) between the internal (D 1 ) and external (D 2 ) diameters, and   that the difference in diameters between said two ports is greater than the thickness (e) of the attachment bushing.       

     Thus, the attachment bushing will not project into the space of the ports reserved for the possible clearance of the energy feed element. 
     Moreover, in order to position the flange in the best possible way and to facilitate its mounting, it is also provided that, in parallel to the axis of said two ports, the attachment bushing has a height (H 1 ) greater than the height of the shank, in order to avoid possible contact between the metal and the refractory material (ceramic). 
     The height (H 1 ) of the attachment bushing may also be greater than the thickness of said first metal connecting wall to which it is thus attached and which surrounds it adjacently. Thus erected, preferably radially outwards above this first metal connecting wall, the bushing can especially serve as a support for mounting the guiding device of the energy feed element in question. 
     In this respect, it is even provided that, in the operational situation, the shank of the flange is supported by the attachment bushing via a flare present on the flange. 
     In addition, for this purpose of support and/or controlled clearance of said energy feed element in its guiding device, it is also provided:
         that the floating bushing has an external edge guided transversely into an internal annular groove of the flange,   that the shank of the flange flares to define a bottom of said groove and peripherally extends to a flanged edge where a cup is attached, so that said edge of the floating bushing is guided between said bottom and the cup, and   the flare of the shank of the flange is located outside the attachment bushing.       

     Another aspect relates to the mechanical strength of said first metal connecting wall and the connection between this wall and the refractory material wall which it covers, which itself has the energy feed element in question passing therethrough. 
     To this end, it is provided that, along said covered annular refractory material wall, the port which passes through said first metal connecting wall is formed in a first covering tab which protrudes relative to a second covering tab:
         in turn protruding relative to a part of said first metal connecting wall which extends circumferentially about said first axis (X), and   where, on either side of the first covering tab, attachment pins attach, by passing therethrough, said first metal connecting wall and the annular refractory material wall which it covers, to each other.       

     Generally speaking, it should be possible to expect from the above:
         a reduction in costs and overall mass,   as well as a better control of the geometric tolerances of the combustion area.       

     Furthermore, in addition to the combustion chamber described above, the invention also relates to an aircraft gas turbomachine equipped with this combustion chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will, if necessary, be better understood and other details, characteristics and advantages of the invention will become apparent upon reading the following description, which is given by way of non-limiting example with reference to the appended drawings, where: 
         FIG.  1    is a schematic axial cross-section half-view (axis X) of a turbomachine “combustion module”, comprising a combustion chamber in accordance with the invention, 
         FIG.  2    is a view identical to that of  FIG.  1   , but angularly offset about the axis X and of the combustion chamber alone, 
         FIG.  3    is detail III of  FIG.  2   , 
         FIG.  4    is detail IV of  FIG.  1   , 
         FIG.  5    shows the zone of  FIG.  4    before mounting the energy feed element and its guiding device, 
         FIG.  6    shows the zone in  FIG.  5    in partial cross-section enlarged view, 
         FIG.  7    is the same view as  FIG.  6   , after mounting the guiding device of the energy feed element, and 
         FIG.  8    is a view as in  FIG.  5   , but after mounting said guiding device and the energy feed element in itself. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the embodiment in  FIG.  1   , the part  1  of an aircraft turbomachine has a space  9  surrounding an annular combustion chamber  10  and which receives a flow of air A, which will, at least in part, supply the combustion area  11  of the chamber  10 . The space  9  is delimited between an outer casing  12  and an inner casing  14 , both annular, coaxial to the axis X of the turbomachine. 
     In the space  9 , the part  1  of the turbomachine comprises a compressor  3 —which may be a high-pressure compressor disposed axially following a low-pressure compressor—the downstream part of which (visible in the figure) comprises a centrifugal stage  5 , an annular diffuser  7  connected downstream of the compressor  3 . The diffuser  7  opens into the space  9 . Together with the outer casing  12  and the inner casing  14 , said part  1  can be called a “combustion module”. 
     Compressed air (A) from the compressor  3  is introduced into the combustion chamber  10  where it is mixed with fuel (C) from injectors, such as the injectors  4  in  FIG.  2   . Gases from the combustion are directed towards a turbine (here a high-pressure turbine) located downstream (DO) of the outlet of the chamber  10 , and first towards a distributor  23  which is a part of the stator of the turbomachine. 
     The chamber  10  comprises an annular outer wall  16  and an annular inner wall  18 , both made of refractory material. 
     As understood, the outer casing  12 , the inner casing  14 , the space  9 , the outer wall  16  and the inner wall  18  each form a ring about the first axis X. 
     The outer  16  and inner  18  walls are therefore not annular about each axis I-I′ which defines the axis of each of the injectors  4  for injecting fuel into the combustion chamber  10 . 
     The annular outer  16  and inner  18  walls are connected upstream to the bottom  21  which is itself annular about the first axis X. 
     The bottom  21  forms the bottom of the combustion area  11 . 
     The combustion chamber  10  is held:
         on the upstream side by spindles  42  attached to the outer casing  12  and to the walls shown below  58 ,  60  and/or to the CB  20 , and   on the downstream side by attachment flanges.       

