DE3782176T2 - REGENERABLE EXHAUST GAS FILTER FOR COMBUSTION ENGINES. - Google Patents

Description

The invention relates to a regenerable filter in the exhaust gases of an internal combustion engine. In particular, the invention relates to a filter which is suitable for arrangement in an exhaust pipe of an internal combustion engine, with a filter member which is suitable for collecting the residual combustion products contained in the exhaust gases and with a device which is suitable for causing combustion of these residues.

Filters are known in which the device for effecting the combustion of the residues is one which uses a catalyst to initiate combustion at low temperatures or which heats the exhaust gases upstream of the filter element to the combustion temperature of the residues.

These filters have certain disadvantages.

In the case of a fully catalytic filter, the so-called "poisoning" of the catalyst is not uncommon due to the presence of chemicals in the exhaust gases, which impair their catalytic activity to the point where the combustion of the residues stops completely; this triggers a gradual clogging of the filter, with the formation of an unacceptable back pressure at the engine outlet.

In the case of heating filters, one of which is described in US-A-4 276 066, the heating of the exhaust gases is usually carried out by means of resistance elements located upstream of the filtering element which, when supplied with an electric current, generate thermal outputs via the Joule effect and increase the temperature of the gases which then hit the filter surface. This solution results in high consumption of electrical energy, with the risk of extreme losses of charge on the vehicle battery.

Another disadvantage of the known heating filters is that the filter temperature can increase excessively, since the regeneration usually takes place over relatively large surfaces and is therefore difficult to control; consequently there is a possibility of serious damage or destruction due to excessive heating.

For the dual purpose of limiting the average temperature of the filter and reducing the electrical current consumed, filters have been manufactured in which the filter element is heated cyclically in relation to successive areas; however, the connection of the filter element can nevertheless allow the initiation of a wide and intense combustion. Moreover, since in the known filters of this type the heating of the successive areas of the filter element is achieved by convection and radiation, the total electrical energy supplied is always considerably greater than the equivalent of the thermal energy actually required to initiate the local combustion of the residues.

Solutions using diesel oil or other fuel burners instead of the resistance element are also known; however, these solutions entail high costs and large dimensions together with the problems associated with regeneration.

It is an object of the present invention to provide a regenerable filter for exhaust gases of an internal combustion engine which does not have the disadvantages of the above-mentioned known filters and in particular is simple and practical and overcomes the risk of clogging or self-destruction due to excessive heating.

This object is achieved by the invention in that it relates to a regenerable filter in the exhaust gases of an internal combustion engine, which comprises an outer housing with at least one inlet line and at least one outlet line, a filter device in the outer housing which has a set of pairs of cavities with walls of porous material, each of the cavities being provided with an open end and a closed end, one cavity of each pair being connected with its open end to the inlet line, the other cavity of each pair being connected with its open end to the outlet line, so that the porous material is provided for collecting the residual combustion products in the gases, an electrical heating device in connection with the pairs of cavities for raising the temperature in the products and a selectively and/or cyclically operable control device for electrically supplying the heating device characterized by a number of elongated honeycomb cells of a prismatic shape each containing at least one pair of said cavities, which porous material is electrically conductive, which plurality of cells are arranged in a net-like structure formed by two orders of intersecting flat walls made of an electrically and thermally insulating material and isolating the cells from each other, which two ends of the cavities of each cell are electrically connected to the control device for selective and/or cyclic electrical supply by means of the control.

For a better understanding of the present invention, a preferred embodiment is described below, by way of non-limiting example and with reference to the accompanying drawings, in which:

- Figure 1 is a perspective view of a regenerable filter according to the present invention;

- Fig. 2 is a front view, partly on an enlarged scale, of a detail in Fig. 1.

In Fig. 1 and 2, 1 generally designates a regenerable filter for an internal combustion engine, in particular a compression internal combustion engine.

The filter 1 comprises a cylindrical outer housing 2, which is shown schematically in dashed lines in Fig. 1, and a filter member 3 which lies in the housing 2.

The filter member 3 comprises a number of honeycomb cells 4, suitably made of a porous and electrically conductive powder, for example of a ceramic type. Each cell 4, which has an elongated, prismatic shape with a square cross-section, comprises four side walls 5 and two inner partition walls 6, which are perpendicular to one another and are arranged along the median planes of the cell 4, so that four cavities 7 with a square cross-section are formed.

The cavities 7 are closed at one end by a front wall 8 and open at the opposite end; in particular, two diagonally opposite cavities of each cell are open at one end 9, and the other two cavities of the same cell 4 are open at the opposite end 10.

According to the present invention, each cell 4 is arranged in a corresponding seat 11 of a mesh structure 12 made of an electrically and thermally insulating material, which is formed by two orders of flat, parallel and equally spaced walls 13 which intersect each other perpendicularly.

The cells 4 are connected at one end 9 to the positive pole of the vehicle battery by means of electric cables; this connection is subjected to the action of an interruption device 15, for example controlled diodes controlled by a conventional control switch 16. The opposite ends 10 of the cells 4 are connected to a metal plate 17 of the casing 2, which is provided with openings 18 facing the corresponding openings of the cells 4, and which is in electrical connection with the negative pole of the vehicle battery.

The filter works as follows.

