Model-based system and method for mitigating diesel emission fluid deposits

Abstract

A system and method for mitigating deposit of diesel emission fluid (DEF) decomposition products on interior surfaces of an internal combustion engine exhaust system ( 14 ). A processor in a controller ( 34 ) contains a model-based control algorithm ( 50 A; 50 B) for controlling DEF injection by a DEF injector ( 24 ) to mitigate deposit formation.

What is claimed is: 
     
         1 . A control system, in a vehicle that is propelled by an internal combustion engine, for mitigating deposit of decomposition products of diesel emission fluid (DEF) on an interior of an exhaust system through which exhaust is flowing from the engine toward a catalyst that promotes chemical reaction between a constituent in the exhaust and a constituent that has become entrained in the exhaust as a consequence of injection of DEF from a DEF injector, the system comprising:
 a processor containing a model-based control algorithm for controlling an aspect of DEF injection by the DEF injector, the control algorithm comprising a model that models convective heat transfer from the exhaust to a given area of an interior surface of the exhaust system that is separated from an exterior surface in contact with atmospheric air by material of the exhaust system, conductive heat transfer from the given area to any liquid DEF on the given area, conductive heat transfer through the material of the exhaust system to the exterior surface, and convective heat transfer from the exterior surface to atmospheric air,   the processor comprising an operating routine that:   processes data relating to the exhaust and to atmospheric air that affect the convective and conductive heat transfers according to the model to calculate temperature (T in wall ) of the given area of the interior surface,   that compares the calculated temperature (T in wall ) of the given area of the interior surface and a temperature (T crit ) below which liquid DEF on the given area has potential to deposit solid material on the given area, and   that uses the result of the comparison to control DEF injection by the DEF injector.   
     
     
         2 . A control system as set forth in  claim 1  in which the model models convective heat transfer from the exhaust to the given area of an interior surface of the exhaust system (Q in ) by the equation:
     Q   in   =h   in ×( T   exh   −T   in wall )
 
 where h in  is the convective heat transfer coefficient for heat transfer from the exhaust to the given area of the interior surface, and T exh  is temperature of the exhaust. 
 
     
     
         3 . A control system as set forth in  claim 2  in which the model models T in wall  by the equation:
     T   in wall   =K   1   ×T   amb   +K   2   ×T   exh   −K   3   ×m   DEF    
 where
     K   1 =1/(1+ h   in   /k   ext ) 
     K   2 =1/(1+ k   ext   /h   in ) 
     K   3   =h   DEFvap /( h   in   +k   ext ) 
 
 and T amb  is the temperature of atmospheric air, 
 m DEF  is the flow rate of DEF being injected by the DEF injector, 
 h DEFvap  is the heat of vaporization and any decomposition of DEF, and k ext  is described by the equation:
     k   ext   =k   wall   ×h   out /( k   wall   +h   out ) 
 
 where k wall  is the thermal conductivity of the material of the exhaust system, and 
 h out  is the convective heat transfer coefficient for heat transfer from the exterior surface to atmospheric air. 
 
     
     
         4 . A control system as set forth in  claim 1  in which, when the result of the comparison of T in wall  with T crit  determines that liquid DEF on the given area has potential to deposit solid material on the given area, the model models
 convective heat transfer from the exhaust to the given area of the interior surface (Q in ) by the equation,
     Q   i   =k   wall ×( T   in wall   −T   out wall )+ m   DEF   ×h   DEFvap  
 
 
 and convective heat transfer from the external surface to atmospheric air (Q out ) by the equation,
     Q   out   =h   out ×( T   out wall   −T   amb )
 
 
 
       where k wall  is the thermal conductivity of the material of the exhaust system, T out wall  is temperature of the exterior surface, m DEF  is the flow rate of DEF being injected by the DEF injector, h DEFvap  is the heat of vaporization and any decomposition of DEF, h out  is the convective heat transfer coefficient for heat transfer from the exterior surface to atmospheric air, and T amb  is the temperature of atmospheric air. 
     
     
         5 . A control system as set forth in  claim 1  in which the operating routine reduces the flow rate at which DEF is being injected by the DEF injector when T in wall ≦T crit , continues reducing the flow rate at which DEF is being injected by the DEF injector until T in wall >T crit , and when the flow rate at which DEF is being injected by the DEF injector ≦0, stops injection of DEF by the DEF injector until T in wall >T crit  whereupon injection of DEF by the DEF injector is resumed. 
     
     
         6 . A control system as set forth in  claim 1  in which the data relating to the exhaust and to atmospheric air that affect the convective and conductive heat transfers include at least temperature of exhaust, flow rate of exhaust, temperature of atmospheric air, and speed of a vehicle that is equipped with the control system. 
     
