Patent Publication Number: US-2005115410-A1

Title: Method and device for avoiding the deposition of soot on the insulating components of an electrostatic filter

Description:
In an electrostatic filter having insulating parts, the invention relates to a method and a system for preventing the deposit of soot particles on the insulating parts.  
      In electrostatic filtration systems comprising a center electrode and a peripheral electrode, a film of soot is observed to form on the surface of the insulators which hold the central and peripheral electrodes. Under the effect of the high voltage applied between these electrodes a conductive channel forms in the soot deposited on the walls. The applied voltage collapses, and the electrostatic filtration effectiveness is diminished, and even lost.  
      The invention has the purpose of preventing the deposit of soot on the insulating parts of the electrostatic filtration system by an effective means of low power consumption.  
     PRIOR ART  
      Various companies concerning themselves with problems of electrostatic filtration applied to the automobile have attempted to offer a solution to the stated problem. Among the different solutions already described, the following are mentioned: 
          The solution taught by U.S. Pat. No. 5,263,317. It consists in projecting a flow of air to release soot from the insulating parts.     The solution taught by U.S. Pat. No. 5,006,134. It consists in using resistances placed in contact with the insulating parts to burn off the soot.        

      The solutions proposed have the major disadvantages of being difficult to embody, not very effective and/or of great power consumption.  
     Solution  
     Process  
      The invention relates to a method, in an electrostatic filter having insulating parts, for preventing the deposit of soot particles on the insulating parts. The method of the invention includes the step of creating a repellent electrical field in the vicinity of the insulating parts. This prevents the formation of a conductive path on the surface of the insulating parts.  
      Preferably, according to the invention the electrostatic filter comprises at least one center electrode and one peripheral electrode held by the insulating parts. In the case of this variant embodiment, to create the repellent electrical field: 
          at least one repelling electrode is placed in or on at least one of the insulating parts,     an adequate voltage is applied to the repelling electrode.        

      The result of the combination of technical characteristics is that the potential difference between the repelling electrode and the center electrode, as well as the potential difference between the repelling electrode and the peripheral electrode creates an electrical field repelling the soot particles. This prevents the formation of a film of soot on the surface of the insulating parts. The formation of a conductive path on the surface of the insulating parts, in and/or on the film of soot, between the center electrode and the peripheral electrode.  
      Preferably, according to the invention the voltage applied to the repelling electrode is between −10 KV and −25 KV.  
      Preferably, according to the invention the potential difference between the repelling electrode and the center electrode is between −35 KV and 0 KV.  
      Preferably, according to the invention, the process is such that the potential difference between the repelling electrode and the peripheral electrode is between −10 KV and −25 KV.  
     System  
      The invention likewise relates to a system for preventing, in an electrostatic filter comprising insulating parts, the deposit of soot particles on the insulating parts. The system comprises means for creating a repellent electrical field in the vicinity of the insulating parts. The formation of a conductive path on the surface of the insulating parts is thus prevented.  
      Preferably, according to the invention the electrostatic filter comprises at least one center electrode and one peripheral electrode held by the insulating parts. In the case of this variant embodiment, the repulsion means for the creation of a repellent electrical field near insulating parts include: 
          a repelling electrode situated in or on the insulating part,     a generator of adequate voltage connected to the repelling electrode.        

      The potential difference between the repelling electrode and the center electrode, as well as the potential difference between the repelling electrode and the peripheral electrode creates an electrical field repelling the particles of soot. The formation of a soot film is thus prevented and, correlatively, the formation of a conductive path on the surface of the insulating parts between the center electrode and the peripheral electrode.  
      Preferably, the repelling electrode is mounted on the center electrode and is of a bell-like shape covering the insulating parts.  
      Preferably, according to the invention the voltage supplied by the voltage generator and applied to the repelling electrode is between −10 KV and −25 KV.  
      Preferably, according to the invention the voltage supplied by the voltage generator is such that the potential difference between the repelling electrode and the center electrode is between −35 KV and 0 KV.  
      Preferably, according to the invention the voltage supplied by the voltage generator is such that the potential difference between the repelling electrode and the peripheral electrode is between −10 KV and −25 KV. 
    