     Through the outer and inner annular flanges  22  and  24  respectively, and at the downstream end, the chamber  10  is axially supported by outer and inner annular shells respectively, of a distributor, here the high-pressure distributor  23 , via sealing lamellas  220 ,  240  connected to said outer and inner annular flanges  22  and  24  respectively. These flanges are axially supported by axial pins  221 ,  241 , respectively, with which the outer  247  and inner  249  annular shells are equipped. As could be done externally by the outer annular flange, the radially inner annular flange  24  extends, radially inwardly with respect to the sealing lamellas  240 , to an annular support member  245  in the form of a peg open downstream side, which is supported by a casing  25 , called the HP distributor support casing. Between the outer and inner annular shells of the distributor  23 , which is attached otherwise, substantially radial blades  251  extend. 
     Moreover, the inner casing  14 , which runs along the chamber  10 , can be considered as being defined by, or comprising, the shell  26  of the diffuser and an inner intermediate web  28  attached upstream of the shell  26  and downstream of the casing  25 . In the upstream part (UP), the combustion chamber  10  can be attached by at least three attachment spindles  42  circumferentially distributed about the axis X, the axis around which the movable vanes of turbine(s) and compressor(s) especially rotate. 
     According to a specificity of the combustion chamber  10  set forth here, the outer annular wall  16 , the inner annular wall  18  and the annular bottom  21  (of the combustion area  11 ) forming a deflector define a single-piece refractory material assembly  100 . 
     Just upstream of the bottom  21  of the combustion area is the annular metal chamber bottom  20  to which devices  2  for injecting an air and fuel mixture, hereinafter referred to as injection devices  2 , are attached and circumferentially distributed about the axis X. 
     This attachment can be done by direct attachment (clamping) of bowls  6  provided on the injection devices  2 . 
     Each injection device  2  may thus comprise a bowl  6  which comprises a venturi  60   a  and terminates towards the combustion area  11  with a divergent  60   b,  for bursting the received air and fuel mixture jet. 
     Following the cross-section of  FIG.  2   , circumferentially offset and passing through the axis of one of the injection devices  2 , it can be seen that the combustion chamber  10  thus comprises such injection devices  2  which (at least for the bowls  6 ) pass through first passage openings  43  formed in the chamber bottom  20  and then, coaxially, through second passage openings  45  formed through the combustion area bottom  21 , without the (bowls  6  of the) injection devices  2  being attached directly with the bottom  21 . 
     The bowl  6 , the fuel outlet engaged therein and the first and second openings  43 ,  45  are coaxially centred on an axis I-I′ which is parallel to an axis X 1  in parallel to which the single-piece outer annular wall  16  and the inner annular refractory material wall  18  generally extend. 
     An intermediate space  56  separates the CB  20  and the combustion area bottom  21 , along this axis I-I′. 
     The bowls  6  are mounted tightly (attached) in coaxial sleeves  13  disposed in the openings  43  and themselves attached to the CB  20 . An axial clearance J can be reserved between each sleeve  13  and the combustion chamber bottom  21 . In this way, contact with fragile refractory material and metal can be avoided. 
     A fuel injector  4  is mounted in the inlet channel  6   a  of the bowl  6  of each of the injection devices  2 , the outlet of which injector, like the channel  6   a,  is oriented along the corresponding axis I-I′. 
     Around its inlet channel  6   a,  the bowl also has one or more air A inlet spinning members  15  passing therethrough. 
     The peripheral spinning member(s)  15  can allow a part of the air A received from the space  9  to be introduced into the bowl  6  (a priori towards the venturi  60   a ) and with a spinning movement (arrow  15   a ). 
     An annular cowl  40  having openings  41  passing therethrough, letting the injectors  4  and the air A pass towards the bowls  6 , can also be provided. The openings  41  may each be coaxial with a so-called axis I-I′. 
     For the supply of air A to the combustion area  11  through the annular volumes  160  and  180  which exist respectively:
         between the outer wall  16  and the outer casing  12 , and   between the inner wall  18  and the inner casing  14 ,       

     (The part(s) forming the inner  18  and/or outer walls  16 , respectively, of the single-piece assembly  100  may furthermore have primary holes  44  and/or dilution holes  46  passing therethrough, which holes open into the combustion area  11 . Some multi-perforation ports  47 ″, for injecting cooling air into the combustion area, have also been shown locally. If they exist, they extend over a much larger surface area, as known. 
     For a connection—with controlled (mechanical/thermal) stresses and manufacture—between the single-piece assembly  100  and the surrounding metal parts of the turbomachine (if they exist: spindles  42 , lamellas  220 , 240  . . . ), it is provided:
         that, towards the upstream end of the combustion chamber  10 , first inner and outer annular metal connecting walls  60  and  58 , respectively, may connect together the CB  20  and the inner and outer walls  18  and  16 , respectively, or even also the cowl  40  located therefore upstream of this CB, and/or   that, towards the downstream end of said chamber, there are provided second inner and outer metal connecting walls  64  and  62  respectively, having inner  24  and outer  22  flanges, respectively, for connection:
           between said inner wall  18  and a metal strain-take-up piece, such as the injector casing  25 , and   between said outer wall  16  and a part of the DHP (outer annular shell  247 ) and/or the outer casing  12 .   
               

     The connecting metal walls  58 , 60 , 62 , 64  will be flexible sheets, more deformable than the refractory material of the assembly  10 , when the turbomachine is operating. 
     Again, for these metal/refractory material connection issues, it is provided, for the connections between said inner  18  and outer  16  walls and the inner  60 , 64  and outer  58 , 62  connecting metal walls, respectively, to use pins  66  and washers  68  attached together (for example welded) and passing through holes provided in the respective walls. 
     By contrast, connections between the (metal) CB  20  and the first metal inner  60  and outer  58  connecting walls respectively (or even with the metal cowling  40 ), will preferably be ensured a priori by screw-nut assemblies  70  which will pass therethrough. 
     Furthermore, at least one further passage double port  72 , 74  passes coaxially through:
         one of the first respectively inner  60  and outer  58  annular metal connecting walls (port  72 ), and, opposite   one of said respectively inner  18  and outer  16  annular refractory material walls (port  74 ), to dispose therein an energy feed element  48  which may typically comprise a spark plug or a fuel injector.       

     The free end  48   a  of the energy feed element  48  is flush with the inner face of the relevant refractory material wall ( 16  in the example), to communicate with the combustion area  11 ;  FIG.  4   . 
     For easier mounting, the wall  58  will cover the wall  16  and the wall  60  will cover the wall  18 , as seen especially in  FIGS.  4    and following for the wall  58  facing the wall  16 , in this example where they have said element  48  passing therethrough, generally along their common axis  75  which intersects the axis X and which may be perpendicular to the axis X 1 . 
     Hereafter, it will be considered for example that the element  48  is a spark plug which opens by its inner end into the combustion area  11 . 
     In fact, the air and fuel mixture injected into the combustion chamber  11  will be ignited by means of at least one spark plug, such as the spark plug  48 , which may extend radially to the axis X 1 , outside the chamber. 
     The radially outer end of the spark plug  48  may be attached to the outer casing  12  outside of which it is connected to power supply means (not represented). 
     At its radially inner end (thus relative to the axis X 1  or I-I′; see  FIG.  4   ), the spark plug  48  is guided into the ports  72 , 74 . 
     For this purpose, a guiding device  76  is attached outside the chamber  10  to the metal wall  58 , around the port  72  to compensate for the relative displacements between the walls of the chamber and the spark plug  48  during the operation of the turbomachine. These relative displacements occur mainly in the longitudinal direction, substantially parallel to the axis X 1 ; see arrows  78  in  FIG.  4   . 
     The guiding device  78  is a priori a metal device. It comprises a flange  80  and a bushing  82  floatingly mounted in the flange  80  to allow the displacement of the spark plug  48  substantially parallel to the axis I-I′, held/secured by a neck  820  of the floating bushing. The floating bushing  82  and the flange  80  have the spark plug  48  passing coaxially (axis  75 ) therethrough. 
     To avoid interference with the refractory material walls  16 ,  18 , the flange  80  is shrink fit or welded to (in the example) the outer metal connecting wall  58 . 
     To facilitate this attachment and to secure it, it is advisable to use an intermediate attachment bushing  84 , which may be a metal bushing, itself shrink fit or welded to the flange  80  and to said metal connecting wall; the outer wall  58  in said example (see  FIG.  6   ). 
     Thus, the intermediate attachment bushing  84  is disposed closely in the port  72 . It may be supported by the wall  16 , just around the port  74 . 
     The floating bushing  82  has an outer edge  830  which moves freely, transversely to the axis  75  (substantially parallel to the axis I-I′), in an internal annular groove  88  of the flange  80 . 
     The neck  820  is at the junction between the central zone of the edge  830  and that of a possible frustoconical part  835  for guiding the initial axial engagement of the element  48  into the floating bushing  82 . 
     The flange  80  may have a shank or chimney  86  by which it is attached to the metal wall  58 , preferably via the intermediate attachment bushing  84  (shank/bushing attachment by brazing, preferably welding). 
     To form the groove  88 , the shank  84  of the flange:
         flares (via a shoulder) to define a bottom  840  of the groove  88 , and   peripherally extends to a flanged edge  860  to which a cup  90  is attached (a priori soldered or welded), so that the edge  830  of the floating bushing can be guided between the bottom  840  and the cup  90 , into the groove  88 .       

     The flare (that is, the bottom  840 ) of the shank of the flange is located outside the attachment bushing  84 : along the axis  75 , beyond the free end of the bushing  84  on which the bottom  840  can rest. 
     The shank  86  may be cylindrical. 
     For secure positioning, it is provided:
         that, in parallel to the axis of the element  48  and said two ports  72 ,  74 , the shank  86  of the flange  80  is interrupted at a distance from the refractory material wall in question: space  92  between the free end of the shank and the facing wall,  16  in this case; and/or   that the port  72  which passes through said (first) metal connecting wall ( 58  in the example) has a diameter D 1  which is larger than the diameter D 2  of the port  74  which passes coaxially through the facing refractory material wall ( 16  in the example); see  FIG.  6   . Thus, there is a support  94  ( FIG.  5   ) on which to push the attachment bushing  84  during positioning before attachment to the wall  58 .       

     Moreover, it may be desired to limit the projection of the flange  80  into the ports  72 ,  74 . For this purpose, it is provided:
         that the attachment bushing  84  has an internal diameter, an external diameter and a thickness e 1  between the internal and external diameters ( FIG.  6   ), and   that the difference in diameters between said two ports  72 , 74  (shoulder formed by the edge  94   FIG.  5   ) is greater than the thickness (e) of the attachment bushing  84 : see  FIG.  6   .       

     And to facilitate the installation of the flange  80 , it is furthermore provided that, in parallel to the axis  75  of said two ports  72 ,  74 , the attachment bushing  84  has a height (H 1 ) greater than the thickness e 2  of said corresponding (first) metal connecting wall ( 58  in the example; see  FIG.  6   ). 
     In this way, it can be provided that the flare (bottom  520 ) of the shank  86  of the flange is supported by the attachment bushing  84 , along the axis  75 ; see  FIG.  4  or  7   . The question of the mechanical strength related to the presence of the single-piece assembly  100  has also arisen with regard to the mechanical weakening caused by the port  72  provided in the relevant metal connecting wall:  58  in the example. 
     To overcome this, it is provided that, along the covered annular refractory material wall (wall  16  in the example), said port  72  is formed in a first covering tab  96  which protrudes downstream relative to a second covering tab  98 :
         in turn protruding relative to a part  102  of said first metal connecting wall ( 58  in the example) which thus extends circumferentially about said first axis X, and   where, on either side of the first covering tab  96 , attachment pins  66  (held by washers  68 ) pass through and attach said first metal connecting wall ( 58  in the example) and the annular refractory material wall ( 16  in the example) which it covers, to each other.       

     This characteristic may be important. 
     Thus, it may be advantageous for the (protrusion formed by the) first covering tab  96 :
         to be only local to enable the guiding device of the energy feed element (spark plug guide) to be attached,   and thus, not to extend continuously over the entire circumference.       

     Thus, the problems of weight, friction and thermal expansion are reduced. 
     Favourably, said first metal connecting wall ( 58  in the example) will thus extend, axially:
         over a first length X 10 , over most of its perimeter about the axis X,   locally along a second length X 20  to enable it to be attached (in the form of a tab: second covering tab  98 ), and   along a third length X 30 , even more locally (again in the form of a tab: first covering tab  96 ) to allow the guiding device of the energy feed element to be attached.       

     Such a double axially protruding zone of said first metal connecting wall may be defined as follows: 
     axially (X 20 +X 30 ): between 1.2×D 2  and 3×D 2  (D 2 : diameter of the port  74 ), circumferentially (cumulative lengths Cl of the first and second covering tabs  96 ,  98 ): between 6×D 2  and 12×D 2 ; see  FIG.  8   . 
     With such an embodiment, weight and friction will be significantly reduced, without affecting the mechanical strength. 
     As understood, in order to dispose the energy feed element  48  in question, it will therefore be possible to extend locally, downstream, said completely circumferential part  102  of the first metal connecting wall ( 58  in the example) through which the mounting port  72  is to pass:
         firstly, to the second (circumferentially quite long) covering tab  98 ,   then to said first covering tab  96  (circumferentially shorter, preferably just surrounding the port  72 ), and which may be circumferentially centred along the second covering tab  98 ; see  FIG.  5  or  8   .       

     It will have been noted that the first and second covering tabs  96 ,  98  may be defined on a portion of a cylinder of axis parallel to the axis X 1 .