Patent Abstract:
An after-treatment system includes, in series along an exhaust gas flow direction through the after-treatment system: a diesel oxidation catalyst (DOC), a diesel exhaust fluid (DEF) delivery device, a soot-reducing device and a selective catalytic reduction (SCR) catalyst.

Full Description:
TECHNICAL FIELD 
     This disclosure relates generally to engine systems and, more particularly, to exhaust after-treatment systems and methods. 
     BACKGROUND 
     One known method for abating certain diesel engine exhaust constituents is by use of an exhaust after-treatment system that utilizes Selective Catalytic Reduction (SCR) of nitrogen oxides. In a typical SCR system, urea or a urea-based water solution is mixed with exhaust gas. In some applications, a urea solution is injected directly into an exhaust passage through a specialized injector device. The injected urea solution, which is sometimes referred to as diesel exhaust fluid (DEF), mixes with exhaust gas and breaks down to provide ammonia (NH 3 ) in the exhaust stream. The ammonia then reacts with nitrogen oxides (NO x ) in the exhaust at a catalyst to provide nitrogen gas (N 2 ) and water (H 2 O). 
     In typical applications, especially for large engines, high efficiency diesel particulate filters (DPF) are used in conjunction with NOx reduction systems such as systems using SCR. Such systems are generally quite effective in filtering soot while also converting NOx emissions from diesel exhaust, but such systems are also relatively large in volume. For example, a typical combined DPF/SCR after-treatment system, which may also include AMOX and DOC catalysts, can be approximately 3-6 times engine displacement in volume, which makes it challenging to design and integrate into a vehicle or engine system and also increases overall machine weight and cost. 
     It has been proposed in the past to coat the SCR catalyst onto the DPF filter substrate to eliminate a separate substrate for the SCR catalyst and allow DEF injection upstream of the DPF, but the low temperature soot oxidation reaction and fast SCR reaction will compete for NO 2  during engine operation, which will generally result in high DPF balance points, i.e., a system balance at high soot loadings on the DPF, which is known to make the DPF prone to cracking or catastrophic failure, and requires DPF regeneration at a high temperature. High temperature regeneration often requires so-called active regeneration, which entails conducting the regeneration using a heat source or a high fuel concentration, both of which reduce fuel economy for the machine. 
     One example of a previously proposed after-treatment system can be seen in U.S. Pat. No. 8,413,432 to Mullins et al. (“Mullins”). Mullins describes a regeneration control system for a vehicle that includes a regeneration control module and a regeneration interrupt module. The regeneration control module selectively provides fuel to an oxidation catalyst for a regeneration event of a particulate filter that occurs during a predetermined melting period for frozen dosing agent. The regeneration interrupt module selectively interrupts the regeneration event and disables the provision of fuel to the oxidation catalyst before the regeneration event is complete when a temperature of a dosing agent injector that is located between the oxidation catalyst and the particulate filter is greater than a predetermined temperature. As can be appreciated, therefore, the system of Mullins requires active regeneration. 
     SUMMARY 
     The disclosure describes, in one aspect, an after-treatment system. The after-treatment system is suitable for use, for example, with a machine that includes an engine having an exhaust conduit, which is adapted to route a flow of exhaust gas from the engine during operation. The after-treatment system may be connected to the exhaust conduit and disposed to receive and treat the flow of exhaust gas from the engine. The after-treatment system includes a diesel oxidation catalyst (DOC) connected to the exhaust conduit and arranged to receive the flow of exhaust gas from the engine, a transfer conduit connected in a downstream end of the DOC, a diesel exhaust fluid (DEF) delivery device associated with the transfer conduit and adapted to selectively inject DEF into the transfer conduit to be carried in a downstream direction by gas passing through the transfer conduit during operation, a soot-reducing device connected to a downstream end of the transfer conduit, the soot-reducing device arranged to receive the gas passing through the transfer conduit during operation, and a selective catalytic reduction (SCR) catalyst connected to a downstream end of the DPF opposite the transfer conduit, the SCR catalyst arranged to receive the gas passing through the soot-reducing device during operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an engine having a known SCR system, and  FIG. 2  is a partially sectioned outline view of a known exhaust treatment module. 
