Patent Publication Number: US-6981368-B2

Title: Exhaust gas aftertreatment systems

Description:
FIELD OF INVENTION 
     The present invention relates to an emission control system for diesel and other lean-burn vehicles and, more specifically, to achieving optimal NOx conversion efficiency while minimizing ammonia slip in a urea-based Selective Catalytic Reduction (SCR) catalyst by controlling the ammonia surface coverage fraction of the catalyst. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas. In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst, are known to achieve high NOx conversion efficiency. Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into hydro cyanic acid (NHCO) and gaseous ammonia (NH 3 ), which is then used to convert NOx. However, in such systems, urea injection levels have to be very precisely controlled. Under-injection of urea may result in sub-optimal NOx conversion, while over-injection may cause tailpipe ammonia slip. In a typical urea-based SCR catalyst system, the amount of urea injected is in proportion to the feedgas NOx concentration that represents a trade-off between maximum NOx conversion and minimum ammonia slip. 
     The inventors herein have recognized a disadvantage with the prior art approach. Namely, this approach requires multiple calibration maps to account for complex behavior of the catalyst with respect to adsorption and desorption of ammonia as a function of engine operating conditions and catalyst deterioration, and is therefore inherently inaccurate. 
     The inventors herein have also recognized that while NOx conversion efficiency of an SCR catalyst is improved in the presence of adsorbed ammonia, it is not necessary that all of the catalyst storage capacity be utilized by ammonia in order to achieve optimal NOx conversion efficiency. Further, the inventors have recognized that, under certain operating conditions, such as at high SCR catalyst temperatures, if the amount of ammonia stored in the catalyst is too high, some of it may desorb and slip from the catalyst or be oxidized to NO x  thereby reducing the overall NO X  conversion efficiency. Therefore, the inventors have determined that in order to achieve optimal NOx reduction and minimize ammonia slip in a urea-based SCR catalyst, it is crucial to control the amount of ammonia stored in the SCR catalyst. Accordingly, since a direct measurement is not possible, the inventors have developed an accurate method to estimate the amount of ammonia stored in the SCR catalyst. 
     In accordance with the present invention, a method for estimating an amount of reductant stored in an exhaust gas aftertreatment device coupled downstream of an internal combustion engine, includes: injecting reductant into the device to react with a component of an engine exhaust gas mixture; and estimating the amount of reductant stored in the device based on an amount of said exhaust gas component downstream of the device. 
     In one embodiment of the present invention, the method is used to estimate an amount of ammonia stored in an SCR catalyst based on an amount of NOx in the exhaust gas downstream of the SCR catalyst. 
     In another embodiment of the present invention, the amount of ammonia stored is represented by the ratio of the number of the catalyst storage sites containing adsorbed ammonia to the total number of catalyst storage sites normally available, i.e., the ammonia surface coverage fraction of the SCR catalyst. 
     In another embodiment of the present invention, the method includes predicting the amount of ammonia exiting the catalyst as slip based on the estimated amount of stored ammonia. 
     In yet another embodiment of the present invention, a method for controlling injection of a reductant into a substance to be reduced by such reductant, such reduction being facilitated by a catalyst, includes: estimating an amount of reductant stored in the catalyst based on an amount of an unreduced substance downstream of the catalyst; and adjusting an amount of reductant injected into the substance based on said estimate. 
     In yet another embodiment of the present invention, the method includes adjusting the amount of ammonia injected into the SCR catalyst based on the estimate of the amount of stored ammonia. 
     An advantage of the present invention is that an accurate estimate of the amount of ammonia stored in the SCR catalyst is obtained. Therefore, improved NOx conversion efficiency can be achieved by maintaining an optimum ammonia storage amounts in the SCR catalyst. 
     Another advantage of the present invention is the ability to accurately predict the ammonia slip, since there are no ammonia sensors currently available commercially. 
     Yet another advantage of the present invention is that tailpipe ammonia slip is minimized. Additionally, since the downstream NOx sensor is cross sensitive to ammonia, minimizing the ammonia slip improves the accuracy of the NOx sensor readings, and thus improves overall system NOx conversion efficiency. 
     The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Description of Preferred Embodiment, with reference to the drawings, wherein: 
