Abstract:
A control system and method for controlling an engine ( 10 ) of an automotive vehicle having a catalyst ( 34 ) is set forth herein. The control system maintains the efficiency of the catalyst by monitoring the catalyst state and driving the catalyst state to a target point. A first oxygen sensor ( 50 ) generates a first oxygen signal. A second oxygen sensor ( 52 ) downstream of the catalyst generates a second oxygen signal. A controller ( 12 ) is programmed to perform the steps of determining a catalyst state having a maximum value, a minimum value, and a target point therebetween; determining a commanded air-fuel ratio to drive the catalyst state to the target point; and operating the engine with the commanded air-fuel ratio.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to an exhaust gas control system for an internal combustion of an automotive vehicle, and more particularly, to a method and apparatus for controlling the catalyst efficiency by monitoring the state of the catalyst.  
         BACKGROUND  
         [0002]    Minimizing tailpipe emission is an objective of closed loop fuel systems. Closed loop fuel systems include a catalytic converter that is used to treat the exhaust gas of an engine. The efficiency of a catalytic converter is affected by the ratio of air to fuel supplied to the engine. At the stoichiometric ratio, catalytic conversion efficiency is high for both oxidation and reduction conversions. The air-fuel stoichiometric ratio is defined as the ratio of air to fuel which in perfect combustion would yield complete consumption of the fuel. The air-fuel ratio Lambda of an air-fuel mixture is the ratio of the amount by weight of air divided by the amount by weight of fuel to the air-fuel stoichiometric ratio. Closed loop fuel control systems are known for use in keeping the air-fuel ratio in a narrow range about the stoichiometric ratio, known as a conversion window of an emission catalyst.  
           [0003]    The difficulty with known systems is that the catalyst is very sensitive to errors in the input air fuel mixture that are less than the resolution of an upstream sensor output signal. Also, the oxygen storage capability of the catalyst can delay the response of a downstream sensor making the determination less accurate due to the time delay. One system estimates the oxygen mass stored in the catalyst by observing the amount of oxygen upstream of the catalyst and downstream of the catalyst to infer the catalyst capacity which may in turn be used to adjust the air-fuel ratio. Such systems suffer from the drawback mentioned above.  
           [0004]    Other known systems use a predicted downstream oxygen sensor voltage to predict the amount of oxygen in the exhaust gas. One problem with this and other prior known systems is that the storage capacity of the catalyst will change with temperature and catalyst age. Therefore, the calculated efficiency in such systems may not correspond to the actual efficiency of the catalyst, particularly in older systems.  
           [0005]    It would therefore be desirable to provide a method and apparatus for maximizing catalyst efficiency that takes into consideration the state of the catalyst.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a method and apparatus for controlling the operation of an engine of the automotive vehicle in response to a catalyst state rather than a calculation based on the amount of oxygen in the exhaust gas.  
           [0007]    In one aspect of the invention, a method for controlling an engine comprises determining a catalyst state having maximum value, a minimum value and a target point therebetween. The method further comprises determining a commanded air fuel ratio to drive said catalyst state to the target and operating the engine with the commanded air fuel ratio.  
           [0008]    In a further aspect of the invention, a system for controlling an engine of an automotive vehicle includes a catalyst as set forth herein. The control system maintains the efficiency of the catalyst by monitoring the catalyst state and driving the catalyst state to a target value. A first oxygen sensor generates a first oxygen signal. A second oxygen sensor downstream of the catalyst generates a second oxygen signal. A controller is programmed to perform the steps of determining a catalyst state having a maximum value, a minimum value, and a target point therebetween; determining a commanded air-fuel ratio to drive the catalyst state to the target; and operating the engine with the commanded air-fuel ratio.  
           [0009]    One advantage of the invention is that factors such as temperature and age may be used in the catalyst state determination. This results in an improved efficiency calculation that does not correspond to a particular stored oxygen mass within the catalyst.  
           [0010]    Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic view of a motor vehicle internal combustion engine together with apparatus for controlling the air-fuel ratio to the engine in accordance with the preferred embodiment of the invention.  