         FIG. 3  is a block diagram of an after-treatment system in accordance with the disclosure. 
         FIGS. 4A and 4B  are schematic configurations of a known system packaging envelope, 
         FIGS. 5A and 5B  are schematic configurations of a first embodiment of an after-treatment system in accordance with the disclosure and relative to the known packaging envelope, and 
         FIGS. 6A and 6B  are schematic configurations of a second embodiment of an after-treatment system in accordance with the disclosure and relative to the known packaging envelope. 
         FIG. 7  is a graph showing operation of an after-treatment system over time in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  are representations of an exhaust after-treatment system  100 , which is known in the art. In the illustrated embodiment, the system  100  includes a first module  104  that is fluidly connected to an exhaust conduit  106  of the engine  102 . During engine operation, the first module  104  is arranged to internally receive engine exhaust gas from the conduit  106 . The first module  104  contains a diesel oxidation catalyst (DOC)  108  arranged in series, upstream from a diesel particulate filter (DPF)  110 , each of which has a relatively large frame. It is noted that the CDPF  110  is a coated DPF (CDPF). Exhaust gas provided to the first module  104  by the engine  102  first passes through the DOC  108  and then through the CDPF  110  before entering a transfer conduit  112 . 
     The transfer conduit  112  fluidly interconnects the first module  104  with a second module  114  such that exhaust gas from the engine  102  may pass through the first and second modules  104  and  114  in series before being released at a stack  120  that is connected to the second module. In the illustrated embodiment, the second module  114  encloses a SCR catalyst  116  and an Ammonia Oxidation Catalyst (AMOX)  118 , each formed on its own respective substrate. The SCR catalyst  116  and AMOX  118  operate to treat exhaust gas from the engine  102  in the presence of ammonia, which is provided after degradation of DEF injected into the exhaust gas in the transfer conduit  112 . A regeneration device  130  is disposed upstream of the first module  104  along the conduit  106 . The regeneration device  130 , which can be implemented as a fuel-fired heater, increases exhaust gas temperature for an active regeneration of the CDPF  110 , selectively during operation as is known. 
     The DEF  121  is injected into the transfer conduit  112  by a DEF injector  122 . The DEF  121  is contained within a reservoir  128  and is provided to the DEF injector  122  by a pump  126 . As the DEF  121  is injected into the transfer conduit  112 , it mixes with exhaust gas passing therethrough and is thus carried to the second module  114 . To promote mixing of DEF with exhaust, a mixer  124  may be disposed along the transfer conduit  112 .  FIG. 2  is a partially sectioned outline view of the system  100 , where same or similar structures as corresponding structures previously described are denoted by the same reference numerals previously used for simplicity. As shown in  FIG. 2 , the first and second modules  104  and  114  are disposed next to one another, with the transfer conduit  112  disposed between them. The DEF injector  122  is disposed on an upstream end of the transfer conduit  112  relative to a direction of exhaust gas flow, F. 
       FIG. 3  is a block diagram of an after-treatment system  200  in accordance with the disclosure. The system  200  is configured to replace the system  100  for the engine  102  ( FIG. 1 ) but without necessarily use of the regeneration device  130  and with a smaller package size, as will be described hereinafter. The system  200  includes a DOC  202 , which in this embodiment has a smaller diameter and an overall smaller volume than the DOC  108  ( FIG. 1 ). The DOC  202  may optionally further include a relatively small NOx absorber to improve flow temperature of exhaust gas temperature flowing there through. The system  200  is arranged such that the exhaust conduit  106  from the engine  102  ( FIG. 1 ) provides exhaust gas from the engine  102  to the DOC  202 , which operates in the known fashion. A transfer conduit  204  fluidly interconnects the DOC  202  to a treatment module  206 , which is connected to the stack  120  (also see  FIG. 1 ) either directly or through a muffler (not shown). The treatment module  206  includes a series-compact device  208 , which in the illustrated embodiment includes a DPF  210 , and a combined SCR plus AMOx (SCR/AMOx)  212 . A DEF injector  214  is disposed along the transfer conduit  204  and arranged to inject DEF therein between the DOC  202  and the series-compact device  208  during operation such that injection of DEF occurs downstream of the DOC  202  and upstream of the series-compact device  208 . 
