FR2864143A1 - Electrostatic filter for removing soot particles from internal combustion engine exhaust gases comprises a deflector sleeve surrounding a central electrode where it passes through the wall of a filtration cell - Google Patents

Abstract

Electrostatic filter for removing soot particles from internal combustion engine exhaust gases, comprising at least one filtration cell that is traversed by the exhaust gases and comprises a high-voltage central electrode (21) between cell wall elements. One end of the electrode passes through a wall element (31) and is surrounded by a deflector sleeve (32) whose inner surface is spaced apart from the electrode.

Description

Electrostatic filtration system for the exhaust gases of a

internal combustion engine

  The present invention relates to systems for the electrostatic filtration of soot particles contained in the exhaust gases of internal combustion engines, in particular diesel engines, for motor vehicles, of the type comprising at least one elementary filter cell traversed by the gases. exhaust.

  At present, the aim is to minimize the polluting emissions of internal combustion engines used in motor vehicles and, in particular, to reduce or eliminate the emission of soot particles by diesel engines.

  The Applicant has developed an electrostatic filtration system which was the subject of the filing of a French patent application 02 06 304 not yet published. This patent application discloses a filtration device comprising a passage for the exhaust gas, an outer electrode surrounding the passage, formed by an agglomerate or entanglement of metal fibers permeable to gas and capable of mechanically retaining particles. An internal electrode is disposed along the axis of the passage and electrode supply means for establishing an electric field with formation of corona discharge for electrostatic filtering of the particles. Thanks to the existence of this external electrode acting as a porous cathode by the entanglement of metal wires, in cooperation with the central anode, simultaneous mechanical filtering and electrostatic type filtering are obtained. Indeed, the particles passing through the filtration cell are caused to pass through the outer porous cathode formed by the agglomeration or the entanglement of metal fibers which constitutes a multitude of obstacles allowing the interception of the particles by a mechanical process intensified by electrostatic interactions between mass-bound fibers and electrically charged particles.

  To improve the efficiency of the filtration, it is important to obtain a corona discharge for which the ionization zone remains localized in the immediate vicinity of the electrode in the form of a luminescent sheath. For this purpose, the anode must have a very small radius of curvature with respect to the cathode and a circular section. A wired type anode of small diameter is therefore preferably used.

  Because of its small diameter, the thermal inertia of such a wire anode is almost zero so that it follows the temperature variations of the gas related to the transients of the engine. The length of the wire electrode is thus subject to rapid variations with respect to the rest of the assembly.

  In addition, it is necessary to provide insulating systems for the high voltage supply to the passage of the walls of the filtration cell. Finally, the anode must be properly maintained on the structural elements of the filtration cell and in particular on the end end wall thereof.

  The fixing of such a wire electrode under high voltage as well as its passage through the walls of the filter cell presents difficulties among which the appearance of surface discharges on the insulators which equip the passage of the electrode through the walls and the appearance of deposits of soot particles on these insulators. This results in the inadvertent appearance of electric arcs and an instability of the regulation of the high electrical voltage which is thus parasitized, the deposited particles forming bridges between the metal wall connected to ground and the input of the high voltage. .

  German Patent Application DE-A-41 14 935 (Nagao) describes various embodiments of fixing a wire electrode in an electrostatic filter by means of insulating sleeves allowing the electrode to pass through the wall of the cell. filtration. In this document, an insulating sleeve is generally used which may for example have ribs preventing the formation of electrical discharges on its surface. The insulating sleeve protrudes inside the filtration cell and the wire electrode simply passes through it. In some embodiments, an airflow is fed into the insulating sleeve to create an axial airflow along the electrode at its passage through the insulating sleeve.

  German Patent Application DE-A-39 27 743 (Bosch) discloses a device for cleaning insulating sleeves for fixing an electrode in an electrostatic filtering cell by means of a jet of pulsed air directed perpendicular to the insulating sleeve in order to remove soot deposits that may form on its surface.

