Abstract:
Particle traps are often used to reduce particulate emissions from internal combustion engines to acceptable levels. In order to maintain the particulate traps, they must be periodically regenerated in order to burn off the trapped particles. Strategies for efficiently regenerating the particle traps have been elusive. The present invention separates the exhaust flow into several flow paths. During regeneration, the flow paths are sequentially partially closed, and the particle traps in each of the flow paths are individually regenerated using electrically conductive filter elements. The present invention can be used to effectively filter particles from any combustion process, especially exhaust from internal combustion engines. The particle trap assembly achieves a relatively low pressure loss and efficient regeneration by supplying an oxidizer via a small cross-flow passage.

Description:
TECHNICAL FIELD 
   The present invention relates generally to particle traps for filtering exhaust from an internal combustion engine, and more particularly to a particle trap assembly with efficient regenerative capabilities. 
   BACKGROUND 
   In order to meet the very stringent requirements on exhaust particulate emissions being imposed by governmental regulators, modifications to the engine and/or exhaust treatment strategies are necessary. Although particle traps are known in the art, many employ cordierite as the filter medium, and use some separate regenerator, such as injected fuel or an electrically resistive heater, to regenerate the filter. Although several filter mediums are known in the art that can effectively capture particulate matter in a wide range of particle sizes suitable for treatment of an engine&#39;s exhaust, efficiently regenerating the particle traps has been much more problematic. For instance, regenerating particle traps by burning fuel in the exhaust section to create the heat necessary to cause regeneration (i.e. burn the particulate matter) can result in an excess usage of fuel, cause uneven regeneration of the filter medium and potentially create even further undesirable emissions. Likewise, particulate trap strategies that employ an electrically resistive heater embedded in, or positioned adjacent, a filter medium can also experience uneven regenerative heating and can consume relatively large amounts of electrical power to perform the regenerative function. 
   Existing systems often suffer from additional energy consumption due to the fact that regenerative heaters must heat the filter medium well above typical exhaust temperatures. When regeneration is performed with the engine running, the exhaust flow blowing over the filter medium tends to cool the same. Thus, even larger amounts of heat energy must be supplied to overcome this continuous cooling of the filter medium that occurs due to exhaust gas flow. In addition, many prior assemblies attempt to regenerate almost the entire particulate filter assembly in a single operation, which can also place large energy demands on an engine&#39;s electrical system. 
   The present invention is directed to one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In one aspect, a particle trap assembly includes a plurality of particle traps positioned in a housing. At least one valve is attached to the housing and is moveable between a first position and a second position. A flow area to one of the particle traps is relatively large when the valve is in its first position, but is a relatively small predetermined flow area that is greater than zero when the valve is in its second position. 
   In another aspect, a particle trap assembly includes a housing with an inlet separated from an outlet by a plurality of flow paths. At least one particle trap separates an upstream portion of one of the flow paths from a down stream portion. A plurality of valves are attached to the housing, and each is operable to open and close a selected one of the flow paths. A plurality of cross flow passages are disposed in the housing and fluidly connect different pairs of the flow paths. Each of the flow paths has a relatively large flow area, and each of the cross flow passages has a relatively small predetermined flow area. 
   In still another aspect, a method of regenerating a particle trap in a particle trap assembly includes a step of changing a flow area to a particle trap from a relatively large flow area to a relatively small predetermined flow area that is greater than zero. The particle trap is regenerated at least in part by heating the particle trap while supplying an oxidizer via the relatively small predetermined flow area. 
   In still another aspect, a particle trap assembly includes a housing with a can pressure loss co-efficient for fluid flow between an inlet and an outlet. At least one particle trap is positioned in the housing and has a filter pressure loss coefficient for fluid flow passing through a filter medium. The can pressure loss coefficient is on a same order as the filter pressure loss coefficient. 
   In another aspect, a particle trap assembly includes a housing. At least one particle trap with an electrically conductive filter element divides the housing into an upstream volume and a downstream volume. The upstream volume is about equal to the downstream volume. 