The exhaust gases emitted by the engine are directed in a known manner into the housing 2 of the filter 1 and enter the cavities 7 on the side of the open end 9. As these cavities are closed at the opposite end by the front walls 8, the gases are forced to pass through the walls 6 which capture the residual combustion products in order to reach the adjacent cavities 7 which are open on the side of the end 10 and allow the gases to exit the filter 1 through the openings 18 of the plate 17. The build-up of the residues in the walls 6 produces a gradual blockage of the filter which creates a back pressure in the engine exhaust. The control switch 16 causes the closure of one of the switch 15. Consequently, one of the cells 4 is electrically powered through the circuit formed by the corresponding cable 14, the cell 4 and the plate 17. The flow of the electric current through the walls 5 and 6 of the cell 8 causes the walls to heat up by the Joule effect until the combustion temperature of the residues is reached, which are oxidized and gasified, releasing the porosity of the cell.

The control switch 16 then interrupts the power supply to the cell 4 and supplies the next cell 4. All the cells are supplied with power in turn, according to an order and for a time that is predetermined or determined from time to time by the control switch 16. For example, the switch 16 can control the intensity of the power and the heating time of each cell 4 as a function of the revolutions of the machine and the flow rate of air passing through the filter 1, by the action of a suitable device, for example a fan or a forced-feed pump.

From an examination of the features of the filter produced according to the invention, the advantages made possible are obvious.

Firstly, the conductive areas 4 of the filter elements 3 are mutually insulated. This makes it possible to achieve selective and cyclic heating of the areas, with the advantage of reducing the electrical current consumed and of being able to control the temperature of the filter element 3 without the risk of clogging or destruction due to overheating. Furthermore, the heating of the residues takes place by conduction, that is to say by direct contact between the residues themselves and the conductive areas 4, so that it is possible to exploit the majority of the energy supplied to trigger combustion without causing major losses due to convection.

The ceramic filter elements 3 can have a good resistance to thermal shocks since they are divided into a number of cells 4 which have reduced dimensions and are thermally insulated from one another.

In this context, the fibres used are subjected to appropriate pre-treatments that make it possible to avoid brittleness and/or possible breakage due to phase transformation or other phenomena caused by thermal shocks. The pre-treatments may be chemical and/or physical and, depending on the type of fibre used, the treatments consist in the introduction or extraction of ions by diffusion into or from the fibre material.

It is therefore obvious that changes or modifications to the filter are possible without departing from the scope of the invention. In particular, the shape, arrangement and composition of the conductive regions 4 and the relatively insulating regions 12 can be changed.

The filter elements can be made of a combination of ceramic fibers, felts or cardboards. For example, the filter element can comprise a series of layered felt elements; in particular, the individual layers of the filter element 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 by introducing electrically conductive fibers or wires into these elements. This solution makes it possible to reduce the effect of thermal shocks by increasing the thermal conductivity.

It may further be mounted on a sliding element arranged substantially on a central plane of an inner chamber of the filter, which in this case is suitably formed with a rectangular cross-section, and can be easily pushed into and pulled out of the outer casing of the filter.

It is possible to change the logic of control of the switch 16. The switches 15 can be controlled according to signals received from the user and/or from process sensors (for example temperature or pressure sensors) located inside or outside the filter; the electric current supplying the conductive areas 4 can be modulated according to the temperature level in the filter.

A device may be provided for introducing air into the filter in order to ensure a sufficient partial pressure of oxygen in the exhaust gases and thus complete combustion of the residues.

Finally, catalyzing additives can be provided which support and optimize the combustion of the solid and burned particles. In particular, the additives can suitably comprise a mixture of one or more metal oxides, for example CuO, Cu₂O, MnO₂, Mn₃O₄, PbO, CeO₂ or the corresponding oxygenated salts, for example Cu(NO₃)₂, CuSO₄, and one or more chlorides of an alkaline or alkaline earth metal, for example NaCl, KCl, LiCl, CuCl, CuCl₂, MgCl₂, BaCl₂, possibly also in hydrated form; preferably, the mixture comprises CuO and NaCl. This mixture can be in solid form (powder) or in the form of a solution in water or another solvent and is deposited on the filter element in a simple manner, such as by suction, spraying or dipping.

Claims (4)

1. Regenerable filter for the exhaust gases of an internal combustion engine, with an outer housing having at least one inlet line and one outlet line, a filter device in the housing having sets of pairs of cavities (7) having walls (5, 6) of porous material, which cavities are each provided with an open end and a closed end, one cavity of each pair being connected with its open end to the inlet line and the other cavity of each pair being connected with its open end to the outlet line, so that the porous material captures the residues of the combustion products of the gases, with an electrical heating device in connection with the pairs of cavities for increasing the temperature of the products, and with selectively and/or cyclically operable control devices (15, 16) for electrically supplying the heating device, characterized by a number of elongated honeycomb cells (4) prismatic shape, each enclosing at least one pair of said cavities (7), which porous material is electrically conductive, the plurality of cells being arranged in a network structure (12) formed by two groups of intersecting flat walls (13) on an electrically conductive and thermally insulating material, which insulate the cells from each other, furthermore the two ends (9, 10) of the cavities (7) of each cell (4) being electrically connected to the control devices (15, 16) and being selectively and/or cyclically supplied electrically via the control devices. 2. Filter according to claim 1, characterized in that each of the cells (4) is provided with outer walls (5) and is divided internally by longitudinal inner walls (6), which outer walls and inner walls consist of the porous and electrically conductive material and form the walls of a group of at least two pairs of the cavities (7). 3. Filter according to claim 1 or 2, characterized in that the cells (4) and the cavities (7) have a polygonal cross-section. 4. Filter according to one of the preceding claims, characterized in that the porous and electrically conductive material is sprayed with a mixture of at least one metal oxide or an oxygenated salt suitable for producing the metal oxide and at least one chloride of an alkaline metal or alkaline earth metal.