FR2823252A1 - Filtering body for i.c. engine exhaust gas particles has several adjacent gas filtering channels with first and second filtering zones with different filtering surfaces - Google Patents

Abstract

The filtering body (7) consists of several adjacent channels through which the exhaust gases are filtered. There are first (12-23) and second (8-11) filtering zones. The gas filtering surface is different in the first and second zones.

Description

<Desc / Clms Page number 1>

 The invention relates to filter bodies for filtering particles contained in the exhaust gases of internal combustion engines, in particular of the Diesel type.

 Porous honeycomb structures are used as filtering bodies for the filtration of particles emitted by Diesel vehicles. Generally, these filter bodies are made of ceramic (cordierite, silicon carbide, etc.). They can be monolithic or else consist of different blocks. In the latter case, the blocks are assembled together by gluing using a ceramic cement. The whole is then machined to take the desired section, round or ovoid in general. The filter body may have a plurality of adjacent channels. It is inserted in a metal envelope.

Each channel is closed at one or other of its ends; the exhaust gases are thus forced to pass through the side walls of the channels; this is how particles or soot are deposited in the filtering body.

 After a certain period of use, soot builds up in the channels of the filter body which increases the pressure drop due to the filter body and deteriorates the performance of the engine. For this reason, the filter body must be regenerated regularly (for example every 500 km).

de 300 C alors que la température d'auto-inflammation des suies est plutôt de l'ordre de 6000C, dans des conditions de fonctionnement classiques. Il est possible d'ajouter des additifs dans le carburant qui permettent de catalyser Regeneration consists of oxidizing the soot. To do this, it is necessary to overheat this soot. Indeed the temperature of the exhaust gases is around

300 C while the auto-ignition temperature of the soot is rather of the order of 6000 C, under conventional operating conditions. Additives can be added to the fuel which catalyze

<Desc / Clms Page number 2>

la réaction d'oxydation des suies et d'abaisser la température d'auto-inflammation de 150oC environ. Le chauffage peut concerner les gaz d'échappement, la face amont du corps filtrant ou bien même directement les suies déposées sur le corps filtrant. Différentes techniques ont été développées mais elles demandent beaucoup d'énergie et sont très souvent difficiles à contrôler.

the soot oxidation reaction and lower the auto-ignition temperature by about 150oC. The heating can relate to the exhaust gases, the upstream face of the filter body or even directly the soot deposited on the filter body. Different techniques have been developed but they require a lot of energy and are very often difficult to control.

 A more recent and advantageous approach consists in heating locally (for example, in front of the filter body) so as to ignite the soot and initiate its combustion, which spreads to the whole of the filter body through the soot. This type of technique is for example described in patent applications FR-A-2 771 449 or DE-A-19530749.

 This solution has drawbacks, in particular because the soot is distributed inhomogeneously in the filter body. Indeed, mainly because the exhaust gas evacuation pipe which feeds the filter body is of smaller section than that of the latter, the flow of exhaust gases is greater at the heart of the filter body than on its outskirts. Thus, the amount of soot deposited is also greater in the heart of the filter body.

 Under these conditions, when combustion is initiated locally, for example on the front face of the filter body, the soot burns well in the central part (longitudinally) but the amount of soot in the peripheral zone is too low to allow the transmission of heat and the spread of combustion to the entire filter body.

 This has two consequences which significantly affect the life of the filter body. Indeed, the bad

<Desc / Clms Page number 3>

 heat transmission, radially inside the filter body, creates strong thermo-mechanical constraints between the hot core and the much colder periphery. These constraints weaken the structure of the filter body. Furthermore, the propagation of the combustion of soot being poor at the periphery, the regeneration of the filter is incomplete and its efficiency after regeneration is reduced.

 There is therefore a need for a body for filtering the particles contained in the exhaust gases of internal combustion engines, in particular diesel engines, which improves the conditions for regeneration.

 The invention aims to satisfy this need.

 More particularly, the invention relates to a filter body of a particulate filter, for the purification of exhaust gases from an internal combustion engine, in particular from a diesel engine, remarkable in that said body comprises at least first and second filtration zones, the gas filtration surface, relative to the surface unit of the gas inlet of said filter body, being different in said first and second zones.