     For achieving desired emissions, the DPF  210  in the illustrated embodiment is a monolithic, wall-flow type substrate that can be made from advanced cordierite (AC) or aluminum titanate (AT) having an asymmetric channel (ACT) construction with larger inlet and smaller outlet channels. The DPF  210  shown has about 300 channels per square inch (cpsi) and is uncoated, uncatalyzed or includes a hydrolysis coating. During operation, the DOC  202  creates NO 2  from NO and O 2  present in the exhaust stream. The NO 2  created by the DOC  202  is carried to the DPF  210  to support a passive regeneration of the DPF  210  at a relatively low temperature of about 200 def. C. 
     The SCR/AMOx  212  of the system  200  in the illustrated embodiment is built on a substrate having about 600 cpsi that is physically connected to the substrate of the DPF  210  or is otherwise in close proximity thereto within the treatment module  206  to act as a single substrate. In the illustrated embodiment, the system  200  operates to remove more than 98% of engine soot on a mass or particulate count basis, and reduces NOx by more than 96% on a mass basis. 
     In general, the after-treatment system  200  may include additional or alternative structures for treating the exhaust gas stream provided from the engine  102 . For example, in an alternative embodiment, a soot-reducing, soot-filtering or soot-removing device such as an electrostatic precipitator, a plasma burner or any other known soot-removing device may be used instead of, or in addition to, the DPF  210  in the after-treatment system  200 . The term soot-reducing device, as used herein, is contemplated to include any structure that operates to at least partially remove soot and/or other particulates from an exhaust stream of an engine as the exhaust stream passes through, over or around the soot-reducing device. Moreover, in an alternative embodiment, the after-treatment system  200  may be configured and/or sized to remove an optimized fraction of soot, for example, between 10% and 90% on a mass or particulate count basis, and to reduce NOx by an optimized fraction, for example, more than 70% on a mass basis, from the flow of exhaust from the engine. 
     INDUSTRIAL APPLICABILITY 
     This disclosure relates to after-treatment systems for diesel engines used alone or in conjunction with other power sources and types in a machine. More particularly, the disclosure describes use of an uncatalyzed or hydrolysis coated low backpressure DPF, which allows DEF dosing upstream of a single can with a series DPF and SCR catalyst. One challenge in designing and integrating a combined DPF/SCR system for an engine in a machine is the requirement for DEF injection to be downstream of the DOC or a catalyzed DPF to avoid ammonia oxidation to NOx. The described embodiments advantageously reduce package size and weight for the after-treatment devices as compared with known systems while maintaining passive soot oxidation capability, i.e., the ability to avoid using active DPF regeneration, which avoid the cost, complexity and fuel consumption increase associated with active regeneration. The described systems and methods, therefore, provide greater flexibility than known systems have to integrate low or high temperature thermal management. Additionally, the systems in accordance with the disclosure provide the capability of moving or relocating the DPF from in-series with the DOC, as is the case in known systems, to a remote location, for example, on the engine. This flexibility also allows the DOC aspect ratio to be optimized for packaging resulting in considerable height and width reductions of 15% or more as compared to previously known systems. Overall, the disclosed systems and methods provide a compact, high efficiency package that works with low or high temperature DPF regeneration. 
     The present disclosure is applicable to internal combustion engines operating in mobile or stationary applications. The disclosed systems are advantageously more compact the systems having comparable emission constituent abatement performance. The systems in accordance with the present disclosure are simpler and more cost effective to operate in that the DPF used is suitable for both passive and active regeneration, which makes use of an active regeneration device optional. 