         [0012]    [0012]FIG. 2 is a control block diagram of the catalyst state controller of the present invention.  
         [0013]    [0013]FIG. 3 is a control block diagrammatic view of the catalyst state controller of FIG. 2.  
         [0014]    [0014]FIG. 4 is a block diagrammatic view of the integrate state block of FIG. 3.  
         [0015]    [0015]FIG. 5 is a control block diagrammatic view of the upstream reference block of FIG. 3.  
         [0016]    [0016]FIG. 6 is a control block diagrammatic view of the Lambda control block of FIG. 3.  
         [0017]    [0017]FIG. 7 is a block diagrammatic view of the catalyst capacity block of FIG. 3. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    In the following example the same reference numerals and signal names will be used to identify the respective same components and the same electrical signals in the various views.  
         [0019]    The present invention seeks to maximize catalyst efficiency based on the assumption that the catalyst is most efficient in an arbitrary state where the active catalyst sites are neither saturated with nor depleted of oxygen. Also, a high or low downstream exhaust gas oxygen sensor voltage indicates that the whole catalyst is effectively saturated rich or lean and is outside a catalyst state window. When the sensor indicate an intermediate value, both oxidation and reduction sites are available.  
         [0020]    Referring now to FIG. 1, internal combustion engine  10  is controlled by electronic controller  12 . Engine  10  has a plurality of cylinders  14 , one of which is shown. Each cylinder has a cylinder wall  16  and a piston  18  positioned therein and connected to a crankshaft  20 . A combustion chamber  22  is defined between piston  18  and cylinder wall  16 . Combustion chamber  22  communicates between intake manifold  24  and exhaust manifold  26  via a respective intake valve  28  and an exhaust valve  30 . Intake manifold  24  is also shown having fuel injector  32  coupled thereto for delivering liquid fuel in proportion to the pulse width of signal (FPW) from controller  12 . The fuel quantity together with the amount of airmass in the intake manifold  24  defines the air-fuel ratio directed into combustion chamber  22 . Those skilled in the art will also recognize that engine may be configured such that the fuel is injected directly into the cylinder of the engine in a direct injection type system.  
         [0021]    A catalyst  34  is coupled to exhaust manifold  26  through exhaust system  36  and may comprise a three-way catalytic converter. Catalyst  34  is used to reduce tail pipe emissions by performing reduction and oxidation reactions with the combustion gasses leaving cylinder  22  through exhaust valve  30 .  
         [0022]    Controller  12  is shown as a conventional microcomputer  41  including a microprocessing unit (CPU)  38 , input/output ports  40 , read-only memory  42 , random access memory  44 , and a conventional data bus  46  therebetween.  
         [0023]    Controller  12  is shown receiving various signals from sensors coupled to engine  10 . The various sensors may include a mass airflow sensor  47  used to provide an airmass signal to controller  12 . An engine speed sensor  48  is used to generate an engine speed signal corresponding to the rotational speed of the crankshaft. An exhaust gas oxygen sensor  50  positioned upstream of catalyst  34  provides a signal corresponding to the amount of oxygen in the exhaust gas prior to the catalyst. A second exhaust gas oxygen sensor  52  may be coupled to the exhaust system after catalyst  34 . Sensors  50 ,  52  may comprise an EGO sensor, an UEGO sensor, or a HEGO sensor. Catalyst  34  may also have a temperature sensor  54  coupled thereto. Catalyst temperature sensor  54  provides an operating temperature signal for the catalyst to controller  12 . Although a physical sensor  54  is illustrated, controller may also indirectly determine a temperature of the catalyst from other sensed inputs. The temperature of the catalyst may be estimated based upon the various engine operating conditions. In particular, catalyst temperature may be estimated using on a normal estimated temperature based on engine operating conditions that represent the catalyst temperature under normal conditions increased by a change in temperature based on the various operating conditions such as engine speed or load.  
