Abstract:
A method and apparatus for initiating regeneration in a particulate trap including the steps of locating microwave-absorbing material in the particulate trap in areas that particulates build up, generating microwaves, absorbing microwaves with the microwave-absorbing material, and controlling the microwaves to initiate a burn-off of particulates.

Description:
[0001]    This application claims priority from U.S. Provisional Application No. 60/256,075 filed Dec. 15, 2000. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to a diesel particulate trap. More specifically, the present invention relates to a method and apparatus for regenerating a diesel particulate trap using microwave radiation.  
         BACKGROUND OF THE INVENTION  
         [0003]    Increased regulation has reduced the allowable levels of particulates generated by diesel engines. The particulates can generally be characterized as a soot that is captured and reduced by particulate filters or traps. Present particulate filters or traps contain a separation medium with tiny pores that capture particles. As trapped material accumulates in the particulate trap, resistance to flow in the particulate trap increases, generating back pressure. The particulate trap must then be regenerated to burn off the particulates/soot in the particulate trap to eliminate the back pressure and allow air flow through the particulate trap. Past practices of regenerating a particulate trap utilized an energy source such as a burner or electric heater to generate combustion in the particulates. Particulate combustion in a diesel particulate trap by these past practices has been found to be difficult to control and may result in an excessive temperature rise.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention is a method and apparatus for regenerating a particulate trap using microwave energy. The present invention directs microwaves to select locations in a particulate trap such as near an inlet channel end plug of a particulate trap to initiate regeneration and prevent particulate build-up. By directing microwaves to select locations, a relatively small amount of energy initiates the particle combustion that regenerates the particulate trap. The exotherm or combustion of a small amount of particulates is leveraged to burn a larger number of particulates.  
           [0005]    The present invention includes a particulate trap placed in the exhaust flow of a diesel engine. The particulate trap includes microwave-absorbing materials configured to absorb microwaves in selected locations in the particulate trap. A microwave source is operatively coupled to a wave guide, and a focus ring may be used to direct the microwaves to the microwave-absorbing materials. The microwave-absorbing material generates heat in response to incident microwaves to burn off particulates. Materials transparent to microwaves are preferably used for the basic construction of the particulate trap housing and other areas in the particulate trap where it would be inefficient to absorb microwave energy.  
           [0006]    In the present invention, the microwave reflecting and guiding materials are configured to guide and reflect the microwaves until they are incident upon the microwave-absorbing material. The microwaves in effect “bounce” around the particulate trap until they are incident upon the microwave-absorbing materials. By strategically locating microwave-absorbing materials, microwaves may be used efficiently at the locations they are most needed to initiate the burn off of particulates.  
           [0007]    The use of microwaves in the present invention further allows the frequency of particulate trap regeneration to be precisely controlled. The present invention may schedule regenerations based on empirically-generated particulate trap operation data and/or utilize a pressure sensor to determine when the particulate trap requires a regeneration.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a diagrammatic drawing of a wall flow monolith particulate trap;  
         [0009]    [0009]FIG. 2 is a diagrammatic drawing illustrating the exhaust flow through a particulate trap;  
         [0010]    [0010]FIG. 3 is a diagrammatic drawing of the microwave regeneration system of the present invention;  
         [0011]    [0011]FIG. 4 is a diagrammatic drawing illustrating end plug heating in a particulate trap;  
         [0012]    [0012]FIG. 5 is a plot detailing the exhaust gas velocity, flame front, and heat release generated by the end plug heating illustrated in FIG. 4;  
         [0013]    [0013]FIGS. 6 and 7 are diagrammatic drawings of a particulate trap utilizing axial channel heating;  
         [0014]    [0014]FIGS. 8 and 9 are diagrammatic drawings of a particulate trap illustrating mid-channel banded heating;  
         [0015]    [0015]FIG. 10 is a diagrammatic drawing illustrating mid-channel heating in a particulate trap;  
         [0016]    [0016]FIG. 11 is a plot detailing the exhaust gas velocity, flame front, and heat release generated by the mid-channel heating of FIG. 10;  
         [0017]    [0017]FIG. 12 is a diagrammatic drawing illustrating mid-channel heating combined with end plug heating in a particulate trap; and  
         [0018]    [0018]FIG. 13 is a plot detailing the exhaust gas velocity, flame front, and heat release generated by the mid-channel and end plug heating of FIG. 12. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    [0019]FIG. 1 is a diagrammatic drawing of a typical wall flow monolith particulate trap  10  “particulate trap” used in diesel applications. The particulate trap  10  includes alternating closed cells/channels  14  and open cells/channels  12 . Exhaust gases such as those generated by a diesel engine enter the closed end channels  14  depositing particulate matter  16  and exit through the open channels  12 . Referring to FIG. 2, a more detailed view of the exhaust flow through closed end  14  and open end  12  channels can be seen. Plugs  18  are used to seal the ends of the channels  12  and  14 . The walls  20  of the particulate trap are preferably comprised of a porous ceramic honeycomb wall of chordierite material, but any ceramic honeycomb material is considered within the scope of the present invention.  
