Abstract:
An exhaust aftertreatment device ( 10 ) includes an aftertreatment element ( 24 ) for treating internal combustion engine exhaust, an injector ( 26 ) for injecting chemical species mixing with the exhaust prior to reaching the aftertreatment element, and a turbulator ( 36 ) turbulating the exhaust to enhance the noted mixing. In a desired combination, a two-stage integrated perforated tube combination structure includes a turbulent mixing tube ( 38 ) disposed in an acoustic tube ( 40 ) and concentrically surrounded thereby.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-part of U.S. patent application Ser. No. 09/981,171, filed Oct. 17, 2001, incorporated herein by reference. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention relates to aftertreatment devices for internal combustion engine exhaust, and more particularly to combined chemical mixing and acoustic effects. 
     To address engine emission concerns, new standards continue to be proposed for substantial reduction of various emissions, including NO X  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 NO X  systems, and additives such as cerium and iron are injected for diesel particulate filter regeneration, and urea solution is injected in selective catalytic reduction (SCR) systems for NO X  reduction. These injected chemical species need to be well mixed with exhaust gas before reaching catalysts or filters for the systems to perform properly. 
     Perforated tubes are widely used in engine exhaust systems for noise reduction. If designed properly, perforated tubes can also create high intensity turbulent flow. The turbulent flow will promote turbulent diffusion of the chemical species and therefor enhance the mixing process. 
     In one aspect of the present invention, improved chemical mixing is provided. 
     In another aspect, the invention integrates a turbulent mixing tube with an acoustic tube into an engine exhaust system. 
     In another aspect, the invention provides an engine exhaust system with two-stage perforated tubes. The system is designed not only to reduce the noise level, but also to enhance the mixing processes of chemical species which are injected into the exhaust system, including for regeneration of diesel particulate filters and for controlling engine NO X  emissions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side schematic sectional view of an exhaust aftertreatment device in accordance with the invention. 
     FIG. 2 is like a portion of FIG.  1  and shows another embodiment. 
     FIG. 3 is like a portion of FIG.  1  and shows another embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an exhaust aftertreatment device  10  including a housing  12  extending axially along an axis  14 , and having an upstream inlet  16  for receiving engine exhaust as shown at arrow  18 , and having a downstream outlet  20  for discharging the exhaust as shown at arrow  22 . An aftertreatment element  24 , for example an SCR catalyst and/or an oxidation catalyst and/or a particulate filter, is provided in the housing for treating the exhaust. An injector  26  is provided in the housing for injecting chemical species as shown at  28  mixing with the exhaust prior to reaching aftertreatment element  24 . For example, in one embodiment, aqueous urea solution is injected from reservoir or tank  30  through tubular conduit  32  and is injected at nozzle or tip  34 , though other chemical species may be used. 
     A turbulator  36  is provided in the housing upstream of aftertreatment element  24  and turbulating the exhaust to enhance the noted chemical mixing upstream of aftertreatment element  24 . The turbulator is provided by a perforated mixing tube  38 . Also provided in the housing is a perforated acoustic tube  40  quieting the exhaust. 
     It has been found that improved performance results from providing the tubes  36  and  40  with different perforation hole sizes, namely by providing the mixing tube  36  with larger perforation hole sizes than acoustic tube  40 . In a particular situation, it has been found that improved performance results when mixing tube  36  has a perforation hole size greater than or equal to one-quarter inch, and when acoustic tube  40  has a perforation hole size less than one-quarter inch, preferably less than or equal to one-eighth inch. It has been found that the noted perforation hole size greater than or equal to one-quarter inch for mixing tube  36  creates improved turbulent diffusion and mixing of the injected chemical species, and that the noted perforation hole size less than one-quarter inch for acoustic tube  40  minimizes aeroacoustic effects. In preferred form, perforation holes  48  of turbulator  36  are square shaped as shown at  47  for generating homogenous and isotropic turbulence, though circular holes are also acceptable as optionally shown at  49 . Perforation holes  52  and  54  of acoustic tube are preferably circular. 
     In the preferred embodiment, mixing tube  36  is conical, preferably frustoconical with a closed nonperforated downstream end  42 . Further in the preferred embodiment, acoustic tube  40  is cylindrical, with a closed nonperforated downstream end  44 . Conical mixing tube  38  has a tapered sidewall  46  with uniform porosity as shown at perforations  48 . Cylindrical acoustic tube  40  has a sidewall  50  with varied porosity, for example as shown at upstream perforations  52  having a higher density than downstream perforations  54 . The varied porosity along a cylindrical sidewall has been found to provide a more even flow therealong. Porosity may also be varied by varying the size, distance, and number of perforation holes. Mixing tube  38  is disposed in acoustic tube  40  and concentrically surrounded thereby. Each of tubes  38  and  40  is upstream of aftertreatment element  24 . In preferred form, mixing tube  38  is upstream of acoustic tube  40 , and mixing tube  38  is within acoustic tube  40 . 
     Mixing tube  38  has an upstream end  56  and a downstream end  58 , and the noted perforated sidewall  46  extending therebetween. Sidewall  46  is perforated at perforations  48  with a porosity selected to provide substantially uniform resistance and even flow along mixing tube  38 . In preferred form, the noted substantially uniform resistance and even flow is provided in combination by a conically tapered sidewall  46  perforated with uniform porosity. The conical shape points downstream such that mixing tube  38  narrows to smaller cross-sectional areas as mixing tube  36  extends from upstream end  56  to downstream end  58 . As above noted, the conical shape is truncated at  42  at downstream end  58 . 
     FIG. 2 uses like reference numerals from above where appropriate to facilitate understanding. In FIG. 2, a screen  60  extends from injector  26  at nozzle or tip  34  and is disposed in mixing tube  38 . Injector  26  at nozzle  34  injects the chemical species along a spray pattern as shown at  28 , FIG. 1, having an injection boundary  62 . Screen  60 , FIG. 2, extends from the injector along injection boundary  62 . As above noted, mixing tube  38  has a conical shape pointing downstream. Screen  60  has a conical shape pointing upstream, namely to an apex or truncated apex at injector tip or nozzle  34 . Conical mixing tube  38  at its tapered sidewall  46  convergingly tapers as it extends downstream. Screen  62  divergingly tapers as it extends downstream. 
     FIG. 3 uses like reference numerals from above where appropriate to facilitate understanding. In FIG. 3, conical screen  60  of FIG. 2 is replaced by a spherical screen  64  around injector tip  34  and extending therefrom. 
     As is known, the injected chemical species undergoes chemical processes in mixing with the exhaust, including chemical decomposition, chemical reaction, and phase change. In a further embodiment, injector  26  is heated by a heat source in addition to heating by the exhaust. In one embodiment, the heat source is provided by a voltage source  70  external of the housing and a pair of electrical conductors  72 ,  74  connecting the voltage source to the injector. Heater  70  is provided for heating the injector and accelerating the noted chemical processes. 
     Also as known, the injected chemical species is subject to coagulation and coalescence. In a further embodiment, a screen such as  60  or  64  is provided, extending from the injector, and a heater is provided for heating the screen to minimize the noted coagulation and coalescence. In one embodiment, such heat source is provided by the same voltage source  70  noted above, and a pair of electrical conductors  76 ,  78  connecting the voltage source to screen  60  or  64 . 
     It is recognized that various equivalents, alternatives, and modifications are possible within the scope of the appended claims.