This invention relates to a regenerable filter for the exhaust gases of an internal-combustion engine. In particular, this invention relates to a filter suited to be placed into an exhaust pipe of an internal-combustion engine, comprising a filter­ing member suited to intercept the residual combustion products contained in the exhaust gases and further comprising means suited to produce the combustion of said residues.

Filters are known in which said means suited to produce the combustion of the residues are of the kind that uses a catalyst for the starting of the combustion at low temperatures, or of the kind that heats the exhaust gases, upstream of the filtering member, to the combustion temperature of said resi­dues.

Said filters suffer from certain drawbacks.

In the case of the entirely catalytic fil­ters, it is not uncommon the so-called "poisoning" of the catalyst, due to the presence in the exhaust gases of chemicals that impair their catalytic acti­vity to the point of completely discontinue the com­bustion of the residues; this originates a gradual clogging of the filter, with the resulting attainement of an unacceptable back-pressure on the engine ex­haust.

In the case of heating filters, the heating of the exhaust gases usually takes place by means of resistive elements positioned upstream of the filter­ing member, which, supplied with an electric current, generate thermal power through Joule effect and rise the temperature of the gases that affect then the filtering surface. This solution entails a high con­sumption of electric power, with the risk of extre­me charge losses of the vehicle battery.

A further drawback of the known heating fil­ters is the fact that the filter temperature can in­crease exceedingly, since the regeneration usually takes place upon rather wide surfaces and so in a scarcely controlled manner; as a result there is the possibility of serious damages or the destruction due to exaggerated heatings.

For the double object of limiting the avera­ge temperature of the filter and reducing the electric power used, filters have been realized in which the filtering member is cyclically heated in relation with consecutive portions; the continuity of the filter­ing member nevertheless can allow the starting of wi­de and intense combustions. Furthermore, since in the known filters of this kind the heating of the consecutive portions of the filtering member is ob­tained through convection and radiation, the electric power overall supplied, is always substantially high­er than the equivalent of the thermal power actually needed to locally start the combustion of the resi­dues.

Solutions in which diesel oil or other fuel burners in substitution for said resistive elements are used, are also known; said solutions have however high costs and dimensions together with said problems connected with the regeneration.

An object of the present invention is to rea­lize a regenerable filter for the exhaust gas­es of an internal-combustion engine, that lacks in the drawbacks connected with the above-mentioned and known filters, and is particularly simple, practical, and removes the risks of clogging or self-destruc­tion due to exaggerated heating.

Said object is attained by the present inven­tion in that it relates to a regenerable fil­ter for the exhaust gases of an internal-combustion engine, of the kind comprising:
an outer casing provided with at least an in­let duct and at least an outlet duct, suited to be placed in series with an exhaust pipe of said engi­ne;
filtering means housed into said outer casing and suited to intercept the residual combustion pro­ducts contained in said exhaust gases; and
heating means suited to bring the residual combustion products to a temperature sufficient to originate the combustion;
characterized in that said filtering means comprise a plurality of portions at least partially realized from an electrically conductive material and forming said heating means; said portions being mutually insulated and electrically supplied in a selective and cyclic manner.

For a better understanding of the present in­vention, some preferred embodiments are described be­low, as a non limiting example and referring to the accompanying drawings, in which:

• figure 1 is a perspective view of a first em­bodiment of a regenerable filter according to the present invention;
• figure 2 is a front view, partial and in en­larged scale of a detail of figure 1;
• figure 3 is an elevation and partial view of a second embodiment of a regenerable filter ac­cording to the present invention;
• figure 4 is a partial cross-section and in enlarged scale of the filter of figure 3;
• figure 5 illustrates a cross-section of a par­ticular arrangement of the filter of figure 3;
• figure 6 is a cross-section of a third embo­diment of a filter according to the present invention; and
• figure 7 is a partial cross-section of a fil­tering member of a further embodiment according to the present invention.

Referring in particular to figures 1 and 2, it is designated generraly by 1 a regenerable filter for an internal-combustion engine, in parti­cular of the compression-ignition type.

