Abstract:
A diesel particulate filter including a microwave-absorbing target housed within a waveguide cavity lined along the walls with a hard-electromagnetic surface (HES). The HES modifies specific electromagnetic boundary conditions for a given design frequency so as to enable the establishment of electromagnetic field patterns which are more uniform across the cavity thereby causing the target material to undergo enhanced uniformity heating. The heating of the microwave absorbing media causes particulate buildup to be vaporized and removed from the filter by the exhaust stream flow.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to diesel particulate filter systems, and more particularly to a diesel particulate filter system utilizing a ceramic filter to trap particulate exhaust in combination with a localized microwave absorbing media positioned in close proximity to regions of particulate buildup within the ceramic filter. The cavity housing the ceramic filter and microwave absorbing media is adapted to accept inputs of microwave-frequency electromagnetic radiation to establish field patterns causing the target absorbing media to undergo substantially uniform heating.  
       BACKGROUND OF THE INVENTION  
       [0002]     The use of ceramic filters to entrap particulates carried by a diesel engine exhaust flow is known. In operation, such ceramic diesel particulate filters accept exhaust flow at one end and trap particulates as exhaust gases diffuse through thin channel walls and exit out the other end. Particulate buildup which is allowed to continue causes the filter to become clogged thereby giving rise to an undesirable increased pressure differential across the filter and leading to back pressure that reduces the engine efficiency. Thus, it is necessary to clear the particulate buildup before critical levels of obstruction are achieved. Such particulate removal may be effected by raising the temperature at the location of particulate buildup to a level above the flash point of the hydrocarbon particulates thereby causing combustion and vaporization of the particulates. Once the particulates are vaporized, the combustion products may be swept out of the filter by the exhaust stream.  
         [0003]     In order for localized heating to efficiently remove particulates from the filter, such heating must be applied across substantially the entire cross-section of the filter. In the event that zones across the filter cross-section are left unheated, the particulates at those zones will not be vaporized and the filter will develop a pattern of plugged zones. Thus, it is desired to provide an efficient method of localized particulate combustion across substantially the entire exit plane of the filter so as to provide uniform heating across that plane.  
         [0004]     The use of microwave-frequency electromagnetic radiation is known to be effective for heating dielectric materials in other environments. However, a challenge in using microwave-frequency radiation is the achievement of uniform temperature distributions across a target material. That is, the use of microwave-frequency radiation is highly susceptible to the creation of target hot sports and cold spots. As explained above, such non-uniformity is generally inconsistent with the requirements for particulate filter regeneration. Moreover, the environment of a particulate filter in a diesel exhaust system provides challenges with regard to space availability and cost constraints. Suitable systems for diesel exhaust filter regeneration based on microwave heating are not believed to have been previously available.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides advantages and alternatives over the known art by providing a diesel particulate filter including a microwave-absorbing target disposed within a waveguide cavity lined along the walls with a hard-electromagnetic surface (HES). The HES modifies specific electromagnetic boundary conditions for a given design frequency so as to enable the establishment of electromagnetic field pattern which are more uniform across the cavity and do not vanish at the boundary walls as they would in an ordinary cavity. Instead, the electromagnetic field patterns are characterized by enhanced uniformity substantially across the cavity cross-section thereby causing the target material to undergo enhanced uniformity heating. The heating of the microwave absorbing media causes the particulate buildup to be vaporized and removed from the filter by the exhaust stream flow. An optional coaxial conductor element may be disposed along the center axis of the cavity so as to further enhance uniformity of the field patterns. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The following drawings which are incorporated in and which constitute a part of this specification illustrate an exemplary embodiment of the present invention and, together with the general description above and the detailed description set forth below, serve to explain the principles of the invention wherein:  
         [0007]      FIG. 1  is a cut-away view of a diesel particulate filter system incorporating a microwave-absorbing target positioned at the outlet of a ceramic filter within a cavity lined with a hard electromagnetic surface;  
         [0008]      FIGS. 2A-2D  illustrate known target heating patterns for different modes achieved in a right circular cavity with conductive walls of traditional construction;  
         [0009]      FIG. 3  illustrates a potentially preferred construction of a hard electromagnetic surface for the walls of the cavity in  FIG. 1 ;  
         [0010]      FIG. 4  illustrates schematically a quarter-section of the waveguide cavity in  FIG. 1  illustrating the arrangement of layers forming a hard electromagnetic surface construction; and  
         [0011]      FIG. 4A  illustrates an alternative construction including grounding elements extending between the outer layer and an interior layer of the hard electromagnetic surface. 
     
