Abstract:
Methods and systems for controlling reductant levels in an SCR catalyst are provided. In one aspect, reductant levels are adjusted in response to a ratio of change of NOx conversion efficiency to a change of reductant level. In another aspect, reductant injection levels are periodically adjusted to see if NOx conversion efficiency is better or worse at the adjusted levels.

Description:
BACKGROUND AND SUMMARY 
       [0001]    The present invention relates generally to Selective Catalytic Reduction (SCR) catalysts and, more particularly, to methods and systems for controlling reductant levels in SCR catalysts. 
         [0002]    Selective catalytic reduction is an important tool in efforts to meet increasingly strict engine emissions standards. Certain techniques for reducing CO emissions result in greater production of nitrogen oxides, also referred to as NOx. SCR is a means of converting NOx with the aid of a catalyst into diatomic nitrogen, N2, and water, H2O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed onto a catalyst. Carbon dioxide, CO2 is a reaction product when urea is used as the reductant. 
         [0003]    A controlled level of NH3 storage buffer in the catalyst is desired in order to maintain high NOx conversion efficiency (μ NOx ), defined here by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       μ 
                       NOx 
                     
                     = 
                     
                       
                         
                           NOx 
                            
                           
                             ( 
                             inlet 
                             ) 
                           
                         
                         - 
                         
                           NOx 
                            
                           
                             ( 
                             outlet 
                             ) 
                           
                         
                       
