Abstract:
A method for controlling diesel engine emissions is disclosed. The diesel engine&#39;s exhaust system has a diesel particulate filter (DPF) and a lean NO x  trap (LNT). The DPF is a filter and is monitored to determine the need for regeneration. The LNT is monitored to determine the need for regeneration or desulfurization. A fuel injector is used to inject fuel upstream of the LNT under certain conditions, and a bypass may be used to bypass exhaust around the DPF to provide richer or hotter exhaust to the LNT. The sensor outputs and controls for providing the appropriate heat or fuel mix for regeneration and desulfurization may be controlled with enhancements to existing engine control circuitry.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part application of application Ser. No. 09/339,080 filed Jun. 23, 1999, entitled, “Multiple Stage Aftertreatment System”, now U.S. Pat. No. 6,293,096. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to emission control for internal combustion engines, and more particularly to a control system for diesel engines that provides integrated control of engine and post-combustion emission control devices, the latter including both DPF and LNT devices. 
     BACKGROUND OF THE INVENTION 
     New emission limits call for major reductions in oxides of nitrogen (NO x ) and particulate matter (PM) emissions from diesel engines. Engine manufacturers have developed systems for exhaust gas recirculation (EGR), and diesel particulate filters (DPF) to achieve low NO x /PM emissions. The use of DPFs has been found to reduce PM below the stringent requirements of the new emission standards proposed for the 2005 through 2010 time frame. However, NO x  emissions are still six to eight times higher than the proposed standards for model year 2007. 
     To achieve the target NO x  emission levels, new post-combustion devices are being experimented with. These devices include selective catalytic reduction (SCR) using urea or ammonia as reductant, and lean NO x  traps (LNT) (otherwise known as NO x  adsorbers) using hydrocarbon as reductant. Although SCR systems have been used in stationary applications for several years they are now being developed for the mobile fleet with good success. However, greater NO x  reduction than SCRs can deliver is still hoped for and the LNT system promises to achieve the desired NO x  conversion efficiency. 
     SUMMARY OF THE INVENTION 
     The invention is directed to methods and systems for controlling diesel engine emissions. In all embodiments, the diesel engine&#39;s exhaust system has at least a diesel particulate filter (DPF) and a lean NOx trap (LNT). The DPF may be one of two types: a first type that uses a catalyzed soot filter or a second type that uses a continuously regenerated trap. In either case, the LNT is monitored to determine the need for regeneration or desulfurization. In the former case, the catalyzed soot filter is also monitored to determine the need for regeneration. A fuel injector is used to inject fuel upstream of the LNT under certain conditions, and a bypass may be used to bypass exhaust upstream of the LNT to provide richer or hotter exhaust to the LNT. The sensor outputs and controls for providing the appropriate heat or fuel mix for regeneration and desulfurization may be controlled with enhancements to existing engine control circuitry. 
     Features of the invention include the capability of integration of engine controls with control of post combustion emission control devices. Intake throttling, existing EGR systems, and post-combustion injection can be used, alternatively or in combination, for the purpose of regenerating the LNT. Existing EGR can be used to reduce NO x , without adversely affecting the ability to maintain low PM emissions. A portion of the exhaust can be diverted to assist in creating a stoichiometric air-to-fuel ratio at the inlet of the LNT to facilitate regeneration. Overall, the system performs all of the above while not affecting drivability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a first embodiment of a multiple stage aftertreatment system. 
     FIG. 2 illustrates a second embodiment of a multiple stage aftertreatment system. 
     FIG. 3 illustrates a NO x  and PM reduction system having a CSF-LNT unit in the exhaust path. 
     FIG. 4 illustrates a NO x  and PM reduction system having a CRT-LNT unit in the exhaust path. 
    
    
     DETAILED DESCRIPTION 
     Tri-Stage Aftertreatment Device 
     U.S. patent application Ser. No. 09/339,080, entitled “Multiple Stage Aftertreatment System” to Khair, et al., now U.S. Pat. No. 6,293,096, and assigned to Southwest Research Institute, describes an aftertreatment system for reducing the amount of nitrogen oxide and particulate matter in engine exhaust gases without the need for electric heating elements to increase the temperature of the exhaust gas to periodically regenerate the particulate filter. The aftertreatment system does not require the injection of additional fuel in each cylinder of the engine to provide additional necessary hydrocarbon to reduce the NO x  to N 2  (nitrogen). It has an internal bypass for the NO 2  trap to control the rate of NO 2  directed to the carbon trap. 
     A first preferred embodiment of the aftertreatment system is generally indicated by reference numeral  8  in FIG. 1, and effectively combines the functions of a CRT (catalytically regenerated trap), a LNT (lean NO x  trap), and a carbon trap in a single system for the reduction of both NO x  (nitrogen oxides) and PM (particulate matter) emissions. The aftertreatment system  8  is particularly adapted to operate in lean air-fuel ratio engines, such as diesel engines, and uses the excess oxygen present in the exhaust stream of such lean burn engines to reduce the amount of NO x  and carbonaceous particulate matter discharged into the atmosphere. The main elements of aftertreatment system  8  are a first stage  10 , a second stage  20 , a third stage  30 , and a hydrocarbon fuel injector  40  interposed between the first stage  10  and the second stage  20 . 
     The first stage  10  has an inlet  12  which is adapted to receive exhaust gases discharged from an internal combustion engine, such as a diesel engine (not shown). Typically, the exhaust gas from a diesel engine contains various oxides of nitrogen (NOx) and particularly NO (nitric oxide) and NO2 (nitrogen dioxide), as well as HC (hydrocarbons), CO (carbon monoxide), CO2 (carbon dioxide), PM (particulate matter), and other products of the combustion process. The first stage  10  of the aftertreatment system includes an oxidation catalyst  14 , preferably a noble metal such as platinum or palladium. The catalyst  14  oxidizes the NO in the exhaust gas stream, in the presence of the catalyst, to NO2. This reaction is represented by Formula 1: 
     
       
         NO+½O 2 →NO 2   1) 
       
     
     Thus, NO 2  (nitrogen dioxide) is formed and is carried into the second stage  20  of the aftertreatment system  8 . 
     The second stage  20  of the aftertreatment system  8  includes a first portion  22  and a second portion  24 . The first portion  22  contains a lean NOx trap (LNT). The lean NOx trap stores NO2 under lean fuel-air mixture (i.e., oxygen-rich) engine operation, then reduces the stored NO2 to N2 and O2 under rich fuel-air conditions. In most diesel applications, rich fuel-air conditions do not frequently occur during normal over-the-road or other relatively steady state operation. The portion of the NO2 stored in the LNT  22  of the second stage  20  combines with the supplemental HC provided by the periodic injection of supplemental HC (hydrocarbon fuel) upstream of the second stage  20  to form N2, H2O and CO2. The reduction of NO 2  in the second stage is represented by Formula 2: 
     
       
         NO 2 +HC+O 2 →N 2 +H 2 O+CO 2   2) 
       
     
     With continued reference to FIG. 1, the hydrocarbon fuel injector  40  is in fluid communication with a source  42  of pressurized hydrocarbon fuel, for example diesel fuel such as that used in the normal operation of the engine. The reduction conversion efficiency of NO 2  to N 2  and O 2  in the second stage is typically somewhat less than 100% and therefore, some NO 2  is expected to escape the LNT  22  and pass on to the third stage  30 . Another portion of the NO 2  formed in the first stage  10  bypasses the LNT  22  of the second stage by being directed through the second portion bypass  24  of the second stage  20 . The size of the bypass  24  can be readily determined by experimentation for specific applications to ensure good NO x  and PM emission reduction. 
     The third stage  30  of the aftertreatment system  8  in the first embodiment of the present invention includes a carbon trap oxidizer  32  such as a ceramic cordierite wallflow trap. In the carbon trap oxidizer  32 , the NO2 reacts with carbon in the trap and forms CO2 and N2. Thus, the unconverted NO 2  from the first portion  22  of the second stage  20 , as well as bypassed NO 2  passing through the second portion  24  of the second stage  22 , is reduced to nitrogen and CO 2  and is represented below by Formula 3: 
     
       
         2NO 2 +2C→N 2 +2CO 2   3) 
       
     
     Thus, in the first stage  10  of the multiple stage aftertreatment system  8 , NO resulting from the diesel combustion process combines with excess oxygen in the exhaust gas stream to form NO 2 , aided by the catalyst  14  in the first stage  10 . In the second stage  20 , a lean NO x  trap stores the NO 2  formed by the oxidation catalyst  14  of the first stage  10 . The stored NO 2  then combines with supplemental HC, injected by the injector  40  to form N 2 , H 2 O, and CO 2  Unconverted as well as bypassed NO 2  then proceed to the carbon trap oxidizer  32  of the third stage  30 , where NO 2  is reduced to N 2  and carbon is oxidized to CO 2 , NO 2  is stored in the LNT portion  22  of a second stage  20  as long as the exhaust is lean (oxygen-rich). As the LNT portion  22  reaches its NO 2  storage capacity limits, the fuel injector  40 , positioned just upstream of the LNT portion  22 , delivers supplemental hydrocarbon in the form of diesel fuel, thereby reducing NO 2  to N 2 . 
     Switching from the NO 2  storage mode to the reducing mode is preferably controlled by the use of a NO x  sensor  50  positioned in the exhaust downstream of the second stage  20 , and preferably between the second stage  20  and the third stage  30 . The NO x  sensor  50  senses the NO x  content of the exhaust stream and is thereby capable of indirectly detecting engine load. The NO x  sensor  50  delivers an electrical signal  52  to a programmable controller  54  which conditions the electrical signal  52  and controls the operation of the hydrocarbon fuel injector  40 . Thus, fuel can be controllably injected into the aftertreatment system  8  under desired engine operating conditions to enhance the performance of the LNT portion  22  of the second stage. An alternative to sensing NO x  is measuring exhaust gas temperature for use as an indicator of engine speed and load. NO x  formation in diesel engines is a function of engine temperature, generally increasing as the combustion temperature increases, and thus it can be inferred that NO x  formation is taking place at a high rate under high temperature engine operating conditions. At such times, supplemental diesel fuel can be injected to reduce the NO x  emissions. It is desirable that the fuel used for engine operation be a low sulfur fuel to prevent damage to catalysts in the system. 
     A second embodiment of the aftertreatment system is indicated by reference numeral  108  in FIG.  2 . In the second embodiment, the lean NO x  trap and carbon trap are reversed with respect to their positions in the first embodiment, but still effectively produce the same desirable reduction in both NO x  and particulate matter in the exhaust emission stream. As in the first embodiment, the first stage  110  includes an oxidation catalyst  114  positioned just downstream of an inlet  112 . The catalyst  114  oxidizes the NO in the exhaust stream in the presence of the catalyst, to NO 2 . Thus, the reaction that takes place in the first stage of the second embodiment is the same as that shown in Formula 1 above. 
     In the second embodiment, the second stage  130  includes a carbon trap oxidizer  132 , such as a ceramic cordierite walliflow trap. In the carbon trap  132 , NO2 in the gas stream discharged from the first stage reacts with the carbon component of the carbonaceous particulate matter in the exhaust gas stream, reducing at least a portion of the NO2 to N2 and simultaneously oxidizing the carbon to form carbon dioxide (CO2). The amount of NO2 reduced is dependent upon the amount of carbon present, and therefore, the reduction of NO2 to N2 may not be 100%. That is, the exhaust gas stream discharged from the second stage  132  usually will contain both reduced NO 2  (N 2 ) and oxidized carbon (CO 2 ) as well as some residual nitrogen dioxide (NO 2 ), as represented below by Formula 4: 
     
       
         2NO 2 +2C+NO 2 →N 2 +2CO 2 +NO 2   4) 
       
     
     With continued reference to FIG. 2, the third stage  120  includes a lean NO x  trap (LNT)  122 . The gas stream emitted from the second stage  130 , containing nitrogen, carbon dioxide and a remaining portion of nitrogen dioxide passes through the third stage  120 . The third stage lean NO x  trap is arranged to store the remaining portion of the nitrogen dioxide discharged from the second stage, and with the addition of a supplemental hydrocarbon fuel, reduce the stored nitrogen dioxide to nitrogen, water in a gaseous state, and carbon dioxide, and discharge a gaseous stream from the multiple stage aftertreatment system  108  that consists essentially of nitrogen, water and carbon dioxide. This reaction is represented above by Formula 2. 
     Aftertreatment system  108  has a hydrocarbon fuel injector  140  that is positioned between the second stage  130  and the third stage  120  at a position adapted to controllably inject selected amounts of hydrocarbon fuel, from a pressurized source  142 , into the gaseous stream discharged from the second stage  130 , prior to the gaseous stream being received by the third stage  120 . As described above, the lean NOx trap  122  stores the remaining portion of the NO2 then combines the stored NO2 with supplemental HC, injected by the injector  140  to form N2, H2O and CO2. NO2 is stored in the lean NO trap portion  122  of the third stage  120  when the exhaust is lean (oxygen-rich). As the lean NO x  trap portion  122  reaches its NO 2  storage capacity limits, the fuel injector  140 , positioned just upstream of the lean NO x  trap  122  delivers supplemental hydrocarbon (HC) in the form of diesel fuel, thereby inducing NO 2  reduction to N 2 . 
     Switching the NO 2  from the storage mode to the reducing mode may be controlled, as described above, by the use of a NO x  sensor  150  positioned in the exhaust downstream of the third stage  120 . The NO x  sensor  150  senses the NO x  content of the exhaust stream and is thereby capable of indirectly determining engine load. The NO x  sensor  150  delivers an electrical signal  152  to a programmable controller  154  which conditions the electrical signal  152  and controls the operation of the hydrocarbon fuel injector  140 . Thus, fuel can be controllably injected into the aftertreatment system  108  under desired engine operating conditions to enhance the performance of the lean NO x  trap portion  122  of the third stage  120 . 
     The multiple stage aftertreatment devices described above combine the functions of a catalytically regenerated trap (CRT) with a lean NO x  trap (LNT) in a single system for the reduction of both NO x  and PM emissions. This effectively maximizes the common features of both systems, such as a noble metal catalyst and its NO 2  formation capability. 
     Integrated Systems for Diesel Engine Control 
     The following description is directed to the integration of an emission control system consisting of a diesel engine equipped with an EGR and intake throttle, and capable of post-injection, with post-combustion exhaust emission control devices. The emission control devices in the exhaust are a DPF, LNT, supplemental fuel (reductant) injector, and a bypass valve. The tri-stage concepts discussed above are integrated into the overall engine emission control system. 
     Two categories of control systems are discussed, each using a different DPF system. A first system uses a Catalyzed Soot Filter (CSF), whereas a second system uses a Continuously-Regenerated Trap (CRT™). The following description describes two systems: a CSF-LNT system and a CRT-LNT system. Both systems have at least seven features that interact to facilitate control of the regeneration and/or desulfurization of the emissions components: 
     1) Intake air throttling 
     2) A venturi used to enhance EGR flow 
     3) Cooled EGR System 
     4) Post combustion fuel injection (in-cylinder and/or in exhaust) 
     5) A bypass valve in the exhaust stream to direct flow during desulfurization 
     6) Placement of the supplemental fuel injector in the exhaust 
     7) Embedded software that contains integration and control logic 
     CSF-LNT Configuration and Control 
     FIG. 3 illustrates the CSF-LNT system  300 , which has the engine controls noted above (i.e., EGR line  301 , EGR valve  302 , EGR cooler  303 , intake throttle  304 , intake venturi  305 , electronic engine controls  322 ). The exhaust system has a CSF  310 , an LNT  311 , a bypass valve  313 , and a fuel injector  317 , as well as CSF pressure and temperature sensors  330  and  333 , and LNT NO, sensors  331  and  332 . Sensor  330  monitors CSF dP (pressure drop), and sensors  331  and  332  monitor NO x  levels before and after the LNT  311 . 
     As explained below, the monitoring sensors  330 - 333  are used to determine the need for regeneration of both systems. Their output signals may be delivered to engine controller  322 , which performs appropriate algorithms for implementing the regeneration and desulfurization described below, including control of various engine and exhaust devices so as to enhance conditions for regeneration and desulfurization. 
     CSF  310  may be any device that acts as a particulate filter, wherein part or all of the surface of the filter is “catalyzed” (i.e., coated with catalytically active materials). NO in the exhaust is oxidized to NO2 on the catalyzed surfaces of the filter, and the NO2 then oxidizes the carbon trapped on the filter. For the rate of regeneration to exceed the rate of trapping in the CSF  31   1 (i.e., the CSF can clean out any accumulated soot particles), the temperature must generally exceed 380° C. If long periods of operation are performed below this temperature (for example sustained idle or light load operation), it may be necessary to artificially elevate the temperature of the CSF  310  for a period of time to clean it. 
     A bypass valve  313  is incorporated upstream of the CSF  310  to allow a small portion of the exhaust to be routed around the CSF  310 , via a bypass line  315 , and reintroduced into the exhaust upstream of the LNT  311 . A supplemental fuel injector (SFI)  317  is located in the exhaust between the CSF  310  and LNT  311  to allow for injection of additional fuel to supplement the air-tofuel ratio of the exhaust entering the LNT  311 . All of these systems are controlled from the electronic control module (ECM)  322 . 
     Under normal operation, all of the engine exhaust passes first through the CSF  310  and then through the LNT  311 . The CSF  310  traps insoluble particulates and oxidizes volatile organic particulates. In addition, if the temperature is high enough (over 380° C.) some of the NO2 generated by the CSF  310  removes carbon particles already trapped in the CSF  310 . Excess NO2 from CSF  310  is stored on LNT  311 . The exhaust enters the LNT  311 , where any remaining NO is oxidized to NO2. The NO2 then adsorbs on the surface of the LNT. 
     When the CSF  310  differential pressure indicates that regeneration is needed, and the temperature of the CSF  310  is not at the required level, the exhaust temperature is increased using control strategies programmed into the ECM  322 . This may be done using intake throttle  304 , increased EGR rate, or in-cylinder post-combustion fuel injection while the exhaust valves are open (referred to as post-injection). Intake throttling or increased EGR rate increases the exhaust gas temperature from the engine  320 . Post-injection increases the CSF temperature by using the exothermic heat generated by the oxidation of the post-injected fuel. These methods may be used separately or in combination, and may be optimized to produce the highest temperature for the smallest amount of performance and fuel penalty. 
     When NO x  monitors at the inlet and outlet of the LNT  311  indicate that NO x  reduction across the LNT  311  has fallen below acceptable levels, regeneration and/or desulfurization will be needed. LNT regeneration requires temperatures above 250° C. and a stoichiometric or slightly rich exhaust gas air-to-fuel ratio. If temperature is not sufficient for regeneration, it may be increased using any or a combination of the methods described above for increasing the CSF temperature. 
     To generate the rich exhaust gas air-to-fuel ratio, a variety of methods may be used. Intake throttling and increased EGR will generate higher CO levels in the exhaust. The bypass valve  313  may be used to pass a portion of this CO laden exhaust around the CSF  310  (which would otherwise oxidize and remove the CO), thus moving the exhaust closer to a rich condition upstream of the LNT  311 . Post-injection could also be used to add more fuel into the exhaust, again in conjunction with the bypass valve  313 . Some of the post-injected fuel may also make it through the CSF (or be partially oxidized to generate CO by the CSF) depending on exhaust gas temperature and flow rate. Finally, fuel injector  317  may be used to inject additional fuel into the exhaust upstream of the LNT  311  to ensure the rich exhaust conditions needed for regeneration. Any combination of these may be used, and the regeneration strategy may be optimized to achieve the fastest and most complete regeneration for the least amount of fuel economy and performance penalty. 
     If, after a period of regeneration, the NO x  sensors indicate that NO x  reduction across the LNT  311  has still not returned to acceptable levels, then a need for desulfurization is indicated. Desulfurization requires temperatures between 400° C. and 600° C., with higher temperatures requiring a shorter period of time to complete desulfurization. This increased temperature may be accomplished by any or a combination of the means described above for the CSF  310 . In addition, the bypass valve  313  may be used to pass some of the hot exhaust around the CSF  310  and into the LNT  311  in order to prevent some of the heat from being lost to the CSF. Finally, the fuel injector  317  may be used to generate heat in the LNT  311  by using the exothermic heat caused by oxidation of the injected fuel over the LNT itself. Any combination of these may be used, and the desulfurization strategy may be optimized to achieve the fastest and most complete desulfurization for the least amount of fuel economy and performance penalty. Desulfurization may be halted periodically to check if NO x  reduction has returned to acceptable levels, and can be resumed if the check indicates more desulfurization is needed. If desulfurization is unsuccessful after several attempts, a problem with the LNT  311  could be indicated. 
     Balancing the above-described regeneration and desulfurization requirements calls for an integrated control strategy. Areas where several requirements overlap can be exploited to accomplish these tasks simultaneously (e.g., temperature increase episode for CSF regeneration can also be used for LNT regeneration and/or desulfurization) to reduce the overall fuel economy and performance penalties associated with such operation. In addition, episodes where exhaust temperatures and flow rates are ideal for regeneration can be exploited by using any of these techniques (e.g., post-injection during deceleration events, to allow a brief period of regeneration without affecting driveability). 
     CRT-LNT Configuration and Control 
     FIGURE 4 illustrates another embodiment of an integrated system, a CRTTM-LNT system  400 , which has then engine  420  and engine controls noted above (i.e., EGR line  401 , EGR valve  402 , EGR cooler  403 , intake throttle intake venturi  405 , electronic engine controls  422 ). As explained above CRT  410  is an alternative to the CSF of system  300 , and both devices have a LNT. The configuration of the system  400  is such that the bypass control valve  413  is placed upstream of the oxidation catalyst (OC)  410   a  and is configured so as to bypass the OC  410   a . The next emissions component in the exhaust flow is the DPF  410   b , followed by the LNT  411 . Together the OC  410   a  and DPF  410   b  make up the CRTTM  410 . 
     CRT  410  may be any device that converts NO to NO2 in a first step, followed by a wallflow DPF where carbonaceous particulate is accumulated. LNT  411  may be any device that has an oxidation catalyst to oxidize nitric oxide (NO) to nitrogen dioxide (NO2), a followed by an NO2 trap (usually a base-metal oxide) wherein NO2 is adsorbed on the surface. Temperature and pressure sensors  430  and  433  are placed at the DPF  410 b to determine the need for DPF regeneration, and NO x  sensors  431  and  432  are used to determine the need for regeneration and desulfurization of the LNT  411 . Regeneration and desulfurization conditions are determined in a manner similar to the methods described above in connection with system  300 . 
     The LNT  411  is periodically regenerated under conditions of sufficient temperature and stoichiometric (or slightly rich) exhaust air-to-fuel ratio. In addition to regeneration, the LNT  411  is “desulfurized” periodically. This is necessary because sulfur (in the form of sulfur trioxide) adsorbs in the surface of the LNT  411 , blocking the sites used to trap NO 2  and thus reducing the efficiency of the LNT. Desulfurization requires high temperature (400° C. to 600° C.) for a sustained period of time (often several minutes-much more time than required for regeneration), and stoichiometric or slightly rich air-to-fuel ratio conditions. 
     Referring to FIG. 4, system  400  operates in the following manner. The OC  410   a  converts NO x  to NO 2  in the exhaust stream. The exhaust then flows through the DPF  410   b , where PM in the exhaust is trapped and the carbon is reacted with NO 2  to form elemental nitrogen and carbon dioxide (CO 2 ). Excess NO 2  emitted from the DPF  410   b  is stored on the LNT. 411 , until the LNT is regenerated. 
     To integrate the CRTTM-LNT system  400 , software logic is added to the engine controller  422  to monitor the system status and to control the regeneration/ desulfurization of the emissions systems as needed. For the LNT  411 , NOx sensors  431  and  432  are monitored and their locations strategically determined, to develop a strategy for indicating when regeneration of the LNT  411  is required. An additional strategy monitors regeneration frequency and deciphers whether a regeneration or desulfurization is required. Once the condition for regeneration/desulfurization is met, an engine control strategy is used to create a rich condition, with adequate carbon monoxide (CO) to initiate regeneration in the NOx trap. To achieve this goal, intake throttling (to increase the vacuum across the venturi  405 , resulting in increased EGR) may be used to create a richer in-cylinder air-to-fuel ratio (increasing CO in the exhaust), the bypass valve  413  may be partially activated to prevent the consumption of the excess CO across the OC  410   a , and in-exhaust (or in-cylinder post combustion) supplemental fuel injection (SFI)  417  will be used to further raise CO and to increase the exhaust temperature to regenerate the LNT  411 . Additional logic may be programmed into the controller  422  to verify that the duration of regeneration/desulfurization event was adequate to achieve complete regeneration. CO emitted during DPF regeneration will also help regenerate the LNT  411 . 
     The control logic also monitors and controls the regeneration of the DPF  411 . For the DPF  411 , filter pressure drop (dP) and inlet exhaust temperature are monitored with pressure and temperature sensors  430  and  433 . A strategy is developed to determine when regeneration is necessary. Ideally, the regeneration strategy achieves continuous regeneration through engine management, and avoids cyclic regeneration. The strategy for continuous DPF regeneration involves identifying the balance point temperature (BPT) (the temperature at which the rate of soot accumulation is equal to the rate of regeneration), predicting in real-time whether the DPF is at or below the BPT, and continuously adjusting in-cylinder, post combustion fuel injection characteristics in an attempt to maintain BPT. 
     Other Embodiments 
     Other aspects, features and advantages of the present invention can be obtained from a study of this disclosure together with the appended claims.