Abstract:
A reconfigurable exhaust system provides selective positioning of exhaust gas treatment devices either near, or spaced from, the exhaust gas outlet port of a turbine of a turbocharged Diesel engine. In some embodiments, auxiliary air or a heat exchanger are arranged to cool the exhaust gas when required.

Description:
This is a continuation application U.S. patent application Ser. No. 11/261,011, filed Oct. 28, 2005 now U.S. Pat. No. 7,251,932, which claims priority to U.S. Provisional Application Ser. No. 60/625,847 filed Nov. 8, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to an exhaust system having a controllably variable flowpath for exhaust gas circulation, and more particularly to the use of such a system to control exhaust gas flow through regenerable exhaust gas treatment devices. 
     2. Background Art 
     Worldwide emissions regulations slated for introduction in the near future impose very stringent emissions regulations. The Tier 2 regulations in the United States require that Diesel vehicles have the same ultra-low emissions levels as spark ignited vehicles. Moreover, Tier 3 requirements, which phase in for different engine levels over the next three years call for a 40% reduction in NO x  (oxides of nitrogen) from the Tier 2 levels now in existence. 
     Various combustion modes, directed to addressing both in-cylinder (engine-out) and exhaust gas treatment device requirements, have been proposed. For example, U.S. Pat. No. 5,732,554, issued Mar. 31, 1998 to Shizuo Sasaki, et al. for an EXHAUST GAS PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE describes a method by which the normal fuel lean operating mode of an engine is switched to a rich premixed charge compression ignition, more accurately and preferably referred to as premixed controlled compression ignition (PCCI), combustion mode. 
     U.S. Pat. No. 5,937,639 granted Aug. 17, 1999 to Shizuo Sasaki, et al. for INTERNAL COMBUSTION ENGINE describes an alternative method for lowering the combustion temperature, i.e., low temperature combustion (LTC) to minimize smoke generation during rich, or near rich, combustion. LTC and PCCI combustion are alternative combustion modes which normal Diesel lean combustion can be transitioned to during engine operation. 
     Perhaps of most concern to the Diesel engine market are the proposed very tight future reductions in terms of oxides of nitrogen (NO x ) and particulate matter (PM) emissions. One of the most promising technologies for NO x  treatment is NO x  absorbers, also known as “lean NO x  traps.” Diesel particulate filters, also known as Diesel particulate traps, and lean NO x  traps are the most likely, at least in the foreseeable future, means by which emissions will be reduced. Lean NOx traps and Diesel particulate filters need to be regenerated periodically to restore their efficiencies. The regeneration of lean NO x  traps is usually done by providing reductants, such as CO and HC under oxygen-free conditions. A regenerated lean NO x  trap not only adsorbs NO x  emissions, but also adsorbs sulfur carried in the exhaust gas stream. Sulfur removal (desulfation) must be undertaken at a temperature above 600° C. under oxygen-free conditions, i.e., combustion of a stoichiometric or richer air/fuel ratio. Under typical Diesel lean combustion operation, such very high temperatures cannot normally be obtained except under very high load conditions. Diesel particulate filter regeneration is carried out by oxidizing soot and other particles “trapped” in the Diesel particulate filter at a high temperature and a lean air/fuel ratio. 
     Frequent lean NO x  trap (LNT) regeneration is necessary when the engine-out NO x  is high, for example, when operating under high loads, but frequent generation at higher loads can cause the temperature of the LNT to increase rapidly. The rapid temperature increase results from the exothermic reaction associated with the rich combustion products carried in the exhaust that are used to regenerate the LNT. Diesel particulate filters (DPF) also require high temperature to be regenerated. When the LNT is located downstream of the DPF, the exothermic reaction taking place in the DPF during regeneration will result in an increase in the outlet exhaust gas temperature of the DPF. The LNT inlet exhaust gas temperature is therefore also increased. When the temperature of the LNT increases above a critical temperature, as a result of the frequent regeneration of the LNT or the regeneration of the DPF, the absorption efficiency of NO x  by the LNT is very low and tailpipe NO x  emissions accordingly are high. 
