Abstract:
A method and system for controlling exhaust emissions from an engine of a motor vehicle includes sensing oxygen levels upstream and downstream of a catalytic converter, predicting an instantaneous oxygen storage amount in the catalytic converter, determining a maximum oxygen storage capacity, selecting a target percentage of the maximum oxygen storage amount, and controlling the motor vehicle engine performance to a state where the oxygen storage amount is approximately the target percentage of the maximum oxygen storage amount. The instantaneous oxygen storage amount is determined from an oxygen storage mass flow rate, which is determined from a converter-in mass flow rate, converter-out mass flow rate, and a predicted oxygen consumption mass flow rate. The converter-in and converter-out mass flow rates are calculated from an upstream and downstream oxygen mass fraction, respectively, based on the sensed upstream and downstream oxygen levels, respectively.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to vehicle engine control, and more particularly, to a catalytic converter control system for oxygen storage management and control. 
     BACKGROUND OF THE INVENTION 
     Increasingly stringent federal and state motor vehicle emissions standards require that specific emissions-related systems on a motor vehicle be controlled and optimized. These systems must be functioning as intended over the life of the vehicle, and if the systems have deteriorated or lose their functionality, the vehicle operator must be informed and the system repaired. For example, a catalytic converter of a motor vehicle is monitored because of its ability to reduce undesirable emissions in exhaust gases from the engine of the motor vehicle. 
     The performance of the catalytic converter depends upon the chemical compositions of the exhaust gases from the engine of the motor vehicle. Maintaining the feed-gas concentration to the catalytic converter close to stoichiometry maximizes catalytic converter efficiency. The oxygen storage capability of the catalytic converter determines the functional performance of the converter, which may also deteriorate over time due to factors such as engine misfire, a faulty oxygen sensor, poisoning or prolonged high-temperature operation. Such deteriorzatioin results in diminished capability to store the oxygen available in the exhaust gases. Active management and control of the amount of oxygen stored in the catalyst during motor vehicle operation helps lower pollutants from motor vehicle emissions. 
     Three-way catalytic converters are designed to have oxygen storage capability to improve their conversion efficiency. Oxygen storage/release is carried out by the precious-metal-assisted transition between Ce 3+  and Ce 4+  of Ceria compound added to the washcoat of the catalyst. The major storage/release reactions are shown below.                           
     Current engines have an on-board control system that applies one or more oxygen sensor outputs to control fuel/air flow rates. Emission control is accomplished by increasing catalyst loading. This results in adding more precious metal particles, thereby increasing the overall volume and cost of the catalytic converter. 
     A major drawback of current engine systems are that no known current engines employ active management of oxygen storage amount or oxygen storage capacity. Knowing the instantaneous oxygen storage amount is key to emission control. A further drawback is that catalysts can be saturated when too much oxygen is coming high-temperature exposure and poisoning due to the decreased surface area of the Ceria and the precious metal particles. 
     Accordingly, active oxygen storage management and control would improve emission control. An engine capable of predicting instantaneous oxygen storage amount is desired to overcome emission breakthroughs, increase catalytic converter efficiency, and generally reduce the overall size and cost of the catalytic converter. 
     SUMMARY OF THE INVENTION 
     The active oxygen storage management and control method and system according to the invention include sensing oxygen levels upstream and downstream of a catalytic converter for a fuel-injected engine of a motor vehicle. An engine control system predicts an oxygen consumption mass flow rate, and then determines an oxygen storage mass flow rate based on the sensed upstream and downstream oxygen levels and the predicted oxygen consumption mass flow rate. The oxygen storage mass flow rate is used to determine an instantaneous oxygen storage amount. 
     To determine the oxygen storage mass flow rate, the upstream and downstream oxygen levels are sensed and used to calculate upstream and downstream oxygen mass fractions, which are then used to determine a converter-in oxygen mass flow rate and a converter-out oxygen mass flow rate. Thus, the oxygen mass flow rate is preferably determined from converter-in mass flow rate, converter-out mass flow rate, and the predicted oxygen consumption mass flow rate. 
     Further, the upstream and downstream oxygen levels are preferably used to determine an upstream and downstream oxygen flow rate by determining an upstream and downstream lambda. In this manner, the determination of the upstream and downstream oxygen mass fractions are achieved by using the upstream or downstream lambda, respectively, as well as a set of reaction constants and a reaction fraction. 
     The method according to the invention is used to control exhaust emissions from a motor vehicle by predicting an instantaneous oxygen storage amount in the catalytic converter, determining a maximum oxygen storage capacity, and selecting a target percentage of the maximum oxygen storage amount. The motor vehicle engine performance is controlled so that the instantaneous oxygen storage amount is approximately the target percentage of the maximum oxygen storage amount. To accomplish this control, the instantaneous oxygen storage amount and the maximum oxygen storage amount are calculated as discussed above. 
