Abstract:
A mixing device for mixing a gas and a liquid in an exhaust system has an injector for injecting the liquid, a shell, an inlet, an outlet and an exhaust gas flow path between the inlet and the outlet. The mixing device defines an axis, and the flow path has a first part, a second part and a third part. The first part extends generally parallel to the axis to direct a flow in a generally forward direction, the first part extending up to a first axial position. The second part extends generally parallel to the axis to direct flow in a generally reverse direction. The third part extends generally parallel to the axis to direct a flow in the generally forward direction. The third part is a volume that allows mixing and extends past the first axial position.

Description:
RELATED APPLICATIONS 
       [0001]    The application is the U.S. National Phase of PCT/GB2007/000844 filed 9 Mar. 2007, which claimed priority to UK Application 0606116.2 filed 28 Mar. 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a mixing device for exhaust systems and also a method of treating exhaust gases of an internal combustion engine. 
         [0003]    To clean up emissions from vehicles such as trucks and cars it is known to use diesel particulate filters (DPF) and selective catalytic reduction (SCR). 
         [0004]    With regard to DPFs, in order to burn off the accumulated carbon on the DPF, it is known to inject diesel fuel or other hydrocarbons in front of a diesel oxidizing catalytic (DOC) to create heat by catalytic oxidation. This heat then passes from the DOC to the DPF raising the temperature of the DPF and hence burning off the accumulated carbon. 
         [0005]    SCR is used to remove oxides of nitrogen (NOx). In this case urea, or a similar liquid is injected upstream of the SCR catalyst to act as a chemical reductant to remove NOx. 
         [0006]    For either system to work reliably and effectively it is necessary that the injected liquids are highly dispersed and are evenly distributed onto the catalyst. However, on a typical installation there is little space available to allow good mixing to occur. It is known to provide devices to create turbulence to assist mixing, however, these devices cause a relatively high back pressure which adversely effects fuel economy and engine durability. 
         [0007]    The present invention seeks to overcome or mitigate some or all of these problems. 
       SUMMARY OF THE INVENTION 
       [0008]    Thus, according to the present invention there is provided a mixing device for mixing a gas and a liquid in an exhaust system. The mixing device has an injector for injecting the liquid, a shell, an inlet, an outlet and an exhaust gas flow path between the inlet and the outlet. The mixing device defines an axis, and the flow path has a first part, a second part and a third part. The first part extends generally parallel to the axis to direct a flow in a generally forward direction, the first part extending up to a first axial position. The second part extends generally parallel to the axis to direct flow in a generally reverse direction. The third part extends generally parallel to the axis to direct a flow in the generally forward direction. The third part is a volume that allows mixing and extends past the first axial position. 
         [0009]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0011]      FIG. 1  shows a cross section of an exhaust system including a mixing device according to the present invention. 
           [0012]      FIG. 2  shows a cross section of another example of a mixing device according to the present invention. 
           [0013]      FIG. 3  shows a cross section of another example of a mixing device according to the present invention. 
           [0014]      FIG. 4  shows a cross section of another example of a mixing device according to the present invention. 
           [0015]      FIG. 5  shows an end view of one portion of  FIG. 4 . 
           [0016]      FIG. 6  shows an end view of another portion of  FIG. 4 . 
           [0017]      FIG. 7  shows an enlarged view of  FIG. 4 . 
           [0018]      FIG. 8  shows another embodiment of an exhaust system including mixing devices according to the present invention. 
           [0019]      FIG. 9  shows another embodiment of an exhaust system including mixing devices according to the present invention. 
           [0020]      FIG. 10  shows another embodiment of an exhaust system including mixing devices according to the present invention. 
           [0021]      FIG. 11  shows a further embodiment of an exhaust system including mixing devices according to the present invention. 
           [0022]      FIG. 12A  shows a further embodiment of an exhaust system including mixing devices according to the present invention. 
           [0023]      FIG. 12B  shows a further embodiment of an exhaust system including mixing devices according to the present invention. 
           [0024]      FIG. 12C  shows a further embodiment of an exhaust system including mixing devices according to the present invention. 
           [0025]      FIG. 12D  shows a further embodiment of an exhaust system including mixing devices according to the present invention. 
           [0026]      FIG. 12E  shows a further embodiment of an exhaust system including mixing devices according to the present invention. 
