Patent Publication Number: US-2011067386-A1

Title: Oxidizing Particulate Filter

Description:
FIELD OF THE INVENTION 
     Exemplary embodiments of the present invention relate to exhaust gas treatment systems for internal combustion engines and, more particularly, to an efficient system for assuring complete regeneration of an exhaust particulate filter. 
     BACKGROUND 
     The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NO x ”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in a diesel engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components. 
     An exhaust treatment technology, in use for high levels of particulate mater reduction, is the Diesel Particulate Filter device (“DPF”). There are several known filter structures used in DPF&#39;s that have displayed effectiveness in removing the particulate matter from the exhaust gas such as ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications. 
     The filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates will have the effect of increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of exhaust gas particulates, the DPF is periodically cleaned, or regenerated. Regeneration of a DPF in vehicle applications is typically automatic and is controlled by an engine or other controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the DPF to levels that are often above 600° C. in order to burn the accumulated particulates. 
     One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver unburned HC to an oxidation catalyst device disposed upstream of the DPF. The HC may be delivered by injecting fuel directly into the exhaust gas system or may be achieved by “over-fueling” the engine. The HC is oxidized in the oxidation catalyst device resulting in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas travels downstream to the DPF and burns the particulate accumulation. Another method for generating temperatures sufficient to regenerate the DPF has involved the placement of an electric heater adjacent to the upstream face of the filter. When energized, the electric heater operates to deliver thermal energy to the upstream face of the filter that is sufficient for the ignition of the filtered particulates. 
     While these methods of heating the DPF are both effective for regenerating an un-catalyzed particulate trap, it has been found that heat loss from the outer surface of the DPF may result in incomplete combustion of the particulate matter in areas of the filter that are closely adjacent to its outer surface. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the present invention, a particulate filter assembly for application to an exhaust system of an internal combustion engine comprises an exhaust gas particulate filter configured to receive and filter carbon and particulates from exhaust gas flowing through an exhaust system. An oxidation catalyst compound is disposed on an outer radial region of the exhaust gas particulate filter and is configured to induce oxidation of carbon monoxide from the combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates therein. 
     In another exemplary embodiment of the present invention, an exhaust gas particulate filter system for an internal combustion engine comprises, an exhaust gas conduit in fluid communication with, and configured to receive an exhaust gas from, an internal combustion engine, a hydrocarbon supply connected to and in fluid communication with the exhaust gas conduit for delivery of a hydrocarbon thereto and formation of an exhaust gas and hydrocarbon mixture therein, an oxidation device downstream of the hydrocarbon supply, and configured to receive the exhaust gas and hydrocarbon mixture and induce a rapid exothermic oxidation reaction of the mixture to thereby heat the exhaust gas, and a particulate filter assembly in fluid communication with the exhaust gas conduit, downstream of the oxidation device, and configured to receive the heated exhaust gas for combustion of carbon and particulates trapped therein. The particulate filter assembly comprises an exhaust gas particulate filter disposed within the particulate filter assembly for removal of particulates from the exhaust gas and an oxidation catalyst compound disposed on an outer radial region of the exhaust gas particulate filter and configured to induce oxidation of carbon monoxide from combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates. 
     In yet another exemplary embodiment of the present invention, an exhaust gas treatment system for an internal combustion engine comprises an internal combustion engine and an exhaust gas conduit in fluid communication with, and configured to receive an exhaust gas from, the internal combustion engine and to conduct the exhaust gas between a plurality of devices of the exhaust gas treatment system. A selective catalyst reduction device, configured for reduction of components of NO x  in the exhaust gas, is disposed in fluid communication with the exhaust gas conduit. A hydrocarbon injector is connected to the exhaust gas conduit and is in fluid communication with the exhaust gas for delivery of hydrocarbon thereto and formation of an exhaust gas and hydrocarbon mixture therein. An oxidation device is located downstream of the hydrocarbon injector, and is configured to receive the exhaust gas and hydrocarbon mixture and to induce a rapid exothermic oxidation reaction of the mixture to thereby heat the exhaust gas. A particulate filter assembly is in fluid communication with the exhaust gas conduit, downstream of the oxidation device, and is configured to receive the heated exhaust gas for combustion of carbon and particulates trapped therein. The particulate filter assembly comprises an exhaust gas particulate filter disposed within the particulate filter assembly for removal of particulates from the exhaust gas and an oxidation catalyst compound disposed on a outer radial region of the exhaust gas particulate filter and configured to induce oxidation of carbon monoxide from combustion of carbon and particulates, to thereby maintain temperatures in the outer radial region at a level sufficient to maintain the combustion of carbon and particulates therein. 
