Patent Publication Number: US-7716921-B2

Title: Exhaust particulate filter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/713,541 filed Sep. 1, 2005, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present disclosure relates generally to an exhaust system, and particularly to a particulate filter for a diesel exhaust system. 
     Automotive exhaust systems for diesel and other internal combustion engines typically include a filtration system that limits the mass of particulate matter emitted with the exhaust gases. In diesel engine systems, this matter typically includes carbonaceous matter (soot) and ash particles. Present filtering methods to trap the exhaust particulates focus on wall-flow filtration. Wall-flow filtration systems typically have a high filtration efficiency not only for exhaust particulates but also for ash particles. Catalyzed diesel particulate filters have been used extensively, where the catalyst is normally applied either to the front end of the diesel particulate filter or applied to the whole filter for the purpose of reducing the regeneration temperature. Catalytic or thermal arrangements within the exhaust system, which serve to effect regeneration of the filtration element, tend to create high temperatures within the filtration body, which tends to limit the choice of materials for the filtration body. In view of present particulate filter arrangements, it is desirable to have a more advanced particulate filter that can operate with effective filtration and improved regeneration. 
     SUMMARY OF THE INVENTION 
     An embodiment of the invention includes a particulate filter for an exhaust system configured to receive an exhaust flow. The filter includes a wall-flow filtration element having a first regeneration zone and a second regeneration zone, the first zone being downstream of the second zone, and a heat source disposed at the first regeneration zone. In response to demand for regeneration, the wall-filtration element regenerates according to a staged regeneration such that the first zone initiates regeneration ahead of the second zone, and each zone regenerates in the direction of the exhaust flow. 
     Another embodiment of the invention includes a method for regenerating a particulate filter for an exhaust system configured to receive an exhaust flow. The particulate filter includes a wall-flow filtration element having a first regeneration zone and a second regeneration zone, the first zone being downstream of the second zone, and a heat source disposed at the first regeneration zone. In response to demand for regeneration, the wall-filtration element is regenerated according to a staged regeneration such that the first zone initiates regeneration ahead of the second zone, and each zone regenerates in the direction of the exhaust flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures: 
         FIG. 1  depicts an exhaust system employing an embodiment of the invention; 
         FIG. 2  depicts an isometric view of a particulate filter in accordance with an embodiment of the invention; 
         FIG. 3  depicts a cross section view of a particulate filter similar to that of  FIG. 2  and in accordance with an embodiment of the invention; 
         FIG. 4  depicts in schematic view an embodiment of a particulate filter in accordance with and embodiment of the invention; and 
         FIGS. 5   a - 5   b,    6   a - 6   b,  and  7 , depict alternative cross section views of a particulate filter similar to that of  FIG. 2  under varying operating conditions and in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the invention provides a particulate filter for an exhaust system of an automotive diesel engine having improved regeneration features. While the embodiment described herein depicts an automotive diesel engine as an exemplary diesel powerplant using a particulate filter, it will be appreciated that the disclosed invention may also be applicable to other diesel powerplants that require the functionality of the particulate filter herein disclosed, such as a diesel powered generator for example. While the disclosed invention is well suited for filtering the combustion byproducts of a diesel engine, it may also be applicable for filtering combustion byproducts of a gasoline powered engine. 
     An exemplary exhaust system  100  for an automotive diesel engine (not shown) is depicted in  FIG. 1  having a manifold exhaust pipe  110  suitably connected at one end to an exhaust manifold (not shown) of the diesel engine (not shown) for receiving an exhaust flow depicted generally as numeral  150 . Turbocharger  140  is suitably connected to intermediate manifold exhaust pipe  110  and intermediate exhaust pipe  120 . Intermediate exhaust pipe  120  is suitably connected to a particulate filter  200  for trapping exhaust particulates present in the exhaust flow  150 , which is suitably connected to an exhaust pipe  130 . A tailpipe (not shown) for exhausting the conditioned exhaust flow to atmosphere is suitably connected to exhaust pipe  130 . Exhaust system  100  manages the exhaust flow  150  by controlling how the exhaust flow  150  passes from exhaust manifolds (not shown) to manifold exhaust pipe  110 , turbocharger  140 , intermediate exhaust pipe  120 , particulate filter  200 , exhaust pipe  130 , and then to atmosphere. Exhaust system  100  has a nominal flow area equal to or greater than the inside cross-sectional flow area of manifold exhaust pipe  110 . 
