Abstract:
An exhaust system for reducing particulate and NO 2  emissions from diesel engine exhaust gases. The exhaust system includes a diesel particulate filter, an inlet for receiving the exhaust gases, a first conduit for providing a first fluid connection between the inlet and the diesel particulate filter, and a second conduit for providing a second fluid connection between the inlet and the diesel particulate filter. The exhaust system also includes a diesel oxidation catalyst in the second conduit for catalysing hydrocarbon combustion and the formation of NO 2  and having high I-IC activity and high NO 2  activity, and a fuel injector for injecting fuel upstream of the diesel oxidation catalyst. A valve mechanism for selectively directing exhaust gases from the inlet to the diesel particulate filter through the first conduit or the second conduit is also included. A method for reducing particulate and NO 2  emissions using the exhaust system is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a National Stage application of International Application No. PCT/EP2013/050236, filed on Jan. 8, 2013, which claims priority of Great Britain patent application number 1200230.9, filed on Jan. 9, 2012, both of which are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    a. Field of the Invention 
         [0003]    The present invention relates to an exhaust system for reducing particulate matter and NO 2  levels in exhaust gases from diesel engines, and to a method for using the system. 
         [0004]    b. Related Art 
         [0005]    Historically, NOx and particulate emissions have been of particular concern when developing diesel engines and aftertreatments. More recently, legislation and local air quality improvement schemes are now also targeting NO 2  reduction. 
         [0006]    SCRT is a technology that can control NOx, particulate, CO and HC (hydrocarbon) emissions. However it is difficult also to control NO 2  emissions because significant quantities of NO 2  are generated within the system in order to facilitate the passive regeneration of the CRT. 
         [0007]    SCR has the potential to control NOx and NO 2 , but particulate reduction might be difficult to achieve downstream of this because there is limited energy available in the exhaust gas to enable particulate burn (regeneration) over the filter. 
         [0008]    It is known to provide a system in which the DPF regenerates at low temperatures. The system is connected downstream of a turbocharger or engine and the exhaust gas is split into a main pipe and a bypass. The bypass has a heating element and means for injecting diesel fuel into the exhaust gas stream. A first DOC provides heating through the exothermic heat from catalytic fuel burning. Downstream of the first DOC, the two pipes join together. Gases from the combined pipes are fed to a second DOC and the DPF. The second DOC also serves to burn hydrocarbons, to increase the temperature to a level at which the filter can regenerate. Sensors are used to measure temperatures at various points, and relative gas flows through the pipes are adjusted, together with pre-heating using the electric heater, to ensure that gases reaching the second DOC are above a threshold temperature required for efficient hydrocarbon conversion. The system meters flow through the first DOC between 0% to 100% in order to achieve the ideal flow rate for maximum performance. The use of supplementary heating and the flow rate adjustment system has significant cost and development disadvantages. Moreover, because exhaust gases flow through the DOC, which generates NO 2  it is necessary to use a special oxidation catalyst which is selected for low NO 2  formation but which is less effective than others for catalysing HC conversion. 
         [0009]    US 2010/0037607 describes a system with parallel exhaust gas flows. The system has a NO oxidation catalyst in one stream and a HC oxidation catalyst in the other. The HC catalyst acts as a heater when fuel is injected upstream. This allows selective heating of the DPF without heating of the NO catalyst. This is said to optimise the catalytic activity of the NO oxidation catalyst. 
         [0010]    US 2003/0089104 describes an exhaust system which has an exhaust line with an oxidation catalyser and a catalytically-coated DPF. During a DPF regeneration phase, post-injection of fuel takes place to provide unburned HC to the oxidation catalyst which oxidises the HC. A bypass circuit and valve arrangement permits exhaust gas with unburned fuel to bypass the oxidation catalyst. When regeneration of the filter is triggered, a proportion of the exhaust gases with injected HC is diverted straight to the DPF without passing through the oxidiser. This arrangement is said to increase the combustion rate of particles trapped in the DPF. 
         [0011]    Other prior art systems are described in the following documents: JP2010043577, US2006/117742, US2010/199634, JP2010209783, EP2014348, US2008/155968, EP2309103, JP2006233947, US2011/192143, US2009/277159, US2011/146233, US2010/242438, US2009/178393, WO07/136148, US2009/260346, US2005/031514. 
         [0012]    An industry challenge is to develop a DPF system that can regenerate at low temperature, with low tailpipe NO 2  and, if necessary, in combination with other aftertreatment systems, such as those for reducing NOx. 
