Abstract:
A method and system for simultaneously regenerating a particulate filter coupled to an internal combustion engine and for desulfating a lean NOx trap disposed downstream of the particulate filter. The method includes adjusting at least one engine operating parameter to maintain a desired air fuel ratio for gases exiting the particulate filter in accordance with a difference between a reference set point air fuel ratio level and the air fuel ratio of gases exiting the particulate filter and wherein the reference set point level is changed between a rich air fuel ratio and a lean air fuel ratio as a function of the air fuel ratio of the gases exiting the lean NOx trap.

Description:
INCORPORATION BY REFERENCE 
     This patent application incorporates herein by reference the entire subject mater of U.S. patent application Ser. No. 10/063454 filed Apr. 24, 2002 entitled “Control for Diesel Engine With Particulate Filter, inventors Michiel van Nieuwstadt and Tom Brewbaker, assigned to the same assignee as the present invention. 
     TECHNICAL FIELD 
     This invention relates generally to methods and systems diesel engines having a diesel particulate filter (DPF) and a lean NOx trap (LNT). 
     BACKGROUND 
     As is known in the art, future diesel powertrains will likely be equipped with diesel particulate filters (DPF) and lean NOx traps (LNT). The DPF traps soot in the exhaust and needs to be regenerated, i.e. the soot needs to be burned off, every 500 miles or so. This is achieved by elevating the temperature (to around 600 deg C.) and providing enough oxygen to combust the soot. LNTs are poisoned by sulfur in the fuel and oil, and need to be desulfated every 3000 miles or so. This is achieved by elevating the temperature (to around 700 degrees C.) and depleting the exhaust gas of oxygen, i.e. running rich. Heating the exhaust gas and depleting oxygen uses extra fuel, which reflects negatively on the fuel economy. 
     The inventors have recognized that it would be desirable to coordinate both processes efficiently and to use the synergies to the largest extent possible. This invention proposes such coordination between DPF regeneration and LNT desulfation (deSOx). 
     SUMMARY 
     In accordance with the present invention, a method and system are is provided for simultaneously regenerating a particulate filter coupled to an internal combustion engine and for desulfating a lean NOx trap disposed downstream of the particulate filter. The method includes producing regeneration in the particulate filter. The regeneration produces an exhaust gas exiting the particulate trap having an elevated temperature and reduced oxygen concentration relative to gases entering such particulate filter. The exiting gases produce desulfation in the lean NOx trap. 
     In one embodiment, the method includes adjusting at least one engine operating parameter to control both regeneration in the particulate filter and the desulfation of the lean NOx trap. 
     In accordance with another feature of the invention, a method is provided for simultaneously regenerating a particulate filter coupled to an internal combustion engine and for desulfating a lean NOx trap disposed downstream of the particulate filter. The method includes adjusting at least one engine operating parameter to maintain a desired air fuel ratio for gases exiting the particulate filter in accordance with a difference between a reference set point air fuel ratio level and the air fuel ratio of gases exiting the particulate filter. The reference set point level is changed between a rich air fuel ratio and a lean air fuel ratio as a function of the air fuel ratio of the gases exiting the lean NOx trap. 
     In one embodiment, the method includes changing the reference set point level as a function of oxygen consumption of oxygen across the particulate filter 
     In one embodiment, the regeneration control comprises: commencing a self-sustaining filter regeneration; monitoring whether said regeneration causes temperature of said particulate filter to become greater than a predetermined value; and, in response to said monitoring, adjusting one or more operating parameters so as to limit exothermic reaction via control of an excess oxygen amount entering said filter and prevent temperature from rising to become greater than a pre-selected value. 
     In accordance with yet another feature of the invention, a method is provided for simultaneously regenerating a particulate filter coupled to an internal combustion engine and for desulfating a lean NOx trap disposed downstream of the particulate filter. The method includes controlling the oxygen concentration of the gas exiting the lean NOx trap by commanding an oxygen concentration setpoint for the gas entering the lean NOx trap, such commanded oxygen concentration being controlled by commanding an oxygen concentration setpoint for the gas entering the particulate filter. 
     In accordance with still another feature of the invention, a method is provided for simultaneously regenerating a particulate filter coupled to an internal combustion engine and for desulfating a lean NOx trap disposed downstream of the particulate filter. The method includes providing an oxygen sensor upstream of the particulate filter and using a signal produced by such sensor to control the particulate filter regeneration rate by metering the oxygen flow sensed by sensor and; providing an oxygen sensor downstream of the particulate filter and using a signal produced by such sensor to control the oxygen content of the gas entering the lean NOx trap. 
