Abstract:
A reductant injection control strategy for controlling an amount of nitrogen oxide reducing agent injected upstream of a selective reduction catalyst uses an ammonia sensor located downstream of the catalyst. An open loop injection quantity is first determined based on operation conditions. Ammonia concentration detected downstream of the catalyst is controlled to a desired value, with the desired value based on catalyst temperature and the open loop injection quantity.

Description:
This application is a continuation of application Ser. No. 09/353,294, filed Jul. 12, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a system and method for controlling ammonia injection upstream of a selective reduction catalyst for use with an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     In order to meet some emission regulations, selective catalytic reduction systems using externally added reducing agents may be used. In such a system, regulated emissions, such as certain nitrogen oxides, or NOx, can be reduced in a oxygen-rich environment to nitrogen and water over a catalyst when a reducing agent, such as ammonia, is added. In addition to controlling nitrogen oxide emissions, the amount of excess ammonia, or ammonia slip, must be managed. Ammonia slip is experienced when ammonia in excess of that used to reduce the nitrogen oxides passes through the catalyst unaffected and exits the catalyst (as ammonia slip). 
     One method for regulating ammonia slip is to use an ammonia sensor located downstream of the catalyst. The detected ammonia concentration is compared with a fixed upper threshold value. This comparison generates a correction signal that is used to control the metering of ammonia upstream of the catalyst. Allegedly, by regulating actual ammonia slip to the upper threshold value, a certain nitrogen oxide reduction is obtained. Such a system is disclosed in U.S. Pat. No. 5,369,956. 
     The inventors herein have recognized a disadvantage with the above system. The above system regulates to a fixed concentration value for the upper threshold ammonia slip. However, this system does not consider NOx conversion efficiency or percentage slip. While NH 3  slip expressed as concentration (ppm) and as a percent are related, there is an important distinction in their use for reductant control strategy. In general, as maximum NOx conversion is approached with increasing ammonia addition (i.e., increasing NH 3 /NOx mole ratio), ammonia starts to slip. After maximum NOx conversion is attained, ammonia slip increases more rapidly with increasing NH 3 /NOx. For example, if ammonia slip is regulated to a constant concentration value, an ammonia setting high enough for sufficient NOx conversion at high NOx feed gas levels is likely excessive for low NOx feed gas levels, thereby wasting ammonia. Conversely, a setting at minimum detectable ammonia concentration is likely insufficient to provide high NOx conversion at high NOx feed gas levels. Further, intermediate settings may still be insufficient to provide high enough NOx conversion at high NOx feed gas levels. Thus, prior approaches can not achieve high NOx conversion with minimal ammonia slip, particularly for vehicle engines where NOx concentration levels varies widely and quickly. 
     In other words, because a catalyst experiences widely varying levels of engine NOx, controlling to an ammonia slip concentration results in widely varying, and less than optimum, NOx conversion efficiency. 
     SUMMARY OF THE INVENTION 
     An object of the invention claimed herein is to provide a system and method for controlling ammonia injection upstream of a selective reduction catalyst using an ammonia sensor located downstream of the catalyst to keep ammonia slip low while achieving a high level of NOx conversion. 
     The above object is achieved and disadvantages of prior approaches overcome by a method for controlling a reductant injection into a catalyst coupled to an internal combustion engine, the method comprising the steps of: determining a temperature region in which the catalyst is operating; generating a reductant injection quantity based on engine operating conditions; generating a desired reductant slip based on a catalyst temperature and said reductant injection quantity; and adjusting said reductant injection quantity so that an actual reductant slip approaches said desired reductant slip. 
     By regulating reductant slip to a desired value that is a fraction of injected reductant, NOx conversion efficiency is kept high and more consistent throughout widely varying NOx concentration levels typical for diesel vehicles. Further, since the desired ammonia slip value is also based on temperature, this additionally improves NOx conversion. 
     It is therefore possible to control ammonia slip with improved NOx reduction, particularly for vehicle engines where NOx concentration levels varies widely and quickly. In other words, when ammonia slip is regulated to a fraction of injected reductant, or ammonia, high NOx conversion is provided without excessive slip throughout the widely varying NOx feed gas concentrations. 
     An advantage of the present invention is improved NOx conversion while keeping ammonia slip low. 
     Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification. 
