US20030113249A1 - System and method for removing SOx and particulate matter from an emission control device - Google Patents

Abstract

A system and method for removing SOx and particulate matter from an emission control device receiving exhaust gases from an engine is provided. The method includes adding a reductant to the exhaust gases to increase a temperature of the emission control device above a threshold temperature. The method further includes ceasing adding the reductant to the exhaust gases to remove particulate matter from the device. The method further includes adding additional reductant to the exhaust gases to remove SOx from the device.

Description

FIELD OF THE INVENTION
• The present invention relates to a system and method for removing SOx and particulate matter from an emission control device coupled to an engine.[0001]
DESCRIPTION OF THE RELATED ART
• A known engine control system for purging SOx and particulate matter from a combined particulate filter and NOx trap is disclosed in Japanese Patent 06-272541A. In particular, the control system operates the engine lean while adding reductant to exhaust gases to increase a temperature of the exhaust gases. The increased temperature is used to oxidize particulate matter stored in the combined filter and trap. This process is called particulate filter regeneration. Thereafter, the control system adds more reductant to purge sulphur-oxides (SOx) from the combined filter and trap.[0002]
• The known control system, however, has a significant drawback. In particular, the control system supplies reductant/fuel to exhaust gases during the purging of particulate matter. As a result, the fuel economy of the engine is decreased. Further, the additional reductant may result in increased hydrocarbon emissions during regeneration of the particulate filter.[0003]
SUMMARY OF THE INVENTION
• The foregoing problems and disadvantages are overcome by a system and method for removing particulate matter and SOx from an emission control device in accordance with the present invention.[0004]
• The inventive method is utilized with an emission control device receiving exhaust gases from an engine. The method includes adding a reductant to the exhaust gases to increase a temperature of the emission control device above a threshold temperature. The method further includes ceasing adding the reductant to the exhaust gases to remove particulate matter from the device. The method further includes adding additional reductant to the exhaust gases to remove SOx from the device.[0005]
• A system for removing particulate matter and SOx from an emission control device is also provided. The system includes a reductant valve selectively supplying reductant to the exhaust gases responsive to a first signal. The system further includes a throttle valve controlling flow of the exhaust gases to the device responsive to a second signal. The system further includes a controller operably connected to the reductant valve and the throttle valve. The controller generates the first and second signals. The controller is configured to add the reductant to the exhaust gases to increase a temperature of the emission control device above a threshold temperature. The controller is further configured to cease adding the reductant to the exhaust gases to remove particulate matter from the device. The controller is further configured to add additional reductant to the exhaust gases to remove SOx from the device.[0006]
• The present system and method provides a substantial advantage over known systems and methods. In particular, the inventive method ceases delivery of reductant during particulate filter regeneration and then subsequently supplies reductant to remove SOx in the combined filter and trap. The process of supplying reductant and then stopping the supply of reductant may be iteratively performed to remove SOx and particulate matter, respectively. Thus, the inventors herein have recognized that an exothermic reaction generated during NOx or SOx removal can be utilized to maintain a desired temperature of exhaust gases for subsequent oxidation of particulate matter—without adding additional reductant during the oxidation process. Thus, the present system and method provides for increased fuel economy and lower hydrocarbon emissions—as compared to known systems that deliver reductant during both the particulate matter oxidation and the SOx reduction from an emission control device.[0007]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a plan view of an exhaust system coupled to an engine.[0008]
• FIG. 2 is a schematic diagram of a dual NOx trap and particulate filter of the exhaust system of FIG. 1.[0009]
• FIGS. 3A, 3B are flowcharts of a method for monitoring and removing stored NOx from the exhaust system of FIG. 1.[0010]
• FIGS. 4A, 4B,[0011]4C are flowcharts for monitoring and removing particulate matter and sulfur oxides (SO_x) stored in the exhaust filters of FIG. 1.
