Abstract:
A mixing device for an exhaust system of a vehicle includes an inlet pipe, through which exhaust gas can enter a mixing chamber of the mixing device, and an outlet pipe for the exhaust gas to leave the mixing chamber. The inlet pipe and the outlet pipe form an angle. The mixing chamber includes an opening for an outlet of a dosage device, by means of which a reducing agent for exhaust gas aftertreatment is introducible into the mixing chamber. A baffle is arranged at a wall of the inlet pipe a baffle, which is induces a turbulence in the exhaust gas flow entering the mixing chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 U.S.C. §119 to UK patent application number 1301152.3, filed on Jan. 22, 2013, the entire disclosure of which is herein expressly incorporated by reference. 
       BACKGROUND AND SUMMARY OF THE INVENTION 
       [0002]    Exemplary embodiments of the present invention relate to a mixing device for an exhaust system of a vehicle. The mixing device comprises an inlet pipe through which exhaust gas can enter a mixing chamber of the mixing device. The exhaust gas can leave the mixing chamber through an outlet pipe of the mixing device. The inlet pipe and the outlet pipe form an angle. The mixing chamber comprises an opening for an outlet of a dosage device, by means of which a reducing agent for exhaust gas aftertreatment is introducible into the mixing chamber. 
         [0003]    PCT patent publication WO 2009/128 885 A1 describes a mixing device designed for the fine distribution of a reducing agent for exhaust gas aftertreatment, for example an urea-water solution. A mixing pipe is arranged within a mixing chamber formed by a pipe with a 90° angle. The mixing pipe has an opening adapted to receive a nozzle of a urea dosage device. A funnel-shaped end portion of the mixing pipe bears against a wall of the mixing chamber. The mixing pipe is provided with a multitude of large-area perforations on its circumference, which enable the exhaust gas to enter the mixing pipe. Other perforations in the funnel-shaped end portion of the mixing pipe lead to an exhaust gas flow along the circumference of an outlet pipe connected to the mixing chamber. 
         [0004]    One disadvantage of this urea mixing device is that it leads to a considerable backpressure of the exhaust system. Additionally, in case of urea as the reducing agent, an enhanced danger for unwanted deposits may exist. 
         [0005]    Extensive mixing of urea with the exhaust gas helps improve the conversion of nitrogen oxides in an SCR catalytic converter (SCR=selective catalytic reduction) arranged downstream of the mixing device in the exhaust system. In the SCR catalytic converter nitrogen oxides (NOx) contained in the exhaust gas react with ammoniac (NH3) released from the urea of the urea containing solution or such a reducing agent. Thus, nitrogen oxides are converted with the ammoniac to nitrogen and water in a selective catalytic reduction reaction. This leads to a decrease of the content of nitrogen oxides in the exhaust gas. 
         [0006]    Exemplary embodiments of the present invention provide a mixing device that achieves an enhanced mixing of the reducing agent with the exhaust gas. 
         [0007]    The mixing device according to the invention comprises a baffle element. This baffle element is arranged at a wall of the inlet pipe, and it is designed to induce a turbulence in the exhaust gas flow entering the mixing chamber. The baffle element guides the exhaust flow and introduces a turbulence or swirl in the flow field downstream of the baffle element. Therefore, turbulences are already induced at a location of the mixing device, where the reducing agent is injected into the mixing device and the mixing of reducing agent and exhaust gas takes place. 
         [0008]    Such a mixing device significantly enhances the mixing of the reducing agent, in particular the mixing of urea with the exhaust gas. With urea as the reducing agent this leads to an improvement of the conversion efficiency regarding nitrogen oxides in an SCR catalytic converter arranged downstream of the mixing device in the exhaust system. 
         [0009]    Due to the performance improvement of the mixing device a cost reduction in the operation of the mixing device can be achieved, as less reducing agent needs to be utilized. As the design of the mixing device with the baffle element arranged inside the inlet pipe is very simple, there are low maintenance costs and relatively few parts are incorporated in the mixing device. 
