Regenerable filter for exhaust gases of an internal-combustion engine

The filter comprises filtering means suited to intercept the residual combustion products, and heating means suited to bring said residues to a combustion temperature; the heating means comprise a plurality of electrically conductive portions of said filtering means, mutually insulated and cyclically and selectively supplied.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF THE INVENTION 
Referring in particular to FIGS. 1 and 2, it is designated generally 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 FIG. 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 FIGS. 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 22, forming the actual filtering member, 
suitably obtained from non-conducting ceramic fibers having a substantial 
heat-resistance. Upon a surface 23 of the intervening layer 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 adjacent 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 FIG. 1. 
In FIG. 5, an example of an embodiment of a filter 35 using the fabric 21 
of the kind described 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 FIG. 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 51 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 FIG. 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 electrodes 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 
front walls 8, gases are constrained to pass through the walls 6, which 
intercept the residual combustion products, in order to go in the adjacent 
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 build-up of residues in the walls 6 originates 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 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 (FIG. 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 FIG. 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 FIG. 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 
resistance 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-treatments 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 material 
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 metal-oxides, for instance 
CuO, Cu.sub.2 O, MnO.sub.2, Mn.sub.3 O.sub.4, PbO, CeO.sub.2 or the 
respective oxygenated salts, for instance Cu(NO.sub.3).sub.2, CuSO.sub.4, 
and of one or more chlorides of an alkaline or alkaline-earth metal for 
instance NaCl, KCl, LiCl, CuCl, CuCl.sub.2, MgCl.sub.2, BaCl.sub.2, 
possibly also in the hydrated form; preterably 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.