 By filtration area is thus meant the total area available to collect the particles per unit area of the front section of the filter body, or "inlet" of this body.

 According to other characteristics of the present invention: said first and second zones respectively occupy a peripheral part and a central part of said filter body, and the peripheral part has a filtration surface

<Desc / Clms Page number 4>

 larger than that of the central part of the filter body, the area with a larger filtration surface is arranged so that, in operation, it is close to a heat source intended to initiate the regeneration of said filter body, the filtration surface of one of said zones is at least 10% greater than that of the other zone.

 The description which follows, made with reference to the accompanying drawings, will make it possible to better understand and appreciate the advantages of the invention. In these drawings: - Figure 1 is a cross-sectional view of a filter body of the prior art made by assembling nine blocks, - Figure 2 shows a cross-sectional view of a monolithic filter body of the prior art consisting of channels of square shape, - Figure 3 is a view of the upstream face of a filter body according to the invention produced by assembling sixteen blocks, - Figure 4 shows a schematic view of part of the upstream face of a block of an area of the filter body of Figure 3, - Figure 5 shows a schematic view of a portion of the upstream face of a block of another area of the filter body of Figure 3 , - Figure 6 shows a schematic view of part of the upstream face of another embodiment of a block of an area of the filter body of Figure 3, and Figure 7 shows yet another mode of realization of a filter body according to the invention, comprising two concentric zones presenting cara different characteristics.

<Desc / Clms Page number 5>

We will not focus here on the characteristics of the material constituting the filter body, to assess the filtration surface. Indeed, if there are differences in permeability, porosity, etc. in different areas of the filter body, these are very quickly compensated for by the thickness of the soot bed that forms. Thus, the filtration surface is only linked to the geometry of the filter body. Given the tolerances for the tools used to make the filter bodies, we consider that the filtration surfaces are different if there is a difference greater than or equal to 5%.

 By "surface density" is meant below, for a given area, the total number of channels divided by the front surface of the area.

 Figure 1 shows a cross-sectional view of a filter body 1 of the prior art surrounded by a metal casing 2. The filter body 1 consists of different blocks 1a, 1b, lc ... bonded to each other by means of ceramic cement 3. Each block consists of a multitude of channels 4, of square section, alternately plugged on the upstream face or on the downstream face of the filter body, as is well known. In this case, the filtration surface is equal to the internal perimeter of the channel multiplied by the surface density of the filter body, divided by two (one channel in two being blocked) and multiplied by the length of the filter body.

Typically, the inner perimeter of a channel measures between 4 and 6 mm, the surface density is between 200 and 300 cpsi (cell per square inch), or between 31 and 47 channels / cm2, and the length of the filter body measures 15 to 30 cm. The filtration surface generally varies between 200 and 350 cm2 per cm2 of front surface, for a filter length of 25cm.

<Desc / Clms Page number 6>

 FIG. 2 illustrates another example of a filter body 5 of the prior art surrounded by its metal envelope 2. The filter body is monolithic and comprises a multitude of channels 6, of square section, and plugged alternately on the upstream face or on the downstream face of the filter body.

 FIG. 3 shows an exemplary embodiment of a filter body 7 according to the invention, stripped of its metal envelope 2. It consists of different blocks 8 to 23 assembled by a ceramic cement. Each block comprises a multitude of channels which are not shown in this figure and whose detailed sectional views are shown diagrammatically in FIGS. 4 and 5.

 In FIG. 4, the channels constituting the central blocks 8 to 11 of FIG. 3 are seen. Each channel wall 24 has a thickness of 0.5 mm and the pitch A between two successive channels is equal to 1.8 mm. The surface density is 200 cpsi, or 31 channels / cm2. For the sake of clarity, the alternative plugging of the channels is not shown in FIGS. 4 to 6. For a structure as described in FIG. 4, and for a 25 cm long filter, the filtration surface is 201cm2 per cm2 of frontal area.

 In FIG. 5, the channels constituting the blocks 12 to 23 in FIG. 3 are seen. Each channel wall 25 has a thickness of 0.2 mm, less than that of the walls 24.