         [0010]    Another advantage is the reduction in the chance of a deposit of the reducing agent inside the mixing chamber and downstream of the mixing chamber. This is also due to the open design of a volume of the mixing chamber close to the dosage device. In other words the mixing chamber is empty, i.e. devoid of any elements that would increase backpressure. 
         [0011]    The turbulences introduced in the exhaust gas entering the mixing chamber very efficiently avoids the deposit of the reducing agent within the mixing chamber. The mixing device is therefore highly effective and a low cost exhaust gas aftertreatment component. 
         [0012]    In an advantageous embodiment of the invention the baffle element is arc-shaped with an outer boundary contacting the wall of the inlet pipe and an inner boundary forming a section of a circular-arc. Such a baffle element has proven to be particularly effective in inducing turbulences in the exhaust gas flow and also guiding the flow direction of exhaust gas in an advantageous manner, while the backpressure is only slightly increased by the baffle element. 
         [0013]    It has further proven to be advantageous if the baffle element is contacting the wall of the inlet pipe substantially along a quarter of an inner circumference of the inlet pipe. This leaves a large part of the inner circumference of the inlet pipe free of any flow guiding element, but nevertheless the baffle element is efficient in inducing turbulences. 
         [0014]    A first section of the wall of the inlet pipe is located on the same side of the mixing device as the opening in the mixing chamber. A second section of the wall of the inlet pipe is located opposite the first section. Preferably at least a major part of the baffle element is arranged at a portion of the wall of the inlet pipe, which is located between this first section and the second section. In other words the baffle element is preferably arranged at other locations of the inlet pipe than the first section and the second section. However, the baffle element can be arranged in a quadrant of the inlet pipe adjoining an imaginary central plane going through the opening in the mixing chamber and dividing the inlet pipe into two halves. It has been found out that a particularly good distribution of reducing agent in the exhaust gas downstream of the mixing device can be achieved by such arrangements of the baffle element. 
         [0015]    In a further advantageous development of the invention a width of the baffle element is less than 25% of a diameter of the inlet pipe. With such a relatively small height of the baffle element the mixing device can achieve particularly good backpressure values. This is particularly true if the width of the baffle element is about 10% of the diameter of the inlet pipe. 
         [0016]    It is further advantageous if a portion of the mixing chamber, which is closer to the inlet pipe than to the outlet pipe, is substantially square-shaped. Such a shape of a main part or body part of the mixing chamber allows the exhaust flow to stay relatively long within the mixing chamber and to interact to a very high degree with the reducing agent. This further enhances the mixing of the reducing agent spray with the exhaust gas. 
         [0017]    In a further advantageous development of the invention the mixing chamber comprises a bulge in a side wall facing the inlet pipe. This leads to an enhanced formation of turbulences or swirls in the mixing chamber itself and in the outlet pipe connected to the mixing chamber. The uniformity of mixing is largely improvable by this design of the mixing chamber. Also, the flow inertia and the characteristics of the exhaust gas-reducing agent interaction are utilized by this design of the mixing chamber. This bulge or pocket, which is located in a prolongation of the inlet pipe, has proven to be very effective in producing strong swirls in the exhaust gas flow. 
         [0018]    During operation of the exhaust system the exhaust gas flow from the inlet pipe is guided by the flat side walls of the mixing chamber to the bulge or pocket at the bottom. The flow thus experiences one or more swirls in a volume close to the bulge before it turns towards the outlet pipe. The strong swirls induced by this design of the mixing chamber are effective even in the outlet pipe. This further enhances the mixing of the reducing agent with the exhaust gas. 
         [0019]    A deposit of reducing agent within the bulge can be minimized, if according to a further advantageous development of the invention a portion of the bulge is bevelled, which is adjacent to a wall of the mixing chamber, in which the outlet opening of the dosage device is located. 
         [0020]    By the sloped design of this portion of the bulge, which is adjacent to the wall of the mixing chamber with the opening, there is no flow recirculation in this region, which could otherwise lead to a deposit of the reducing agent within the mixing chamber. As such a deposit is critical for the performance of the aftertreatment device, bevelling this portion of the bulge improves the performance. 
         [0021]    In order to minimize backpressure and to ease the flow from the inlet pipe into the mixing chamber and from the mixing chamber to the outlet pipe, it has further proven to be advantageous if the inlet pipe and the outlet pipe are smoothly fitted in respective accesses of the mixing chamber. Such blended transitions considerably help reducing backpressure. 
         [0022]    For a simplified introduction of the reducing agent into the mixing chamber and a good mixing performance it has further turned out to be advantageous if the inlet pipe and the outlet pipe form an angle of about 90°. 
         [0023]    The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination, but also in other combinations or alone without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0024]    Further advantages, features and details of the invention are apparent from the claims, the following description of preferred embodiments as well as based on the drawings, in which identical or functionally identical elements are provided with identical reference characters. Therein show: 
           [0025]      FIG. 1  a urea mixer for an exhaust system of a vehicle, comprising an inlet pipe, a mixing chamber and an outlet pipe, wherein inside the inlet pipe a baffle is arranged; 
           [0026]      FIG. 2  a schematic of components of the urea mixer shown in  FIG. 1 ; 
           [0027]      FIG. 3  the mixing chamber of the urea mixer shown in  FIG. 1  in a perspective view; 
           [0028]      FIG. 4  three potential locations of the baffle within the inlet pipe; 
           [0029]      FIG. 5  a side sectional view and a front view of parts of the urea mixer with a first arrangement of the baffle inside the inlet pipe according to  FIG. 4 ; 
           [0030]      FIG. 6  a side sectional view and a front view of parts of the urea mixer with a second arrangement of the baffle inside the inlet pipe according to  FIG. 4 ; 
           [0031]      FIG. 7  a side sectional view and a front view of parts of the urea mixer with a third arrangement of the baffle inside the inlet pipe according to  FIG. 4 ; and 
           [0032]      FIG. 8  the inlet pipe with a baffle which has a reduced height compared to the baffles shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 1  shows a urea mixer  10  that is part of an exhaust system of a vehicle. The urea mixer  10  comprises an inlet pipe  12  connected to a mixing chamber  14 . Through the inlet pipe  12  the exhaust gas enters the mixing chamber  14 , and from the mixing chamber  14  the exhaust gas flows to an outlet pipe  16  of the urea mixer  10 . The mixing chamber  14  has an elbow-shape. Consequently a center axis Ml of the inlet pipe  12  and a center axis M 2  of the outlet pipe  16  form an angle, in particular an angle of about 90° (see  FIG. 2 ). 
         [0034]    An opening  22  is arranged in a rear wall  18  of a portion  20  of the mixing chamber  14 . This opening  22  is designed to accommodate a nozzle of a dosage device (not illustrated) for introducing liquid urea into the mixing chamber  14 . As the rear wall  18  faces the outlet pipe  16 , the urea-water solution is injected in the direction of the center axis M 2  of the outlet pipe  16  upon entering the mixing chamber  14 . 
         [0035]    A line  24  in  FIG. 1  marks a location inside the inlet pipe  12  where a baffle  26  (see  FIG. 4 ) is arranged at a wall  28  of the inlet pipe  12 . This baffle  26  leads to the formation of turbulences or swirls in the exhaust gas flow entering the mixing chamber  14 . 
         [0036]    As is apparent in particular from the schematic illustration of components of the urea mixer  10  shown in  FIG. 2 , the portion  20  of the mixing chamber  14  that is closer to the inlet pipe  12  than to the outlet pipe  16  is substantially square-shaped or box-shaped. A bottom wall of this box-shaped portion  20  forms a bulge or pocket  30 . A corner of this pocket  20  is cut away. In other words, a portion  32  of the pocket  30  adjacent to the rear wall  18  is preferably bevelled. This square-shape of the portion  20  or main body of the mixing chamber  14  together with the pocket  30  having the cut away corner region or slanted portion  32  leads to the formation of further swirls in the exhaust flow out of the inlet pipe  12  when it reaches the pocket  30  at the bottom of the mixing chamber  14 . The cut away corner at the portion  32  of the pocket  30  reduces the chance of urea deposit within the mixing chamber  14 . 
