Patent Application
20080110341
The invention relates generally to ceramic honeycomb articles, and more particularly to systems and methods for purifying diesel exhaust gases including such honeycomb articles. More specifically, the invention relates to flow-through honeycomb substrates and to methods and systems including combinations of flow-through substrates and wall-flow particulate filters.
Combustion of diesel fuel produces particulates including soot. These particulates are in addition to traditional fuel combustion emissions such as carbon monoxide, hydrocarbons, and nitrogen oxides. Wall-flow particulate filters are often used in diesel engine systems to remove particulates from exhaust gas. These wall-flow particulate filters are typically made of a honeycomb substrate with parallel flow channels and internal porous walls. The flow channels are plugged, usually in a checkerboard pattern, so that exhaust gas, once inside the honeycomb substrate, is forced to pass through the internal porous walls, whereby the porous walls retain a portion of the particulates in the exhaust gas.
Wall-flow particulate filters have been found to be effective in removing particulates from exhaust gas. However, pressure drop across the honeycomb filter increases as the particulates trapped in the porous walls increase. In a diesel-powered vehicle, this increasing pressure drop results in a gradual rise in back pressure against the diesel engine. When the pressure drop across the honeycomb substrate reaches a certain level, the wall-flow particulate filter may be thermally regenerated in-situ. Thermal regeneration involves subjecting the wall-flow particulate filter to a temperature sufficient to fully combust soot.
During thermal regeneration, excessive temperature spikes at various points in the honeycomb filter can occur due to poor control of the thermal regeneration. These excessive temperature spikes may produce thermal stress in the honeycomb filter. If the thermal stress exceeds the internal mechanical strength, the wall-flow particulate filter may crack, which may, in some cases, degrade performance. Therefore, means of better controlling regeneration temperatures in the wall-flow particulate filter during thermal regeneration is desirable.
In one broad aspect, the invention is an exhaust after treatment system comprising a flow-through honeycomb substrate positioned upstream of a wall-flow particulate filter. The flow-through honeycomb substrate has an inlet face and an outlet face and a plurality of longitudinal cell walls extending between the inlet face and the outlet face. The longitudinal cell walls define a plurality of parallel cell channels extending between the inlet and outlet faces. The flow-through honeycomb substrate has a flow-through region including a first portion of the parallel cell channels, and a flow-control region including a second portion of the parallel cell channels. The first portion of the parallel channels includes unplugged channels wherein flow passes straight through the channels, and the second portion of the parallel channels includes a plugged channels. The plugged cell channels in the flow-control region adjust flow through the honeycomb substrate such that flow having a first flow distribution presented at the inlet face emerges at the outlet face with a second flow distribution that is different than the first flow distribution. In particular, the adjusted flow results from a radial plug density of the plugged channels which is non-uniform. The resultant flow may be, for example, made more uniform than the first flow distribution. Alternatively any desired flow profile may be developed and presented to the downstream particulate filter. Accordingly, radial soot distribution within the downstream wall-flow filter may be controlled.
According to further embodiments of the invention, plugs are distributed in the flow-through honeycomb substrate such that a radial plug density is non-uniform. In more detail, the radial plug density may be non-uniform in relation to a radial centroid of area of the inlet face. According to further embodiments of the invention, the flow-control region may include a higher radial density of plugs than the flow-through region. Certain embodiments include an inner region that includes a relatively higher radial density of plugged cell channels than an outer region located radially outward from the inner region. In another embodiment, an intermediate region includes a relatively higher plug density than regions located radially inward and outward therefrom. In yet other exemplary embodiments, the minimum density of plugs is located other than at the centroid of area. For example, the minimum plug density may be located in an intermediate region in between an inner and outer region. Accordingly, these embodiments modify the flow velocity profile through the honeycomb substrate such that the flow pattern presented to the downstream wall-flow particulate filter includes a desired modified flow velocity profile. For example, the flow velocity profile may include a relatively higher flow velocity level in a radially outward region thereof, as compared to a like flow-through substrate without plugs, i.e., with an unmodified flow profile. Optionally, the maximum flow velocity may coincide with an intermediate region with an annular region of lower flow velocity.
In another broad aspect, the invention is an exhaust system comprising a non-uniformly plugged flow-through honeycomb substrate, and a downstream wall-flow particulate filter. The wall-flow filter is presented with, and receives, a modified flow velocity profile generated from flow initiated through the non-uniformly plugged flow-through honeycomb substrate. In particular, the flow velocity profile through the substrate may be substantially modified, as compared to a like (same cell structure, wall thickness, cell density, etc.) unplugged flow through substrate. The flow velocity profile may be modified, for example, such that high velocity region(s) in the flow profile exiting the flow-through honeycomb substrate are reduced in magnitude, as compared to a system with a like cell structure unmodified flow-through substrate. In other embodiments, the flow velocity profile is modified, by providing suitable plug patterns in the flow through substrate, to provide any desired flow profile at the inlet to the downstream wall-flow filter. In some embodiments, the flow is modified such that relatively more soot is distributed radially outward from the center of area of the wall-flow filter. This may reduce temperature peaks within the filter during active regeneration events.
