Abstract:
A method to control NH3 slippage and NO x  conversion in an electronic controlled internal combustion engine exhaust system equipped with a selective catalyst reducer, a diesel particulate filter, a diesel oxidation catalyst and a urea doser.

Description:
TECHNICAL FIELD 
     Emission control for compression or diesel engines has been a subject of great interest, especially with the advent of new emission control regulations and the need to operate cleaner engines to reduce overall global pollution levels. As a part of this effort, many diesel engine manufacturers have resorted to using exhaust system after treatments that include diesel particulate filers to trap particulate emissions and hydrocarbons, a diesel oxidation catalyst to convert NO x  to N 2 , (HC to H 7 O and CO 7 ) and a selective catalyst reducer with a urea doser to trap NO x  in the SCR until operating conditions of the SCR permit the NO x  to be treated with exposure to ammonia, such as from urea, to change NO x  to N 2  gas for emission to the atmosphere. 
     There is a need for a model based method for developing and implementing a SCR urea dosing strategy. 
     BACKGROUND 
     It has become understood that SCR performance will degrade over time, so that as an SCR ages, it is less efficient than it was when installed new in the vehicle exhaust system. In order to maintain required emission standards, it has become necessary to understand the aging process of the SCR and how to adapt the engine operation, particularly the urea dosing to a strategy that takes into account the age of the SCR. 
     When the SCR is operating at low temperature, ammonia is absorbed by the SCR, whereas at high temperatures, there is an increased ammonia slip past the SCR. At low temperatures, it is desirable to have a very high storage of ammonia in the SCR. At high temperatures, it is desirable to have low ammonia storage in the SCR. It has been determined that an SCR ages as a function of temperature of operation. It has been determined that the storage capacity of the SCR for ammonia degrades with SCR age. As the temperature of the SCR rises to about 500° C. or more, the performance degrades. Understanding the amount of time the SCR operates above a predetermined temperature can be used to map or populate a data table with expected levels of SCR efficiency, so that NO x  and ammonia are not vented to the atmosphere, and so that a warning alert may be made to the vehicle operator once it is determined that the SCR is too old to be effective. Such information may be developed using a map or data points in a table. The map or data points may further be developed according to a one dimension model of the operation of the SCR and a one dimension model inverse logic model for the SCR. There is a need for a method to determine how urea dosing can be adjusted and the engine exhaust gas flow will meet emission standards regardless of the age of the SCR. 
     SUMMARY 
     In one embodiment, the present application is directed to a method to control NH3 slippage and NO x  conversion in an electronic controlled internal combustion engine exhaust system equipped with a selective catalyst reducer (SCR) and a urea doser. One method includes determining the SCR operating condition; determining engine out NO x  exhaust flow rate into the SCR; adapting urea dosing conditions to conform to the SCR operating condition; determining ammonia storage, ammonia slip and NO x  conversion in the exhaust gas flow out of the SCR; and recalibrating the SCR operating condition in response to ammonia storage slip and NO x  conversion. 
     In another embodiment, the method may include determining the SCR operating condition by using temperature of the exhaust and the exhaust flow rate through the SCR to determine SCR age. Generally, the SCR reduced age may be determined by the amount of time the SCR operates above a predetermined temperature. More particularly, the predetermined temperature is in the range of from about 500° C. to about 700° C. 
     The temperature operation of the SCR may be contained as data points within a map or table of an electronic control module memory, and the SCR condition is predictable by the electronic controller based upon data contained in the map or table. 
     When the SCR has reached a stable operating condition, urea dosing may be controlled by determining the amount of ammonia storage and slippage in the SCR exhaust gas flow. Generally, the urea dosing may be controlled by an engine control module having memory and urea control strategies resident therein. 
     When the engine is a compression ignition or diesel engine, adapting urea dosing conditions to current SCR conditions includes considering at least engine air mass flow rate, engine total air flow rate; engine NO x  flow rate; SCR inlet NO 2  over NO x  ratio; SCR inlet exhaust pressure; SCR inlet temperature; diesel oxidation temperature; ambient air temperature; diesel particulate filter oxygen flow rate and vehicle speed to develop ammonia rate for urea dosing control. 
     The ammonia dosing rate of the SCR is controlled by targeting both critical ammonia slip and ammonia storage in the SCR and is targeted to prevent ammonia slip during step acceleration of the vehicle and may vary based on operating conditions. Generally, the engine is operated for a predetermined period of time to determine a stable engine operating condition and ammonia slip. Ammonia storage, ammonia slip, NO x  reduction efficiency may be modeled under one dimension SCR model and desired urea dosing rate with a desired ammonia storage and ammonia slip may be modeled with a one dimension SCR inverse logic model. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an engine with an exhaust system including a diesel particulate filter (DPF) a selective catalyst reducer (SCR) and a diesel oxidation catalyst (DOC). 
