Abstract:
Embodiments described herein relate to internal combustion engine exhaust after-treatment systems and to methods of treating exhaust of an internal combustion engine.

Description:
TECHNICAL FIELD 
     This disclosure relates to internal combustion engines, especially diesel engines like those used to propel trucks, busses, motor coaches and similar large vehicles. In particular the disclosure relates to the treatment of diesel engine exhaust (sometimes referred to simply as after-treatment or exhaust after-treatment) by selective catalytic reduction (SCR) using urea injection upstream of an SCR catalyst. 
     BACKGROUND 
     Combustion processes that occur within internal diesel combustion engines create exhaust gases that contain nitric oxides, or NOx, an exhaust gas constituent that is the subject of certain governmental regulations. 
     One known process for reducing the nitric oxides content in engine exhaust is SCR. SCR involves chemical reaction, promoted by a suitable catalytic system, between nitric oxides present in the exhaust and a reductant that is introduced into the after-treatment system specifically as a reducing agent. It is known to introduce reductant as an aqueous urea solution that is able to release ammonia by hydrolysis under suitable temperature conditions or due to the action of specific catalysts directly into the exhaust gas stream upstream of an SCR catalyst. The introduction of urea solution may be closed-loop controlled by a processor that analyzes relevant data, such as backpressure, NOx, temperature, and ammonia leakage collected from corresponding sensors, and causes dosing apparatus to meter the solution based on results of the analysis. 
     The urea solution may be introduced by any of various methods, such as by nebulization in which the liquid is mixed with pressurized air and the mixture is allowed to expand into the exhaust stream, or by injection using a specific injection device, or injector, to flow the liquid at a certain pressure through a nozzle or valve into the exhaust stream without the use of air. The latter method, unlike the former, doesn&#39;t seem to promote the unwanted precipitation of solids out of solution. Nebulizing systems also tend to be less cost-effective, and more functionally complex, than injection systems. 
     On the other hand, an airless process may not disperse the liquid within the exhaust stream as well as one that is air-assisted. To improve dispersion in an airless process, a static mixer may be employed upstream of the SCR catalyst but the extent of improvement may be limited. Furthermore, the inclusion of such a device makes a usually unwanted contribution to system backpressure. Some static mixers impart tangential velocity components to the exhaust with respect to the exhaust flow axis and those components tend to promote concentration around the outer margin of the flow stream which can propagate downstream even as far as the SCR catalyst. 
     Non-uniform dispersion and incomplete dissolving of solution within the exhaust flow stream are known to impair efficiency of chemical and catalytic processes, and consequently, should be avoided in a commercial product. 
     SUMMARY 
     Embodiments described herein relate to internal combustion engine exhaust after-treatment systems and to methods of treating exhaust of an internal combustion engine. According to one embodiment, an internal combustion engine comprises an exhaust system through which exhaust gas created by combustion in engine combustion chambers passes to atmosphere and an after-treatment system that treats the exhaust gas before the exhaust gas leaves the exhaust system. The after-treatment system comprises an exhaust flow path having an entrance through which exhaust gas enters the after-treatment system and an exit through which exhaust gas exits the after-treatment system. An SCR catalyst is disposed in the flow path. A partition wall structure is disposed in the flow path upstream of the SCR catalyst for causing exhaust gas flowing toward the SCR catalyst to separate into multiple detached exhaust gas flow streams. One or more ports via which data for one or more characteristics of each detached exhaust gas flow stream can be obtained, and a port through which reductant can be introduced into each detached exhaust gas flow stream are included. 
     Another embodiment provides an engine exhaust after-treatment device for reducing NOx content in engine exhaust. The device comprises an exhaust flow path having an entrance through which exhaust gas enters the device and an exit through which exhaust gas exits the device. An SCR catalyst is disposed in the flow path. A partition wall structure is disposed in the flow path upstream of the SCR catalyst for causing exhaust gas flowing toward the SCR catalyst to separate into multiple detached flow streams. One or more ports via which data about one or more characteristics of each detached exhaust gas flow stream can be obtained, and a port through which reductant can be introduced into each detached exhaust gas flow stream are included. 
     A further embodiment provides a method for treating exhaust gas flowing through an exhaust system of an internal combustion engine. In one embodiment, the method comprises causing exhaust gas flowing toward an SCR catalyst to separate into multiple detached exhaust gas flow streams. Data for one or more characteristics of each detached exhaust gas flow stream is obtained. Introduction of reductant into each detached exhaust gas flow stream is controlled using the data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a generally schematic diagram showing a diesel engine that includes an exhaust after-treatment system. 
         FIG. 2  is a side view of a portion of the after-treatment system of  FIG. 1 . 
         FIG. 3  is an axial end view of  FIG. 2 . 
         FIG. 4  is a view similar to  FIG. 2  showing a modified form. 
         FIG. 5  is an axial end view of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a turbocharged diesel engine  10  having an intake system  12  through which charge air enters and an exhaust system  14  through which exhaust gas resulting from combustion exits, not all details of those two systems that are typically present being shown. Engine  10  is shown in the drawing by way of example as an eight-cylinder version in which cylinders  16  form combustion chambers into which fuel is injected by fuel injectors (not shown) to combust with the charge air that has entered through intake system  12 . Energy released by combustion powers the engine via pistons connected to a crankshaft. 
     When used in a motor vehicle, such as a truck, engine  10  is coupled through a drivetrain to driven wheels that propel the vehicle. Intake valves control the admission of charge air into cylinders  16 , and exhaust valves control the outflow of exhaust gas through exhaust system  14  and ultimately to atmosphere. Before entering the atmosphere however, the exhaust gas is treated by an after-treatment system  18 . 
     After-treatment system  18  comprises several treatment devices in axial succession forming an exhaust treatment flow path  20  having an entrance  22  at which engine exhaust gas that is to be treated enters flow path  20  and an exit  24  through which exhaust gas that has been treated by system  18  exits flow path  20 . 
     The first treatment device comprises a housing containing a diesel oxidation catalyst (DOC)  26  followed by a diesel particulate filter (DPF)  28  through which exhaust gas flow that enters entrance  22  is constrained to pass. DOC  26  oxidizes hydrocarbons and the soluble organic fraction of diesel soot and can accomplish any of several purposes including compliance with tailpipe emission regulations, increasing exhaust gas temperature for DPF regeneration, SCR catalyst preheating, and oxidizing NO into NO 2  in order to a) promote NO 2 -induced soot oxidation and b) create a NO-to-NO 2  ratio favorable for SCR catalyst reaction. DPF  28  traps particulate matter. 
     The second treatment device comprises a housing containing an SCR catalyst  30  for catalytic reaction of reductant and nitric oxides to reduce the nitric oxides content in exhaust gas. 
     The third treatment device comprises a housing containing a slip catalyst  32  for reducing the content of any excess reducant that may be present in exhaust gas passing through it before leaving exit  24  and flowing through one or more tail pipes into the atmosphere. 
     Exhaust gas that has been treated by the first device is conveyed to the second device through a tubular-walled assembly  34  that contains several arrays  341 ,  342 ,  343  in axial succession and will be more fully described later with reference to  FIGS. 2-5 . 
     After its treatment by the second device, exhaust gas is conveyed to the third device through a tubular-walled assembly  36  that contains arrays  361 ,  362  in axial succession and will also be more fully described later. 
       FIG. 1  further shows a reductant system  38  comprising a supply tank  40 , a dosing control unit  42 , and a processor  44 . Tank  40  holds a supply of urea solution that is delivered by dosing control unit  42  to after-treatment system  18  with processor  44  providing control over the quantity of solution introduced into after-treatment system  18 . A conduit  46  carries the urea solution from control unit  42  to array  343 , and a conduit  47  keeps unit  42  supplied with solution from tank  40 . 
     Electric cables  48 ,  50 ,  52 ,  54  are associated with the arrays of assembly  34 . Cables  52 ,  54  have connections to processor  44  in reductant system  38 , which further includes electric cables  56 ,  58  that connect processor  44  and tank  40 . Electric cables  60 ,  62  are associated with the arrays of assembly  36 . As will be more fully explained later, cables  48 ,  50 ,  60 ,  62  also have connections to processor  44  although actual connections are not apparent in  FIG. 1 . 
     Detail of assembly  34  that is presented in  FIGS. 2 and 3  shows a tubular wall  64  of circular cross section that is open at opposite axial ends to which circular annular mounting rings  66  are joined to provide attachment flanges  68  containing threaded through-holes  70  that allow respective axial ends of assembly  34  to be attached to the respective housings of the first and second treatment devices by fasteners (not shown). Seals that are also not shown are disposed between end faces of rings  66  and mating surfaces of the respective treatment device housings to prevent leakage through those joints. 
     The cylindrical space bounded by wall  64  is partitioned by a partition wall structure to create multiple independent parallel channels running lengthwise through assembly  34 . In this embodiment the partition structure comprises a closed cylindrical wall  71  of circular cross section concentric with wall  64 , and four planar walls  72 ,  74 ,  76 ,  78  extending between walls  64  and  71  at 90° intervals about the common axis of walls  64  and  71 . Consequently this embodiment comprises five independent lengthwise channels  80 ,  82 ,  84 ,  86 ,  88  with channel  80  having a circular cross section while the others have substantially identical arcuate cross sections whose circumferential extents are substantially 90° each. Channel  80  has substantially the same transverse cross sectional area along its length as each of the other four. 
     Wall  64  contains three sets  90 ,  92 ,  94  of five through-openings  96  each. Each set accommodates a respective one of the three arrays  341 ,  342 ,  343 . 
     Each array comprises a set of five tubes  98 . An outer end of each tube  98  has sealed communication with a respective through-opening  96 . The tubes of array  341  have inner ends each disposed within a respective channel  80 ,  82 ,  84 ,  86 ,  88  and facing toward entering exhaust gas flow. So do the tubes of array  342  which are spaced downstream of the tubes of array  341 . While the inner ends of the tubes of array  343  are also each disposed within a respective channel  80 ,  82 ,  84 ,  86 ,  88 , they however face away from entering exhaust gas flow. 
