Abstract:
A reductant injection control strategy for controlling an amount of nitrogen oxide reducing agent injected upstream of a first and second serially placed nitrogen oxide catalyst, calculates the reductant injection For the first catalyst and second catalyst. The reductant injection for the first catalyst is based on engine operating conditions. The reductant injection for the second catalyst is based on the reductant injection for the first catalyst and a conversion efficiency of the first catalyst.

Description:
FIELD OF THE INVENTION 
     The invention relates to a system and method for controlling reductant injection upstream of an active lean NOx catalyst for use with an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     In order to meet current emission regulations, active lean NOx catalyst systems with externally added reducing agents are used. In such systems, regulated emissions, such as certain nitrogen oxides, are reduced to nitrogen and water in the catalyst when a reducing agent containing hydrocarbons is added. In order to obtain maximum fuel economy while meeting emission regulations, it is necessary to inject just enough reductant to promote the chemical reactions without increasing hydrocarbon emissions by injecting too much reductant. 
     In certain circumstances, it may be desirable to have multiple lean NOx catalysts coupled in series to accommodate, for example, packaging and manufacturing constraints. In this case, dual reductant injection may be used for injecting reducing agent upstream of each catalyst. 
     One method for controlling reductant injection upstream of first and second lean NOx catalysts uses a NOx sensor located downstream of each catalyst. In this method, reductant is injected upstream of the first NOx catalyst in accordance with a control dependent on engine operating parameters. Similarly, reductant is injected upstream of the second NOx catalyst in accordance with a strategy identical to the first NOx catalyst. Both the reductant injection strategies rely on a downstream NOx sensor for the reductant control, and thereby the exhaust air/fuel ratio control. Such a system is described is U.S. Pat. No. 5,771,686. 
     The inventors herein have recognized a disadvantage with the above system. The above system does not indicate a method for determining the quantity of NOx entering the second catalyst. In addition, the above system does not exploit available benefits of having multiple catalyst with individual reductant injection control. In other words, the above method uses the same reductant control strategy for each catalyst, thereby requiring sensors for each catalyst. The inventors herein have recognized that improvements are possible by recognizing the physical couplings and eliminating unnecessary sensor duplications. 
     SUMMARY OF THE INVENTION 
     An object of the invention claimed herein is to provide a system and method for controlling multiple reductant injection upstream of serially placed lean NOx catalysts that maximizes nitrogen oxide conversion with minimum reductant injection. 
     The above object is achieved, and disadvantages of prior approaches is overcome, by the instant invention. 
     By realizing that the product of the first catalyst efficiency and the first reductant injection quantity is proportional to the necessary reductant injection quantity for the second catalyst, it is possible to eliminate the need for additional sensors. In other words, based on the efficiency of the first catalyst and the amount of reductant injected into the first catalyst, it is possible to calculate the amount of reductant necessary for the second catalyst. This intrinsically takes into account the amount of nitrogen oxides exiting the first catalyst, and the amount of unused reductant exiting the first catalyst, each of which will enter the second catalyst. 
     An advantage of the present invention is improved emission control. 
     Another advantage of the present invention is improved nitrogen oxide conversion efficiency. 
     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. 2-4 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, is controlled by electronic engine controller  12  as shown in FIG.  1 . Engine  10  is coupled to exhaust manifold  30 , where burnt combustion gases (not shown) exit engine  10  through exhaust manifold  30 . Exhaust manifold  30  is coupled to first lean NOx catalyst  32 . Exhaust gases (not shown) pass through exhaust manifold  30  and then enter first catalyst  32 . First catalyst  32  is coupled to second catalyst  36  via exhaust pipe  34 . Exhaust gases exit first catalyst  32 , pass through exhaust pipe  34 , then pass through second catalyst  36 , before finally passing through exit pipe  38 . 
     Reductant is injected upstream of first catalyst  32  in exhaust manifold  30  by first reductant injector  40 . Reductant is injected upstream of second catalyst  36  in exhaust pipe  34  by second reductant injector  42 . First and second injectors  40  and  42  receive reductant via reductant pipe  44 , which is supplied by pump  50 . Pump  50  is coupled to tank  52  via second tube  54 . In a preferred embodiment, reductant is diesel fuel, and tank  52  also supplies engine  10  with diesel fuel for combustion (not shown). First and second reductant injectors  40  and  42  receive signal ird 1  and ird 2  from controller  12  respectively. 
     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  120  coupled to engine  10  and sending various signals to actuators  122 . In addition, controller  12  receives an indication of exhaust temperature (Tm) from temperature sensor  62 . Alternatively, temperature (Tm) may be estimated using various methods known to those skilled in the art. Controller  12  also sends signal fpwr to fuel injectors  20  and  22  and sends signal fpwl to fuel injectors  24  and  26 . 
     As stated above, Engine  10  receives diesel fuel for combustion in a fuel injection amount represented by signal FI. The fuel injection amount is in proportion to a driver actuated element (not shown). The fuel is injected via a convention diesel fuel injection system (not shown) with a variable start of injection time according to operating conditions, where signal SOI represents the start of injection time. 
     Referring now to FIG. 2, a routine for calculating signal ird 1  sent to reductant injector  40  is described. First, in step  210 , a base reductant demand (ird 1 b) is calculated from a predetermined function of engine speed (N) and fuel injection amount (FT). Then, in step  220 , this base value is modified by a correction factor (CF). The correction factor is a compilation of multiple corrections for various engine operating conditions, such as, for example, exhaust gas recirculation amount, engine coolant temperature, intake air temperature, and start of injection time (SOI). For example, correction factor CF can be formed from the product of the correction for each of the engine operating conditions, with correction factor equal to 1 when the operating condition is at a base condition. In a preferred embodiment, the individual correction factors range between 0 and 2. The corrected reductant demand (ird 1 c) is calculated from the product of correction factor (CF) and based reductant demand (ird 1 b). In step  230 , first injector reductant demand (ird 1 ) is calculated based on corrected reductant demand (ird 1 c) and temperature (Tm) 
     Referring now to FIG. 3, a routine for calculating signal ird 2  sent to reductant injector  42  is described. First, in step  312 , the efficiency (η 1 ) of first catalyst  32  is determined based operating conditions. In a preferred embodiment, efficiency (η 1 ) is determined from a predetermined characteristic map as a function of engine speed (N) and fuel injection amount (FI). 
     In step  314 , the first injector reductant demand (ird 1 ) from step  230  is read. In step  316 , temporary second injector demand (ird 2 t) for injector  42  is calculated based on first injector reductant demand (ird 1 ) and first catalyst efficiency (η 1 ). More specifically, temporary second injector demand (ird 2 t) for injector  42  is calculated by multiplying first injector reductant (ird 1 ) demand and unity minus first catalyst efficiency (η 1 ). Then, in step  318 , second injector reductant demand (ird 2 ) is calculated based temporary second injector demand (ird 2 t) and temperature (Tm), where function f is calibrated based on engine testing data. 
     In this way, first and second reductant demand are calculated for controlling reductant injection into catalysts  32  and  36 . By performing the calculations in this way, the reductant injection control of second catalyst  36  takes into account the operating characteristics of first catalyst  32  as well as the operating characteristics of engine  10 . For example, when first catalyst  32  is operating at high efficiency and a low reductant demand, little reductant is needed for second catalyst  36 . However, first catalyst  32  may be operating at high efficiency and high reductant demand, indicating larger reductant injection is needed for second catalyst  36 . 
     In an alternative embodiment, the temperature used in steps  230  and  318  are adjusted values of temperature (Tm). In particular, when temperature sensor  62  is located between first catalyst  32  and second catalyst  36 , the temperature value used in step  230  is an increased value of temperature (Tm) to account for heat loss and more accurately represent the temperature of first catalyst  32 . Similarly, the temperature value used in step  318  is a decreased value of temperature (Tm) to account for heat loss and more accurately represent the temperature of second catalyst  36 . The amount of increase or decrease in temperature is based on engine speed and throttle position to account for exhaust gas flow velocity. 
     Referring to FIG. 4, a routine is described for controlling injectors  40  and  42 . In step  410 , injector  40  is controlled according to first injector reductant demand (ird 1 ). Then, in step  412 , injector  42  is controlled according to second injector reductant demand (ird 2 ). 
     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. Also, other methods of adding reductant to the catalysts may be used. For example, reductant can be added by injecting fuel from cylinder fuel injectors during an exhaust stroke of the engine thereby allowing unburned hydrocarbons to enter the first catalyst. Also, the method is applicable to a single structure having multiple catalyst segments located therein, with reductant added between the segments. Further, the method is applicable where multiple parallel upstream catalysts lead a single underbody catalyst. For example, when the engine has multiple banks, each having a catalyst, that lead to a single underbody catalyst, the efficiency of both upstream catalysts is taken into account for adding reductant to the single underbody catalyst. The invention is therefore to be defined only in accordance with the following claims.