EP1506347B1 - Device and method for filtering particle-loaded exhaust gases - Google Patents

Description

The present invention relates to a device for filtering particulate exhaust gases, particularly for motor vehicles, a filter assembly and a particle-laden exhaust filtering method.

Efforts are being made to reduce the polluting emissions of internal combustion engines used in motor vehicles.

We know, by the document WO 01/19525 , a particle-laden gas filtering assembly comprising an electrostatic precipitator, or electrostatic filter, corona or corona discharge. The electrostatic filter comprises a cylindrical cage internally covered on an axial portion by a knitted wire. The cage comprises two openings formed near its axial ends. A central rod extends axially in the cage, being held at its ends by insulators. The cage is maintained at zero potential, while the rod is brought to a high negative potential. The stem carries stars extending perpendicularly towards the cage.

The exhaust gases enter the cage through an opening and flow axially to exit through the opposite opening. The central rod provided with stars and brought to a high negative potential constitutes an emissive structure for electrically charging particles carried by the exhaust gases. A volume surrounding the stem and the stars is ionized. Electrons are emitted and move towards the outer cage, hit particles, and eventually associate with these particles to form negatively charged particles. The particles thus charged move radially under the effect of the electric field created between the central rod and the outer cage, and are trapped by the knit of metal son.

Such a device has the disadvantage that the particles are mainly negatively charged by electron emission and attachment of electrons emitted with the particles flowing in a gas flow. Given the low particle concentration of the exhaust gases, the probability of attachment of the particles with an emitted electron remains low. It is necessary to power the central electrode with a high energy level to cause significant electron emission. In addition, to be trapped, the charged particles must migrate radially to the metal wire knit. However, these charged particles move axially while being driven by the exhaust gases. It is therefore necessary to provide power supply of the central rod ensuring rapid radial migration under the effect of the electric field created between the electrodes.

It is also known from the patent US-A-5,557,923 , an electrostatic filtering device for soot particles conveyed by exhaust gases from an internal combustion engine, in which the gases are passed through an agglomerate of metal fibers surrounding an axially closed central passage on the opposite side to the admission of gases into the device.

In this document, the soot particles are first electrically charged by means of an annular electrode disposed perpendicularly to the path of the exhaust gas. The annular electrode is placed in a radial plane, outside the zone occupied by the cylindrical or frustoconical external electrode. The ionization of the gaseous medium takes place at the location of the annular electrode where almost all of the energy is injected.

This document thus proposes a sequential mechanism for loading particles upstream of their attachment.

This document does not provide for crown discharges between two electrodes so that the overall efficiency of the device remains low.

The present invention relates to a filtering device for improving the filtering of particles, and requiring a low energy input.

The invention also relates to a compact filtering device, simple, and can be obtained with a low manufacturing cost.

The invention also relates to a filtering device for eliminating filtered particles, without interrupting particle filtration, and can easily be integrated into an exhaust line of a motor vehicle.

As claimed, such an exhaust gas filtering device for motor vehicles, the exhaust gases being charged with particles, comprises filtering means comprising a central passage for the exhaust gas, an external electrode formed by an agglomerate of metal fibers surrounding the passage, a second electrode, and electrical supply means of the electrodes. The central passage is axially closed on the side opposite to the admission of the exhaust gases in the filtering means so that the exhaust gases pass, before being discharged, through the external electrode which is permeable to gases. and able to mechanically retain the particles.

According to the invention, the second electrode is an internal electrode of the wire type, arranged along the axis of the central passage, the power supply means being able to establish between the electrodes a radial electric field with formation of crown discharges between the external electrode and the wire internal electrode.

The filtering device simultaneously allows mechanical filtering and filtering of the electrostatic type. The particles passing through the passage are caused to pass through the external electrode formed by an agglomerate or an entanglement of metal fibers having interstices capable of trapping the particles, which allows mechanical filtering and promotes electrostatic filtering. Since the outer electrode is gas permeable, the charged particles must not migrate in a direction different from the flow direction of the gases, but are trapped by mechanical or electrostatic filtration as they pass through the outer electrode.

