Title of Invention	TUNINIG PARTICULATE FILTER PERFORMANCE THROUGH SELECTIVE PLUGGING
Abstract	Modified exhaust aftertreatment filters for filtering engine exhaust are provided as are methods of design and manufacturing modified exhaust aftertreatment filters. The modified filters are subject to reduced back pressure and reduced thermal gradients experienced during regeneration as compared to unmodified filters. The modified filters include flow-through channels obtained by unplugging channels which normally are plugged in an unmodified filter.

Title of Invention

TUNINIG PARTICULATE FILTER PERFORMANCE THROUGH SELECTIVE PLUGGING

Abstract

Modified exhaust aftertreatment filters for filtering engine exhaust are provided as are methods of design and manufacturing modified exhaust aftertreatment filters. The modified filters are subject to reduced back pressure and reduced thermal gradients experienced during regeneration as compared to unmodified filters. The modified filters include flow-through channels obtained by unplugging channels which normally are plugged in an unmodified filter.

Full Text	TUNING PARTICULATE FILTER PERFORMANCE THROUGH SELECTIVE PLUGGING AND USE OF MULTIPLE PARTICULATE FILTERS TO REDUCE EMISSIONS AND IMPROVE THERMAL ROBUSTNESS BACKGROUND [0001] The invention relates to exhaust aftertreatment filters for filtering exhaust from internal combustion engines, including diesel engines. [0002] Exhaust aftertreatment filters for diesel engines are known in the prior art. The filter traps contaminant particulate present in exhaust, and in order to remove the trapped particulate, the filter is heated to burn-off the trapped contaminant particulate as gas. Accordingly, the filter is regenerable and is composed of material on which trapped contaminant particulate from the engine exhaust is removed by addition of heat. Commonly used particulate filter materials include cordierite, silicon carbide, mullite, or aluminum titanate, which are manufactured as filter elements to capture the soot and other particulate generated by the engine. [0003] Diesel particulate filters (DPF) are subject to high temperatures during use. The design of the DPF consists of a honeycomb structure with opposing channels blocked to force exhaust gases to flow through the porous channel walls, while trapping soot. The soot (composed primarily of carbon) accumulates in the DPF and must be removed periodically. Typically, the soot is removed from the filter by oxidation reactions between carbon in the soot and either oxygen (i.e., burning) or nitrogen dioxide, both of which are constituents of the exhaust. The carbon may react with oxygen or nitrogen dioxide according to the following reactions: Reaction (1) is the primary reaction that occurs during an active regeneration. Reaction (2) is the primary reaction that occurs during passive regeneration. Heat is a significant by-product of the reaction shown in Reaction (1) and, if not controlled, can cause thermal runaway of the filter, leading to fractures and/or melting of the filter and rendering it ineffective as a filter. [0004] Although thermal runaway may be prevented by controlling the rate at which carbon is burned in the filter, nonetheless the DPF may be subject to thermal gradients caused by differential heating patterns, which also may lead to fractures. Differential heating may occur during active regeneration of the filter where carbon may be unequally distributed, either radially or axially, within the DPF. For example, carbon may be more highly distributed in the DPF at locations where the largest volume of exhaust passes through the filter (i.e., at locations where exhaust velocity is highest). These locations may exhibit a relatively high temperature during active regeneration as compared to other locations. [0005] Although carbon distribution may be altered by modifying the design of a filter, particulate filter manufacturers are hampered by material strength issues that limit the maximum porosity that can be obtained in an extruded honeycomb structure. Filter manufacturers typically design filters that have the lowest back pressure and suitable filtration efficiency as required for a particular engine. However, ceramic filter manufacturers have difficulty designing filters that have the lowest back pressure and suitable filtration efficiency without greatly weakening the honeycomb structures. Furthermore, it is commonly believed that filtration efficiency drops precipitously with even a single unblocked or broken channel in the honeycomb structure of the filter, even though unblocked channels might lower back pressure. [0006] Therefore, there is a need for filters having modified design characteristics in order to minimize back pressure and thermal gradients during regeneration. Furthermore, it is desirable that these modified design characteristics can be combined with control techniques to create filters that are more durable and resistant to structural damage which may occur during use of the filter (e.g., during regeneration). SUMMARY [0007] Disclosed are exhaust aftertreatment filters and systems for filtering engine exhaust flowing along an axial direction. The filters include a plurality of flow channels comprising a plurality of wall-flow channels and a plurality of flow-through channels. The plurality of wall-flow channels and the plurality of flow-through channels may be positioned in the filter in order to improve back pressure, reduce temperature, and/or increase velocity of exhaust through the filter while maintaining filtering efficiency. Also disclosed are methods of manufacturing filters having modified performance characteristics. [0008] The disclosed filters may be regenerable and composed of monolithic material on which trapped contaminant particulate from the engine exhaust is removed by addition of heat. In some embodiments, the filters are composed of material comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends. The wall segments may define a plurality of axial flow channels including wall-flow channels and flow-through channels. Typically, the wall segments define a first set of first wall-flow channels where the wall segments are alternately sealed to each other by a first set of plugs to define a first set of wall-flow channels closed by the plugs and having open downstream ends; and the wall segments define a set of second wall-flow channels interdigitated with the first set of flow channels and having open upstream ends, the wall segments being alternately sealed to each other by a second set of plugs closing the second set of flow channels. The wall segments further define a third set of flow-through channels positioned in the filter for modifying the performance of the filter (e.g., achieving reduced back pressure, reduced temperature, and/or increased velocity of exhaust) while maintaining adequate filtering efficiency (e.g., efficiency of at least about 95%, 90%, 85%, 80%, 75%, 70%, 60%, or 50% for soot concentrations of about 2.0 gL or less). [0009] The number of flow-through channels may be adjusted to achieve a desired filter performance. In some embodiments, the number of flow- through channels represents at least about 1% of total channels (or at least about 2% of total channels in some embodiments) while the maintained filtering efficiency is at least about 90% (e.g., for soot concentrations of about 2.0 g/L or less). In other embodiments, the number of flow-through channels represents at least about 5% of total channels (or at least about 10% of total channels in some embodiments) while the maintained filtering efficiency is at least about 80% (e.g., for soot concentrations of about 2.0 g/L or less). [0010] The filter may include a coating. For example, the wall segments of the filter may be coated with a coating material, which optionally may include a catalytic agent. In some embodiments of the filters, wall segments defining flow-through channels are coated with a coating material that comprises a catalytic agent. Catalytic agents may include oxidizing catalysts and reducing catalysts. Catalytic agents may include catalysts for at least one reaction selected from the group consisting of C + O2 → CO2 and 2NO + O2 → 2NO2. Catalytic agents may include noble metals (e.g., platinum, rhodium, and palladium.) [0011] The wall segments defining the flow-through channels may have an average thickness different than an average thickness of the wall segments defining the first set of wall-flow channels or the wall segments defining the second set of wall-flow channels. In some embodiments of the filters, the wall segments defining the flow-through channels have an average thickness greater than an average thickness of the wall segments defining the first set of wall-flow channels or the wall segments defining the second set of wall-flow channels. [0012] The flow-through channels may be distributed in the filter in any suitable arrangement. In some embodiments, the flow-through channels are distributed in the filter in a gradient, where the filter has an increasing concentration of open channels in sections located at peripheral positions in the filter as compared to sections located at central positions of the filter. [0013] The filter may include wall segments further defining a fourth set of channels having closed downstream ends and closed upstream ends (i.e., closed channels). The closed channels may be positioned in the filter in order to reduce physical damage to the periphery of the filter. In some embodiments of the filters, the closed channels form a peripheral ring in the filter. Optionally, the wall segments defining the closed channels have an average thickness greater than an average thickness of the wall segments defining the first set of wall-flow channels or the wall segments defining the second set of wall-flow channels. In further embodiments, the flow-through channels may be arranged in a ring adjacent to a peripheral ring formed by the closed channels. [0014] The filter may be composed of any suitable material. In some embodiments, the filter is composed of a ceramic material, examples of which are cordierite, silicon carbide, mullite, and aluminum titanate. The filter may be monolithic (i.e., composed of single piece of material), or segmented (i.e., composed of multiple pieces of material bonded together). [0015] The filters may be utilized alone or may be combined with additional components in an exhaust aftertreatment system for filtering engine exhaust flowing along an axial direction. For example, an exhaust aftertreatment system as disclosed herein may include the following components in series along an axial direction: a diesel oxidation catalyst, a first filter as disclosed herein, and optionally a second filter. [0016] The disclosed exhaust aftertreatment systems for filtering engine exhaust flowing along an axial direction may comprise at least a first filter and a second filter arranged in series along the axial direction, where the first filter is positioned upstream of the second filter and has a lower filtration efficiency than the second filter. The first and second filter may be composed of a monolithic ceramic material such as cordierite, silicon carbide, mullite, and aluminum titanate (optionally having a honeycomb or ceramic bead structure). The first filter, second filter, or both filters may include a coating material that comprises a catalytic agent (e.g., an oxidizing catalyst for converting nitric oxide to nitrogen dioxide). Typically, at least one of the first and second filters includes a plurality of wall-flow channels and flow-through channels. Optionally, at least one of the first and second filters includes closed channels. The flow-through channels and closed channels, if present, may be arranged in any suitable formation, including a formation where a peripheral ring of closed channels surrounds an adjacent ring of flow-through channels. [0017] In the disclosed systems, the first filter may be composed of a first regenerable material and the second filter may be composed of a second regenerable material that is different than the first regenerable material. For example, the first regenerable material may have a larger pore diameter than the second regenerable material. In some embodiments, the first filter is a high cell density flow-through element having a cell density of greater than 200 per square inch. In other embodiments, the first filter may be a partially plugged filter. In further embodiments, the second filter may have a high cell density (e.g., a cell density of greater than 200 per square inch), which may be higher than the first filter. [0018] In the disclosed systems, the first filter may have a soot filtration efficiency that is lower than the second filter. In some embodiments, the first filter has a soot filtration efficiency of at least about 50% (e.g., about 50-60% in some embodiments) and the second filter has a soot filtration efficiency of at least about 90% (or at least about 95% in some embodiments). [0019] The disclosed systems optionally include a catalytic converter element, such as a diesel oxidation catalyst element, which may be arranged in series with the first filter and the second filter along the axial direction. The catalytic converter element may be positioned upstream of the first filter and may include an oxidizing catalyst for at least one reaction selected from the group consisting of 2CO + O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O (where in some embodiments x is an integer from 1-25 and y is an integer from 0-52). In some embodiments, the first filter may include an oxidizing catalyst for at least one reaction selected from the group consisting of 2CO + O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O (where in some embodiments x is an integer from 1-25 and y is an integer from 0-52). Optionally, the second filter may include an oxidizing catalyst for at least one reaction selected from the group consisting of 2CO + O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)02 → (4x)CO2 + (2y)H2O (where in some embodiments x is an integer from 1-25 and y is an integer from 0-52). [0020] Also disclosed are methods for manufacturing a modified exhaust aftertreatment filter for filtering engine exhaust flowing along an axial direction and having modified performance. The modified filter has a structure as described herein. [0021] In some embodiments of the methods of manufacture, an unmodified filter is composed of regenerable material comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends. The wall segments may define a plurality of axial flow channels including wall-flow channels and flow-through channels. Typically, the wall segments define a first set of first wall-flow channels where the wall segments are alternately sealed to each other by a first set of plugs to define a first set of wall- flow channels closed by the plugs