    
       [0012]     While embodiments of the invention have been illustrated and generally described above and will hereinafter be described in connection with certain potentially preferred embodiments and procedures, it is to be understood and appreciated that in no event is the invention to be limited to such embodiments and procedures as may be illustrated and described herein. On the contrary, it is intended that the present invention shall extend to all alternatives and modifications as may embrace the broad principles of this invention within the true spirit and scope thereof.  
       DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     Reference will now be made to the various drawings wherein to the extent possible like elements are designated by corresponding reference numerals in the various views. In  FIG. 1 , there is illustrated a diesel particular filter assembly  10  for disposition along the exhaust gas flow path down stream from a diesel engine (not shown). The direction of gas flow is illustrated by the directional arrow within the figure. According to the illustrated construction, the diesel particulate filter includes a cavity portion  12  that serves to contain a porous ceramic filter  14  and microwave-absorbing material  16  disposed in embedded contacting relation substantially across the cross-section of the filter  14 . The microwave-absorbing material  16  may be any one or a combination of well known substances which undergo heating upon exposure to microwave radiation. By way of example, such materials may include SiC (Silicon Carbide), ITO (Indium-Tin Oxide), various ferrites, and the like as will be well know to those of skill in the art.  
         [0014]     As illustrated, the diesel particulate filter assembly  10  is provided with one or more microwave coupling ports  20  at one or both ends for operative connection to a microwave power source  22  of greater than 1 kW such as a standard 2.45 GHz/2 kW source as will be well known and readily available. Preferably, a single microwave power source will be utilized although multiple sources can be utilized if desired. Microwave reflectors  24  which permit gas flow but which prevent excess of trapped microwave energy are provided at the inlet and outlet ends of the diesel particulate filter assembly  10 .  
         [0015]     As will be described further hereinafter, the cavity portion  12  is lined with a so-called hard-electromagnetic surface or metasurface structure which will hereinafter be referred to as an HES lining  30  so as to permit the cavity portion  12  to function as a waveguide in which electromagnetic field patterns are not caused to vanish at the cavity walls but rather extend in substantially uniform fields across the cavity cross-section. The cavity  12  and filter  14  with embedded microwave-absorbing material  16  are all preferably of a substantially cylindrical configuration having a substantially round cross-section. However, other cross-sectional geometries such as square, rectangular or elliptical and the like may also be utilized if desired.  
         [0016]     In operation, diesel exhaust enters through an inlet aperture  32 , passes into the filter  14  through intake channels, diffuses through the filter channel walls, flows out of the filter output channels and exits the cavity through the exhaust output aperture  34 . In the flow process, particulates carried by the exhaust flow are deposited where the gases diffuse through the channel walls upon exiting the filter. As the engine continues to run, the particulate mass builds up until the exhaust gas flow is impeded. At a selected optimum point based on measured back pressure within the system, the microwave power source  22  is activated and energy enters the chamber thereby heating the microwave-absorbing material  16 . The microwave-absorbing material  16  is disposed within the area of particulate buildup and as it absorbs energy, it heats to a point beyond the flash point of the accumulated hydrocarbon particulates. The particulates are thus ignited and are removed in vaporized form by the flow of exhaust gas.  
         [0017]     To facilitate the uniform heating of the target microwave-absorbing material  16 , the HES lining  30  is utilized within the cavity portion  12  so as to make applied field energy substantially uniform across the entire cross-section where particulates are deposited. In the illustrated embodiment, field uniformity is further promoted by the inclusion by the use of a coaxial conductor  36  which promotes the development of transverse electromagnetic (TEM) waves within the cavity portion. The use of the HES lining  30  in the cavity portion  12  either with or without an additional coaxial conductor has been found to provide greatly enhanced heating uniformity across the cross-section of the filter  14  as compared to heating profiles documented in targets within right circular cavities with standard conductive wall boundaries.  
         [0018]     In standard microwave heating applications, non-uniform heating of the target material arises due to electromagnetic modes that constrain the heating energy to specific patterns within the heating cavity. For example, it is well documented that when microwave radiation is introduced into a cavity of cylindrical geometry housing a target element, the electromagnetic radiation within the cavity is distributed among several orthonormal cavity modes. Each of these modes is a solution to the Maxwell&#39;s wave equation for the cavity&#39;s particular boundary conditions. When the walls of the cavity are made of metal, the high conductivity dictates that the tangential electric field approaches zero at the cavity walls. The result is that there are spots within the cavity near the walls where the field is very small. At those locations, a target material experiences little or no heating.  
         [0019]     To illustrate the traditional non-uniform heating,  FIGS. 2A-2D  illustrate heating profiles for circular targets at four different modes in a right circular cavity with conductive walls. In these figures, the lighter regions correspond to higher temperatures while the darker regions correspond to lower temperatures. In particular,  FIG. 2A  illustrates the heating pattern for the TE11 mode,  FIG. 2B  illustrates the heating pattern for the TE21 mode,  FIG. 2C  illustrates the heating pattern for the TE22 mode and  FIG. 2D  illustrates the heating pattern for the TE12 mode. Thus, as can be seen, there is a substantial non-uniformity of heating across the cavity cross-section.  
         [0020]     The use of the HES lining  30  of appropriate construction at the walls within the cavity portion  12  has been found to substantially enhance field uniformity across the cavity and thereby enhance uniformity of heating a target within that cavity. The present invention incorporates a unique HES lining  30  of triple-layer construction to form an electromagnetic band gap (EBG) lining. The construction utilized permits the HES lining to be extremely thin and unobtrusive within the heating cavity. As illustrated in  FIG. 3 , the HES lining  30  incorporates a ground plane  40  in the form of the standard conductive cavity wall, a first layer of conducting strips  42  running in the length direction of the cavity offset from the ground plane  40  and laterally spaced apart from one another with a second layer of conducting strips  44  disposed in laterally spaced relation to one another such that the strips  44  in the second layer cover the spaces between the strips  42  in the first layer. A quarter section end view illustrating is arrangement in the cavity is illustrated in  FIG. 4 .  
         [0021]     Layers of dielectric materials  47  are preferably disposed between the layers. The dielectric material acts as a spacer between the layers while also increasing the field breakdown voltage between layers since it is harder for a spark to go through a dielectric than through air. By way of example only, and not limitation, one contemplated dielectric material with desired high breakdown voltage character is believed to be available from DuPont under the trade designation KAPTON. It is contemplated that the placement of dielectric materials of different permittivity between the layers of conducting strips and/or the ground plane may be desirable to prevent electrical breakdown between adjacent conductive strips.  
         [0022]     As illustrated in  FIG. 3 , wave propagation is allowed along the direction of the strips, when the wave frequency is near the HES&#39;s resonant frequency. Thus, by designing the HES lining  30  to yield a resonant frequency which closely approximates the frequency of the heating source used, much more uniform cross-sectional heating field can be achieved. In this regard, substantially complete uniform heating is achieved when the resonant frequency matches the heating radiation frequency. As microwave frequency is moved away from the resonance frequency, the heating patterns become more non-uniform.  
         [0023]     The resonant frequency of the HES is determined by the HES&#39;s characteristic inductance (L) and capacitance (C) by the following formula. 
 