The external electrode is preferably connected to an electrical ground, the internal electrode being able to be raised to a positive potential or a negative potential.

Advantageously, the external electrode comprises conductive reinforcing and electrical continuity parts embedded in the agglomeration of metal fibers. The conductive parts are metallic and have a certain rigidity, making it possible to strengthen the external electrode. The conductive parts preferably extend radially and axially in the electrode while in contact with multiple metal fibers. The conductive parts thus provide electrical continuity between the metal fibers, and more generally throughout the electrode.

In one embodiment, the device comprises means for heating the external electrode. Advantageously, the heating means of the external electrode may be conductors embedded in the external electrode and able to heat the external electrode by Joule heat diffusion. The means for heating the external electrode may also comprise means for circulating a current in the external electrode to cause a heating directly Joule entangled fibers. It is also possible to provide heating of the matrix by induction, in particular by means of an induction generator.

In one embodiment, the internal electrode has protruding asperities directed substantially towards the external electrode. The protruding asperities make it possible locally to create high electrical potential gradients and thus important electric field values favoring the generation of crown discharges.

In one embodiment, the device includes a filtered exhaust gas collection manifold surrounding the outer electrode. Particle-laden exhaust gases pass through the entanglement of metal fibers forming the outer electrode. The recovery casing makes it possible to route the filtered exhaust gases to an exhaust outlet, or to other filtering means arranged downstream of the filtering device, on the exhaust line.

An exhaust gas filter assembly for a motor vehicle, the exhaust gas being charged with particles may comprise a plurality of filter cells comprising a passage for the exhaust gas, an external electrode surrounding the passage formed by an agglomerate or an entanglement of metal fibers permeable to gases, being able to mechanically filter the exhaust gases, an inner electrode of the wire type arranged generally along the axis of the passage, means for supplying the electrodes with electricity to establish an electric field with formation of crown discharges for electrostatic particle filtering.

Filtration cells may be associated in parallel or in series.

The invention also relates to a method for filtering exhaust gas for a motor vehicle, the exhaust gases being charged with particles, in which the gases are mechanically and electrostatically filtered at the same time by passing through a permeable external electrode. to the gas formed by entanglement of metal fibers and establishes an electrostatic field with corona discharge between an inner wire electrode and the outer electrode.

• la figure 1 est une vue en coupe axiale d'une cellule de filtrage selon l'invention ;
• la figure 2 est une vue d'un ensemble de filtrage comprenant des cellules de filtrage selon la figure 1 ; et
• la figure 3 est une vue d'une variante de l'ensemble de filtrage selon la figure 2.

The present invention and its advantages will be better understood on studying the detailed description of an embodiment taken as a nonlimiting example and illustrated by the accompanying drawings, in which:

• the figure 1 is an axial sectional view of a filter cell according to the invention;
• the figure 2 is a view of a filtering set comprising filtering cells according to the figure 1 ; and
• the figure 3 is a view of a variant of the filter set according to the figure 2 .

On the figure 1 , a filter cell, referenced 1 as a whole, comprises an intake pipe 2, a collector 4, and a filter unit 3 between the intake pipe 2 and the collector 4.

The filter unit 3 comprises an outer electrode 5 of generally cylindrical shape and formed by an agglomerate or an entanglement of metal fibers. The external electrode 5 comprises metal braces 6 embedded in the thickness of the external electrode 5. On the figure 1 four metal braces 6 can be seen. The metal braces 6 are distributed circumferentially and axially in the thickness of the external electrode 5 to increase its rigidity and to ensure electrical continuity of the electrode 5. The electrode 5 forms a axial passage 7 that it surrounds.

The external electrode 5 comprises radial front surfaces 8, 9. An end portion 10 of the external electrode 5 situated on the side of the end face 8 is inserted into the intake duct 2. The front face 8 comes into axial contact with a ring 11 for fixing the external electrode 5 on the inlet pipe 2. The fixing ring 11 comprises an outer surface 12 in contact with a bore 13 of the inlet pipe 2. The ring 11 is crossed by a conductor 14 radially out of the ring 11 and connected to the outer ring 5. The conductor 14 electrically connects the outer electrode 5 to a mass of zero potential.