     The above features and advantages, and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, advantages and details appear, by way of example only, in the following detailed description of the embodiments, the detailed description referring to the drawings in which: 
         FIG. 1  is a schematic view of an exhaust gas treatment system for an internal combustion engine; 
         FIG. 2  is a schematic, sectional view of an exemplary embodiment of a diesel particulate filter device embodying aspects of the present invention; and 
         FIG. 3  is a perspective view of the diesel particulate filter of  FIG. 2  taken at section  3 - 3 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     Referring now to  FIG. 1 , an exemplary embodiment of the invention is directed to an exhaust gas treatment system, referred to generally as  10 , for the reduction of regulated exhaust gas constituents of an internal combustion engine, such as diesel engine  12 . It is appreciated that the diesel engine  12  is merely exemplary in nature and that the invention described herein can be implemented in various engine systems implementing a particulate filter. Such engine systems may include, but are not limited to, gasoline direct injection systems and homogeneous charge compression ignition engine systems. For ease of description and discussion, the disclosure will be discussed in the context of a diesel engine. 
     The exhaust gas treatment system includes an exhaust gas conduit  14 , which may comprise several segments that function to transport exhaust gas  16  from the diesel engine  12  to the various exhaust treatment devices of the exhaust gas treatment system  10 . The exhaust treatment devices may include a first Diesel Oxidation Catalyst device (“DOC1”)  18 . The DOC1 may include a flow-through metal or ceramic monolith substrate  20  that is wrapped in an intumescent mat (not shown) that expands when heated, securing and insulating the substrate which is packaged in a stainless steel shell or canister  21  having an inlet and an outlet in fluid communication with exhaust gas conduit  14 . The substrate  20  has an oxidation catalyst compound (not shown) disposed thereon. The oxidation catalyst compound may be applied as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combinations thereof. The DOC1  18  is useful in treating unburned gaseous and non-volatile HC and CO, which are oxidized to form carbon dioxide and water. 
     A Selective Catalytic Reduction device (“SCR”)  22  may be disposed downstream of the DOC1  18 . In a manner similar to the DOC1, the SCR  22  may also include a flow-through ceramic or metal monolith substrate  24  that is wrapped in an intumescent mat (not shown) that expands when heated, securing and insulating the substrate which is packaged in a stainless steel shell or canister  25  having an inlet and an outlet in fluid communication with exhaust gas conduit  14 . The substrate  24  has an SCR catalyst composition (not shown) applied thereto. The SCR catalyst composition preferably contains a zeolite and one or more base metal components such as iron (“Fe”), cobalt (“Co”), copper (“Cu”) or vanadium (“V”) that can operate to effectively convert NO x  constituents in the exhaust gas  16  in the presence of a reductant such as ammonia (‘NH 3 ”). The NH 3  reductant  23 , supplied from reductant supply tank  19  through conduit  17 , may be injected into the exhaust gas conduit  14  at a location upstream of the SCR  22  using an injector  26 , in fluid communication with conduit  17 , or other suitable method of delivery of the reductant to the exhaust gas  16 . The reductant may be in the form of a gas, a liquid or an aqueous urea solution and may be mixed with air in the injector  26  to aid in the dispersion of the injected spray. A mixer or turbulator  27  may also be disposed within the exhaust conduit  14  in close proximity to the injector  26  to further assist in thorough mixing of the reductant with the exhaust gas  16 . 
     Referring to  FIGS. 1 ,  2  and  3 , an exhaust gas particulate filter assembly, in this exemplary case a Diesel Particulate Filter device (“DPF”)  28 , is located within the exhaust gas treatment system  10 , downstream of the DOC1  18  and the SCR  22 , and operates to filter the exhaust gas  16  of carbon and other particulates. The DPF  28  may be constructed using an exhaust gas particulate filter such as ceramic wall flow monolith filter  30  that is wrapped in an intumescent mat  33  that expands when heated, securing and insulating the filter which is packaged in a stainless steel shell or canister  31  having an inlet and an outlet in fluid communication with exhaust gas conduit  14 . The ceramic wall flow monolith has a plurality of longitudinally extending passages  32  that are defined by longitudinally extending walls  34 . The passages  32 ,  FIG. 2 , include a subset of inlet passages  36  that have an open inlet end  38  and a closed outlet end  40 , and a subset of outlet passages  42  that have a closed inlet end  44  and an open outlet end  46 . Exhaust gas  16  entering the filter  30  through the inlet ends  38  of the inlet passages  36  is forced to migrate through adjacent longitudinally extending walls  34  to the outlet passages  42 . It is through this wall flow mechanism that the exhaust gas  16  is filtered of carbon and other particulates  48 . The filtered particulates are deposited on the longitudinally extending walls  34  of the inlet passages  36  and, over time, will have the effect of increasing the exhaust gas backpressure experienced by the diesel engine  12 . It is appreciated that the ceramic wall flow monolith filter  30  is merely exemplary in nature and that the DPF may include other exhaust gas particulate filters such as wound or packed fiber filters, open cell foams, sintered metal fibers, etc. 
     In an exemplary embodiment, the increase in exhaust backpressure caused by the accumulation of particulate matter  48  requires that the DPF  28  is periodically cleaned, or regenerated. Regeneration involves the oxidation or burning of the accumulated carbon and other particulates  48  in what is typically a high temperature (&gt;600° C.) environment. For regeneration purposes, an electrically heated catalyst device (“EHC”)  50  may be disposed within canister  31  of the DPF  28 . The EHC  50  may be constructed of any suitable material that is electrically conductive such as a wound or stacked metal monolith  52 . An electrical conduit  54  that is connected to an electrical system, such as a vehicle electrical system, supplies electricity to the EHC  50  to thereby heat the device, as will be further described below. In an exemplary embodiment, an oxidation catalyst compound (not shown) may be applied to the EHC  50  as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof. 
     In an exemplary embodiment, the ceramic wall flow monolith filter  30  has an oxidation catalyst compound  58  disposed about the perimeter thereof. The oxidation catalyst compound  58  extends, from the outermost surface of the wall flow monolith filter  30 , radially inwardly a distance (“r”). The oxidation catalyst compound may be applied as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof. Application of the oxidation catalyst compound  58  may be through a precision coating process in which a mold of the inlet end of the wall flow monolith filter  30  is made and through which the catalyst is machine dispensed or vacuum applied or both. Simpler coating techniques may include masking of the central portion  60  of the wall flow monolith filter  30  but such methods are more suitable to lower volume productions. The application of the oxidation catalyst compound  58  to the outer radial region  56  of the wall flow monolith filter  30  is for the purpose of assisting in the regeneration of the carbon and other particulates  48  that are trapped near the outer surface of the DPF  28  in regions that may be subjected to heat transfer rates from the canister  31  that may lower the regeneration temperatures to a level below which the particulates will not combust. The radial distance “r” or thickness of application of the oxidation catalyst compound  58  as well as the axial extent to which the oxidation catalyst compound  58  is applied to the wall flow monolith filter  30  is selected based on the heat transfer characteristics of a given configuration of DPF  28 . In some cases, it may be necessary to apply the catalyst compound  58  along the entire length of the outer radial region  56  of the wall flow monolith filter  30  and in others; a partial or axially zoned application will be suitable for complete combustion of particulates  48 . 
     Referring again to  FIG. 1 , disposed upstream of the DPF  28 , in fluid communication with the exhaust gas  16  in the exhaust gas conduit  14 , is an HC or fuel injector  62 . The fuel injector  62 , in fluid communication with HC  65  in fuel supply tank  63  through fuel conduit  61 , is configured to introduce unburned HC  65  into the exhaust gas stream for delivery to the DPF  28 . A mixer or turbulator  64  may also be disposed within the exhaust conduit  14 , in close proximity to the HC injector  62 , to further assist in thorough mixing of the HC with the exhaust gas  16 . 
     A controller such as vehicle controller  66  is operably connected to, and monitors, the exhaust gas treatment system  10  through signal communication with one or more sensors. As used herein the term controller may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     In an exemplary embodiment, a backpressure sensor  68 , located upstream of DPF  28 , generates a signal indicative of the carbon and particulate loading in the ceramic wall flow monolith filter  30 . Upon a determination that the exhaust system backpressure has reached a predetermined level indicative of the need to regenerate the DPF  28 , the controller  66  activates EHC  50  and raises the temperature of the EHC to a level suitable for rapid HC oxidation. A temperature sensor  70 , disposed within the shell  31  of the DPF  28  monitors the exhaust gas temperature downstream of the EHC  50 . When the EHC  50  has reached the desired operational temperature, the controller  66  will activate the HC injector  62  to deliver fuel into the exhaust gas conduit  14  for mixing with the exhaust gas  16 . The fuel/exhaust gas mixture enters the DPF  28  and flows through the heated EHC  50  that induces a rapid oxidation reaction and resultant exotherm to thereby raise the exhaust gas temperature to a level (&gt;600° C.) suitable for regeneration of the carbon and particulate matter  48  in the ceramic wall flow monolith filter  30 . Following its exit from the EHC  50 , the heated exhaust gas  16  flows downstream through the ceramic wall flow monolith filter  30  where it effectively combusts the carbon and other particulates  48  trapped therein. During the regeneration event, carbon (“C”) is oxidized in the presence of oxygen (“O 2 ”) to generate carbon dioxide (“CO 2 ”) and carbon monoxide (“CO”): 
       C+O 2 →CO 2 +CO
 
     In the outer radial region  56  of the wall flow monolith  30  in which the ring of oxidation catalyst compound  58  has been applied, the carbon monoxide (“CO”) generated from the oxidation of the carbon particulates  48  is oxidized by the oxidation catalyst compound  58 , in an exothermic reaction to create carbon dioxide (“CO 2 ”): 
       CO+O 2 →CO 2 +Thermal Exotherm
 
     As a result of the exothermic reaction in the outer radial region  56  of the wall flow monolith  30  the temperatures required to achieve complete combustion of the carbon and particulates  48  is maintained and the DPF  28  is fully regenerated across its entire cross-section. 
     In another exemplary embodiment, it is contemplated that, in some circumstances the fuel injector  62  may be dispensed with in favor of engine control of the HC levels in the exhaust gas  16 . In such an instance the controller such as vehicle controller  66  is operably connected to, and monitors, the exhaust gas treatment system  10  through signal communication with a number of sensors such as backpressure sensor  68 . The backpressure sensor generates a signal indicative of the carbon and particulate loading  48  in the ceramic wall flow monolith filter  30  and, upon a determination that the backpressure has reached a predetermined level indicative of the need to regenerate the DPF  28 , the controller  66  activates EHC  50  and raises the temperature of the EHC to a level suitable for rapid HC oxidation (about 450° C.). Temperature sensor  70  monitors the exhaust gas temperature downstream of the EHC  50  and when the EHC  50  has reached the desired operational temperature, the controller  66  will adjust the engine timing and rate/frequency of fueling to deliver excess, unburned HC into the exhaust gas conduit  14  for mixing with the exhaust gas  16 . The reactions described in the previous discussion, however, remain the same. In either case, the controller  66  may monitor the temperature of the exothermic oxidation reaction in the EHC  50  and the ceramic wall flow monolith filter  30  through temperature sensor  70  and adjust the HC delivery rate of injector  62  to maintain a predetermined temperature. 
     The use of an oxidation catalyst compound  58  disposed in limited region of the wall flow monolith filter  30 , particularly the outer radial region  56  of the filter, provides for complete regeneration of the filter  30  while minimizing or thrifting the quantity of precious metals necessary for application to each DPF  28 . In addition, the radially inner area  60  of the wall flow monolith filter  30  is protected from thermal extremes that would result in the case of application of the oxidation catalyst compound  58  to the entire filter  30  and could lead to sub-optimal durability or even failure. 
     While the invention has been described using an electrically heated catalyst device (“EHC”)  50  to initiate the oxidation of the exhaust gas and hydrocarbon mixture to heat the exhaust gas  16  to a temperature sufficient to regenerate the DPF  28 , it is contemplated that other oxidation devices such as a passive upstream oxidation catalyst device, (ex. DOC1  18 ) may also be used for the same purpose without deviating from the scope or intent of the present invention. 
     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 embodiments 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 present application.