The present invention relates generally to systems and methods for purifying exhaust gases from internal combustion engines. More specifically, the invention relates to methods and systems including combinations of flow-through substrates and wall-flow particulate filters.
Combustion of fuel produces particulates such as 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 engine systems to remove particulates from the exhaust gas. These wall-flow particulate filters are typically made of a honeycomb-like substrate with parallel flow channels or cells separated by internal porous walls. Inlet and outlet ends of the flow channels are selectively plugged, such as in a checkerboard pattern, so that exhaust gas, once inside the substrate, is forced to pass through the internal porous walls, whereby the porous walls retain a portion of the particulates in the exhaust gas.
In this manner, wall-flow particulate filters have been found to be effective in removing particulates from exhaust gas. However, the pressure drop across the wall-flow particulate filter increases as the amount of particulates trapped on and with the porous walls increases. The increasing pressure drop results in a gradual rise in back pressure against the engine, and a corresponding decrease in the performance of the engine. When the pressure drop across the particulate filter reaches a certain level, the filter may be thermally regenerated in-situ.
Thermal regeneration involves subjecting the particulate filter to a temperature sufficient to fully combust particulates such as soot trapped in the filter, thereby reducing the pressure drop across the filter. In some instances, only partial regeneration of the filter occurs, such that a residual amount of trapped soot remains at the outer periphery of the wall-flow filter element due to inadequate heating in this region. Inadequate heating at the outer periphery of the wall-flow filter may result from, for example, heat loss to the environment and/or inadequate exhaust gas flow (and its associated thermal energy) to the periphery of the filter.
Residual soot in the filter has several undesirable effects, such as inefficient use of regeneration energy, loss of filter capacity, and increased backpressure of the filter during operation. In addition, as residual soot is allowed to concentrate at the periphery of the filter substrate over sequential regeneration cycles, the soot in that region may reach a critical concentration, thereby allowing it to regenerate in a manner that causes excessive temperature spikes within the filter substrate. Excessive temperature spikes may produce thermal stress in the structure of the particulate filter. If the thermal stress exceeds the mechanical strength of the particulate filter, the filter may crack, which may, in some cases, degrade performance and/or life of the filter. Therefore, means of better controlling the soot distribution and thermal energy distribution in the wall-flow particulate filter is desirable.
In one broad aspect, embodiments according to the invention provide an exhaust after treatment system comprising a wall-flow particulate filter, and a flow-through substrate positioned upstream of the wall-flow particulate filter, the flow-through substrate having an inlet face and an outlet face and a plurality of channels extending between the inlet face and the outlet face, the plurality of channels defining a mean channel length, the flow-through substrate having a first flow-through region including a first portion of the channels and a second flow-through region including a second portion of the channels, wherein the first flow-through region includes unplugged channels having lengths less than the mean channel length and the second flow-through region includes unplugged channels having lengths greater than the mean channel length, wherein at least one of the inlet face and outlet face possess a non-planar contour.
In another broad aspect, embodiments according to the invention provide a method of purifying exhaust gas from an internal combustion engine, the method comprising the steps of: directing an exhaust gas at an inlet face of a flow-through substrate having a plurality of channels, wherein the exhaust gas is presented to the inlet face with a first flow distribution and emerges at an outlet face of the flow-through substrate with a second flow distribution that is different than the first flow distribution, wherein at least one of the inlet face and the outlet face of the flow-through substrate is non-planar; and passing the exhaust gas with the second flow distribution through a wall-flow particulate filter in-line with the flow-through substrate.
In yet another broad aspect, embodiments according to the invention provide a flow-through honeycomb substrate, comprising a honeycomb structure having an inlet face and an outlet face and a plurality of longitudinal walls extending between the inlet face and the outlet face, the longitudinal walls defining a plurality of parallel channels extending between the inlet face and the outlet face, the plurality of channels each having a channel length, wherein at least one of the inlet face and the outlet face are contoured to provide a range of channel lengths.
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 exemplary embodiments illustrated in the accompanying drawings. In describing the exemplary embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention as set forth in the accompanying claims. 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 described herein, the invention provides a flow-through substrate having a honeycomb-like structure with longitudinally-oriented through-channels or cells of different lengths for passage of exhaust gas therethrough. During engine operation, exhaust gas approaches and is presented to the inlet face of the flow-through substrate with an incoming flow distribution, passes through the channels of the flow-through substrate, and exits the flow-through substrate with an outgoing flow distribution. The different lengths of the channels in the flow-through substrate present different flow resistances to the exhaust gas passing therethrough. The different flow resistances of the channels act to modify the flow distribution through the flow-through substrate (as compared to a flow-through substrate having channels of equal length) such that the outgoing flow distribution is different than the incoming flow distribution. In particular, the channel lengths are designed and positioned to provide an outgoing flow distribution that provides a desired soot distribution and/or desired thermal energy distribution to a filter element downstream from the flow-through substrate. In one embodiment, the desired soot and/or thermal energy distributions in the filter element may be achieved by a uniform outgoing flow distribution. In another embodiment, the desired soot and thermal energy distributions may be achieved by a non-uniform outgoing flow distribution.
In an exhaust system including a wall-flow particulate filter, the flow-through substrate may be positioned upstream of the wall-flow particulate filter and may be used to generate and provide a desired soot distribution and/or desired thermal energy distribution to the inlet of the wall-flow particulate filter. The desired soot distribution and desired thermal energy distribution can produce a thermal profile in the wall-flow filter element which improves the regeneration efficiency of the filter element and improves (i.e., reduces) the thermal gradients within the filter. During the soot loading process, the improved thermal energy distribution may also increase passive regeneration efficiency by reducing or eliminating cold regions of the filter substrate. The improved thermal energy distribution may reduce or eliminate excessive local temperature spikes that produce differential thermal stresses in the wall-flow particulate filter during regeneration events. As noted above, such differential thermal stresses may cause internal cracking of the particulate filter. Accordingly, reductions in differential thermal stress during regeneration intervals are much sought after.