           [0027]      FIG. 12F  shows a further embodiment of an exhaust system including mixing devices according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]      FIG. 1  shows part of an exhaust system  10  including a mixing device  12  and a catalyst  14 . The mixing device  12  includes an inlet  16  and an outlet  18 . Between the inlet  16  and the outlet  18  there is defined a gas flow path F 1 , F 2 , F 3 . The mixing device  12  also includes an injector  20 . The exhaust system  10  is connected to an exhaust manifold (not shown) of an internal combustion engine. When the engine is running, exhaust gases pass down the exhaust system  10  generally from left to right as shown by the exhaust gas arrows EG. When required, a reagent is injected into the exhaust gas flow by injector  20 . The injected reagent and exhaust gas then pass through the mixing device  12  and on to the catalyst  14 . 
         [0029]    Where the catalyst  14  is a DOC, the reagent injected by injector  20  is diesel fuel or another type of hydrocarbon fuel. Where the catalyst  14  is an SCR the reagent injected by injector  20  is urea or an equivalent reagent. 
         [0030]    In summary, the mixing device  12  is designed such that reverse flow of the exhaust gas and reagent occurs as they pass through the mixing device  12 . Thus, it can be seen that the flow path F 1  is generally left to right, the flow path F 2  is generally right to left, and the flow path F 3  is generally left to right. In this case, there are two general reversals of gas flow, though in further embodiments the mixing device could be designed to have a single reversal of gas flow or three reversals of gas flow or four reversals of gas flow or more than four reversals of gas flow. 
         [0031]    The gas flow reversal creates a homogeneous distribution of liquid within the exhaust gas which then passes onto the catalyst  14  with minimum loss of space and minimum back pressure. 
         [0032]    In more detail, the mixing device  12  includes a shell  22  having an outer portion  24  made from sheet steel and an inner portion  26  made from a thermal insulation material. 
         [0033]    A central tube  30  is positioned within the shell  22  and a sleeve  50  is positioned between the central tube  30  and the shell  22 . The left hand (when viewing  FIG. 1 ) end  31  of tube  30  defines the inlet  16 . The right hand (when viewing  FIG. 1 ) end  32  of tube  30  is blanked off by blanking plate  33 . Between ends  31  and  32  the tube  30  has a perforated region  34 . In this case the perforated region comprises over 200 holes  35 . 
         [0034]    The sleeve  50  includes a central cylindrical portion  51 , a frustoconical end portion  52 , and a frustoconical end portion  53 . 
         [0035]    The frustoconical end portion  53  connects the cylindrical portion  51  with the right hand end  32  of central tube  30 . There are no perforations in either the cylindrical portion  51  or the frustoconical end portion  52 , and as such the frustoconical end portion  52  acts to blank off the right hand end of sleeve  50 . 
         [0036]    However, the frustoconical end portion  53  defines a perforated region  54  of the sleeve  50 . The perforated region  54  includes holes  55 . In one example, there are over 200 holes  55 . The right hand end  32  of the central tube  30  and the right hand end of the sleeve  50  are both supported by a baffle  60 . Baffle  60  includes holes to allow the passage of the exhaust gases, these holes are similar to those shown on baffle  360  on  FIG. 5 . 
         [0037]    A support member  62  includes an outer frustoconical region  63  and an inner frustoconical region  64 . There are no perforations in the outer frustoconical region  63  and there are no perforations in the inner frustoconical region  64 . The left hand end of the perforated region  54  of the sleeve  50  is connected to the right hand end of the outer frustoconical region  63 , and is therefore supported by the outer frustoconical region  63 . The left hand end  31  of the central tube  30  is connected to and supported by the right hand end of the inner frustoconical region  64 . The inner frustoconical region is, in turn, supported at its left hand end by the outer frustoconical region. The outer frustoconical region  63  includes a portion  65  which supports the injector  20 . 
         [0038]    Consideration of the perforated region  34  of the central tube  30  and the perforated region  54  of the sleeve  50  show that they do not axially overlap, i.e. there is a gap G between the axial position of the left most hole of the perforated region  34  and the right most hole of the perforated region  54 . 
         [0039]    It will be appreciated that the exhaust gas and reagent must initially enter the central tube  30  via the inlet  16 . At this point, all the exhaust gas is traveling from left to right when viewing  FIG. 1 . When the exhaust gas has reached the perforated region  34 , the exhaust gas flow direction will turn and the exhaust gas will flow radially outwardly through the holes  35  into an inner annular region  27  defined between the central tube  30  and sleeve  50 . When in this inner annular region  27  exhaust gases will be forced to move from right to left towards the perforated region  54 . Upon reaching the perforated region  54  the exhaust gas will again turn and flow generally radially outwardly through the holes  55  into an outer annular region  28  defined between cylindrical portion  51  and the shell  22 . When in this outer annular region  28 , the exhaust gases will be forced to move from left to right. 