The filter 1 comprises a cylindric outer ca­sing 2, schematically shown in dotted line in figu­re 1, and a filtering member 3 housed into said ca­sing 2.

The filtering member 3 comprises a plurali­ty of honey-comb cells 4, suitably obtained from a porous and electrically conductive material, for in­stance of the ceramic type. Each cell 4, having an elongate prismatic shape with square section, com­prises four side walls 5 and two inner division walls 6, mutually orthogonal and located along the middle planes of the cell 4, so as to define four square section cavities 7. Cavities 7 are closed at one end by a front wall 8, and open at the opposite end; in particular, two diagonally opposed cavities 7 of each cell 4 are open at one end 9, the other two ca­vities of the same cell 4 being open at an opposite end 10.

According to the present invention, each cell 4 is arranged into a corresponding seat 11 of a reticulated structure 12, realized in an electri­cally and thermally insulating material, defined by two orders of flat, parallel and equidistant walls 13 perpendicularly intersecting one another.

Cells 4 are connected at one end 9 by means of respective electric cables 14, to the positive po­le of the vehicle battery; said connection is subject to the action of interruption means 15, for instan­ce controlled diodes, controlled by a conventional control exchange 16. The opposite ends 10 of cells 4 are connected to a metal plate 17 of the casing 2, which is provided with apertures 18 facing cor­responding apertures of the cells 4 themselves, and is in electric connection with a negative pole of the vehicle battery.

In figure 3 and 4, is designated by 21 a portion of a filtering member realized in electrical­ly conductive fabric. In particular, fabric 21 com­prises an intervening layer 2, forming the actual filtering member, suitably obtained from non-con­ducting ceramic fibers having a substantial heat­resistance. Upon a surface 23 of the intervening la­ yer 22, facing in use the inflow of the exhaust gas­es, a second fabric layer 24 is arranged, formed by bundles 25 of microthreads (afterwards named shortly "threads") in non-conductive ceramic fibre, arranged longitudinally, and by conductive threads 26 alter­nate with non-conductive threads 27 arranged tran­sversally. Conductive threads 26 are suitably pro­duced from metallic materials (for instance stain­less steel, Ni-Cr or other alloys) or conductive ce­ramics (for instance SiC), resistant to high tempe­ratures, or ceramics plated with oxidation-resistant metals. The fabric 21 further comprises a layer 28 having the role of simple support, applied on the op­posite surface 29 of the intervening layer 23.

Conductive threads 26 are connected adiacent an edge 30 of the fabric 21 to an electrode 31 form­ed by a plurality of metal foils 32 arranged perpen­dicularly to the threads 26 and in turn connected to a pole of the battery of the vehicle or directly to the alternator; the conductive threads 26 are con­nected in groups, at an opposite end, to another po­le of the vehicle battery, through a control device similar to that described with reference to figure 1.

In figure 5, an example of an embodiment of a filtre 35 using the fabric 21 of the kind describ­ ed is shown. Filter 35 comprises an outer casing formed by two coaxial tubular bodies 36, 37, de­fining an annular chamber 38 inbetween. The fabric 21, the surface provided with the conductive layer 24 of which is shown in phantom, is arranged into the annular chamber 38 substantially according to a closed polygonal line, the corners of which, angularly e­quidistant, are alternately joined to the tubular bodies 36 and 37. The fabric 21 forms with the tu­bular body 36 and with the tubular body 37, a plura­lity of spaces 39, 40, respectively communicating, in a not shown way, with an inlet aperture and with an outlet aperture of filter 35.

In figure 6 a second example of an embodi­ment of a filter 50 using the fabric 21 of the kind described is partially illustrated. The fabric 21 is pleated and is fixed on a support layer 51 spiral wound into a not shown cylindric casing. The fabric 21 forms with the support layer 44 a plurality of spaces 52, 53 respectively communicating, in a not shown way, with an inlet aperture and with an out­let aperture of filter 50.