In yet another broad aspect, the invention is directed to a method of purifying exhaust gas from an internal combustion engine, such as a diesel engine, which comprises directing an exhaust gas at an inlet face of a flow-through honeycomb substrate having a combination of plugged and unplugged (flow-through) channels, wherein the exhaust gas is presented to, and received at, the flow-through honeycomb substrate with a first flow velocity distribution and emerges at an outlet face of the flow-through honeycomb substrate with a second flow velocity distribution that is modified and different than the first flow distribution. The exiting flow velocity distribution may be, for example, more uniform than the received flow velocity distribution. Optionally, the location and number of plugs in the flow-through substrate may be arranged such that any desired flow velocity profile exiting the flow through substrate is achieved. To achieve the desired exiting flow distribution, the density of plugs may be applied to be radially non-uniform. In particular, higher density of plugs in various radial regions may be provided, as measured relative to a plane perpendicular to the flow direction. For example, as shown in
According to further embodiments, the invention is a flow-through honeycomb substrate, comprising a honeycomb structure having an inlet face and an outlet face and a plurality of longitudinal cell walls extending between the inlet and outlet faces, said longitudinal cell walls defining a plurality of parallel cell channels extending between the inlet face and the outlet face, said honeycomb substrate having a non-uniform density of plugged cell channels. The radial non-uniformity is measured relative to a radial centroid of area of the honeycomb structure. In a preferred implementation, a ratio of the total number of plugged cell channels to the total number of cell channels is less than or equal to 45%, or even less than or equal to 35%, or even less than or equal to 25%. Some embodiments include a relatively higher density of plugged channels in an inner radial region as compared to other regions located radially outward therefrom. Other plug patterns include an intermediate region with the relatively higher plug density. Additionally, multiple regions of varying plug densities may be provided.
Other features and advantages of the invention will be apparent from the following description and the appended claims.
The accompanying drawings, described below, illustrate typical embodiments of the invention and are not to be considered limiting of the scope of the invention, for the invention may admit to other equally effective embodiments. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.
The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in the accompanying drawings. In describing the preferred embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals are used to identify common or similar elements.
According to embodiments, the invention provides a flow-through honeycomb substrate having longitudinally-oriented through channel cells for passage of exhaust gas. Exhaust gas approaches, and is presented to, the inlet face of the flow-through honeycomb substrate with an incoming flow velocity distribution, passes through the flow-through honeycomb substrate, and exits the flow-through honeycomb substrate with an outgoing flow velocity distribution. The plug pattern in the flow through honeycomb substrate is such that it modifies the flow velocity pattern and flow distribution through the flow-through substrate (as compared to an unplugged flow-through substrate of the same cell structure) such that the outgoing flow velocity distribution is different than the incoming flow velocity profile. In particular, the outgoing flow velocity distribution may be made more uniform than the incoming flow velocity distribution. The invention may achieve the different (e.g., more uniform) outgoing flow velocity distribution by reducing the flow area, by selectively plugging of the flow-through substrate, in a region(s) where a maximum of the incoming flow velocity distribution impinges on the inlet face of the flow-through monolith. The changed or modified flow profile is achieved by non-uniformly plugging the flow-through substrate in the radial direction. In certain embodiments, the interior surfaces of the flow-through honeycomb substrate may include active catalytic species, which would then allow the flow-through substrate of the invention to double up as a flow-through honeycomb substrate catalyst. In particular, the catalysts may be a diesel oxidation catalyst comprising a platinum group metal(s) dispersed on a ceramic support in order to convert both HC and CO gaseous pollutants and particulates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOC's) which are placed in the exhaust train of diesel power systems to treat the exhaust before it vents to the atmosphere.
In an exhaust system of the invention including a wall-flow particulate filter, the flow-through honeycomb substrate may be positioned upstream of the wall-flow particulate filter and used to generate and provide a desired, possibly more uniform, flow velocity distribution across the inlet to the wall-flow particulate filter. More uniform flow distribution can promote more uniform distribution of particulates (including soot) inside the wall-flow particulate filter. Relatively more uniform soot distribution in the wall-flow particulate filter may promote more uniform soot combustion within the wall-flow particulate filter. This, in turn, may then reduce or eliminate excessive local temperature spikes that may produce differential thermal stresses in the wall-flow particulate filter during regeneration events. Such differential stresses may cause internal cracking. Accordingly, reductions in thermal stress during regeneration intervals are much sought after.