The external electrode 5 is open on the side of its front surface 8, so that the central passage 7 communicates with the intake pipe 2. The inlet pipe 2 comprises an orifice 15 of the opposite side to the outer electrode 5. A disc 16 of insulating material bears axially against a radial surface 17 against the front surface 9 of the external electrode 5 opposite the intake pipe 2. The disc 16 axially closes the central passage 7 on the opposite side to the intake pipe 2.

The filter unit 3 also comprises a central electrode 18 in the form of a rod 19 coaxial with the external electrode 5 and one end of which is plugged into the insulating disk 16. The internal electrode 18 extends axially from its end 20 beyond the fixing ring 11 being bent to form a radial portion 21 radially out of the intake pipe 2 through an opening 22 formed in a wall of the intake pipe 2. An insulator 23 is disposed in the opening 22 to electrically isolate the wall of the intake pipe 2 from the internal electrode 18. The radial portion 21 is electrically connected to a voltage source 24.

The filter unit 3 comprises means for heating the external electrode 5 in the form of conductors 31, insulated or not, and arranged axially in the outer electrode 5 being embedded in the thickness of the latter. The conductors 31 are fed with a view to current flow in a manner not shown in the drawing for the sake of clarity.

The collector 4 comprises a cylindrical envelope 25 surrounding the portion of the external electrode 5 situated outside the intake duct 2. The cylindrical envelope 25 extends axially beyond the insulating disc 16. The envelope 25 comprises an inner diameter greater than the outer diameter of the outer electrode 5, so that there is an empty annular space 26 between the cylindrical shell 25 and the outer electrode 5. The collector 4 comprises a radial wall 27 connecting one end 25a of the casing 25 located on the side of the intake pipe 2 at the end of the intake pipe 2, sealingly. The collector 4 is closed on the opposite side to the intake pipe 2 by a radial wall 28 provided with a central opening 29 into which a discharge pipe 30 opens.

Particle-laden exhaust gases enter the intake pipe 2 through the inlet 15. Particle-laden exhaust gases enter the central passage 7 of the outer electrode 5 which communicates with the pipe. 2. The insulating disc 16 prevents the axial passage of the exhaust gas. The exhaust gas is deflected radially and passes through the outer electrode 5 which is gas permeable. The exhaust gas passes into the annular space 26, then flows into the manifold 4 towards the exhaust pipe 30. The exhaust pipe 30 can be connected to an exhaust outlet directly, or via different catalyst volumes, for example oxidation type reduction of nitrogen oxides.

By passing through the external electrode 5, the particulate loaded exhaust gases are mechanically filtered. Indeed, the outer electrode 5 is formed by an agglomerate or entanglement of metal son comprising interstices capable of trapping particles circulating in the gas.

In addition, the filter cell 1 also allows electrostatic filtering of the particle-laden gases. The external electrode 5 is maintained at a zero potential, the internal electrode 18 being brought to a positive or negative potential. The potential difference created between the external and internal electrodes 5 induces the presence of an electric field in the axial passage 7. If this electric field has a sufficient intensity, in particular in the very near vicinity of the internal electrode 18, it a partial or total ionization of the gases, or medium, occurs between the internal and external electrodes 18 and 5.

An ionized gaseous medium comprises free electrons and positive ions. Particles in the ionized medium combine with electrons or ions to form charged particles. Similarly, an electronic avalanche from an electrode to the other causes collisions between the electrons and the particles that combine to form negatively charged particles.

The radial circulation of the gases causes the charged particles to the external electrode 5. The charged particles passing through the external electrode 5 are trapped by electrostatic forces, Van Der Walls forces, capillary forces and electric forces.

In the case of an internal electrode 18 brought to a negative potential, the corona discharges have a spatial propagation limited to the near vicinity of the central electrode 18. The volume of ionized gas is low. The electrons are torn off the middle molecules are pushed back to the outer electrode 5. The predominant phenomenon is an electronic avalanche phenomenon. The particles present in the exhaust gases are mainly negatively charged by collision and combination with free electrons.