and having open downstream ends; and the wall segments define a set of second wall-flow channels interdigitated with the first set of flow channels and having open upstream ends, the wall segments being alternately sealed to each other by a second set of plugs closing the second set of flow channels. [0022] The methods of manufacture typically include selecting and removing at least one plug of the first set of plugs and the second set of plugs to provide open flow in at least one channel of the modified filter. In some embodiments, the selected plug for removal is located at a position in the unmodified filter whereby removing the plug reduces back pressure on the modified filter during operation while maintaining filtering efficiency. In other embodiments, the selected plug for removal is located at a position in the unmodified filter which is subject to relatively high temperature during operation of the unmodified filter as compared to a non-selected plug, thereby reducing the relatively high temperature during operation of the modified filter while maintaining filtering efficiency. In further embodiments, the selected plug for removal is located at a position in the unmodified filter where exhaust flow exhibits relatively low velocity during operation of the unmodified filter as compared to a position of a non-selected plug, thereby increasing the relatively low velocity during operation of the modified filter while maintaining filtering efficiency. [0023] Also disclosed are exhaust aftertreatment filters for filtering engine exhaust prepared by the disclosed methods of manufacture. In some embodiments, the filters prepared by the methods of manufacture include a number of flow-through channels representing at least about 1% of total channels (or at least about 2% of total channels in some embodiments) while the maintained filtering efficiency of the modified filter is at least about 90% (or at least about 80% in some embodiments) (e.g., for soot concentrations of about 2.0 g/L or less). BRIEF DESCRIPTION OF THE DRAWINGS [0024] Figure 1 is a perspective view of an exhaust aftertreatment filter. [0025] Figure 2 is a sectional view of the exhaust aftertreatment filter of Figure 1. [0026] Figure 3 illustrates the effects on filtering efficiency (top) and filter restriction (bottom) versus percentage channels open (i.e., percentage flow- through channels) in a modified filter having a 12 inch diameter by 12 inch length with 200 cells per square inch. [0027] Figure 4a illustrates the velocity of exhaust through a filter exhibiting a parabolic flow profile. Figure 4b illustrates a potential distribution of open channels in a filter designed to increase flow at locations of relatively low exhaust velocity in view of the parabolic flow profile of Figure 4a. [0028] Figure 5a illustrates the velocity of exhaust through a filter exhibiting a turning flow profile. Figure 5b illustrates a potential distribution of open channels in a filter designed to increase flow at locations of relatively low exhaust velocity in view of the turning flow profile of Figure 5a. [0029] Figure 6a illustrates the velocity of exhaust through a filter exhibiting a flat velocity profile. Figure 6b illustrates a potential distribution of open channels in a filter designed to increase flow at locations of relatively low exhaust velocity in view of the flat flow profile of Figure 6a. [0030] Figure 7 illustrates two filters having sample unplugging patterns {i.e., modified filters). The sample patterns include a 2-cell (or 4-cell) peripheral ring of double-plugged channels adjacent to a 2-cell (or 4-cell) ring of flow-through channels. [0031] Figure 8 illustrates an exhaust aftertreatment system including a diesel oxidation catalyst (DOC) in series with a first filter element and a second filter element. DETAILED DESCRIPTION [0032] Figure 1 shows an exhaust aftertreatment filter 10 for filtering exhaust from an internal combustion engine, such as diesel engine 12, flowing along an axial flow direction 14. Figure 2 shows a section view of the filter of Figure 1. The filter is composed of particulate filtration material 16, as known in the prior art, for example ceramic such as a cordierite, silicon carbide, mullite, or aluminum titanate on which trapped contaminant particulate from the engine exhaust is removed by addition of heat. The filter 10 includes wall-flow channels formed by wall segments having an upstream plug 34 or a downstream plug 36. Contaminant particulate such as soot is trapped and accumulates in the filter, which trapped contaminant particulate is burned-off during regeneration. The filter includes a filter body 18 having an outer periphery 20 surrounding a central core 22. Outer periphery 20 and central core 22 may be subject to differential thermal expansion during thermal cycling during regeneration, due to outer periphery 20 being cooler than central core 22. For example, Figure 2 shows central hot spot 24, which is hotter than outer periphery 20, and which may be more dominant at the downstream side of the filter where particulate contaminant may accumulate and clog. With or without clogging or a downstream hot spot such as 24, outer periphery 20 may run cooler than central core 22, as is known. The filter is typically mounted in a housing 26, such as a stainless steel canister, having a mat mounting material 28 surrounding the filter body and performing a number of functions including thermal resistance, dampening of vibration, and resistance to movement. The mat material is typically compressed between housing 26 and filter body 18. [0033] The noted differential thermal expansion between hotter central core 22 and cooler outer periphery 20 may subject the filter body to separational axial tensile stress in the axial direction which in turn subjects the filter body to separational fracture and cracking, for example as shown at fracture or crack line 30 in Figure 1. It is known by catalyst and filter manufacturers that the radial compressive stress applied by pressure obtained from an expanding mat material 28 assists in reducing the fracture probability of filter body 18. However, the radial compressive stress reduces the probability of fracture along a fracture line parallel to axis 31 of the filter perpendicular to crack line 30), and does little to prevent fractures along a fracture or crack line such as 30. Furthermore, the mat material can degrade over time, resulting in loss of pressure. With larger and heavier filters and longer lifetimes, particularly for diesel particulate filters in wall- flow application versus automotive flow-through catalyst application, the noted pressure and compressive stress applied by mat material 28 will decrease more rapidly, particularly than that observed for automotive catalysts. Furthermore, an automotive catalyst can still function after cracking because of its flow-through application, whereas a contaminant particulate filter loses effectiveness if cracked because of the bypass flow path created. The filter may include a pre-stressed layer 32 bonded to filter body 18 at outer periphery 20 and is compressively axially pre- stressed in the opposite axial direction to the noted separational axial tensile stress to counteract the latter during regenerative heating. [0034] The modified exhaust aftertreatment filters disclosed herein include a plurality of flow-through channels obtained by removing an upstream plug 34 or downstream plug 36 in what otherwise would be a wall-flow channel in an unmodified filter. The selected flow-through channels may be located at any suitable position in the modified filter and may be patterned in the filter in order to modify the performance of the filter. In some embodiments, open-flow channels may be provided at locations that are normally are subject to relatively low exhaust gas flow {e.g., locations at the outer diameter of the filter), which may result in a decreased pressure drop. The modified filters may include a percentage of open- flow channels relative to total channels that does not significantly impair filtering efficiency as illustrated in Figure 3. [0035] For a filter subject to low speed laminar flow with a parabolic exhaust velocity distribution, channels may be opened in such a pattern that the number of the open channels is inversely related to the pipe velocity as shown in Figure 4, and according to the equation N = kl/(Velocity Profile), where N is the number of open channels within a selected area of the filter and kl is a coefficient which can be tuned to optimize the flow distribution. If a filter is located right after a fitting due to space constrain (such as an elbow or an expansion tube) and the fitting causes sudden changes of flow pathlines, channels may be opened against the velocity profile as shown in Figure 5. For fully developed turbulent flows as shown in Figure 6, the following equation may be used to determine a suitable number of open channels for a given area of the filter: N = k2/(Velocity Profile)I/n, where N is the number of open channels within a selected area of the filter; k2 is a coefficient which can be tuned to optimize the flow distribution; and 1 [0036] The modified filters typically include open-flow channels {i.e., unplugged channels). Optionally, the modified filters may include double-plugged channels {i.e., channels having both an upstream plug 34 and a downstream plug 36, as compared to a single-plug channel having only an upstream plug 34 or a downstream plug 36 and defining a wall-flow channel). The unplugged channels and double-plugged channels may be arranged in any suitable formation. Figure 7 illustrates an arrangement in filter having a peripheral ring of double-plugged channels (either 2-cells or 4-cells wide) adjacent to a ring of unplugged channels (either 2-cells or 4-cells wide). For both double-plugged and unplugged cells (optionally in a ring formation), the wall thickness may be higher than single- plugged cells. A filter having double-plugged or unplugged cells with thicker walls may be more robust to handling, regeneration thermal shock, or ringoff failures. In some embodiments of the filters, double-plugged and unplugged cells may have lesser wall porosity relative to single-plugged cells. [0037] A variety of distributions of plugs in the filters could be used to change gas flow, particularly in combination with a series filtration approach. For example, the disclosed filters may be used in a modified diesel particulate filter design to lower engine back pressure and improve soot distribution. In some embodiments, the design includes two filter elements, where the first filter element may have lower filtration efficiency than the second filter element. [0038] By modifying the element architecture, it may be possible to improve the soot distribution on the filter, reduce thermal gradients during an active regeneration, and increase the level of passive regeneration. In a modified diesel particulate filter design, the filter element may be broken into two or more elements with progressively increasing filtration efficiency. Commonly, DPFs have about 90% filtration efficiency. In a modified diesel particulate filter design, the filter may be separated into two separate filter elements. For example, the first filter may have a filtering efficiency on the order of 50-60%, while the second filter may have a filtration efficiency of about 90%. The reduction in filtration efficiency for the first filter may be attained by several methods. One method may be to reduce the percentage of plugged channels, either randomly, or in a specific pattern in the first filter. A second method may be to increase the pore diameter of the filter material of the first filter, thus allowing more soot particles to pass through the walls. A third method may be to use a high cell density flow-through element for the first filter (e.g., an element having a cell density greater than about 200 cells per square inch). [0039] Potential methods for decreasing filtration efficiency for the first filter element could be to use a high cell density flow-through element, higher pore size filters, or selective plugging of channels (partially plugged filter). The use of a partially plugged filter may be used to affect the flow distribution and temperature distribution within a modified diesel particulate filter design. [0040] A filtration system, as shown in Figure 8, incorporates a DOC to heat the exhaust gases to burn the accumulated soot in the filters. The first filter may also incorporate a catalyst to burn hydrocarbons not burned in the DOC and to oxidize NO (nitric oxide). Optionally, the second filter may incorporate a catalyst. [0041] Catalysts, as described herein, may include oxidation catalysts and reduction catalysts. Catalysts may include NOx adsorbers (e.g., where x is 1 or 2). In some embodiments, the combustion product of diesel particulate matter is a soot oxidation product, e.g., CO, and the noted downstream NOx adsorber is regenerated with the assistance of CO derived from the oxidation of the diesel particulate matter. The downstream NOx adsorber is provided in sufficiently close proximity to the diesel particulate filter to maximize the probability that the CO will assist in regeneration of the NOx adsorber. Preferably, the CO assists NOx adsorber regeneration by releasing stored NOx, for example according to the reaction Ba(N03)2 + 3CO -> BaC03 + 2NO + 2C02. Furthermore, the CO preferably assists in regeneration of the NOx adsorber by reducing the released NOx to benign N2, for example according to the reaction NO + CO -> 1/2N2 + C02. Furthermore, the CO preferably assists in regeneration of the NOx adsorber by oxidizing CO (either through one of the above two reactions, or by reaction with 02 over the noble metal component of the NOx adsorber according to CO + l/202 -$ C02) with substantial heat release. Close proximity of the particulate filter to the NOx adsorber allows efficient utilization of this heat to assist regeneration of the filters and systems disclosed herein. [0042] In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. The different configurations, systems and method steps described herein may be used alone or in combination with other configurations, systems and method steps. It is to be expected that various equivalents, alternatives and modifications are possible within the scope of the appended claims. WE CLAIM 1. An exhaust aftertreatment filter for filtering engine exhaust flowing along an axial direction, said filter being composed of monolithic regenerable material and comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends, said wall segments defining axial flow channels therebetween, said wall segments being alternately sealed to each other by a first set of plugs to define a first set of flow channels closed by said plugs and having open downstream ends, and a second set of flow channels interdigitated with said first set of flow channels and having open upstream ends, said wall segments being alternately sealed to each other by a second set of plugs closing said second set of flow channels, said wall segments further defining a third set of channels having open flow, said third set of channels having selected positions in said filter for reducing back pressure on said filter during operation while maintaining filtering efficiency. 2. The filter of claim 1, wherein said channels of said third set represent at least about 1% of total channels in said filter and said maintained filtering efficiency is at least about 75%. 3. The filter of claim 1, further comprising a coating material along at least some of said wall segments defining said third set of flow channels. 4. The filter of claim 3, wherein said coating material comprises a catalytic agent for at least one reaction selected from the group consisting of 2CO + O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O, where x is an integer selected from 1-25 and y is an integer selected from 0-52. 5. The filter of claim 1, wherein said wall segments defining said third set of flow channels have an average thickness greater than an average thickness of said wall segments defining said first set of flow channels or said wall segments defining said second set of flow channels. 6. The filter of claim 1, said wall segments further defining a fourth set of flow channels having closed downstream ends and closed upstream ends, said fourth set of flow channels forming a peripheral ring in said filter. 7. The filter of claim 6, wherein said wall segments further defining said fourth set of flow channels have an average thickness greater than an average thickness of said wall segments defining said first set of flow channels or said wall segments defining said second set of flow channels. 8. The filter of claim 6, wherein said third set of flow channels are arranged in a ring adjacent to the peripheral ring formed by the fourth set of flow channels. 9. The filter of claim 1, wherein said third set of flow channels are distributed in said filter in a gradient, said filter having an increasing concentration of open channels in sections located at peripheral positions in said filter as compared to sections located at central positions of said filter. 10. The filter of claim 1, wherein said filter is composed of a ceramic material selected from the group consisting of cordierite, silicon carbide, mullite, and aluminum titanate. 11. An exhaust aftertreatment filter for filtering engine exhaust flowing along an axial direction, said filter being composed of monolithic regenerable material and comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends, said wall segments defining axial flow channels therebetween, said wall segments being alternately sealed to each other by a first set of plugs to define a first set of flow channels closed by said plugs and having open downstream ends, and a second set of flow channels interdigitated with said first set of flow channels and having open upstream ends, said wall segments being alternately sealed to each other by a second set of plugs closing said second set of flow channels, said wall segments further defining a third set of channels having open flow, said third set of channels having selected positions in said filter for reducing high temperature in said filter during operation while maintaining filtering efficiency. 12. An exhaust aftertreatment filter for filtering engine exhaust flowing along an axial direction, said filter being composed of monolithic regenerable material and comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends, said wall segments defining axial flow channels therebetween, said wall segments being alternately sealed to each other by a first set of plugs to define a first set of flow channels closed by said plugs and having open downstream ends, and a second set of flow channels interdigitated with said first set of flow channels and having open upstream ends, said wall segments being alternately sealed to each other by a second set of plugs closing said second set of flow channels, said wall segments further defining a third set of channels having open flow, said third set of channels having selected positions in said filter for increasing velocity of exhaust flow through said filter during operation while maintaining filtering efficiency. 13. An exhaust aftertreatment system for filtering engine exhaust flowing along an axial direction, said system comprising in series along said axial direction: a diesel oxidation catalyst, the exhaust aftertreatment filter of claim 1, and a second filter. 14. An exhaust aftertreatment system for filtering engine exhaust flowing along an axial direction, said system comprising in series along said axial direction: a diesel oxidation catalyst, the exhaust aftertreatment filter of claim 11, and a second filter. 15. An exhaust aftertreatment system for filtering engine exhaust flowing along an axial direction, said system comprising in series along said axial direction: a diesel oxidation catalyst, the exhaust aftertreatment filter of claim 12, and a second filter. 16. An exhaust aftertreatment system for filtering engine exhaust flowing along an axial direction, said system comprising at least a first and a second filter arranged in series along said axial direction, said first filter being positioned upstream of said second filter and having a lower filtration efficiency than said second filter, wherein at least one of said first and second filter is composed of monolithic regenerable filtration material, said material comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends, said wall segments defining axial flow channels therebetween, said wall segments being alternately sealed to each other by a first set of plugs to define a first set of flow channels closed by said plugs and having open downstream ends, and a second set of flow channels interdigitated with said first set of flow channels and having open upstream ends, said wall segments being alternately sealed to each other by a second set of plugs closing said second set of flow channels, and said wall segments further defining a third set of channels having open flow. 17. The system of claim 16, wherein said first filter is composed of said filtration material. 18. The system of claim 16, wherein said second filter is composed of said filtration material. 19. The system of claim 16, wherein said first filter and said second filter are composed of said filtration material. 20. The system of claim 16, wherein said first filter and said second filter are composed of a ceramic material selected from cordierite, silicon carbide, mullite, and aluminum titanate. 21. The system of claim 16, wherein said wall segments further defining at least one of said first, second, or third set of flow channels comprise a coating material. 22. The system of claim 21, wherein said coating material comprises a catalytic agent for a reaction wherein nitric oxide is oxidized to nitrogen dioxide. 23. The system of claim 16, wherein the wall segments defining said third set of flow channels have an average thickness greater than an average thickness of said wall segments defining said first set of flow channels or said wall segments defining said second set of flow channels. 24. The system of claim 17, said wall segments of said first filter further defining a fourth set of flow channels having closed downstream ends and closed upstream ends, said fourth set of flow channels forming a peripheral ring in said first filter. 25. The system of claim 24, wherein said third set of flow channels are arranged in a ring adjacent to the peripheral ring formed by the fourth set of flow channels. 26. The system of claim 17, wherein said third set of flow channels are distributed in said first filter in a gradient, said filter having an increasing concentration of open channels in sections located at peripheral positions in said first filter as compared to sections located at central positions of said first filter. 27. The system of claim 16, wherein said first filter is composed of a first filtration material and said second filter is composed of a second filtration material that is different than said first filtration material. 28. The system of claim 27, wherein said first filtration material has a larger pore diameter than said second filtration material. 29. The system of claim 16, wherein said first filter has a cell density of greater than 300 per square inch and comprises flow through channels. 30. The system of claim 16, wherein said second filter has a cell density of greater than 200 per square inch. 31. The system of claim 16, wherein said first filter has a soot filtration efficiency of at least about 50% and said second filter has a soot filtration efficiency of at least about 90%. 32. The system of claim 32, wherein said first filter has a soot filtration efficiency of about 50-60%. 33. The system of claim 16, further comprising a diesel oxidation catalyst element arranged in series with said first filter and said second filter along said axial direction, said diesel oxidation catalyst element being positioned upstream of said first filter and comprising a catalytic agent for at least one reaction selected from the group consisting of 2CO + O2 → 2CO2; 2NO + O2 →2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O, where x is an integer selected from 1-25 and y is an integer selected from 0-52. 34. The system of claim 33, wherein said first filter comprises a catalytic agent for at least one reaction selected from the group consisting of 2CO + O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O, where x is an integer selected from 1-25 and y is an integer selected from 0-52. 35. The system of claim 33, wherein said second filter comprises a catalytic agent for at least one reaction selected from the group consisting of 2CO + O2 →2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O, where x is an integer selected from 1-25 and y is an integer selected from 0-52. 36. A method of manufacturing a modified exhaust aftertreatment filter for filtering engine exhaust flowing along an axial direction and having modified performance as compared to an unmodified filter, said filter being composed of monolithic regenerable material, said material comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends, said wall segments defining axial flow channels therebetween, said wall segments being alternately sealed to each other by a first set of plugs to define a first set of flow channels closed by said plugs, and a second set of flow channels interdigitated with said first set of flow channels and having open upstream ends, said wall segments being alternately sealed to each other by a second set of plugs closing said second set of flow channels, said first set of flow channels having open downstream ends, said method comprising selecting and removing at least one plug of said first set of plugs and said second set of plugs to provide open flow in at least one channel of said modified filter, said selected plug being located at a position in said unmodified filter whereby removing said plug reduces back pressure on said modified filter during operation while maintaining filtering efficiency. 37. A method of manufacturing a modified exhaust aftertreatment filter for filtering engine exhaust flowing along an axial direction and having modified performance as compared to an unmodified filter, said filter being composed of monolithic regenerable material, said material comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends, said wall segments defining axial flow channels therebetween, said wall segments being alternately sealed to each other by a first set of plugs to define a first set of flow channels closed by said plugs, and a second set of flow channels interdigitated with said first set of flow channels and having open upstream ends, said wall segments being alternately sealed to each other by a second set of plugs closing said second set of flow channels, said first set of flow channels having open downstream ends, said method comprising selecting and removing at least one plug of said first set of plugs and said second set of plugs to provide open flow in at least one channel of said modified filter, said selected plug being located at a position in said unmodified filter which is subject to relatively high temperature during operation of said unmodified filter as compared to a non-selected plug, thereby reducing said relatively high temperature during operation of said modified filter while maintaining filtering efficiency. 38. A method of manufacturing a modified exhaust aftertreatment filter for filtering engine exhaust flowing along an axial direction and having modified performance as compared to an unmodified filter, said filter being composed of monolithic regenerable material, said material comprising an axially extending filter element having wall segments extending axially between upstream and downstream ends, said wall segments defining axial flow channels therebetween, said wall segments being alternately sealed to each other by a first set of plugs to define a first set of flow channels closed by said plugs, and a second set of flow channels interdigitated with said first set of flow channels and having open upstream ends, said wall segments being alternately sealed to each other by a second set of plugs closing said second set of flow channels, said first set of flow channels having open downstream ends, said method comprising selecting and removing at least one plug of said first set of plugs and said second set of plugs to provide open flow in at least one channel of said modified filter, said selected plug being located at a position in said unmodified filter where exhaust flow exhibits relatively low velocity during operation of said unmodified filter as compared to a position of a non-selected plug, thereby increasing said relatively low velocity during operation of said modified filter while maintaining filtering efficiency. 39. A modified exhaust aftertreatment filter for filtering engine exhaust prepared by the method of claim 36. 40. A modified exhaust aftertreatment filter for filtering engine exhaust prepared by the method of claim 37. 41. A modified exhaust aftertreatment filter for filtering engine exhaust prepared by the method of claim 38. Modified exhaust aftertreatment filters for filtering engine exhaust are provided as are methods of design and manufacturing modified exhaust aftertreatment filters. The modified filters are subject to reduced back pressure and reduced thermal gradients experienced during regeneration as compared to unmodified filters. The modified filters include flow-through channels obtained by unplugging channels which normally are plugged in an unmodified filter.

Full Text

TUNING PARTICULATE FILTER PERFORMANCE THROUGH SELECTIVE
PLUGGING AND USE OF MULTIPLE PARTICULATE FILTERS TO REDUCE
EMISSIONS AND IMPROVE THERMAL ROBUSTNESS
BACKGROUND
[0001] The invention relates to exhaust aftertreatment filters for
filtering exhaust from internal combustion engines, including diesel engines.
[0002] Exhaust aftertreatment filters for diesel engines are known in the
prior art. The filter traps contaminant particulate present in exhaust, and in order to
remove the trapped particulate, the filter is heated to burn-off the trapped contaminant
particulate as gas. Accordingly, the filter is regenerable and is composed of material
on which trapped contaminant particulate from the engine exhaust is removed by
addition of heat. Commonly used particulate filter materials include cordierite,
silicon carbide, mullite, or aluminum titanate, which are manufactured as filter
elements to capture the soot and other particulate generated by the engine.
[0003] Diesel particulate filters (DPF) are subject to high temperatures
during use. The design of the DPF consists of a honeycomb structure with opposing
channels blocked to force exhaust gases to flow through the porous channel walls,
while trapping soot. The soot (composed primarily of carbon) accumulates in the
DPF and must be removed periodically. Typically, the soot is removed from the filter
by oxidation reactions between carbon in the soot and either oxygen (i.e., burning) or
nitrogen dioxide, both of which are constituents of the exhaust. The carbon may react
with oxygen or nitrogen dioxide according to the following reactions:

Reaction (1) is the primary reaction that occurs during an active regeneration.
Reaction (2) is the primary reaction that occurs during passive regeneration. Heat is a
significant by-product of the reaction shown in Reaction (1) and, if not controlled, can
cause thermal runaway of the filter, leading to fractures and/or melting of the filter
and rendering it ineffective as a filter.

[0004] Although thermal runaway may be prevented by controlling the
rate at which carbon is burned in the filter, nonetheless the DPF may be subject to
thermal gradients caused by differential heating patterns, which also may lead to
fractures. Differential heating may occur during active regeneration of the filter
where carbon may be unequally distributed, either radially or axially, within the DPF.
For example, carbon may be more highly distributed in the DPF at locations where
the largest volume of exhaust passes through the filter (i.e., at locations where exhaust
velocity is highest). These locations may exhibit a relatively high temperature during
active regeneration as compared to other locations.
[0005] Although carbon distribution may be altered by modifying the
design of a filter, particulate filter manufacturers are hampered by material strength
issues that limit the maximum porosity that can be obtained in an extruded
honeycomb structure. Filter manufacturers typically design filters that have the
lowest back pressure and suitable filtration efficiency as required for a particular
engine. However, ceramic filter manufacturers have difficulty designing filters that
have the lowest back pressure and suitable filtration efficiency without greatly
weakening the honeycomb structures. Furthermore, it is commonly believed that
filtration efficiency drops precipitously with even a single unblocked or broken
channel in the honeycomb structure of the filter, even though unblocked channels
might lower back pressure.
[0006] Therefore, there is a need for filters having modified design
characteristics in order to minimize back pressure and thermal gradients during
regeneration. Furthermore, it is desirable that these modified design characteristics
can be combined with control techniques to create filters that are more durable and
resistant to structural damage which may occur during use of the filter (e.g., during
regeneration).
SUMMARY
[0007] Disclosed are exhaust aftertreatment filters and systems for
filtering engine exhaust flowing along an axial direction. The filters include a

plurality of flow channels comprising a plurality of wall-flow channels and a
plurality of flow-through channels. The plurality of wall-flow channels and the
plurality of flow-through channels may be positioned in the filter in order to
improve back pressure, reduce temperature, and/or increase velocity of exhaust
through the filter while maintaining filtering efficiency. Also disclosed are methods
of manufacturing filters having modified performance characteristics.
[0008] The disclosed filters may be regenerable and composed of
monolithic material on which trapped contaminant particulate from the engine
exhaust is removed by addition of heat. In some embodiments, the filters are
composed of material comprising an axially extending filter element having wall
segments extending axially between upstream and downstream ends. The wall
segments may define a plurality of axial flow channels including wall-flow
channels and flow-through channels. Typically, the wall segments define a first set
of first wall-flow channels where the wall segments are alternately sealed to each
other by a first set of plugs to define a first set of wall-flow channels closed by the
plugs and having open downstream ends; and the wall segments define a set of
second wall-flow channels interdigitated with the first set of flow channels and
having open upstream ends, the wall segments being alternately sealed to each other
by a second set of plugs closing the second set of flow channels. The wall
segments further define a third set of flow-through channels positioned in the filter
for modifying the performance of the filter (e.g., achieving reduced back pressure,
reduced temperature, and/or increased velocity of exhaust) while maintaining
adequate filtering efficiency (e.g., efficiency of at least about 95%, 90%, 85%,
80%, 75%, 70%, 60%, or 50% for soot concentrations of about 2.0 gL or less).
[0009] The number of flow-through channels may be adjusted to
achieve a desired filter performance. In some embodiments, the number of flow-
through channels represents at least about 1% of total channels (or at least about 2%
of total channels in some embodiments) while the maintained filtering efficiency is
at least about 90% (e.g., for soot concentrations of about 2.0 g/L or less). In other

embodiments, the number of flow-through channels represents at least about 5% of
total channels (or at least about 10% of total channels in some embodiments) while
the maintained filtering efficiency is at least about 80% (e.g., for soot
concentrations of about 2.0 g/L or less).
[0010] The filter may include a coating. For example, the wall
segments of the filter may be coated with a coating material, which optionally may
include a catalytic agent. In some embodiments of the filters, wall segments
defining flow-through channels are coated with a coating material that comprises a
catalytic agent. Catalytic agents may include oxidizing catalysts and reducing
catalysts. Catalytic agents may include catalysts for at least one reaction selected
from the group consisting of C + O2 → CO2 and 2NO + O2 → 2NO2. Catalytic
agents may include noble metals (e.g., platinum, rhodium, and palladium.)
[0011] The wall segments defining the flow-through channels may
have an average thickness different than an average thickness of the wall segments
defining the first set of wall-flow channels or the wall segments defining the second
set of wall-flow channels. In some embodiments of the filters, the wall segments
defining the flow-through channels have an average thickness greater than an
average thickness of the wall segments defining the first set of wall-flow channels
or the wall segments defining the second set of wall-flow channels.
[0012] The flow-through channels may be distributed in the filter in
any suitable arrangement. In some embodiments, the flow-through channels are
distributed in the filter in a gradient, where the filter has an increasing concentration
of open channels in sections located at peripheral positions in the filter as compared
to sections located at central positions of the filter.
[0013] The filter may include wall segments further defining a fourth
set of channels having closed downstream ends and closed upstream ends (i.e.,
closed channels). The closed channels may be positioned in the filter in order to
reduce physical damage to the periphery of the filter. In some embodiments of the
filters, the closed channels form a peripheral ring in the filter. Optionally, the wall

segments defining the closed channels have an average thickness greater than an
average thickness of the wall segments defining the first set of wall-flow channels
or the wall segments defining the second set of wall-flow channels. In further
embodiments, the flow-through channels may be arranged in a ring adjacent to a
peripheral ring formed by the closed channels.
[0014] The filter may be composed of any suitable material. In some
embodiments, the filter is composed of a ceramic material, examples of which are
cordierite, silicon carbide, mullite, and aluminum titanate. The filter may be
monolithic (i.e., composed of single piece of material), or segmented (i.e.,
composed of multiple pieces of material bonded together).
[0015] The filters may be utilized alone or may be combined with
additional components in an exhaust aftertreatment system for filtering engine
exhaust flowing along an axial direction. For example, an exhaust aftertreatment
system as disclosed herein may include the following components in series along an
axial direction: a diesel oxidation catalyst, a first filter as disclosed herein, and
optionally a second filter.
[0016] The disclosed exhaust aftertreatment systems for filtering
engine exhaust flowing along an axial direction may comprise at least a first filter
and a second filter arranged in series along the axial direction, where the first filter
is positioned upstream of the second filter and has a lower filtration efficiency than
the second filter. The first and second filter may be composed of a monolithic
ceramic material such as cordierite, silicon carbide, mullite, and aluminum titanate
(optionally having a honeycomb or ceramic bead structure). The first filter, second
filter, or both filters may include a coating material that comprises a catalytic agent
(e.g., an oxidizing catalyst for converting nitric oxide to nitrogen dioxide).
Typically, at least one of the first and second filters includes a plurality of wall-flow
channels and flow-through channels. Optionally, at least one of the first and second
filters includes closed channels. The flow-through channels and closed channels, if
present, may be arranged in any suitable formation, including a formation where a

peripheral ring of closed channels surrounds an adjacent ring of flow-through
channels.
[0017] In the disclosed systems, the first filter may be composed of a
first regenerable material and the second filter may be composed of a second
regenerable material that is different than the first regenerable material. For
example, the first regenerable material may have a larger pore diameter than the
second regenerable material. In some embodiments, the first filter is a high cell
density flow-through element having a cell density of greater than 200 per square
inch. In other embodiments, the first filter may be a partially plugged filter. In
further embodiments, the second filter may have a high cell density (e.g., a cell
density of greater than 200 per square inch), which may be higher than the first
filter.
[0018] In the disclosed systems, the first filter may have a soot
filtration efficiency that is lower than the second filter. In some embodiments, the
first filter has a soot filtration efficiency of at least about 50% (e.g., about 50-60%
in some embodiments) and the second filter has a soot filtration efficiency of at
least about 90% (or at least about 95% in some embodiments).
[0019] The disclosed systems optionally include a catalytic converter
element, such as a diesel oxidation catalyst element, which may be arranged in
series with the first filter and the second filter along the axial direction. The
catalytic converter element may be positioned upstream of the first filter and may
include an oxidizing catalyst for at least one reaction selected from the group
consisting of 2CO + O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 →
(4x)CO2 + (2y)H2O (where in some embodiments x is an integer from 1-25 and y is
an integer from 0-52). In some embodiments, the first filter may include an
oxidizing catalyst for at least one reaction selected from the group consisting of
2CO + O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 +
(2y)H2O (where in some embodiments x is an integer from 1-25 and y is an integer
from 0-52). Optionally, the second filter may include an oxidizing catalyst for at

least one reaction selected from the group consisting of 2CO + O2 → 2CO2; 2NO +
O2 → 2NO2; and 4CxHy + (4x+y)02 → (4x)CO2 + (2y)H2O (where in some
embodiments x is an integer from 1-25 and y is an integer from 0-52).
[0020] Also disclosed are methods for manufacturing a modified
exhaust aftertreatment filter for filtering engine exhaust flowing along an axial
direction and having modified performance. The modified filter has a structure as
described herein.
[0021] In some embodiments of the methods of manufacture, an
unmodified filter is composed of regenerable material comprising an axially
extending filter element having wall segments extending axially between upstream
and downstream ends. The wall segments may define a plurality of axial flow
channels including wall-flow channels and flow-through channels. Typically, the
wall segments define a first set of first wall-flow channels where the wall segments
are alternately sealed to each other by a first set of plugs to define a first set of wall-
flow channels closed by the plugs and having open downstream ends; and the wall
segments define a set of second wall-flow channels interdigitated with the first set
of flow channels and having open upstream ends, the wall segments being
alternately sealed to each other by a second set of plugs closing the second set of
flow channels.
[0022] The methods of manufacture typically include selecting and
removing at least one plug of the first set of plugs and the second set of plugs to
provide open flow in at least one channel of the modified filter. In some
embodiments, the selected plug for removal is located at a position in the unmodified
filter whereby removing the plug reduces back pressure on the modified filter during
operation while maintaining filtering efficiency. In other embodiments, the selected
plug for removal is located at a position in the unmodified filter which is subject to
relatively high temperature during operation of the unmodified filter as compared to a
non-selected plug, thereby reducing the relatively high temperature during operation
of the modified filter while maintaining filtering efficiency. In further embodiments,

the selected plug for removal is located at a position in the unmodified filter where
exhaust flow exhibits relatively low velocity during operation of the unmodified filter
as compared to a position of a non-selected plug, thereby increasing the relatively low
velocity during operation of the modified filter while maintaining filtering efficiency.
[0023] Also disclosed are exhaust aftertreatment filters for filtering
engine exhaust prepared by the disclosed methods of manufacture. In some
embodiments, the filters prepared by the methods of manufacture include a number of
flow-through channels representing at least about 1% of total channels (or at least
about 2% of total channels in some embodiments) while the maintained filtering
efficiency of the modified filter is at least about 90% (or at least about 80% in some
embodiments) (e.g., for soot concentrations of about 2.0 g/L or less).
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Figure 1 is a perspective view of an exhaust aftertreatment filter.
[0025] Figure 2 is a sectional view of the exhaust aftertreatment filter of
Figure 1.
[0026] Figure 3 illustrates the effects on filtering efficiency (top) and
filter restriction (bottom) versus percentage channels open (i.e., percentage flow-
through channels) in a modified filter having a 12 inch diameter by 12 inch length
with 200 cells per square inch.
[0027] Figure 4a illustrates the velocity of exhaust through a filter
exhibiting a parabolic flow profile. Figure 4b illustrates a potential distribution of
open channels in a filter designed to increase flow at locations of relatively low
exhaust velocity in view of the parabolic flow profile of Figure 4a.
[0028] Figure 5a illustrates the velocity of exhaust through a filter
exhibiting a turning flow profile. Figure 5b illustrates a potential distribution of open
channels in a filter designed to increase flow at locations of relatively low exhaust
velocity in view of the turning flow profile of Figure 5a.
[0029] Figure 6a illustrates the velocity of exhaust through a filter

exhibiting a flat velocity profile. Figure 6b illustrates a potential distribution of open
channels in a filter designed to increase flow at locations of relatively low exhaust
velocity in view of the flat flow profile of Figure 6a.
[0030] Figure 7 illustrates two filters having sample unplugging
patterns {i.e., modified filters). The sample patterns include a 2-cell (or 4-cell)
peripheral ring of double-plugged channels adjacent to a 2-cell (or 4-cell) ring of
flow-through channels.
[0031] Figure 8 illustrates an exhaust aftertreatment system including a
diesel oxidation catalyst (DOC) in series with a first filter element and a second filter
element.
DETAILED DESCRIPTION
[0032] Figure 1 shows an exhaust aftertreatment filter 10 for filtering
exhaust from an internal combustion engine, such as diesel engine 12, flowing
along an axial flow direction 14. Figure 2 shows a section view of the filter of
Figure 1. The filter is composed of particulate filtration material 16, as known in
the prior art, for example ceramic such as a cordierite, silicon carbide, mullite, or
aluminum titanate on which trapped contaminant particulate from the engine
exhaust is removed by addition of heat. The filter 10 includes wall-flow channels
formed by wall segments having an upstream plug 34 or a downstream plug 36.
Contaminant particulate such as soot is trapped and accumulates in the filter, which
trapped contaminant particulate is burned-off during regeneration. The filter
includes a filter body 18 having an outer periphery 20 surrounding a central core 22.
Outer periphery 20 and central core 22 may be subject to differential thermal
expansion during thermal cycling during regeneration, due to outer periphery 20
being cooler than central core 22. For example, Figure 2 shows central hot spot 24,
which is hotter than outer periphery 20, and which may be more dominant at the
downstream side of the filter where particulate contaminant may accumulate and
clog. With or without clogging or a downstream hot spot such as 24, outer

periphery 20 may run cooler than central core 22, as is known. The filter is
typically mounted in a housing 26, such as a stainless steel canister, having a mat
mounting material 28 surrounding the filter body and performing a number of
functions including thermal resistance, dampening of vibration, and resistance to
movement. The mat material is typically compressed between housing 26 and filter
body 18.
[0033] The noted differential thermal expansion between hotter
central core 22 and cooler outer periphery 20 may subject the filter body to
separational axial tensile stress in the axial direction which in turn subjects the filter
body to separational fracture and cracking, for example as shown at fracture or
crack line 30 in Figure 1. It is known by catalyst and filter manufacturers that the
radial compressive stress applied by pressure obtained from an expanding mat
material 28 assists in reducing the fracture probability of filter body 18. However,
the radial compressive stress reduces the probability of fracture along a fracture line
parallel to axis 31 of the filter perpendicular to crack line 30), and does little to
prevent fractures along a fracture or crack line such as 30. Furthermore, the mat
material can degrade over time, resulting in loss of pressure. With larger and
heavier filters and longer lifetimes, particularly for diesel particulate filters in wall-
flow application versus automotive flow-through catalyst application, the noted
pressure and compressive stress applied by mat material 28 will decrease more
rapidly, particularly than that observed for automotive catalysts. Furthermore, an
automotive catalyst can still function after cracking because of its flow-through
application, whereas a contaminant particulate filter loses effectiveness if cracked
because of the bypass flow path created. The filter may include a pre-stressed layer
32 bonded to filter body 18 at outer periphery 20 and is compressively axially pre-
stressed in the opposite axial direction to the noted separational axial tensile stress
to counteract the latter during regenerative heating.
[0034] The modified exhaust aftertreatment filters disclosed herein
include a plurality of flow-through channels obtained by removing an upstream

plug 34 or downstream plug 36 in what otherwise would be a wall-flow channel in
an unmodified filter. The selected flow-through channels may be located at any
suitable position in the modified filter and may be patterned in the filter in order to
modify the performance of the filter. In some embodiments, open-flow channels
may be provided at locations that are normally are subject to relatively low exhaust
gas flow {e.g., locations at the outer diameter of the filter), which may result in a
decreased pressure drop. The modified filters may include a percentage of open-
flow channels relative to total channels that does not significantly impair filtering
efficiency as illustrated in Figure 3.
[0035] For a filter subject to low speed laminar flow with a parabolic
exhaust velocity distribution, channels may be opened in such a pattern that the
number of the open channels is inversely related to the pipe velocity as shown in
Figure 4, and according to the equation N = kl/(Velocity Profile), where N is the
number of open channels within a selected area of the filter and kl is a coefficient
which can be tuned to optimize the flow distribution. If a filter is located right after
a fitting due to space constrain (such as an elbow or an expansion tube) and the
fitting causes sudden changes of flow pathlines, channels may be opened against
the velocity profile as shown in Figure 5. For fully developed turbulent flows as
shown in Figure 6, the following equation may be used to determine a suitable
number of open channels for a given area of the filter: N = k2/(Velocity Profile)I/n,
where N is the number of open channels within a selected area of the filter; k2 is a
coefficient which can be tuned to optimize the flow distribution; and 1 [0036] The modified filters typically include open-flow channels {i.e.,
unplugged channels). Optionally, the modified filters may include double-plugged
channels {i.e., channels having both an upstream plug 34 and a downstream plug
36, as compared to a single-plug channel having only an upstream plug 34 or a
downstream plug 36 and defining a wall-flow channel). The unplugged channels
and double-plugged channels may be arranged in any suitable formation. Figure 7
illustrates an arrangement in filter having a peripheral ring of double-plugged

channels (either 2-cells or 4-cells wide) adjacent to a ring of unplugged channels
(either 2-cells or 4-cells wide). For both double-plugged and unplugged cells
(optionally in a ring formation), the wall thickness may be higher than single-
plugged cells. A filter having double-plugged or unplugged cells with thicker walls
may be more robust to handling, regeneration thermal shock, or ringoff failures. In
some embodiments of the filters, double-plugged and unplugged cells may have
lesser wall porosity relative to single-plugged cells.
[0037] A variety of distributions of plugs in the filters could be used
to change gas flow, particularly in combination with a series filtration approach.
For example, the disclosed filters may be used in a modified diesel particulate filter
design to lower engine back pressure and improve soot distribution. In some
embodiments, the design includes two filter elements, where the first filter element
may have lower filtration efficiency than the second filter element.
[0038] By modifying the element architecture, it may be possible to
improve the soot distribution on the filter, reduce thermal gradients during an active
regeneration, and increase the level of passive regeneration. In a modified diesel
particulate filter design, the filter element may be broken into two or more elements
with progressively increasing filtration efficiency. Commonly, DPFs have about
90% filtration efficiency. In a modified diesel particulate filter design, the filter
may be separated into two separate filter elements. For example, the first filter may
have a filtering efficiency on the order of 50-60%, while the second filter may have
a filtration efficiency of about 90%. The reduction in filtration efficiency for the
first filter may be attained by several methods. One method may be to reduce the
percentage of plugged channels, either randomly, or in a specific pattern in the first
filter. A second method may be to increase the pore diameter of the filter material
of the first filter, thus allowing more soot particles to pass through the walls. A
third method may be to use a high cell density flow-through element for the first
filter (e.g., an element having a cell density greater than about 200 cells per square
inch).

[0039] Potential methods for decreasing filtration efficiency for the
first filter element could be to use a high cell density flow-through element, higher
pore size filters, or selective plugging of channels (partially plugged filter). The use
of a partially plugged filter may be used to affect the flow distribution and
temperature distribution within a modified diesel particulate filter design.
[0040] A filtration system, as shown in Figure 8, incorporates a DOC
to heat the exhaust gases to burn the accumulated soot in the filters. The first filter
may also incorporate a catalyst to burn hydrocarbons not burned in the DOC and to
oxidize NO (nitric oxide). Optionally, the second filter may incorporate a catalyst.
[0041] Catalysts, as described herein, may include oxidation catalysts
and reduction catalysts. Catalysts may include NOx adsorbers (e.g., where x is 1 or
2). In some embodiments, the combustion product of diesel particulate matter is a
soot oxidation product, e.g., CO, and the noted downstream NOx adsorber is
regenerated with the assistance of CO derived from the oxidation of the diesel
particulate matter. The downstream NOx adsorber is provided in sufficiently close
proximity to the diesel particulate filter to maximize the probability that the CO will
assist in regeneration of the NOx adsorber. Preferably, the CO assists NOx adsorber
regeneration by releasing stored NOx, for example according to the reaction
Ba(N03)2 + 3CO -> BaC03 + 2NO + 2C02. Furthermore, the CO preferably assists
in regeneration of the NOx adsorber by reducing the released NOx to benign N2, for
example according to the reaction NO + CO -> 1/2N2 + C02. Furthermore, the CO
preferably assists in regeneration of the NOx adsorber by oxidizing CO (either
through one of the above two reactions, or by reaction with 02 over the noble metal
component of the NOx adsorber according to CO + l/202 -$ C02) with substantial
heat release. Close proximity of the particulate filter to the NOx adsorber allows
efficient utilization of this heat to assist regeneration of the filters and systems
disclosed herein.
[0042] In the foregoing description, certain terms have been used for
brevity, clearness, and understanding. No unnecessary limitations are to be implied

therefrom beyond the requirement of the prior art because such terms are used for
descriptive purposes and are intended to be broadly construed. The different
configurations, systems and method steps described herein may be used alone or in
combination with other configurations, systems and method steps. It is to be expected
that various equivalents, alternatives and modifications are possible within the scope
of the appended claims.

WE CLAIM
1. An exhaust aftertreatment filter for filtering engine exhaust
flowing along an axial direction, said filter being composed of monolithic
regenerable material and comprising an axially extending filter element having wall
segments extending axially between upstream and downstream ends, said wall
segments defining axial flow channels therebetween, said wall segments being
alternately sealed to each other by a first set of plugs to define a first set of flow
channels closed by said plugs and having open downstream ends, and a second set
of flow channels interdigitated with said first set of flow channels and having open
upstream ends, said wall segments being alternately sealed to each other by a
second set of plugs closing said second set of flow channels, said wall segments
further defining a third set of channels having open flow, said third set of channels
having selected positions in said filter for reducing back pressure on said filter
during operation while maintaining filtering efficiency.
2. The filter of claim 1, wherein said channels of said third set
represent at least about 1% of total channels in said filter and said maintained
filtering efficiency is at least about 75%.
3. The filter of claim 1, further comprising a coating material
along at least some of said wall segments defining said third set of flow channels.
4. The filter of claim 3, wherein said coating material comprises a
catalytic agent for at least one reaction selected from the group consisting of 2CO +
O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O,
where x is an integer selected from 1-25 and y is an integer selected from 0-52.
5. The filter of claim 1, wherein said wall segments defining said

third set of flow channels have an average thickness greater than an average
thickness of said wall segments defining said first set of flow channels or said wall
segments defining said second set of flow channels.
6. The filter of claim 1, said wall segments further defining a
fourth set of flow channels having closed downstream ends and closed upstream
ends, said fourth set of flow channels forming a peripheral ring in said filter.
7. The filter of claim 6, wherein said wall segments further
defining said fourth set of flow channels have an average thickness greater than an
average thickness of said wall segments defining said first set of flow channels or
said wall segments defining said second set of flow channels.
8. The filter of claim 6, wherein said third set of flow channels
are arranged in a ring adjacent to the peripheral ring formed by the fourth set of
flow channels.
9. The filter of claim 1, wherein said third set of flow channels
are distributed in said filter in a gradient, said filter having an increasing
concentration of open channels in sections located at peripheral positions in said
filter as compared to sections located at central positions of said filter.
10. The filter of claim 1, wherein said filter is composed of a
ceramic material selected from the group consisting of cordierite, silicon carbide,
mullite, and aluminum titanate.
11. An exhaust aftertreatment filter for filtering engine exhaust
flowing along an axial direction, said filter being composed of monolithic
regenerable material and comprising an axially extending filter element having wall

segments extending axially between upstream and downstream ends, said wall
segments defining axial flow channels therebetween, said wall segments being
alternately sealed to each other by a first set of plugs to define a first set of flow
channels closed by said plugs and having open downstream ends, and a second set
of flow channels interdigitated with said first set of flow channels and having open
upstream ends, said wall segments being alternately sealed to each other by a
second set of plugs closing said second set of flow channels, said wall segments
further defining a third set of channels having open flow, said third set of channels
having selected positions in said filter for reducing high temperature in said filter
during operation while maintaining filtering efficiency.
12. An exhaust aftertreatment filter for filtering engine exhaust
flowing along an axial direction, said filter being composed of monolithic
regenerable material and comprising an axially extending filter element having wall
segments extending axially between upstream and downstream ends, said wall
segments defining axial flow channels therebetween, said wall segments being
alternately sealed to each other by a first set of plugs to define a first set of flow
channels closed by said plugs and having open downstream ends, and a second set
of flow channels interdigitated with said first set of flow channels and having open
upstream ends, said wall segments being alternately sealed to each other by a
second set of plugs closing said second set of flow channels, said wall segments
further defining a third set of channels having open flow, said third set of channels
having selected positions in said filter for increasing velocity of exhaust flow
through said filter during operation while maintaining filtering efficiency.
13. An exhaust aftertreatment system for filtering engine exhaust
flowing along an axial direction, said system comprising in series along said axial
direction: a diesel oxidation catalyst, the exhaust aftertreatment filter of claim 1,
and a second filter.

14. An exhaust aftertreatment system for filtering engine exhaust
flowing along an axial direction, said system comprising in series along said axial
direction: a diesel oxidation catalyst, the exhaust aftertreatment filter of claim 11,
and a second filter.
15. An exhaust aftertreatment system for filtering engine exhaust
flowing along an axial direction, said system comprising in series along said axial
direction: a diesel oxidation catalyst, the exhaust aftertreatment filter of claim 12,
and a second filter.
16. An exhaust aftertreatment system for filtering engine exhaust
flowing along an axial direction, said system comprising at least a first and a second
filter arranged in series along said axial direction, said first filter being positioned
upstream of said second filter and having a lower filtration efficiency than said
second filter, wherein at least one of said first and second filter is composed of
monolithic regenerable filtration material, said material comprising an axially
extending filter element having wall segments extending axially between upstream
and downstream ends, said wall segments defining axial flow channels
therebetween, said wall segments being alternately sealed to each other by a first set
of plugs to define a first set of flow channels closed by said plugs and having open
downstream ends, and a second set of flow channels interdigitated with said first set
of flow channels and having open upstream ends, said wall segments being
alternately sealed to each other by a second set of plugs closing said second set of
flow channels, and said wall segments further defining a third set of channels
having open flow.
17. The system of claim 16, wherein said first filter is composed of
said filtration material.

18. The system of claim 16, wherein said second filter is composed
of said filtration material.
19. The system of claim 16, wherein said first filter and said
second filter are composed of said filtration material.
20. The system of claim 16, wherein said first filter and said
second filter are composed of a ceramic material selected from cordierite, silicon
carbide, mullite, and aluminum titanate.
21. The system of claim 16, wherein said wall segments further
defining at least one of said first, second, or third set of flow channels comprise a
coating material.
22. The system of claim 21, wherein said coating material
comprises a catalytic agent for a reaction wherein nitric oxide is oxidized to
nitrogen dioxide.
23. The system of claim 16, wherein the wall segments defining
said third set of flow channels have an average thickness greater than an average
thickness of said wall segments defining said first set of flow channels or said wall
segments defining said second set of flow channels.
24. The system of claim 17, said wall segments of said first filter
further defining a fourth set of flow channels having closed downstream ends and
closed upstream ends, said fourth set of flow channels forming a peripheral ring in
said first filter.

25. The system of claim 24, wherein said third set of flow channels
are arranged in a ring adjacent to the peripheral ring formed by the fourth set of
flow channels.
26. The system of claim 17, wherein said third set of flow channels
are distributed in said first filter in a gradient, said filter having an increasing
concentration of open channels in sections located at peripheral positions in said
first filter as compared to sections located at central positions of said first filter.
27. The system of claim 16, wherein said first filter is composed of
a first filtration material and said second filter is composed of a second filtration
material that is different than said first filtration material.
28. The system of claim 27, wherein said first filtration material
has a larger pore diameter than said second filtration material.
29. The system of claim 16, wherein said first filter has a cell
density of greater than 300 per square inch and comprises flow through channels.
30. The system of claim 16, wherein said second filter has a cell
density of greater than 200 per square inch.
31. The system of claim 16, wherein said first filter has a soot
filtration efficiency of at least about 50% and said second filter has a soot filtration
efficiency of at least about 90%.
32. The system of claim 32, wherein said first filter has a soot
filtration efficiency of about 50-60%.

33. The system of claim 16, further comprising a diesel oxidation
catalyst element arranged in series with said first filter and said second filter along
said axial direction, said diesel oxidation catalyst element being positioned
upstream of said first filter and comprising a catalytic agent for at least one reaction
selected from the group consisting of 2CO + O2 → 2CO2; 2NO + O2 →2NO2; and
4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O, where x is an integer selected from 1-25
and y is an integer selected from 0-52.
34. The system of claim 33, wherein said first filter comprises a
catalytic agent for at least one reaction selected from the group consisting of 2CO +
O2 → 2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O,
where x is an integer selected from 1-25 and y is an integer selected from 0-52.
35. The system of claim 33, wherein said second filter comprises a
catalytic agent for at least one reaction selected from the group consisting of 2CO +
O2 →2CO2; 2NO + O2 → 2NO2; and 4CxHy + (4x+y)O2 → (4x)CO2 + (2y)H2O,
where x is an integer selected from 1-25 and y is an integer selected from 0-52.
36. A method of manufacturing a modified exhaust aftertreatment
filter for filtering engine exhaust flowing along an axial direction and having
modified performance as compared to an unmodified filter, said filter being
composed of monolithic regenerable material, said material comprising an axially
extending filter element having wall segments extending axially between upstream
and downstream ends, said wall segments defining axial flow channels therebetween,
said wall segments being alternately sealed to each other by a first set of plugs to
define a first set of flow channels closed by said plugs, and a second set of flow
channels interdigitated with said first set of flow channels and having open upstream
ends, said wall segments being alternately sealed to each other by a second set of
plugs closing said second set of flow channels, said first set of flow channels having

open downstream ends, said method comprising selecting and removing at least one
plug of said first set of plugs and said second set of plugs to provide open flow in at
least one channel of said modified filter, said selected plug being located at a position
in said unmodified filter whereby removing said plug reduces back pressure on said
modified filter during operation while maintaining filtering efficiency.
37. A method of manufacturing a modified exhaust aftertreatment
filter for filtering engine exhaust flowing along an axial direction and having
modified performance as compared to an unmodified filter, said filter being
composed of monolithic regenerable material, said material comprising an axially
extending filter element having wall segments extending axially between upstream
and downstream ends, said wall segments defining axial flow channels therebetween,
said wall segments being alternately sealed to each other by a first set of plugs to
define a first set of flow channels closed by said plugs, and a second set of flow
channels interdigitated with said first set of flow channels and having open upstream
ends, said wall segments being alternately sealed to each other by a second set of
plugs closing said second set of flow channels, said first set of flow channels having
open downstream ends, said method comprising selecting and removing at least one
plug of said first set of plugs and said second set of plugs to provide open flow in at
least one channel of said modified filter, said selected plug being located at a position
in said unmodified filter which is subject to relatively high temperature during
operation of said unmodified filter as compared to a non-selected plug, thereby
reducing said relatively high temperature during operation of said modified filter
while maintaining filtering efficiency.
38. A method of manufacturing a modified exhaust aftertreatment
filter for filtering engine exhaust flowing along an axial direction and having
modified performance as compared to an unmodified filter, said filter being
composed of monolithic regenerable material, said material comprising an axially

extending filter element having wall segments extending axially between upstream
and downstream ends, said wall segments defining axial flow channels therebetween,
said wall segments being alternately sealed to each other by a first set of plugs to
define a first set of flow channels closed by said plugs, and a second set of flow
channels interdigitated with said first set of flow channels and having open upstream
ends, said wall segments being alternately sealed to each other by a second set of
plugs closing said second set of flow channels, said first set of flow channels having
open downstream ends, said method comprising selecting and removing at least one
plug of said first set of plugs and said second set of plugs to provide open flow in at
least one channel of said modified filter, said selected plug being located at a position
in said unmodified filter where exhaust flow exhibits relatively low velocity during
operation of said unmodified filter as compared to a position of a non-selected plug,
thereby increasing said relatively low velocity during operation of said modified filter
while maintaining filtering efficiency.
39. A modified exhaust aftertreatment filter for filtering engine
exhaust prepared by the method of claim 36.
40. A modified exhaust aftertreatment filter for filtering engine
exhaust prepared by the method of claim 37.
41. A modified exhaust aftertreatment filter for filtering engine
exhaust prepared by the method of claim 38.

Modified exhaust aftertreatment filters for filtering engine exhaust are
provided as are methods of design and manufacturing modified exhaust
aftertreatment filters. The modified filters are subject to reduced back pressure and
reduced thermal gradients experienced during regeneration as compared to
unmodified filters. The modified filters include flow-through channels obtained by
unplugging channels which normally are plugged in an unmodified filter.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=0SLeLA23f+4Eun6alI9PYQ==&loc=wDBSZCsAt7zoiVrqcFJsRw==

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Patent Number

271331

Indian Patent Application Number

604/KOLNP/2010

PG Journal Number

08/2016

Publication Date

19-Feb-2016

Grant Date

17-Feb-2016

Date of Filing

16-Feb-2010

Name of Patentee

CUMMINS FILTRATION IP, INC.

Applicant Address

1400-73RD AVENUE NE, MINNEAPOLIS, MN 55432

Inventors:

#	Inventor's Name	Inventor's Address
1	MATTHEW L. ANDERSON	6010 CONESTOGA TRAIL COLUMBUS, IN 47203
2	THOMAS M. YONUSHONIS	4412 MALLARD POINT COLUMBUS, IN 47201
3	WILLIAM C. HABERKAMP	511 NORTH DIXIE AVENUE COOKEVILLE, TN 38501-2631
4	Z. GERALD LIU	917 TRAMORE TRAIL MADISON, WI 53717
5	BRYAN E. BLACKWELL	2747 S 25TH ST. W FRANKLIN, IN 46131
6	ROGER D. ENGLAND	706 STERLING DRIVE CHARLESTON, SC 29412
7	MATTHEW P. HENRICHSEN	1440 GUTHRIE WAY APPLE VALLEY, MN 55124

PCT International Classification Number

B01D 51/00

PCT International Application Number

PCT/US2008/065195

PCT International Filing date

2008-05-30

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	11/836,233	2007-08-09	U.S.A.