Patent Publication Number: US-7707824-B2

Title: Excess NH3 storage control for SCR catalysts

Description:
FIELD 
     The present disclosure relates to exhaust treatment systems, and more particularly to an excess NH3 storage control for an exhaust treatment system including a selective catalytic reduction (SCR) catalyst. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Internal combustion engines combust an air and fuel mixture to generate drive torque. The combustion process generates exhaust that is exhausted from the engine to atmosphere. The exhaust contains nitrogen oxides (NOx), carbon dioxide (CO 2 ), carbon monoxide (CO) and particulates. NOx is a term used to describe exhaust gases that consist primarily of nitrogen oxide (NO) and nitrogen dioxide (NO 2 ). An exhaust after-treatment system treats the exhaust to reduce emissions prior to being released to atmosphere. In an exemplary exhaust after-treatment system, a dosing system injects a dosing agent (e.g., urea) into the exhaust upstream of a selective catalytic reduction (SCR) catalyst. The exhaust and dosing agent mixture reacts over the SCR catalyst to reduce the NOx levels released to atmosphere. 
     The dosing agent reacts with NOx on the SCR catalyst to accomplish the NOx reduction. More specifically, the dosing agent breaks down to form ammonia (NH3), which is the reductant utilized to react with the NOx. The following exemplary, chemical relationships describe the NOx reduction:
 