         FIG. 1  is a functional block diagram of an engine exhaust system having a processor configured to reduce NOx in engine exhaust according to the invention; and 
         FIG. 2  is a truth table for selecting the proper control strategy for reductant injection. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT(S) 
     Referring now to  FIG. 1 , an engine exhaust system  10  is shown for controlling injection of a reductant, here aqueous urea, into the exhaust gas mixture of an engine  11 . More particularly, the reductant is injected into the exhaust gas mixture by an injector  12 , the amount of such injection being in response to a control signal from processor  26  fed to the injector  12  on line  13 . An exhaust gas aftertreatment device  14 , here a urea-based SCR catalyst, is provided downstream of the reductant injection into the engine exhaust to facilitate the reaction between the reductant injected into the engine exhaust and NOx in the engine exhaust gas mixture to thereby reduce such NOx. 
     A pair of NOx sensors  15 ,  16  are provided upstream and downstream of the SCR catalyst, respectively. Measurements of the concentration of NOx in the exhaust gas mixture upstream (C NOx     —     in ) and downstream (C NOx     —     out ) of the SCR catalyst  14  provided by the NOx sensors are fed to a processor  26 . Alternatively, NOx sensor  15  can be eliminated and the amount of NOx in the exhaust gas mixture entering the SCR catalyst can be estimated based on engine speed, load, exhaust gas temperature or any other parameter known to those skilled in the art to affect engine NOx production. 
     Temperature measurements upstream (T u ) and downstream (T d ) of the SCR catalyst are provided by temperature sensors  17  and  18 . Processor  26  calculates catalyst temperature, T cat , based on the information provided by sensors  17  and  18 . Alternatively, any other means known to those skilled in the art to determine catalyst temperature, such as placing a temperature sensor mid-bed of the catalyst, or estimating catalyst temperature based on engine operating conditions, can be employed. 
     Processor  26  comprises a feedforward controller  19  and a feedback controller  20  and selects between the two based on operating conditions to produce the control signal, m urea , on line  13 . The control strategy for reductant injection is described in detail with particular reference to  FIG. 2  below. 
     The feedforward control is based on map-based nominal reductant injection. The nominal amount of reductant injection, RED — NOM, is determined from a look-up table  20  as a function of a plurality of operating parameters, including engine operating conditions and catalyst temperature, T cat . More particularly, here RED — NOM is a function of engine speed, engine load, EGR level, start of fuel injection (SOI), catalyst temperature, T cat , space velocity (SV), and concentration of NOx upstream (C NOx     —     in ) and downstream (C NOx     —     out ) of the SCR catalyst. 
     The feedback control is accomplished via a model-based observer  22 . The model of the SCR catalyst is developed from first principles and uses global kinetics of the relevant reactions. The influence parameters of the model are estimated from empirical data for the catalyst of interest. The inputs to the observer are the concentration of injected ammonia and concentration of NOx upstream of the SCR catalyst. The outputs of observer  22  are estimates of the amount of stored ammonia in the SCR catalyst and of the concentration of gas phase ammonia downstream of the SCR catalyst. In this embodiment, the stored ammonia amount is represented by the ammonia surface coverage fraction, which can be defined as the ratio of the number of SCR catalyst sites with adsorbed ammonia to the total number of ammonia storage sites available. The SCR catalyst model includes the adsorption, desorption and surface coverage dynamics, along with the NOx reduction and ammonia oxidation dynamics based on the relevant chemical reaction rates and can be represented by the following equations: 
               [           C   NO               θ   .                 C   .     NH3           ]     =       [             -       C   NO     ⁡     (         Θ   SC     ⁢       R   _     RED     ⁢   θ     +     F     V   cat         )         +       Θ   SC     ⁢       R   _     OX     ⁢   θ                   -     θ   ⁡     (           R   _     ADS     ⁢     C   NH3       +       R   _     DES     +         R   _     RED     ⁢     C   NO       +       R   _     OX       )         +       R   ADS     ⁢     C   NH3                     -       C   NH3     ⁡     (         Θ   SC     ⁢         R   _     ADS     ⁡     (     1   -   θ     )         +     F     V   cat         )         +         R   _     DES     ⁢     Θ   SC     ⁢   θ             ]     +             (     1   ⁢   a     )                       ⁢         [         0           0             F     V   cat             ]     ⁢   U     +       [           F     V   cat               0           0         ]     ⁢   d                               Y   =       [         1       0       0         ]     ⁡     [           C   NO             θ             C   NH3           ]               (     1   ⁢   b     )             
 