    
    
     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 block diagram of an embodiment wherein the invention is used to advantage; and 
     FIGS. 2-3 are high level flow charts of various operations performed by a portion of the embodiment shown in FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Internal combustion engine  10 , comprising a plurality of cylinders, one cylinder of which is shown in FIG. 1, is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  40 . Combustion chamber  30  is known communicating with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Intake manifold  44  is also shown having fuel injector  80  coupled thereto for delivering liquid fuel in proportion to the pulse width of signal FPW from controller  12 . Both fuel quantity, controlled by signal FPW and injection timing are adjustable. Fuel is delivered to fuel injector  80  by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). Alternatively, the engine may be configured such that the fuel is injected directly into the cylinder of the engine, which is known to those skilled in the art as a direct injection engine. 
     Reducing agent, for example, ammonia, is stored in storage vessel  130  coupled to exhaust manifold  48  upstream of catalyst  97 . Control valve  134  controls the quantity of reducing agent delivered to the exhaust gases entering catalyst  97 . Pump  132  pressurizes the reducing agent supplied to control valve  134 . Both Pump  132  and control valve  134  are controlled by controller  12 . Ammonia sensor  140  is shown coupled to exhaust manifold  48  downstream of catalyst  97 . Temperature sensor  142  coupled to catalyst  97  provides an indication of the temperature (T) of catalyst  97 . Alternatively, catalyst temperature (T) could be estimated using methods known to those skilled in the art and suggested by this disclosure. Ammonia sensor  140  provides an indication of ammonia concentration [NH 3 ] to controller  12  for determining a control signal sent to control valve  134  as described later herein with particular reference to FIGS. 2-3. 
     Controller  12  is shown in FIG. 1 as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , read-only memory  106 , random access memory  108 , and a conventional data bus. Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a measurement of manifold pressure (MAP) from pressure sensor  116  coupled to intake manifold  44 ; a measurement (AT) of manifold temperature from temperature sensor  117 ; an engine speed signal (RPM) from engine speed sensor  118  coupled to crankshaft  40 . 
     Referring now to FIG. 2, a routine for determining a control signal for control valve  134  for controlling reductant addition is described. Reductant is ammonia in a preferred embodiment, but can be any nitrogen (N) containing substance, such as, for example, urea. During step  200 , a determination is made as to whether temperature (T) of catalyst  97  is below first threshold temperature T 1 . Calculation of first threshold temperature T 1  is described later herein with particular reference to FIG.  3 . When the answer to step  200  is YES, the desired mole ratio (Rdes) is set to zero in step  201  and the total quantity of reductant (Qtot) to be injected by control valve  134  is set to zero in step  203 . Thus no reductant is added to the exhaust gases entering catalyst  97  to give a mole ratio (R) equal to first desired mole Ratio (R 1 ) of zero. 
     Mole ratio (R) is the ratio of the number of moles of nitrogen (N) in the reductant to the number of moles of nitrogen oxide in engine out exhaust gas. The moles of nitrogen oxide in engine out exhaust gas is calculated based on experimentally determined relationships between nitrogen oxide quantity and engine operating conditions known to those skilled in the art to be indicative of estimated engine out nitrogen oxide quantity (Nox est ) such as, for example, engine speed, manifold pressure (MAP), intake air temperature (AT), injection timing, injection quantity (FPW), and engine coolant temperature (ECT). 
     When the answer to step  200  is NO, a determination is made in step  204  as to whether temperature (T) is below second threshold temperature T 2 . Calculation of second threshold temperature T 2  is described later herein with particular reference to FIG.  3 . 
     When the answer to step  204  is NO, a determination is made in step  208  as to whether temperature (T) is below third threshold temperature T 3 . Calculation of third threshold temperature T 3  is described later herein with particular reference to FIG.  3 . When the answer to step  208  is YES, the desired mole ratio (Rdes) is set to third desired mole ratio (R 3 ) in step  210 . Then, in step  212 , a determination is made as to whether the measured ammonia concentration from sensor  140  is less than limit amount FR 1 . First limit amount FR 1  is based on a fraction of reductant quantity previously injected. Further, first limit amount FR 1  is determined for the specific temperature range. Alternatively, first limit amount FR 1  can be a ratio of ammonia slip concentration to engine out (or catalyst-in) NOx quantity. Thus, according to the present invention, the ammonia slip is kept within a limit where the limit is a fraction of the amount of injected reductant. 