     
         7 . A method for mitigating deposit of decomposition products of diesel emission fluid (DEF) on an interior of an exhaust system through which exhaust is flowing from a motor vehicle internal combustion engine toward a catalyst which promotes chemical reaction between a constituent in the exhaust and a constituent that has become entrained in the exhaust as a consequence of injection of DEF into the exhaust system by a DEF injector, the method comprising:
 using a processor to control an aspect of injection of DEF by the DEF injector by repeatedly executing in the processor a model-based control algorithm comprising a model that models convective heat transfer from the exhaust to a given area of an interior surface of the exhaust system that is separated from an exterior surface in contact with atmospheric air by material of the exhaust system, conductive heat transfer from the given area to any liquid DEF on the given area, conductive heat transfer through the material of the exhaust system to the exterior surface, and convective heat transfer from the exterior surface to atmospheric air,   in which executing the model-based control algorithm comprises processing data relating to the exhaust and to atmospheric air that affect the convective and conductive heat transfers to calculate temperature (T in wall ) of the given area of the interior surface, comparing the calculated temperature (T in wall ) of the given area of the interior surface and a temperature (T crit ) below which liquid DEF on the given area has potential to deposit solid material on the given area, and using the result of the comparison to control injection of DEF by the DEF injector.   
     
     
         8 . A method as set forth in  claim 7  in which the model models convective heat transfer from the exhaust to the given area of an interior surface of the exhaust system (Q in ) by calculating:
   h   in ×( T   exh   −T   in wall ) 
 where h in  is the convective heat transfer coefficient for heat transfer from the exhaust to the given area of the interior surface, T exh  is temperature of exhaust, and T in  wall is temperature of the given area of the interior surface. 
 
     
     
         9 . A method as set forth in  claim 8  in which the model models temperature of the given area of the interior surface T in wall  by calculating:
     T   in wall   =K   1   ×T   amb   +K   2   ×T   exh   −K   3   ×m   DEF    
 where
     K   1 =1/(1+ h   in   /k   ext ) 
   K 2 =1/(1+ k   ext   /h   in ) 
     K   3   =h   DEFvap /( h   in   +k   ext ) 
 
 and T amb  is the temperature of atmospheric air, 
 m DEF  is the flow rate of DEF being injected by the DEF injector, 
 h DEFvap  is the heat of vaporization and any decomposition of DEF, and k ext  is described by the equation:
     k   ext   =k   wall   ×h   out ( k   wall   +h   out ) 
 
 where k wall  is i the thermal conductivity of the material of the exhaust system, and 
 h out  is the convective heat transfer coefficient for heat transfer from the exterior surface to atmospheric air. 
 
     
     
         10 . A method as set forth in  claim 7  in which, when the result of the comparison determines that liquid DEF on the given area of the interior surface has potential to cause formation of deposits on the given area of the interior surface, the model models convective heat transfer from the exhaust to the given area of the interior surface (Q in ) by the equation,
     Q   in   =k   wall ×( T   in wall   −T   out wall )+ m   DEF   ×h   DEFvap  
 
 and convective heat transfer from the external surface to atmospheric air (Q out ) by the equation,
     Q   out   =h   out ×( T   out wall − T   amb )
 
 
 
       where k wall  is the thermal conductivity of the material of the exhaust system, T out wall  is temperature of the exterior surface, m DEF  is the flow rate of DEF being injected by the DEF injector, h DEFvap  is the heat of vaporization and any decomposition of DEF, h out  is the convective heat transfer coefficient for heat transfer from the exterior surface to atmospheric air, and T amb  is the temperature of atmospheric air. 
     
     
         11 . A method as set forth in  claim 7  comprising reducing the flow rate at which DEF is being injected by the DEF injector when T in wall ≦T crit , continuing to reduce the flow rate at which DEF is being injected by the DEF injector until T in wall >T crit , and when the flow rate at which DEF is being injected by the DEF injector ≦0, stopping injection of DEF by the DEF injector until T in wall >T crit  when injection of DEF by the DEF injector is resumed. 
     
     
         12 . A method as set forth in  claim 7  in which the data relating to the exhaust and to atmospheric air that affect the convective and conductive heat transfers include at least temperature of exhaust, flow rate of exhaust, temperature of atmospheric air, and speed of a motor vehicle that is propelled by the internal combustion engine and is equipped with the control system. 
     