     In accordance with another feature of the invention, a method is provided for simultaneously regenerating a particulate filter coupled to an internal combustion engine and for desulfating a lean NOx trap disposed downstream of the particulate filter. The method includes adjusting the oxygen level into the particulate filter, comprising: reducing the oxygen content of the gas entering the particulate filter if the oxygen concentration measured by downstream oxygen sensor is greater than a predetermined level, such latter oxygen content being measured by the upstream oxygen sensor; increasing the oxygen content of the gas entering the particulate filter if the oxygen concentration measured by downstream oxygen sensor is less than the predetermined level, such latter oxygen content being measured by the upstream oxygen sensor. 
     In one embodiment the method includes monitoring the temperature of the gas exiting the particulate filter and reducing the oxygen concentration into the particulate filter if such measured temperature becomes greater than a predetermined level. 
     In one embodiment, the method includes monitoring the temperature of the gas exiting the lean NOx trap and increasing the oxygen concentration into the particulate filter if such measured temperature becomes greater than a predetermined level. 
     The inventors have observed that, in general, the oxygen content of the gas exiting the particulate filter will be lower than that entering the particulate filter, since soot combustion removes oxygen. By adjusting the oxygen level into the particulate filter there is a resulting increase the CO level out of the particulate filter. The CO acts as a reductant for desulfation. Lower oxygen concentration into the particulate filter results in a higher CO concentration out of the particulate filter and vice versa. 
     If the oxygen measured by oxygen sensor upstream of the lean NOx trap is too high, the oxygen content of the gas entering the particulate filter can be reduced (by the means set forth in the above-referenced patent application). This will increase the flow of reductant and decrease the oxygen flow into the lean NOx trap. 
     If the gas entering the lean NOx trap is too rich, sulfur is released preferentially as H2S, which is undesirable. If the oxygen sensor upstream of the lean NOx trap measures exhaust gas that is too rich, the oxygen concentration into the particulate filter can be increased. This may lead to excessive exotherms, since higher oxygen concentrations allow a higher soot bum rate. The control strategy herein described monitors the temperature of the gas exiting the particulate filter and reduces the particulate filter inlet oxygen concentration if this temperature becomes too high. The optimal oxygen flow into the particulate filter is therefore a trade-off between particulate filter temperature, soot burn rate, and H 2 S release by the lean NOx trap. 
     Thus, the invention utilizes the heat generated already for particulate filter regeneration and the removal of oxygen from the exhaust stream by soot combustion to create rich exhaust gas, and to achieve lean NOx trap desulfation. The lean NOx trap desulfation then only takes a minimal penalty on fuel economy above that for particulate filter regeneration. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of an engine system according to the invention; 
         FIG. 2  is a diagram showing in more detail the control system for the engine system of  FIG. 1 ; 
         FIG. 3  is a diagram showing a portion of the engine system of  FIG. 1 ; 
         FIG. 4  is a flow diagram of a program stored in the engine system of  FIG. 1 ; 
         FIG. 5  shows time histories of parameters generated by the engine system of  FIG. 1  in the absences of temperature limit controls; and 
         FIG. 6  shows a controller block diagram of an oxygen controller used in the system of  FIG. 1 . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a schematic diagram of the engine system is shown. Engine  10  is shown coupled to a turbo charger  12 . Turbo charger  12  can be any number of types, including a single stage turbo charge, a variable geometry turbo charger, a dual fixed geometry (one for each bank), or a dual variable geometry turbo charger (one for each bank). 
     Intake throttle  14  is shown for controlling manifold pressure and air flow entering the engine  10 . In addition, EGR valve  16  is shown for controlling recirculated exhaust gas entering the intake manifold of engine  10 . In the exhaust system, downstream of turbocharger  12 , is HC injector  18 . Located downstream of injector  18  is an oxygen sensor  20 , which provides signal O 2 U representative of the stoichiometry of the gases passing through it. 
     Downstream of oxygen sensor  20  is located a first oxidation catalyst  22 . A second oxidation catalyst  24  may also be used but may also be eliminated. The oxidation catalyst can be of various types, such as, for example, an active lean NOx catalyst. Further downstream of catalyst  24  is located a diesel particulate filter (DPF)  26 . Downstream of the DFP  26  is a lean NOx trap (LNT)  72 . 