     The open inner ends of the three tubes  98  that are within channel  80  are disposed on the common axis of walls  64  and  71 , and wall  71  has three through-openings through which each of those three tubes can pass in a sealed manner. The open inner ends of the remaining twelve tubes  98  are arranged both circumferentially and radially centrally of the respective channel. The three tubes  98  that pass through wall  71  may appear to interfere with that wall in  FIG. 3  because of the scale, but they do not do so and may lie to one side of the wall. 
     Each of the five tubes of array  341  provides for temperature of the exhaust gas that enters the respective channel  80 ,  82 ,  84 ,  86 ,  88  to be measured by a respective sensor. Each of the five tubes of array  342  provides for measurement for nitric oxides content of the exhaust gas whose temperature has been measured by the corresponding sensor of array  341  by a respective NOx sensor. Each piece of data from the five sensors of array  341  is transmitted via cable  48  to processor  44 , as is each piece of data from the five sensors of array  342  via cable  50 . Each of the five tubes of array  343  is used to introduce reductant into the respective channel for entrainment with the respective detached exhaust gas flow headed toward SCR catalyst  30 . 
     The arrangement described defines straight parallel channels which are upstream of SCR catalyst  30 , and through which the respective detached exhaust gas streams flow. Each of the ports at which the temperature sensors are disposed have has an opening to the respective channel that lies substantially in a common plane that is transverse to flow through the channels. Each of the ports at which the NOx sensors are disposed has an opening to the respective channel that lies substantially in a common plane that is transverse to flow through the channels and downstream of the temperature sensing ports. Each of the ports through which reductant can be introduced has an opening to the respective channel that lies substantially in a common plane that is transverse to flow through the channels and downstream of the NOx sensors. 
     The temperature and nitric oxides content data of exhaust gas flowing through each channel are processed in processor  44  according to an algorithm for calculating an appropriate quantity of reductant that should introduced through the respective tube  98  of array  343  to render the subsequent catalytic reaction promoted by SCR  30  effective to reduce the nitric oxides content of the corresponding detached stream to a target level as the stream flows axially through SCR catalyst  30  without contributing to excess ammonia in exhaust gas exiting the SCR catalyst housing. 
     Because of certain transients, disruptions, or the like, the after-treatment system may on occasion not always reduce the nitric oxides content of the corresponding detached stream to the target level as just described, leaving an unwanted excess of ammonia in the flow leaving SCR catalyst  30 . When it is appropriate to remove such excess ammonia, slip catalyst  32  may be employed. 
     Assembly  36  provides a useful sensing and diagnostic aid both when slip catalyst  32  is and isn&#39;t present in an after-treatment system, and both in commercial vehicles and in laboratory testing and development. 
     Assembly  36  has a construction like assembly  34  in that it comprises a tubular wall of circular cross section that is open at opposite axial ends to which circular annular mounting rings are joined to provide attachment flanges containing threaded through-holes that allow respective axial ends to be attached to the respective housings of the second and third treatment devices by fasteners, and seals that are disposed between end faces of the rings and mating surfaces of the respective treatment device housings to prevent leakage through those joints. The interior cylindrical space comprises a partition wall structure that creates multiple independent parallel channels running lengthwise through assembly  36  downstream of SCR  30 . The geometry may be like that of assembly  34 , or different. 
     Assembly  36  accommodates arrays  361 ,  362  in the same way as arrays  341 ,  342 ,  343  are accommodated in assembly  34 , with each array  361 ,  362  comprising a set of tubes  98  whose inner ends are disposed within respective channels to provide for respective sensors of the respective arrays to obtain exhaust gas stream measurements. Sensors in array  361  measure temperature, and sensors in array  362  measure NOx and ammonia content. Each piece of data from the sensors of array  361  is transmitted through the array&#39;s tubes  98  and via cable  60  to processor  44 , as is each piece of data from the sensors of array  362  via its tubes  98  and cable  62 . 
     The measurements of post-SCR NOx and ammonia content by assembly  36  can be used for analyzing effectiveness of an after-treatment system in the laboratory. They can also provide feedback to processor  44  for closed-loop control of reductant introduction into each channel of assembly  34 . 
     The modified form shown in  FIGS. 4 and 5  differs from assembly  34  in that the walled partition structure creates twelve independent parallel channels running lengthwise through the assembly instead of five. The modified structure comprises a closed cylindrical wall  71  of circular cross section concentric with wall  64  whose interior is divided by walls  100 ,  102 ,  104 ,  106  into substantially identical side-by-side sectors of a circle each defining a respective channel through which a respective detached exhaust gas stream can flow. Halfway between each pair of immediately adjacent walls  72 ,  74 ,  76 ,  78  is an additional radial wall  108 ,  110 ,  112 ,  114  that sub-divides the four arcuate channels into a total of eight.