     Continuing with FIG. 2, if the answer to step  212  is YES, then in step  214 , adjusted reductant quantity (DQ) is set to a positive calibration amount (r). If the answer to step  212  is NO, then in step  218  adjusted reductant quantity (DQ) is set to a negative calibration amount (−r). Then, from either step  214  or  218 , the base reductant quantity (Qbase) is determined from the product of the desired mole ratio (Rdes) and the estimated engine nitrogen oxide production (Nox est ) in step  220 . 
     When the answer to step  208  is NO, a determination is made in step  226  as to whether temperature (T) is below fourth threshold temperature T 4 . Calculation of fourth threshold temperature T 4  is described later herein with particular reference to FIG.  3 . When the answer in step  226  is YES, the desired mole ratio (Rdes) is set to fourth desired mole ratio (R 4 ) in step  228 . Then, a determination is made in step  230  as to whether the measured ammonia concentration from sensor  140  is greater than second limit amount FR 2 . Limit amount FR 2  is calculated as a second fraction of reductant quantity previously injected. In a preferred embodiment, second limit amount FR 2  is less than first limit amount FR 1 . In an alternative embodiment, limit amounts FR 1  and FR 2  can be set to constant levels or adjusted to give a specified parts per million (ppm) of ammonia slip. Further, if urea were used in place of ammonia, appropriate adjustment of the fractions is needed to account for the different molecular structure. Alternatively, second limit amount FR 2  can also be a ratio of ammonia slip concentration to engine out (or catalyst-in) NOx concentration. According to the present invention, different limit amounts (FR 1  and FR 2 ) are used in different temperature ranges to maximize NOx conversion and minimize ammonia slip. 
     Continuing with FIG. 2, if the answer to step  230  is YES, then in step  218  adjusted reductant quantity (DQ) is set to a negative calibration amount (−r). Otherwise, adjusted reductant quantity (DQ) is set to a postive calibration amount (−r) in step  214 . 
     When the answer to step  204  is YES, the desired mole ratio (Rdes) is set to second desired mole ratio (R 2 ) in step  236 . Then in step  232  adjusted reductant quantity (DQ) is set zero. Then, base reductant quantity (Qbase) is determined from the product of the desired mole ratio (Rdes) and the estimated engine nitrogen oxide production (Nox est ) in step  220 . Then, in step  222 , total desired reductant quantity (Qtot) is determined from the sum of the base reductant quantity (Qbase) and the adjusted reductant quantity (DQ). The total desired reductant quantity (Qtot) is converted to a control signal sent to control valve  134  for delivering the reductant in proportional thereto. 
     In this way, open loop reductant control is used to calculated the base reductant quantity (Qbase) from the product of the desired mole ratio (Rdes) and the estimated engine nitrogen oxide quantity (Nox est ). Also, desired mole ratio is adjusted based on catalyst temperature (T) to account for changes in catalyst efficiency. 
     Adjustment is made to this open loop value in two temperature ranges when the measured ammonia concentration from sensor  140  deviates from a desired value based on a fraction of reductant injection. Limit values FR 1  and FR 2  represent the allowable limits of ammonia slip. Thus, the reductant is controlled for maximum nitrogen oxide conversion with minimum slip. In an alternative embodiment (not shown), different calibration amounts can be used in different temperature ranges. Further, positive and negative calibration amounts can be different (not shown). 
     Referring now to FIG. 3, a routine for calculating temperature thresholds is now described. First based temperatures (T 1 B, . . . , T 4 B) are determined based on predetermined calibration values in step  310 . Then in step  312 , the space velocity (SV) of the exhaust gas flow entering catalyst  97  is calculated based on the mass flow rate (m), density (r), and catalyst Volume (V). Then, in step  314 , adjustment values, (KA 1 , . . . , KA 4 ), are determined based on space velocity (SV) of the flow entering catalyst  97  and calibration functions (f 1  . . . f 4 ). In a preferred embodiment, functions f 1  . . . f 4  act to reduce temperatures as space velocity decreases and increase temperatures as space velocity increases. 
     Although one example of an embodiment which practices the invention has been described herein, there are numerous other examples which could also be described. For example, the invention may be used to advantage with both lean burning diesel and gasoline engines in which nitrogen oxide emissions are produced. Further, the present invention can be used in diagnostic applications where the invention is therefore to be defined only in accordance with the following claims.