       SUMMARY OF THE INVENTION 
       [0013]    The invention provides a system and method for reducing tailpipe NO 2  emissions while providing for DPF regeneration. According to an aspect of the invention, an exhaust system for reducing particulate and NO 2  emissions from diesel engine exhaust gases is provided. The exhaust system includes a diesel particulate filter, an inlet for receiving exhaust gases from a diesel engine, a first conduit capable of providing a first fluid connection between the inlet and the diesel particulate filter, and a second conduit capable of providing a second fluid connection between the inlet and the diesel particulate filter. The exhaust system also includes a diesel oxidation catalyst in the second conduit for catalysing hydrocarbon combustion and the formation of NO 2  and having high HC activity and high NO 2  activity. A valve mechanism for selectively directing exhaust gases from the inlet to the diesel particulate filter through the first conduit or the second conduit is also included, wherein exhaust gases that pass from the inlet to the diesel particulate filter through the first conduit but not the second conduit will not encounter a diesel oxidation catalyst. The exhaust system also includes a fuel injector for injecting fuel upstream of the diesel oxidation catalyst. 
         [0014]    A method for reducing particulate and NO 2  emissions from diesel engine exhaust gases using the exhaust system according to an aspect of the invention is also provided. According to an aspect of the invention, the method includes, in normal mode, operating the valve mechanism to direct most or all of the exhaust gases entering the inlet, to the diesel particulate filter via the first conduit; and in regeneration mode, operating the valve mechanism to direct most or all of the exhaust gases entering the inlet, to the diesel particulate filter via the second conduit. 
       Definitions 
       [0015]    When used herein, the following definitions define the stated term: 
         [0016]    “CO” is carbon monoxide. 
         [0017]    A “CRT” is a Continuously Regenerating Trap system for the removal of PM from the exhaust gas stream using a wall-flow filter. The system operates passively and is self cleaning. It achieves this by using a catalyst upstream of the filter to produce exhaust gas conditions that enable the carbon fraction of the PM to be burnt off at typical diesel exhaust gas temperatures. 
         [0018]    A “DOC” is a Diesel Oxidation Catalyst, which is used to promote burning of diesel hydrocarbons in an exhaust gas flow. 
         [0019]    A “DPF” is a Diesel Particulate Filter, which is used to remove PM from exhaust gases. 
         [0020]    “HC” means hydrocarbon. 
         [0021]    “High HC activity” means a catalyst which when fresh will cause or promote combustion of at least 70% (preferably at least 80%) of HC in an exhaust gas at 300° C. 
         [0022]    “High NO 2  activity” means a catalyst which when fresh will cause or promote conversion of at least 60% (preferably at least 70%) of NO to NO 2  in an exhaust gas at 300° C. 
         [0023]    “NO” is nitric oxide. 
         [0024]    “NO 2 ” is nitrogen dioxide, a major contributor to photochemical smog and acid rain. It can be removed from an exhaust gas stream by SCR. 
         [0025]    “NOx” is a generic term for all oxides of nitrogen. 
         [0026]    “PM” is Particulate Matter, the solid content of exhaust gases, primarily soot (carbon) and ash. 
         [0027]    “SCR” is Selective Catalytic Reduction, a process for removing NOx by reducing with a reductant such as ammonia over a catalyst. 
         [0028]    “SCRT”® is a combination of SCR and CRT in a single exhaust emissions reduction system. It is capable of removing NOx, PM, HC and CO. SCRT is a registered trade mark of Johnson Matthey PLC. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The invention will now be further illustrated, by way of example only, with reference to the following drawings, in which: 
           [0030]      FIG. 1  is a schematic representation of an exhaust system in accordance with an aspect of the present invention; 
           [0031]      FIG. 2  is a schematic representation corresponding to  FIG. 1 , in by-pass mode; 
           [0032]      FIG. 3  is a graph illustrating decrease in NO 2  output relative to the engine output level in bypass mode; 
           [0033]      FIGS. 4 and 5  correspond to  FIGS. 2 and 3  for the exhaust system in light-off mode; 
           [0034]      FIGS. 6 and 7  correspond to  FIGS. 2 and 3  for the exhaust system in regeneration mode; 
           [0035]      FIG. 8  is a graph showing net tailpipe NO 2  reduction over an eight hour operation; and 
           [0036]      FIGS. 9 to 13  illustrate alternative embodiments of exhaust systems in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    An exhaust system  2  comprises an inlet  6  for receiving exhaust gases from a diesel engine (not shown) and a first conduit  4  connected to a DPF module  10  via an outlet  8 . The first conduit  4  is capable of providing a first fluid connection between the inlet  6  and the DPF  10 . A second conduit  12  is capable of providing a second fluid connection between the inlet  6  and the DPF  10 . The second conduit  12  houses a DOC  14 . The DOC catalyst has high HC activity and high NO 2  activity; when fresh, the DOC converts over 80% HC and over 70% NO 2 at 300° C. Suitable DOCs with high HC activity and high NO 2  activity will be well known to those skilled in the art of exhaust emission control. 