4NO+4NH 3 +O 2 →4N 2 +6H 2 O
 
4NH 3 +2NO+2NO 2 →4N 2 +6H 2 O
 
3NO 2 +4NH 3 →3.5N 2 +6H 2 0
 
     To perform the above-described NOx reduction, the SCR catalyst stores NH3 therein. For an SCR catalyst to perform effectively, the NH3 storage level must be maintained at an adequate level. More specifically, the NOx reduction or conversion efficiency is dependent upon the NH3 storage level. In order to maintain high conversion efficiency under various operating conditions, the NH3 storage must be maintained. However, as the temperature of the SCR catalyst increases, the NH3 level must be reduced to avoid NH3 slip (i.e., excess NH3 being released from the SCR catalyst), which can reduce the conversion efficiency of the catalyst. 
     SUMMARY 
     Accordingly, the present disclosure provides a method of regulating an amount of NH3 stored in a catalyst of an exhaust after-treatment system. The method includes determining a mass of NH3 into the catalyst based on a dosing rate of a dosing agent that is injected into an exhaust stream upstream of the catalyst and determining a mass of NH3 out of the catalyst (i.e., consumed in the catalyst). An accumulated mass of NH3 within the catalyst is calculated based on the mass of NH3 into the catalyst and the mass of NH3 out of the catalyst. The dosing rate is regulated based on the accumulated mass of NH3 within the catalyst. 
     In one feature, the mass of NH3 out of the catalyst is determined based on signals generated by NOx sensors that are located upstream and downstream of the catalyst, respectively. 
     In another feature, the method further includes determining a conversion efficiency of the catalyst based on a temperature of the catalyst. The mass of NH3 out of the catalyst is determined based on a base dosing rate (i.e., stoichiometric) and the conversion efficiency. 
     In still another feature, the method further includes monitoring a catalyst temperature and setting the accumulated mass of NH3 within the catalyst equal to zero when the catalyst temperature exceeds a threshold temperature. In this manner, the areas of operation, in which the catalyst does not have any storage potential, are accounted for. 
     In yet other features, the method further includes defining a maximum NH3 storage mass of the catalyst based on a catalyst temperature. The dosing rate is regulated based on the maximum NH3 storage mass. An excess NH3 storage ratio is calculated based on the accumulated mass of NH3 within the catalyst and the maximum NH3 storage mass, wherein the dosing rate is regulated based on the excess NH3 storage ratio. For example, an adjustment factor is determined based on the excess NH3 storage ratio, wherein the dosing rate is regulated based said adjustment factor. The dosing agent is regulated to maintain the excess NH3 storage ratio to be less than 1. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a functional block diagram of an engine system including an exhaust treatment system including a selective catalytic reduction (SCR) catalyst; 
         FIG. 2  is a flowchart illustrating exemplary steps that are executed by the excess NH3 storage control of the present disclosure; 
         FIG. 3  is a functional block diagram of exemplary modules that execute the excess NH3 storage control; 
         FIG. 4  is a functional block diagram of exemplary modules that are used to determine a cumulative NH3 value into the SCR catalyst; 
         FIG. 5A  is a functional block diagram of exemplary modules that are used to determine a cumulative NH3 value out of the SCR catalyst; 
         FIG. 5B  is a functional block diagram of exemplary, alternative modules that are used to determine the cumulative NH3 value out of the SCR catalyst; and 
         FIG. 6  is a functional block diagram of exemplary modules that are used to determine an excess NH3 storage multiplier in accordance with the excess NH3 storage control of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality. 
     Referring now to  FIG. 1 , an exemplary vehicle system  10  is schematically illustrated. The vehicle system  10  includes an engine system  12 , an exhaust after-treatment system  14 . The engine system  12  includes an engine  16  having a cylinder  18 , an intake manifold  20  and an exhaust manifold  22 . Air flows into the intake manifold  20  through a throttle  24 . The air is mixed with fuel and the air and fuel mixture is combusted within the cylinder  18  to drive a piston (not shown). Although a single cylinder  18  is illustrated, it is appreciated that the engine  12  may include additional cylinders  18 . For example, engines having 2, 3, 4, 5, 6, 8, 10, 12 and 16 cylinders are anticipated. The fuel is provided from a fuel source  26  and is injected into the air stream using an injector  28 . A fuel level sensor  30  is responsive to the amount of fuel within the fuel source  26 . It is anticipated that the present disclosure can be implemented in both lean burn gasoline engines and diesel engines. 
     Exhaust is produced through the combustion process and is exhausted from the cylinder  18  into the exhaust manifold  22 . The exhaust after-treatment system  14  treats the exhaust flowing therethrough to reduce emissions before being released to the atmosphere. The exhaust after-treatment system  14  includes a dosing system  32 , a diesel oxidation catalyst (DOC)  34 , a NOx sensor  36 , a NOx sensor  37  and a catalyst  38  that is preferably provided as a selective catalytic reduction (SCR) catalyst. 
     The NOx sensor  36  is deemed the upstream NOx sensor and the NOx sensor  37  is deemed the downstream NOx sensor, relative to the catalyst  38 . Both NOx sensors  36 ,  37  are responsive to a NOx level of the exhaust and generate respective signals based thereon. An upstream NOx mass flow rate ({dot over (m)} NOXUS ) is determined based on the signal generated by the NOx sensor  36 . Similarly, a downstream NOx mass flow rate ({dot over (m)} NOXDS ) is determined based on the signal generated by the NOx sensor  37 . 
     Temperature sensors T A , T B  and T C  are located at various points along the emissions path. For example, the temperature sensor T A  is located upstream of the DOC  34 , the temperature sensor T B  is located upstream of the catalyst  38  and the temperature sensor T C  is located downstream of the catalyst  38 . The DOC  34  reacts with the exhaust to reduce emission levels of the exhaust. It is also anticipated that a diesel particulate filter (DPF)  40  may be located downstream from the catalyst  30  that filters diesel particulates to further reduce emissions. It is anticipated that the order of the SCR catalyst and the DPF can be reversed. 
     The dosing system  32  includes a dosing agent injector  42 , a dosing agent storage tank  44  and a dosing agent supply sensor  46 . The dosing system  32  selectively injects a dosing agent (e.g., urea) into the exhaust stream to further reduce emissions. More specifically, the rate at which the dosing agent is injected into the exhaust stream ({dot over (m)} DA ) is determined based on the signals generated by one or more of the various sensors described herein. The exhaust and dosing agent mixture reacts within the catalyst  38  to further reduce exhaust emissions. 
     A control module  50  regulates flow of the dosing agent based on the excess NH3 storage control of the present disclosure. The excess NH3 storage control keeps track of the mass of NH3 supplied into (m NH3IN ) and out of (m NH3OUT ) the catalyst  38 . Furthermore, the excess NH3 storage control makes corrections based on where the calculated storage amount is with respect to a maximum NH3 storage capacity (m NH3MAX ) of the catalyst  38 . 
     m NH3IN  is determined based the dosing agent or reductant (e.g., urea) input mass flow rate (i.e., {dot over (m)} DA ). {dot over (m)} DA  is known and is determined based on the signal generated by the upstream NOx sensor  36 . m NH3IN  is further determined based on the exhaust flow rate, which is calculated based on MAF, a known fuel flow rate and other constants. m NH3OUT  is the amount of NH3 that reacts with NOx within the catalyst  38  and is calculated based on the difference between {dot over (m)} NOXUS , {dot over (m)} NOXDS  and a time delta (dt). A set of constants is used to convert this difference to an NH3 mass out of the catalyst  38  (m NH3OUT ) (i.e., NH3 consumed). The difference (Δm NH3 ) between m NH3IN  and m NH3OUT  is provided as the mass of NH3 stored in the catalyst  38 . 
     The stored NH3 (Δm NH 3) is compared to M NH3MAX , which is determined based on a temperature of the catalyst  38  (T CAT ). m NH3IN  is adjusted to keep Δm NH3  at a desired fraction of m NH3MAX . In one embodiment, a simple ratio (i EXCSNH3 ) is implemented. As another embodiment, a closed-loop control setpoint is provided as a fraction of m NH3MAX . In this manner, NH3 release from the catalyst  38  that results from thermal transients can be reduced. 
     The mass flow rate of NH3 supplied into the catalyst  38  ({dot over (m)} NH3IN ) (e.g., provided in g/s) is calculated based on {dot over (m)} DA , provided in g/hour, the concentration of the dosing agent (DA CONC ), the molecular weight of the dosing agent (DA MW ) (e.g., 60.06 g/mol in the case of urea), the molecular weight of NH3 (NH3 MW ) (e.g., 17.031 g/mol) and the known decomposition factor of the dosing agent with respect to NH3 (k DEC ). DA CONC  is determined as the percentage of dosing agent to dosing agent solution (e.g., 32.5% indicates 0.325 parts dosing agent to 1 part dosing agent solution). k DEC  is provided in mol NH3 per mol dosing agent (e.g., in the case of urea, 1 mol of urea decomposes to 2 moles of NH3; k DEC =2). {dot over (m)} NH3IN  is calculated in accordance with the following relationship: 
                         m   .       NH   ⁢           ⁢   3   ⁢   IN       ⁡     (     g   /   s     )       =             m   .     DA     ·     DA   CONC     ·     k   DEC     ·   NH     ⁢           ⁢     3     M   ⁢           ⁢   W           3600   ·     DA     M   ⁢           ⁢   W                   (   1   )               
where 3600 is a time conversion factor (k TIME ) of seconds per hour.
 