                       
                         NOx 
                          
                         
                           ( 
                           inlet 
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where NOx(inlet) is the NOx level proximate an inlet of the SCR catalyst and NOx(outlet) is the NOx level proximate and outlet of the SCR catalyst. As seen in the schematic graph of  FIG. 1A , NOx conversion efficiency can be reduced due to too low or too high an amount of stored NH3. 
         [0004]    The known technique for controlling NH3 levels is not considered to produce acceptable results. By this technique, a device such as an electronic control unit (ECU) (various suitable devices are hereinafter referred to generically as a controller) estimates the amount of NH3 stored in the SCR catalyst by keeping track of how much NH3 has been added to the system via dosing and estimating how much NH3 has been consumed by reaction with NOx. The first component—addition of NH3—is quite simple because the amount of NH3 added is directly proportional to urea dosing because the urea decomposes to NH3 and CO2 under high temperature conditions with adequate humidity. The second component—consumption—can be somewhat more difficult because it uses an estimated exhaust mass flow in addition to NOx sensor measurements both before and after the SCR to estimate how much NOx is reduced. The technique assumes that the amount of NH3 that is used is directly proportional to the NOx that is reduced. 
         [0005]    A problem with the known technique is that error accumulates over time in the stored NH3 calculation, which leads to reduced NOx conversion efficiency. The controller uses the modeled stored NH3 mass as a feedback to a controller that tries to maintain stored NH3 at the target. However with nothing to correct this model over time, there is a risk that the model will diverge from actual NH3 levels. In this case failure to properly control stored NH3 directly leads to reduced NOx conversion efficiency. 
         [0006]    The only mechanism to keep the modeled stored NH3 from diverging from actual NH3 levels is to periodically start over by using all up of the NH3 in the SCR and then resetting the model. In addition to having a direct impact on emissions from the time the SCR begins to operate at low efficiency as the actual stored NH3 approaches zero, emissions control can be dramatically compromised if the model diverges from actual levels before the calibration is triggered. 
         [0007]    It is desirable to provide a method and a system for controlling NH3 levels to better ensure NH3 levels in an SCR catalyst are kept within a desired range. 
         [0008]    In accordance with an aspect of the present invention, a method of controlling reductant levels in an SCR catalyst comprises measuring a change of NOx conversion efficiency (dμ NOx ) across the SCR catalyst, measuring a change of reductant level (dB) in the SCR catalyst, comparing a measured ratio dμ NOx /dB to a target ratio, and adjusting reductant injection to cause the measured ratio to approach the target ratio. 
         [0009]    In accordance with another aspect of the present invention, a system for controlling reductant levels in an SCR catalyst comprises an injector for injecting reductant upstream of the SCR catalyst, and a controller arranged to measure a change of NOx conversion efficiency (dμ NOx ) across the SCR catalyst, measure a change of reductant level (dB) in the SCR catalyst, compare a measured ratio dμ NOx /dB to a target ratio, and control the injector to adjust reductant injection to cause the measured ratio to approach the target ratio. 
         [0010]    In accordance with another aspect of the present invention, a method of controlling reductant levels in an SCR catalyst comprises a) calculating a quantity of reductant in the SCR catalyst as a function of an amount of reductant injected over a first period of time minus an amount of NOx reduced over the first period of time, b) determining a first NOx conversion efficiency (μ NOx1 ) at an end of the first period of time, c) changing reductant injection by a first change amount for a second period of time to a second injection rate different from an injection rate at the end of the first period of time, d) determining a second NOx conversion efficiency (μ NOx2 ) at the end of the second period of time and, if μ NOx2 &gt;μ NOx1 , maintaining the second injection rate, and if μ NOx2 &lt;μ NOx1 , changing reductant injection by a second change amount 
         [0011]    In accordance with another aspect of the present invention, a system for controlling reductant levels in an SCR catalyst comprises an injector for injecting reductant upstream of the SCR catalyst, and a controller arranged to calculate a quantity of reductant in the SCR catalyst as a function of an amount of reductant injected over a first period of time minus an amount of NOx reduced over the first period of time, determine a first NOx conversion efficiency (μ NOx1 ) at an end of the first period of time, control the injector to change reductant injection by a first change amount for a second period of time to a second injection rate different from an injection rate at the end of the first period of time, determine a second NOx conversion efficiency (μ NOx2 ) at the end of the second period of time and, if μ NOx2 &gt;μ NOx1 , control the injector to maintain the second injection rate, and if μ NOx2 &lt;μ NOx1 , control the injector to change reductant injection by a second change amount in a direction opposite a direction of the change amount. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawings in which like numerals indicate similar elements and in which: 
           [0013]      FIG. 1A  is a schematic graph of NOx conversion efficiency (μ NOx ) versus reductant level in a SCR catalyst; 
           [0014]      FIG. 1B  is a schematic graph of a ratio of change of NOx conversion efficiency to change of reductant level in an SCR catalyst versus reductant level in an SCR catalyst; 
           [0015]      FIG. 1C  is a schematic graph of a ratio of a ratio of change of NOx conversion efficiency to change of reductant level in an SCR catalyst to NOx conversion efficiency versus reductant level in an SCR catalyst; 
           [0016]      FIG. 2  is a schematic view of a system for controlling reductant levels in an SCR catalyst according to an aspect of the present invention; 
           [0017]      FIG. 3  is a flow chart illustrating steps in a method for controlling reductant levels in an SCR catalyst according to an aspect of the present invention; and 
           [0018]      FIG. 4  is a flow chart illustrating steps in a method for controlling reductant levels in an SCR catalyst according to an aspect of the present invention. [0000] 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 2  shows features of a system  21  for controlling reductant levels in an SCR catalyst  23  according to an aspect of the present invention. The system  21  includes an injector  25  for injecting reductant at a point upstream of the SCR catalyst  23 . The reductant is typically stored in a reservoir  26  and the injector includes a suitable pump  28  for injection. The system  21  also includes a NOx sensor  27  proximate an inlet  29  of the SCR catalyst  23  for measuring inlet NOx levels (NOx(inlet) and a NOx sensor  31  proximate an outlet  33  of the SCR catalyst for measuring outlet NOx levels (NOx(outlet)). The system  21  further comprises a controller  35 , such as an electronic control unit (ECU) (various suitable devices are hereinafter referred to generically as a controller). 
         [0020]    The system  21  and SCR catalyst  23  are part of an exhaust aftertreatment system of a diesel engine  37  such as might be used as a vehicle engine or for other purposes. Typically, the system  21  and SCR catalyst  23  are arranged downstream of a diesel particulate filter  39  in the aftertreatment system. The aftertreatment system may include other features not illustrated. 
         [0021]    The controller  35  can be arranged to determine NOx conversion efficiency (μ NOx ) by the equation 
         [0000]    
       
