Abstract:
A filter body ( 3 ) which is intended, in particular, for the filtration of particles contained in the exhaust gases from an internal combustion engine. The filter body ( 3 ) is formed by bonding filter blocks ( 11 ) using a joint ( 17 ). An outer surface ( 16 ) of the first of the blocks ( 11 ), which is disposed opposite the second of the blocks ( 11 ) and which is in contact with the joint ( 17 ), includes at least one projection- and/or cavity-type irregularity ( 30   a,    30   b ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a body formed by assembling a plurality of units and used in particular to filter particles contained in the exhaust gases of an internal combustion engine, in particular of the diesel type. 
     Conventionally, before being exhausted to the atmosphere, exhaust gases may be purified by means of a particle filter such as the prior art filter shown in  FIGS. 1 and 2 . 
       FIG. 1  shows a particle filter  1  in cross section taken along the line B-B in  FIG. 2  and  FIG. 2  shows the particle filter  1  in longitudinal section taken along the line A-A in  FIG. 1 . 
     2. Description of the Related Art 
     The particle filter  1  conventionally includes at least one filter body  3  inserted into a metal enclosure  5 . The filter body  3  is produced by assembling and machining a plurality of units  11 , referenced  11   a - 11   i , produced by extruding a ceramic material (cordierite, silicon carbide, etc.) to form porous honeycomb structures. 
     Before assembly and machining, a unit  11  (see  FIG. 3 ) conventionally has the shape of a rectangular parallelepiped extending along an axis D-D between a substantially square upstream face  12  and a substantially square downstream face  13  onto which a plurality of rectilinear square section passages  14  parallel to the axis D-D discharge. 
     The exterior faces  15  of the passages  13   p  located at the periphery of the unit  11 , referred to as “peripheral passages”, form an exterior surface  16  of the unit  11  (see  FIG. 3 ). Because of the substantially square cross section of the unit  11 , the exterior surface  16  has four faces  16   a - 16   d , pairs of which are mutually perpendicular. 
     After extrusion, the units  11  are alternately blocked at the upstream face (outlet passages  14   s ) or the downstream face (inlet passages  14   e ). This is known in the art. 
     The units  11   a - 11   i  are then assembled by bonding them together by means of ceramic cement joints  17  generally consisting of silica and/or silicon carbide and/or aluminum nitride. The resulting assembly can then be machined to a round section, for example. This produces a cylindrical filter body  3  with axis C-C which can be inserted into the enclosure  5 , a peripheral seal  18  that is impermeable to the exhaust gases being disposed between the exterior filter units  11   a - 11   h  and the enclosure  5 . 
     As shown by the arrows in  FIG. 2 , the flow F of exhaust gases enters the filter body  3  via the inlet passages  14   e , passes through the filter walls  20  of those passages into the outlet passages  14   s , and is then exhausted to the exterior. 
     After a certain time of use the performance of the engine is degraded by particles or “soot” accumulated in the inlet passages  14   e  of the filter body  3 . For this reason, the filter body  3  must be regenerated regularly, for example every 500 kilometers. Regeneration or “unclogging” consists in oxidizing the soot by heating it to a temperature at which it can ignite. 
     During regeneration phases, the exhaust gases carry in the downstream direction all the heat energy given off by the combustion of the soot. Moreover, because the soot is not deposited uniformly in the various passages, the combustion areas are not uniformly distributed in the filter body  3 . Finally, the peripheral areas of the filter body  3  are cooled by the surrounding air through the metal enclosure  5 . 
     As a result of this the temperature differs in the various areas of the filter body  3  and does not vary uniformly. The non-homogeneous temperatures in the filter body  3  and the different kinds of materials used for the filter units  11   a - 11   i , on the one hand, and the joints  17 , on the other hand, generate high local stresses that can cause local breaks or cracks. In particular, the local stresses at the interfaces between the units  11   a - 11   h  and the enclosure  5  and between the units  11   a - 11   i  and the joints  17  may lead to the units  11   a - 11   i  becoming unstuck, thereby reducing the service life of the particle filter  1 . 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a new filter body adapted to reduce this risk of unsticking. 