     The present invention is directed to overcoming the problems set forth above with respect to the critical temperature requirements associated with catalyzed and other exhaust gas after treatment device operation and regeneration. It is desirable to have an exhaust system in which LNTs, selective catalytic reduction (SCR) catalysts, and like regenerable exhaust gas treatment devices can be optimally positioned within the exhaust system to provide efficient operation over a wide range of engine operating conditions and regeneration requirements. It is also desirable to have a flexible exhaust system in which the exhaust gas flowpath can be selectively varied to control regeneration temperatures to meet differing operation and regeneration requirements. It is also desirable to have a method by which LNT temperature can be managed for the best efficiency during cold start, regeneration and DPF regeneration. It is also desirable to have an exhaust system and method of temperature control by which unregulated emissions can be reduced by management of the exhaust gas temperature passing through various components of the exhaust system. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, an exhaust system having a variable flowpath has a three-way engine-out exhaust control valve adapted to direct exhaust gas flow through selectable first and second outlet ports. A regenerable gas treatment device has an inlet in direct communication with the first outlet port of the three-way engine-out exhaust control valve, and a second exhaust gas treatment device has an inlet in direct communication with the second outlet port of the three-way engine-out exhaust control valve. A second three-way exhaust control valve has a first inlet port in communication with the outlet of the second exhaust gas treatment device, and a second inlet port in communication with the inlet of the regenerable exhaust gas treatment device. The second three-way exhaust control valve has an outlet port in direct communication with the ambient environment. A third three-way exhaust control valve has an inlet port in direct communication with the outlet of the regenerable exhaust gas treatment device. 
     Other features of the exhaust gas system embodying the present invention, include a third exhaust gas treatment device having an inlet in direct communication with the outlet of the second exhaust gas treatment device. 
     In accordance with another aspect of the present invention, a method for controlling exhaust gas flow through a regenerable exhaust gas treatment device includes passing exhaust gas through a relatively short passageway from the engine to an inlet of the regenerable exhaust gas treatment device by which the internal temperature of the regenerable exhaust gas treatment device, during engine operation, is maintained above a predetermined low temperature. The method further includes sending exhaust gas produced by the engine through a relatively longer second exhaust duct to the regenerable gas treatment device by which the internal temperature of the regenerable exhaust gas treatment device, during engine operation, is maintained below a predefined high temperature. 
     Other features of the method for controlling exhaust gas flow through a regenerable exhaust gas treatment device, in accordance with the present invention, includes passing exhaust gas produced by the engine through a relatively shorter first exhaust duct when the engine is operating in a low to light load condition, and through the relatively longer second duct when the engine is operating in a medium to high load condition. 
     Yet another feature of the method for controlling exhaust gas flow through a regenerable exhaust gas treatment device, embodying the present invention, includes passing exhaust gas produced by the engine through the relatively short first exhaust duct during cold start-up of the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the exhaust system and method for controlling exhaust gas flow through a regenerable exhaust gas after treatment device, in accordance with the present invention, may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of an exhaust treatment system embodying the present invention, showing the controlled flowpath of exhaust gas during cold start and warm-up; 
         FIG. 2  is a schematic diagram of the exhaust treatment system embodying the present invention, showing the controlled flowpath of exhaust gas during normal engine operation; 
         FIG. 3  is a schematic diagram of the exhaust treatment system embodying the present invention, showing the controlled flowpath of exhaust gas at high load engine operation, using cooled auxiliary air to regulate the exhaust gas temperature; 
         FIG. 4  is a schematic diagram of the exhaust treatment system embodying the present invention, showing the controlled flowpath of exhaust gas at high load engine operation, in which an exhaust cooler is used to regulate the temperature of the exhaust gas; 
         FIG. 5  is a schematic diagram of the exhaust treatment system embodying the present invention, showing the controlled flowpath of exhaust gas during simultaneous regeneration Diesel particulate filter and desulfation of a lean NO x  trap; 
         FIG. 6  is a schematic diagram of the exhaust gas treatment system embodying the present invention, showing an alternate controlled pathway for exhaust gas during simultaneous regeneration of a lean NO x  trap and a Diesel particulate filter; 
         FIG. 7  is a graphical representation of exhaust gas temperatures at various locations in the exhaust treatment system, illustrating thermal management of exhaust temperatures by auxiliary air injection. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     At cold start, the best way to increase LNT substrate temperature is to increase the exhaust gas temperature so that hot exhaust gas will quickly heat the LNT and raise its substrate temperature. In addition to using in-cylinder or external means to increase exhaust gas temperature as fast as possible, such as by increasing engine idle speed, increasing exhaust back pressure whereby engine load is increased at idle, retarding combustion so that the exhaust temperature will be higher, or by electrical heating, optimizing the location of each treatment device in the exhaust system is also very important. 