where: 
             R   _     j     =       k   j     ⁢           ⁢     exp   ⁡     (       -     E   j       RT     )           ;       
     j=ads, des, red, ox.   E j : Activation energy for reaction j=ads, des, red, ox.   R j : Reaction rate for reaction j=ads, des, red, ox.   k j : Pre-exponential terms for reaction j. [vol/mole].   C x : Concentration of species x [mole/vol].   θ NH3 : Surface coverage fraction [dimensionless].   Θ SC : Total ammonia storage capacity [moles/vol].   R: Universal gas constant [kJ/kmole/K].   T: Temperature [Kelvin]   Y: Output concentration of NO, (C NO ) available as measurement [mole/vol].   U: Inlet concentration of NH 3 , (C NH3   in ), [mole/vol].   d: Is a known and bounded disturbance input → inlet concentration of NO, (c NO   in ), [mole/vol].   F: Constant flow rate through catalyst [m 3 /sec].   V cat : Catalyst volume [m 3 ].   

     The activation energies may be functions of temperature, inlet concentrations, or other quantities that can be measured or estimated. The model for ammonia storage may also be a different one than expressed by above Arrhenius Equations (1a) and (1b). 
     Since the only measurement available in this system is the NOx concentration downstream of the SCR catalyst, the observer for the surface coverage fraction and the ammonia slip concentration takes the form: 
           ⅆ     ⅆ   t       ⁢     x   ^       =       f   ⁡     (       x   ^     ,   U   ,   d     )       +     L   ⁢           ⁢     (         C   ^     NOx     -     C   NOx       )             
 
where {circumflex over (x)}=[Ĉ NOx , {circumflex over (θ)}, Ĉ NH3 ] denotes the observer states, f denotes the nonlinear system dynamics of the SCR catalyst as shown in Equations 1a and 1b, and L is any function of the error ε=(Ĉ NOx −C NOx ) which renders the dynamics of the error system asymptotically stable. The two outputs of the observer  22  (estimated ammonia surface coverage fraction, {circumflex over (θ)}, and the estimated concentration of gas phase ammonia downstream of the SCR catalyst, Ĉ NH     3 out   ) are provided to the feedback controller  21 . Further, feedback controller  21  determines desired ammonia surface coverage fraction, O des , from either a look-up table  23  based on operating conditions, such as catalyst temperature, engine speed, load, start of injection, space velocity, etc or from some finite or infinite horizon optimal control law, and generates a control signal on line  13  based on {circumflex over (θ)} and θ des . Alternatively, feedback controller  21  can generate a control signal based on estimated and desired amount of ammonia downstream of the SCR catalyst.
 