  The object of the present invention is to improve the assembly of a wire electrode on the fixed elements of an electrostatic filtration cell as well as to pass through the wall of the cell.

  The present invention particularly relates to a mounting of the wire electrode which allows to avoid or significantly reduce the soot particle deposits on the insulating elements so as to avoid short circuits and the formation of electric arcs.

  The invention also relates to various improvements in the fixing of a wire electrode in order to keep it properly tensioned regardless of the variations in speed of the heat engine of which the electrostatic filtration device is equipped.

  The electrostatic filtration system according to the invention, for soot particles contained in the exhaust gas of an internal combustion engine, is of the type comprising at least one elementary filter cell traversed by the exhaust gas and comprising a high voltage central electrode held between at least two wall elements of the cell. At at least one of its ends, the central electrode passes through a wall element and is surrounded by a deflector sleeve portion whose inner surface is spaced from the electrode.

  Thanks to this arrangement, the deposits of soot particles on the insulating parts, at the location of the wall penetration, are very small since the flow of the exhaust gas practically no longer comes into contact with these insulating parts. An open chamber defining a dead zone is created inside the deflector sleeve and the few surface electrical discharges that occur cause an automatic cleaning of the surfaces of the elements near the point where the electrode passes through the wall element. .

  The deflector sleeve portion may be formed as a localized depression of the wall member, in the form of a projecting piece integral with the wall member or in any other suitable form.

  In a preferred embodiment, an air supply line opens into the deflector sleeve portion. A small flow of clean air into the chamber formed by the baffle improves the result obtained by further preventing the penetration of soot-laden exhaust gases into the area of the insulating portions.

  In an advantageous embodiment, the end of the central electrode passes through the wall element inside an insulating cylindrical insert whose outer surface is spaced from the deflector sleeve to create the aforementioned dead zone.

  In a preferred embodiment, the central electrode is of the wired type, with a diameter of, for example, between 0.1 mm and 1 mm.

  The wire electrode advantageously traverses the wall element inside an insulating cylindrical insert internally coated with a conductive tube, the electrode being spaced from the inner surface of the conductive tube and said tube protruding beyond the free end of the insert. The corona discharge which originates along the wire electrode is then stopped by the end of the conductive tube. The diameter of the conductive tube and its suitably smooth surface state are preferably chosen to prevent the formation of electrical discharges upon contact.

  The fixing of the ends of the electrodes must make it possible to keep them properly tensioned. Various montages are possible.

  In one embodiment, the end of the electrode is held in abutment with the insert on the side of the wall element opposite to the deflector sleeve, by means of a retaining member, for example a metal ball.

  In another embodiment, at least one of the ends of the wire electrode is held by means of an elastic member capable of catching the dimensional variations due to the thermal expansions.

  When the system comprises a set of several elementary filtration cells connected in parallel with the flow of the exhaust gas, it is advantageous for the ends of the set of wire electrodes of the different elementary cells to be held by a single frame.

  Such a frame preferably comprises several branches placed opposite the different elementary cells.

  The various elementary cells are advantageously fixed by their front ends on at least one mounting flange and preferably on two flanges, one downstream and the other upstream of the flow of the exhaust gas.

  The present invention will be better understood on studying the detailed description of some embodiments taken by way of non-limiting examples and illustrated by the appended drawings in which: FIG. 1 is an axial sectional view of a transmission cell; filtration according to the invention; - Figures 2 to 4 illustrate different variants of the passage of an electrical conductor through the wall of the filter cell; - Figures 5 to 6 illustrate different variants of fixing a wire electrode on the end flange of the filter cell; - Figure 7 schematically illustrates a filter assembly comprising a plurality of filter cells according to Figure 1; FIG. 8 is a perspective view showing an exemplary practical embodiment of a filtration assembly of the type illustrated in FIG. 7; FIG. 9 is a front view of an alternative embodiment of a filtration assembly; FIG. 10 is a partial view from above showing an embodiment of the fixation of the wire electrodes; - Figure 11 is a sectional view along XI / XI of Figure 10; - Figure 12 is a view similar to Figure 11 illustrating another embodiment; and - Figure 13 is a sectional view along XIII / XIII of Figure 12.