     Each particulate filter  200  has a housing  210 , which may be any form of construction and configuration suitable for the purpose, and a filter element  220  suitably contained within housing  210 , best seen by now referring to  FIG. 2 . In an embodiment, filter element  220  is a ceramic monolith structure. Filter element  220  is of the wall-flow filtration type, meaning that exhaust flow  150  passes from the inlet channels  230 , through the porous internal walls  240 , to the outlet channels  250 . Filtering of the exhaust flow  150  primarily occurs as exhaust flow  150  passes through the pores of internal walls  240 , hence the term wall-flow filtration. Filter element  220  is configured to trap exhaust particulates. 
     In an exemplary embodiment, inlet channels  230  each have an inlet port  260  at one end  310  and a non-porous end-plug  270  at the opposite end  320 . In an embodiment, the non-porous end-plugs  270  are substantially thicker (such as 0.25-0.5 inches for example) than the filter wall (such as 0.010-0.020 inches for example). In an alternative embodiment, non-porous end-plug  270  may be replaced by a porous end-plug  270 ′. End-plug  270  is also herein referred to as a standard end-plug for purposes of distinction. Embodiments of the invention may be applied to a particulate filter  200  having either a standard end-plug  270  or a porous end-plug  270 ′. In the various drawings, reference numeral  270  may be replaced with reference numeral  270 ′ when reference is made to a porous end-plug. Outlet channels  250  each have an outlet port  280  at one end  320  and an end-plug  290  at the opposite end  310 . Exhaust flow  150  enters filter element  220  at inlet ports  260 , passes through porous internal walls  240 , and is discharged from filter element  220  at outlet ports  280 . In this manner, inlet channels  230  and outlet channels  250  are referred to as being in fluid communication with each other via internal walls  240 . Internal walls  240  of filter element  220  are fabricated with a pore size less than about 30 micrometers, thereby enabling the entrapment of exhaust particulates. In an embodiment, porous end plugs  270  have a pore sized equal to or greater than about 30 micrometers, and equal to or less than about 60 micrometers. End-plugs  290  may be solid or may have a porosity similar to that of internal walls  240 . In this manner, the artisan will readily recognize that in general, porous end-plugs  270  have a greater porosity than end-plugs  290 . 
     In an embodiment depicted in  FIG. 2 , filter element  220  is a ceramic monolith structure having a plurality of porous internal walls  240  that define and separate the inlet and outlet channels  230 ,  250 . Inlet and outlet channels  230 ,  250  are arranged parallel to the direction of exhaust flow  150 , resulting in a sideways flow (depicted generally by arrows  300  in  FIG. 3 ) as exhaust flow  150  passes through internal walls  240 . Housing  210  includes a first end  310  and a second end  320 . Inlet ports  260  and end-plugs  290  are arranged at first end  310 , and outlet ports  280  and porous end-plugs  270  are arranged at second end  320 . In an embodiment, and as depicted illustratively in  FIGS. 2 and 3 , the overall surface area of porous end-plugs  270  is substantially less than the total surface area of internal walls  240 , with an exemplary ratio being less than about 1:240. 
     Outlet channels  250  have outlet ports  280  at second end  320  to discharge exhaust flow  150  and end-plugs  290  at first end  310  to block the incoming exhaust flow  150 . Exhaust flow  150  is filtered at the ceramic monolith structure  220  as it passes through the porous walls  240  between inlet and outlet channels  230 ,  250 . Exhaust byproducts, such as metallic particles and carbonaceous matter, are trapped at porous walls  240 , end-plugs  290 , and porous end-plugs  270 . The filtered exhaust flow  150  is then discharged at outlet ports  280 . 
     As discussed above, porous end-plugs  270  may be replaced with standard end-plugs  270 ′, and unless otherwise specified the discussion that follows applies to both. 
     A diesel particulate filter (dpf), such as the particulate filter  200  and more particularly filter element  220 , requires regeneration from time to time. Normally regeneration is initiated by increasing the inlet temperature of the exhaust gases at first end  310  to a temperature higher than 650° C. At this temperature, soot deposited on the filter walls  240  will react with the oxygen in the exhaust gases and will be converted into CO and CO 2 . This reaction is strongly exothermic. The reaction and the associated heat will propagate toward the downstream side of the filter to second end  320 , which causes high temperature near the second end  320  of the filter. As the soot deposited at the first end  310  of the filter is oxidized, some of the exhaust gases will flow through the filter wall  240  and flow out the filter through the exit channels (outlet channels)  250 . Consequently, less flow will pass through the yet to be regenerated part of the inlet channels  230 . 