   In still another aspect, a method of reducing pressure losses through a particle trap assembly includes a step of separating an upstream volume of the housing from the downstream volume with at least one particle trap having an electrically conductive filter element. The upstream volume is sized relative to the downstream volume so that a can pressure loss coefficient is on a same order as a filter pressure loss coefficient. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of a particle trap according to one aspect of the present invention; 
       FIG. 2  is an isometric view of a partially assembled particle trap sub-assembly according to another aspect of the present invention; 
       FIG. 3  is an isometric view of a particle trap exhaust section according to another aspect of the present invention; and 
       FIG. 4  is a sectioned isometric view of the exhaust section of  FIG. 3  as viewed along section lines A—A. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , an individual particle trap  30  according to the present invention includes a metallic box that surrounds a conduit  31  and an electrically conductive filter element  32  on four sides. The box is preferably constructed from a suitable corrosion resistant material, such as stainless steel, and includes an arcuate shaped top separated from a like shaped bottom  37  by a pair of sides  38  and  39 . Thus, trap  30  can be said to have a long dimension that deviates from a straight line and, the long axis extends between sides  38  and  39 , which are angled to one another. Top  36  and bottom  37  preferably have a relatively planar outer surface to facilitate stacking a plurality of particle traps  30 , as shown in  FIG. 2 . The filter element  32  could be of a type manufactured by Bekaert Metal Fiber Technologies of Belgium to include a relatively fine mesh of an alloy of iron, chromium and aluminum that is electrically conductive. In this example, the filter medium comes in an elongated piece that is formed into a serpentine pattern for appropriate sizing in the box of  FIG. 1 . This serpentine pattern allows for a large filtration surface area in a manner well known in the art. The sides  38  and  39  separate the filter element  32  from electric terminals  34  and  35 , respectively. The individual particle trap  30  is regenerated by passing a suitable electric current through the electrically conductive filter element  32  via electric terminals  34  and  35 . When a sufficient electric current is passed through electrically conductive filter element  32 , it glows red from heat and regenerates the filter by burning off any trapped particulate matter. Preferably, terminals  34  and  35  and the filter element  32  are electrically isolated from the surrounding box using an appropriate insulator, such as by coating the inner surfaces of top  36 , bottom  37  and sides  38 ,  39  with an aluminum oxide coating. The filter element  32  can be secured in the box and attached to the electrical terminals  34  and  35  in any suitable manner known in the art. 
   Referring now to  FIG. 2 , a plurality of individual particle traps  30  are stacked atop one another and attached to a housing quadrant  41  in order to produce a particle trap subassembly  40 .  FIG. 2  shows a particle trap subassembly  40  partially assembled to include three stacked particle traps  30  separated from one another by ceramic mats  45 . When subassembly  40  is completely assembled, it will include eight stacked particle traps  30  in this illustrative example embodiment. In order to electrically isolate particle traps  30  from one another, the sides of the particles traps are preferably separated from the housing segment  41  by a pair of ceramic mats  46 . Mats  46  preferably include holes to receive the electrical terminals  34  and  35  so that the same are exposed outside of housing segment  41  for connection to an appropriate electrical circuit. In order to further the electrical isolation, the individual particle traps  30  are also preferably separated from one another by suitably shaped ceramic mats  45 . When the complete stack  44  of particle traps  30  are attached to housing segment  41 , an individual exhaust flow path  50  is bound on one side by the inner surface of housing segment  41  and on its other surface by the stack  44  particle traps  30 . 
   Referring now to  FIGS. 3 and 4 , a particle trap exhaust section  10  according to the illustrated embodiment includes an inlet section  12  separated from an outlet section  16  by a filter section  14  with an external surface  11 . Filter section  14  is made up of four separate particle trap subassemblies  40  which are attached at their flanges in a suitable manner, such as by bolts. Exhaust section  10  preferably includes a suitably sized inlet  20  for a connection to an exhaust pipe from an internal combustion engine, and an outlet  21  for fluid connection to a tail pipe or another exhaust after treatment section from an internal combustion engine. In the illustrated embodiment, the inlet section  12  includes an inlet  20  and four outlets  28 , which correspond to the four flow paths  50   a–d  defined by the four particle trap subassemblies  40 . The inlet section  12  and the outlet section  16  are preferably attached to the trap or the filter section  14  via a plurality of bolts located at the perimeter flanges of the respective housing segments  41 , or the sections can be welded together. In order to control which of the flow paths  50   a–d  are open, and which are at least partially closed, the inlet section  12  is provided with an individual flow path valve  22  for each of the flow paths  50   a–d . In the preferred embodiment, each of the flow path valves  22  is equipped with a poppet valve member  26 , a pneumatic actuator  25  and a biasing spring  27 . Preferably, the biasing springs  27  bias the individual valve members  26  toward an open position so that the respective flow path  50  is biased to a normally open position. When compressed air is supplied to pneumatic actuator  25 , the individual valve member  26  is driven downward into contact with a seat to close that individual flow path. Those skilled in the art will appreciate that any other type of actuator could be substituted in place of the pneumatic actuators  25  without departing from the intended scope of the present invention. However, the illustrated embodiment is preferably intended for use with on highway trucks, which have compressed air available, for such items as the vehicle&#39;s air brakes. The pneumatic actuators  25  would be preferably controlled in a conventional manner via an electronic control module associated with the vehicle and/or engine. 
   It is important to note that the electrical terminals  34  and  35  of the individual particle traps  30  are preferably exposed on the outside of the external surface  11  of individual particle trap subassemblies  40 . Particle traps  30  are positioned in the internal space  15  of filter section  14 . This prevents the electrical connections from becoming corroded due to exposure to exhaust flow. In addition, this structure allows for each of the individual particle traps  30  to be placed on a selectively energizable electric circuit of a type well known in the art. In the preferred embodiment, only one flow path  50  is partially closed by a flow path valve  22  at a time, and only one particle trap  30  within that flow path is electrically regenerated at a time. 
   In order to ensure an adequate supply of oxygen or oxidizer during regeneration of the particle traps in a given flow path, the present invention also preferably includes some means for supplying oxygen in the region where the particle trap is being regenerated. In the illustrated embodiment, this is accomplished by including cross passages  51  that extend between the separate flow paths so that some small amount of exhaust flow continues to flow through a closed flow path during regeneration. For instance, and in reference to  FIG. 4 , if the valve  26  were closed to partially closed within flow path  50   a , some oxygen and/or oxidizer would be supplied to assist in the regeneration of particle traps in that flow path via cross passage  51  that is fluidly connected to flow path  50   c . Those skilled in the art will appreciate that supplying oxygen to assist in the regeneration process could be accomplished in a number of ways. For instance, oxygen from the ambient environment could be supplied in some manner known in the art. Another alternative might be to simply have a small passage through the valves  26  so that when they are seated, a small passage remains. 
   In order to minimize pressure losses through the particle trap assembly of the present invention, the housing is divided into an upstream volume and a downstream volume by the filter medium. By appropriately shaping the housing and the internal particle traps, the upstream flow speed can be made about equal to the downstream flow speed. In the illustrated embodiment, this is accomplished by making the upstream and downstream flow areas about equal. As a result, in the illustrated embodiment the upstream volume is set to be about equal to the downstream volume. By about equal, it means that the upstream volume, speed or flow area is within plus or minus 5% of the downstream volume speed or flow area, or vice versa, respectively. In addition, a can pressure loss coefficient is made to be on the same order as a filter pressure loss coefficient. The can pressure loss coefficient can be determined by known methods, such as by determining a pressure drop through the housing without the filter medium being present. The filter pressure loss coefficient relates to a pressure drop that could be expected for exhaust traversing the filter medium, ignoring pressure losses due to the housing. This can also be determined by known methods of calculation and/or measurement. By utilizing an electrically conductive filter element and shaping the housing as described, the can pressure loss coefficient is preferably on the same order as the filter pressure loss coefficient. Being on the same order means that neither coefficient is greater than ten times the other coefficient. This is accomplished at least in part, by utilizing an electrically conductive filter element while adjusting the relative sizes of the upstream and downstream flow areas and/or volumes within the housing to provide an overall reduction in pressure loss across the particle trap assembly when used to filter exhaust from an internal combustion engine. In other words, the upstream flow speed is made about equal to the downstream flow speed. 
   INDUSTRIAL APPLICABILITY 
   The present invention is generally applicable to trapping combustible particles that are typically a bi-product of a combustion process. The invention finds specific applicability to particle traps for internal combustion engines. The invention is illustrated in its preferred application, which relates to particle traps for over the road type trucks, off road vehicles and stationary power generators equipped with diesel engines. 
   During normal operation, all of the flow path valves  22  are biased toward their normally open position, and exhaust from the engine enters at inlet  20  and is divided substantially evenly between the four outlets  28  of the inlet section  12 . As the exhaust flows down each of the flow paths  50   a–d , particulates are filtered as the exhaust moves radially inward toward a central passage, then downward and out of the particle trap exhaust section  10  via outlet  21 . As the particle traps  30  continue to operate, they collect more and more particles and become more and more clogged. Eventually, the individual traps will become so clogged with particles that they are in need of being regenerated. This condition can be sensed in a conventional manner, such as by use of one or more pressure sensors that can sense pressure in the exhaust gas flow, and that information can be used in a conventional manner to determine the state of the overall particle trap assembly or the state of particle traps in any of the individual flow paths. For instance, by measuring a pressure differential across a stack of particle traps, one can determine whether the particle traps are clogged using formulas and/or empirical data. 
   When it is determined that one or more of the particle traps are in need of being regenerated, one of the flow path valves  22  is actuated to close that individual flow path  50 . By closing the flow path, the relatively cool exhaust gases will not continuously pass through the filter elements, cooling the same, during the regenerative process. In other words, the heat generated in order to regenerate the individual particle traps need not overcome the cooling influence of the exhaust gas flow as in many regenerating particle traps of the past. However, cross passages  51  insure that some minimum amount of exhaust flow continues in the substantially closed flow path in order to provide oxygen or another oxidizer for the regeneration process. In order to avoid undermining the engine&#39;s performance, the remaining open flow paths are preferably sized such that no excessive back pressure burden is placed on the engine when one of the flow paths is closed. Thus, although not necessary, the combined flow area of all of the flow paths, when open, is preferably larger than that necessary for effective operation of the engine and particle traps. Thus, regenerating the particle traps  30  should not undermine engine performance. However, the present invention does contemplate the possibility that the overall system could be sized such that some back pressure would be placed upon the engine when one of the flow paths is closed, but this burden would preferably be relatively short in duration, as the particle traps can be regenerated relatively quickly. 
   After closing one of the flow paths  50 , the regenerating process proceeds by selectively energizing an electrical circuit associated with one of the particle traps associated with the closed flow path. This causes the electrically conductive filter element  32  ( FIG. 1 ) to glow brightly with heat, which in turn burns the trapped particles caught in the filter medium. Burning of the trapped particles is further facilitated via oxygen supplied to the substantially closed flow path via the cross passage  51 . After some predetermined duration, the electrical energy is ended to that particular particle trap and then another particle trap is regenerated sequentially in a similar manner. Thus, regenerating the particle traps in one flow path in the illustrated embodiment would include selectively energizing an electrical circuit of each of the eight stacked particle traps  44  in some predetermined sequence. After all of the particle traps in that flow path are regenerated, the flow path valve  22  associated with that flow path is de-actuated and allowed to reopen. Next, another flow path is closed by actuating a different flow path valve  22 . The individual particle traps  30  in that closed flow path are then regenerated one at a time until the entire stack  44  has been regenerated. Nevertheless, those skilled in the art will appreciate that not all of the particle traps in each flow path need necessarily be regenerated in each regeneration sequence. Thus, in the illustrated embodiment, each of the four flow paths are sequentially closed, and eight individual particle traps are individually regenerated in each of the four flow paths. Thus, the overall process of regenerating the particle trap exhaust section  10  illustrated would include regenerating  32  individual particle traps  30 . After all of the of the particle traps  30  are regenerated, all of the flow path valves  22  are returned to their normally biased open positions and filtering of the exhaust flow continues as normal. 
   Although the present invention has been illustrated in the context of using individual particle traps  30  that include an electrically conductive filter element, those skilled in the art will appreciate that a non-conductive filter element, such as one using a ceramic filter element, could be substituted without departing from the intended scope of the present invention. In addition, although the present invention preferably contemplates heat regenerating the individual filter traps  30  using an electrically conductive filter element, a separate electrically conductive resistive heater could accompany the filter medium without departing from the present invention. For instance, an electrically conductive resistive heater could be imbedded in or positioned adjacent a ceramic filtering element. In addition, the present invention also contemplates generating heat for regenerating the individual particle traps in another manner besides using electricity. For instance, the individual traps could be regenerated by burning fuel in the vicinity of the particle traps in order to regenerate the same. 
   The present invention is advantageous in that the electrical demands on the engine can be made to acceptable levels due to a number of strategies associated with the present invention. First, less electrical energy is required since the individual particle traps are partially isolated from the exhaust flow during the regenerating process. This avoids the filter elements being cooled by the exhaust flow during the regeneration process. In addition, each particle trap exhaust section is preferably made up of many individual particle traps  30  that may be heat regenerated selectively. Thus, less power is necessary because only a small fraction of the entire particle trap is regenerated at any given time. This aspect of the invention also makes the individual particle traps potentially serviceable in that one failed individual particle trap could be replaced without affecting the remaining particle traps. Still another advantage of the present invention relates to the fact that the electrical terminals for the individual particle traps are preferably located outside the housing holding those particle traps. This allows the electrical connections to be made away from the corrosive affect of the exhaust flow. This strategy can also facilitate in trouble shooting potential problems associated with the particle traps or the regenerators associated therewith. 
   Still another advantage of the present invention is accomplished by reducing the overall pressure loss across the particle trap assembly when in operation. This can be accomplished by arranging the particle traps to divide the housing between an upstream volume that is about equal to a downstream volume, and making the upstream flow area about equal to the downstream flow area. The goal being uniform exhaust flow speed upstream and downstream of the particle trap. This avoids energy waste associated with accelerating and/or decelerating the exhaust flow. In addition, by using an electrically conductive filter element, the filter pressure loss coefficient can be set to be on the same order as the can pressure loss coefficient, resulting in overall pressure loss coefficient that is lower than that believed possible with prior art designs. This strategy is accomplished at least in part by utilizing individual particle traps that have a long dimension that deviates from a straight line so that the housing can be divided into four quadrants that each include a plurality of stacked individual particle traps. 
   It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. Thus, those skilled in the art will appreciate that other aspects, objects, and advantages of the invention can be obtained from a study of the drawings, the disclosure and the appended claims.