The pitch A of the channels as well as the surface density are identical to those of the channels of FIG. 4. Compared to the zone represented in FIG. 4, the filtration surface is here much larger. Here it is equal to 247 cm2 for 1 cm2 of frontal area. This filtration surface is actually

<Desc / Clms Page number 7>

 proportional to the cumulative internal perimeter of all channels.

 By thus placing blocks with a larger filtration surface on the external periphery of the filter body 7, a greater proportion of the flow of the exhaust gases is forced to pass through this external filtration zone. The deposit of soot takes place all the more in this zone and the quantity of particles deposited is thus distributed more homogeneously throughout the filter body. The conditions necessary for good thermal regeneration are thus established. The transmission of heat, step by step, through the soot, is significantly improved and the combustion of the soot can spread throughout the filter body.

 In addition, a more homogeneous distribution of soot in the filter induces a reduced pressure drop. This constitutes an important advantage of the present invention.

 As a variant of these examples, it is possible to envisage an increase in the filtration surface by the increase in the surface density. In fact, in part thanks to the improvement of the manufacturing processes, changes are possible towards greater surface densities and smaller wall thicknesses. For example, for a filter element 25 cm in length as shown in FIG. 4, with a wall thickness equal to 0.2 mm, the filtration surface is equal to 181 cm2 for 1 cm2 of front surface if the density surface area is equal to 100 cpsi or 15.5 channels / cm2, (= 2.54 mm) whereas it is 331 if the surface density is equal to 400 cpsi, 62 channels / cm2 (X = 1.27 mm) . It can be seen that this parameter makes it possible to considerably increase the filtration surface while keeping a constant wall thickness.

<Desc / Clms Page number 8>

 It can be noted here that the reduction in the size of the channels will however be limited by the problem of the potential blockage of the channel by combustion residues (after burning of the soot).

 FIG. 6 illustrates another embodiment of the blocks 11 to 23 of the filter body of FIG. 3. In this case, the channels have a section of triangular shape, with a surface density of 200 cpsi (31 channels / cm2) and a wall thickness of 0.36 mm. In this case, the filtration surface is 245 cm2 for 1 cm2 of front surface, for a filter 25 cm long. By way of comparison, a filter body having a geometry of the type shown in FIGS. 4 and 5, with a wall thickness of 0.36 mm and a surface density equal to 200 cpsi, ie 31 channels / cm2, (À. = 1.8 mm), has a filtration area equal to 222 cm2 for 1 cm2 of front surface, for a filter 25 cm long. The embodiment of FIG. 6 therefore makes it possible to obtain a larger filtration surface and the deposit of soot is thus more homogeneous. It can however be noted that under these conditions of comparison, the triangular geometry leads to a greater mass for the filter body, which can be troublesome in the automotive industry.

 As a variant of this embodiment, and in order to avoid this drawback, the same type of channels is envisaged as the arrangement illustrated in FIG. 6, but with a thickness of the wall of the channels which is less. This could indeed make it possible to keep the same mass for a filter body entirely produced with blocks made up as in FIG. 4, and for a filter body according to the

<Desc / Clms Page number 9>

 Figure 6, at least one block of which is made up of smaller channels with thinner walls. The reduction in the thickness of the walls is however limited by difficulties in producing the channels as well as by a certain fragility of the blocks made up of channels having very thin walls; currently the walls cannot have a thickness of less than 50 μm.

 It is particularly advantageous to use channels of triangular section because the thermal conductivity of this type of geometry is better than that of a network made up of channels of square section.

 Another alternative embodiment relates to a filter body as described in FIG. 3, in which the blocks 14, 17, 20 and 23 have a larger filtration surface than the other blocks - for example with channels like those represented in the figures 5 or 6 for blocks 14, 17, 20 and 23, and as shown in FIG. 4 for the other blocks. This variant is particularly advantageous if hot spots intended to initiate combustion are located near the blocks 14, 17, 20 and 23. In fact, the local increase in the filtration surface ensures good accumulation of soot and guaranteed a good start to regeneration in these areas.