  In FIG. 1, a filter cell referenced 1 as a whole comprises an intake duct 2, a collector 4 and a filtration unit 3 between the intake duct 2 and the collector 4.

  The filtration unit 3 comprises an outer electrode 5 of generally cylindrical shape and formed by an agglomerate or an entanglement of fibers or metal son. The electrode 5 forms an axial passage 7 which it surrounds.

  The outer electrode 5 comprises radial front surfaces 8, 9. The end face 8 comes into axial contact with a ring 11 for fixing the external electrode 5 to the inlet pipe 2. The fixing ring 11 comprises a surface external 12 in contact with a bore 13 of the intake pipe 2.

  The outer electrode 5 is open on the side of its front surface 8 so that the central passage 7 communicates with the inlet pipe 2. The inlet pipe 2 comprises an inlet 15 on the opposite side to the electrode external 5. A disk 16 made of an electrically conductive 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 passage central 7 opposite the side of the intake pipe 2. The conductive disk 16 and the external electrode 5 are connected to a ground of zero potential by a conductor 14.

  The filtration unit 3 also comprises a central electrode 18 made in the example illustrated in the form of a rod 19 coaxial with the external electrode 5 and one end 20 of which is fixed in an insulating insert 16a mounted in the central portion. The central electrode 18 extends axially from its end 20 beyond the fixing ring 11 by being bent to form a radial portion 21 radially out of the inlet pipe 2. An insulator 23 electrically isolates the wall of the intake pipe 2 of the portion of the electrode 18. The radial portion 21 is electrically connected to a high voltage power supply 24.

  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 disc 16. The envelope 25 has a 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 and the outer electrode 5. The collector 4 comprises a radial wall 27 ensuring the sealing of the envelope 25 located on the side of the inlet pipe 2. The manifold 4 is closed on the opposite side to the intake pipe 2 by a radial wall 28 provided with a central opening into which a discharge pipe 30 opens.

  The exhaust gas charged with soot particles enters the intake pipe 2 through the inlet orifice 15 and then enters the central passage 7 of the external electrode 5 which communicates with the intake pipe 2. The 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 gases pass into the annular space 26 and then flow into the manifold 4 towards the exhaust pipe 30. The exhaust pipe 30 can be connected to an exhaust outlet directly or via different volumes. catalyst for example oxidation / 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 which form multiple obstacles capable of intercepting the particles. This mechanical filtering process is intensified by electrostatic filtering resulting from the interactions between the entanglement of grounded metal wires and electrically charged particles.

  Moreover, the filtration cell 1 also has a direct electrostatic filtering effect of the particle-laden gases. The external electrode 5 is in fact maintained at a zero potential, the central electrode 18 being brought to a positive or negative potential.

  The potential difference created between the external and central electrodes 18 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 central electrode 18, it partial or complete ionization of the gases or the medium between the central and outer electrodes 18 and 5 occurs. An ionized gaseous medium comprises free electrons, positive and negative ions. Particles in the ionized medium combine with electrons or ions to form charged particles. Likewise, an electronic avalanche from one 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 interception on the entanglement of metal wires and by phenomena of electrostatic interaction due to the electric charges carried. by said particles. Van der Walls forces and capillary forces keep the particles attached to the wires even after they give up their electrical charge.