     That object is achieved by a filter body intended in particular for filtering particles contained in the exhaust gases of an internal combustion engine, said filter body being formed by bonding filter units by means of a joint, noteworthy in that an exterior surface of a first of said units face to face with a second of said units and in contact with said joint includes at least one irregularity of the boss and/or recess type. 
     As will emerge in more detail in the remainder of the description, the presence of irregularities on the exterior surface of a unit encourages the adhesion of the joints  17  and therefore limits the risk of unsticking in the event of high thermomechanical stresses. 
     According to preferred other features of the invention:
         said irregularity extends along the longitudinal axis of said unit over the whole length of said first unit, which advantageously allows it to be formed during extrusion;   said irregularity is a recess carried by an exterior face of a gas outlet passage at the periphery of said first unit;   said irregularity is a recess in the thickness of an exterior wall of a passage at the periphery of said first filter unit, which advantageously reduces the quantity of material necessary for fabricating the unit;   said irregularity is a boss carried by an exterior face of a gas inlet passage at the periphery of said first filter unit, which advantageously increases the interior volume of the inlet passages and therefore their soot storage capacity;   the width of said irregularity measured in a transverse plane of said first unit is substantially equal to the width of said exterior face of said passage;   said exterior surface includes a plurality of said irregularities regularly spaced from each other;   at least one portion of said exterior surface of said first unit has a sinusoidal shape in cross section;   said irregularity is conformed so that it can be accommodated in an irregularity of complementary shape of said second unit;   said irregularity is a recess in the thickness of an exterior wall of a passage at the periphery of said first unit.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The following description, which is given with reference to the appended drawings, explains and assesses the advantages of the invention. In the drawings: 
         FIG. 1  represents a prior art particle filter in cross section taken along the line B-B in  FIG. 2 ; 
         FIG. 2  represents the same particle filter in longitudinal section taken along the line A-A in  FIG. 1 ; 
         FIG. 3  is a perspective view of a unit that can be used to fabricate a filter body of the invention; 
         FIGS. 4 and 5  represent details of the unit represented in  FIG. 3  in cross section in the plane P shown in  FIG. 3 ; and 
         FIGS. 6 and 7  represent in cross section details of different variants of units of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the nonlimiting figures, the various items are not necessarily represented to the same scale. The same references are used in the various figures to designate identical or similar items. 
       FIGS. 1 and 2  have been described in the preamble. Refer to  FIG. 3 , also described in part hereinabove. 
     According to the invention, the first and second faces  16   a  and  16   b  of the exterior surface  16  of the filter unit  11  include first and second irregularities  30   a  and  30   b , respectively, that extend along the longitudinal axis D-D of the unit  11 . The irregularities  30   a  and  30   b  preferably have a length “L” equal to that of the unit  11  and extend from the upstream face  12  to the downstream face  13  on exterior faces  32   a  and  32   b  of peripheral passages  14   a  and  14   b , respectively. 
     The irregularities  30   a  and  30   b , represented in section in  FIGS. 4 and 5 , respectively, are respectively a boss and a groove. The width “I” of these irregularities is substantially that of the exterior faces  32   a  and  32   b  of the peripheral passages  14   a  and  14   b , respectively. The irregularities  30   a  and  30   b  are in line with a single passage  14   a  and  14   b , respectively. 
     The irregularities  30   a  and  30   b  can have any height “h”. The height “h” is preferably less than the local thickness of the exterior walls  34   a  and  34   b  of the peripheral passages  30   a  and  30   b  on the exterior faces  32   a  and  32   b  whereof the irregularities  30   a  and  30   b , respectively, extend. 
     The thickness “e” of the exterior walls of the peripheral passages forming the boss  30   a  and/or the groove  30   b  is preferably substantially constant and substantially equal to the thickness “e′” of the exterior walls of the adjacent peripheral passages. 
     The thickness “e” is preferably never zero; in other words, the irregularity does not create a lateral opening in the peripheral passage(s) in which it is formed. 
     The boss  30   a  then takes the form of an outward deformation of the exterior wall  34   a  of the passage  14   a  ( FIG. 4 ). This advantageously increases the useful interior volume of the passage  14   a.    
     After each regeneration, ash accumulates in the inlet passages, which limits their subsequent efficiency and limits the time of use of the filter unit before the next regeneration. To extend the service life of the filter, it is preferable for the passage  14   a  to be an inlet passage, i.e. a passage through which the gases to be filtered are introduced into the filter unit  11 . 