         
           
             
               
                 
                   
                     μ 
                     NOx 
                   
                   = 
                   
                     
                       
                         
                           NOx 
                            
                           
                             ( 
                             inlet 
                             ) 
                           
                         
                         - 
                         
                           NOx 
                            
                           
                             ( 
                             outlet 
                             ) 
                           
                         
                       
                       
                         NOx 
                          
                         
                           ( 
                           inlet 
                           ) 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    The controller  35  can also be arranged to measure an amount of reductant injected over time (Bi) and measure an amount of NOx reduced (NOx red ) over time. The controller  35  can also be arranged to measure a change of reductant level (dB) in the SCR catalyst as a function of the amount of reductant (Bi) added over a period of time and the amount of NOx reduced (NOx red ) over the period of time. 
         [0022]    In an aspect of the invention referred to as “perturbation control”, the controller  35  can also be arranged to measure a change of NOx conversion efficiency (dμ NOx ) across the SCR catalyst  23  and to measure a change of reductant level (dB) in the SCR catalyst. The controller  35  can be arranged to compare a measured ratio 
         [0000]      dμ NOx /dB  (2)
 
         [0000]    to a target ratio, usually “0” (zero) in the graph of  FIG. 1B , and to control the injector  25  to adjust reductant injection to cause the measured ratio to approach the target ratio. 
         [0023]    The controller  35  can further be arranged to compare a second measured ratio of the first measured ratio to the NOx conversion efficiency 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         μ 
                         NOx 
                       
                     
                     dB 
                   
                   
                     μ 
                     NOx 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    to a second target ratio, usually “0” (zero) in the graph of  FIG. 1C , and to control the injector  25  to adjust reductant injection to cause the second measured ratio to approach the second target ratio. In this way, it is possible to better ensure that feedback control will converge to a stable limit cycle. 
         [0024]    A method of controlling reductant levels in the SCR catalyst  23  will be further described in connection with the flow chart seen in  FIG. 3 . In the method, step  101  includes measuring NOx levels proximate the inlet  29  (NOx(inlet)) and proximate an outlet  33  (NOx(outlet)) of the SCR catalyst  23  and determining NOx conversion efficiency by the equation (1), above. 
         [0025]    In step  103 , an amount of reductant (Bi) injected over time is measured, and in step  105 , an amount of NOx reduced (NOx red ) over time is measured. In step  107 , the change of reductant level (dB) in the SCR catalyst  23  is measured as a function of B and NOx red . Technically, the change of reductant level in the SCR catalyst  23  can only be estimated or modeled with the inputs of amount of reductant (B) injected over time and the amount of NOx reduced (NOx red ) over time, however, for purposes of the present discussion, the change of reductant level (dB) in the SCR catalyst  23  shall be referred to as being measured using these inputs. 
         [0026]    In a perturbation control aspect, in step  109 , a change of NOx conversion efficiency (dμ NOx ) across the SCR catalyst  23  is determined. In step  111 , the measured ratio 
         [0000]      dμ NOx /dB  (2)
 
         [0000]    is compared to a target ratio. In step  113 , reductant injection is adjusted, if necessary, to cause the measured ratio to approach the target ratio. 
         [0027]    If desired (as reflected by dotted lines), in step  115 , the second measured ratio 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         μ 
                         NOx 
                       
                     
                     dB 
                   
                   
                     μ 
                     NOx 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    is compared to a second target ratio and, in step  117 , reductant injection is adjusted, if necessary, to cause the second measured ratio to approach the second target ratio. Also, if desired (as reflected by dotted lines), in step  119 , the third measured ratio 
         [0000]      ƒ(dμ NOx /dB)  (4)
 