In certain embodiments, the interior surfaces of the flow-through substrate and/or the wall-flow filter may include active catalytic species. In particular, the catalysts may be oxidation catalysts 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 the exhaust system of internal combustion power systems to treat the exhaust before it vents to the atmosphere.
In embodiments according to the invention, thermal energy is transferred to the filter element during soot loading and during regeneration, through convection (provided by heat-carrying exhaust gas entering the filter) and chemical energy (provided by the exothermic conversion of CO, HC to CO2 and H2O in the catalyzed flow-through substrate and/or catalyzed filter substrate). During the regeneration cycle, additional hydrocarbons may be added to the exhaust stream to be oxidized (either in the catalyzed flow-through substrate or filter element) to produce additional heating to enable the oxidation of carbonaceous soot and other organics which are trapped in the filter element.
An optional exhaust system 100A, such as shown in
Within the exhaust system, the flow-through substrate 200 and wall-flow particulate filter 300 may be either aligned or misaligned. For example, in
In
Preferably, the spacing (d) between the opposing faces 206, 304 of the flow-through substrate 200 and the wall-flow particulate filter 300 is not so large that the flow profile 117 exiting the flow-through substrate 200 has a chance to significantly change (due to, e.g., laminar pipe flow) prior to entering the wall-flow particulate filter 300. In one example, the spacing (d) is less than about 15 cm. In another example, the spacing (d) is less than about 8 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 is shown in
Again referring to
Exhaust gas flow 114 having a first flow distribution 115 (with an associated soot distribution and thermal energy distribution) is received at the inlet face 204. The exhaust gas flow 114 passes through the substrate 200 via the channels 208 to the outlet face 206. The non-uniform lengths of the channels 208 present non-uniform flow resistance to exhaust gas flow 114, and thereby alter or modify the flow distribution 115 of exhaust flow 114 (as compared to a flow-through substrate having equal length channels). The altered or modified exhaust gas flow 114a has a second flow distribution 117 (with an associated soot distribution and thermal energy distribution) different from first flow distribution 115. Exhaust gas flow 114a having second flow distribution 117 thus exits the honeycomb substrate 200 through the outlet face 206. As will be described in further detail below, the second flow distribution 117 is tailored to provide a desired soot distribution and/or thermal energy distribution to filter 300. In one embodiment, the second flow distribution optimizes the regeneration efficiency of filter 300.
The intersecting walls 210 of the honeycomb substrate 202 defining the channels 208 are porous, and exemplary embodiments exhibit a total porosity of less than about 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. The coefficient of thermal expansion (CTE) is, in one embodiment, between 1.0×10−7/° C. up to about 9×10−7/C measured between 25° C. and 800° C. In another embodiment, the CTE is greater than 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 substrate 200 may incorporate any known active catalytic species for purifying exhaust gas, such as oxidation catalytic species for reducing the quantities of carbon monoxide, hydrocarbons, and soluble organic fraction of particulates in the exhaust gas. 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 substrate 200 includes a first flow-through region 212 (corresponding to first portion 208a of channels 208) and a second flow-through region 214 (corresponding to second portion 208b of channels 208). In one embodiment, none of the channels 208 are plugged in the first and second flow-through regions 212, 214, and exhaust gas passes straight through the unplugged channels. The longer channels of second portion 208b have the effect of increasing flow resistance in the second flow-through region 214 (or conversely, the shorter channels of first portion 208a have the effect of decreasing flow resistance in the first flow-through region 212). This differential flow resistance is tailored to redirect exhaust flow 114 from the second flow-through region 214 to and through the first flow-through region 212 in a desired manner. Accordingly, this modifies the flow distribution 115 entering flow-through honeycomb substrate 200 to create the desired flow distribution 117 exiting the substrate 200. This may be used to produce desired (i.e., optimized) soot and/or thermal energy distributions to the inlet face 304 of filter 300.
Returning to
Two different exemplary locations of the second flow-through region 214 are illustrated in
In the system 100, 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 with CTE generally closely matched to the CTE of the honeycomb structure. Exemplary plugging materials are 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. 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
In one embodiment, 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 900 cells/in2 (1.5 and 135 cells/cm2), more typically between approximately 100 and 600 cells/in (15.5 and 93 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
As shown in
As described herein, embodiments according to the invention enable improved or optimal exhaust flow profile (and thereby improved or optimal soot distribution and associated improvements in thermal profiles) into a wall-flow filter in an exhaust gas after-treatment system, and thereby enable (through convection and/or chemical energy) optimal heat distribution for passive and active filter regeneration which produces improved regeneration efficiencies and thermal profiles. Notably, the improved efficiency of the exhaust flow profile allows for more efficient use of catalysts in the substrate 200. In particular, the improved exhaust flow profile decreasing the typically high velocities in central regions of the substrate and directs gas flow to typically underutilized peripheral regions of the substrate. Prior art devices without benefit of the invention described herein have higher local gas velocities which produce shorter residence time of exhaust gases in the catalyzed regions, thereby requiring a correspondingly higher precious metal loading to ensure catalytic conversion of undesirable species. However, the more even flow distribution with a lower maximum velocity provided by the invention enables use of less catalyst, through both a reduction in the overall available surface area of the substrate, and also through lower catalyst loadings on the remaining substrate.
While the invention has been described herein with respect to a limited number of exemplary 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.