• FIGS. 5A and 5B are schematics of signals illustrating the operation of the flowchart of FIG. 4B for removing particulate matter from an exhaust filter.[0012]
• FIGS. 6A and 6B are schematics of signals illustrating the operation of the flowchart of FIG. 4C for removing particulate matter and SO[0013]_xfrom an exhaust filter.
DESCRIPTION OF AN EMBODIMENT
• Referring now to FIG. 1, an[0014]exhaust system10 is illustrated in operational relationship with aninternal combustion engine12 such as a diesel engine for a vehicle such as an automotive vehicle (not shown). Theengine12 has anexhaust manifold14 to direct the exhaust gases fromengine12 toexhaust system10. Theexhaust manifold14 is divided into twoexhaust intake conduits16,18. The exhaust intake conduits16,18 direct exhaust gases through two integrated NO_xtrap/particulate filters19,20. The outputs of thefilters19,20 are directed through twoexhaust output conduits21,22 to a muffler ortail pipe23. The amount of exhaust gases flowing to filters19,20 is controlled bycontrol valves24,25, respectively.Conventional oxidation catalysts26,27 are located upstream andproximate filters19,20, respectively.
• The[0015]oxidation catalysts26,27 serve several functions. First,catalysts26,27 are utilized to accurately control temperatures infilters19,20, respectively. Fuel injected into the exhaust gases upstream ofcatalysts26,27 can be used to create exothermic reactions incatalysts26,27 to provide exhaust gases at desired temperatures to filters19,20, respectively. Thus, temperature spikes produced by the injected fuel only occur withincatalysts26,27 instead offilters19,20—thereby protectingfilters19,20 from degrading due to excess temperatures. Further, controlled exothermic reactions incatalysts26,27 allowfilters19,20 to be maintained within desired temperature ranges and air-fuel ranges for optimally removing NOx, SO_x, and particulate matter. It should be understood, however, that an oxidation catalyst as taught by this specification can be used to control a temperature of any proximate downstream emission control device such as a NOx absorber, a catalytic converter, and a particulate filter, for example. Second,catalysts26,27 begin burning the fuel injected into the exhaust gases and rapidly vaporize the remaining fuel before the mixture of fuel and exhaust gases reachfilters19,20, respectively. The vaporized fuel more effectively reduces NOx withinfilters19,20 as compared to injecting liquid fuel directly intofilters19,20.
• A reducing agent supply tank[0016]28 and associatedfuel nozzles29,30 enable precise quantities of reductant, such as diesel fuel, to be injected into the exhaustpath intake conduits16,18. In the case of a diesel engine, the reductant is preferably the diesel fuel supply in the vehicle fuel tank. It should be understood, however, that other reductants such as gasoline for example could be used as the injected reductant.
• In addition,[0017]temperature sensors31,32 generate feedback signals to the engine control module (ECM)34. The signals are indicative of temperatures of exhaustgases exiting catalysts26,27, respectively—obtained from exothermic reactions inoxidation catalysts26,27. TheECM34, in turn, controls the flow of exhaust gases throughexhaust system10 as well as the regeneration process offilters19,20. Logic control ofsystem10 is carried out inECM34 by way of a central processing unit (CPU)36 such as amicroprocessor36 and an associatedmemory38.
• Referring to FIGS. 1 and 2, each of the exhaust filters[0018]19,20 may include afilter substrate40 extending along a longitudinal axis. Thefilter substrate40 has a plurality ofwalls42 extending longitudinally and forming a plurality of alternating first andsecond channels44 and46. Thewalls42 may be made of a ceramic material such as ceramic cordierire. The ceramic material is porous and has a pore size of approximately 60 microns. Thewalls42 have a thickness of 10 to 20 mils. Thewalls42 are configured to provide 50 to 200 channels per square inch.Channels44,46 have a generally rectangular cross-section, although it should be appreciated that the cross-sectional area of thechannels44,46 is dictated by flow and filtering requirements.
• Each of[0019]channels44,46 have aninlet end48 and anoutlet end50. The first channels44 have a blockingmember52 to close theoutlet end50 and thesecond channels46 have a blockingmember52 to closeinlet end48. The blockingmember52 is made of ceramic material such as ceramic cordierite.Second channels46 also have a NO_xabsorbent wash coat54 extending from theoutlet end50 along thewalls42 toward theinlet end48. Thewash coat54 is a NO_xabsorbent applied by conventional procedures. The NO_xabsorbent may be (i) a precious metal such as Pt—Rh and an alkali metal such as potassium or lithium, or (ii) alkaline earth metals such as barium or strontium or (iii) lanthanides such as cerium—dispersed into an alumina support deposited ontowalls42 ofsecond channels46.