     Ideally, an LNT should be positioned as close as possible to the exhaust manifold or turbocharger outlet so that the exhaust gas will heat the LNT first. However, a close-coupled LNT will have lower efficiency at higher engine loads when the LNT has warmed up, increasingly high LNT substrate temperatures will result in a reduced capability to the LNT to absorb NO x  emissions. When the LNT is mounted remotely, i.e., further away, from the turbocharger outlet, it is easier to maintain the LNT substrate temperature for best efficiency under most speed-load condition after the LNT warms up. However, the remote location also contributes to cool-down of the LNT during low or light load conditions and, accordingly, the LNT will lose its conversion efficiency. 
     In the following description of preferred embodiments of the present invention, a flexible configuration of exhaust gas treatment system components permits the functional location of the LNT, or other after treatment device, in the exhaust system to be readily changed by controlling the exhaust gas flowpath. A close-coupled LNT can be provided for fast warm-up during cold start and warming up operation and LNT substrate temperatures can be maintained within a desirable temperature range under light load engine operation. By lengthening the exhaust gas flowpath and behind other components of exhaust gas treatment system, the LNT can be advantageously positioned for optimum efficiency under higher engine speed and load operation. 
     The first embodiment of the present invention is illustrated in  FIG. 1 . In this embodiment, the lean NO x  trap is close-coupled to the engine exhaust manifold, a position particularly desirable during cold start and warm-up periods as well as for maintenance of the LNT substrate temperatures under light engine load operation. 
     As shown in  FIG. 1 , an exhaust gas treatment system  10  embodying the present invention provides a controllably variable flowpath for exhaust gas produced by a Diesel engine  12 . The Diesel engine  12  has a plurality of combustion chambers  14  and an intake air duct  16  in communication with the intake port of a compressor  18 . The compressor  18  provides compressed air to a boost air duct  20  having an inter-cooler  22  positioned between the compressor  18  and an intake throttle valve  24  which regulates, or controls, the flow of intake air into an intake manifold  26 . An exhaust manifold  28  is connected to the inlet port of a turbine  30 . The engine  12  also has an exhaust gas recirculation duct  32  by which controlled amounts of exhaust gas may be recirculated from the exhaust manifold  28  into the intake manifold  26 . The amount of recirculated exhaust gas is controlled by an exhaust gas recirculation valve  34 . 
     The exhaust system  10  has a three-way engine-out exhaust valve  38  positioned in close proximity to the turbine  30 . A conduit  36 , preferably having a very short length, extends between an outlet of the turbine  30  and an inlet port of the three-way engine-out exhaust valve  38 . If desired, the three-way engine-out exhaust valve  38  could even be mounted directly on the outlet of the turbine  30 . The three-way engine-out exhaust valve  38  has first and second outlet ports arranged so that the valve can controllably direct all, or portions, of the exhaust gas flow through either of the outlet ports. 