         [0040]    It will therefore be appreciated that the flow path includes the first general reversal of the direction of exhaust flow which general reversal will occur as the exhaust gas passes through holes  35 . The flow path also includes a second general reversal of the direction of exhaust gas flow, which will occur typically as the exhaust gas passes through the holes  55 . 
         [0041]    It will also be appreciated that since the central tube  30 , the cylindrical portion  51  and the shell  22  are all cylindrical, and are all concentric, the exhaust gas flow path is substantially symmetrical about a center line CL of the mixing device  12 , and this is in spite of the fact that the injector  20  is positioned asymmetrically relative to the center line CL. 
         [0042]    It will also be appreciated that there is a space between the catalyst  14  and the blanking plate  33  which defines the outlet  18 . Note that final mixing occurs in flow path F 3  and in outlet  18  prior to the gas entering the catalyst  14 . 
         [0043]    Note that flow path F 2  generally surrounds flow path F 1  and that flow path F 3  generally surrounds flow path F 2 . 
         [0044]      FIG. 2  shows an exhaust system  110  in which components that fulfill substantially the same function as those of exhaust system  10  are labeled  100  greater. 
         [0045]    In exhaust system  110 , the injector  120  injects the reagent at the center line CL of the mixing device  112  whereas the injector  20  ( FIG. 1 ) injects the reagent from an edge of the mixing device  12 . It will be appreciated that the flow path F 1 , F 2 , F 3  shown in  FIG. 2  is identical to the flow path F 1 , F 2 , F 3  shown in  FIG. 1 . 
         [0046]      FIG. 3  shows an exhaust system  210  in which components that fulfils substantially the same function as those of exhaust system  10  are labeled  200  greater. In summary the mixing device  212  is a “mirror image” version of mixing device  12 . Thus, the exhaust gas flow is generally from left to right as shown by arrows EG. However, the exhaust gas enters the mixing device at inlet  216  and leaves the mixing device  212  at outlet  218 . It will be appreciated that flow F 1  is from left to right along outer annular region  228 . The flow then reverses by flowing radially inwardly through holes  255  into inner annular region  227  at which point the gas flow is from right to left. The flow then reverses again by flowing radially inwardly through holes  235  and then flows from left to right along the center of central tube  230  and out of the mixing device. The injector is positioned upstream of the mixing device  212 , and positions X, Y and Z show examples of where the injector might be positioned. 
         [0047]    Note that because the mixing device  212  is a “mirror image” version of mixing device  12 , the flow path F 1  generally surrounds the flow path F 2  and the flow path F 2  generally surrounds the flow path F 3 . 
         [0048]      FIGS. 4 to 7  show an exhaust system  310  in which components that fulfill substantially the same function as those of exhaust system  10  are labeled  300  greater. 
         [0049]    In this case the injector is not shown, but will be positioned upstream of inlet  316 . Holes  335  and  355  are only shown schematically (as crosses). 
         [0050]      FIG. 5  shows an end view of the baffle  360  which includes several holes  361  which allow the exhaust gases to pass through the baffle  360  and onto the catalyst  314 . A central region  366  of baffle  360  acts to blank off the end  332  of central tube  330 . The cross section area of the central tube  330  is A 1  (see  FIG. 6 ). The cross section area of the inner annular region  327  is A 4  and the cross section area of the outer annular region  328  is A 5 . 
         [0051]    The open area (i.e. the gas flow area) of the holes  335  is A 2  and the open area (i.e. gas flow area) of the holes  355  is A 3 . 
         [0052]    Preferably A 2  approximately equals A 3 . 
         [0053]    Preferably A 2  is greater than or equal to 1.5 times A 1 . 
         [0054]    Preferably A 2  is greater than or equal to 1.5 times A 4 . 
         [0055]    Preferably A 2  is greater than or equal to 1.5 times A 5 . 
         [0056]    Preferably A 3  is greater than or equal to 1.5 times A 1 . 
         [0057]    Preferably A 3  is greater than or equal to 1.5 times A 4 . 
         [0058]    Preferably A 3  is greater than or equal to 1.5 times A 5 . 
         [0059]    Preferably A 4  is approximately equal to A 1  or is greater than A 1 . 
         [0060]    Preferably A 5  is approximately equal to A 1  or greater than A 1 . 