In figure 7, a further embodiment of the con­ductive fabric is illustrated. The fabric 43 is sub­divided in a plurality of portions 44, separated one another by insulating elements 45, formed for instance by strips of fabric from non conductive and oxidation resistant ceramic fibres. Portions 44 are realized from non conductive fibres, into which conductive fibres 46 schematically shown in phantom and arrang­ed at random are incorporated. The fabric 43 has at an edge a plurality of elctrodes 47, for instan­ce metal foils, insulated each other by the elements 45 and apt to be connected through a control devi­ce of the kind described to a pole of the vehicle battery; fabric 43 further has, at an opposite edge, a further electrode 48 connecting portions 45 to the other pole of the battery.

The operation of the filter is as follows.

The exhaust gases emitted by the engine are conveyed in a know manner into the casing 2 of the filter 1 and enter in the cavities 7 open on the si­de of the end 9. Since these cavities are closed at the opposite end by the fron walls 8, gases are constrained to pass through the walls 6, which in­tercept the residual combustion products, in order to go in the adiacent cavities 7 which are open on the side of the end 10 and allow the discharge of gases from filter 1 through the apertures 18 of the plate 17. The buil-up of residues in the walls 6 o­ riginates a gradual clogging of the filter, creat­ing a back-pressure in the engine exhaust. The con­trol exchange 16, as a result of a signal received, for instance, from a pressure sensor detecting the pressure difference of the exhaust gas between inlet and outlet from the filter 1, causes the closing of one of the switches 15. Consequently one of the cells 4 is electrically supplied through the circuit formed by the respective cable 14, the cell 4 and the plate 17. The flow of electric current through the walls 5 and 6 of the cell 4 gives rise to a heat­ing by Joule effect of the walls till the combustion temperature of the residues is attained, which are oxidized and gassified freeing the porosity of the cell.

Subsequently, the control exchange 16 discon­nects the the supply to the cell 4 and supplies the subsequent cell 4. All the cells are supplied in se­quence, according to an order and for a time pre­determined or governed from time to time by the con­trol exchange 16. For instance, the exchange 16 can control the intensity of current and the heating ti­me of each cell 4 as a function of the revolutions of the engine and of the rate of flow of the air passing through the filter 1 by the action of an ap­propriate device, for instance a fan or a positive-­ displacement pump.

The operation of filter 35 is similar; in this case, the filtering and heating function is as­signed to the fabric 21. The exhaust gases (fig. 5) enter the filter 35 through an inlet aperture, not illustrated, and enter the spaces 39 included bet­ween the outer tubular body 36 and the fabric 21. The gases pass then through the fabric 21, which in­tercepts the residual combustion products, and enter the spaces 40 included between said fabric 21 and the inner tubular body 37, from which they exit through a not shown outlet aperture.

As relates the filter 50 (figure 6), the exhaust gases enter through an inlet aperture, not illustrated, and enter the spaces 52; they pass then through the fabric 21, that intercepts the residual combustion products, and enter the spaces 53 from which they exit through a not shown outlet aperture.

The conductive threads 26 of the fabric 21 can be supplied by groups according to a predetermin­ed program, so as to assure in use the combustion of the residues accumulated in a well defined portion of the fabric 21; while this portion is regenerated, the residues build-up in other portions of the fa­bric 21, which will be regenerated in sequence. At the end of the regeneration of the last portion, the cycle starts again with the regeneration of the first regenerated portion, which in the meantime will have intercepted new residues.

It should be noted, in particular, that the "meshes" of the conductive layer 24 are suitably wide enough to allow the passage of the residues, that are intercepted, as schematically illustrated in fi­gure 4, by the filtering layer 22. The concentration of the residues results substantially distributed around the conductors 26, which provide for the heat­ing by direct conduction of those residues.

In the fabric 43 illustrated in figure 7, the conduction between the opposite electrodes 47, 48, is assigned not to conductors having a definite geo­metry, such as the cells 4 or the threads 26, but to conductive fibres 46 irregularly scattered in por­tions 44 mutually insulated of non-conductive fa­bric.

From an examination of the features of the filters realized according to the present invention, the advantages they allow to attain are obvious.