     An engine control system according to the invention disposes oxygen sensors upstream and downstream from a catalytic converter. The engine control system monitors engine operating parameters including an output signal on the upstream and downstream oxygen sensors, determines an instantaneous oxygen storage amount based on the monitored sensor output signals, and controls engine operation to maintain the determined instantaneous oxygen storage amount in a predicted oxygen storage capacity. The engine control system monitors a plurality of engine control terms including a target instantaneous oxygen storage amount selected within a range from zero oxygen storage capacity to about a predicted maximum oxygen storage capacity. 
     The engine control system controls engine operation to maintain the instantaneous oxygen storage amount at approximately the target instantaneous oxygen storage amount. A plurality of fuel injectors receive a control signal from the engine control system to supply fuel to the engine at a rate where the instantaneous oxygen storage amount is approximately the target instantaneous oxygen storage amount. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of an emission control system according to the present invention for catalytic converter control; 
     FIG. 2 is an algorithm block diagram illustrating a method according to the present invention for catalytic converter control; and 
     FIG. 3 is a graph representing how instantaneous oxygen storage changes over time within the maximum oxygen storage capacity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to FIG. 1, an emission control system  10  for a motor vehicle (not shown) is illustrated. The emission control system  10  includes an engine  12  and an engine controller  14  in communication with the engine  12 . The engine controller  14  includes a microprocessing unit  13 , memory  15 , inputs  16 , outputs  18 , communication lines and other hardware and software (not shown but known in the art) necessary to control the engine  12  and related tasks. The engine controller  14  may control tasks such as maintaining a fuel-to-air ratio, spark timing, exhaust-gas recirculation and on-board diagnostics. The emission control system  10  may also include other sensors, transducers or the like that are in communication with the engine controller  14  through the inputs  16  and outputs  18  to further carry out a method according to the present invention as described below. 
     The emission control system  10  also includes at least one fuel injector  20 , and preferably a plurality of fuel injectors  20 , which receive a signal from the engine controller  14  to precisely meter an amount of fuel to the engine  12 . As a result of the combustion process that takes place in the engine  12 , exhaust gasses are created and passed out of the engine  12 . Constituents of the exhaust gas include hydrocarbons, carbon monoxide and oxides of nitrogen, which are generally believed to have a potentially detrimental effect on air quality. 
     The emission control system  10  includes a catalytic converter  22  for receiving the exhaust gas from the engine  12 . The catalytic converter  22  contains material that serves as a catalyst to reduce or oxidize the components of the exhaust gas into harmless gasses. The emission control system  10  includes an exhaust pipe  24  connected to the catalytic converter  22  and to the atmosphere. 
     The emission control system  10  further includes an upstream oxygen sensor  26  and downstream oxygen sensor  28 , each of which measure the level of oxygen in the exhaust gas. The upstream oxygen sensor  26  is positioned in front or upstream of the catalytic converter  22 . Similarly, the downstream oxygen sensor  28  is positioned after or downstream of the catalytic converter  22 . It should be appreciated that as part of the emission control system  10 , the oxygen sensors  26 ,  28  are in communication with the engine controller  14 . 
     Referring to FIG. 2, an algorithm block diagram  30  illustrating the computational process of the present invention is described. Input module  32  receives conventional control terms such as engine speed, engine load, and λ values from upstream and downstream O 2  sensors  26 ,  28 . Input vector  32  distributes upstream λ, downstream λ, fuel composition, and engine operating condition variables to modules  34  and  40  to calculate converter-in and converter-out O 2  mass fraction and a predicted O 2  consumption mass flow rate respectively. The output of module  34  is then used to calculate converter-in O 2  mass flow rate in module  36  and converter-out O 2  mass flow rate in module  38 . Subtracting the output of modules  38  and  40  from the output of module  36  yields the O 2  storage mass flow rate in module  42 . 
     Module  44  represents the integration calculation of the output of module  42 , which is provided to module  46  for calculating the net O 2  storage amount. Modules  48 ,  50 , and  52  are control algorithms while module  46  provides an extra control term for the fuel control algorithm module  48 , On Board Diagnostic (OBD) algorithm module  50 , and the fuel cutoff algorithm module  52 . The output of module  48  is fed back into the integrator module  44  to adjust fuel control to meet target operation. The control algorithm outputs of modules  46 ,  50 , and  52  are distributed by the output module  54  for incorporation into overall engine control. 
     The following equations describe the detailed calculations illustrated in FIG.  2 : 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 {dot over (m)} = {dot over (m)} 1  − {dot over (m)} 2  − {dot over (m)} 3   
                 (5) 
               
               
                 {dot over (m)} 1  = v 1*{dot over (m)}   4   
                 (6) 
               
               
                 {dot over (m)} 2  = v 2*{dot over (m)}   4   
                 (7) 
               
               
                 {dot over (m)} 3  =f(λ 1 , {dot over (m)} 4 , T 1 , RPM, MAP, i) 
                 (8) 
               
               
                 v 1  = [a 1 *(1 + b*y)(λ 1  − x)]/[(a 2  + a 3 *y) + a 4 *(1 + b*y)λ 1 ] 
                 (9) 
               
               
                 v 2  = [a 1 *(1 + b*y)(λ 2  − x)]/[(a 2  + a 3 *y) + a 4 *(1 + b*y)λ 2 ] 
                 (10) 
               
               
                 x = f(y, RPM, MAP) 
                 (11) 
               
               
                 y = hydrogen to carbon ratio of the fuel 
                 (12) 
               
               
                 O 2str  = ∫mdt = ∫(m i  − m 2  − m 3 )dt 
                 (13) 
               
               
                 OSC = f(λ 1 , T 1 , RPM, MAP) 
                 (14) 
               
               
                   
               
             
          
         
       
     
     where {dot over (m)} is the O 2  storage mass flow rate; {dot over (m)} 1  is the converter-in O 2  mass flow rate; {dot over (m)} 2  is the converter-out O 2  mass flow rate; {dot over (m)} 3  is the O 2  consumption rate inside the converter; {dot over (m)} 4  is the total exhaust mass flow rate; v 1  is the upstream O 2  mass fraction; 
     It should be noted that equation (7) represents the best mode of the invention as practiced by the inventor. The total exhaust mass flow rate at the converter outlet will actually be slightly less than at the converter inlet since a mass flow of oxygen will have been stored within the catalyst. In practice, for the purposes of equation (7) and those dependent upon it, the inventor considers this mass of stored oxygen to be negligible when compared to the total exhaust mass flow rate at the converter outlet. 
     Referring to equations (9) and (10), O 2  mass fraction is modeled. Constants a 1 , a 2 , a 3 , a 4,  and b are defined as: a1 =molecular weight of O 2 ; a 2 =atomic weight of carbon; a 3  =atomic weight of hydrogen; a 4 =molecular weight of O 2  +(N 2  to O 2  ratio in air)*molecular weight of N 2 ; and b=¼ derived from the stoichiometry of the complete combustion reaction. The complete combustion reaction is: 
     
       
         CH y +(1+y/4)O 2 →CO 2 +y/2H 2 O. 
       