         [0024]    A throttle body  56  having a throttle plate  58  and a throttle position sensor  60  is illustrated. Throttle position sensor  60  provides controller  12  with an electrical signal corresponding to the desired driver demand.  
         [0025]    Referring now to FIG. 2, a simplified block diagrammatic view of a portion of controller  12  is illustrated coupled to various blocks and illustrate various signals provided from the sensors to the controller  12 . Block  70  is the catalyst state controller which is used to generate an air-fuel ratio lambse based upon the various inputs. The air-fuel ratio lambse is used to measure a catalyst state (CatState). The catalyst state is a normalized number between 1 and −1 indicative of a drift in the downstream oxygen level. That is, when the catalyst state is between 1 and −1, both reduction and oxidation sites in the catalyst are available. At 1 and −1, neither oxidation nor reduction sites are available. Lambse is a signal that averages to unity when the engine operates at stoichiometry with no air-fuel errors or offsets. Typically, lambse ranges between 0.75 and 1.25. A commanded state output (Out 2 ) may also be provided from block  70  corresponding to the commanded state of the catalyst. Engine  12  receives the lambse signal and the airmass signal that controls the fuel injector as described above to provide an air-fuel ratio output to the engine (AirFuelOut). The combustion gasses are coupled to catalyst  54  and are output through the exhaust system at output one (Out 1 ).  
         [0026]    Inputs to block  70  include the output from exhaust gas sensor  50  upstream of catalyst  54  (UEGOLambda). The output of downstream exhaust gas sensor  52  is coupled to block  70  as DSHEGO volts. The airmass (AM) from throttle body  56  is also coupled to block  70 . The catalyst temperature (CatTemp) is also coupled to block  70 .  
         [0027]    Referring now to FIG. 3, block  70  is illustrated in further detail. As described in FIG. 2, the outputs of block  70  are the catalyst state (CatState), lambse, and commanded state. The catalyst state (CatState) is determined in the IntegrateState block  72 . The IntegrateState block  72  integrates the rate of the state change that results in an estimated catalyst state (CatState) between 1 and −1. The estimated catalyst state may actually extend beyond −1 and 1 due to estimation and measurement errors. Further details of the IntegrateState block  72  will be further described in FIG. 4 below. The IntegrateState block  72  has various inputs including airmass, lambda, lambda 1 KAM, current capacity, CatSaturated, and DSHEGOState inputs. The airmass is generated from the mass airflow sensor  47  in FIG. 1. Lambda is derived from the upstream exhaust gas sensor  50 . Lambda 1 KAM is derived in upstream reference block  74 . The current capacity of the catalyst is determined by catalyst capacity block  76 . The catalyst saturated signal (CatSaturated) and the DSHEGOState signal are provided from state limits block  78 . A lambda control block  80  receives the catalyst state output from IntegrateState block  72  and generates the CommandedState and lambse signals as will be further described below.  
         [0028]    Block  76  receives the airmass signal and the catalyst temperature signal and determines a catalyst capacity (BaseCapacity). The units of capacity may, for example, be termed as delta lambda times the pounds of air.  
         [0029]    Block  78  enables the IntegrateState in block  72  by monitoring the output of the downstream exhaust gas sensor  52  shown in FIG. 1. An Enable Integrate (Enab_Integrate) signal and a state reset value (StateResetVal) are generated. By monitoring the exhaust gas sensor  52 , the determination whether or not the catalyst is saturated rich or lean may be determined. When the voltages exceed the predetermined limits, block  78  resets the states to 1 or −1 using the state reset value. Both sensor and stoichiometric chemistry errors are compensated for in block  74 . The details of block  74  will be further described below.  
         [0030]    Block  80  is coupled to the airmass signal and current capacity signal generated from block  76 . Also, the target state is provided from a target state block  81 . In the present example, the target state is set at zero. That is, between the catalyst states 1 and −1. Target state zero corresponds to the target state which will drive the catalyst to the most efficient state. Although zero is used, various numbers could be used and the number may also be adjustable based on load or other engine operating conditions.  