     Referring also to  FIG. 3 , a first temperature sensor  28  which produces a temperature signal T 1  is located upstream of the particulate filter  26 , a second temperature sensor  30  which produces a temperature signal T 2  is located downstream of the particulate filter  26  and represents the temperature of gases exiting the DPF  26  and entering the LNT  72 , and a third temperature sensor  31  downstream of the LNT produces a temperature T 3  representative of the temperature of gases exiting the LNT  72 . 
     Also provided are UEGO sensors  33 ,  35  and  37 . UEGO sensor  33  produces a signal UEGO 1  representative of the oxygen concentration of gases entering the DPF  26 . UEGO sensor  35  produces a signal UEGO 2  representative of the oxygen concentration of gases exiting the DPF  26  and entering the LNT  72 . UEGO sensor  37  produces a signal UEGO 3  representative of the oxygen concentration of gases exiting the LNT  37 . 
     The particulate filter  26  is typically made of SiC, NZP and cordierite, with SiC being the most temperature resistant, and cordierite the least. Further, independent of the material used, self-sustained filter regeneration can be obtained simply by raising the particulate filter to a high enough temperature. 
     Each of the sensors described above provides a measurement indication to controller  34  as described below herein. Further, throttle position and EGR valve position are controlled via the controller  34  as described later herein. 
       FIG. 2  shows additional details of components shown and described in  FIG. 1 . Direct injection compression ignited internal combustion engine  10 , comprising a plurality of combustion chambers, is controlled by electronic engine controller  34 . Combustion chamber  40  of engine  10  is shown in  FIG. 2  including combustion Intake manifold  42  is shown communicating with throttle body  44  via throttle plate  14 . 
     In this particular example, throttle plate  14  is coupled to electric motor  46  so that the position of throttle plate  14  is controlled by controller  34  via electric motor  46 . This configuration is commonly referred to as intake throttle (ITH). In diesels, the ITH is frequently vacuum actuated; however, it could also be electrically actuated. chamber walls  48  with piston  50  positioned therein and connected to crankshaft  52 . 
     Combustion chamber or cylinder  40  is shown communicating with intake manifold  42  and exhaust manifold  52  via respective intake valves  54 , and exhaust valves  56 . Fuel injector  58  is shown directly coupled to combustion chamber  40  for delivering liquid fuel directly therein in proportion to the pulse width of signal fpw received from controller  34  via electronic driver  60 . Fuel is delivered to fuel injector  58  by a high pressure fuel system (not shown) including a fuel tank, fuel pumps, and a fuel rail. 
     Exhaust gas oxygen sensor  62  is shown coupled to exhaust manifold  52  upstream of active lean NOx catalyst  70 . In this particular example, sensor  62  provides signal EGO to controller  34 . This oxygen sensor is a so-called UEGO, or linear oxygen sensor, and provides continuous oxygen readings. 
     Controller  34  causes combustion chamber  40  to operate in a lean air-fuel mode. Also, controller  34  adjusts injection timing to adjust exhaust gas temperature. 
     As noted above, the diesel particulate filter (DPF)  26  is shown positioned downstream of catalyst  70 . DPF  70  retains particles and soot to be later regenerated (burned) at high temperatures as described herein. As noted above, downstream of the DPF  26  is the lean NOx trap (LNT)  72 . 
     Controller  34  is shown in  FIG. 2  as a conventional unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values shown as read-only memory semiconductor chip  106  in this particular example, random access memory  108  for storing a computer program which controls the engine  10 . Included in such computer program is a set of instructions for executing a method described below in connection with  FIG. 4 . Also included are a keep-alive memory  110 , and a conventional I/O data bus. 
     Controller  34  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor  100  coupled to throttle body  44 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a profile ignition pickup signal (PIP) from variable reluctance sensor (VRS)  118  coupled to crankshaft  40 ; throttle position TP from throttle position sensor  120 ; and absolute manifold pressure signal (MAP) from sensor  122 . Engine speed signal RPM is generated by controller  34  from signal PIP in a conventional manner and manifold pressure signal MAP from a manifold pressure sensor provides an indication of boost pressure in the intake manifold. 