 f   r =1/(2 π√{square root over (LC)} ) 
 
         [0024]     The HES capacitance and inductance in turn are determined from the surface&#39;s geometrical dimensions. L=μ o t is the approximate sheet inductance, and the sheet capacitance is the sum of the contributions from the parallel plates between layers, the edges from strip to strip within the layers, and the fringe capacitance from the edges strips of one layer to the body of the strips on the other layer.  
         [0025]     The parallel-plate capacitance is  
           C   parallel     ≈       E   d     ⁢       (     w   -       1   2     ⁢   L       )     2         ,       
 
 while the edge-to-edge capacitance is  
           C   edge     ≈     w   π     &lt;   E   &gt;           ⁢       cosh     -   1       ⁢       (     L   +   w     )       (     L   -   w     )           ,       
 
 and the fringe capacitance is  
         C   fringe     ≈       2   ⁢   w     π     &lt;   E   &gt;           ⁢       cosh     -   1       ⁢       (   w   )       (   d   )             
 
 The bandwidth of the resonance is  
           Δ   ⁢           ⁢   ω       ω   r       =       2   ⁢           ⁢   π   ⁢           ⁢   t       λ   r           
 
         [0026]     By way of example, for an HES lining  30  utilizing the geometry of  FIG. 3 , with d=0.100 inches, w=0.380 inches, L=0.440 inches and t=0.250 inches, e=1.23, the resonant frequency is approximated by the above formulation to be 2.38 GHz with a bandwidth of 32%. A simulation of the HES lining using HFSS yields a resonant frequency of 2.5 GHz with a bandwidth of 33%. Thus, such an HES lining has a resonant frequency such that heating using a standard commercially available 2.45 GHz microwave source will be substantially uniform. Of course, the HES lining  30  may be the subject of substantial variation by adjustment of the various identified parameters.  
         [0027]     In order to provide yet a further degree of uniformity it is also contemplated that the HES lining may include conductive grounding elements between the ground plane forming the wall of the cavity and the conductive strips. Such a construction is illustrated in  FIG. 4A  wherein elements corresponding to those previously described are designated by like reference numerals with a prime. As shown, the construction is identical to that illustrated in  FIG. 4 , with the exception that grounding elements  46 ′ such as conductive studs or the like may extend between the first layer of conductive strips  42 ′ and the ground plane  40 ′ of the cavity wall. Such a construction may give rise to slightly improved uniformity relative to the non-grounded construction but may also be slightly more complex to produce.  
         [0028]     It is to be understood that while the present invention has been illustrated and described in relation to potentially preferred embodiments, constructions, and procedures, that such embodiments, constructions, and procedures are illustrative only and that the invention is in no event to be limited thereto. Rather, it is contemplated that modifications and variations embodying the principles of the invention will no doubt occur to those of ordinary skill in the art. It is therefore contemplated and intended that the present invention shall extend to all such modifications and variations as may incorporate the broad aspects of the invention within the true spirit and scope thereof.