         FIG. 2  is a representation of a model based open loop SCR control system I/O. 
         FIG. 3  is a representation of a model showing how ammonia dosing rate is determined. 
         FIG. 4A  is a graph showing ammonia storage in the SCR as a function of SCR temperature 
         FIG. 4B  is a graph showing ammonia storage in the SCR as a function of time and temperature of the SCR. 
         FIG. 5  is a graph demonstrating a model based SCR Control at step acceleration condition. 
         FIG. 6  is graph demonstrating a One Dimension ammonia storage distribution based upon SCR inlet temperature and time. 
         FIG. 7  is a graph showing model based SCR Control at transient and steady state conditions. 
         FIGS. 8A and 8B  is a graph showing Constant Dosing Alpha Strategy ammonia slip. 
         FIGS. 8C and 8D  form a graph showing model based dosing strategy ammonia slip according to one embodiment of the present disclosure. 
         FIG. 9A  is a graph showing SCR age as a function of SCR Temperature 
         FIG. 9B  is a graph showing SCR deNO x  efficiency as a function of SCR aging function time. 
         FIG. 10  is a software flow diagram showing one method according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings wherein like numbers refer to like structures,  FIG. 1  schematically illustrates a compression ignition engine  10  for an on-highway vehicle  12 . The engine  10  includes an engine control unit  14  that controls operation of the engine  10  and also controls exhaust component urea dosing according to the present invention as described below. 
     Exhaust manifold sensors  16  and tail pipe sensors  18  provide information to the engine control unit (ECU)  14 , that may be comprised of an engine control module and a component control module in communication with each other over an engine common area network (ECAN) that is used to ensure that the component control module and the ECU functions in a coordinated manner to operate the engine and attendant systems. The ECU controls the engine and the exhaust component operation, including urea dosage as will hereinafter be described. 
     The exhaust manifold sensors  16  may provide information regarding NO x  levels, air/fuel ratios, temperature, and pressure at any of the exhaust system components. More specifically, the exhaust manifold sensors  16  and tail pipe sensors  18  may provide information regarding NO R , and temperature that enable the ECU to detect an impending need for ammonia storage in the SCR or urea dosage. The ECU may also monitor other engine operating parameters to determine the need for urea dosage or ammonia storage. For example, the ECU may contain data tables or maps populated with data. The map or data points may further be developed according to a one dimension model of the operation of the SCR and a one dimension model inverse logic model for the SCR. The ECU, based upon input from sensors at the SCR inlet and SCR outlet uses the tables or maps to determine how urea dosing can be adjusted and the engine exhaust gas flow will meet emission standards regardless of the age of the SCR. The exhaust system is seen with conduit  19  and particulate filter  22 , catalyzed soot filter  24 , or NO absorber catalyst, such as the SCR  20 . Urea doser  26  is in close proximity to the SCR inlet for the administration of urea according to a method of the present disclosure. A warning light  28  may be provided to alert an operator that the SCR is too old to operate efficiently and should be replaced. 
     Turning to  FIG. 2 , there is illustrated a model based open loop SCR control System I/O  30  according to one embodiment of the present disclosure. Specifically, the model illustrates that engine air mass flow rate  32 , engine total air flow rate  34 , engine NO flow rate  36 , SCR inlet NO 2  over NO ratio  38 , SCR inlet pressure  40 , SCR inlet temperature  42 , DOC inlet temperature  44 , ambient temperature  46 , O 2  flow rate from diesel particulate filter (DPF)  48 , and vehicle speed  50  are input into the model. The model considers sensor input indicative of ammonia storage of the SCR  52 , ammonia slip from the SCR  54 , SCR outlet NO 56, SCR deNO x  efficiency  58  and the requested ammonia rate in order to determine and the ammonia rate for dosing and thereby control the urea doser to ensure that the proper amount of urea is used at all stages of the SCR operation as indicated at  59 . 
       FIG. 3  is a schematic representation of model  60  showing the inputs as described in relation to  FIG. 2  above, and their consideration by a one dimensional model  62  that then inputs its determinations to model inversion  64  which, together with the input regarding critical ammonia storage and slip  66 , is considered in the model inversion  64  to determine ammonia dosing rate  59 . Note that the ammonia dosing rate is in a feedback loop with the one dimensional SCR model  62  as an input therein. Generally, the urea dosing rate is controlled by targeting the critical ammonia storage and slip in the model schematically presented in  FIG. 3 . 
     Specifically, one example to explain the inverse logic of a one dimensional SCR model may be represented by the equation (1)
 