         [0000]    is compared to a third target ratio and, in step  121 , reductant injection is adjusted, if necessary, to cause the third measured ratio to approach the third target ratio. ƒ(dμ NOx /dB) is a function of dμ NOx /dB that is defined such that it has a near constant negative slope across the buffer level (similar to the one shown in  FIG. 1C ). 
         [0028]    A system  21  for controlling reductant levels in an SCR catalyst  23  according to another aspect of the present invention referred to as “storage correction” can be structurally similar to the system described above, but is arranged to operate differently. In the system according to this further aspect of the invention, an amount of NOx reduced over a first period of time is determined at step  201  and an amount of reductant required for the reduction is subtracted from an amount of reductant injected (Bi) over a first period of time determined at step  203 , and, at step  205 , the controller  35  is arranged to calculate a quantity of reductant (B) in the SCR catalyst as a function of the values determined at steps  201  and  203  according to a conventional technique for measuring (or, perhaps more accurately, estimating or modeling) reductant levels in an SCR. 
         [0029]    The controller  35  is arranged to determine a first NOx conversion efficiency (μ NOx1 ) at an end of the first period of time at step  207  while the injector  25  injects reductant at a rate R 1 . At step  209 , the controller  35  is arranged to control the injector  25  to change reductant injection by a change amount X 1  for a second period of time to a second injection rate R 2  (R 2 =R 1 −X) different from the injection rate R 1  at the end of the first period of time. At step  211 , the controller  35  is arranged to determine a second NOx conversion efficiency (μ NOx2 ) at the end of the second period of time. 
         [0030]    At step  213 , the controller  35  is arranged to compare μ NOx2  and μ NOx1 . If μ NOx2 &gt;μ NOx1 , at step  215 , the controller  35  is arranged to control the injector  25  to maintain the second injection rate R 2 . If μ NOx2 ≦μ NOx1 , at step  217 , the controller  35  controls the injector to change reductant injection a second amount X 2  in a direction opposite a direction of the change amount (i.e., if the change amount X 1  was a reduction of injection rate, then the change amount X 2  will be an increase of injection rate). If, by a comparison at step  219 , the NOx conversion efficiency μ NOx3  at this further dosing rate R 3  is better than μ NOx1 , i.e., μ NOx3 &gt;μ NOx1 , then, at step  221 , dosing remains at this changed rate and if, at step  223 , μ NOx3 ≦μ NOx1 , then, at step  225 , dosing will return to R 1  and, ordinarily, the process will repeat to attempt to obtain increased NOx conversion efficiency. Typically, change amount X 2  will be twice change amount X 1 . For example, if the injector  25  injects reductant at a rate of 1 unit reductant per unit time, the controller  35  might reduce the rate of reductant injection by 10%, or 0.1 units reductant per unit time, and, if NOx conversion efficiency decreases, the controller might then increase the rate of reductant injection by 0.2 units reductant per unit time. 
         [0031]    The method for using the system  21  according to this aspect can be triggered to operate so as to change injection and, as appropriate, maintain injection at the changed level or change injection again in an opposite direction by any number of events, such as automatically after a predetermined period of operation or when NOx conversion efficiency falls below a target value. The method permits the conventional mass-based model of calculating reductant level in the SCR as shown in steps  201 - 205  to be substantially maintained, however, it provides for a correction that will permit the system to be operated for a substantially longer period of time than is typical in a conventional system without resetting the entire system. 
         [0032]    It will be appreciated that perturbation control and storage control as described above are not mutually exclusive and can be run at the same time. 
         [0033]    In the present application, the use of terms such as “including” is open-ended and is intended to have the same meaning as terms such as “comprising” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” is intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such. 
         [0034]    While this invention has been illustrated and described in accordance with a preferred embodiment, it is recognized that variations and changes may be made therein without departing from the invention as set forth in the claims.