In
Preferably, the spacing (d) between the opposing faces of the flow-through honeycomb substrate 200 and the wall-flow particulate filter 300 is not so large that the flow velocity profile distribution exiting the flow-through honeycomb substrate 200 has a chance to significantly reform (due to pipe flow) to a comprise a substantially parabolic shape prior to entering the wall-flow particulate filter 300. In one example, the spacing (d) is less than 6 inches (15.2 cm). In another example, the spacing (d) is less than 3 in. (7.6 cm). In yet another example, the spacing (d) is less than (D), the largest diameter of the flow through substrate 200, i.e., d<D. As shown in
Again referring to
The intersecting walls 210 of the honeycomb substrate 202 defining the channels 208 are preferably porous, and exemplary embodiments exhibit a total porosity of less than 65%, or even between about 20% and 55%, or even between 25% and 40%. Mean pore size of the walls may be between 1 μm and 15 μm, or even between 5 μm and 10 μm. CTE is preferably between 1.0×10−7/° C. Up to about 9×10−7/° C. measured between 25° C. and 800° C. The walls 210 may or may not carry active catalytic species, such as oxidation catalytic species. Where the walls 210 carry active catalytic species, the active catalytic species may be provided in a porous wash coat applied on the walls 210 or otherwise incorporated on the walls 210. Where wash coated, the wash coat may include a material such as alumina, zirconia, or ceria. The flow-through honeycomb substrate 200 may incorporate any known active catalytic species for purifying exhaust gas, such as oxidation catalytic species for reducing carbon monoxide, hydrocarbons, and soluble organic fraction of particulates. The catalyst can be any type of oxidation catalyst, including PGM (mainly Pt, Pd, Rh or RuO2) or other types of mixed oxide catalysts, such as perovskite, oxygen storage materials, and supported metal catalysts.
The flow-through honeycomb substrate 200 of the invention includes a flow-through region 212 and a flow-control region 214 (inside the illustrative circle). In this embodiment, none of the channels 208 are plugged in the flow-through region 212, and exhaust gas passes straight through the unplugged channels. In the flow-control region 214 of this embodiment, a first set 208a of the channels 208 are plugged, while a second set 208b of the channels 208 are unplugged, and exhaust gas does not pass through (or is significantly restricted through) the plugged channels 208a but only passes through the unplugged channels 208b. The channels 208a may be plugged by inserting filler material 209 at one or both ends of the channels 208a or somewhere within the channel 208a along the length. Optionally, the channel may be completely filled. To avoid creating large turbulences at the inlet face 204, the filler material 209 is preferably inserted in the plugged channels 208a at or near the outlet face 206. In this case, the plugged channels 208a may also serve to collect some particulates from the exhaust gas. The unplugged channels 208b in the flow-control region 214 and the unplugged channels 208 in the flow-through region do not contain filler material.
The plugged channels 208a have the effect of reducing the flow area in the flow-control region 214 and, thus, add a flow restriction in the flow control area. This redirects flow from the flow-control region 214 to and through the flow-through region 212. Accordingly, this modifies the flow velocity profile exiting the flow through honeycomb substrate. This may be used to produce a more uniform flow distribution exiting the outlet face 206 of the flow-through honeycomb substrate 200.
Returning to
Outside the flow control region, i.e., in the flow-through region 212, the density of plugged channels is less than in the flow control region 214, thereby resulting in a radially non-uniform plug density. The flow through regions may be placed in the structure at any location where it is desired to locally increase the flow.
It should be recognized that the plugging of the channels in the flow-control regions(s) may be accomplished by any know plugging means, such as by applying a thin transparent mask, laser cutting holes in the cell channels to be plugged, and injecting plugging cement into the cells to a desired depth, such as between about 3 to 25 mm. Any suitable plugging material may be used, such as taught and described in U.S. patent application Ser. No. 11/486,699 dated Jul. 14, 2006 and entitled “Plugging Material For Aluminum Titanate Ceramic Wall Flow Filter Manufacture,” WO 2005/051859, WO/074599, U.S. Pat. No. 6,809,139, and U.S. Pat. No. 4,455,180, for example.
Two different locations for the flow-control region 214 are illustrated in
In the system, the wall-flow particulate filter (300 in
The honeycomb structure 302 of the filter may be made by extrusion from, for example, ceramic batch precursors and forming aids and fired to produce ceramic honeycombs of cordierite, aluminum titanate, or silicon carbide. The plugging material 312 for plugging the channels 310 may also include any suitable ceramic forming material, such as a cordierite- or aluminum titanate-based composition as described above, with CTE generally closely matched to the CTE of the honeycomb structure. For passive regeneration, the porous walls 308 of the filter may include active catalytic species. Further, an oxidative catalyst, such as a lean NOx catalyst 500A, may be added to the system at one of the end faces of the wall-flow particulate filter 300A such as shown in
For diesel exhaust purification, the porous walls 308 of the filter 300 may incorporate pores having mean diameters in the range of 1 to 60 μm, more typically in the range of 10 to 50 μm, or even 10 to 25 μm, and the honeycomb substrate 302 may have a cell density between approximately 10 and 300 cells/in2 (1.5 and 46.5 cells/cm2), more typically between approximately 100 and 200 cells/in2 (15.5 and 31 cells/cm2). The thickness of the porous walls 308 may range from approximately 0.002 in. to 0.060 in. (0.05 mm to 1.5 mm), more typically between approximately 0.010 in. and 0.030 in. (0.25 mm and 0.76 mm). The channels 310 may have a square cross-section or other type of cross-section, e.g., triangle, rectangle, octagon, hexagon or combinations thereof.
Returning to
Additionally, as shown in
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