 aX   2   +bX +( c−Y )=0
 
     Wherein:
         α=f a (T,time resi )   b=f b (θ star ,c nox )   c=f a (ratio NO2 ,C 02 )   θ star =f θ (t,T,time resi ,ratio NO2 ,C 02 ,C NOx ,C NH3  . . . )       

     One example of the inverse model, as depicted in  FIG. 3 , may be represented by the equation 
     
       
         
           
             X 
             = 
             
               
                 
                   - 
                   b 
                 
                 ± 
                 
                   
                     
                       b 
                       2 
                     
                     - 
                     
                       4 
                       ⁢ 
                       
                         a 
                         ⁡ 
                         
                           ( 
                           
                             c 
                             - 
                             Y 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 2 
                 ∂ 
               
             
           
         
       
     
     wherein the variables have the same values as set forth in regard to equation (1) above. 
     θ=1 is the ammonia storage capacity of the SCR. If the SCR is fully stored with ammonia, there will be ammonia slippage from the SCR. The higher the ammonia storage levels, the higher the conversion of ammonia and NO X  to N 2  will occur, but there will also be higher ammonia slip past the SCR. In operation, based upon engine and SCR conditions, a particular ammonia storage level is targeted so that there can be a higher NO X  conversion rate to N 2 , thereby reducing ammonia slippage. 
       FIGS. 4A through 9B  are not taken from actual test data, but are merely predictive and provide to illustrate the concept of the instant application. 
       FIG. 4A  is a graph showing ammonia storage capacity in the SCR as a function of SCR temperature, based upon the model developed according to one embodiment of the present disclosure. Specifically, model data points  70 ,  72 ,  24 ,  76  and  78  form a curve  80 , that is almost identical with observed data points  82 ,  84 ,  86 ,  88  and  90  which form an almost identical curve  92  as curve  80 . This correlation indicates that the model is a very good predictor of ammonia storage as a function of SCR temperature, and may be relied upon instead of the actual observed data points. 
       FIG. 4B  is a graph showing ammonia storage level in the SCR as a function of time and temperature of the SCR. It can be seen that as SCR inlet temperature  92  increases to a spike  93  of about 400° C., ammonia storage  94  increases until the SCR inlet temperature reaches about 400° C., at which point  95  ammonia storage decreases, and ammonia slippage increases. Considering the data from the two graphs of  FIGS. 4A and 4B , it may be seen that ammonia storage should be limited to prevent ammonia slip past the SCR during step-acceleration operation of the vehicle. The graph shows that the NH 3  dosing strategy is best determined by noting when the NH 3  slip is equal to NH 3  slip critical  93 , should be that ammonia slippage should equal ammonia slip critical and the NH 3  storage  96  is less than or equal to ammonia storage critical 
       FIG. 5  is a reading of a model based SCR control at step acceleration condition. Basically, the graphs show SCR substrate temperature, dosing alpha, deNO x  efficiency, ammonia slippage past the SCR and ammonia storage percent. It can be seen that under dosing due the lower deNO x  efficiency results in higher ammonia storage critical, whereas overdosing due to ammonia oxidation results in an increase in the ammonia slip critical. 
       FIG. 6  is graph demonstrating a One Dimension ammonia storage distribution based upon SCR inlet temperature and time. It can be seen that as the SCR inlet temperature changes from 200 to 350° C., at 2000 RPMS, ammonia storage distribution decreases and assumes an almost steady state as indicated at  97 . 