• It should be appreciated that[0020]alternate channels44,46 are blocked to force all of the exhaust gas flow throughwalls42 havingwash coat54 thereby filtering the exhaust gas particulate matter and absorbing the NO_x. Exhaust gases fromengine12 enter through theinlet openings48, pass throughporous walls42 of thesubstrate40, and exit through thesecond channels46 atopen outlets50. Since the particulate matter is too large to pass through the pores withinsubstrate wall42, it deposits ontosurface55 of the open channels44. The NO_x, HC, and CO pass readily through thesubstrate wall42. NO_xis stored as a nitrate complex while HC and CO are oxidized over the platinum within NO_xtrap washcoat54.
• Referring to FIGS. 1 and 2, under the control of[0021]ECM34, all of the exhaust gases flowing fromengine12 may be directed, for example, to firstexhaust intake conduit16 andcorresponding catalyst26 andfilter19 by controllingvalves24,25. In this example,control valve24 is completely open andcontrol valve25 is completely closed. In this manner, all of the exhaust gases fromengine12 will flow throughexhaust intake conduit16, throughoxidation catalyst26,filter19, exhaust output conduit21, andexit tail pipe23. When a mixture of exhaust gases and fuel are rich of stoichiometry, the majority of the CO and HC in the exhaust gases are oxidized inoxidation catalyst26, and the remaining CO and HC is oxidized over the platinum contained within the NO_xtrap-particulate filter19.
• When the quantity of absorbed NO[0022]_xinfilter19 approaches the absorption capacity of the NO_xtrap washcoat54 offilter19, the NO_xregeneration routine for this filter is initiated. At such time,control valve25 is actuated to a fully opened position, and controlvalve24 is set to a partially opened position to restrict the flow of exhaust gases throughintake conduit16. Diesel fuel from the supply28 is injected by nozzle orvalve29 intointake conduit16 and is carried towardfilter19 by the restricted exhaust flow inconduit16. Fuel is injected such that the fuel quantity exceeds the stoichiometric amount required to completely react and consume all of the oxygen contained within the exhaust gases flowing throughconduit16. In other words, the mixture of exhaust gases and fuel is rich of stoichiometry. Catalytic combustion of the injected fuel occurs inoxidation catalyst26. Excess fuel is carried downstream intofilter19 and reacts with the absorbed NO_x, thereby regenerating NO_xtrap washcoat54 withinfilter19.
• During the NO[0023]_xtrap wash coat regeneration process, a substantial exothermic temperature rise occurs withinoxidation catalyst26 whencatalyst26 is oxidizing a rich mixture of exhaust gases and fuel. By locating thecatalyst26 upstream of NO_xtrap-particulate filter19, relatively large temperature spikes caused by an exothermic energy release occur primarily incatalyst26. Thus, a temperature of exhaustgases exiting catalyst26 are maintained at a relatively uniform desired temperature which prevents thermal damage to filter19. Becausefilter19 isproximate catalyst26, the temperature offilter19 corresponds to the temperature of exhaustgases exiting catalyst26.
• Further, locating[0024]catalyst26 adjacent and upstream offilter19 allow for high molecular weight hydrocarbons within the diesel fuel to be cracked and partially oxidized by catalyst26 (producing shorter chained hydrocarbons and CO/H₂) before enteringfilter19. The shorter chained hydrocarbons vaporize at lower temperatures than non-oxidized hydrocarbons allowing for more effective NO_xregeneration offilter19.
• When regeneration of NO[0025]_xtrap wash coat offilter19 is complete, the diesel fuel injection is discontinued, and thecontrol valve24 is closed. At such time, all of the exhaust gases created byengine12 will be flowing throughexhaust intake conduit18,oxidation catalyst27,filter20,output conduit22, andtail pipe23. When the NO_xtrap washcoat54 offilter20 becomes saturated, the above process is repeated forfilter20.
• At the same time the level of absorbed NO[0026]_xis being monitored infilters19,20, the amount of accumulated particulate matter and accumulated SO_x(within NO_xtrap wash coat54) is monitored as well. The removal of particulate matter and desulfation processes can be accomplished in a single step. For example, when the accumulation of particulate matter and the buildup of SO_xwithin the NO_xtrap wash coat offilter19 exceeds a predetermined level,control valve25 is fully opened and thecontrol valve24 is set to a partially opened position. Diesel fuel from the supply28 is introduced through the nozzle orvalve29 intointake conduit16 and carried towardsfilter19 by the restricted exhaust flow. The rate of diesel fuel injection is set to obtain a rich air-fuel ratio needed to remove the absorbed SO_x. The time interval for the fuel injection and the time interval between subsequent injections are determined to provide the exothermic temperature rise required to desorb SOx from the NOx trap and to initiate oxidation of the particulate matter or soot infilter19. Thetemperature sensor31 provides a feedback control signal toECM34 to provide the fuel metering and timing control. Once the temperature of thefilter19 is maintained above a threshold temperature for removing SO_xand particulate matter, fuel is delivered at periodic intervals to the exhaust gases. Thus, the exhaust gases are alternated between rich of stoichiometry and lean of stoichiometry. During delivery of rich exhaust gases, the SO_xis removed fromfilter19. During delivery of lean exhaust gases, particulate matter is removed fromfilter19.
• Referring to FIGS. 3A, 3B,[0027]4A-4C, the method executed byECM34 to control the exhaustgas purification system10 will now be described. FIGS. 3A and 3B describe a preferred control arrangement for monitoring NO_xabsorption infilters19,20 and regeneration of the same. As shown in FIG. 3A, the primary NO_xcontrol scheme begins with a series of initializations. In these logic control diagrams, the parallel filter arrangement as shown in FIG. 1 is considered to have two sides or paths. Thus, logic variables ending in the numeral “1” refer to the exhaustpath containing filter19 and logic variables ending in the numeral “2” refer to the exhaustpath containing filter20.
• At[0028]step60, the regeneration flags REGNFLG1 and RGENFLG2 forfilters19,20, respectively are initialized to zero—indicating that particulate matter (PM) removal and desulfation (SO_xregeneration) is not taking place.
• At[0029]step62, the values CUMNO1 and CUMNO2 corresponding to the cumulative NO_xstored infilters19,20, respectively, are initialized to zero.
• At[0030]step64, the NOx regeneration counter DNOXCNT and the maximum value of the NOx regeneration counter are initialized to zero.
• At step[0031]66, the status ofcontrol valves24,25 are initialized. In this example, thecontrol valve24 is fully opened (VFLG1=1) andvalve25 is completely closed (VFLG2=0). The flags VFLG1 and VFLG2 indicate a partially open position when equal to the value of two. After initializing the foregoing variables, all of the exhaust gases will be flowing throughfilter19.
• At block[0032]68, the mass of feed gas NO_x(MNOX) generated byengine12 is estimated as a function of the engine speed andload70. The value MNOX can be readily determined from lookup tables indexed by engine speed and load created during engine mapping.
• Because VFLG1 is initially equal to “1”, the method advances from[0033]step72 to step74. Atstep74, the value MNOX is added to the cumulative NO_x(CUMNO1) absorbed byfilter19 through which the exhaust gases are flowing.
• At[0034]step76, the value RGENFLG2 indicating whether particulate matter and SO_xregeneration is occurring forfilter20 is checked. If regeneration and particulate matter burn-off is not occurring infilter20, then the method advances to step78. Otherwise, the method returns to step68.
• At step[0035]78, a determination is made as to whether the total mass of absorbed NO_x(CUMNO1) is greater than or equal to a predetermined maximum value (CUMNO_MAX) forfilter19. If the value of step78 equals “No”, the NO_xregeneration program (DNOX2) for thefilter20 is executed. Otherwise, thesteps80,82,84,86 are performed.
• At[0036]step80,control valve24 is partially closed. Atstep82, the duration of the NO_xregeneration (DNOXCNT_MAX) forfilter19 is set as a function of the total NO_xabsorbed. Next atstep84, the regeneration timer DNOXCNT is reset. Finally, atstep86,control valve25 is fully opened.
• Referring to FIG. 3B, the DNOX2 routine for removing NOx from[0037]filter20 will now be described. Before entering the DNOX2 routine,control valve24 will be fully open andcontrol valve25 will be partially open.
• At[0038]step94, a determination is made as to whether the regeneration counter DNOXCNT is greater than or equal to the count DNOXCNT_MAX. The value of the maximum count DNOXCNT_MAX corresponds to the amount of time required to remove the stored NO_xin filter20 (FIG. 3A, step92). If the value ofstep94 equals “Yes”, the method if exited atstep110. Otherwise, the method advances to step95. Atstep95, a partially open position ofvalve25 is determined. As shown, the position ofvalve25 is determined fromexhaust flow rate97 andexhaust oxygen concentration100. Theexhaust flow rate97 is determined as a function ofengine speed96. Theexhaust oxygen concentration100 is determined as a function ofengine load99. Afterstep95, the method advances to step98.
• At[0039]step98, the amount of fuel flow F2 that is injected byinjector30 is determined. The fuel flow amount is calculated based on theexhaust flow rate97 and theexhaust oxygen concentration100.
• At[0040]step101, a determination is made as to whether the temperature T1 ofoxidation catalyst32 is greater than a threshold temperature T_THRESHOLD for optimal removal of NOx. The temperature T_THRESHOLD may be 270° C. for example. If the value ofstep101 equals “Yes”, the method advances to step102 which determines whether the temperature is greater than a maximum temperature T_MAX. The value of T_MAX may be 400° C. for example. If the value ofstep102 equals “Yes”, the opened position V2 ofvalve25 is reduced using the following equation:
• V2=V2−ΔV
• where ΔV is a predetermined adjustment amount of[0041]valve25 that is empirically determined. Afterstep103, the method returns to step98 to recalculate the fuel flow F2 based on the new position ofvalve25.
• Referring again to step[0042]101, if the temperature T1 is less than threshold temperature T_THRESHOLD for optimal removal of NOx, the method advances to step104.
• At[0043]step104, the fuel flow F2 is increased based on the following equation:
• F2=F2+ΔF
• where ΔF is a predetermined fueling adjustment amount for[0044]valve30 that is empirically determined. Thestep104 increases the amount of fuel combusted inoxidation catalyst27 to thereby increase the temperature ofcatalyst27 andfilter20.
• Next at[0045]step105, the counter DNOXCNT is incremented using the following equation:
• DNOXCNT=DNOXCNT+DT
• where DT corresponds to the time interval that has elapsed since the value DNOXCNT was previously incremented in this execution of the DNOX2 routine.[0046]
• Next at[0047]step106, a determination is made as to whether DNOXCNT is greater than or equal to maximum count DNOXCNT_MAX. If the value ofstep106 equals “No”, the method is exited atstep110. Otherwise, the method advances to step107.
• At[0048]step107, the fuel flow F2 is set equal to zero thereby stopping the fuel flow fromvalve30.
• Next at[0049]step108, thethrottle valve25 is closed and the flag VFLG2 is set equal to zero.
• Next at[0050]step109, the cumulative stored NOx value CUMNO2 is set equal to zero. Thereafter, the method is exited atstep110.
• Although not shown, the DNOX1 regeneration routine for[0051]filter19 is performed in a similar manner as described in routine DNOX2, except thatvalves24,29 are controlled instead ofvalves25,30, respectively. The DNOX1 regeneration routine forfilter19 is called fromstep91 of FIG. 3A.
• Referring to FIGS. 4A, 4B,[0052]4C, the routines for removing particulate matter and SO_xfromfilter19 will now be described. The variables ending in the numeral “1” refer to the exhaust path communicating withfilter19 and variables ending in the numeral “2” refer to the exhaust path communicating withfilter20.
• Referring to FIG. 4A, the Main Particulate Matter and SOXREG Routine is illustrated. At[0053]step200, the regeneration flags are REGNFLG1 and REGNFLG2 are initialized to zero.
• At[0054]step202, the accumulated particulate matter counts CUMPM1 and CUMPM2 forfilters19,20, respectively are initialized to zero.
• At[0055]step204, the accumulated sulfur counts CUMSOX1 and CUMSOX2 forfilters19,20, respectively, are initialized to zero. Atstep206, thefirst valve24 is opened (VFLG1=1) and thesecond valve25 is closed (VFLG2=0).
• At[0056]step208, the mass of particulate matter flowing through the first path is determined as a function of the engine speed/load210.
• Next at[0057]step212, the mass of sulfur flowing throughfilter19 is estimated as a function of theengine fuel flow214. Since all of the exhaust is flowing through thefirst path16, these estimated values will be attributed to filter19 atsteps218 and220. If either the accumulated total particulate matter infilter19 as governed bydecision step222, or the total sulfur absorbed byfilter19 as dictated bydecision step224 exceeds a predetermined maximum, the PMREG1 routine is executed to regeneratefilter19.
• Before proceeding with a detailed discussion of the PMREG1 routine for removing particulate matter from[0058]filter19, a general overview of the methodology will be explained. Referring to FIG. 5A, the lambda value (λ) corresponding to an exhaust gas-fuel ratio or an air-fuel ratio of exhaust gases flowing intooxidation catalyst26 over time is illustrated. As shown, prior to time T₁, lambda (λ) is lean of stoichiometry. Between times T₁-T₂, lambda (λ) is reduced to a more rich value. Lambda (λ) may be made more rich by (i) throttlingvalve24 to reduce the amount of exhaust gases flowing intocatalyst26 and (ii) injecting reductant, such as diesel fuel, viavalve29 into the exhaust gases flowing intocatalyst26. Referring to FIG. 5B, during the injection of fuel during time interval T₁-T₂, the fuel is completely combusted incatalyst26 increasing the temperature of exhaustgases entering filter19. At time T₂, the temperature T1 ofcatalyst26 increases above a temperature TCRIT. The temperature TCRIT corresponds to a temperature above which particulate matter and SO_xcan be removed fromfilter19. For example, TCRIT may be greater than or equal to 600° C. As illustrated, the lean mixture of exhaust gases burns off the particulate matter infilter19 over the time period DPMTIME_MAX.
• Referring to FIG. 4B, the PMREG1 routine for removing particulate matter from[0059]filter19 is illustrated. The PMREG1 routine is called bystep226 of FIG. 4A.
• At[0060]step230, the regeneration flag RGENFLG1 is initialized to a value of one indicating regeneration has commenced.
• Next at[0061]step232, thefirst valve24 is partially opened (VFLG1=2) and thesecond valve25 is opened fully (VFLG2=1).
• Next at[0062]step234, the particulate matter burn-off time DPMTIME is initialized and the SO_xregeneration time DSOXTIME is initialized. The value DPMTIME corresponds to an amount of time that particulate matter has been burned off offilter19. The value DSOXTIME corresponds to an amount of time that SO_xhas been removed fromfilter19.
• Next at[0063]step236, a determination is made as to whether the cumulative amount of stored SO_xvalue CUMSOX1 is greater than or equal to the value CUMSOX1_MAX. If the value ofstep236 equals “Yes”, the method advances to step238 that initializes the values DPMCNT and DSOXCNT. Thereafter, the routine SOXREG1-PMREG1 is executed to purge both SO_xand particulate matter fromfilter19. Alternately, if the value ofstep236 equals “No”, only particulate matter regeneration is required and the method advances to step240.
• Next at[0064]step240, the partially open position V1 forvalve24 is determined as a function of theexhaust flow rate242 andexhaust concentration246. Theexhaust flow rate242 is determined fromengine speed244. Theexhaust concentration246 is determined based onengine load248.
• Next at[0065]step250, the fuel flow rate F1 that is delivered byvalve29 into the exhaust gases upstream ofoxidation catalyst26 is determined based onexhaust flow rate242 andexhaust oxygen concentration246. The injected fuel results in catalytic combustion overoxidation catalyst26 in the first exhaust path and a corresponding exothermic reaction. The reaction is allowed to continue until the temperature reaches a threshold temperature TCRIT where sustained oxidation of particulate matter on infilter19 is achieved. The mixture of exhaust gases and fuel enteringoxidation catalyst26 may be lean of stoichiometry. Alternately, when a faster temperature increase is desired in oxidation catalyst, the mixture may initially be rich of stoichiometry until a temperature above TCRIT is achieved.
• At[0066]step252, a determination is made as to whether temperature T1 downstream ofoxidation catalyst26 is greater than temperature TCRIT. If the value ofstep252 equals “No”, the methodre-executes steps240 and250 to increase the temperature T1. Otherwise, the method advances to step254.
• At[0067]step254, fuel flow throughvalve29 is shut off. In other words, no further reductant is provided tooxidation catalyst26 once sustained oxidation of the soot has been achieved infilter19.
• Next at[0068]step256, the counter DPMTIME is incremented utilizing the following equation:
• DPMTIME=DPMTIME+DT
• where DT corresponds to the amount of elapsed time since the value DPMTIME was last incremented during this execution of the PMREG1 routine.[0069]
• Next at[0070]step258, a determination is made as to whether the value DPMTIME is greater than or equal to a maximum allowable time DPMTIME_MAX for removing the particulate matter. If the value ofstep258 equals “No”, the method advances back to step240 for continued oxidation of particulate matter infilter19. Otherwise, the method advances to step260 which sets the total particulate matter value CUMPM1 equal to zero. Further, the flag RGENFLG1 is set equal to zero. Thereafter atstep262, the routine is exited and the method returns to the MAIN PM and SOXREG program.
• Before proceeding with a detailed discussion of the SOXREG1-PMREG1 routine for removing particulate matter and SO[0071]_xfromfilter19, a general overview of the methodology will be explained. Referring to FIG. 6A, the lambda value (λ) corresponding to an exhaust gas-fuel ratio of exhaust gases flowing intooxidation catalyst26 over time is illustrated. As shown, lambda (λ) is alternated between being lean of stoichiometry and rich of stoichiometry. Particulate matter is removed fromfilter19 when lambda (λ) is lean of stoichiometry. SO_xis removed fromfilter19 when lambda (λ) is rich of stoichiometry.
• During each time period DSOXCNT_PRD, lambda (λ) is maintained rich of stoichiometry. A portion of the rich mixture is combusted on[0072]oxidation catalyst26 to increase the temperature of thecatalyst26. Further, as illustrated in FIG. 6B, the temperature T1 ofcatalyst26 is maintained above the temperature TCRIT. A remaining non-combusted portion of the rich mixture removes SO_xstored infilter19. During each time period DPMCNT_PRD, lambda (λ) is maintained lean of stoichiometry. The lean mixture (containing excess oxygen by definition) burns off particulate matter stored infilter19.
• Referring to FIG. 4C, the routine SOXREG1-PMREG1 for removing SO[0073]_xand particulate matter fromfilter19 is illustrated.
• At[0074]step264, the partially open position V1 forvalve24 is determined as a function of theexhaust flow rate242 andexhaust oxygen concentration246.
• Next at[0075]step266, the fuel flow rate F1 delivered into the exhaust gases upstream ofcatalyst26 is determined based onexhaust flow rate242 andexhaust oxygen concentration246.
• Next at[0076]step268, the intermediate period (DSOXCNT_PRD) for removing SO_x, and the intermediate period (DPMCNT_PRD) for removing particulate matter is determined. It should be noted that the sum of the DSOXCNT_PRD values corresponds to the De-SOx regeneration period DSOX_TIME_MAX. Similarly, the sum of the DPMCNT_PRD values correspond to the total particulate matter regeneration period DPMTIME_MAX. The values DSOXCNT_PRD and DPMCNT_PRD may be obtained from a table stored inmemory38 indexed byexhaust flow rate242,exhaust oxygen concentration246, and fueling rate F1.
• As shown in FIG. 6A, the periods DSOXCNT_PRD and DPMCNT_PRD are utilized to create fuel injection pulses to obtain the fuel flow rate F1 (and a desired lambda value). The fuel injection pulses create a mixture of exhaust gases and fuel that alternate between being rich of stoichiometry and lean of stoichiometry. When the mixture is rich of stoichiometry, a portion of the fuel is burned in[0077]oxidation catalyst26 increasing the temperature offilter19. The remaining un-combusted fuel decomposes stored SOx within the NO_xtrap wash coat offilter19.
• Next at[0078]step270, a determination is made as to whether temperature T1 ofoxidation catalyst26 is greater than temperature TCRIT. If the value ofstep270 equals “No”, the methodre-executes steps264 and266 to increase the temperature T1. Otherwise, the method advances to step272.
• At[0079]step272, the time DSOXCNT is incremented utilizing the following equation:
• DSOXCNT=DSOXCNT+DT
• where DT corresponds to the elapsed time since the value DSOXCNT was last incremented in this execution of the SOXREG1-PMREG1 routine.[0080]
• Next at[0081]step274, a determination is made as to whether the value DSOXCNT is greater than or equal to the value DSOXCNT_PRD. If the value ofstep274 equals “No”, the method returns to step264 for continued delivery of reductant to filter19 to remove SO_xfromfilter19. Otherwise, the method advances to step276.
• At[0082]step276, the value DSOXTIME is incremented using the following equation:
• DSOXTIME=DSOXTIME+DSOXCNT_—PRD
• Next at[0083]step278, the fuel flow rate F1 is set equal to zero to create a lean mixture of exhaust gases for removing particulate matter fromfilter19.
• Next at[0084]step280, the valve DPMCNT is incremented using the following equation:
• DPMCNT=DPMCNT+DT
• where DT corresponds to the amount of elapsed time since the value DPMCNT was last incremented in this execution of the SOXREG1-PMREG1 routine.[0085]
• At[0086]step282, a determination is made as to whether DPMCNT is greater than or equal to regeneration period DPMCNT_PRD. If the value ofstep282 equals “Yes”, the method returns to step280 for continued removal of particulate matter fromfilter19. Otherwise, the method advances to step284.
• At[0087]step284, the total measured regeneration time DPMTIME is incremented using the following equation:
• DPMTIME=DPMTIME+DPMCNT_—PRD
• Next at[0088]step286, a determination is made as to whether the measured SO_xregeneration time DSOXTIME is greater than or equal to the value DSOXTIME_MAX. If the value ofstep286 equals “Yes”, the method advances to step288 explained below. Otherwise, the method returns to step264 described above.
• At[0089]step288, a determination is made as to whether the measured particulate matter removal time DPMTIME is greater than or equal to value DPMTIME_MAX. If the value ofstep288 equals “No”, the method returns tosteps278 for continued removal of particulate matter fromfilter19. Otherwise, the method advances to step290.
• At[0090]step290, the values CUMSOX1, CUMPM, REGNFLG1 are all set to zero. Thereafter, the routine is exited atstep292.
• The present system and method provides a substantial advantage over systems and methods. In particular, the inventors herein have recognized that an exothermic reaction generated during NOx or SOx removal can be utilized to maintain a desired temperature of exhaust gases for subsequent oxidation of particulate matter—without adding additional reductant during the oxidation process. Thus, the present system and method provides for increased fuel economy as compared to known systems that deliver reductant during both the particulate matter oxidation and the SOx reduction from the combined filter and trap.[0091]

Claims (20)

We claim:

1. A method for removing SOx and particulate matter from an emission control device receiving exhaust gases from an engine, the method comprising:

adding a reductant to said exhaust gases to increase a temperature of said emission control device above a threshold temperature;

ceasing adding said reductant to said exhaust gases to remove particulate matter from said device; and

adding additional reductant to said exhaust gases to remove SOx from said device.

2. The method ofclaim 1 wherein said step of adding a reductant to said exhaust gases to increase a temperature of said emission control creates a mixture lean of stoichiometry.

3. The method ofclaim 1 wherein said step of ceasing adding said reductant to remove particulate matter from said device creates a mixture lean of stoichiometry that combusts said particulate matter in said device.

4. The method ofclaim 1 wherein said step of adding additional reductant to said exhaust gases to remove SOx from said device creates a mixture rich of stoichiometry that removes SOx from said device.

5. The method ofclaim 1 wherein said temperature of said emission control device is maintained above said threshold temperature during said step of ceasing adding said reductant to said exhaust gases.

6. The method ofclaim 1 wherein said temperature of said emission control device is maintained above said threshold temperature during said step of adding said additional reductant to said exhaust gases.

7. The method ofclaim 1 further including indicating when SOx and particulate matter need to be removed from said device.

8. The method ofclaim 1 wherein said method alternates between said steps of (i) ceasing adding said reductant to said exhaust gases to remove particulate matter from said device; and (ii) adding additional reductant to said exhaust gases to remove SOx from said device.

9. The method ofclaim 8 wherein each iteration of said step of ceasing adding said reductant to said exhaust gases is performed for a time interval based on said temperature of said exhaust gases.

10. The method ofclaim 8 wherein each iteration of said step of adding additional reductant to said exhaust gases is performed for a time interval based on said temperature of said exhaust gases.

11. The method ofclaim 1 wherein said emission control device is coupled proximate and downstream of an oxidation catalyst, wherein said step of adding a reductant to said exhaust gases to increase a temperature of said emission control device includes:

adding said reductant to said exhaust gases flowing into said oxidation catalyst to create an exothermic reaction in said catalyst to increase said temperature of said emission control device.

12. A method for removing SOx and particulate matter from an emission control device receiving exhaust gases from an engine, the method comprising:

iteratively supplying a reductant to said exhaust to remove SOx from the device and then ceasing the supply of reductant to said exhaust gases to remove particulate matter from said device.

13. The method ofclaim 12 wherein a temperature of said emission control device is maintained above a threshold temperature.

14. A system for removing SOx and particulate matter from an emission control device receiving exhaust gases from an engine, the method comprising:

a reductant valve selectively supplying reductant to said exhaust gases responsive to a first signal;

a throttle valve controlling flow of said exhaust gases to said device responsive to a second signal; and

a controller operably connected to said reductant valve and said throttle valve, said controller generating said first and second signals, said controller configured to add said reductant to said exhaust gases to increase a temperature of said emission control device above a threshold temperature, said controller further configured to cease adding said reductant to said exhaust gases to remove particulate matter from said device, said controller further configured to add additional reductant to said exhaust gases to remove SOx from said device.

15. The system ofclaim 14 wherein said controller is further configured to indicate when said SOx and particulate matter need to be removed from said device.

16. The system ofclaim 14 further including a temperature sensor generating a third signal indicative of a temperature of said exhaust gases, said third signal being received by said controller.

17. The system ofclaim 14 further including an oxidation catalyst being coupled adjacent and upstream of said device.

18. An article of manufacture, comprising:

a computer storage medium having a computer program encoded therein for removing SOx and particulate matter from an emission control device receiving exhaust gases from an engine, the computer storage medium comprising:

code for adding a reductant to said exhaust gases to increase a temperature of said emission control device above a threshold temperature;

code for ceasing adding said reductant to said exhaust gases to remove particulate matter from said device; and

code for adding additional reductant to said exhaust gases to remove SOx from said device.

19. The article of manufacture ofclaim 18 wherein said computer storage medium further includes:

code for indicating when SOx and particulate matter need to be removed from said device.

20. The article of manufacture ofclaim 18 wherein said computer storage medium alternates between executing said code for ceasing adding said reductant to said exhaust gases to remove particulate matter from said device, and said code for adding said reductant to said exhaust gases to remove SOx from said device.

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