System and method for lean NOx trap control and diagnosis

Abstract

A system and method for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles. The device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle. The device releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase of the cycle. The method and system determine, during the purge phase of the cycle, a difference between a predicated time required to purge the device with actual time required to purge the device. The predicted time is computed as a function of a parameter of the device. The parameter varies over time. The method and system modify the parameter used to determine the predicted time during a subsequent one of the series of storage-purge cycles.

What is claimed is:  
     
         1 . A method for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles, wherein the device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle and the device releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase of the cycle, the method comprising: 
 determining, during the purge phase of the cycle, a difference between a predicted time required to purge the device with actual time required to purge the device, such predicted time being computed as a function of a parameter of the device, such parameter varying over time; and    modifying the parameter used to determine the predicted time during a subsequent one of the series of storage-purge cycles.    
     
     
         2 . The method recited in  claim 1  wherein the determining comprises: 
 calculating the amount of stored in the device, W S , comprising: 
 (A) Obtaining NOx in-coming flow information, W in , from either: (1) a stored model where W in  is a function of engine speed, fuel quantity fed to the engine, and EGR rate and ignition time; or (2) and a NOx measured by a sensor downstream of the device; and  
 (B) Calculating the amount of stored NOx in the device normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time.  
 
 
     
     
         3 . The method recited in  claim 2  including switching from the storage phase to the subsequent purge phase when the calculated W S , exceeds a predetermined threshold W threshold .  
     
     
         4 . The method recited in  claim 1  wherein the determining comprises: 
 calculating x, where x is the stored amount of NOx in the trap normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time by the capacity, such calculating using a stored model of x as a function of W in  where W in , is either: (A) a function of engine speed, fuel quantity fed to the engine, EGR rate and ignition time or; (B) a NOx measurement obtained from an NOx sensor upstream of the device.  
 
     
     
         5 . The method recited in  claim 4  including: 
 determining from the calculated x, a predicted device conversion efficiency, η c, predicted ; and.  
 switching from the storage phase to the purge phase when the predicted conversion efficiency, η c, predicted , is less than a predetermined conversion efficiency, η c, threshold .  
 
     
     
         6 . The method recited in  claim 1  including: 
 predicting the purge phase time (t predicted ) as a function of a time when x reaches a pre-defined threshold;  
 using a determination of air-fuel ratio to determine the actual time duration of purge mode storage mode, t actual .  
 
     
     
         7 . The method recited in  claim 5  including: 
 predicting the purge phase time (t predicted ) as a function of a time when x reaches a pre-defined threshold;  
 using a determination of air-fuel ratio to determine the actual time duration of purge mode storage mode, t actual ;  
 comparing the actual purge time t actual  and the predicted purge time t predicted ; when the difference, e, between the actual purge time t actual  and the predicted purge time t predicted  is greater than a prescribed threshold, e threshold, updating the time varying parameter of the device.  
 
     
     
         8 . The method recited in  claim 6  wherein the time varying parameter is the storage capacity, C, of the device, and wherein the updating is in accordance with:  
         C   new   =C   original ×θ new   (3a) θ new =θ old   +Ke   (3b)  e=t   actual   −t   predicted   (3c)  where C new  is the updated storage capacity, C original  is the original capacity, θ is a multiplier and K is an adaptation gain.    
     
     
         9 . The method recited in  claim 8  including periodically monitoring the status of the device, comprising determining whether the new capacity, C new  becomes less than a prescribed threshold.  
     
     
         10 . The method recited in  claim 8  including periodically monitoring the status of the device, comprising determining a difference in capacity, ΔC=C new −C original  and a predetermined threshold.  
     
     
         11 . The method recited in  claim 8  including periodically monitoring the status of the device, comprising comparing the absolute value of (θ−1) with a predetermined threshold, θ TH    
     
     
         12 . A system for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine during each of a series of storage-purge cycles, wherein the device stores the quantity of the exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry during a storage phase of the cycle and the device releases a previously-stored amount of the exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry during a subsequent purge phase of the cycle, the system having a processor programmed to: 
 determine, during the purge phase of the cycle, a difference between a predicted time required to purge the device with actual time required to purge the device, such predicted time being computed as a function of a parameter of the device, such parameter varying over time; and    modify the parameter used to determine the predicted time during a subsequent one of the series of storage-purge cycles.    
     
     
         13 . The system recited in  claim 12  wherein the determining comprises: 
 calculating the amount of stored in the device, W S , comprising: 
 (A) Obtaining NOx in-coming flow information, W in , from either: (1) a stored model where W in  is a function of engine speed, fuel quantity fed to the engine, and EGR rate and ignition time; or (2) and a NOx measured by a sensor downstream of the device; and  
 (B) Calculating the amount of stored NOx in the device normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time.  
 
 
     
     
         14 . The system recited in  claim 13  including switching from the storage phase to the subsequent purge phase when the calculated W S , exceeds a predetermined threshold W threshold .  
     