       FIG. 7  is a graph showing model based SCR Control at transient and steady state conditions. Note that when the SCR substrate reaches a predetermined temperature, in this case of about 350° C., the dosing alpha, deNO x  efficiency ammonia slip and ammonia storage percentage each assumed a steady state, as indicated at  81 ,  83 ,  85  and  87  respectively. 
       FIGS. 8A and 8B  are graphs showing Constant Dosing Alpha Strategy ammonia slip. As seen therein the dosing alpha is equal to 1, and ammonia slip past the SCR depends upon cycles. As is apparent in the graphs, a longer low temperature period permits higher ammonia slip past the SCR. The graph  100  is comprised of two parts. Section  102  is the temperature of the SCR over operating on engine and  104  is the temperature of the SCR in Celsius. Section  106  is NH 3  slip as measured in parts per million  108 . Time in seconds is shown at  110 . As can be seen by reference to graphs  8 A &amp;  8 B, as CR temperature increases to beyond about 650° C., the NH 3  slip, as measured in ppm past the SCR spikes, and then decreases, and then decreases as the SCR temperature decreases due to dosing with fuel. In addition, the longer the period of time the SCR remains at a low temperature, the greater the ammonia slip past the SCR. In addition, ammonia slip past the SCR is independent of engine operation. Rather, it is dependent upon temperature of the SCR. 
       FIGS. 8C and 8D  form a graph showing a model based dosing strategy ammonia slip according to one embodiment of the present application. Specifically, the model shows that as SCR temperature passes approximately 650° C., the NH 3  slippage spikes, and decreases when the SCR temperature is reduced. Moreover, the model further shows that the NH 3  slip is independent of engine cycle time. 
       FIG. 9A  is graph  112  showing a model of SCR aging as a function of SCR Temperature. The X axis  114  is SCR temperature in Celsius, and the Y asix  116  is the SCR aging as a function of SCR temperatures. Basically, the aging of the SCR may be presented by the equation:
 
Age SCR =Σfactor aging     —     equiv   ×t   step  
 
     Using the formula, it is possible to create a SCR aging factor function based on SCR aging test results by assuming aging factor is unit at 700° C., and normalize aging rate at other temperatures to establish a correlation between SCR age and NO x  reduction efficiency. 
       FIG. 9B  is a graph  118  showing SCR deNO x  efficiency as a function of SCR aging time. To create a SCR aging factor function based on actual SCR test results, it is helpful to assume that the aging factor is a predetermined temperature, in this case, the unit is at about 700° C. The SCR aging rate may be normalized at other temperatures as well. A correlation between the SCR age and the NOX reduction efficiency is established and the plot  120  set forth in  FIG. 9A  indicates that as SCR Temperature rises, the SCR aging factor rises as well. Similarly,  FIG. 9B  the plots  122 ,  124  and  126  indicate that when the SCR is operated at 700° C., 600° C. and 500° C. respectively, the deNO x  efficiency decreases as the SCR aging cycle time advances. 
       FIG. 10  is a software flow diagram showing one method  128  according to the present disclosure. Specifically, step  130  is determining the condition of the SCR. In this regard, temperature and time operated at specific temperature above a predetermined temperature are factors that are considered. Step  132  is determining engine out NO x  flow rate into the SCR. This may be accomplished by sensor input at the SCR inlet. Step  134  is adapting a urea dosing condition to current SCR conditions, according to the model and inverse models as set forth above. Step  136  is determine the ammonia slip, and NO x  conversion at the SCR and step  138  is recalibrate the SCR condition to a pretargeted ammonia storage based upon ammonia slip and NO x  conversion, and the software loops back to step  130 . 
     The words used in the specification are words of description and not words of limitation. Many variations and modifications are possible without departing from the scope and spirit of the invention as set forth in the appended claims.