Patent Publication Number: US-7712305-B2

Title: Exhaust aftertreatment system with spiral mixer

Description:
BACKGROUND AND SUMMARY 
   The invention relates to aftertreatment systems for internal combustion engine exhaust, and more particularly to chemical species injection mixing. 
   To address engine emission concerns, new standards continue to be proposed for substantial reduction of various emissions, including NOx and particulate emissions. Increasingly stringent standards will require installation of aftertreatment devices in engine exhaust systems. Some of the aftertreatment technologies require certain chemical species to be injected into the exhaust system. For example, HC or fuel is injected in some active lean NOx systems for NOx reduction, or in active diesel particulate filters (DPF) for regeneration to take place (oxidizing the soot and cleaning the filter), and urea solution is injected in selective catalytic reduction (SCR) systems for NOx reduction. These injected chemical species need to be well mixed with exhaust gas before reaching catalysts or filters for the systems to perform properly. 
   The present invention arose during continuing development efforts directed toward the above exhaust aftertreatment systems. In one aspect, a compact mixer is provided. In a system with exhaust flow along an axial direction, a longer mixing distance/time is enabled without increasing axial length. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view of an exhaust aftertreatment system in accordance with the invention. 
       FIG. 2  is a sectional view taken along line  2 - 2  of  FIG. 1 . 
       FIG. 3  is like  FIG. 1  and shows another embodiment. 
       FIG. 4  is a sectional view taken along line  4 - 4  of  FIG. 3 . 
   