     {dot over (m)} NH3OUT  (e.g., provided in g/s) is the mass flow rate of NH3 consumed in the catalyst  38  and is calculated based on {dot over (m)} NOXUS , provided in g/s, {dot over (m)} NOXDS , provided in g/s, the molecular weight of the NOx (NOx MW ) and NH3 MW  (e.g., 17.031 g/mol). NOx MW  is variable, however, any NOx MW  can be used (e.g., NO 2 =46.055 g/mol), because it cancels in the relationships described herein. {dot over (m)} NH3OUT  is calculated in accordance with the following relationship: 
                       m   .       NH   ⁢           ⁢   3   ⁢   OUT       =           [         m   .       NO   ⁢   XUS       -       m   .     ⁢     NO   ⁢   XDS         ]     ·   NH     ⁢           ⁢       3     M   ⁢           ⁢   W       ·   X     ⁢           ⁢   mol   ⁢           ⁢   NH   ⁢           ⁢   3             NO   ⁢   x       M   ⁢           ⁢   W       ·   1     ⁢           ⁢     mole   ⁢   NO   ⁢   X                 (   2   )               
X varies from 1 to 1.333 depending on the upstream % of NO 2 . {dot over (m)} NOXUS  and {dot over (m)} NOXDS  are calculated in accordance with the following relationship:
 
                       m   .         NO   ⁢   XUS     ,   DS       =                   NO   ⁢   x     ⁡     (     1   ⁢   ppm     )       ·     10     -   6         ⁢       (     mol   ⁢           ⁢       NO   ⁢   x     /   mol     ⁢           ⁢   Exhaust     )     ·                     NO   ⁢   x       M   ⁢           ⁢   W       ·       m   .     EXH               EXH     M   ⁢           ⁢   W                 (   3   )               
where {dot over (m)} EXH  is the mass flow rate of the exhaust and EXH MW  is the molecular weight of the exhaust gas (e.g., provided in g of exhaust/mol of exhaust).
 
     Both {dot over (m)} NH3IN  and {dot over (m)} NH3OUT  are multiplied by a time increment (dt) (e.g., 1 second) to provide m NH3IN  and m NH3OUT , respectively, which are provided in grams. Δm NH3  is determined as the difference between m NH3IN  and m NH3OUT  and is deemed the excess NH3 that is stored in the catalyst  38 . Δm NH3  (e.g., or m NH3IN  and m NH3OUT  before calculating Δm NH3 ) can be integrated to provide a cumulative value over time (Δm NH3CUM ). Δm NH3CUM  is divided by m NH3MAX  to provide i EXCSNH3 . 
     i EXCSNH3  is used as an input to a look-up table to look up an excess storage multiplier value (k EXCSSTORE ), which is fed back to the control module  50  to trim {dot over (m)} DA . The look-up table is stored in memory and is calibrated in such a way to make the k EXCSSTORE  equal to 1 at some desired storage ratio (i DSR ) of NH3 STOREMAX . For example, if i EXCSNH3  is less than i DSR , k EXCSSTORE  is set to be greater than 1 and vise versa. In one embodiment, this function is executed by a closed-loop control (e.g., a PID control module). 
     It is preferable to control the i DSR  to be sufficiently below 1 to avoid NH3 slip from occurring. In order to reduce accumulated errors, Δm NH3CUM  is reset during high temperature catalyst operation where no significant NH3 storage occurs (i.e., when T CAT  is greater than a threshold temperature (T THR )). The catalyst temperature (T CAT ) is determined based on a temperature sensor signal (e.g., from one or more of the temperature sensors T A , T B , T C  and/or a temperature sensor integrated into the catalyst (not shown)). As T CAT  increases NH3 STOREMAX  decreases, thereby raising the i EXCSNH3 . This causes less dosing agent, and thus less NH3, to be dosed to the catalyst  38 . By resetting Δm NH3CUM , NH3 release from the catalyst  38  is reduced. 
     As mentioned above, NH3 STOREMAX  is the maximum possible NH3 stored at a given T CAT . Described below is a method of determining NH3 STOREMAX . The catalyst  38 , and exhaust after-treatment system for that matter, is stabilized to a constant temperature and the catalyst is purged of all stored NH3 (i.e., by providing no dosing agent, and thus no incoming NH3, to the catalyst). At this point, Δm NH3CUM  is reset to 0 g. At some time (t 0 ), the dosing agent, and thus NH3, supply is turned back on with an excess NH3 to NOx molar ratio. The conversion efficiency of the downstream NOx sensor  37  and the upstream NOx sensor  36  will stabilize at a maximum value and at some latter time (t 1 ) will start decreasing (i.e., when the downstream NOx sensor  37  detects NH3). At this point, Δm NH3CUM  is read to provide an approximate NH3 STOREMAX  value. The conversion efficiency is determined in accordance with the following relationship: 
     
       
         
           
             
               
                 
                   
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                                 NO 
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                               DS 
                             
                           
                           
                             
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     Referring now to  FIG. 2 , exemplary steps that are executed by the excess NH3 control will be described in detail. In step  200 , control determines whether T CAT  is greater than T THR . If T CAT  is greater than T THR , control sets Δm NH3CUM  equal to zero in step  202  and loops back to step  200 . If T CAT  is not greater than T THR , control determines m NH3IN  in step  204 . Control determines m NH3OUT  in step  206 . In step  208 , control calculates Δm NH3CUM . In step  210 , control determines m NH3MAX . 
     Control calculates i EXCSNH3  based on m NH3MAX  and Δm NH3CUM  in step  212 . In step  214 , control determines k EXCSSTORE  based on i EXCSNH3 . Control regulates {dot over (m)} DA  based on k EXCSSTORE  in step  216  and control ends. It is anticipated, however, that the above-described, exemplary control will continue to loop through steps  200  to  216  at a pre-determined time interval or rate while the engine is running. 
     Referring now to  FIG. 3 , exemplary modules that execute the excess NH3 control will be described in detail. The exemplary modules include a m NH3IN  module  300 , a m NH3OUT  module  302 , a summer module  304 , a dosing agent control module  306  and a comparator module  308 . The m NH3IN  module  300  determines m NH3IN  based on {dot over (m)} DA , as described in detail above. The m NH3OUT  module  302  determines m NH3OUT  based on {dot over (m)} NOXUS  and {dot over (m)} NOXDS , as described in detail above and in further detail with respect to  FIG. 5A  below. Alternatively, the m NH3OUT  module  302  determines m NH3OUT  based on {dot over (m)} DA(BASE) , as described in further detail with respect to  FIG. 5B  below. {dot over (m)} DA(BASE)  is the stoichiometric NH3 quantity. 
     The summer module  304  determines Δm NH3  as the difference between m NH3IN  and m NH3OUT . The dosing agent control module  306  monitors Δm NH3CUM  and regulates {dot over (m)} DA  based thereon. The dosing agent control module  306  also selectively resets Δm NH3CUM , as described in detail above, based on a signal from the comparator module  308 . More specifically, the comparator module  308  compares T CAT  to T THR . If T CAT  is greater than T THR , the signal from the comparator module  308  indicates that Δm NH3CUM  should be reset. If T CAT  is not greater than T THR , the signal from the comparator module  308  indicates that Δm NH3CUM  should not be reset. 
     Referring now to  FIG. 4 , exemplary modules that are used to calculate m NH3IN  will be described in detail. The exemplary modules include a first multiplier module  400 , a first divider module  402 , second and third multiplier modules  404 ,  406 , respectively, a second divider module  408 , a fourth multiplier module  410  and an addition module  412 . The modules  400 ,  402 ,  404 ,  406 ,  408  process {dot over (m)} DA , DA CONC , DA MW , k DEC , NH3 MW  and k TIME  in accordance with Equation 1, described above, to provide {dot over (m)} NH3IN . The fourth multiplier module  410  multiplies {dot over (m)} NH3IN  by dt to provide m NH3IN . The addition module  412 , which may be optionally provided, accumulates the m NH3IN  values to provide a cumulative m NH3IN  (m NH3INCUM ). 
     Referring now to  FIG. 5A , exemplary modules that are used to calculate m NH3OUT  will be described in detail. The exemplary modules include a summer module  500 , a divider module  502 , first, second and third multiplier modules  504 ,  505 ,  506 , respectively, and an addition module  508 . The modules  500 ,  502 ,  504 ,  505  process {dot over (m)} NOXUS , {dot over (m)} NOXDS , NOx MW  and NH3 MW  to provide {dot over (m)} NH3OUT . The third multiplier module  506  multiplies {dot over (m)} NH3OUT  by dt to provide m NH3OUT . The addition module  508 , which may be optionally provided, accumulates the m NH3OUT  values to provide a cumulative m NH3OUT  (m NH3OUTCUM ). It is again noted that the molar ratio X between NH3 and NOx varies from 1 to 1.333 depending on the upstream % of NO 2 . 
     Referring now to  FIG. 5B , alternative exemplary modules that are used to calculate m NH3OUT  will be described in detail. As discussed in further detail below, the exemplary modules initially calculate {dot over (m)} NH3OUT  based on {dot over (m)} NH3IN  and a conversion efficiency (CE(%)) of the catalyst. CE(%) is determined based on several factors including, but not limited to, T CAT , space velocity and NO 2  ratio. 
     The exemplary modules include a first multiplier module  510 , a first divider module  512 , second, third and fourth multiplier modules  514 ,  516 ,  517  respectively, a second divider module  518 , a fifth multiplier module  520  a third divider module  522 , a sixth multiplier module  524  and an addition module  526 . The modules  510 ,  512 ,  514 ,  516 ,  517 ,  518  process {dot over (m)} DA(BASE) , DA CONC , DA MW , k DEC , NH3 MW  and k TIME  in accordance with Equation 1, described above, to provide {dot over (m)} NH3IN . The third divider module divides CE(%) by 100 to provide a decimal value of the conversion efficiency, which is then multiplied by {dot over (m)} NH3IN  in the fifth multiplier module  520  to provide {dot over (m)} NH3OUT . The sixth multiplier module  524  multiplies {dot over (m)} NH3OUT  by dt to provide m NH3OUT . The addition module  526 , which may be optionally provided, accumulates the m NH3OUT  values to provide m NH3OUTCUM . Again, the molar ratio X between NH3 and NOx varies from 1 to 1.333 depending on the upstream % of NO 2 . 
     Referring now to  FIG. 6 , exemplary modules that are used to determine k EXCSSTORE  will be described in detail. The exemplary modules include a m NH3MAX  module  600 , an addition module  602 , a division module  604  and a k EXCSSTORE  module  606 . The m NH3MAX  module  600  determines m NH3MAX  based on T CAT  and V CAT , as discussed above. The addition module  602  accumulates the Δm NH3  values to provide Δm NH3CUM . If, however, the addition modules  412  and  508 ,  526  of  FIGS. 4 and 5A ,  5 B are included, the addition module  602  can be foregone because Δm NH3CUM  will be provided based on m NH3INCUM  and m NH3OUTCUM  from the addition modules  412  and  508 ,  526 . If the addition modules  412  and  508 ,  526  are not provided, the addition module  602  is provided. The division module  604  determines i EXCSNH3  as a ratio between Δm NH3CUM  and m NH3MAX . The k EXCSSTORE  module  606  k EXCSSTORE  determines k EXCSSTORE , as discussed in detail above. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.