Patent Publication Number: US-7721536-B2

Title: Particulate filter having expansible capture structure for particulate removal

Description:
FIELD OF THE INVENTION 
   This invention relates generally to particulate filters, especially those that are used to trap particulate matter in engine exhaust, and to systems and methods for removing trapped particulates. 
   BACKGROUND OF THE INVENTION 
   A known system for treating exhaust gas passing through an exhaust system of a diesel engine comprises a diesel oxidation catalyst (DOC) associated with a diesel particulate filter (DPF). The combination of these two exhaust gas treatment devices promotes chemical reactions in exhaust gas and traps diesel particulate matter (DPM) as exhaust flows through the exhaust system from the engine, thereby preventing significant amounts of pollutants such as hydrocarbons, carbon monoxide, soot, SOF, and ash, from entering the atmosphere. 
   While an engine is running, the existence of certain conditions enables regeneration of a DPF to be initiated. Various techniques are available for developing temperatures sufficiently high to initiate regeneration and thereafter control on-going regeneration. Regeneration is essentially a chemical process that cleans a DPF by burning off trapped DPM. For any of various reasons, not all trapped DPM may be burned off by regeneration. Moreover, the burning of trapped DPM may contribute to the build-up of ash, a non-combustible particulate. 
   Consequently, it may be either necessary or desirable to occasionally use a physical or mechanical process, rather than a chemical process, to remove particulate matter, such as DPM and/or ash, from a DPF. The use of compressed air has been proposed as one way to remove the particulate matter. 
   Compressed air is an appropriate medium because it is readily available in service facilities and shops and it is environmentally friendly. Cleaning a DPF by compressed air has involved certain manual operations such as removing the actual filter module from a casing and manually manipulating a compressed air nozzle across a face of the module. Dislodged matter is ejected from an opposite face and collected in some type of collector for subsequent disposal. 
   When a DPF has been used to an extent where regeneration and mechanical cleaning are unable to sufficiently clean it, it must be replaced. 
   In light of this background, it is believed that improvements in the mechanical cleaning of diesel particulate filters would enjoy commercial acceptance. For example, a cleaning device and method that would minimize the amount of labor required would be beneficial. Likewise, a device and method that could clean a diesel particulate filter more thoroughly and that could extend the useful life of the filter would be desirable. The ability to satisfactorily clean a diesel particulate filter without having to remove the actual filter module from its casing also would have obvious advantages. 
   An improvement that would allow an engine to keep running with the exhaust treatment system remaining effective to trap DPM during on-going mechanical cleaning could also be considered desirable. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a system and method for mechanically removing particulate matter that has been trapped by a particulate filter through which engine exhaust has passed before entering the surrounding atmosphere. 
   One general aspect of the invention relates to a combustion engine that when running generates exhaust containing particulate matter and that comprises an exhaust system containing a particulate filter that traps particulate matter in exhaust passing through the exhaust system. The particulate filter comprising a particulate trapping medium that when the engine is running has a relatively lesser porosity for trapping particulate matter in exhaust, and that is operable to have a relatively greater porosity for facilitating mechanical removal of trapped particulate matter. 
   A further aspect relates to a method for trapping particulate matter entrained in exhaust generated by a combustion engine and for removing trapped particulate matter. The method comprises, when the engine is running, operating a particulate trapping medium to a condition of relatively lesser porosity to trap particulate matter in exhaust flowing through the medium, and when the medium needs to be mechanically cleaned, operating the particulate trapping medium to a condition of relatively greater porosity to allow trapped particulate matter to be removed mechanically from the medium. Cleaning can be performed with the engine off, or in accordance with a still further aspect of the invention while the engine continues running. 
   According to that still further aspect, a combustion engine comprises an exhaust system containing particulate filters in parallel flow relationship. Each particulate filter comprises a casing containing a medium for trapping particulate matter in engine exhaust passing through the exhaust system. A valve is used for shutting off exhaust to one of the particulate filters while the engine is running. A particulate collector is communicated to the casing of the one particulate filter. A compressed air source delivers compressed air into the casing of the one particulate filter and through its medium to the particulate collector to entrain trapped particulates in the air flow and deposit the entrained particulates in the collector. 
