Patent Publication Number: US-2007095038-A1

Title: Filtration structure, in particular a particulate filter for exhaust gases of an internal combustion engine and a reinforcement element intended for such a structure

Description:
The present invention relates to a filtration structure, in particular a particulate filter for exhaust gases of an internal combustion engine, of the type comprising:  
      at least first and second filtration elements which have a first and second face which are arranged opposite each other, respectively;  
      a joint for connecting the faces which extends between the faces, this joint comprising a binding agent and reinforcement means which are embedded in this binding agent.  
      Structures of this type are used in particular in devices for cleaning the exhaust gases of internal combustion engines. These devices comprise an exhaust silencer which comprises in series a catalytic purification element and a particulate filter. The catalytic purification element is suitable for processing polluting emissions in a gaseous phase, whilst the particulate filter is suitable for retaining the particulates of soot discharged by the engine.  
      In a known structure of the above-mentioned type (see, for example, EP-A-0 816 065), the filtration elements comprise a group of adjacent conduits which have parallel axes and which are separated by means of porous filtration walls. These conduits extend between an inlet face for the exhaust gases to be filtered and a discharge face for the filtered exhaust gases. These conduits are further closed at one or other of the ends thereof in order to delimit inlet chambers which open at the inlet face and outlet chambers which open at the discharge face.  
      This structure operates in accordance with a series of filtration and regeneration phases. During the filtration phases, the soot particulates discharged by the engine are deposited on the walls of the inlet chambers. The pressure drop through the filter increases gradually. Beyond a predetermined value for this pressure drop, a regeneration phase is carried out.  
      During the regeneration phase, the soot particulates, which substantially comprise carbon, are burnt on the walls of the inlet chambers using auxiliary heating means in order to restore the original properties of the structure.  
      However, the combustion of the soot in the filter is not carried out in a homogeneous manner (the combustion begins at the front and at the centre of the filter and then spreads) Consequently, high temperature gradients appear in the filter during the regeneration phases.  
      The temperature gradients within the filtration structure produce local occurrences of expansion of different magnitudes and consequently longitudinal and transverse stresses in and/or between the various filtration elements.  
      These high levels of thermomechanical stress bring about cracks in the filtration elements and/or in the connection joints between these filtration elements.  
      In order to limit the risk of these cracks appearing, patent application EP-A-0 816 065 proposes that connection joints be used which comprise a three-dimensional network of ceramic fibres embedded in a mineral cement. The cohesion of the network of fibres and the connection between this network and the cement are brought about by substances for adhesively-bonding the fibres, one of which is mineral, the other organic.  
      Current structures are not entirely satisfactory. The use of a joint of this type between the filtration elements is not very practical owing in particular to the rheology of the joint.  
      The main object of the invention is to overcome this disadvantage, that is to say, to provide, for a particulate filter, a porous filtration structure which comprises a reinforced connection joint and which is easy to use.  
      To this end, the invention relates to a filtration structure of the above-mentioned type, characterised in that the reinforcement means comprise at least one mesh-like reinforcement element which has independent coherence and which comprises at least one active portion which is generally of substantially planar form.  
      The filtration structure according to the invention may comprise one or more of the following features, taken in isolation or according to any technically possible combination:  
      the active portion comprises a plurality of beams which are arranged substantially parallel with a first direction;  
      the active portion comprises a plurality of cross-members which connect the beams and which are arranged substantially parallel with a second direction, distinct from the first direction;  
      the total volume of the apertures delimited by the beams and the cross-members is greater than the total volume of the beams and the cross-members;  
      the reinforcement element is produced from a metal material;  
      the reinforcement element is produced from a material which degrades at temperatures greater than 150° C.;  
      the reinforcement element comprises an active portion opposite two adjacent faces of the filtration element, the active portions being connected to each other;  
      it comprises at least one cell which comprises four filtration elements, and a common reinforcement element, having a sinuous shape, for the filtration elements, the common reinforcement element comprising at least three successive active portions which are arranged opposite adjacent faces of the filtration elements of the cell;  
      it comprises at least first and second cells, at least one active portion of the reinforcement element of the first cell being arranged opposite a face of a filtration element of the second cell.  
      The invention further relates to a reinforcement element which is intended for a filtration structure as defined above. 
    
    
      Application examples of the invention will now be described with reference to the appended drawings, in which:  
       FIG. 1  is a perspective view of a first filtration structure according to the invention;  
       FIG. 2  is an exploded partial perspective view of the filtration structure of  FIG. 1 ;  
       FIG. 3  is an end view of the filtration structure of  FIG. 1 ; and  
       FIG. 4  is a view similar to  FIG. 3 , of a second filtration structure according to the invention.  
    
    
      The particulate filter  11  illustrated in  FIG. 1  is arranged in a partially illustrated exhaust tract  13  of a motor vehicle diesel engine.  
      This exhaust tract  13  extends beyond the ends of the particulate filter  11  and delimits a passage for circulation of the exhaust gases.  
      The particulate filter  11  extends in a longitudinal direction X-X′ for circulation of the exhaust gases. It comprises a plurality of filtration units  15  which are connected to each other by means of connection joints  17 .  
      Each filtration unit  15  has a substantially parallelepipedal rectangular form which is elongate in the longitudinal direction X-X′.  
      The term “filtration unit” more generally refers to an assembly comprising an inlet face, an outlet face, and at least three lateral faces (four lateral faces in the example illustrated) which connect the inlet face to the outlet face.  
      As illustrated in  FIG. 2 , in which two superimposed filtration units  15 A and  15 B are illustrated, each filtration unit  15  comprises a porous filtration structure  19 , an inlet face  21  for the exhaust gases to be filtered, a discharge face  23  for the filtered exhaust gases and four lateral faces  24 .  
      The porous filtration structure  19  is produced from a filtration material which is constituted by a monolithic structure, in particular ceramic material (cordierite or silicon carbide).  
      This structure  19  is sufficiently porous to allow the exhaust gases to pass through. However, as known per se, the diameter of the pores is selected to be sufficiently small to ensure that the soot particulates are retained.  
      The porous structure  19  comprises an assembly of adjacent conduits having axes which are parallel with the longitudinal direction X-X′. These conduits are separated by porous filtration walls  25 . In the example illustrated in  FIG. 2 , these walls  25  are of a constant thickness and extend longitudinally in the filtration structure  19 , from the inlet face  21  to the discharge face  23 .  
      The conduits are distributed in a first group of inlet conduits  27  and a second group of outlet conduits  29 . The inlet conduits  27  and the outlet conduits  29  are arranged transposed.  
      The inlet conduits  27  are closed in the region of the discharge face  23  of the filtration unit  15  and are open at their other end.  
      Conversely, the outlet conduits  29  are closed in the region of the inlet face  21  of the filtration unit  15  and open along the discharge face  23  thereof.  
      In the example illustrated with reference to  FIG. 2 , the inlet conduits  27  and outlet conduits  29  have constant cross-sections along the entire length thereof.  
      Furthermore, the opposing lateral faces  24 A and  24 B of the filtration units  15 A and  15 B are planar.  
      As illustrated in  FIG. 2 , the connection joint  17  is arranged between the opposing planar faces  24 A and  24 B of the filtration units  15 A and  15 B. This connection joint  17  comprises a binding agent  41  and reinforcement means  43  which are embedded in this binding agent  41 .  
      The binding agent  41  is produced based on ceramic cement which is generally constituted by silica and/or silicon carbide and/or aluminium nitride. After sintering, this cement has an elastic modulus of from 500 to 5000 MPa.  
      As illustrated in  FIG. 3 , the reinforcement means comprise sleeves  43  which are arranged alternately around every other filtration unit  15  when moving parallel with a first transverse axis Y-Y′ of the filtration structure  11  (horizontal in  FIG. 3 ). Furthermore, the sleeves  43  are arranged alternately around every other filtration unit  15 A when moving parallel with a second transverse axis Z-Z′ of the structure  11  (vertical in  FIG. 3 ).  
