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.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to internal combustion engines, especially diesel engines like those used to propel large trucks, and in particular the disclosure relates to engine exhaust after-treatment that comprises injecting diesel emission fluid (DEF), such as a urea solution, into an engine exhaust system for promoting selective catalytic reduction (SCR) of certain constituents, NOx for example, in engine exhaust. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    An example of a diesel engine exhaust after-treatment system that uses selective catalytic reduction comprises an injector through which DEF is injected into the exhaust flow. DEF is a solution that either comprises, or as it entrains with the exhaust flow is converted into, one or more constituents that promote catalytic action that treats certain exhaust constituents such as NOx. Ideally DEF should completely vaporize and thoroughtly mix with the exhaust before the flow passes across catalytic surfaces. 
         [0003]    The geometry of an exhaust after-treatment system and the spray pattern of a DEF injector may cause some of the injected DEF to wet interior surfaces of the exhaust system before it vaporizes. When the temperature of those surfaces is low enough, a potential exists for solute (urea for example) to come out of solution and form deposits on those surfaces. Accumulations of solid deposits may, over time, impair the effectiveness of the after-treatment system, such as by altering flow characteristics of the exhaust and/or the spray pattern of the injector, and/or they may damage exhaust and after-treatment system components. 
         [0004]    Removal of significant deposits typically requires disassembly of components because of lack of acceptable ways to satisfactorily remove them without such disassembly. 
         [0005]    In order to avoid wetting surfaces that are cold enough to cause solute to precipitate out of solution and deposit on those surfaces, an injection of DEF may be temporarily delayed when a cold engine is first started, especially during cold ambient conditions. That delay however postpones the onset of SCR treatment of the exhaust. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    The present disclosure provides a system and method for mitigating the potential for formation of DEF deposits in an engine exhaust system. 
         [0007]    A general aspect of the disclosure relates to 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. 
         [0008]    The system comprises 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. 
         [0009]    The processor comprises 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. 
         [0010]    Another general aspect of the disclosure relates to 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. 
         [0011]    The method comprises 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. 
         [0012]    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. 
         [0013]    Another general aspect of the disclosure relates to 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. 
         [0014]    The system comprises 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. 
         [0015]    The processor comprises 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 for 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. 
         [0016]    Another general aspect of the disclosure relates to 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. 
         [0017]    The method comprises 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. 
         [0018]    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. 
         [0019]    The foregoing summary, accompanied by further detail of the disclosure, will be presented in the Detailed Description below with reference to the following drawings that are part of this disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a general schematic diagram of an engine and its exhaust system, including after-treatment. 
           [0021]      FIG. 2  is a more detailed schematic diagram of exhaust after-treatment useful in understanding the disclosed model-based system and method. 
           [0022]      FIG. 3  is a first embodiment of DEF injection control algorithm. 
           [0023]      FIG. 4  is a second embodiment of DEF injection control algorithm. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows an example of a turbocharged diesel engine  10  having an intake system  12  through which charge air enters and an exhaust system  14  through which exhaust gas resulting from combustion exits, not all details of those two systems that are typically present being shown. Engine  10  comprises a number of cylinders  16  forming combustion chambers into which fuel is injected by fuel injectors to combust with the charge air that has entered through intake system  12 . Energy released by combustion powers the engine via pistons connected to a crankshaft. 
         [0025]    When used in a motor vehicle, such as a truck, engine  10  is coupled through a drivetrain to driven wheels that propel the vehicle. Intake valves control the admission of charge air into cylinders  16 , and exhaust valves control the outflow of exhaust through exhaust system  14  and ultimately to atmosphere. Before entering the atmosphere however, the exhaust is treated by one or more after-treatment devices in an after-treatment portion of exhaust system  14 . 
         [0026]    After-treatment portion of exhaust system  14  comprises a walled enclosure  18  circumscribing an exhaust flow path through which exhaust from cylinders  16  passes. The interior of enclosure  18  contains a diesel particulate filter (DPF)  20  and a mixer  22  downstream of DPF  20 . 
         [0027]    A DEF injector  24  is mounted in a boss  26  on the wall of enclosure  18  for spraying DEF from a nozzle  28  into exhaust flowing along the exhaust flow path. Flow that has passed through mixer  22  subsequently passes across catalytic surfaces of an SCR catalyst  30  that promotes treatment of an exhaust constituent by a chemical in the DEF and/or a decomposition product of a chemical in the DEF before the flow exits exhaust system  14  through a tailpipe. 
         [0028]    A supply of DEF is stored in a tank  32 . An example of a DEF is an aqueous urea solution that has approximately a 32.5% concentration by weight and that can reduce NOx in exhaust. 
         [0029]    The injection of DEF into the exhaust flow is controlled by execution of a DEF injection control algorithm in a controller  34  that is associated with the supply in tank  32  and with injector  24 .  FIG. 1  shows an example of an exhaust system design in which injector nozzle  28  lies substantially on an imaginary centerline aimed downstream of the exhaust flow, but at an acute angle to the prevailing axial direction of flow coming from DPF  20  to inject DEF as a spray  36  that contains droplets small enough to completely vaporize in exhaust that is sufficiently hot. The reference numeral  36  depicts the spray pattern only generally and is not intended to imply that it will necessarily strike wall  18  or any other portions of the exhaust system. 
         [0030]    Mixer  22  is intended to promote thorough mixing of the DEF with the exhaust flow during transit to SCR catalyst  30  which comprises catalytic surfaces for promoting the reaction of exhaust with product(s) in, and/or decomposition product(s) of product(s) in, vaporized DEF. Mixer  22  can promote the vaporization of any DEF droplets that may strike it and decomposition of urea. 
         [0031]      FIG. 2  shows that the wall of enclosure  18  comprises an interior surface  40  and an exterior surface  42 . A sensor  44  senses temperature of exhaust that flows through enclosure  18  in the sense indicated by the arrow labeled “Exhaust”. An area of interior surface  40  lies within the pattern of spray  36 . Limited vaporization of injected DEF occurs in the exhaust and consequently some droplets will impinge on internal exhaust system surfaces. If those surfaces are sufficiently hot, the impinging DEF will quickly evaporate. Sufficiently quick evaporation does not lead to surface wetting that can create deposits. 
         [0032]    When the temperature of interior surface  40  is greater than the temperature of droplets wetting the surface, heat will transfer from the wall to the droplets to vaporize them as indicated by the arrow labeled H DEF vap . 
         [0033]    A parameter Q in  represents thermal energy (heat) input to the area of interior surface  40  in the path of spray  36 . Assuming that the temperature of interior surface  40  is greater than that of exterior surface  42 , a quantity of heat Q thru  will be conducted through the wall of enclosure  18  to exterior surface  42 . Assuming that the temperature of exterior surface  42  is greater than that of atmospheric air that is in contact with exterior surface  42 , then a quantity of heat Q out will be transferred to the air. 
         [0034]    The equations that follow describe relevant relationships assuming one-dimensional, steady-state heat transfer and neglecting radiation. 
         [0035]    Convective heat transfer from exhaust to interior surface  40  may be described by the equation: 
         [0000]        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 interior surface  40  (based on sensor measurements, empirical data, and calculations),   T exh  is temperature of the exhaust gas as measured by sensor  44 , and   T in wall  is the temperature of interior surface  40  and is described by the equation:       
 