  Various embodiments of the insulator 23 are illustrated in FIGS. 2 to 4. In the first embodiment illustrated in FIG. 2, the radial portion 21 of the central electrode 18 passes through the wall 31 of the intake duct. 2 being surrounded by a deflector sleeve portion 32 which is constituted by a localized depression of the wall 31. The deflector sleeve 32 is closed at its end 33 and allows the attachment and maintenance of a cylindrical insulating insert 34, made by The electrode portion 21, which is here in the form of a cylindrical rod, is inserted inside the insulating insert 34 and thus passes In an electrically insulated manner, the wall 31. The cylindrical insert 34 projects inside the intake duct 2 beyond the wall 31.

  The deflector sleeve 32 defines around the insulating insert 34 a chamber 36 open towards the intake pipe 2. The exhaust gases flowing in the intake pipe 2 and which are loaded with soot particles practically do not penetrate into this chamber 36, which constitutes a dead zone, so that the soot particles are practically not deposited on the outer surface of the insulating insert 34, which is in a way deported with respect to the flow of the exhaust gas. The few surface discharges that can still appear keep the surface of the insulating insert perfectly clean.

  Note that it is preferable that the metal walls and in particular the wall 31 are polished to provide a minimum probability of development of parasitic discharges by peak effects that would be due to surface roughness.

  FIG. 3 illustrates an alternative embodiment in which the same elements bear the same references. A dead zone 36 is also produced by means of a deflector sleeve 32 which is this time in the form of a projecting piece integral with the wall element 31 and directed towards the inside of the intake duct 2 .

  In Figure 4, there is illustrated an alternative embodiment from the embodiment of Figure 3, the same elements bearing the same references. In this variant, a pressurized air inlet duct 37 passes through the wall 31 and the deflector sleeve 32 so as to open into the dead zone 36. The injection of a small clean air flow into the chamber 36 formed by the deflector sleeve 32 further avoids any penetration of the flow of exhaust gas loaded with soot particles into the dead zone at the location of the passage of the electrode 21 through the wall 31.

  The central electrode 19 is preferably made in the form of a conducting rod of very small diameter, for example between 0.1 mm and 1 mm. Such a wire electrode 19 is illustrated in particular in FIG. 5 which shows the attachment of the downstream end of the electrode 19 to the metal wall of the disk 16 already illustrated in FIG. 1. In this embodiment, provision is made for , as in the embodiments of Figures 2 to 4, a deflector sleeve 32 which defines a dead zone 36 around the insulating insert 34 made for example of ceramic. Fixing the end of the wire electrode 19 which passes through the insulating insert 34 is made by means of a metal ball 38 which bears on the end of the insulating insert 34, the ball 38 being a diameter greater than the internal diameter of the insulating insert 34.

  The assembly can be further improved by placing an electrically conductive tube 39 as shown in FIG. 6 inside the insulating insert 34. The tube 39 which covers the entire inner surface of the insulating insert 34 protrudes beyond of the free end of the insert 34. In this way the electric discharges are stopped by the conductive tube 39 on which no crown discharge can originate. The fixing of the wire electrode 19 as well as the structure of the deflector sleeve 32 is the same as in the embodiment of FIG. 5.

  Figure 7 illustrates a filtration assembly 40 which includes a distributor 41 connected to a common inlet 42 which carries the exhaust gas loaded with soot particles. The distributor 41 has a plurality of outputs 43, here four in number. Each outlet 43 is connected to the inlet of a filtration cell 1 of the type illustrated in FIG. 1. A general collector 44 is connected to the outlets of the different filtration cells 1. The assembly also comprises a control unit, not illustrated. in the figure comprising power supply means for supplying the different filtering cells 1.