      Each filtration unit  15 A surrounded by a sleeve  43  is thus adjacent to filtration units  15 B which are free, that is to say, which are not surrounded by a sleeve  43 . Furthermore, each free filtration unit  15 B is adjacent to filtration units  15 A which are surrounded by sleeves.  
      Each sleeve  43  comprises four active portions  45  which generally have a substantially planar form and each of which extends substantially over the entire adjacent surface of the corresponding unit  15 A.  
      “Active portion generally having a substantially planar form” is understood to be a portion  45  whose dimension, taken parallel with a transverse horizontal or vertical axis Y-Y′ or Z-Z′ is less than at least twice the dimension of the portion  45  taken parallel with the other transverse vertical or horizontal axis and the dimension of the portion  45 , taken parallel with the longitudinal direction X-X′ of the filtration structure  11 .  
      As illustrated in  FIG. 3 , each active portion  45  is arranged between a face  24 A of a unit  15 A which is surrounded by a sleeve  43  and a face  24 B of a free unit  15 B.  
      With reference to  FIG. 2 , each active portion  45  comprises a plurality of metal beams  47  which are arranged parallel with the longitudinal direction X-X′ of the structure. Furthermore, the active portion  45  comprises a plurality of metal cross-members  49  which connect the beams  47 . These cross-members  49  are arranged parallel with the transverse axis Y-Y′, perpendicular relative to the longitudinal direction X-X′ of the structure.  
      The beams  47  and the cross-members  49  thus delimit a plurality of apertures  51 . The active portion  45  is thus mesh-like, which allows it to be embedded in the cement  41 , and has its own or independent coherence or mechanical strength, in contrast to a mass of fibres which are embedded in the cement in a random manner.  
      In the example illustrated in  FIG. 2 , the beams  47  and the cross-members  49  are constituted by rods having a diameter which is smaller than the distance which separates two successive rods, taken parallel with the longitudinal direction X-X′ of the structure or the transverse axis Y-Y′. Thus, the volume of the apertures  51  is greater than the total volume of the beams  47  and the cross-members  49 .  
      These apertures  51  thus define a periodic structure in the longitudinal direction X-X′ and along the axis Y-Y′.  
      The orientation of the beams  47  and the cross-members  49  enhances the mechanical properties of the joint  17  in a plane parallel with the opposing faces  24 A and  24 B of the filtration units  15 A and  15 B.  
      Furthermore, since the beams  47  and the cross-members  49  are produced from a metal material, they constitute preferred axes for propagation of thermal fluxes within the joint  17 . They thus allow the heat released by the combustion of soot to be distributed in a more uniform manner within the joint  17  and the formation of hot spots within this joint  17  to be reduced.  
      If the levels of thermomechanical stress are too great in the structure  11 , the cracks produced in the joint  17  by the relaxation of the structure  11  are orientated along the beams  47  and the cross-members  49 .  
      As illustrated in  FIG. 3 , the active portions  45 C and  45 D opposite two adjacent faces  24 C and  24 D of each unit  15  surrounded by a sleeve  43  are connected to each other. This specific arrangement also improves the cohesion of the joint  17  between two opposing faces  24 C and  24 E in a direction which is orthogonal relative to the plane defined by the active portion  45 C which is arranged between these two faces  24 C and  24 E.  
      The operation of the first filtration structure according to the invention will now be described.  
      During a filtration phase ( FIG. 1 ), the exhaust gases which are loaded with particulates are guided as far as the inlet faces  21  of the filtration units  15  via the exhaust tract  13 . They then enter the inlet conduits  27  and pass through the walls  25  of the porous structure  19  ( FIG. 2 ). During this movement, soot is deposited on the walls  25  of the inlet conduits  27 . This soot is preferably deposited at the centre of the particulate filter  11  and towards the discharge face  23  of the filtration units  15  (on the right-hand side in the drawing).  