First of all, the conductive portions (4; 26; 44) of the filtering members (3; 21; 43) are mutual­ly insulated. This enables to obtain a selective and cyclic heating of the portions, with the advantage of reducing the electric power used and keeping under con­trol the temperature of the filtering elements (3; 21; 43), without any risk of clogging or of destruc­tion due to over-heating. Further, the heating of the residues takes place through conduction, that is through direct contact between the residues themselves and the conductive portions (4; 26; 44), which ena­bles to exploit the most of the power supplied for the starting of the combustion, without great losses due to convection. Lastly, the filtering fabric ele­ments (21; 43), thanks to their deformability, are particularly resistant to thermal shocks produced due to the sudden temperature variations during the heat­ing and the cooling of the conductive portions (26; 44). The ceramic filtering members 3 as well can ha­ve a good resistence to thermal shocks, since they a­re subdivided in a plurality of cells 4 having re­duced dimensions and being mutually thermally insu­lated.

In this connection, the fibres used are sui­tably subjected to pre-tratments apt to avoid the embrittlement and/or the possible breakage due to phase transformation or anyway to other phenomena pro­duced by thermal shocks. The pre-treatments can be of chemical and/or physical nature, and depending on the type of fibre used said treatments consist in the introduction or extraction of ions through diffusion in the matter of the fibre.

It is then obvious that to the filters 1, 35, 50 described can be introduced changes or varia­tions, without departing from the scope of the present invention. In particular, the shape, the arrangement and the composition of the conductive portions (4; 26; 44) and of the relative insulating portions (12; 27; 45) can change. In the fabric 43, said insulat­ing portions (45) can also be omitted, since the pre­ferential orientation of the conductive fibres 46 establishes paths having a relatively reduced resi­stance in the traverse direction to the electrodes 47, 48, while the resistance increases indefinitely moving away from said paths; therefore, supplying only one electrode 47, the electric conduction, and so the heating, is obtained substantially in the por­tion 44 facing said electrode 47, while the surround­ing portions behave substantially as insulators.

The filtering member can be produced with a combination of ceramic fabrics, felts or boards. For instance, the filtering member can comprise a series of stratified felt members; in particular, the single layers of the filtering member can be provided with pores having a geometric distribution, different di­mensions and shapes, and arranged according to a po­rosity gradient. The electrification of the various elements can be carried out introducing in such ele­ments electrically conductive fibres or threads. This solution enables to reduce the effect of the thermal shocks, in that the thermal conductivity is increas­ed.

It can further be fixed upon a sliding ele­ment substantially arranged on a central plane of an inner chamber of the filter, in this case suita­bly formed with a quadrangular cross-section, and which can readily be introduced and extracted from the outer casing of the filter.

It is possible to change the logic of the con­trol of the exchange 16, that can control the switches 15 in response to signals received from the user and/or from process sensors (for instange temperatu­re or pressure sensors) arranged inside or outside the filter; the electric current supplying the con­ductive portions (4; 26; 44) can be modulated accord­ing to the temperature levels established in the fil­ter.

Means for the introduction of air into the filter can be provided, in order to assure a suffi­cient partial pressure of oxygen in the exhaust gases and so a complete combustion of the residues.

At last catalysing additives can be provided suited to aid and optimize the combustion of the so­lid unburned particles. In particular said additives can suitably comprise a mixture of one or more metal­oxides, for instance CuO, Cu₂O, MnO₂, Mn₃O₄, PbO, CeO₂ or the respective oxygenated salts, for instan­ce Cu(NO₃)₂, CuSO₄, and of the one or more chlorides of an alkaline or alkaline-earth metal for instance NaCl, KCl, LiCl, CuCl, CuCl₂, MgCl₂, BaCl₂, possibly also in the hydrated form; preferably said mixture comprises CuO and NaCl. Said mixture can be in a solid form (pow­der) or in the form of a solution in water or other sol­vent, and is deposited on the filtering member in the more convenient manner, such as insufflation, spraying or immersion.