     Feedback controller  21  could be defined as any controller, U, that renders the closed loop system asymptotically stable such as, for example:
 
 U=K·ε , or
 
 U=K ·sign(ε)
 
     Additionally, observer  22  provides convergence time information (the time that it takes for the measured and estimated amounts of NOx in the exhaust gas mixture downstream of the SCR catalyst to come within a small predetermined value, ε, of each other) to a diagnostic/adaptation section  24 . If the convergence time is greater than a predetermined time constant, t conv , indicating that the catalyst model accuracy has been reduced due to such factors as, for example, aging of the catalyst or catalyst poisoning, section  24  initiates an adaptation scheme to reflect the altered behavior by adjusting, for example, the total storage capacity of the catalyst to account for the loss of storage sites due to catalyst aging or poisoning. The adjustment value is generated based on operating conditions and the length of convergence time. Additionally, diagnostic/adaptation section  24  may set a catalyst degradation signal in response to an increased convergence time. 
     Referring now to  FIG. 2 , a truth table for switching between feedforward controller  19  or feedback controller  21  is described. 
     Condition  1  occurs when the exhaust gas temperature at the reductant injection point, T inj , is below a first predetermined threshold, T 1  (170° C. in a preferred embodiment). Since the injected urea does not decompose to ammonia and hydro-cyanic acid until T inj  is above T 1 , any urea injection will result in urea accumulation in the exhaust pipe and/or deposition of urea on the catalyst face. This will lead to inefficient conversion and excess ammonia slip. T inj  can be estimated based on T 1 , or based on operating conditions such as engine speed, load, coolant temperature, etc. Therefore, for Condition  1 , controller  26  disables reductant injection. 
     Condition  2  describes a condition wherein T inj  is above T 1 , but T cat  is below a second predetermined temperature threshold, T 2  (200° C. in a preferred embodiment). Condition  2  applies at engine start up or at stop (idle) conditions. In this temperature range, NOx conversion efficiency of the SCR catalyst is very low and the feedforward controller is used to inject reductant to allow ammonia storage in the catalyst. θ threshold  is a threshold amount of ammonia storage that is required for improved NOx conversion once the SCR catalyst is within the optimum NOx conversion temperature range (50% in a preferred embodiment). At start up conditions the θ value will be available from KAM (keep alive memory of the processor  26 ) of the last operational engine state prior to key off. For idle conditions, since the engine has already been running the predicted value of θ will be available for comparison with θ threshold . Once the desired amount of ammonia storage is achieved, reductant injection may be discontinued until Condition  3  is satisfied. This prevents over-storage of ammonia on the catalyst and thereby reduces ammonia slip during accelerations. 
     Condition  3  occurs when T cat ≧200 deg C. and model-based observer convergence&gt;ε (i.e., the difference between measured and estimated amounts of NOx in the exhaust gas mixture downstream of the SCR catalyst is greater than a small predetermined value, ε). In other words, processor  26  does not switch to the feedback control of reductant injection until acceptable convergence between the measured and estimated NOx concentration downstream of the SCR catalyst is achieved (i.e., the difference between the two is less than or equal to a small predetermined error value, ε). The achievement of convergence between the measured and estimated NO x  concentration values implies that the predicted value for the surface coverage fraction is accurate and can now be used in feedback control. 
     Condition  4  occurs when T cat ≧200 deg C. and model-based observer convergence&lt;ε. Under these conditions processor  26  switches to feedback control of reductant injection using the model-based observer  22 . 
     Therefore, according to the present invention, it is possible to obtain an accurate estimate of the amount of ammonia storage in a catalyst by using a model-based observer of catalyst behavior. In a preferred embodiment, the observer generates an estimate of the ammonia surface coverage fraction of the SCR catalyst based on NOx concentration upstream and downstream of the catalyst, and the concentration of ammonia injected into the catalyst to facilitate NOx reduction. The output of the observer can then be used to adjust the amount of reductant injection into the catalyst thereby achieving desired ammonia surface coverage fraction to minimize ammonia slip and optimize NOx conversion efficiency of the catalyst. Further, by monitoring observer convergence time, the model is continuously updated to account for poisoning or thermal aging of the catalyst as well as for different operating temperatures. Additionally, switching between feedforward and model-based feedback control of reductant injection based on operating conditions prevents over and under injection of reductant, minimizes tailpipe ammonia slip, and improves overall emission control system efficiency. 
     This concludes the description of the invention. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the invention. Accordingly, it is intended that the scope of the invention be defined by the following claims: