Abstract:
An engine exhaust treatment system using a control system is described. In one form of the present invention, engine exhaust gas contaminates are passed through a selective reduction catalyst and a contaminate reducing agent is injected into the catalyst. The amount of contaminates present in the exhaust proximate to the selective reduction catalyst are sensed using at least one sensor. In one embodiment, input signals are sent to a control system from the sensor or sensors and a feedforward control combined with a feedback loop is used to transform these signals and a predetermined catalyst output contaminate value into an output signal. The output signal instructs a provider to inject contaminate reducing agents in a manner to efficiently track the desired predetermined catalyst output contaminate value.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     This invention relates generally to control systems, particularly to a feedforward control system with feedback for urea Selective Catalytic Reduction regeneration.  
       BACKGROUND OF THE INVENTION  
       [0002]     Reducing the exhaust NOx emission of diesel engines has become a major challenge over the past decade and will continue to be the major focus in the future due to the continuing stringent emission requirements for diesel engines. Engines exhaust NOx reduction can be achieved by combustion optimization and/or exhaust gas aftertreatment. In reality, combustion optimization with Exhaust Gas Recirculation (EGR) can only reduce the exhaust NOx to a certain level, further NOx reduction requires exhaust gas aftertreatment. The urea Selective Catalytic Reduction (SCR) Engine Aftertreatment System (EAS) is one of the main aftertreatment technologies with a high potential for reducing NOx.  
         [0003]     The urea SCR technology is a very efficient steady state NOx reduction approach that has been successfully applied to stationary electrical generation sets powered by diesel engines with very stringent emission requirements. One of the greatest challenges in NOx reduction for SCR EAS is when the target engine exhaust NOx level reduces to a very low level. The steady state and transient control must be accurate enough to avoid ammonia (NH 3 ) slip, otherwise an alternate pollutant is released. A baseline control, basically a step control as a function of the desired NOx reduction quantity, has been developed and evaluated demonstrating poor transient performance and large steady state error.  
         [0004]     Accordingly, improved control systems are needed to (a) improve transient NOx reduction and (b) reduce ammonia slip, especially when the engine is in transition. The present invention is directed towards meeting these and many other needs.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is described solely in claims and the present section is not intended to limit or expand the scope of protection described in the claims. Some forms of the present invention include a method and a system to reduce the initial transient lag in contaminate reduction and reduce the steady state error occurring upon the injection of a contaminate reducing agent into a selective reduction catalyst.  
         [0006]     One form of the present invention is a method of first providing a selective reduction catalyst having a catalyst input and a catalyst output. A first catalyst condition is then determined at the catalyst input and a second catalyst condition is determined at the catalyst output. Next, a predetermined ideal catalyst output condition is provided Data relating to said catalyst conditions are inputted into a control system that generates an output signal utilizing a feedforward control. A feedback control updates the output signal. The updated output signal then instructs a provider to supply a contaminate reducing agent to the selective reduction catalyst.  
         [0007]     An alternate form of the present invention includes a system with an engine that produces contaminates having an output, a selective reduction catalyst having an input and an output, wherein the input is operatively coupled to the exhaust gas output, at least one sensor operatively coupled to the selective reduction catalyst, a provider for providing a contaminate reducing agent, and a control system utilizing a feedforward control with a feedback loop. The control system transforms data from the sensor into an output signal that instructs the provider to inject said contaminate reducing agent.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an illustration of a block diagram of one form of the present invention.  
         [0009]      FIG. 2  is a graph describing the decrease in NOx gas in the exhaust over time after injection according to one embodiment of the present invention.  
         [0010]      FIG. 3  is a block diagram of one embodiment of a forward model according to the present invention.  
         [0011]      FIG. 4  is a table of constants that may be used in an embodiment of the present invention.  
         [0012]      FIG. 5  is a schematic diagram of another embodiment of the present invention.  
         [0013]      FIG. 6  is a block diagram of a feedforward control that may be used in one embodiment of the present invention.  
         [0014]      FIG. 7  is a graph describing the transient NOx reduction error and the transient response time in one embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations, modifications, and further applications of the principles of the present invention as illustrated herein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0016]     Referring now to the drawings, one embodiment of the present invention is described in  FIG. 1 . Control system  20  includes feedforward control  21  and feedback control  22 . Inputs  23  enter control system  20  and are transformed into an output  24 . In this embodiment, the inputs may relate to present operating conditions of the engine, exhaust gas, catalyst or injection systems. The output may be a signal to the injection systems, or the pump supplying the NOx reduction agent.  
         [0017]     Feedforward control  21  includes a forward model  25  and a controller  26 . Forward model  25  is a mathematical model that approximates how quickly the decrease in NOx in the exhaust gas occurs after the injection of a NOx reducing agent such as urea. Preferred embodiments of these mathematical models are described in more detail hereinbelow with respect to  FIG. 3 . Controller  26  is preferably a proportional-plus-integral (PI) controller. A PI controller is a mathematical model to help reduce the steady state error of a system. This mathematical model is described in more detail hereinbelow with respect to  FIG. 6 . Signal  27  enters controller  26  and is transformed by controller  26  to produce signal  28 . Signal  28  splits at point  29  and branch signal  30  enters forward model  25 . Forward model  25  transforms branch signal  30  into signal  31 . Signal  31  is combined with signal  43  at junction  32  to reproduce a new signal  27 .  
         [0018]     Feedback control  22  includes a feedback controller  33 . Feedback controller  33  is a mathematical model to transform control signal  34  to produce a feedback signal  35 . Control signal  34  is preferably formed by the combination of two of the three inputs  23  to control system  20 .  
         [0019]     In one embodiment, three input signals provide data to be processed by control system  20 . In this embodiment, input signal  36  provides data concerning the amount of NOx present in the exhaust gas at the catalyst input. Also in this embodiment, input signal  37  provides a value indicative of the desired NOx to be present in the exhaust gas at the catalyst output. Input signal  37  splits at connection point  38  into first branch signal  39  and second branch signal  40 . First branch signal  39  is combined with input signal  36  at junction  42 . This combination of first branch signal  39  and input signal  36  provides the data necessary to produce a signal  43  that references the amount of NOx reduction necessary based on differences between the amount of NOx present in the exhaust line and the desired amount. Second branch signal  40  is combined with input signal  41  at junction  44 . The third signal of this embodiment, input signal  41 , provides the amount of NOx present in the exhaust gas at the catalyst output. The combination of second branch signal  40  and input signal  41  provides the data necessary to form control signal  34  representing the difference between the amount of NOx exiting the catalyst and the desired amount of NOx to exit the catalyst. Control signal  34  is transformed by feedback controller  33  to produce feedback signal  35 . Feedback signal  35  is combined with signal  28  at junction  45  to produce output signal  46 . In this embodiment, output signal  46  controls the injection of urea by the contaminate reducing agent injecting system.  
         [0020]     The operation of control system  20 , according to the present embodiment is as follows. In this embodiment, control system  20  operates to produce an output  24  based on inputs  23 . Input signal  36  and input signal  37  are combined at junction  42 . Input signal  36  provides the NOx present at the inlet of the catalyst through the use of a NOx sensor or other NOx sensing apparatus commonly known to one skilled in the art. Input signal  37  is the amount of NOx desired to exit the outlet of the catalyst to be treated using the SCR EAS system. This difference is used to determine the amount of NOx that must be reduced by the catalyst. This information is provided to feedforward control  21  via signal  43 .  
         [0021]     The data provided by signal  43  does not accurately account for the true amount of NOx reducing agent to be injected to reduce the level of NOx referenced by signal  43 . Upon injection of the NOx reducing agent, the amount of NOx gas present in the exhaust does not instantaneously reduce to the target NOx level, achieving the target takes time. In this embodiment, feedforward control  21  therefore corrects signal  43  to accommodate for the failure of the NOx gas to instantaneously move toward the target NOx level. Moreover, in this embodiment the feedforward control  21  also corrects for the potential to over or under shoot the desired level. The more detailed complexities of feedforward control  21  are discussed hereinbelow with reference to  FIG. 6 . Signal  43  is corrected and signal  28  is output to junction  45 .  
         [0022]     A feedback signal  35  is also provided to junction  45  to account for the present effectiveness of the catalyst or the NOx reducing agent injection to reduce the NOx gas present in the exhaust. Feedback signal  35  is developed by input signal  37  and input signal  41  combining at junction  44  to produce control signal  34 . Input signal  37  provides data concerning the desired target NOx amount and input signal  41  provides data concerning the amount of NOx present at the output of the catalyst. This difference informs control system  20  how effective the catalyst or the injected NOx reducing agent is in achieving the desired target NOx amount. This data is provided to feedback controller  33  via control signal  34 . Feedback controller  33  transforms control signal  34  into feedback signal  35 . Feedback signal  35  is combined with signal  28  at junction  45 . This combination updates signal  28  to accommodate for present effectiveness of the catalyst and/or injection of the NOx reducing agent to push the level of NOx in the exhaust to an acceptable level. In this embodiment, output signal  46  then instructs the NOx reducing agent injection system to inject an amount of NOx reducing agent to move the amount of NOx gas towards the target NOx amount quickly and effectively.  
         [0023]     Referring now to  FIG. 2 , the development of the feedforward control  21  of one embodiment of the present invention is described in more detail.  FIG. 2  displays tests that were performed to determine the transient catalyst output NOx based on different control systems and pumps. The data provided by  FIG. 2  was used to create a mathematical model to approximate the behavior of the level of NOx gas present at the catalyst output. The system dynamics can be appropriately fitted as a time delay from a urea injection point to catalyst output plus a first order dynamics.  
         [0024]     Referring now to  FIG. 3 , a schematic diagram is provided to describe the creation of the mathematical model of the present embodiment. Signal  50  represents a NOx reducing agent, such as urea, injection into system  49 . Upon injection, the decay of the NOx gas present in the exhaust indicated in  FIG. 2  as beginning at Time=20 is appropriately fitted by a first order mathematical system represented by block  51 . This mathematical system is defined as: 
 
e −STd  
 
 where T d  is the dead time from urea injection point to catalyst output point. This equation is inverted through the use of a Laplace transform to arrive forward model  52 . Forward model  52  may be the forward model  25  described in the embodiment described in  FIG. 1 , but forward model  25  is not intended to be limited by forward model  52 . The inverse of the mathematical system of block  51  provides the equation as depicted in block  52 :  
         P   ⁡     (   s   )       =         α   ⁢           ⁢   s     +   31.12         β   ⁢           ⁢   S     +   1           
 
 where α and β are coefficients of the first-order lead/lag filter. T d  and β are functions of exhuast flow rate and α is set to be either zero when no emission control is required under low exhaust flow rate or a constant. The higher the constant value of α is selected, the more conservative the control. As described in block  52 , in one embodiment, the DC gain is 31.12. For this embodiment two assumptions were made to achieve this value of DC gain: (a) a typical NO to NO 2  ratio in the turbo outlet exhaust is 9:1 and the stoichiometric urea requirement per unit NOx mass is 0.67 gram urea per NOx gram; and (b) the urea solution mass concentration is 32 percent and urea solution density is 1.086 grams/cc. By simple unit conversion and calculation, 1 cc per minute urea solutions eliminate 31.12 grams per hour of NOx by chemical reaction at a steady state. These values and assumptions are for the present embodiment described by system  49  but various other assumptions, mathematical models, and operating parameters may be used in accordance with the present invention. Block  52  thereby produces a signal  53  to provide data relating to the amount of NOx reduced. 
 
         [0025]     Referring now to  FIG. 4 , a table describing different values for α and β in relation to different exhaust flow rates in lbs/hr depicting various values that may be used in differing embodiments of the present invention.  
         [0026]     Referring now to  FIG. 5 , a physical embodiment showing the engine system that may be used in accordance with the present invention is described. An engine  55  having an output  56  produces exhaust gas containing contaminates. Selective reduction catalyst  57  includes an input  58  and an output  59 . This embodiment also includes a supply tank  60  containing a contaminant reducing agent including an injector  61  and a supply line  62  to supply the reducing agent to the catalyst input  58 . Preferably, injector  61  atomizes the contaminant reducing agent upon injection. A pump (not shown) may be used to move the contaminant reducing agent through the supply line. A sensor  63  determines the amount of contaminant present in the exhaust gas at the catalyst input  58  and a sensor  64  determines the amount of contaminant in the exhaust gas present at catalyst output  59 . Controller  65  receives input from sensors  63  and  64  and outputs signals to reducing agent supply tank  60  in order to inject contaminate reducing agent via injector  61  into the exhaust gas entering catalyst  57  at input  58 . This configuration describes one embodiment of the present invention; however, the present invention contemplates differing sensor positions and injection areas while still falling under the invention as claimed. Moreover, alternate physical systems may be used in accordance with the present invention.  
         [0027]     Referring now to  FIG. 6 , one potential block diagram describing a mathematical model for one embodiment of the feedforward control  21  is described. Other block diagrams and mathematical models of feedforward control  21  known to those skilled in the art may also be used in accordance with the present invention. One embodiment of feedforward control  21  is described as a feedback inverse dynamic control  69  in  FIG. 6 . Feedback inverse dynamic control  69  includes a forward model represented by block  70  and a controller represented by block  71 . Feedback inverse dynamic control  69  operates by first receiving an input signal  72 . Input signal  72  is combined with signal  73  at junction  74 . Signal  73 , in one embodiment of the present invention, may relate to the correction for the non-linear aspects of the reduction of NOx gas in the exhaust upon injection. The combination of signal  72  and signal  73  results in signal  75 .  
         [0028]     Signal  75  is then transformed by controller  71  using a mathematical function described in  FIG. 6  as G(s). In one embodiment of the present invention, the function G(s) can be represented mathematically as  
           G   ⁡     (   s   )       =       k   p     +       k   i     s         ,       
 
 where k p  and k i  are proportional and integral gains, respectively. Controller  71  then transforms signal  75  into output signal  76 . Output signal  76  is split at connection point  77  into a branch signal  78  to be input into forward model  70 . Output signal  76  also instructs the NOx reducing agent injection system to inject. The combination of the controller  71  and forward model  70  results in a feedback inverse dynamic control  69 . In one embodiment of the present invention feedback inverse dynamic control  69  may be the mathematical model used in feedforward control  21  of  FIG. 1 . Feedback inverse dynamic control  69  may be represented mathematically as C(s) with the following equation:  
           C   ⁡     (   s   )       =         G   ⁡     (   s   )         1   +       G   ⁡     (   s   )       ⁢     P   ⁡     (   s   )             ≅       1     P   ⁡     (   s   )         ⁢           ⁢   as   ⁢           ⁢     G   ⁡     (   s   )       ⁢     P   ⁡     (   s   )             &gt;&gt;   1       
 
       EXAMPLE NO. 1  
       [0029]     The first simulation was performed to determine the effect of this combination of the feedforward control and the feedback control and its effect on improving the transient and steady state response of the NOx reduction upon injection of the urea.  FIG. 7  describes a transient performance index. The graph provided is a function of time and NOx at the catalyst output in (g/hr) and it shows the decay in the amount of NOx exiting the catalyst upon the injection of urea at 15 seconds.  FIG. 7  defines the transient response time as the amount of time that passes from the injection of the urea until the 90 percent of the NOx reduction to target occurs. The target NOx in this case is below 200 (g/hr) shown by the horizontally orientated dotted line. Also,  FIG. 7  defines the transient NOx reduction error. The transient NOx reduction error is the integration of the NOx amount over the target during the transient response time.  
         [0030]     Three control system simulations were run at various engine speeds. One control used a simple step function. Another control used an inverse control. The last control used a feedforward control with feedback. The first simulation was performed using an engine speed of 1500 revolutions per minute (rpm) and torque at 292 pounds per feet (lb.-ft). The second simulation case was performed at the speed of 2000 revolutions per minute and a torque of 381 lb.-ft. The third simulation run utilized an engine speed of 2500 rpm and torque at 303 lb.-ft. The results of the experimentations showed the feedforward control with feedback averaging a 47.5 percent improvement in the quickness of reaching the target over base-line (step function) control. The inverse control showed an average of 30.5 percent improvement in the quickness of reaching the target over the base-line control. Therefore, the feedforward control with feedback shows the best performance among the three control systems used.  
         [0031]     Moreover, the transient NOx reduction error was also shown to be reduced more effectively by the feedforward control with feedback. The feedforward control with feedback averaged a 44 percent improvement over the step control. The inverse control only averaged a 32.2 percent improvement over the step control. Overall, the feedforward control with feedback exhibited the best transient performance over the step control.  
       Example 2  
       [0032]     In this test, the control system of one embodiment of the present invention was performed for the simulation of a real excavator. The excavator was run at 800 rpms, then shifted to 2000 rpms, and then returned to 800 rpms to simulate a real operating condition where the excavator idles and works. The total NOx reduction error by the step control over the test cycle was 6.88 grams NOx and the total NOx reduction error by the feedforward control with feedback was 5.27 grams. Thus, the feedforward control with feedback showed a 23.36 percent improvement over the step control in the ability to reduce NOx during the transient response.  
         [0033]     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only a few embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.