         [0000]    
       
      
       T 
       in wall 
       =K 
       1 
       ×T 
       amb 
       +K 
       2 
       ×T 
       exh 
       −K 
       3 
       ×m 
       DEF  
      
       
         
           
             where 
           
         
       
     
         [0000]        K   1 =1/(1+ h   in   /k   ext ) 
         [0000]        K   2 =1/(1+ k   ext   /h   in ) 
         [0000]        K   3   =h   DEFvap /( h   in   +k   ext )       and T amb  is the temperature of atmospheric air (based on sensor measurements),   m DEF  is the flow rate of DEF being injected by injector  24  (based on sensor measurements, empirical data, &amp; calculations).         
         [0042]    h DEFvap  is the heat of vaporization (and decomposition, if any) of DEF, and 
         [0043]    k ext  is described by the equation: 
         [0000]        k   ext   =k   wall   ×h   out /( k   wall   +h   out )       where k wall  is i the thermal conductivity of the wall of enclosure  18 , and   h out  is the convective heat transfer coefficient from exterior surface  42  to atmospheric air (based on sensor measurements, empirical data, &amp; calculations).         
         [0046]    If no liquid DEF is on interior surface  40 , then all heat transferred from the exhaust to interior surface  40  (Q in ) will be transferred through the wall of enclosure  18  to exterior surface  42 . 
         [0047]    However, if liquid DEF is present on interior surface  40 , only a portion of Q in  will be transferred through the wall of enclosure  18  to exterior surface  42  and the remainder will vaporize, and possibly also decompose, the DEF. This condition is described by the equation: 
         [0000]    
       
      
       Q 
       in 
       =Q 
       thru 
       +H 
       DEFvap  
      
       
         
           
             where Q thru  is the heat transferred through the wall of enclosure  18  to exterior surface  42  and H DEFvap  is heat transferred to liquid DEF present on interior surface  40 . 
           