     
         15 . The system recited in  claim 12  wherein the determining comprises: 
 calculating x, where x is the stored amount of NOx in the trap normalized by the storage capacity of the trap, such storage capacity being the parameter which varies over time by the capacity, such calculating using a stored model of x as a function of W in  where W in , is either: (A) a function of engine speed, fuel quantity fed to the engine, EGR rate and ignition time or; (B) a NOx measurement obtained from an NOx sensor upstream of the device.  
 
     
     
         16 . The system recited in  claim 15  including: 
 determining from the calculated x, a predicted device conversion efficiency, η c, predicted ; and.  
 switching from the storage phase to the purge phase when the predicted conversion efficiency, η c, predicted , is less than a predetermined conversion efficiency, η c, threshold .  
 
     
     
         17 . The system recited in  claim 12  including: 
 predicting the purge phase time (t predicted ) as a function of a time when x reaches a pre-defined threshold;  
 using a determination of air-fuel ratio determine the actual time duration of purge mode storage mode, t actual .  
 
     
     
         18 . The system recited in  claim 17  including: 
 predicting the purge phase time (t predicted ) as a function of a time when x reaches a pre-defined threshold;  
 using a determination of air-fuel ratio to determine the actual time duration of purge mode storage mode, t actual ;  
 comparing the actual purge time t actual  and the predicted purge time t predicted ; when the difference, e, between the actual purge time t actual  and the predicted purge time t predicted  is greater than a prescribed threshold, e threshold , updating the time varying parameter of the device.  
 
     
     
         19 . The system recited in  claim 18  wherein the time varying parameter is the storage capacity, C, of the device, and wherein the updating is in accordance with:  
         C   new   =C   original ×θ new   (3a) θ new =θ old   +Ke   (3b)  e=t   actual   −t   predicted   (3c)  where C new  is the updated storage capacity, C original  is the original capacity, θ is a multiplier and K is an adaptation gain.    
     
     
         20 . The system recited in  claim 19  including periodically monitoring the status of the device, comprising determining whether the new capacity, C new  becomes less than a prescribed threshold.  
     
     
         21 . The system recited in  claim 19  including periodically monitoring the status of the device, comprising determining a difference in capacity, ΔC=C new −C original  and a predetermined threshold.  
     
     
         22 . The system recited in  claim 19  including periodically monitoring the status of the device, comprising comparing the absolute value of (θ−1) with a predetermined threshold, θ TH .  
     
     
         23 . A method for determining an amount of NOx, W S , stored in a lean NOx trap, comprising: 
 (A) obtaining NOx in-coming flow information to the trap, W in ,    (B) determining the storage capacity, C, of the trap, such storage capacity varying over time; and    (C) determining the storage efficiency of the trap θ S ;    (D) calculating the amount of NOx stored in the trap, W S , from the obtained W in , the determined storage capacity, C, and the determined storage efficiency, η S  of the trap.    
     
     
         24 . The method recited in  claim 23  wherein the determining comprises: 
 calculating x, where x is the stored amount of NOx in the trap normalized by the storage capacity of the trap, such calculating using a stored model of x as a function of W in  where W in , is either: (A) a function of engine speed, fuel quantity fed to the engine, EGR rate and ignition time or; (B) a NOx measurement obtained from a NOx sensor upstream of the device.  
 
     
     
         25 . The method recited in  claim 23  including: 
 calculating x, where x is the stored amount of NOx in the trap normalized by the determined storage capacity, C, of the trap,  
 determining from the calculated x, a predicted device conversion efficiency, η c, predicted ; and.  
 switching from a storage phase for the trap to the purge phase for the trap when the predicted conversion efficiency, η c, predicted , is less than a predetermined conversion efficiency, η c, threshold .  
 
     
     
         26 . The method recited in  claim 23  wherein the storage capacity, C, is updated is in accordance with:  
         C   new   =C   original ×θ new   (3a) θ new =θ old   +Ke   (3b)  e=t   actual   −t   predicted   (3c)  where C new  is the updated storage capacity, C original  is the original capacity, θ is a multiplier and K is an adaptation gain.    
     
     
         27 . The method recited in  claim 26  including periodically monitoring the status of the device, comprising determining whether the new capacity, C new  becomes less than a prescribed threshold.  
     
     
         28 . The method recited in  claim 23  including periodically monitoring the status of the device, comprising determining a difference in capacity, ΔC=C new −C original  and a predetermined threshold.  
     
     
         29 . The method recited in  claim 23  including periodically monitoring the status of the device, comprising comparing the absolute value of (θ−1) with a predetermined threshold, θ TH .  
     
     
         30 . The method recited in  claim 23  including switching from the storage phase to the subsequent purge phase when the calculated W S , exceeds a predetermined threshold W threshold .  
     
     
         31 . The method recited in  claim 30  including: 
 predicting the purge phase time (t predicted ) when x reaches a pre-defined threshold;  
 using a determination of air-fuel ratio to determine the actual time duration of purge mode mode, t actual ;  
 comparing the actual purge time t actual  and the predicted purge time t predicted ; when the difference, e, between the actual purge time t actual  and the predicted purge time t predicted  is greater than a prescribed threshold, e threshold , updating the time varying parameter of the device.