     To illustrate the package size benefit of the system in accordance with the present disclosure, various qualitative representations are compared. In general, while the DPF  210  and SCR/AMOx  212  may have a diameter that is comparable to the SCR catalyst  116  and AMOX  118  ( FIG. 1 ), the combined series-compact device  208  has an overall length that is quite shorter than the overall combined substrate length of all devices used in the system  100  ( FIG. 1 ), which greatly reduces the overall package size of the various systems. This is discussed below relative to  FIGS. 4A and 4B  as compared to  FIGS. 5A, 5B, 6A and 6B . 
     More specifically,  FIG. 4A  qualitatively shows a packaging envelope  302  for the components of the system  100  ( FIG. 1 ), in block form, from a top perspective, and  FIG. 4B  shows the packaging envelope  302  from a front perspective. As can be seen from these figures, where arrows “F” denote the flow of exhaust gas through each system, a footprint of the packaging envelope  302  is defined by the space that is required to accommodate arrangement of the DOC  108  and CDPF  110  on the right side, and the SCR catalyst  116  and AMOx  118  on the left side of  FIG. 4A  in the orientation shown. A cross sectional area of the packaging envelope  302  is similarly defined by the diameters of the various substrates mentioned above, as well as by the diameter of the transfer conduit  112 , as shown in  FIG. 4B . 
       FIGS. 5A and 5B  show the components of the system  200  in accordance with the disclosure arranged within the packaging envelope  302  of the system  100  for comparison and to illustrate the space-saving nature of the system  200  over the system  100 . As shown, the arrangement of the smaller-diameter DOC  202  allows the centerline of the series-combined substrates for the DPF  210  and the SCR/AMOx  212  to move closer to a centerline of the DOC  202 , which results in an overall narrower combined width for these components and more space being available to route the transfer conduit  112 . Therefore, and as can be seen from  FIGS. 5A and 5B , the volume  304  required to contain or package the system  200  is about 15% less than the volume occupied by the packaging envelope  302 , with additional space being available around the components to route other machine components, add shielding, and the like. 
       FIGS. 6A and 6B  show the components of the system in accordance with an alternative embodiment of the disclosure, in which the DOC  202  is mounted remotely from the remaining components of the system  200 , for example, on the engine or anywhere along the exhaust conduit supplying exhaust gas from the engine to the system  200 . In this embodiment, the DOC  202  is placed outside from the packaging envelope  302  due to its remote mounting. The series-combined substrates for the DPF  210  and the SCR/AMOx  212  are moved to one side of the envelope thus reducing the volume  306  required to contain or package the system  200  by about 50% or more relative to the volume occupied by the packaging envelope  302 . 
     A qualitative graph showing the soot loading in the DPF of the system  200  as compared to the system  100  over time is shown in  FIG. 7 . In the graph, the horizontal axis  308  represents time, for example, in hours, and the vertical axis  310  represents soot loading, for example, as a percentage of a critical soot loading  312  at which the DPF plugging with soot particles is beyond a desired extent and may render the DPF essentially plugged. The graph shows two curves, a first curve  314  and a second curve  316 . The first curve  314  represents a soot loading over time of the CDPF  110  ( FIG. 1 ) of the system  100 , which is considered as the baseline system. The second curve  316  represents a soot loading over time of the DPF  210  of the system  200  ( FIG. 3 ) of the system  200  in accordance with the disclosure. 
     As can be seen from the graph, the soot loading in both DPFs increases initially before stabilizing and reaching a balance point over time because in both systems  100 ,  200  the DPF continuously regenerates during operation and reaches a steady-state soot loading. When comparing the curves  314  and  316 , it can be seen that the loading in the DPF  210  in the system  200  settles at a soot loading that is higher than the corresponding soot loading in the CDPF  110  in the system  100 . However, although the soot loading in the DPF  210  is higher than the loading in the CDPF  110 , both are still below the critical soot loading  312 . As a practical matter, the higher soot loading in the DPF  210 , which may increase the pressure drop across the DPF, will not appreciably affect engine operation given the relatively higher cell density of the SCR/AMOx  212  used in the system  200  as compared to the system  100 . 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

Technology Classification (CPC): 5