     
     Upstream and downstream λ are represented by λ 1  and λ 2  respectively. At the optimum stoichiometric point, λ=1.0. The O 2  sensor is designed and calibrated to respond to differing levels of O 2  generated during combustion. Using such a sensor, it can be determined whether the air-to-fuel mixture is “rich” (not enough air for the amount of fuel; generally λ&lt;1.0) or “lean” (excess air for the amount of fuel; generally λ&gt;1.0). 
     During operation of a vehicle, an output voltage is based on sensor calibration and the level of O 2  detected. One use of the sensor is as an on/off switch. That is, if the output is above some predetermined target voltage, the air-to-fuel mixture is rich and if it is below the target voltage, the mixture is lean. Another use involves processing the actual sensor output through a closed-loop feedback-control system, which compares sensor output to a target value, generates an error, and then develops a correction factor for upcoming combustion cycles. Both applications use O 2  sensor output to adjust the amount of fuel used for subsequent combustion cycles, thereby attempting to achieve a stoichiometric air-to-fuel ratio. The conventional way to adjust the amount of fuel is by lengthening or shortening the time pulse of the fuel injectors. 
     The equations listed above correspond to the modules illustrated in FIG.  2 : 
     Module  32 →λ 1 , λ 2 , {dot over (m)} 4 , T 1 , RPM, MAP, i, x, y. 
     Module  34 →equation (9) and (10) 
     Module  36 →equation (6) 
     Module  38 →equation (7) 
     Module  40 →equation (8) 
     Module  42 →equation (5) 
     Module  44 →equation (13). 
     A preferred embodiment of the present invention includes a method of predicting the instantaneous oxygen storage amount (O 2str ) and the maximum oxygen storage capacity (OSC). With this method, the O 2str  can be controlled within a calibratable band to maximize the catalyst conversion efficiency with a minimum volume of the converter, thus preventing any transient NOx, CO, and hydrocarbon (HC) breakthroughs. Furthermore, the O 2str  and OSC may also be used as OBD, and provide smarter fuel cutoff. The present invention also provides cost savings in precious metal loading. 
     The OSC is determined based on O 2str  predictions. When downstream O 2  breakthrough occurs, an algorithm is triggered to determine whether it is caused by catalyst saturation or by a sharp lean spike. The OSC is updated when the downstream breakthrough is the result of catalyst saturation, which is used to determine when an OBD alarm should be triggered. 
     Fuel enrichment and lean-out air-to-fuel ratio are triggered based on the estimated O 2str  to clean up excess oxygen or replenish oxygen so that the amount of oxygen stored can be controlled within the ideal range to prevent NO x , CO, or HC breakthroughs. 
     The OSC, which can be used to monitor catalyst deterioration, is estimated based on λ 1 , T 1 , RPM, and MAP. When the maximum OSC is detected to reach the point at which the catalyst conversion efficiency is below a designated threshold, an OBD alarm will be triggered. 
     Referring to FIG. 3, a graph  60  representing how O 2str  without active control changes over time within the OSC is illustrated. Time is measured on the horizontal axis and mass of O 2  is measured vertically. Line  62  represents the predicted OSC. The OSC gets smaller over time as the catalyst deteriorates and ages. Target operation  64  is calibrated as a percentage of OSC. Therefore, over time, as the catalyst ages and the OSC decreases, the target value will be adapted, preferably within capacity. Target-hysterisis  66  defines deviation from target amount  64  in which the extra feedback term to the overall engine control is set to zero or is running at optimum condition. Target-hysterisis  66  represents the optimum O 2str  range during vehicle operation. The control objective is to maintain the O 2str  within target-hysterisis  66 . 
     Trace line  72  illustrates the path in which O 2str  changes over time of vehicle operation. When the O 2str  is above target-hysterisis  66  and below upper control limit  68 , the catalyst has too much O 2  stored and excess O 2  needs to be “cleaned up,” i.e., removed. This is accomplished by adding more fuel, commonly known as “enrichment.” Alternatively, if the O 2str  is below target-hysterisis  66  and above lower control limit  70 , the catalyst has too little O 2  stored and O 2  must be replenished in the engine system. This is accomplished by adding more air (which includes O 2 ), commonly known as “lean out.” If O 2str  reaches above upper control limit  68  or below lower control limit  70 , the engine control will respond more aggressively through enrichment or lean out. Upon resuming the supply of fuel after deceleration-fuel-cut-off, fuel enrichment will be conducted based on OSC to remove excess. O 2 , and thus prevent NO x  breakthrough. 
     Direct measurements of O 2  flowing into and out of the converter  22  and the prediction of the O 2  consumption rate determine the O 2str . The method and system according to the invention computes a reasonable amount of chemical reaction data and is implemented for instantaneous on-board control purposes. This method and system may be implemented into any on-board vehicle control unit without incorporating any new hardware or adding new parts to the vehicle. The inventive method and system generally adds an additional feedback control term to existing PID control. More particularly, the total O 2str  is controlled based on OSC via fueling modifications. Different fueling strategies are used based on the difference between the O 2str  and the oxygen storage control target. The feature outputs a number of control terms, which will be added to a conventional O 2 -feedback fuel control. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.