         [0038]    A fuel injector  16  is arranged to inject fuel into the exhaust gas stream upstream of the DOC. A valve mechanism  18  is adjustable for selectively directing exhaust gases from the inlet  6  to the DPF  10  through the first conduit  4  or the second conduit  12 . The fuel injector  16  is between the valve mechanism  18  and the DOC  14 . In this example, the fuel injector  16  is upstream of the valve mechanism  18  but could alternatively be in a separate stream within the second conduit  12 . Gases which pass through the second conduit  12  encounter the DOC  14  before reaching the DPF  10 . Gases which pass through the first conduit  4  reach the DPF  10  without encountering a diesel oxidation catalyst. Filtered exhaust gases exit the DPF  10  via a tailpipe  28 . 
         [0039]    In this embodiment, the exhaust system  2  is illustrated in combination with an upstream SCR unit  20 . The SCR  20  has an injector  22  for introducing a reductant such as ammonia, and an optional inlet module oxidation catalyst  24  and an optional outlet module  26  with slip catalyst. It will be appreciated that the invention is not limited to use with an SCR. It may be used as a standalone active DPF system or may be combined with other aftertreatment or exhaust technologies. 
         [0040]    Referring to  FIG. 2 , in bypass (soot filter) operating mode the valve mechanism  18  is open, and exhaust gases entering via the inlet  6  substantially bypass the DOC  14  and reach the DPF  10  without encountering an oxidation catalyst. This limits or eliminates NO 2 production over the DOC. A reduction in engine-out NO 2  occurs over the DPF  10  by passive reduction of NO 2  over accumulated particulate matter. The decrease in NO 2  relative to the engine-out level is illustrated in  FIG. 3 ; both soot and NO 2  levels are reduced by the DPF. 
         [0041]    Referring now to  FIG. 4 , in light-off mode, the valve mechanism  18  closes off the path through the first conduit  4 , diverting exhaust gases through the second conduit  12  and DOC  14 . Combustible components of the hot exhaust gases, notably CO and HC, are catalytically oxidised over the DOC  14 . We have found that it is during this brief light-off period that the most significant quantity of NO 2  is produced, as shown in  FIG. 5 . 
         [0042]    When the DOC achieves light-off, the system switches to regeneration mode, and fuel is injected via the fuel injector  16  to oxidise over the DOC ( FIG. 6 ). The resulting exotherm increases the temperature of the exhaust gas sufficiently to combust the organic fraction of PM (predominantly carbon/adsorbed HCs). This enables burning of the PM accumulated within the DPF and regeneration of the DPF filtering capacity. Combustion of the organic fraction of PM on the DPF could optionally be catalysed by use of a fuel-borne catalyst so that the temperature required to initiate PM combustion is lowered, requiring less energy to start regeneration. In this embodiment, the fuel injector  16  is arranged and adapted to direct most or substantially all of the injected fuel directly into the second conduit  12 . By ‘directly’ we mean without substantial axial travel in the first conduit  4 . As illustrated, the fuel injector  16  is positioned directly opposite the entrance to the second conduit  12  where it branches from the first conduit  4 . The injected fuel is directed straight across the first conduit  4  into the second conduit  12 . The fuel injector  16  could alternatively be located in a wall of the second conduit  12  upstream of the DOC  14 . 
         [0043]    As shown in  FIG. 7 , NO 2  production is suppressed when fuel injection takes place. The catalyst has preferred selectivity towards HC oxidation rather than NO oxidation. 
         [0044]    Tailpipe NO 2  output is relatively high during the light-off mode, as illustrated in  FIG. 5 , but this mode is operated for only a small proportion of the duty cycle. This offsets some, but not all, of the NO 2  reduction that occurs during the rest of the duty cycle in bypass (soot filter) mode. Results for an eight hour shift at steady state operation are shown in  FIG. 8 . Cumulative tailpipe NO 2  is substantially reduced with respect to cumulative engine-out NO 2 . The invention provides a regenerative DPF system with low tailpipe NO 2 . The system may regenerate at low temperatures, particularly if a fuel-borne catalyst is used, and it may be used in combination with other aftertreatment systems, such as those for reducing NOx. 
