Dosing and mixing arrangement for use in exhaust aftertreatment

TECHNICAL FIELD

0001The present invention relates to an exhaust aftertreatment device according to the preamble to appended claim 1 which is based on <patcit id="pcit0001" dnum="EP2128398A1"><text>EP2128398A1</text></patcit>.

BACKGROUND

0002Vehicles equipped with internal combustion engines (e.g. diesel engines) typically include exhaust systems that have aftertreatment components such as selective catalytic reduction (SCR) catalyst devices, lean NOx catalyst devices, or lean NOx trap devices to reduce the amount of undesirable gases, such as nitrogen oxides (NOx) in the exhaust. In order for these types of aftertreatment devices to work properly, a doser injects reactants, such as urea, ammonia, or hydrocarbons, into the exhaust gas. As the exhaust gas and reactants flow through the aftertreatment device, the exhaust gas and reactants convert the undesirable gases, such as NOx, into more acceptable gases, such as nitrogen and oxygen. However, the efficiency of the aftertreatment system depends upon how evenly the reactants are mixed with the exhaust gases. Therefore, there is a need for a flow device that provides a uniform mixture of exhaust gases and reactants.

0003SCR exhaust treatment devices focus on the reduction of nitrogen oxides. In SCR systems, a reductant (e.g., aqueous urea solution) is dosed into the exhaust stream. The reductant reacts with nitrogen oxides while passing through an SCR substrate to reduce the nitrogen oxides to nitrogen and water. When aqueous urea is used as a reductant, the aqueous urea is converted to ammonia which in turn reacts with the nitrogen oxides to covert the nitrogen oxides to nitrogen and water. Dosing, mixing and evaporation of aqueous urea solution can be challenging because the urea and by-products from the reaction of urea to ammonia can form deposits on the surfaces of the aftertreatment devices. Such deposits can accumulate over time and partially block or otherwise disturb effective exhaust flow through the aftertreatment device.

0004Examples of background art arrangements are disclosed in <patcit id="pcit0002" dnum="EP0470361A1"><text>EP0470361A1</text></patcit>, <patcit id="pcit0003" dnum="DE202007010324U1"><text>DE202007010324U1</text></patcit> and <patcit id="pcit0004" dnum="US20090320726A1"><text>US2009/0320726A1</text></patcit>.

SUMMARY

0005Subject to the present invention is an aftertreatment device having the features set forth in the appended independent claims; preferred embodiments being defined in the dependent claims.

0006An aspect of the present disclosure relates to a dosing and mixing unit for use in exhaust aftertreatment. The dosing and mixing unit includes a mixing tube having a generally constant diameter along the length of the mixing tube. The mixing tube includes a first portion having a plurality of apertures (e.g., perforations) and a second portion having a solid wall without any apertures. The mixing tube includes a first end portioned adjacent the first portion of the mixing tube and a second end positioned adjacent the second portion of the mixing tube. The first end of the mixing tube is closed to exhaust flow and a doser is mounted at the first end of the mixing tube. The second end of the mixing tube is open and functions as an outlet for the mixing tube. The dosing and mixing unit also includes a swirling structure for swirling exhaust generally circumferentially (i.e., tangentially) around an exterior of the first portion of the mixing tube. The swirling exhaust enters the first portion of the mixing tube through the apertures of the mixing tube. The exhaust entering the mixing tube through the apertures has a tangential flow component that causes the flow to swirl within the mixing tube about a central axis of the mixing tube. The swirling exhaust then flows from the first portion of the mixing tube to the second portion of the mixing tube and exits the mixing tube through the second end of the mixing tube. The doser injects reactant into the interior of the mixing tube and the swirling motion of the exhaust within the mixing tube assists in uniformly mixing the reactant into the exhaust while the exhaust is within the mixing tube.

0007Another aspect of the present disclosure relates to a dosing and mixing unit for use in exhaust aftertreatment. The dosing and mixing unit includes a mixing tube having a plurality of apertures. The dosing and mixing unit also includes a swirl housing partially surrounding the mixing tube. The dosing and mixing unit further includes an inlet pipe attached to a side of the swirl housing and extending out from the side of the swirl housing in an angled tangential direction in relation to a central axis of the mixing tube. The dosing and mixing unit also includes a swirl structure for causing exhaust flow to swirl along a flow path around the central axis of the mixing tube. In addition, the dosing and mixing unit includes a doser for dispensing a reactant into the interior of the mixing tube.