  This filtration assembly illustrated schematically in FIG. 7 can be made for example by grouping several elementary filtration cells into a filtration matrix 45 as illustrated in FIG. 8, the various cells, here ten in number, being arranged in two parallel rows of five cells. In this embodiment, the different filtering cells are in the form of cylindrical tubes 46 open at both ends and held at each of their ends by a flange 47, 48. The two flanges 47, 48 are suitably perforated at the right. As a variant, it is possible to use only one flange 47 or 48, the cylindrical cells being held at their other end by guiding devices not shown in the figure. The ends of the various wire electrodes 19 are maintained, in the example illustrated in FIG. 8, by a single rigid frame 49. The frame 49 is made of an electrically conductive material and is connected to the high voltage so as to play the common feeding role for all the electrodes 19 of the same matrix 45. The frame 49 can be mounted forward of the flange 48 located upstream of the flow, with its four ends 56.

  The frame 49 has stiffening struts 51 and branches 52 at the end of which are fixed the respective electrodes 19.

  FIG. 9 illustrates another embodiment in which a matrix 53 of elementary filtration cells, also ten in number, are arranged in three rows instead of the two rows of the matrix 45 of FIG. embodiment, the frame 54 is fixed only by three attachment points 55. The frame 54 has a general rectangle shape and further comprises a central upright 54a so as to allow the attachment of the ends of each of the electrodes 19 of the different cells filtering the matrix 53. In the figure will be seen the outer frame 56 of the filtration system inside which is mounted the matrix 53 constituted by the different filter cells.

  FIGS. 10 and 11 illustrate an embodiment of the fixing of the ends of the electrodes 19 in the case of the variant of FIG. 8. FIGS. 10 and 11 show the same parts as in FIG. references. In order to compensate for the thermal expansion of the electrodes 19, the branches 52 secured to the frame 49 are made in the form of resilient or flexible blades and are capable of bending at short distances without causing significant decentering of the electrode. Fixing the end of the electrode 19 is for example as illustrated in Figure 5 by means of a metal ball 38.

  In the variant of FIGS. 12 and 13, the electrodes 19 are fixed on a rigid branch 53 of the frame 49. The thermal expansion of the electrodes 19 is obtained by mounting the balls 38 on individual springs 54.

Claims (11)

  1. System for the electrostatic filtration of soot particles contained in the exhaust gas of an internal combustion engine, of the type comprising at least one elementary filtration cell (1) traversed by the exhaust gas and comprising a central electrode (19) high voltage held between at least two wall elements of the cell, characterized in that at least one of its ends, the central electrode passes through a wall element surrounded by a portion of deflector sleeve (32) whose inner surface is spaced from the electrode (19, 21).   2. System according to claim 1 characterized in that the deflector sleeve portion is formed in the form of a localized depression of the wall element.   3. System according to claim 1 characterized in that the deflector sleeve portion is formed in the form of a projecting piece integral with the wall element.   4. System according to any one of the preceding claims characterized in that an air supply line (37) opens into the deflector sleeve portion (32).   5. System according to any one of the preceding claims, characterized in that the end of the electrode passes through the wall element inside a cylindrical insulating insert (34) whose outer surface is spaced from the sleeve. deflector (32).   6. System according to any one of the preceding claims, characterized in that the end of the central electrode (19) is of the wire type and passes through the wall element inside a cylindrical insulating insert (34). ) coated internally with a conductive tube (39), the electrode being spaced from the inner surface of the conductive tube and said tube protruding beyond the free end of the insert.   7. System according to claim 6 characterized in that the end of the electrode is held in abutment on the insert on the side of the wall element opposite to the deflector sleeve, by means of a retaining member, by example a metal ball (38).   8. System according to any one of the preceding claims, characterized in that at least one of the ends of the electrode is held by means of an elastic member (52, 54) capable of catching the dimensional variations due to thermal expansion.   9. System according to any one of the preceding claims comprising a plurality of elementary filtration cells connected in parallel to the flow of the exhaust gas, characterized in that the ends of the set of electrodes of the different elementary cells are maintained by a single frame (49).   10. System according to claim 9 characterized in that the frame comprises branches (52, 53) placed opposite different elementary cells.   11. System according to one of claims 9 or 10 characterized in that the different elementary cells are fixed by their front ends on at least one mounting flange (47, 48).