         [0061]      FIG. 9  shows an exhaust system  410  in which components that fulfill substantially the same function as those shown in exhaust system  10  are labeled  400  greater. In this case the injector  420  is positioned in a pipe  470 , the diameter of the pipe  470  is smaller than the diameter of the shell  422 . A further pipe  471  connects the mixing device  412  with the catalyst  414 . The diameter of the pipe  471  is smaller than the diameter of shell  422 . The diameter of pipe  471  is also smaller than the casing  472  of the catalyst  414 . Pipe  471  can vary in length, and can include one or more bends, depending upon the particular installation. 
         [0062]      FIG. 8  shows an exhaust system  510  in which components that fulfill substantially the same function as those shown in  410  are labeled  100  greater. In this case exhaust system  510  is a modified version of exhaust system  410 . It can be seen that the mixing device  512  and the catalyst  514  have been integrated into a single unit. Thus, the exhaust system  510  does not include a pipe that would be the equivalent of pipe  471  of exhaust system  410 . The integrated exhaust system  510  is more compact than the exhaust system  410 , and also includes fewer components (as it does not include pipe  471 ). 
         [0063]    Thus, the exhaust system  510  is integrated because the outlet  418  from the mixing device  512  passes directly to the inlet  573  to the catalyst  514 . In other words, the diameter of the shell  522  of the mixing device  512  is substantially the same as the diameter of the casing  572  of the catalyst  514 , i.e. when the exhaust gases pass from the mixing device  512  to the catalyst  514 , there is no significant reduction in cross section area of flow path. 
         [0064]      FIG. 10  shows an exhaust system  610  incorporating two mixing devices  612 A,  612 B according to the present invention. The exhaust system  610  also incorporates four catalysts  614 A-D and a DPF  675 . 
         [0065]    Thus, catalyst  614 A is a DOC, catalyst  614 B is an SCR, catalyst  614 C is DOC, and catalyst  614 D is DOC. The DPF  675  is provided between catalyst  614 C and  614 D. Injector  620 A is a urea injector and injector  620 B is a diesel fuel injector. As the exhaust gases pass through the exhaust system  610 , they are treated as follows:
       (a) the DOC catalyst  614 A oxidizes NO to NO2.   (b) Urea is injected at injector  620 A and mixed with the exhaust gas in mixing device  612 A.   (c) The SCR catalyst  614 B then removes NOx.   (d) The diesel injector  620 B injects diesel into the gas stream and the mixing device  612 B mixes the exhaust gas and the diesel.   (e) The exhaust gas/diesel mixture pass into the DOC  614 C and the diesel fuel is oxidized thereby generating heat.   (f) The heat is passed into the DPF  675 , thereby burning off accumulative carbon.   (g) The exhaust gas then passes into the DOC catalyst  614 D to oxidize any remaining hydrocarbons.       
 
         [0073]    It will be appreciated by those skilled in the art the injector  620 A and  620 B only inject reagent as and when required. Various sensors on the engine and within the exhaust system will determine when injection of a particular reagent is required and this injection is controlled by a control system. 
         [0074]      FIGS. 11 to 12F  show an exhaust system  710  in which components that fulfill substantially the same function as those of exhaust system  10  are labeled  700  greater. 
         [0075]    The mixing device  712  includes a shell  722 . A central tube  730  is positioned partly within the shell  722  and a sleeve  750  is positioned between the central tube  730  and the shell  722 . The tube  730  defines an inlet  16 . The right hand (when viewing FIG.  11 ) end  732  of tube  730  is blanked off by blanking plate  733 . Tube  730  has a perforated region  734  (shown schematically as a cross). 
         [0076]    Sleeve  750  is connected to an extension of blanking plate  733  at its right hand end and includes a perforated region  754  at its left hand end (shown schematically as a cross). 
         [0077]    Consideration of the perforated region  734  of the central tube  730  and the perforated region  754  of the sleeve  750  show that they do not axially overlap, i.e. there is a gap G′ between the axial position of the left most hole of the perforated region  734  and the right most hole of the perforated region  754 . 
         [0078]    An injector (not shown) is included in central tube  730  to inject a reagent. 
         [0079]    In use, exhaust gas and reagent are mixed in the mixing chamber and the flow is similar to that of exhaust system  10 , i.e. the exhaust gas and reagent initially travel from left to right until the perforated region  734  is reached, whereupon the exhaust gas flow production will turn and the exhaust gas will flow radially outward through the holes in the perforated region  734  and into the annular region  727  defined between the central tube  730  and the sleeve  750 . When in this inner annular region  727  exhaust gases will be forced to move from right to left towards the perforated region  754 . Upon reaching the perforated regions  754  the exhaust gas will again turn and flow generally radially outwardly through the holes in the perforated region  754  into an outer annular region  728  defined between the cylindrical portion  751  and the shell  722 . When in this outer annular region  728 , the exhaust gases will be forced to move from left to right, and will ultimately pass the blanking plate  733 . 
         [0080]      FIG. 11  shows various axial positions A 1 ″, B 1 ′, C 1 ′, D 1 ′, E 1 ′ and F 1 ′ of the mixing chambers at the appropriate positions. For the purposes of explanation, we can consider a portion (or slug) of exhaust gas S traveling through the mixing chamber. At position A 1 ′, the slug is contained within the central tube  730  which has cross section area A 1 ′. As the slug of gas moves to the B 1 ′ position, it expands, since it is no longer constrained by the central tube  730 , and is only constrained by the sleeve  750 . As shown in  FIG. 12B , the slug occupies a cross section area of the mixing chamber equivalent to A 1 ′ plus A 4 ′. 
         [0081]    Once the slug of gas has reached position C 1 ′, whilst it is still constrained within sleeve  750 , it is constrained on its inner diameter by tube  730 . The gas is therefore flowing through an area A 4 ′ which is necessarily smaller than the volume at position B 1 ′. 
         [0082]    Once the slug of gas reaches the position D 1 ′, it is no longer radially constrained by sleeve  750  and can therefore expand radially outwardly to occupy the volume as shown in  FIG. 12D , i.e. the volume cross section area equivalent to A 4 ′ plus A 5 ′. Continued flow of the exhaust gas to position E 1 ′ again causes a narrowing of the cross section of flow area to A 5 ′. Finally, as the slug of exhaust gas passes blanking plate  733 , it can expand into the outlet (position F 1 ′) which has a cross section flow area equivalent to A 1 ′ plus A 4 ′ plus A 5 ′. 
         [0083]    In summary, the slug S, starting at position A 1 ′, will expand when it reaches position E 1 ′, and then will contract when it reaches position C 1 ′, and then will expand when it reaches position D 1 ′, and then will contract when it reaches position E 1 ′ and then will expand when it reaches position F 1 ′. 
         [0084]    The mixer therefore causes the gas to expand then contract then expand then contract then expand, and this process of repeated expansion and contraction attenuates the exhaust gas noise. 
         [0085]    Consideration of  FIG. 11  shows that the diameter of tube  730  is L 1 . The radial distance between tube  730  and sleeve  750  is L 2 . The radial distance between sleeve  750  and shell  722  is L 3 . 
         [0086]    The length over which tube  730  is perforated is M 1 . The length over which sleeve  750  is perforated is M 2 . It will be noted that M 1  is larger than L 1  and is also larger than L 2 . 
         [0087]    Furthermore, the open area of the perforated region  734  is larger than A 1 ′ and is also larger than A 4 ′. The open area of perforated region  754  is larger than area A 4 ′ and is also larger than area A 5 ′. 
         [0088]    In this manner, the mixing chamber  712  can be arranged to expand, then contract, then expand, then contract, then expand exhaust gas as it passes through the mixing chamber  712 . A similar process of expansion and contraction and expansion and contraction and expansion occurs as exhaust gas pass through the other embodiments shown in the accompanying figures. 
         [0089]    Turning to  FIG. 1 , there are over 200 holes  35 , and there are also over 200 holes  55 . Depending upon the particular circumstances, there may be more or less holes  35  and there may be more or less holes  55 . Typically there will be more than 100 holes  35  to provide for perforated region  34 , and typically there will be over 100 holes  55  to provide for perforated region  54 . However, in further embodiments the perforated region  34  of central tube  30  could be completely removed, thereby creating a simple gap for the gases to pass through. Similarly, the perforated region  54  of the sleeve  50  could be completely removed, thereby creating a simple gap for the exhaust gases to pass through. Clearly, such modifications could be applied to any of the embodiments shown i.e. any of the set of perforations could be removed to create a simple gap for the gases to pass through. 
         [0090]    Whilst the embodiments shown provide a substantially symmetrical flow path, in further embodiments this need not be the case. 
         [0091]    As shown in  FIG. 1 , the perforated region  34  does not axially overlap with the perforated region  54 . However, in further embodiments these perforated regions could axially overlap whilst nevertheless ensuring that there is still a general reversal of the exhaust gas flow within the mixing device. 
         [0092]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.