Method for urea injection control

Abstract

A urea injection control system ( 10 ) for a vehicle having an exhaust line ( 16 ) in fluid communication with a SCR catalyst ( 18 ) and a urea injector ( 14 ) for injecting urea into the exhaust line includes a controller ( 12 ) for controlling the flow rate of urea injection at the urea injector. The controller ( 12 ) compares an optimum urea flow rate ( 20 ) with a flexible urea flow rate ( 22 ) determined by a flexible flow rate calculator. The controller ( 12 ) selects the minimum value of the optimum urea flow rate ( 20 ) and the flexible urea flow rate ( 22 ), and controls the urea injector ( 14 ) to inject urea at the minimum value.

1 . A urea injection control system for an exhaust line in fluid communication with a SCR catalyst and a urea injector for injecting urea into the exhaust line, the urea injection control system comprising:
 a controller for controlling the flow rate of urea injection at the urea injector, wherein the controller compares an optimum urea flow rate with a flexible urea flow rate determined by a flexible flow rate calculator, wherein the controller selects the minimum value of the optimum urea flow rate and the flexible urea flow rate, and controls the urea injector to inject urea at the minimum value.   
     
     
         2 . The urea injection control system of  claim 1  wherein the flexible value calculator receives inputs of exhaust flow rate and exhaust temperature. 
     
     
         3 . The urea injection control system of  claim 2  wherein the flexible urea flow rate calculator determines the flexible urea flow rate with the equation:
     F   urea     —     max   =A*F   exh ( T   exh   −T   min ) 
 where, F urea     —     max =flexible urea flow rate, g/hr
 F exh =exhaust flow rate, kg/hr 
 T exh =exhaust temperature, ° C. 
 T min =cut-off temperature, ° C. 
 A=coefficient 
 
 
     
     
         4 . The urea injection control system of  claim 3  wherein the coefficient A is between 0.35 and 0.45. 
     
     
         5 . The urea injection control system of  claim 3  wherein Tmin is in the range of 130 to 180° C. 
     
     
         6 . The urea injection control system of  claim 1  wherein the flexible value calculator receives inputs of at least one of intake airflow, fuel flow rate, oxygen concentration, and fuel flow rate to calculate the exhaust flow rate and the exhaust temperature. 
     
     
         7 . The urea injection control system of  claim 6  wherein the flexible urea flow rate calculator determines the flexible urea flow rate with the equation:
     F   urea     —     max   =A*F   exh ( T   exh   −T   min ) 
 where, F urea     —     max =flexible urea flow rate, g/hr
 F exh =exhaust flow rate, kg/hr 
 T exh =exhaust temperature, ° C. 
 T min =cut-off temperature, ° C. 
 A=coefficient 
 
 
     
     
         8 . The urea injection control system of  claim 7  wherein the coefficient A is between 0.35 and 0.45. 
     
     
         9 . The urea injection control system of  claim 7  wherein Tmin is in the range of 130 to 180° C. 
     
     
         10 . The urea injection control system of  claim 1  wherein the controller further comprises a selector for selecting the minimum value of the optimum urea flow rate and the flexible urea flow rate, and for controlling the urea injector to inject urea at the minimum value. 
     
     
         11 . A urea injection control system for an exhaust line in fluid communication with a SCR catalyst and a urea injector for injecting urea into the exhaust line, the urea injection control system comprising:
 a controller for controlling the flow rate of urea injection at the urea injector, wherein the controller compares an optimum urea flow rate with flexible urea flow rate calculator, wherein the flexible urea flow rate calculator receives inputs of exhaust flow rate and exhaust temperature; and   a selector at the controller that selects the minimum value of the optimum flow rate and the flexible flow rate, wherein the urea injector is instructed by the selector to inject urea at the minimum value.   
     
     
         12 . The urea injection control system of  claim 11  wherein the flexible urea flow rate calculator determines the flexible urea flow rate with the equation:
     F   urea     —     max   =A*F   exh ( T   exh   −T   min ) 
 where, F urea     —     max =flexible urea flow rate, g/hr 
 F exh =exhaust flow rate, kg/hr 
 T exh =exhaust temperature, ° C. 
 T min =cut-off temperature, ° C. 
 A=coefficient 
 
     
     
         13 . The urea injection control system of  claim 12  wherein the coefficient A is between 0.35 and 0.45. 
     
     
         14 . The urea injection control system of  claim 12  wherein Tmin is in the range of 130 to 180° C.