Given the low concentration of particles in the exhaust gas, the probability that an electron moving rapidly from the inner electrode 18 to the outer electrode 5 hits a particle and combines with the latter is low. In order to properly charge the particles for electrostatic filtering, it is necessary to provide sufficient electronic avalanche. The internal electrode 18 must therefore be brought to a high potential, for example between 50 and 150 kV. The increase in the potential at which the internal electrode 18 is carried also has the effect of ionizing a gaseous medium in a larger volume. However, the ionization of the entire volume between the inner 18 and outer 5 electrodes requires considerable energy.

In the case of an internal electrode 18 brought to a positive potential, the corona discharge is similar to an ionization wave propagating from the internal electrode 18 to the external electrode 5. The highly inhomogeneous electric field in the vicinity close to the central electrode 18, that is to say with strong gradients of electrical potential in the radial direction, causes, initially, the ionization of the gas in the close vicinity of said central electrode 18. The free electrons are attracted by the central electrode 18. It creates a local inhomogeneity of electric field the difference in mobility between ions and electrons between the ionized volume and the volume portion surrounding the ionized volume. This inhomogeneity of the electric field in the gas propagates as a wave from the internal electrode 18 to the external electrode 5. Each portion of ionized volume creates a local gradient of electric field causing the ionization of the surrounding medium surrounding the volume portion ionized. Behind the corona discharge wave, the gases are ionized, i.e. the gas is electrically neutral as a whole, but includes free electrons torn from molecules that have become positive ions. The ionization wave propagates to the external electrode 5. In this way, the entire volume between the internal electrode 18 and the external electrode 5 is crossed by the ionization wave.

When the entire volume between the inner electrode 18 and the outer electrode 5 is ionized, the path separating these two electrodes is considered as an ohmic conductor having a certain resistivity which increases with the recombination of electrons and ions present in the medium. This recombination phenomenon gradually leads to the extinction of the ionized channel. In the vicinity of the central electrode 18, once the medium has recovered its insulating nature, a new ionization wave starts, thus starting a new cycle. The ionization wave phenomenon repeating itself with a high frequency makes it possible to preserve an ionized medium almost permanently. The ionization phenomenon can be self-maintained or maintained by acting on the potential of the internal electrode 18.

The inner electrode 18 may be powered by a DC positive voltage source, or a positive pulse voltage source. In the case of a supply by a positive voltage source, ionization waves are created on a regular basis spatially and temporally but little controllable by the external circuit. The characteristics of the medium impose its repetition frequency. The supply of the internal electrode 18 by a source of positive voltage per pulse makes it possible to be able to apply larger amplitudes of higher voltage than continuous and to control the frequency of repetition of the phenomenon.

The particles entrained by the gases thus pass through an ionized medium. There is a greater probability, in this case, that a particle encounters an ionized molecule or a free electron and combines to form a positively or negatively charged particle. A positive internal electrode 18 allows an ionization of the volume between the internal electrode 18 and the external electrode 5 with a low energy input, being brought to a lower positive potential, for example between 1 kV and 50 kV.

The internal electrode 18 of the wire type makes it possible to create in the near vicinity of the internal electrode a highly inhomogeneous radial electric field, that is to say with a variation of the amplitude of the electric field that is very important, favorable to the initiation of a corona discharge. The smaller the radius of curvature of the inner electrode, the greater the electrical potential gradient and hence the larger and inhomogeneous the electric field. In other words, in the case of a wire-type electrode, the smaller the diameter of the electrode, the greater the electrical potential gradient and therefore the greater the electric field is and inhomogeneous. Advantageously, the wire internal electrode 18 may have an outer diameter of between 0.1 mm and 10 mm. A strongly inhomogeneous field allows the initiation of a corona discharge and an ionization wave with a low positive potential of the internal electrode 18, with a lower energy input.

Moreover, in a homogeneous field configuration, the difference between the potential allowing the formation of ionization waves and the potential leading to the formation of arcs between the electrodes is small. The use of a central electrode of the wired type makes it possible to obtain an inhomogeneous field presenting a gradient sufficient for the formation of ionization waves without risking the establishment of arcs between the inner electrode 18 and outer electrode 5. The inner electrode 18 brought to a positive potential allowing the formation of crown discharges with a low contribution of energy further decreases the risk of arcing.