     
         13 . A control system, in a vehicle that is propelled by an internal combustion engine, for mitigating deposit of decomposition products of diesel emission fluid (DEF) on an interior of an exhaust system through which exhaust is flowing from the engine toward a catalyst that promotes chemical reaction between a constituent in the exhaust and a constituent that has become entrained in the exhaust as a consequence of injection of DEF from a DEF injector, the system comprising:
 a processor containing a model-based control algorithm for controlling an aspect of DEF injection by the DEF injector, the control algorithm comprising a model that models convective heat transfer from the exhaust to a given area of an interior surface of the exhaust system that is separated from an exterior surface in contact with atmospheric air by material of the exhaust system, conductive heat transfer from the given area to any liquid DEF on the given area, conductive heat transfer through the material of the exhaust system to the exterior surface, and convective heat transfer from the exterior surface to atmospheric air,   the processor comprising an operating routine that:   processes, according to the model, data relating to the exhaust and to atmospheric air that affect the convective and conductive heat transfers to calculate a desired flow rate for injection of DEF by the DEF injector,   that selects, for the actual flow rate of DEF injected by the DEF injector, the lower of a flow rate based on a temperature of the given area of the interior surface below which liquid DEF on the given area has potential to deposit solid material on the given area and the desired flow rate calculated according to the model, and   that uses the result of the selection to set the actual flow rate of injection of DEF by the DEF injector.   
     
     
         14 . A control system as set forth in  claim 13  in which the desired flow rate calculated according to the model (m DEF ) is modeled by the equation:
     m   DEF =( T   in wall   −K 1 ×T   amb   −K 2 ×T   exh )/ K 3 
 and the flow rate based on a temperature of the given area of the interior surface below which liquid DEF on the given area has potential to deposit solid material on the given area (m DEFcrit ) is calculated by the equation:
     m   DEFcrit =( T   crit   −K 1 ×T   amb   −K 2 ×T   exh )/ K 3 
 
 where
     K   1 =1/(1+ h   in   /k   ext ) 
     K   2 =1/(1+ k   ext   /h   in ) 
     K   3   =h   DEFvap /( h   in   +k   ext ) 
 
 T wall  is the temperature of the given area of the interior surface, 
 T crit  is the temperature below which liquid DEF on the given area has potential to deposit solid material on the given area, 
 T amb  is the temperature of atmospheric air, 
 h DEFvap  is the heat of vaporization and any decomposition of DEF, and k ext  is described by the equation:
     k   ext   =k   wall   ×h   out /( k   wall   +h   out ) 
 
 where k wall  is i the thermal conductivity of the material of the exhaust system, and 
 h out  is the convective heat transfer coefficient for heat transfer from the exterior surface to atmospheric air. 
 
     
     
         15 . A method for mitigating deposit of decomposition products of diesel emission fluid (DEF) on an interior of an exhaust system through which exhaust is flowing from a motor vehicle internal combustion engine toward a catalyst which promotes chemical reaction between a constituent in the exhaust and a constituent that has become entrained in the exhaust as a consequence of injection of DEF into the exhaust system by a DEF injector, the method comprising:
 using a processor to control an aspect of injection of DEF by the DEF injector by repeatedly executing in the processor a model-based control algorithm comprising a model that models convective heat transfer from the exhaust to a given area of an interior surface of the exhaust system that is separated from an exterior surface in contact with atmospheric air by material of the exhaust system, conductive heat transfer from the given area to any liquid DEF on the given area, conductive heat transfer through the material of the exhaust system to the exterior surface, and convective heat transfer from the exterior surface to atmospheric air,   in which executing the model-based control algorithm comprises:   processing, according to the model, data relating to the exhaust and to atmospheric air that affect the convective and conductive heat transfers to calculate a desired flow rate for injection of DEF by the DEF injector,   selecting, for the actual flow rate of DEF injected by the DEF injector, the lower of a flow rate based on a temperature of the given area of the interior surface below which liquid DEF on the given area has potential to deposit solid material on the given area and the desired flow rate calculated according to the model, and   using the result of the selection to set the actual flow rate of injection of DEF by the DEF injector.   
     
     
         16 . A method as set forth in  claim 15  in which the model models the desired flow rate (m DEF ) by calculating:
   ( T   in wall   −K 1 ×T   amb   −K 2 ×T   exh )/ K 3 
 and models the flow rate based on a temperature of the given area of the interior surface below which liquid DEF on the given area has potential to deposit solid material on the given area (m DEFcrit ) by calculating:
   ( T   crit   K 1 ×T   amb   −K 2 ×T   exh )/ K 3 
 
 where 
 
         K   1 =1/(1+ h   in   /k   ext )
     K   2 =1/(1+ k   ext   /h   in )       K   3   =h   DEFvap /( h   in   +k   ext )   T in wall  is the temperature of the given area of the interior surface,   T crit  is the temperature below which liquid DEF on the given area has potential to deposit solid material on the given area,   T amb  is the temperature of atmospheric air,   h DEFvap  is the heat of vaporization and any decomposition of DEF, and k ext  is described by the equation:
     k   ext   =k   wall   ×h   out /( k   wall   +h   out ) 
   where k wall  is i the thermal conductivity of the material of the exhaust system, and   h out  is the convective heat transfer coefficient for heat transfer from the exterior surface to atmospheric air.