         
       
     
         [0049]    The last equation may be expanded to: 
         [0000]        Q   in   =k   wall ×( T   in wall   −T   out wall )+ m   DEF   ×h   DEFvap  
 
         [0050]    Convective heat transfer from exterior surface  42  to atmospheric air is described by: 
         [0000]        Q   out   =h   out ×( T   out wall   −T   amb )
       where h out  is the convective heat transfer coefficient for heat transfer from exterior surface  42  to atmospheric air (based on sensor measurements, empirical data, and calculations), and T out wall  is the temperature of exterior surface  42 .       
 
         [0052]    A parameter T crit  represents a temperature below which the liquid phase of a particular DEF on a surface have the potential to form deposits on that surface. 
         [0053]    By calculating the temperature T in wall  using the above equation and then comparing the result to T crit , it can be determined if the temperature of interior surface  40  is high enough to avoid liquid DEF forming deposits on the surface. 
         [0054]    The calculation of temperature T in wall  utilizes the constants, h DEFvap  and k wall  and the variables h in , h out , T amb , T exh , and m DEF . The parameter h in  is a variable because it is a function of the rate of exhaust flow through enclosure  18 . Parameter h out  is a variable because it is a function of the rate of air flow along exterior surface  42 . 
         [0055]      FIG. 1  shows various input data, represented generally by reference numeral  38 , are processed by the DEF injection control algorithm in controller  34 , a first embodiment  50 A of which is shown in  FIG. 3 . 
         [0056]    DEF injection control algorithm  50 A comprises certain processing steps, a first one of which (step  52 ) determines if a present value for DEF flow rate m def  needs to be updated. After that, a second step  54  is performed to calculate energy balance and a temperature T in wall  of interior wall surface  40 . After that, a third step  56  is performed to compare T in wall  with a temperature T crit  representing a temperature of the area of interior surface  40  in the path of spray  36  below which liquid DEF on the area have potential to deposit solid material on the area. 
         [0057]    In performing its calculation, step  54  processes data  57  representing temperature and atmospheric pressure of ambient air, data  58  representing speed at which a vehicle that is being propelled by engine  10  is traveling (this affects air flow along exterior surface  42 ), data  60  representing temperature of engine exhaust at any suitable location in exhaust system  14 , typically upstream of injector  24  but downstream of DPF  20 , as provided by sensor  44 , and data  62  representing flow rate of engine exhaust. Data  57 ,  58 ,  60 , and  62  are all variables. Engine speed data  70  and engine fueling data  72  are used to calculate flow rate of engine exhaust. 
         [0058]    Additional data  64 ,  66 ,  68  are also processed by step  54 . Data  64 ,  66 , and  68  are typically non-variable for a given exhaust and after-treatment system and can therefore be embedded in controller  34 . Data  64  defines certain thermodynamic properties of atmospheric air. Data  66  defines certain properties of walled enclosure  18  relevant to heat transfer through its wall between interior surface  40  across which exhaust flows and exterior surface  42  that is in contact with atmospheric air. Data  68  defines certain thermodynamic properties of the particular DEF that is injected by injector  24 . 
         [0059]    The algorithm routine comprises: 
         [0060]    Calculating T in wall    
         [0061]    If T in wall &gt;T crit  injecting DEF at m def  (step  98  in  FIG. 3 ) 
         [0062]    If T in wall ≦T crit , then reducing m def  until T in wall &gt;T crit  (step  100  in  FIG. 3 ) and injecting DEF at the reduced m def . 
         [0063]    If the reduced m def ≦0, stopping injection of DEF, until 
         [0064]    T in wall &gt;T crit . 
         [0065]      FIG. 4  shows another DEF injection control algorithm  50 B that comprises processing steps  90 ,  92 ,  94 , and  98 . Each iteration of step  90  calculates a desired flow rate for injection of DEF by DEF injector  24  m DEF  for securing optimum performance of SCR catalyst  30 , by rearranging the above equation for T in wall  the equation: 
         [0000]        m   DEF =( T   in wall   −K 1 ×T   amb   −K 2 ×T   exh )/ K 3 
         [0066]    If an iteration produces a result different from that of an immediately preceding iteration, then the value of m DEF  is updated (step  92 ). 
         [0067]    Step  94  calculates energy balance and a parameter m DEFcrit  where 
         [0000]        m   DEFcrit =( T   crit   −K 1 ×T   amb   −K 2 ×T   exh )/ K 3 
         [0068]    If the calculated value for m DEF  is not too high for the temperature T in wall , meaning that deposits will not form, then that calculated value is used instead of m DEFcrit  to set the actual flow rate of DEF injected by DEF injector  24 . On the other hand, if the calculated value for m DEF  is too high for the temperature T wall , meaning that deposits can form, then m DEFcrit  is used to set the actual flow rate of DEF injected by DEF injector  24 . The selection of which is lower, m DEF  or m DEFcrit , is made by step  98 , and it is that selected value which is used as the actual flow rate of DEF injection.