   The foregoing, along with further aspects, features, and advantages of the invention, will be seen in the following disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. The disclosure includes drawings, briefly described as follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view illustrating a particulate filter embodying principles of the present invention. 
       FIG. 2  is an enlarged fragmentary perspective view of a particulate trapping medium inside the filter showing a condition of relatively lesser porosity. 
       FIG. 3  is an enlarged fragmentary perspective view of a particulate trapping medium showing a condition of relatively greater porosity and removal of trapped particulate matter. 
       FIG. 4  is a strategy diagram showing steps for operating the filter to the respective conditions. 
       FIG. 5  is a perspective pictorial of a further embodiment in various degrees of detail. 
       FIGS. 6 ,  7 , and  8  disclose an embodiment of exhaust filter system. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a particulate filter  10  suitable for placement in an engine exhaust system for trapping diesel particulate matter in exhaust passing through the filter. Filter  10  comprises a casing  12  having an exhaust inlet  14  through which exhaust enters and an exhaust outlet  16  through which exhaust exits. A particulate trapping medium  18  is disposed within the interior of casing  12  between inlet  14  and outlet  16 . As exhaust passes through medium  18 , the medium traps diesel particulate matter (DPM) when in a relatively less porous condition shown in  FIG. 2  where the DPM is marked by the reference numeral  20 . The relatively lesser porosity condition allows exhaust gas, and some DPM having sizes smaller than the porosity of the medium, to pass through to outlet  16  and then into the surrounding atmosphere. 
   Medium  18  is constructed to be expansible and contractible so as to vary its porosity. A condition of relatively greater porosity is shown in  FIG. 3 . The material forming medium  18  provides interstices whose sizes and shapes change depending on the extent to which the medium is expanded or contracted. When the medium is maximally contracted, the interstices are relatively smaller and create more tortuous paths for the exhaust gas as it flows through the medium, thereby trapping particulate matter. When the medium is maximally contracted as shown by  FIGS. 1 and 2 , it has an overall length less than that of casing  12  thereby leaving an interior void  22  inside casing  12  between the medium and exhaust outlet  16  into which the medium can expand. 
   Medium  18  is constructed to selectively expand and contract as a function of a magnetic field applied to it. An electromagnetic device  24  is disposed in association with medium  18  to provide the magnetic field. Device  24  has a bi-directional capability for selectively creating opposite magnetic fields, one of which is effective to contract medium  18  to relatively lesser porosity and the other of which is effective to expand the medium to relatively greater porosity. If the medium possesses elasticity, then device  24  need have only uni-directional capability. 
   One example of a trapping medium comprises a multitude of strands or filaments arranged in random and/or ordered pattern. The material of those elements may be chosen to be magnetically responsive to the applied magnetic field. If the material is not so chosen, then the elements may be attached to one or more magnetically responsive pieces that are arranged to move within casing  12  in response to the applied magnetic field and either expand or contact the medium in the process by virtue of suitable attachment to the elements. For instance, application of a certain magnetic field may cause a magnetically responsive piece to pull on ends of elements that are attached to it while opposite ends remain anchored. In the absence of any resiliency, an opposite field may be used to restore the elements to their prior condition. Because magnetic properties of certain materials are temperature-dependent, it may not be possible to use a magnetic field to change the porosity of medium  18  when the particulate filter is hot. 
   A control system  26  controls the application of electric current to device  24  selectively to cause medium  18  to selectively expand and contract. A strategy  28  for control of the current is shown in  FIG. 4 . 
   In a motor vehicle, an engine  30  whose exhaust system contains filter  10  consumes fuel supplied from a tank  32 . Exhaust  34  resulting from combustion of fuel in the engine passes through the exhaust system where DPM is captured by a particulate capture device, namely filter  10 . The filter is in a relatively lesser porosity condition when the engine runs. A particulate sensor  36  is disposed to sense the extent to which the filter is loaded with DPM. This can be done by measuring exhaust back-pressure on the running engine in relation to engine speed. 
   When the loading is indicated sufficiently great that mechanical removal of DPM is needed, the engine is shut off, and it and the exhaust system are allowed to cool. Electromagnetic device  24  can then be operated to expand medium  18  to a greater porosity condition. Compressed air from a source of compressed air  38  is introduced into casing  12  upstream of medium  18  and flowed to a collector  40  that is communicated to the downstream side of the medium, such as via a separate outlet  42 . Trapped DPM entrains with the air flow and is carried into the collector. 