    
     DETAILED DESCRIPTION  
      Other features and advantages of the invention will appear in the reading of the description of variant embodiments of the invention given by way of suggestive and non-restrictive example, and from:  
       FIG. 1  which represents a schematic, partially cut-away view, of a variant embodiment of the invention,  
       FIG. 2   a  and  FIG. 2   b , representing respectively a left-hand view and a view partially in section of the variant shown in  FIG. 1 .  
       FIG. 3  representing a schematic view in section of another variant embodiment of the invention. 
    
    
      The electrostatic filter  1  has at least one center electrode  6  and a peripheral electrode  7  (forming the outer wall of the electrostatic filter  1 ). The center electrode  6  and the peripheral electrode  7  are held by insulating parts  2 . The exhaust gases of the motor from which the soot is to be eliminated enter through inlet pipes  10  and leave through outlet pipes  11 . The electrostatic filter  1  likewise contains repulsion means to create a repulsion electrical field  4  in the vicinity of the insulating parts  2 . These repulsion means comprise a repelling electrode  8  and  12  situated in the insulating part, and a voltage generator  9  connected to the repelling electrodes  8  and  12 . In a variant embodiment the metal repelling electrode  8  and  12  is covered with an insulating material, particularly an enamel coating.  
      The potential difference between the repelling electrode  8  and the central electrode  6 , as well as the potential difference between the repelling electrode  8  and the peripheral electrode  7  creates a repulsion electrical field driving back the soot particles. Thus the deposit of soot particles  3  on the surface of the insulating parts  2  is prevented. This prevents the formation of a conductive pathway  5  on the surface of the insulating parts  2  in or on the soot particles  3  between the center electrode  6  and the peripheral electrode  7 .  
      The voltages delivered by the voltage generator  9  and applied to the repelling electrode  8  are appropriate for this purpose. The repelling electrode  8  can be: 
          at the same potential as the center electrode  6 ,     at a potential different from that of the center electrode  6 ; in this case the potential of the repelling electrode  8  is of the same sign as the potential of the center electrode  6 ,     at a potential lower than the potential of the center electrode  6 .        

      Preferably, the voltage supplied by the voltage generator  9  and applied to the repelling electrode  8  is between −10 KV and −25 KV.  
      Preferably, the voltage supplied by the voltage generator  9  is such that the potential difference between the repelling electrode  8  and the center electrode  6  is between −35 KV and 0 KV.  
      Preferably, the voltage supplied by the voltage generator  9  is such that the potential difference between the repelling electrode  8  and the peripheral electrode is between −10 KV and −25 KV.  
      A description will now be given of  FIG. 3  representing a schematic view in section of another variant embodiment of the invention. Most of the elements described will be seen in this figure by referring to  FIGS. 1, 2   a  and  2   b . They bear the same reference numbers.  
      In the case of this other variant embodiment, the repelling electrode  8  is mounted on the center electrode  6 , close to the cylindrical insulating portion  2  bearing the center electrode  6 . The repelling electrode  8  is of a bell-like shape extending over the insulating parts  2 . More precisely, the repelling electrode of bell-like shape is closed by a disk  8   a  affixed to the center electrode  6 . The plane of disk  8   a  is perpendicular to the center electrode  6 . The disk  8   a  is prolonged by a cylindrical tube  8   b  surrounding the insulating part  2   a  bearing the center electrode  6  and extending to the vicinity of the annular portion  2   b  of the insulating portion bearing the peripheral electrode  7 . An annular disk  8   c  is affixed to the tube  8   b , parallel to the annular portion  2   b  of the insulating part. A cylindrical skirt  8   d  is affixed to the annular disk  8   c . The cylindrical skirt  8   d  is concentric with the peripheral electrode  7  and is situated near the latter. The elements  8   a ,  8   b ,  8   c  and  8   d  form the bell  8 .