     The groove  30   b  preferably extends over an outlet passage  14   b  ( FIG. 5 ). This has the advantage of avoiding the loss of volume of an inlet passage. 
     Moreover, the reduction of the volume of a peripheral outlet passage adapts it to the reduced volumes of filtered gas that it receives. Indeed, a peripheral outlet passage does not receive filtered gases through its face(s) in contact with the joints  17  and therefore receives a lesser volume of gas than passages inside the filter unit, or “interior passages”, the four faces of which have a filter action. The groove  30   b  represented in  FIG. 5  reduces the section of the peripheral outlet passage  14   b  and advantageously makes the ratios between the section of a passage and the volume of gas that it receives homogeneous between the various outlet passages. This facilitates the flow of gas through the filter unit and reduces the head loss. 
     To improve further the adhesion of the joint  17  to the exterior face  16  of the unit  11 , the exterior surfaces of the boss  30   a  and/or the groove  30   b  may themselves have microroughnesses  36   a  and  36   b , respectively. 
     As represented in  FIG. 6 , a groove  30   b ′ may result from a local reduction in the thickness “e” of an exterior wall  34   b ′ of a passage  14   b ′. This advantageously reduces the quantity of material necessary for fabricating the filter unit  11 . Furthermore, this embodiment enables grooves to be produced on the inlet passages  14   a ′ without reducing their interior volume. 
     There is no limit on the number of irregularities  30   a  and  30   b . In one embodiment of the invention, bosses  30   a  and grooves  30   b  follow on alternately over the width of at least one face  16   a - 16   d  of the exterior surface  16  of the unit  11 , preferably covering respective successive inlet and outlet passages. The longitudinal grooves  30   b  and bosses  30   a  are preferably regularly spaced from each other. 
     The transition between bosses and grooves may be progressive, with no projecting corners. For example, the exterior surface  16  of the unit may have a sinusoidal shape in cross section, at least locally, as represented in  FIG. 7 . The thickness “e” of the exterior walls  34  of the peripheral passages is preferably substantially constant. 
     The boss  30   a  or the grooves  30   b  may also straddle two passages, which is preferable because it reinforces the mechanical coherence of the unit  11 . 
     The shape, dimensions and number of the irregularities  30   a  and  30   b  are preferably determined as a function of the support, i.e. the joints  17  for fastening them together. The shape, dimensions and number of the irregularities  30   a  and  30   b  may in particular depend on the nature and/or the thickness of the joints  17 , the position of the irregularities on the exterior surface  16  of the unit  11  and/or the position of the unit  11  within the filter body  3 . Thus not all the faces  16   a - 16   d  of the same filter unit  11  are necessarily provided with the same irregularities. 
     However, the width “I” of the irregularities is preferably less than or equal to that of the passages over which they extend, preferably substantially equal thereto. 
     Neither the width “I”, nor the length “L”, nor the thickness “e”, nor the orientation of an irregularity is limiting on the invention. For example, according to the invention, the exterior surface  16  of the filter unit  11  can have diagonal striations in one or more directions, holes, notches, etc. The width, thickness or orientation may also vary along the same irregularity. 
     In a variant of the invention that is not represented, two units intended to be assembled with two respective faces placed once against the other have irregularities on said faces that have complementary shapes and are disposed so that they can be accommodated one within the other. 
     The irregularities extending longitudinally may be fabricated during extrusion of the unit  11  by means of an appropriate die, using techniques known to the person skilled in the art. It is equally possible to form the irregularities on the surface of the unit  11  solid by “sculpting” the exterior surface  16  of the unit  11  and/or by fixing beads  30   a  of material thereto by gluing, welding or any other technique known in the art. The material of the attached beads  30   a  may be the same as or different from the material of the unit  11 . 
     Of course, the present invention is not limited to the embodiments described hereinabove and represented by way of illustrative and nonlimiting example. 
     The joint  17  disposed between the respective exterior faces of two units facing each other may be continuous or discontinuous, provided that they fasten the units together. 
     The filter unit  11  can have any shape. 
     The cross section of the passages  14  is not limited to a square shape. Equally, the section of the inlet passages could be different from that of the outlet passages. The cross section of a passage could also evolve along the passage, periodically or otherwise.