   DETAILED DESCRIPTION 
     FIGS. 1 and 2  show an exhaust aftertreatment system  10  including an exhaust conduit  12  carrying internal combustion engine exhaust from engine  14  to an aftertreatment element  16 ,  FIG. 2 , treating the exhaust, for example a selective catalytic reduction (SCR) catalyst and/or an oxidation catalyst (e.g. a diesel oxidation catalyst, DOC). An injector  18  is provided upstream of aftertreatment element  16  and injects chemical species mixing with the exhaust prior to reaching aftertreatment element  16 . For example, in one embodiment, aqueous urea solution is injected from reservoir or tank  20 . A mixer  22  is provided in the exhaust system upstream of aftertreatment element  16  and mixing the chemical species and the exhaust. The injected chemical species needs to be well-mixed with the exhaust gas prior to reaching aftertreatment element  16  to ensure optimal performance for chemical reaction. Mixer  22  is a spiral chamber  24 . 
   Spiral chamber  24  has a spiral exhaust flow passage  26  around a central axis  28 . The spiral exhaust flow passage has an outer reach  30  spaced radially outwardly of central axis  28 , and has an inner reach  32  spaced radially inwardly of outer reach  30 . Spiral chamber  24  has first and second exhaust flow ports  34  and  36  for exhaust flow therethrough. In the disclosed embodiment, exhaust flow port  34  is an inlet exhaust flow port receiving exhaust from engine  14  as shown at arrow  38 , and exhaust flow port  36  is an outlet exhaust flow port discharging exhaust to aftertreatment element or catalyst  16  as shown at arrow  40 . Inner reach  32  provides the center of the spiral at central axis  28 . Exhaust flow port  34  is at outer reach  30 . Exhaust flow port  36  is at inner reach  32 . Exhaust flows from inner reach  32  of the spiral through outlet exhaust flow port  36  along an axial flow direction  40  along central axis  28 . In the embodiment of  FIGS. 1 ,  2 , an outlet exhaust pipe  42  extends axially from spiral chamber  24  at outlet exhaust flow port  36 . Outlet exhaust pipe  42  has an outer portion  44  extending axially externally of spiral chamber  24  and conducting exhaust axially therethrough for transmission to aftertreatment element  16 . Outlet exhaust pipe  42  has an inner portion  46  extending axially internally of spiral chamber  24 . Inner portion  46  of outlet exhaust pipe  42  is perforated as shown at  48  and receives exhaust through such perforations from spiral chamber  24  at inner reach  32  thereof. 
   Exhaust flows through exhaust flow port  34  along a first flow direction as shown at arrow  38 . Exhaust flows through exhaust flow port  36  along a second flow direction as shown at arrow  40 . Flow directions  38  and  40  are non-parallel to each other. Exhaust flows through exhaust flow port  36  along an axial flow direction  40 . Exhaust flows through exhaust flow port  34  along a lateral flow direction  38  along a lateral plane transverse to axis  28 . Spiral exhaust passage  26  guides exhaust flow along a spiral pattern lying in the noted lateral plane. Exhaust flows through exhaust flow port  34  along the noted flow direction  38  radially relative to axis  28 . An angled guidance wall  49  may optionally be provided at the spiral entrance adjacent port  34 . In another embodiment, exhaust flow port  34  is instead oriented as shown in dashed line at  34   a  such that exhaust flows through exhaust flow port  34   a  along flow direction  38   a  tangentially relative to the noted spiral of spiral exhaust passage  26 , for reduced pressure drop. 
   In the embodiment of  FIGS. 1 ,  2 , an inlet exhaust pipe  50  extends from spiral chamber  24  at inlet exhaust flow port  34 , and injector  18  is in inlet exhaust pipe  50  and injects chemical species into the exhaust prior to and upstream of spiral chamber  24 . In an alternate embodiment, injector  18   a  is in spiral chamber  24  and injects the chemical species from tank  20   a  into exhaust flowing in spiral chamber  24 . 
   Spiral chamber  24  has an inner scroll wall  52  defining spiral exhaust flow passage  26 . Scroll wall  52  may optionally be heated by a heater, e.g. by electrical resistance heating from a voltage source such as a battery  54 , heating the scroll wall to enhance interaction of the chemical species and the exhaust, and to assist evaporation and hydrolysis. In another embodiment, scroll wall  52  may be perforated, for example as shown at  56 , for improved acoustic performance. Spiral chamber  24  has first and second axially spaced chamber end walls  58  and  60 ,  FIG. 2 , and has an outer circumferential housing wall  62  extending axially therebetween. Inner scroll wall  52  is disposed axially between chamber end walls  58  and  60 . 
     FIGS. 3 ,  4  show another embodiment and use like reference numerals from above where appropriate to facilitate understanding. Exhaust aftertreatment system  70  includes exhaust conduit  72  carrying exhaust from engine  14  to aftertreatment element  16 ,  FIG. 4 , treating the exhaust. Injector  18  injects chemical species from tank  20  mixing with the exhaust prior to reaching aftertreatment element  16 . A mixer  74  mixes the chemical species and the exhaust. Mixer  74  is a spiral chamber  76  having a spiral exhaust flow passage  78  around central axis  28 . Spiral exhaust flow passage  78  has an outer reach  80  spaced radially outwardly of central axis  28 , and has an inner reach  82  spaced radially inwardly of outer reach  80 . Spiral chamber  76  has first and second exhaust flow ports  84  and  86  for exhaust flow therethrough. In the embodiment of  FIGS. 3 ,  4 , exhaust flow port  84  is an inlet exhaust flow port receiving exhaust from engine  14  as shown at arrow  88 . Exhaust flow port  86  is an outlet exhaust flow port, and exhaust flows from spiral chamber  76  through outlet exhaust flow port  86  along an axial flow direction  90 . Inner reach  82  provides the center of the spiral at central axis  28 . Exhaust flow port  84  is at outer reach  80 . Exhaust flow port  86  is at inner reach  82  and also along the downstream chamber end wall  92  spanning between inner reach  82  and outer reach  80 , to be described. In the embodiment of  FIGS. 3 ,  4 , outlet exhaust pipe  42  of  FIG. 2  is eliminated, and instead chamber wall  92  is perforated and provides exhaust flow therethrough to aftertreatment element  16 . 
   In  FIGS. 3 ,  4 , exhaust flows through exhaust flow port  84  along flow direction  88 , and exhaust flows through exhaust flow port  86  along flow direction  90 . First and second flow directions  88  and  90  are non-parallel to each other. Exhaust flows through exhaust flow port  86  along axial flow direction  90 . Exhaust flows through exhaust flow port  84  along a lateral flow direction  88  along a lateral plane transverse to axis  28 . Spiral exhaust passage  78  guides exhaust flow along a spiral pattern lying in the noted lateral plane. Exhaust flows through exhaust flow port  84  along the noted flow direction  88  radially relative to axis  28 . In an alternate embodiment, exhaust flow port  84  may instead by oriented like that shown in dashed line at  34   a  in  FIG. 1  such that exhaust flows through the exhaust flow port in a flow direction tangentially relative to the spiral. Injector  18  may be provided in an inlet exhaust pipe  94  extending from the spiral chamber at inlet exhaust flow port  84 , such that injector  18  is in inlet exhaust pipe  94  and injects chemical species into the exhaust prior to and upstream of spiral chamber  76 . Alternatively, the injector may be provided in spiral chamber  76 , for example as shown in dashed line at  18   a  in  FIG. 1 , such that the injector injects the chemical species into the exhaust flowing in spiral chamber  76 . 
   Spiral chamber  76  in  FIGS. 3 ,  4  has an inner scroll wall  96  defining spiral exhaust flow passage  78 . A heater, such as heater  54  in  FIG. 1 , may be provided for heating scroll wall  96  to enhance interaction of the chemical species and the exhaust, e.g. by assisting evaporation and hydrolysis of urea. Scroll wall  96  may be perforated, for example as shown at  98 , to gain additional acoustic performance. Spiral chamber  76  has the noted first and second exhaust flow ports  84 ,  86  for exhaust flow therethrough. Spiral chamber  76  has first and second axially spaced chamber end walls  100  and  92  and an outer circumferential housing wall  102  spanning axially therebetween. Inner scroll wall  96  is disposed axially between chamber end walls  100  and  92 . Chamber wall  92  is perforated at  104  and provides the noted exhaust flow port  86  for exhaust flow therethrough as shown at arrows  90 . This provides improved flow distribution prior to entering aftertreatment catalyst section  16 , to assist optimization of catalyst performance. The perforations  104  of chamber end wall  92  span at least partially between the noted inner and outer reaches  82  and  80  of spiral exhaust flow passage  78 , and provide the noted exhaust flow port  86 . In the embodiment of  FIGS. 3 ,  4 , exhaust flow port  86  is an outlet exhaust flow port supplying exhaust to aftertreatment element  16 , and perforations  104  of chamber end wall  92  distribute flow from outlet exhaust port  86  to aftertreatment element  16 . 
   In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. The different configurations, methods and systems described herein may be used alone or in combination with other configurations, methods, and systems. It is to be expected that various equivalents, alternatives and modifications are possible within the scope of the appended claims.