In the case of an internal electrode 18 brought to a positive potential, the negatively charged particles are attracted to the internal electrode 18 where they can undergo a chemical treatment leading to their destruction.

Positively charged particles, which have been combined with positive ions, are driven radially outward by the gases by the electric forces. A positively charged particle migrates radially outward and can be trapped in the outer electrode electrostatically and / or mechanically. A charged particle will settle more easily in a porosity of the agglomerate. A charged particle, positively or negatively, will electrostatically interact with a metal fiber connected to the ground potential. This capacitive coupling effect between the particle and the fiber substantially improves the efficiency of capture of the particles by the fibers.

In the case of an internal electrode 18 brought to a negative potential, the particles are mainly negatively charged and migrate to the outside. Particles can also be positively charged and migrate inward or outward. The more the medium between the electrodes is ionized, the more charged particles will be found. However, the ionization of a large volume with a negatively charged internal electrode 18 requires a large amount of energy. In this case also, the particles migrating inward remain trapped, unlike a radial migration filter and axial flow of gases. Filtering is improved.

The combination of a mechanical and electrostatic filtering of the particles circulating in the exhaust gas allows a better filtering efficiency, so that the exhaust gases contain fewer particles at the outlet of the filtering cell 1.

In addition, the combination of mechanical filtering and electrostatic filtering makes it possible to use an external electrode having a high porosity coefficient, so that it offers less resistance to the passage of gases. In this way, the back pressure formed by the passage of gases through the filter is less important, and less troublesome for the internal combustion engine located upstream of the filter cell 1. The integration of the filter cell 1 in an exhaust line of a motor vehicle is facilitated.

The heating conductors 31 passing through the external electrode 5 allow a regeneration of the filter cell 1. The accumulation of particles trapped in the external electrode 5 leads in the long run to a decrease in the filtering efficiency. To regenerate the external electrode 5, an increase in temperature of the external electrode 5 is caused, causing combustion of the particles trapped in the external electrode 5. The heating elements are supplied with electricity, which leads to the increase of their temperature by Joule effect. The heating elements embedded in the external electrode 5 heat the latter.

In a variant, the external electrode 5 can be directly supplied with current to heat it by Joule energy dissipation. For this purpose, a current flow circuit will be provided through the external electrode 5.

In another variant, the temperature rise of the external electrode 5 can be obtained by means of an induction heating system.

In another variant, a nickel foam can be used in the external electrode 5. The nickel foam allows mechanical filtering of the particles passing through it while being carried by the exhaust gases, and also serves as a catalyst between the Molecular oxygen contained in the exhaust gas and hydrogen injected upstream of the filter cell 1. The strongly exothermic reaction between oxygen and hydrogen releases heat causing the combustion of the particles and the regeneration of the oxygen. external electrode 5.

It is also conceivable that these particles are removed by reaction with chemical molecules formed during the ionization of the medium. Such elimination will be facilitated with an internal electrode 18 brought to a positive potential and allowing better ionization of the medium between the electrodes.

The regeneration of a filter cell can be performed periodically, according to a predetermined duration of use. Also, probes for measuring particulate matter in the exhaust gas and / or the unfiltered exhaust gas may be provided to determine when regeneration is needed.

In an ionization cell variant, it is conceivable to provide an ionization section upstream of the cell. Such an ionization stage makes it possible to improve the ionization of the particles before their circulation between the internal and external electrodes 18 and 5. For example, it is possible to provide at an upstream end of the internal electrode 18 surrounded by the external electrode 5 a ball or sphere of small radius, possibly being formed by an agglomerate of metal fibers.

In another variant, an ionization section is formed by an alternation of wires and plates arranged transversely to the flow of the exhaust gas.