0008In certain embodiments, the swirling structure can include different types of structures for causing the exhaust to swirl about the mixing tube. In one embodiment, the swirling structure can include an outer housing that at least partially encloses the mixing tube and that directs exhaust flow in a swirling motion about the mixing tube. In another embodiment, the swirling structure can include a baffle that directs exhaust flow in a swirling motion about the mixing tube.

0009In certain embodiments, dosing and mixing units in accordance with the principles of the present disclosure can be used as part of an SCR exhaust treatment system for reducing nitrogen oxides to nitrogen and water. In such embodiments, the dosing and mixing units can be used to dose and mix reductants such as aqueous urea or ammonia at locations upstream from SCR substrates. In other embodiments, dosing and mixing units in accordance with the principles of the present disclosure can be used to mix other types of reactants such as hydrocarbons (e.g., fuels such as diesel fuel or syngas) upstream from other types of substrates such as lean NOx catalyst devices, lean NOx traps, catalytic converters such as diesel oxidation catalyst (DOC) substrates and diesel particulate filter (DPF) substrates.

0010A variety of additional aspects will be set forth in the description that follows. These aspects can relate to individual features and to combinations of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad concepts upon which the embodiments disclosed herein are based.

DRAWINGS

0011<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a front view of a doser and mixing unit having features that are examples of aspects not forming part of the invention;</li><li><figref idref="f0002">FIG. 2</figref> is a top view of the doser and mixing unit of <figref idref="f0001">Fig. 1</figref>;</li><li><figref idref="f0003">FIG. 3</figref> is a cross-sectional view taken along section line 3-3 of <figref idref="f0001">Fig. 1</figref>;</li><li><figref idref="f0003">FIG. 4</figref> is a cross-sectional view taken along section line 4-4 of <figref idref="f0001">Fig. 1</figref>;</li><li><figref idref="f0004">FIG. 5</figref> is a front view of an exemplary doser and mixing unitnot forming part of the invention;</li><li><figref idref="f0005">FIG. 6</figref> is a front view of another exemplary doser and mixing unit not forming part of the invention;</li><li><figref idref="f0006">FIG. 7</figref> is a front view of still another exemplary doser and mixing unit not forming part of the invention;</li><li><figref idref="f0007">FIG. 8</figref> is a front view of a further exemplary doser and mixing unit not forming part of the invention;</li><li><figref idref="f0008">FIG. 9</figref> is a front view of yet an exemplary doser and mixing unit not forming part of the invention;</li><li><figref idref="f0008">FIG. 10</figref> is a top view of the doser and mixing unit of <figref idref="f0008">Fig. 9</figref>;</li><li><figref idref="f0009">FIG. 11</figref> is a front view of an aftertreatment device having features that are examples of aspects in accordance with an embodiment of the invention;</li><li><figref idref="f0010">FIG. 12</figref> is a top view of the aftertreatment device of <figref idref="f0009">Fig. 11</figref>;</li><li><figref idref="f0011">FIG. 13</figref> is a front view of a doser and mixing unit having features that are examples of aspects not forming part of the invention;</li><li><figref idref="f0012">FIG. 14</figref> is a schematic representation of a first exhaust treatment system incorporating a doser and mixing unit not forming part of the invention;</li><li><figref idref="f0013">FIG. 15</figref> is a schematic representation of a second exhaust treatment system incorporating a doser and mixing unit not forming part of the invention;</li><li><figref idref="f0014">FIG. 16</figref> is a schematic representation of a third exhaust treatment system incorporating a doser and mixing unit not forming part of the invention; and</li><li><figref idref="f0015">FIGS 17a-17f</figref> are top views of a dosing and mixing unit showing different means for creating swirl not forming part of the invention.</li></ul>

DETAILED DESCRIPTION

0012Reference will now be made in detail to the exemplary aspects of the present disclosure that are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like structure.

0013<figref idref="f0001 f0002 f0003 f0004 f0005 f0006 f0007">Figures 1-8</figref> show a dosing and mixing unit 20 not forming part of the invention. The dosing and mixing unit 20 includes an inlet 22 and an outlet 24. The inlet 22 is formed by an inlet pipe 26 that extends to a swirl housing 28. The dosing and mixing unit 20 also includes a mixing tube 30 having a first end 32 positioned within the swirl housing 28 and a second end 34 that forms the outlet 24 of the dosing and mixing unit 20. The inlet pipe 26 is attached to a side 29 of the swirl housing 28 and extends out from the side 29 of the swirl housing 28 in an angled tangential direction in relation to a central axis 42 of the mixing tube 30. As is illustrated in <figref idref="f0001">Figures 1</figref>, <figref idref="f0005">6</figref> and <figref idref="f0006">7</figref>, the inlet pipe 26 can be attached to a top portion of the side 27 of swirl housing 28 (see <figref idref="f0001">Figure 1</figref>) or a lower portion of the side 27 of swirl housing 28 (see <figref idref="f0005">Figure 6</figref>. The inlet pipe 26 can also have different angles in relation to the central axis 42 such that the exhaust flow enters the swirl housing 28 in a direction towards a bottom of the swirl housing 28 (see <figref idref="f0001">Figures 1</figref> and <figref idref="f0005">6</figref>) or towards a top end 27 of swirl housing 28 (see <figref idref="f0006">Figure 7</figref>). In other embodiments (not shown), the inlet pipe 26 is attached to the top end 27 of swirl housing 28 in an angled tangential direction in relation to a central axis 42 of the mixing tube 30. The angle between the inlet pipe 26 and the central axis 42 is in some embodiment an oblique angle.

0014The mixing tube 30 has a first portion 36 positioned adjacent to the first end 32 of the mixing tube 30 and a second portion 38 positioned adjacent to the second end 34 of the mixing tube 30. The first portion 36 has a plurality of apertures 37 (e.g. perforations) and the second portion 38 has a solid wall without any apertures. The apertures 37 can be formed as circles, squares, slots or any other shape. The dosing and mixing unit 20 also includes a doser 40 mounted to the top end 27 of the swirl housing 28 adjacent to the first end 32 of the mixing tube 30. The doser 40 is adapted for dispensing reactant into an interior region of the mixing tube 30.

0015In use of the dosing and mixing unit 20, exhaust enters the dosing and mixing unit 20 through the inlet 22 and is swirled circumferentially (i.e., tangentially) through a swirl structure about the exterior of the first portion 36 of the mixing tube 30 by the swirl housing 28. As the exhaust flow swirls circumferentially around the first portion 36 of the mixing tube 30, the exhaust gas enters the interior of the mixing tube 30 through the apertures 37. The exhaust flow entering the interior of the mixing tube 30 through the apertures 37 has a tangential/circumferential flow component that causes the exhaust to swirl within the interior of the mixing tube 30. The doser 40 dispenses reactant into the swirling exhaust within the interior of the mixing tube where the swirling action of the exhaust assists in uniformly mixing the reactant within the exhaust. Swirling flow of the exhaust continues from the first portion 36 of the mixing tube 30 to the second portion 38 of the mixing tube 30 whereby mixing is enhanced as the exhaust moves through the length of the mixing tube 30. After the swirling exhaust has traveled through the mixing tube in a direction extending from the first end 32 to the second end 34 of the mixing tube 30, the exhaust exits the dosing and mixing unit 20 through the outlet 24. As is seen in <figref idref="f0002">figure 2</figref>, the swirl structure has in some embodiments a cross-section that gradually decreases along the exhaust flow path.

0016The mixing tube 30 of the dosing and mixing unit 20 defines the central axis 42 and has a length that extends along the central axis 42 from the first end 32 to the second end 34 of the mixing tube 30. The mixing tube 30 is cylindrical in shape and has in some embodiments (shown in <figref idref="f0001 f0002 f0003 f0004 f0005 f0006">Figures 1-7</figref>) a constant diameter along the entire length of the mixing tube 30. Thus, the first and second portions 36, 38 of the mixing tube 30 have constant diameters along their respective lengths. Also, the first and second portions 36, 38 of the mixing tube 30 are shown having equal diameters. In some embodiments (shown in <figref idref="f0007">Figure 8</figref>), the first end 32 of the mixing tube 30 has a larger diameter than the second end 34. In some embodiments, the first portion 36 of the mixing tube 30 has a diameter that gradually decreases along its length towards the second portion 38. The first end 32 of the mixing tube 30 is blocked by the top end 27 of swirl housing 28 so that exhaust flow can not pass through the first end 32 of the mixing tube 30. In some embodiments, as is seen in <figref idref="f0004">Figure 5</figref> and <figref idref="f0007">8</figref>, the first end 32 of the mixing tube 30 is arranged at a distance from the top end 27 of the swirl housing 28, forming a gap 52. The doser 40 is positioned at the first end 32 of the mixing tube 30 and is aligned along the central axis 42. As shown at <figref idref="f0003">Figures 3 and 4</figref>, the swirling motion of the exhaust can increase in intensity as the swirling exhaust moves axially through the first portion 36 of the mixing tube in a direction toward the second portion 38 of the mixing tube 38. Thus, by the time the exhaust enters the second portion 38 of the mixing tube 30, the exhaust is swirling at an increased rate.

0017In certain embodiments, not forming part of the invention, the doser 40 can include an injector that injects reactant in a spray cone aligned along the central axis 42 of the mixing tube 30. The swirling action of the exhaust and the converging flow passing through the apertures 37 (see <figref idref="f0003">Figure 3</figref>) assists in narrowing the spray cone angle thereby inhibiting wetting of the interior of the mixing tube 30 and minimizing deposit formation within the mixing tube and downstream from the mixing tube. The swirling action is particularly suited for breaking-up, mixing and evaporating aqueous urea in a relatively short time frame/distance.

0018The swirl housing 28 at least partially encloses the first portion 36 of the mixing tube 30 and has an arrangement that directs exhaust flow tangentially relative to the outer surface of the mixing tube 30 such that the exhaust swirls circumferentially around the exterior of the mixing tube 30. In one embodiment, the exhaust flows in a single direction (e.g., clockwise relative to the central longitudinal axis as shown at <figref idref="f0002">Figure 2</figref>) around at least 75 percent of the outer diameter of the mixing tube 30. In other words, the exhaust flow direction turns at least 270 degrees around the outer diameter of the mixing tube 30. As the exhaust flows around the mixing tube 30, portions of the exhaust progressively enter the interior of the first portion 36 of the mixing tube 30 through the apertures 37. The swirl housing 28 has a curved/bent surface 44 that curves along and opposes the outer surface of the first portion 36 of the mixing tube 30. The surface 44 is arranged to transition progressively closer to the outer surface of the first portion 36 of the mixing tube 30 as the surface extends in the circumferential direction of exhaust flow. In some embodiments (not shown), the mixing tube 30 is arranged such in the swirl housing 28 that the central axis 42 of the mixing tube 30 is not in parallel with a longitudinal axis of the swirl housing 28, i.e. the second portion 28 of the mixing tube 30 extends from the swirl housing 28 in an angled direction.

0019<figref idref="f0008">Figures 9 and 10</figref> show a dosing and mixing unit 20, which does not form part of the invention and which is configured in the same manner and includes the same features as described in conjunction with <figref idref="f0001 f0002 f0003 f0004 f0005 f0006 f0007">Figures 1-8</figref>. The unit 20 further comprises indents 62 or some other kind of profile for causing the exhaust to swirl within the interior of the mixing tube 30. In some embodiments (not shown), the swirl housing 28 includes indents or other kinds of profiles on the inside of the top end 27 for causing the swirl. In other embodiments, the unit 20 includes a screw or helical shaped device arranged around the mixing tube 30 for causing the exhaust to swirl.

0020<figref idref="f0009">Figures 11</figref> and <figref idref="f0010">12</figref> show an aftertreatment device 120 in accordance with an embodiment of the invention. The aftertreatment device 120 includes an inlet 122 and an outlet 124. The inlet 122 is formed by an inlet pipe 126 that extends to a substrate housing 128. A substrate 129 (e.g., a DPF substrate or DOC substrate) is positioned within the substrate housing 128 adjacent to the inlet pipe 126. The aftertreatment device 120 also includes a mixing tube 130 having the same configuration as the mixing tube 30. The mixing tube 130 has a first end 132 positioned within the substrate housing 128 and a second end 134 that forms the outlet 124 of the aftertreatment device 120. The mixing tube 130 has a first portion 136 positioned adjacent to the first end 132 of the mixing tube 130 and a second portion 138 positioned adjacent to the second end 134 of the mixing tube 130. The first portion 136 has a plurality of apertures 137 (e.g., perforations) and the second portion 138 has a solid wall without any apertures. The apertures 137 can be formed as circles, squares, slots or any other shape. The aftertreatment device 120 also includes a doser 140 mounted to the housing 128 adjacent to the first end 132 of the mixing tube 130. The doser 140 is adapted for dispensing reactant into an interior region of the mixing tube 130. A deflector baffle 150 is positioned between the substrate 129 and the first portion 136 of the mixing tube 130. The deflector baffle 150 is configured to cause the exhaust to flow circumferentially in one direction around at least 270 degrees of the exterior of the outer diameter of the first portion 136 of the mixing tube 30. The baffle 150 directs the flow in a tangential direction relative to the outer diameter of the mixing tube 30. In certain embodiments, the mixing tube 130 can be bent so that an out put end of the tube angles away from the inlet 122. In other embodiments, the tube 130 can be straight and the entire tube 130 can be angled at angle θ relative to a central axis of the housing 128. In certain embodiments, the angle θ is in the range of 60-90 degrees. In other embodiments, the angle θ is less than 90 degrees, or less than 80 degrees, or in the range of 60-80 degrees. In other embodiments, the angle θ is 90 degrees. The angle θ is measured between the outer end of the tube 130 and the main body of the housing 128.

0021In another embodiment, the tube 130 can be offset from the center of the housing 128 so as to be closer to a first side 131 (e.g., a top side) of the housing as compared to a second side 133 (e.g., a bottom side) of the housing 128.

0022In use of the aftertreatment device 120, exhaust enters the device 120 through the inlet 122 and passes through the substrate 129 where the exhaust is initially treated (e.g., contaminants removed by filtration or chemically through a catalyzed reaction at the substrate). After the exhaust passes through the substrate 129, the baffle 150 causes the exhaust to swirl circumferentially (i.e., tangentially) through a swirl structure about the exterior of the first portion 136 of the mixing tube 130. As the exhaust flow swirls circumferentially around the first portion 136 of the mixing tube 130, the exhaust gas enters the interior of the mixing tube 130 through the apertures 137. The exhaust flow entering the interior of the mixing tube 130 through the apertures 137 has a tangential/circumferential flow component that causes the exhaust to swirl within the interior of the mixing tube 130. The doser 140 dispenses reactant into the swirling exhaust within the interior of the mixing tube where the swirling action of the exhaust assists in uniformly mixing the reactant within the exhaust. Swirling flow of the exhaust continues from the first portion 136 of the mixing tube 130 to the second portion 138 of the mixing tube 130 whereby mixing is enhanced as the exhaust moves through the length of the mixing tube 130. After the swirling exhaust has traveled through the mixing tube in a direction extending from the first end 132 to the second end 134 of the mixing tube 130, the exhaust exits the device 120 through the outlet 124. As is seen in <figref idref="f0010">figure 12</figref>, the swirl structure has a cross-section that gradually decreases along the exhaust flow path.

0023<figref idref="f0011">Figure 13</figref> shows a doser and mixing unit 220 not forming part of the invention. The unit 220 includes an inlet 222 and an outlet 224. The inlet 222 is formed by an inlet pipe 226 that extends to a swirl housing 128. The unit 220 also includes a mixing tube 230 having the same configuration as the mixing tube 30 shown and described in conjunction with <figref idref="f0001 f0002 f0003 f0004 f0005 f0006 f0007 f0008">Figures 1-10</figref>. As can be seen in <figref idref="f0011">Figure 13</figref>, the inlet pipe 226 is arranged with a radial offset from a central axis of the mixing tube 230 such that the incoming exhaust through the inlet 222 has a flow direction that is generally in parallel of the direction of the outgoing exhaust through outlet 224. The mixing tube 230 has a first end 232 positioned within the swirl housing 228 and a second end 234 that forms the outlet 224 of the unit 220. The mixing tube 230 has a first portion 236 positioned adjacent to the first end 232 of the mixing tube 230 and a second portion 238 positioned adjacent to the second end 234 of the mixing tube 230. The first portion 236 has a plurality of apertures 237 (e.g., perforations) and the second portion 238 has a solid wall without any apertures. The apertures 237 can be formed as circles, squares, slots or any other shape. The unit 220 also includes a doser 240 mounted to the housing 228 adjacent to the first end 232 of the mixing tube 230. The doser 240 is adapted for dispensing reactant into an interior region of the mixing tube 230. A deflector baffle 250 is positioned within the swirl housing 228 between the inlet pipe 226 and the first portion 236 of the mixing tube 230. The deflector baffle 250 is configured to cause the exhaust to flow circumferentially in one direction around at least 270 degrees of the exterior of the outer diameter of the first portion 236 of the mixing tube 230. The baffle 250 directs the flow in a tangential direction relative to the outer diameter of the mixing tube 230.

0024As the exhaust flow swirls circumferentially around the first portion 236 of the mixing tube 230, the exhaust gas enters the interior of the mixing tube 230 through the apertures 237. The exhaust flow entering the interior of the mixing tube 230 through the apertures 237 has a tangential/circumferential flow component that causes the exhaust to swirl within the interior of the mixing tube 230. The doser 240 dispenses reactant into the swirling exhaust within the interior of the mixing tube where the swirling action of the exhaust assists in uniformly mixing the reactant within the exhaust. Swirling flow of the exhaust continues from the first portion 236 of the mixing tube 230 to the second portion 238 of the mixing tube 230 whereby mixing is enhanced as the exhaust moves through the length of the mixing tube 230. After the swirling exhaust has traveled through the mixing tube 230 in a direction extending from the first end 232 to the second end 234 of the mixing tube 230, the exhaust exits the unit 220 through the outlet 224.

0025<figref idref="f0012">Figure 14</figref> shows a system 300, not forming part of the invention, including the dosing and mixing unit 20. The system includes an internal combustion engine 302. A pipe 304 carries exhaust from the engine 302 to the dosing and mixing unit 20 where reactant (e.g., aqueous urea) is injected into the exhaust stream and mixed with the exhaust stream. A pipe 306 carries the exhaust stream containing the reactant to an SCR device 308 where nitrogen oxides are reduced to nitrogen and water. <figref idref="f0013">Figure 15</figref> shows a system 400 that is the same as the system 300 except a separate aftertreatment substrate (e.g., a DPF or DOC) is positioned between the engine 300 and the dosing and mixing unit 20. <figref idref="f0014">Figure 16</figref> shows a system 500 that is the same as the system 300 except the aftertreatment device 120 has been substituted for the dosing and mixing unit 20.

0026A selective catalytic reduction (SCR) catalyst device is typically used in an exhaust system to remove undesirable gases such as nitrogen oxides (NOx) from the vehicle's emissions. SCR's are capable of converting NOx to nitrogen and oxygen in an oxygen rich environment with the assistance of reactants such as urea or ammonia, which are injected into the exhaust stream upstream of the SCR through the doser 40. In alternative embodiments, other aftertreatment devices such as lean NOx catalyst devices or lean NOx traps could be used in place of the SCR catalyst device, and other reactants (e.g., hydrocarbons) can be dispensed by the doser.

0027A lean NOx catalyst device is also capable of converting NOx to nitrogen and oxygen. In contrast to SCR's, lean NOx catalysts use hydrocarbons as reducing agents/reactants for conversion of NOx to nitrogen and oxygen. The hydrocarbon is injected into the exhaust stream upstream of the lean NOx catalyst. At the lean NOx catalyst, the NOx reacts with the injected hydrocarbons with the assistance of a catalyst to reduce the NOx to nitrogen and oxygen. While the exhaust treatment systems 400 and 500 will be described as including an SCR, it will be understood that the scope of the present disclosure is not limited to an SCR as there are various catalyst devices that can be used in accordance with the principles of the present disclosure.

0028The lean NOx traps use a material such as barium oxide to absorb NOx during lean burn operating conditions. During fuel rich operations, the NOx is desorbed and converted to nitrogen and oxygen by reaction with hydrocarbons in the presence of catalysts (precious metals) within the traps.

0029<figref idref="f0015">Figures 17a-17f</figref> are top views of a dosing and mixing unit 20, not forming part of the invention, as described and shown in conjunction with <figref idref="f0001 f0002 f0003 f0004 f0005 f0006 f0007 f0008">Figures 1-10</figref> and show different means for creating exhaust flow swirl. <figref idref="f0015">Figure 17a</figref> illustrates a swirl structure that has the same cross-section around the outer diameter of the mixing tube 30. In the embodiment shown in <figref idref="f0015">Figure 17b</figref>, the swirl housing 28 includes a stop wall 63 that forces the exhaust flow to enter the mixing tube 30. In the embodiment shown in <figref idref="f0015">Figure 17c</figref>, the dosing and mixing unit 20 includes one or more flow guide devices 64 arranged between an outer wall of the mixing tube 30 and an inner wall of the swirl housing 28. The flow guide devices 64 may extend from a bottom of the swirl housing 28 or from a top of the swirl housing 28. The flow guide device(s) 64 may also be arranged on the inner wall of the swirl housing 28 (as shown in <figref idref="f0015">Figure 17d</figref>) or on the outer wall of the mixing tube 30 (not shown). Further, according to some embodiments, the mixing tube 30 may be provided with fins 65 inwardly extending from an inner wall of the mixing tube 30 (see <figref idref="f0015">Figure 17e). Figure 17f</figref> illustrates that swirl structure transits from a circular cross-section into an oval or elliptical cross-section along the flow path.