   The remainder of  FIG. 4  shows how the magnetic field is adjusted as DPM removal proceeds. 
     FIG. 5  shows another embodiment of medium  18  that comprises a random pattern of elements  44 . Like the prior embodiment, the one shown in  FIG. 5  is selectively operable to conditions of relatively greater and relatively lesser porosity. Elements  44  are metal filaments containing various kinks similar to what is commonly known as steel wool although the material of the elements is one that is suited for high temperatures. Rather than using a magnetic field to change the porosity of the medium, the embodiment of  FIG. 5  uses an electric field. By suitably connecting elements  44  to respective electrodes (not shown), and applying a potential difference across the electrodes, the kinking can be reduced, making the elements relatively straighter and increasing the porosity of the medium in the process. 
   In  FIG. 5 , the reference  5 A shows enlarged detail of a portion of the medium while the reference  5 B 1  is rescaled even larger to show a condition of relatively lesser porosity. The reference  5 B 2  is on the same scale as that of  5 B 2 , but shows a condition of relatively greater porosity. 
     FIGS. 6 ,  7 , and  8  disclose an embodiment of exhaust filter system  50  that utilizes any medium  18  that is selectively operable to relatively greater and relatively lesser porosities. System  50  comprises two chambers  52 ,  54  that are arranged in parallel flow configuration. Engine exhaust enters through an inlet pipe  55  with flow in the direction of arrows  56 . A valve  58  is selectively operable to direct the entering flow to chambers  52 ,  54  depending on whether system  50  is to assume a principal DPM capture mode or an auxiliary DPM capture mode that allows chamber  52  to be cleaned. 
   In the principal capture mode, valve  58  directs exhaust to flow via a pipe  60  into chamber  52  where it is filtered by the medium  18  that is inside chamber  52 . In the auxiliary capture mode, valve  58  directs the flow into chamber  54  through a pipe  62  instead of into chamber  52 . In the principal capture mode, DPM is trapped in medium  18  inside chamber  52 , with treated exhaust exiting through a pipe  64  leading to a tailpipe  68 . 
   A collector container  70  is associated with chamber  52  by having an entrance communicated to the interior of chamber  52  via a pipe  72 . Another pipe  74  is communicated to the interior of chamber  52  upstream of the location of pipe  72 . Container  70  is used in the auxiliary capture mode to allow system  50  to continue trapping DPM while chamber  52  is being cleaned. 
   System  50  may be placed under the control of a control system such as control system  26 . When the DPM loading of chamber  52  increases to a level at which cleaning is called for while the engine is running, valve  58  is operated to divert exhaust gas to chamber  54  so that no exhaust flows through chamber  52 . The medium in chamber  54  now traps DPM. 
   When exhaust was flowing through only chamber  52 , a valve system (not shown) prevented exhaust from flowing through pipes  72 ,  74 . With valve  58  now diverting the flow through chamber  54 , the valve system associated with pipes  72 ,  74  can be operated to allow air to flow into chamber  52  through pipe  74 , to pass through medium  18  and exit the chamber through pipe  72 . 
   Air from a compressed air source (not shown) is communicated to pipe  74 . Container  70  is vented to atmosphere but has a filter medium covering the vent opening. When compressed air from the source is allowed to flow through chamber  52 , trapped DPM entrains with the air flow and is conveyed through pipe  72  to the interior of container  70 . The filter medium in container  70  allows the air to vent through the vent opening, but contains the DPM within the container interior. The cleaning process continues until stopped. Thereafter the valve system associated with the cleaning process can be operated to block flow through pipes  70 ,  72 , and valve  58  can be operated to restore engine exhaust flow through chamber  52 . 
   Should container  70  need to be emptied, suitable provision for emptying is made in its construction, and such emptying is preferably made when the system is cold and the engine is not running. 
   An advantage of system  50  is that it allows the principal DPF, i.e. chamber  52 , to be cleaned while the engine continues running. To the extent that chamber  54  might need to be cleaned, a similar system could be associated with it. 
   While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention are applicable to all embodiments that fall within the scope of the following claims.