         [0020]    [0020]FIG. 3 is a diagrammatic drawing of the microwave system  22  of the present invention. The system  22  includes a particulate trap  10  placed in the exhaust flow of a diesel engine. The particulate trap  10  includes a microwave-absorbing material  24  such as silicon carbide configured to absorb microwaves in selected locations in the particulate trap  10 , but any known microwave-absorbing materials are considered within the scope of the present invention. A microwave power source  26  and microwave antenna  28  are operatively coupled to a wave guide  30  and an optional focus ring  32  to direct the microwaves to the microwave-absorbing material  24 . In alternate embodiments of the present invention, the microwave antenna  28  is directly coupled to the housing of the particulate trap  10 . The microwave-absorbing material  24  generates heat in response to incident microwaves to initiate the burn-off of particulates in the particulate trap  10 . Materials such as chordierite that are transparent to microwaves are preferably used for the basic construction of the particulate trap  10  housing and other areas in the particulate trap  10  where it would be inefficient to absorb microwave energy. As the chordierite does not absorb microwave energy, the microwaves will “bounce” around until they are incident upon the microwave-absorbing material  24 . The channels  12  and  14  are further configured to guide the microwaves to the microwave-absorbing material  24 . The temperature of the particulate trap  10  may be regulated by the properties and location of the microwave-absorbing materials and by controlling the application of the microwave energy.  
         [0021]    [0021]FIGS. 4 and 5 illustrate end plug heating in a particulate trap  10  of the present invention. The end plug  18  in FIG. 4 is comprised of a microwave-absorbing material. The diesel exhaust is filled with particulates  34  and flows through the honeycomb ceramic walls  20  depositing soot  16  upon the upstream walls  20  of the particulate trap  10 . Microwaves incident upon the microwave-absorbing plug  18  heat the plug  18 , and the heated plug  18  initiates the burn-off of the soot  16  to clear the walls  20  of the particulate trap  10 , as seen by waves  17  that represent the flame front of the particulate burn off. In an end plug heating configuration of the present invention, the burn-off will initially occur where the particulate mass or soot  16  is the highest, at the end of the closed end channel  14 , and propagate to the rest of the closed end channel  14 . The exotherm of a relatively small amount of particulates, that are ignited by the end plug  18 , will be leveraged to burn a relatively large amount of soot.  
         [0022]    [0022]FIG. 5 illustrates the performance of the particulate trap shown in FIG. 4. The exhaust gas velocity will decrease as a function of the distance of the closed end channel  14 . The heat generated by the particulate heat release will initially be localized near the end plug  18  and then propagate as a burn-off flame front shown by arrow  19 .  
         [0023]    [0023]FIGS. 6 and 7 are diagrammatic drawings of a particulate trap  10  utilizing axial channel heating. The particulate trap is similar to the particulate trap  10  shown in FIG. 1 with microwave-absorbing material  38  added to the closed end channels  14 . The microwave-absorbing material  38  is deposited linearly along a wall or walls of the closed end channels  14 , as seen in FIGS. 6 and 7.  
         [0024]    [0024]FIGS. 8 and 9 are diagrammatic drawings of a particulate trap  10  utilizing mid-channel band heating. The particulate trap is similar to the particulate trap  10  shown in FIG. 1 with bands  40  of microwave-absorbing material added to the closed end channels  14 . The microwave-absorbing material bands  40  are deposited in selected areas along the axial flow length of the closed end channels  14 , as seen in FIGS. 9 and 10. The exact location of the microwave-absorbing bands  40  on the channel walls and the pattern of channels that are banded can be determined experimentally for the application.  
         [0025]    [0025]FIGS. 10 and 11 illustrate the mid-channel or banded heating in a particulate trap  10  of the present invention. The diesel exhaust is filled with particulates  34  and flows through the honeycomb ceramic walls  20  depositing soot  16  upon the walls  20  of the particulate trap  20 . Microwaves incident upon the microwave-absorbing band  40  heat the band  40 , and the heated band  40  initiates the burn-off of the soot  16  to clear the walls  20  of the particulate trap  10 . In a mid-channel or banded heating configuration of the present invention, the initial burn-off will occur where the bands  40  are placed in a closed end channel  14 , as seen in FIG. 10.  
         [0026]    [0026]FIG. 11 illustrates the performance of the particulate trap  10  shown in FIG. 10. The exhaust gas velocity will decrease as a function of the distance of the closed end channel  14 . The heat generated by the particulate heat release will initially be localized near the bands  40  and then propagate as a burn-off flame front shown by arrow  41 .  
         [0027]    [0027]FIGS. 12 and 13 are diagrammatic drawings of a particulate trap  10  utilizing a combination of banded heating and end plug heating. The particulate trap is similar to the particulate trap  10  shown in FIG. 1 with bands  40  of microwave-absorbing material added to the closed end channels  14  and a microwave-absorbing end plug  18 . This combination of microwave-absorbing bands  40  and microwave-absorbing end plugs  18  initiate the burn-off of particulates substantially along the entire length of the closed end channel  14 .  
         [0028]    [0028]FIG. 13 illustrates the performance of the particulate trap  10  shown in FIG. 12. The exhaust gas velocity will decrease as a function of the distance of the closed end channel  14 . The heat generated by the particulate heat release will initially be localized near the band  40  and end plug  18  and then propagate as burn-off flame fronts shown by arrows  51  and  53 .  
         [0029]    It is to be understood that the invention is not limited to the exact construction illustrated and described above, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.