     A first exhaust duct  40 , also preferably having a short length, extends between the first outlet port of the three-way engine-out exhaust valve  38  and an inlet to a regenerable exhaust gas treatment device  42  which, for the purpose of illustrating the present invention, is a lean NO x  trap. A second exhaust duct  44  extends between the second outlet port of the three-way engine-out exhaust valve  38  and a second exhaust gas treatment device  46 , for example, a Diesel oxidation catalyst. In the illustrated embodiments, the exhaust gas treatment system  10  also has a third exhaust gas treatment device  48 , such as a Diesel particulate filter. A third exhaust duct  50  provides fluid communication between an outlet of the Diesel particulate filter  48  and a first inlet port of a second three-way exhaust valve  52 . An outlet port of the second three-way exhaust valve  52  provides direct communication with the ambient environment. A third three-way exhaust valve  54  has an inlet port in direct communication with an outlet of the lean NO x  trap  42 . A fourth exhaust duct  56  extends from a first outlet port of the third three-way exhaust valve  54  to the second exhaust duct  44 . A fifth exhaust duct  58  extends from the inlet of the LNT  42  to the second inlet port of the second three-way exhaust valve  52 . A sixth exhaust duct  59  extends from a second outlet port of the third three-way exhaust valve  54  to the third exhaust duct  50 . 
     During cold start and warm-up if an engine control unit, not shown, detects temperatures in the LNT  42  that are less than a predefined lower value, the configuration of the flexible exhaust system  10 , illustrated in the  FIG. 1  embodiment, will provide fast warm-up of the LNT  42 , as illustrated by arrows indicating the direction of exhaust flow. Exhaust gas discharged from the outlet of the compressor  30  is directed through the three-way engine-out exhaust valve  38  directly to the inlet of the LNT  42 . The flexible exhaust system  10  is configured in this embodiment in such a manner that the exhaust gas exiting the LNT  42  is directed, as indicated, through the third three-way exhaust valve  54 , the fourth exhaust duct  56 , and the second exhaust duct  44  to the inlet of the Diesel oxidation catalyst  46 . Exhaust gas flow continues from the outlet of the Diesel oxidation catalyst  46  to the inlet of the Diesel particulate filter  48 , and then from the outlet of the Diesel particulate filter  48  and through the third exhaust duct  50  to the first inlet port of the second three-way exhaust valve  52  and subsequently into the ambient environment. The configuration provided by the first embodiment assures that the LNT is the closest exhaust system component to the outlet of the turbocharger. 
     Under normal operating conditions, the flexible exhaust system  10  embodying the present invention is reconfigured to the arrangement illustrated in  FIG. 2 . In this embodiment, the LNT  42  is desirably remotely positioned from the outlet of the turbine  30 . As indicated by the exhaust flow arrows, exhaust gas discharged from the turbine  30  is directed through the second outlet port of the three-way engine-out exhaust valve  38  and then through the second exhaust duct  44  to the inlet of the Diesel oxidation catalyst  46 . After passing through the Diesel oxidation catalyst  46 , the Diesel particulate filter  48 , and a portion of the third exhaust duct  50 , the exhaust is directed through the sixth exhaust duct  59  and the second inlet port of the third three-way exhaust valve  54  into the LNT  42 . From the LNT  42 , the exhaust gas is directed through the fifth exhaust duct  58  to the second inlet port of the second three-way exhaust valve  52 , and thence into the ambient environment. 
     During low to light load operation, if the LNT temperature is less than a desired minimum LNT efficiency temperature, the variably configurable exhaust system  10  directs the flow of exhaust gas through the close-coupled LNT configuration shown in  FIG. 1 , so that the hotter exhaust gases will be provided to the LNT  42 . When the internal temperature in the LNT  42  is sufficiently high, the three-way engine-out exhaust valve  38  is controllably switched to the normal operating position, illustrated in  FIG. 2 , by which the exhaust gas is directed first to other treatment devices and then lastly to the LNT. 
     During regeneration of the LNT  42 , the close-coupled configuration illustrated in  FIG. 1  is particularly useful in enabling fast air/fuel ratio switching. Increased regeneration efficiency also requires fewer reductants, such as CO and HC during regeneration and the breakthrough of CO and HC from LNT  42  can be treated by the oxidation catalyst  46 , and the subsequent treatment of these reductants in the Diesel oxidation catalyst  46  of the Diesel particulate filter  48  is greatly reduced. Moreover the fuel penalty generally attendant with regeneration is reduced and drivability is improved as a result of more efficient and fewer required LNT regeneration cycles. Importantly, the variably configurable exhaust system  10  embodying the present invention permits alternate operation between the LNT close-coupled configuration illustrated in  FIG. 1  and the LNT remotely mounted configuration shown in  FIG. 2 . Importantly, alternate operation between the two configurations enables the LNT  42  to be thoroughly regenerated alternately from both sides. 
     During LNT regeneration, the regeneration frequency, duration and air/fuel ratio are determined by a specific regeneration strategy that is a function of engine-out NO x  emissions and current engine operating conditions. At high load, due to the high engine-out NO x  emissions, regeneration must be carried out at more frequent intervals. Accordingly, the temperature of the LNT  42  increases rapidly if the period between two adjacent regenerations is not long enough for the LNT substrate temperature to be cooled. The only way to reduce the internal temperature of the LNT is to cool down the exhaust gas entering the LNT between two consecutive LNT regenerations. Ways in which the exhaust gas can be cooled during LNT regeneration are discussed below in the embodiments illustrated in  FIGS. 3-6 . 
     During LNT desulfation at high load operation, in which PCCI is the preferred combustion mode for increasing LNT temperature while providing exhaust gas consisting of products of stoichiometric combustion for regeneration of the LNT  42 , in the remotely-mounted LNT configuration illustrated in  FIG. 2  the DPF  48  is desirably positioned in front of the LNT  42  thereby permitting DPF regeneration only during the lean combustion period. However, the DPF  48  will collect particulate matter during the rich combustion period, and therefore LNT desulfation and DPF regeneration cannot be desirably carried out simultaneously under the remote mounted configuration of  FIG. 2 . In the readily reconfigurable exhaust system  10  embodying the present invention, by switching the reconfigurable exhaust system to the closed coupled configuration, the LNT  42  is desirably positioned before the Diesel oxidation catalyst  46  and the Diesel particulate filter  48 , and as shown in subsequently described embodiments, external air can be introduced before the Diesel oxidation catalyst  46  and/or the Diesel particulate filter  48  to provide abundant oxygen during the LNT desulfation process. Thus, by introducing external air upstream of the Diesel oxidation catalyst  46  and/or the Diesel particulate filter  48 , the exhaust gas fed into the Diesel particulate filter  48  can always be lean regardless of whether or not the exhaust gas fed into the LNT  42  is rich or lean. 
     The variably reconfigurable exhaust system  10  embodying the present invention, provides at least two methods by which exhaust gas temperature can be cooled down between consecutive LNT regenerations.  FIG. 3  illustrates an embodiment of the variably reconfigurable exhaust system  10 _in which the internal temperature of the LNT  42  can be controlled during high load engine operation by introducing cooled auxiliary air through an auxiliary air conduit  60  extending between the boost air duct  20  and third exhaust duct  50 . The flow of auxiliary air through the auxiliary air conduit  60  is controlled by an auxiliary air control valve  68 . Thus, between regenerations, external low temperature air can be used to dilute the exhaust gas entering the LNT  42 , enabling the internal temperature of the LNT to be lowered. The addition of auxiliary air must be interrupted prior to the next regeneration period to prevent dilution of the rich exhaust gas needed for NO x  conversion, i.e., LNT regeneration. 
       FIG. 4  illustrates another embodiment of the readily reconfigurable exhaust system  10 ″ embodying the present invention in which the thermal management of the LNT  42  during high load engine operation is enabled by the use of an exhaust cooler  66 . In this embodiment, a fourth three-way exhaust valve  62  has an inlet port in direct communication with the outlet of the DPF  48 . A seventh exhaust duct  64  extends from a first outlet port of the fourth three-way exhaust valve  62  to an inlet of the exhaust cooler  66 . The outlet exhaust gas flows out of the exhaust gas cooler through a modified third exhaust duct  50 ′ and then through the sixth exhaust duct  59  and through the third three-way exhaust valve  54  to the LNT  42 . After passing through the LNT  42 , the exhaust gas is directed through the fifth exhaust duct  58  to the second intake port of the second three-way exhaust valve  52 , and subsequently discharged into the ambient environment. During LNT NO x  regeneration, directing exhaust gas through the exhaust cooler  66  provides cooler exhaust gas to lower the internal temperature in the LNT between regenerations. 
       FIG. 5  illustrates a fourth embodiment of the configurable exhaust system  10 ″ embodying the present invention. In this embodiment, the second auxiliary air conduit  72  extends between the first auxiliary air conduit  60  and the second exhaust duct  44  at a position adjacent the inlet to the Diesel oxidation catalyst  46  in which auxiliary air can be introduced before the Diesel particulate filter  48  during rich combustion operation for desulfation of the LNT  42 . Auxiliary airflow through the second auxiliary air conduit  72  is controlled by a second auxiliary air control valve  70 . If desired, the auxiliary air may be conducted through an alternately positioned second auxiliary air conduit  72 ′ to provide auxiliary air to the inlet of the Diesel particulate filter  48 . 
       FIG. 6  illustrates a fifth embodiment of the variably configurable exhaust system  10 ″ embodying the present invention in which auxiliary air can be introduced before the Diesel particulate filter  48  during rich combustion operation for desulfation of the LNT  42 . In this configuration, the exhaust cooler  66  can provide additional exhaust gas cooling prior to introduction into the LNT  42  if desired. 
       FIG. 7  illustrates the effectiveness of the variably configurable exhaust system embodying the present invention in providing control of LNT temperature at high load conditions. In the upper portion of the graph, labeled LNT substrate temperature, ° C. without auxiliary air injection, the LNT substrate temperatures, i.e., inlet center line and middle center line overlap with each other, represented by the solid thin line rises to a temperature above 500° C. and keeps increasing during regeneration. However, with auxiliary air injection between successive regenerations, the LNT inlet center line substrate temperature represented by the heavy solid line, the LNT middle center line substrate temperatures represented by the thin dash line can be maintained at a temperature below 500° C. 
     Moreover, as illustrated in the lower portion of the  FIG. 7  graph, during regeneration, auxiliary air injection significantly reduces the tailpipe NO x  emissions. The spikes in tailpipe NO x  indicated as cooled when auxiliary air is injected, is measurably less than the spikes in the absence of auxiliary air injection. From the foregoing discussion, it can be seen that the variably configurable exhaust system embodying the present invention provides a readily configurable exhaust flowpath that can be changed as required by different engine operating conditions. 
     Although the present invention is described in terms of preferred illustrative embodiments, those skilled in the art will recognize that variations on, or combinations of, the described embodiments can be made in carrying out the present invention. For example, LNT desulfation can be carried out in the close-coupled LNT configuration illustrated in  FIG. 1  by introducing external air upstream of the Diesel oxidation catalyst or Diesel particulate filter during the rich combustion period, so that the oxidation of particulate matter can be carried out simultaneously during rich combustion with LNT desulfation and DPF regeneration. Such arrangements embodying the present invention are intended to fall within the scope of the following claims. 
     Other aspects, features and advantages of the present invention may be obtained from the study of this disclosure and the drawings, along with the appended claims.