         [0037]    It is particularly apparent from the perspective view of the mixing chamber  14  in  FIG. 3  that an access  34  of the mixing chamber  14 , which is designed to accommodate the inlet pipe  12 , guarantees a smooth fit of the inlet pipe  12  when connected to the mixing chamber  14 . Another connecting region  36  of the mixing chamber  14 , which serves as an access for fitting the outlet pipe  16  to the mixing chamber  14  is also smoothly blended in order to reduce backpressure (see  FIG. 1 ). 
         [0038]      FIG. 4  shows potential locations of the baffle  26  inside the inlet pipe  12 , wherein the view direction is along the center axis Ml of the inlet pipe  12 . The baffle  26  is arc-shaped with an inner boundary  38  being in contact with the wall  28  of the inlet pipe  12  and an outer boundary  40  forming a section of a circular arc. 
         [0039]    In all three alternatives shown in  FIG. 4  this arc-shaped baffle  26  contacts the wall  28  of the inlet pipe  12  along a quarter of an inner circumference of the inlet pipe  12 . The width of the baffle  26  is about 25% of the diameter of the inlet pipe  12 . 
         [0040]    In a first example shown in the left part of  FIG. 4  this quarter baffle  26  is located in the middle between a first point  42  and a second point  44  located on the wall  28  of the inlet pipe  12 . The first point  42  lies on the same side of the urea mixer  10  as the opening  22  in the rear wall  18  of the mixing chamber  14 . The second point  44  is located opposite the first point  42 . 
         [0041]    In other words these two points  42 ,  44  are located on the intersection of an imaginary plane  46  dividing the inlet pipe  12  into two halves, wherein this plane  46  also goes through the opening  22 . In  FIG. 4  a second imaginary plane  48  is indicated, which also divides the inlet pipe  12  into two halves and which is perpendicular to first plane  46 . In the example shown in the left part of  FIG. 4  the baffle  26  is symmetrical with respect to this second plane  48 . 
         [0042]    In a second example shown in the middle of  FIG. 4  the baffle  26  is arranged between the first plane  46  and the second plane  48 , but in a quadrant adjacent to the first point  42 . In a third example shown in the right part of  FIG. 4  the baffle  26  is also arranged between the first plane  46  and the second plane  48  but in the lower right quadrant, i.e. adjacent to the second point  44 . 
         [0043]    The exact location of this partial baffle  26  or quarter baffle  26  within the inlet pipe  12  depends on the exhaust flow conditions in the inlet pipe  12  and can be determined by computational fluid dynamics (CFD) analysis. CFD analysis is also utilized in order to determine the shape of the mixing chamber  14 . 
         [0044]      FIG. 5  shows droplet trajectories  50  within the mixing chamber  14  in a case when the baffle  26  is arranged in the inlet pipe  12  as shown in the left part of  FIG. 4 . 
         [0045]      FIG. 6  shows the droplet trajectories  50  when the baffle  26  is arranged as shown in the example represented in the middle of  FIG. 4 . 
         [0046]      FIG. 7  shows the droplet trajectories  50  in a case when the baffle  26  is arranged at the wall  28  of the inlet pipe  12  as presented in the right part of  FIG. 4 . 
         [0047]    All these arrangements of the baffle  26  lead to a good uniformity of distribution of ammoniac in the outlet pipe  16 . The exhaust flow produces strong swirls in the region of the pocket  30 . Furthermore swirls or turbulences exist as well in the portion  20  of the mixing chamber  14  as in the straight outlet pipe  16 . 
         [0048]      FIG. 8  shows an example, where the baffle  26  has a width or height of about 10% of the diameter of the inlet pipe  12 . This design further reduces backpressure of the urea mixer  10 . This baffle  26  with a lower width is arranged at the location of the wall  28  of the inlet pipe  12  as represented in  FIG. 6  and in the middle of  FIG. 4  respectively. 
         [0049]    This design of the urea mixer  10  shows an acceptable backpressure and a very uniform distribution of urea and/or ammoniac within the outlet pipe  16  of the urea mixer  10 . 
         [0050]    The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.