     In this particular example, the temperature Tdpf of DPF  26  is inferred from engine operation. In an alternate embodiment, and temperature Tdpf is provided by temperature sensor  126 . Continuing with  FIG. 2 , a variable camshaft system is described. However, the present invention can also be used with non-VCT engines. Camshaft  130  of engine  10  is shown communicating with rocker arms  132  and  134  for actuating intake valve  54  and exhaust valve  56 . Camshaft  130  is directly coupled to housing  136 . Housing  136  forms a toothed wheel having a plurality of teeth  138 . Housing  136  is hydraulically coupled to an inner shaft (not shown), which is, in turn, directly linked to camshaft  130  via a timing chain (not shown). Therefore, housing  136  and camshaft  130  rotate at a speed substantially equivalent to the inner camshaft. The inner camshaft rotates at a constant speed ratio to crankshaft  40 . However, by manipulation of the hydraulic coupling as will be described later herein, the relative position of camshaft  130  to crankshaft  52  can be varied by hydraulic pressures in advance chamber  142  and retard chamber  144 . By allowing high pressure hydraulic fluid to enter advance chamber  142 , the relative relationship between camshaft  130  and crankshaft  40  is advanced. Thus, intake valves  54  and exhaust valves  56  open and close at a time earlier than normal relative to crankshaft  52 . Similarly, by allowing high pressure hydraulic fluid to enter retard chamber  144 , the relative relationship between camshaft  130  and crankshaft  40  is retarded. Thus, intake valves  54  and exhaust valves  56  open and close at a time later than normal relative to crankshaft  52 . 
     In addition, controller  34  sends control signals (LACT,RACT) to conventional solenoid valves (not shown) to control the flow of hydraulic fluid either into advance chamber  142 , retard chamber  144 , or neither. Relative cam timing is measured using the method described in U.S. Pat. No. 5,548,995, which is incorporated herein by reference. 
     In general terms, the time or rotation angle between the rising edge of the PIP signal and receiving a signal from one of the plurality of teeth  138  on housing  136  gives a measure of the relative cam timing. Sensor  160  provides an indication of oxygen concentration in the exhaust gas. Signal  162  provides controller  34  a voltage indicative of the O 2  concentration. 
     Note that  FIG. 2  merely shows one cylinder of a multi-cylinder engine, and that each cylinder has its own set of intake/exhaust valves, fuel injectors, etc. 
     In  FIG. 1 , an EGR system is included. In particular, EGR Valve  16  ( FIG. 1 ) (which can be electrically, pneumatically or magnetically controlled) is positioned in a recirculation tube that transmits exhaust gas from manifold  52  to intake manifold  42 . 
     A method and system are provided for controlling the rate of DPF regeneration by metering the oxygen in the inlet gas to the DPF is described in the above referenced patent application, the entire subject mater thereof heaving been incorporated herein by reference. The current invention builds on that invention. More particularly, when the DPF  26  is regenerated, the DPF exit temperature is high enough for LNT desulfation. To achieve desulfation of the LNT there is also a need to achieve sufficient oxygen depletion of the gas entering the LNT. The inventors have discovered the use of an oxygen sensor (UEGO sensor  33 ) upstream of the DPF  26  to control the DPF  26  regeneration rate by metering the oxygen flow of the gases entering the DPF  26  using UEGO  33 . The inventors have also discovered the use of an oxygen sensor  35  downstream of the DPF  26  to control the oxygen content of the gas entering the LNT  72 . The goal is to remove the oxygen from the gas that exits the DPF  26 . 
     In general, the oxygen content of the gas exiting the DPF  26  will be lower than that entering the DPF  26 , since soot combustion removes oxygen. By adjusting the oxygen level into the DPF  26  to increase the CO level out of the DPF  26 . The CO acts as a reductant for desulfation by the LNT  72 . Lower oxygen concentration into the DPF results in a higher CO concentration out of the DPF  26  and vice versa. 
     If the oxygen measured by UEGO sensor  35  (i.e., UEGO 2 ) is too high, the oxygen content of the gas entering the DPF  26  is reduced (by the means set forth in the above-referenced patent application). This will increase the flow of reductant and decrease the oxygen flow into the LNT  72 . 
     If the gas entering the LNT  72  is too rich for too long a time, sulfur is released preferentially as H 2 S, which is undesirable. If the UEGO sensor  35  (i.e., UEGO 2 ) measures exhaust gas that is too rich, the oxygen concentration into the DPF  26  is increased by the means set forth in the above-referenced patent application. This may lead to excessive exotherms, since higher oxygen concentrations allow a higher soot burn rate. The control strategy herein described monitors the temperature T 2  (using sensor  30 ) of the gas exiting the DPF and reduce the DPF  26  inlet oxygen concentration if this temperature becomes too high. The optimal oxygen flow into the DPF  26  is therefore a trade-off between DPF temperature, soot burn rate, and H 2 S release by the LNT  72 . 
     Thus, the invention utilizes the heat generated already for DPF  26  regeneration and the removal of oxygen from the exhaust stream by soot combustion to create rich exhaust gas, and to achieve LNT desulfation. The LNT desulfation then does not take a large penalty on fuel economy above that for DPF regeneration. 
     Referring now to the flow diagram in  FIG. 4 , DPF regeneration begins in Step  400 . In Step  402 , a check is made of normalized backpressure (using the differential pressure sensor  32 ). In Step  404 , soot-loading SL in grams/liter is inferred from a previously calibrated table. Next, in Step  406 , RICH — TIME is set equal to 0. 
     In Step  408 , a determination is made as to whether SL&lt;TBD — MAX, where TBD — MAX is determined experimentally. If SL is not less than TBD — MAX, the process returns to Step  402 ; otherwise, the process proceed to Step  410 . In Step  410 , the combined DPF regeneration and LNT deSOx commences. 
     Thus, the process, in Step  412 , sets UEGO 2  lean setpoint; i.e., a predetermined level of oxygen concentration, UEGO 2 , is established. More particularly, UEGO 2   — DES=UEGO 2   — DES — LEAN, where UEGO 2   — DES — LEAN is established for a particular engine type a priori. 
     In Step  413 , a determination is made as to whether the oxygen concentrations, i.e., air fuel ratio, sensed by UEGO sensor  35  (i.e., UEGO 2 ) is less than UEGO 2   — DES. If it is, the oxygen concentration as sensed by UEGO sensor  33  (i.e., UEGO 1 ) is increased reduced by the means set forth in the above-referenced patent application. For example, the increase in oxygen concentration is produced by engine measures (e.g. increase O 2  level in feed gas, for example by means of a PI controller, as described in the above referenced patent application, Step  415 . On the other hand, if UEGO 2 &gt;UEGO — DES, the oxygen concentration as sensed by UEGO sensor  33  (i.e., UEGO) 1  is decreased by such engine measures, Step  416 . Thus, Steps  413 ,  415  and  416  adjust the oxygen concentration as sensed by UEGO sensor  33  (i.e., UEGO 1 ) of the gases into the DPF  26  in accordance with the oxygen concentration of the gases exiting the DPF, such exiting oxygen concentration being sensed by UEGO sensor  35  (i.e., UEGO 2 ) relative to setpoint UEGO 2   — DES — LEAN. 
     During this control process, the temperature, T 2 , at the output of the DPF  26  is measured by sensor  30  and compared with a predetermined temperature level T 2   — SAFE established for a particular engine type a priori, Step  118 . If T 2  exceeds T 2   — SAFE, the set point UEGO 2   — DES is reduced in order to slow down the regeneration, Step  419 . 
     During this control process, the oxygen concentration of the gases exiting the LNT is measured by UEGO sensor  37  (i.e., UEG 03 ) and such concentration is compared with an a priori established predetermined level UEGO 3   — LEAN — MAX — LEAN in Step  420 . If the oxygen concentration of the gases exiting the LNT is less than UEGO 3   — LEAN — MAX — LEAN, the process returns to Step  404 ; otherwise, the UEGO 2  set point, UEGO 2   — DES is set is changed to an a priori established predetermined set point UEGO 2   — DES — RICH, Step  421 . 
     While the process proceeds as described above, a determination as made as to whether the oxygen concentration as sensed by LEGO sensor  35  (i.e., UEGO 2 ) is less than UEGO 2   — DES, Step  422 . If UEGO 2 &lt;UEGO 2   — DES, the oxygen concentration as sensed by UEGO sensor  33  (i.e., UEGO 1 ) is increase. The increase in oxygen concentration is produced by engine measures (e.g. increase O 2  level in feed gas, for example by means of a PI controller, as described in the above referenced patent application), Step  423 . On the other hand, if UEGO 2 &gt;UEGO 2   — DES, the oxygen concentration as sensed by UEGO sensor  33  (i.e., UEGO 1 ) is decreased by such engine measures, Step  424 . Thus, Steps  422 ,  423  and  424  adjust the oxygen concentration as sensed by LEGO sensor  33  (i.e., LEGO 1 ) of the gases into the DPF in accordance with the oxygen concentration of the gases exiting the DPF, such exiting oxygen concentration being sensed by UEGO sensor  35  (i.e., UEGO 2 ) relative to setpoint UEGO 2   — DES which may be either UEGO 2   — DES — LEAN, UEGO 2   — DES — LEAN reduced if T 2  exceeds T 2   — SAFE, or UEGO 2   — DES — RICH if the oxygen concentration of the gases exiting the LNT is less than UEGO 3   — LEAN — MAX — LEAN. 
     A determination is made in Step  426  as to whether the temperature T 3  of the gases exiting the LNT (sensed by sensor  31 ) is greater than an a priori predetermined established level T 3   — SAFE, Step  126 . If T 3 &gt;T 3   — SAFE, the set point UEGO 2   — DES is increased to slow down LNT deSOx, Step  427 . 
     Next, in Step  428 , the RICH — TIME is incremented; RICH — TIME=RICH — TIME+Ts, where Ts is the processing interval. If, in Step  128 , RICH — TIME&gt;RICH — TIME — MAX, the process terminates regeneration and deSOx, Step  434 ; otherwise, the process continues and, in Step  432 , a determination is made as to whether the oxygen concentration of the gases exiting the LNT  72  as sensed by UEGO sensor  37  (i.e., UEGO 3 ) is greater than an a priori set point UEGO 3   — RICH — MIN, Step  432 . If it is, the process returns to Step  422  and the deSOx process continues; otherwise the process returns to Step  412  to continue with the combination DFP regeneration and LNT deSOx. 
     Thus, from the flow diagram it is noted that, neglecting the effects of T 2   — SAFE and T 3   — SAFE, one of two different set points are used for UEGO 2   — DES; i.e., UEGO 2   — DES=UEGO 2   — DES — LEAN (Step  412 ,  FIG. 4 ) or UEGO 2   — DES UEGO 2   — DES — RICH (Step  412 ,  FIG. 4 ). 
     Thus, referring to  FIG. 5 , when UEGO 3  is less than UEGO 3   — RICH — MIN (i.e., at times indicated by in  FIG. 5  by A) the set point UEGO 2   — DES changes from UEGO 2   — DES — RICH to UEGO 2   — DES — LEAN and when UEGO 3  is greater than UEGO 3   — RICH — MIN (i.e., at times indicated by in  FIG. 5  by B) the set point UEGO 2   — DES changes from UEGO 2   — DES — LEAN to UEGO 2   — DES — RICH. 
     It is also noted from  FIG. 5  that T 3  increases when UEGO 3  is rich and T 3  decreases when UEGO 2  is lean. Further, it is noted that T 2  increases when UEGO 2   — DES has the set point UEGO 2   — DES — LEAN and T 2  decreases when UEGO 2   — DES has the set point UEGO 2   — DES — RICH. Finally, it is noted that LNT stores oxygen during the intervals between times indicated by A. 
     Referring now to  FIG. 6 , a high level schematic of the oxygen controller is shown. In particular in this embodiment, three actuators are used to limit the supply of oxygen delivered to the DPF  26 : an exhaust gas recirculation valve (EGR), an intake throttle (ITH) and a (hydrocarbon) (HC) injector located in the exhaust feedback. EGR and ITH are used in feedback control to account for slowly varying changes in the oxygen flow rate supply to the DPF. 
     As described above, the UEGO sensors  35  (i.e., UEGO 2 ) and  37  (UEGO 3 ) are used as the feedback sensors. In the present embodiment, quick changes in oxygen flow rate are compensated using the HC injector in a feed-forward control. While injecting hydrocarbons can supply additional heat to the DPF, there are instances where this additional heat will be more than compensated for by reducing the exothermic reaction rate (by limiting excess oxygen). Part of the heat added to the system upstream is rejected by heat transfer to the environment through the exhaust system. Adding heat upstream also gives a much more uniform heat distribution that is less likely to damage the DPF than local hot spots resulting from local burning on the DPF  26 . In particular, the hydrocarbon feed-forward controller, in one embodiment, simply calculates the quantity of fuel necessary to stoichiometrically combust with the high pass oxygen flow rate error. However, the control authority of the HC injection is one-sided since HC injection can only remove excess oxygen. 
     Note, in an alternative embodiment, other control structures can be used. For example, rather than using the EGR valve, the intake throttle, or a hydrocarbon injector, the oxygen concentration in the exhaust can be modified by changing intake or exhaust valve timing on an engine equipped with an appropriate actuator. If the engine is equipped with a variable geometry turbocharger (VGT), the vane setting on the VGT can be modified. If the engine is equipped with an exhaust brake, its position can be modified. 
     Referring now specifically to  FIG. 6 , the oxygen flow rate error (which is the error between the desired and actual oxygen flow rate. More particularly, UEGO  2  and UEGO 3  measure air fuel ratio or oxygen concentration. They are equivalent. To be precise: the measurement is O 2  concentration=0.2*(AFR−14.6)/(AFR+1) and is fed to a low-pass filter. The cutoff frequency of the low-pass filter is preferably selected as the bandwidth of the EGR/ITH controller, defined from the oxygen flow rate error to the oxygen flow rate. In one example, the cutoff frequency was selected as 0.5 RAD/S. However, various factors such as controller stability and feedback control performance effect the selection of this frequency. Therefore, various values may be used according to the present invention. In another example, the cutoff frequency is made a calibratable function of engine operating conditions. Also, it may be desirable to increase this cutoff frequency as high as possible, thereby improving controller performance and minimizing control action necessary from the HC injector. The highest possible cutoff frequency is equal to the bandwidth of the EGR/ITH controller. Then, the oxygen flow rate error minus the low-pass filtered error is fed to the feed-forward controller to determine the HC injection quantity. Further, the low-pass filtered oxygen flow rate error is fed to the EGR/ITH PI controller, which determines the control action for the EGR valve and the throttle valve. 
     Also, the present invention is described with particular reference to a self-sustaining DPF regeneration. Such self-sustaining regeneration is used to refer to the regeneration of stored particles in the DPF that continues without additional control action beyond normal other engine operation. For example, the engine control system may need to adjust fuel injection timing, or other operating parameters, to initiate increased exhaust temperatures. Thus, these conditions would include non-normal operation required to start particulate filter regeneration. However, once the self-sustaining regeneration is reached, the engine operating parameters can be returned to whatever normal conditions require. As such, the particulate filter regeneration will continue as long as enough excess oxygen is present and there are stored particles left to be burned. 
     As another example, an external burner could be used to raise particulate filter temperature above the self-sustained regeneration temperature. After this point, the burner is no longer necessary and the self-sustaining reaction can proceed without any special control action by the engine controller. According to the present invention, this self-sustaining regeneration is monitored via, for example, the particulate filter temperature, and, in one example, when the temperature is greater than a predetermined temperature control, action is taken to limit excess oxygen and thereby limit the diesel particulate filter regeneration reaction rate. This limits the self-sustaining reaction, thereby limiting temperature and minimizing any potential degradation. 
     A number of embodiments of the invention have been described. For example, as described above, there are various parameters that can be used to limit oxygen entering a DPF during a self-sustained filter regeneration interval. Also note that it is not necessary and not intended to completely stop filter regeneration to prevent DPF temperature from becoming greater than an allowable temperature. In particular, during some operating conditions, excess oxygen fed to the DPF can be reduced thereby slowing the exothermic reactions in the DPF, but still providing enough gas flow rate through the DPF to carry away enough excess heat from this continued regeneration so that DPF temperature is maintained at or below an allowable temperature. Further, the upstream UEGO sensor can be replaced by an estimator of feed gas oxygen, based on engine operating conditions. The deSOx typically takes less time than DPF regeneration, hence one only needs to run the LNT oxygen control for part of the DPF regeneration. Preferably this is towards the end of the DPF regeneration, when there is still enough soot to generate reductant (CO) in the DPF, but not so much soot that to risk an uncontrolled DPF regeneration resulting in excessive exotherms. The oxygen content into the DPF can be decreased by throttling the engine, increasing EGR level, retarding timing accompanied by increasing fuel quantity, changing valve timing, post injecting fuel into the cylinder, injecting fuel via a downstream injector, etc. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention Accordingly, other embodiments are within the scope of the following claims.