On the figure 2 , a filter assembly 34 comprises a distributor 35 comprising an inlet 36 intended to receive particulate loaded exhaust gases, and a plurality of outlets 37, here four in number. Each output 37 is connected to an input of a filter cell 38 of the type described above. The filter assembly 34 associates a plurality of filter cells 38 of small dimensions. A general collector 42 is connected to the outputs of the filtering cells 42.

The device comprises a control unit 39 comprising means for supplying electrical energy. The control unit is connected by electrical connections 40 to the internal electrodes of the ionization cells, and by electrical connections 41 to the external electrodes of the ionization cells 38.

By associating several filtering cells 38, the regeneration of the different filtering cells 38 can be performed at different times. During the regeneration of a filter cell 38, the passage of gases through the filtering cell prevents rapid heating of the filter cell and its rapid regeneration. The use of several filtering cells 38 makes it possible to isolate a filtering cell, the exhaust gases continuing to flow into the other filtering cells, in order to regenerate the isolated cell.

In a variant, filtering cells can be associated in series to increase filtering efficiency, by successive filtering of the gases.

Cell associations in parallel and in series can be envisaged.

On the figure 3 , on which references to elements similar to those of the figure 2 have been taken, a filtering assembly 34 comprises subset 43 of cells, here two in number, associated in series, the subsets 43 being themselves associated in parallel. A distributor 35 comprises an input 36 and two outputs 37 each connected to an input of a subassembly 43. Each subassembly 43 here comprises two filtering cells 38 arranged in series between an output of the distributor 35 and an input of a general collector 42.

The filtering device according to the invention makes it possible to obtain compact filtering cells of small dimensions, which facilitates the association of a plurality of filtering cells with the advantages which result therefrom.

Thanks to the invention, there is obtained a filtering device combining a mechanical and electrostatic filtering. The filtering device notably allows the operation of an electrostatic filter with a central electrode brought to a positive potential with formation of a highly inhomogeneous electric field more favorable to the formation of ionization waves. The internal electrode raised to a positive potential allows ionization of the entire volume between the inner electrode and the outer electrode. A internal wire electrode, having a small radius of curvature, allows the formation of a highly non-homogeneous electric field and the formation of ionization waves with low potentials, and with a low electrical energy input.

Claims (8)

1. Device for filtering the exhaust gases of motor vehicles, the exhaust gases being laden with particulates, comprising filtering means having a central passage (7) for the exhaust gases, an external electrode (5) formed by an agglomerate of metal fibres surrounding the passage, a second electrode and means for supplying electrical power to the electrodes (5, 18), the central passage (7) being closed axially on the opposite side to the intake of the exhaust gases into the filtering means, so that the exhaust gases before being discharged, pass through the external electrode (5) which is permeable to the gases and capable of mechanically retaining the particulates, characterized in that the second electrode is an internal electrode of the wire type, placed along the axis of the central passage, the electrical supply means being capable of establishing a radial electric field between the electrodes with the formation of corona discharges between the external electrode (5) and the wire internal electrode (18).
2. Device according to Claim 1, characterized in that the external electrode (5) is connected to an electrical earth, the internal electrode (18) being at a positive or negative potential.
3. Device according to either of Claims 1 and 2, characterized in that the external electrode (5) comprises conductive pieces (6) for reinforcement and for electrical continuity, said pieces being embedded in the agglomerate of metal fibres.
4. Device according to any one of the preceding claims, characterized in that the device includes means (31) for heating the external electrode (5).
5. Device according to any one of the preceding claims, characterized in that the internal electrode (18) has protruding asperities directed substantially towards the external electrode.
6. Device according to any one of the preceding claims, characterized in that it includes surrounding the external electrode (5), a manifold (4) for recovering the filtered exhaust gases.
7. Assembly for filtering particulate-laden exhaust gases of motor vehicles, characterized in that it comprises a plurality of filtering devices in accordance with one of the preceding claims.
8. Method of filtering particulate-laden exhaust gases of a motor vehicle, in which the gases are mechanically and electrostatically filtered at the same time by passing between the electrodes and through the gas-permeable electrode formed by an agglomerate of metal fibres, characterized in that an electrostatic field is established, forming a corona discharge between a wire internal electrode and the external electrode.

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