      The filtered exhaust gases are discharged via the discharge conduits  29  and are guided to the outlet of the exhaust silencer.  
      When the vehicle has travelled approximately 500 km, the pressure loss through the filter  11  increases significantly. A regeneration phase is then carried out.  
      During this phase, the soot is oxidised by means of the temperature of the filter  11  being increased. This oxidation is exothermic. The propagation of the regeneration and the non-homogeneous distribution of the soot in the filter  11  brings about a temperature gradient between the zones in which there is a significant accumulation of soot and zones in which there is little accumulation of soot.  
      Furthermore, the filtration units  15  and the joints  17  expand under the effect of the temperature. The local extent of this expansion depends on the temperature.  
      These variations in the magnitude of expansion, under the effect of the temperature gradients, produce high levels of thermomechanical stress.  
      As set out above, the sleeves  43  bring about the cohesion of the joint  17  when it is subjected to these high levels of, stress.  
      If the levels of thermomechnical stress are too great in the structure, the cracks produced in the joint  17  by the relaxation of the structure  11  are orientated along the beams  47  and the cross-members  49  of the sleeves  43 .  
      Furthermore, the extent of the temperature gradients is reduced by a better diffusion of the thermal fluxes through the sleeves  43 .  
      In the variant which is illustrated with reference to  FIG. 4 , the structure comprises cells  61  which comprise four adjacent filtration units  15 .  
      Within a cell, each filtration unit  15 C comprises two adjacent faces  24  opposite two faces of two other filtration units  15 D,  15 E of the cell  61 , respectively.  
      Each cell  61  further comprises a common reinforcement element  43  for the four filtration units  15 .  
      As illustrated in  FIG. 4 , the reinforcement element  43  of each cell has a sinuous form and comprises a plurality of successive active portions  45  of substantially planar form which are connected to each other in series.  
      Each active portion  45  is thus connected to a maximum of two other active portions  45  of the reinforcement element  43 .  
      Furthermore, the active portions  45  which are connected to each other extend along orthogonal planes.  
      Consequently, within each cell  61 , the reinforcement element  43  comprises at least two active portions  45  opposite two adjacent faces  24  of each filtration unit  15 , respectively.  
      The cohesion within a filtration cell  61  is thus enhanced parallel with the longitudinal direction X-X′ of the structure  11 , parallel with the horizontal axis Y-Y′ and parallel with the vertical axis Z-Z′ of this structure  11 .  
      Furthermore, the filtration structure  11  comprises a plurality of cells  61 . As illustrated in  FIG. 4 , for each pair of adjacent cells, at least one active portion  45 A of the reinforcement element  43 A of a first cell  61 A is arranged opposite a face  24 B of a filtration unit  15 B of a second adjacent cell  61 B, in order to provide the mechanical cohesion between the various cells  61 .  
      In a variant, the beams  47  and the cross-members  49  may have other orientations, for example, at 45° relative to the axes X-X′ and Y-Y′ or at 30° relative to one of these axes.  
      Also in a variant, the reinforcement element comprises active portions which are formed from a woven web. The woven web is produced from fibres which are, for example, organic and which degrade at temperatures greater than 150° C.  
      This reinforcement element disappears owing to combustion, either during the production of the filtration structure, or during local heating within the joint. However, the passages which are created in the space which was previously occupied by the organic fibres of the reinforcement element promote the relaxation of the stresses in the filtration joint and, if the levels of thermomechanical stress are too great, ensure that any cracks are spread along these passages.  
      In another variant, the active portions of the reinforcement element comprise mesh-like plates or undulating mesh-like sheets in order to reduce the magnitude of the thermal gradients within the structure.  
      Owing to the invention which has been described above, it is possible to have a filtration structure which can withstand a multitude of regeneration phases whilst retaining its mechanical strength and sealing with respect to the soot.  
      In this structure, the relaxation of the thermomechanical stresses and the possible formation of cracks in the joint are orientated in preferred directions.  
      This structure further provides a better distribution of the temperatures within the joint, if the reinforcement element is produced from a metal material.