     To improve upon the regeneration of particulate filter  200 , an embodiment of the invention provides for staged regeneration, that is, the length of particulate filter  200 , from first end  310  to second end  320 , is arranged into zones, such as first zone  410  and second zone  420  for example, best seen by referring to  FIG. 4 , with regeneration occurring in first zone  410  and then in second zone  420 . While an embodiment of the invention is depicted and described herein having only two zones, it will be appreciated that any number of zones may be applied in accordance with embodiments of the invention, and that the scope of the invention is not limited to only the two-zone arrangement depicted and described herein. 
     Each zone  410 ,  420  has a front end  411 ,  421  and a back end  412 ,  422 , respectively. In response to a demand for regeneration, the downstream first zone  410  is caused to regenerate first, beginning at its front end  411  and progressing with the flow to its back end  412 , and then the upstream second zone  420  is caused to regenerate second, beginning at its front end  421  and progressing with the flow to its back end  422 . With the regeneration progressing from downstream first zone  410  (front to back) then to upstream second zone  420  (front to back), the regeneration of particulate filter  200  is said to be staged. 
     From the foregoing, it will be appreciated that no matter how many regeneration zones there are in particulate filter  200 , the staged regeneration is caused to take place beginning at the downstream zone with progression toward the upstream zone, with each zone regenerating from front to back in the direction of the flow. 
     The regeneration of each zone may be caused by heaters  425 ,  430  or activation of a catalyst  405 , which will be discussed in more detail below. 
     While  FIG. 4  is depicted having heating elements  425 ,  430  along the entire length of first and second zones  410 ,  420 , respectively, it will be appreciated that only the first zone  410  may have a heater  425 , and that heater  425  may only be disposed proximate the front end  411  of first zone  410 , since the generated heat will naturally flow in the direction of the exhaust flow toward the rear end  412  of first zone  410 . In an alternative embodiment, a similar arrangement may also be applied for the second zone  420 . 
     Reference is now made to  FIGS. 5   a  and  5   b,  which depict a conventional dpf regeneration.  FIG. 5   a  illustrates uniform accumulation of soot  400  on filter walls  240  with an inlet exhaust gas temperature of less than about 500° C.  FIG. 5   b  illustrates the initiation of regeneration at the first end  310  of the dpf  220 , where the inlet exhaust gas temperature has been elevated to greater than about 650° C. Here, the exhaust temperature may be raised by introducing some fuel into the exhaust system, or an oxidation catalyst upstream from the dpf may be used to oxidize the fuel and increase the exhaust temperature, or the exhaust temperature may be raised by an electrical heater located upstream from the dpf. In  FIG. 5   b,  dpf  220  experiences a high temperature and a high oxidant concentration at the first end  310 , and a respectively lower temperature and lower oxidant concentration at the second end  320 . Consequently, and with reference still to  FIG. 5   b,  the soot  400  at first end  310  would burn, without the assistance of an embodiment of the invention, before the soot  400  at second end  320 . This in turn causes the exhaust flow through walls  240  from inlet channels  230  to outlet channels  250  to be concentrated toward the first end  310  of dpf  220 , causing a lower flow rate through walls  240  toward the second end  320 . As a consequence, the lower flow rate reduces the capacity for the exhaust gases to carry away the heat generated by oxidation of the soot  400 . This situation may result in thermal run away for the soot deposits near the closed end (second end  320 ) of the inlet channels  230 , which may lead to the filter degradation (melting or cracking of the filter). 
     To avoid a thermal run away condition and protect the integrity of the diesel particulate filter  200 , an embodiment of the invention includes a catalyzed filter element  220  having an oxidation catalyst  405  disposed at the last 25-50% of the filter element  220  (first zone  410 ). While embodiments are disclosed herein having an oxidation catalyst disposed over a defined percentage of the filter element length, it will be appreciated that this is for illustration purposes only, and that other embodiments may have a different percentage of catalyst coverage.  FIG. 6   a  illustrates a zone-coated catalyzed filter  220  having an oxidation catalyst  405  disposed at first zone  410  on about the last 25% of the internal walls  240  toward the second end  320 . Since the catalyst  405  can lower the ignition temperature of the soot deposits  400 , the soot-oxygen reaction can be initiated proximate the back end (second end  320 ) of the filter  220  first, which will serve to remove the soot  400  deposited near the closed end (second end  320 ) of the inlet channel  230  first. More specifically, and as discussed previously, regeneration of filter element  220  at first zone  410  takes place in a direction with the flow from front end  411  toward back end  412  of first zone  410  (see also  FIG. 4  depicting reference numerals  411  and  412 ). As a consequence, more exhaust gases will flow along the inlet channels  230  before they cross the internal walls  240  to the outlet channels  250 . The resulting higher flow rate down the inlet channels  230  allows better heat transfer through convection, and thus, serves to lower the peak temperature and the associated thermal stress on the filter element  220  during filter regeneration. Furthermore, by the time the first end  310  is ignited, that is, the second zone  420  being elevated above the temperature of about 650° C., there is little or no soot  400  remaining at the first zone  410  near the closed end (second end)  320  of filter element  220 . When the thermal energy associated with regeneration propagates to the closed end (second end)  320 , there is little or no additional energy to be released on the closed end, which is where the temperature is normally the highest with a conventional regeneration method. 
       FIG. 6   b  illustrates the dpf  200  of  FIG. 6   a,  but with an inlet exhaust temperature of about 550° C. or greater. With the catalyst  405  at the first zone  410  proximate the second end  320  effectively lowering the ignition temperature of the soot  400  by about 100° C. from about 650° C. to about 550° C., ignition of the soot  400  occurs first at the first zone  410 . The second zone  420  is regenerated when the inlet exhaust temperature reaches or exceeds about 650° C. 
     As previously discussed, embodiments of the invention may employ standard end-plugs  270  or porous end-plug  270 ′.  FIG. 7  illustrates the dpf  200  with porous end-plugs  270 ′ and a catalyst  405  disposed over the last 25-50% of the internal walls  240  toward the second end  320 . The porous end-plugs  270 ′ allow more flow to pass through the inlet channels  230  to the closed end, thereby further lowering the peak temperature near the closed end (second end)  320 . 
     As previously discussed, regeneration at first and second zones  410 ,  420  may be initiated by auxiliary heaters  425 ,  430 , rather than by a catalyst  405 , which may be controlled by a control system  435  for providing controlled heating (best seen by referring to  FIG. 4 ). In another embodiment, a combination of heaters and a catalyst may be employed. Heaters  425 ,  430  may be electric heaters, microwave heaters, or any heating device suitable for the purposes disclosed herein. Collectively, heaters  425 ,  430 , catalyst  405 , or other means of heating, such as activated soot for example, are herein referred to as heat sources. 
     When used as herein disclosed, filter element  220  may be made from Cordierite (Mg 2 Al 4 Si 5 O 18 , Magnesium Aluminum Silicate) or SiC (Silicon Carbide), which are two ceramic materials that may be used for manufacturing ceramic dpfs. Regarding Cordierite with forced regeneration, however, the peak temperature of conventional regeneration may be too high for the Cordierite dpf, which may cause it to either crack or melt. Consequently, this characteristic tends to dissuade the use of Cordierite for dpf&#39;s despite its low cost. Only from the teachings disclosed herein does the unexpected advantage arising from embodiments of the invention provide for the use of a Cordiertie dpf. 
     In view of the foregoing, some embodiments of the invention may include some of the following advantages: reduced peak temperature and therefore reduced thermal stress of the particulate filter  200  through staged regeneration that regenerates the filter beginning at a downstream zone and proceeding to an upstream zone; employing staged regeneration from a downstream zone to an upstream zone allows for regeneration in a direction of the exhaust flow, which is the natural direction of heat flow; less heat accumulation at the rear (exhaust) end of the filter; lowered peak regeneration temperature thereby allowing less frequent regeneration of particulate filter  200 ; the potential for providing a more durable diesel particulate filter (dpf); and, the option of using a Cordierite dpf which is much cheaper and weaker, but suitable for the intended purpose disclosed herein using staged regeneration, than the a SiC dpf. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.