 FIG. 7 shows another embodiment of the invention, obtained by concentrically bonding two parts defining filtration zones 26 and 27, with a ceramic seal 28. Zone 26 can be made up of channels as shown on the FIG. 4. Zone 27 can be made up of channels as shown in FIGS. 5 or 6. In this embodiment again, the outermost part of the filter body has a larger filtration surface,

<Desc / Clms Page number 10>

 which allows to homogenize the soot deposit and thus improve the regeneration yield, while reducing thermo-mechanical stresses.

 As a variant of this example, the two parts 26 and 27 could be two filtration zones of the same monolithic filter body. One could also envisage a smoother transition between the two zones and have a gradual increase in the filtration surface of the core towards the periphery of the filter body. In this case, the peripheral zone has a filtration surface greater by at least 5% greater than that of the central zone having the smallest filtration surface.

 These embodiments of the invention provide the aforementioned advantages without disadvantages as regards their manufacture or their use. In particular, they do not require additional machining or bonding steps, compared with a filter body of the prior art as described in FIG. 1. On the other hand, the overall shape of the filter body remains unchanged and therefore has no impact on automated assembly lines in the automotive industry.

 These different embodiments are described only by way of example and in no way limit the scope of the invention. This indeed extends to any type of filter body, whatever the shape and dimensions of the channels, provided that this filter body comprises at least two distinct zones differentiated by their filtration surfaces, one of the zones having a filtration area at least 5% greater than that of the other and, preferably, at least 10%.

 We have noticed that these filter body designs according to the invention make it possible to optimize the distribution of soot in the filter body. Through

<Desc / Clms Page number 11>

optimisation on entend meilleur contrôle des zones de dépôt. Ceci peut se traduire, selon les besoins, par une homogénéisation de la quantité de suies déposée dans les différents zones du corps filtrant, ou bien par une accumulation forcée de la quantité de suies à un point précis, par exemple à proximité d'un point chaud par lequel la régénération doit démarrer.

optimization means better control of the drop zones. This can be translated, as necessary, by a homogenization of the amount of soot deposited in the various zones of the filtering body, or by a forced accumulation of the amount of soot at a precise point, for example near a point by which regeneration should start.

Without wishing to link the invention to any theory, we believe that these results are linked to the balancing of the pressure drop at any point of the filtering body. Indeed, at equilibrium, the pressure drop is identical at all points of the filtering body. However, the pressure drop is essentially due to the passage of gases through the side walls of the channels constituting the filter body. Since the gas passage speed is identical at all points, the exhaust gas flow must compensate for the differences in filtration surfaces.

By proposing zones differentiated by their filtration surfaces we create zones where the exhaust gas flow is increased by an increase in the filtration surface, and therefore where the quantity of soot deposited is greater than in the absence of this increase.

Claims (6)

 CLAIMS 1. Filter body of a particle filter, for purifying the exhaust gases of an internal combustion engine, in particular of a Diesel engine, characterized in that said body (7; 26,27, 28 ) comprising at least first (12 to 23; 27) and second (8 to 11; 26) filtration zones, the gas filtration surface, relative to the surface unit of the gas inlet of said body (7 ), being different in said first (12 to 23; 27) and second (8 to 11; 26) zones.  2. Filter body according to claim 1, characterized in that said first (12 to 23; 27) and second (8 to 11; 26) zones occupy respectively a peripheral part and a central part of said filter body (7), and in that the peripheral part has a larger filtration surface than that of the central part of the filter body (7).  3. Filter body according to claim 1, characterized in that it comprises at least one zone with a larger filtration surface arranged to, in operation, be close to a heat source intended to initiate the regeneration of said filter body .  4. Filter body according to one of claims 1 to 3, characterized in that the filtration surface of one of said zones is at least 10% greater than that of the other zone.  5. Filter body according to any one of claims 1 to 4, characterized in that it consists of a plurality of adjacent channels through the side walls (24; 25) from which the filtration of said gases is carried out. exhaust. <Desc / Clms Page number 13>  6. Filter body according to any one of claims 1 to 5, characterized in that it is made of silicon carbide or cordierite.