Abstract:
A reductant injection control strategy for controlling an amount of nitrogen oxide reducing agent injected upstream of a selective reduction catalyst uses a NOx sensor located downstream of the catalyst. An open loop injection quantity is first determined based on operation conditions. Nitrogen oxide conversion efficiency of the catalyst is controlled by controlling the reductant injection based on after catalyst NOx sensor reading and engine out nitrogen oxide concentration.

Description:
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 can 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 occurs 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 nitrogen oxide emissions and ammonia slip is to use an after-catalyst NOx sensor to detect nitrogen oxide concentration. Control of NOx emissions are allegedly achieved by varying reductant injection until the level or quantity of nitrogen oxides as measured by the sensor falls within an acceptable limit. The amount of reductant injected to keep NOx emissions within the acceptable limit needs to be balanced with an ammonia slip limit. This can be measured and controlled by an after-catalyst ammonia sensor. Such a system is disclosed in U.S. Pat. No. 5,233,934. Alternatively, ammonia slip can be calculated and controlled using an algorithm. Such a system is disclosed in U.S. Pat. No. 4,751,054. 
     The inventors herein have recognized a disadvantage with the above systems. The above systems attempt to control nitrogen oxide emission level, while limiting ammonia slip. However, these systems do not consider NOx conversion efficiency. While NOx conversion efficiency and after-catalyst NOx emission levels 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 a NOx emission level is regulated to a specific concentration value, then at high feed gas NOx levels, the demand for NOx reduction can easily result in attaining a NOx conversion where ammonia slip is likely excessive and prone to go out of control. 
     In other words, because a catalyst experiences widely varying levels of engine NOx, controlling to a specific concentration value results in widely varying, and less than optimum, NOx conversion efficiency. Thus, prior art methods are insufficient. 
     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 to obtain a desired level of nitrogen oxide conversion efficiency while keeping ammonia slip as low as possible. 
     The above object is achieved, and disadvantages of prior approaches overcome by the method of controlling a reductant injection upstream of a catalyst coupled to an internal combustion engine, the method comprising the steps of: generating a reductant injection quantity based at least on an engine operating condition; determining a nitrogen oxide conversion efficiency of the catalyst; and adjusting said injection quantity to obtain a predetermined value of said nitrogen oxide conversion efficiency. 
     By controlling reductant injection based on operating the catalyst at a desired nitrogen oxide conversion efficiency value, low nitrogen oxide emissions are obtained, and ammonia slip is kept low, even when the operating conditions vary widely and rapidly such as those for vehicle driving. 
     In other words, it is possible to reduce NOx significantly and keep ammonia slip low by regulating NOx conversion efficiency rather than NOx emission level. Controlling NOx conversion efficiency is particularly useful where NOx production and flow rate vary widely and quickly such as for vehicle engines. 
     An advantage of the present invention is optimum reduction in NOx while keeping ammonia slip low without need for an ammonia sensor or an algorithm estimate to adjust ammonia slip. 
     Another advantage of the present invention is improved reduction in NOx emissions 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 . NOx 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. NOx sensor  140  provides an indication of nitrogen oxide concentration [NO x ] 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  10  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. 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  202 . 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 ammonia 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 YES, the desired mole ratio (Rdes) is set to second desired mole ratio (R 2 ) in step  206 . Then, adjusted reductant quantity (DQ i ) for step i is set to zero in step  208 . Then, 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  210 . Then, in step  212 , total desired reductant quantity (Qtot) is determined from the sum of the base reductant quantity (Qbase) and the adjusted reductant quantity (DQ i ). The total desired reductant quantity (Qtot) is converted to a control signal sent to control valve  134  for delivering the reductant in proportional thereto. 
     When the answer to step  204  is NO, a determination is made in step  220  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 in step  220  is YES, the desired mole ratio (Rdes) is set to third desired mole ratio (R 3 ) in step  222 . 
     Continuing with FIG. 3, in step  224 , the value of the nitrogen oxide conversion efficiency (NOxConv i ) at step i is determined from sensor  140  and estimated engine out nitrogen oxide quantity (Nox est ). In step  226 , a determination is made as to whether the nitrogen oxide conversion efficiency at step i is greater than a desired NOx conversion efficiency. The desired NOx conversion efficiency (NOxDES) is determined as a fraction of estimated engine out nitrogen oxide quantity (Nox est ). In addition, the desired NOx conversion efficiency can be changed versus temperature (T). The optimum desired NOx conversion as a function of engine out NOx and catalyst temperature is determined from engine testing and stored as predetermined values. Thus, according to the present invention, both the base reductant injection quantity and the desired NOx conversion control value are adjusted based on temperature to improve overall NOx conversion and ammonia slip. In an alternative embodiment, the desired NOx conversion efficiency can be calculated based on a base reductant injection quantity. More specifically, the desired NOx conversion efficiency can be calculated based on a predetermined percentage of base reductant injection quantity, where the predetermined percentage is mapped versus engine operating conditions. 
     Continuing with FIG. 2, if the answer to step  226  is YES, then the adjusted reductant quantity (DQ i ) is set to a negative calibration amount (−r) in step  228 . Otherwise, in step  230  the adjusted reductant quantity (DQ i ) is set to a positive calibration amount (r). 
     When the answer to step  220  is NO, a determination is made in step  236  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  236  is YES, the desired mole ratio (Rdes) is set to fourth desired mole ratio (R 4 ) in step  238 . Then, the routine continues to step  224  previous described herein. 
     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 to attain desired nitrogen oxide conversion efficiency based on measured nitrogen oxide from sensor  140  and estimated engine nitrogen oxide quantity. Further, desired nitrogen oxide conversion efficiency is determined based on both catalyst temperature and engine out NOx production. 
     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 flow 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. The invention is therefore to be defined only in accordance with the following claims.