         [0045]    In the embodiment illustrated in  FIG. 1 , the valve mechanism  18  is a simple butterfly valve located in the first conduit. When the valve  18  is open, most exhaust gases pass through the first conduit to the DPF. 
         [0046]    The relatively low gas flow through the second conduit  12  can be eliminated completely by use of a second valve  19  in the second conduit, as illustrated in the embodiment shown in  FIG. 9 . The first valve  18  and the second valve  19  are independently controllable to allow exhaust gases to flow through only the first conduit  4  or the second conduit  12  according to the operating mode. 
         [0047]    Referring now to  FIG. 10 , another embodiment is illustrated, which uses a three-way valve  18  at the junction of the conduits  4 , 12  for selectively directing exhaust gases through either or both conduits. 
         [0048]    The first conduit  4  and the second conduit  12  may be arranged in any convenient manner. In the embodiment illustrated in  FIG. 11 , the conduits  4 , 12  are coaxial. A valve  18  in the central second conduit  12  selectively permits exhaust gas flow through the DOC  14  during light-off and regeneration modes. Because the second conduit  12  is aligned with the exhaust gas inlet  6 , gases quickly reach the DOC  14  when the valve is opened. Accordingly, this embodiment provides fast light-off when light-off mode is selected. It will be understood that the DOC  14  could alternatively be located in the outer annular passage, which would function as the second conduit. It will be further understood that the valve mechanism  18  could alternatively be provided in the outer annular passage. 
         [0049]    The embodiment illustrated in  FIG. 12  is a similar fast light-off arrangement to that of  FIG. 11 , but with an alternative valve mechanism  18  that allows all exhaust gases to be diverted through either the inner conduit or the outer annular conduit. The valve mechanism  18  comprises a first valve disc  18   a  and a second valve disc  18   b,  each of which is provided with at least one outer aperture  30  and at least one inner aperture  32  which correspond respectively with the first conduit  4  and the second conduit  12 . Relative rotation of the valve discs selectively brings the outer apertures  30  into alignment and the inner apertures  32  out of alignment. Further relative rotation of the valve discs selectively brings the outer apertures  30  out of alignment and the inner apertures  32  into alignment. When the inner apertures  32  are aligned, the central (second) conduit  12  is opened, and when the outer apertures  30  are aligned, the annular (first) conduit  4  is opened. At intermediate alignments the valve mechanism  18  optionally prevents gas flow through either conduit. 
         [0050]    As illustrated, the DOC  14  is located in the inner conduit which functions as the second conduit  12 , while the annular conduit functions as the first conduit  4 . It is appreciated that the DOC could alternatively be provided in the annular conduit. 
         [0051]    It will be appreciated that various sensors and control systems may be employed to optimise performance of the exhaust system. For example, the temperature of exhaust gases may be monitored at or downstream of the DOC to determine when light-off occurs, and to trigger fuel injection at or after light-off. Accurate control of the fuel injection rate is preferred in order to limit unburned fuel passing through the DOC. The pressure difference over the DPF may be monitored, and regeneration mode triggered when a threshold Δp value is reached, corresponding to a threshold level of channel blockage within the DPF. 
         [0052]    The system may be open loop, closed loop, feedforward, feedback or use other suitable monitoring and control methods. 
         [0053]    The system is simple in terms of hardware and control, providing cost and operating advantages. For example, whereas the prior art system requires control of flow through the first catalyst anywhere between 0% and 100% in order to achieve desired flow rates through the DOC, this is not necessary in the present system. The valve mechanism can simply be switched between zero and maximum flow depending on the operating mode required. 
         [0054]    The system uses fewer components than prior art systems, and operates differently to achieve both DPF regeneration and low NO 2  levels at the tailpipe. The described prior art system cannot achieve low tailpipe NO 2  because there will always be exhaust gas flowing through a catalyst that will generate NO 2 . 
         [0055]    The prior art system requires a special oxidation catalyst selected for good HC conversion and low NO 2  formation. An advantage of the present system is that it can use a very active catalyst that typically has both high HC and NO 2  conversion, lighting off at lower temperatures and having higher efficiency. The layout and operation of the system is such that low tailpipe NO 2  is achieved relative to NO 2  going into the system. 
         [0056]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination. 
         [0057]    What has been described above are preferred aspects of the present invention. It is of course not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, combinations, modifications, and variations that fall within the spirit and scope of the appended claims.