         [0031]    Referring now to FIG. 4, IntegrateState block  72  of FIG. 3 is illustrated in further detail. The IntegrateState block  72  has a calculate state rate (CalcStateRate) block  82  that is used to determine the rate of change in the catalyst state. Block  82  receives lambda, lambda 1 KAM, airmass and current capacity signals as described above. The catalyst state dot (CatStateDot) is determined in block  82 . The difference between lambda and lambda 1 KAM is the catalyst input air-fuel ratio error. Thus, the change in the catalyst state is determined as a function of the catalyst input air-fuel ratio error. That is, (CatStateDot)=[am*(Lambda−Lambda 1 Kam)]/Current Capacity. The output of block  82  is coupled with the catalyst saturated signal (CatSaturated) and the DSHEGOState signal. By integrating the catalyst state dot signal an estimate of the current catalyst state (CatState) is determined in block  84 .  
         [0032]    Referring now to FIG. 5, block  74  of FIG. 3 is illustrated in further detail. A target downstream voltage block  88  is coupled to a summing block  90 . Summing block  90  is also coupled to the downstream heated exhaust gas oxygen voltage. The 0.6 value within the target downstream voltage block  88  represents the desired operating value of the downstream sensor. This value, of course, may change based upon a function of the engine operating condition such as engine speed and load.  
         [0033]    The output of the summing block  90  is the downstream voltage error. Saturation block  92  receives the downstream voltage error. Saturation block  92  is used because the downstream voltage error may be larger in one direction than the other since the target voltage may not be exactly in the center of the sensor voltage range. Saturation block  92  limits the signal maximum and minimum values to provide a symmetric output.  
         [0034]    A gain block  94  receives the downstream voltage error signal after passing through saturation block  92 . The downstream voltage gain block  94  is coupled to integral controller  96  to calculate the lambda reference. Of course, those skilled in the art would recognize that other types of controllers may be used. As an alternative, the lambda reference may also be stored in a table as a function of engine load and speed. Other values may also be coupled to integral controller  96  to allow the proper integration of the signal and enable the integration. It should also be noted that the voltage gain of block  94  may be a function of catalyst capacity.  
         [0035]    The lambda 1  reference signal generated by integral controller  96  refers to the use of one upstream sensor. In various other applications, such as a “V” style engine, multiple sensors may be used.  
         [0036]    As can be seen, a lambda reference is used to determine the measured upstream lambda that corresponds to the desired downstream heated exhaust gas oxygen voltage.  
         [0037]    Referring now to FIG. 6, the CatState is used as an input to the fuel control where it is compared to a target catalyst state. FIG. 6 illustrates block  80  of FIG. 3 in further detail. The target state and the catalyst state are provided to a summing block  104 . As mentioned above, the catalyst state is preferably between −1 and +1, while the target state is preferably a value therebetween such as zero. The difference in the signal is the catalyst state error (CatStateError) signal. This signal is provided to a proportional integrator controller (PI) having a proportional block  108  and a discrete-time integrator block  110 . Those skilled in the art will recognize that other types of controllers could be used to control the target state and the estimated catalyst state. The PI controller  106  sums the proportional signal provided by block  106  in block  112 . The sum of the proportional and integrated signal is the commanded state (CommandedState). The commanded state signal is added together with a constant from block  114  in block  116 . The current capacity, the commanded state signal, and the airmass are combined together in product block  118  to obtain lambse. That is, by multiplying the commanded state times the current capacity and dividing it by the airmass, some limits are provided on the oxygen in and out rate. The storage reaction rate is limited and the high deviation from stoichiometric will have a tendency for breaking through the predefined limit. The commanded rate may therefore be clipped or limited by constant  114 .  
         [0038]    Referring now to FIG. 7, block  76  of FIG. 3 is described in further detail. In block  76 , the capacity estimate of the catalyst is determined. It should be noted that if the catalyst estimate is in error the catalyst will not be operating at an optimum state contrary to the goals of the present invention. Various methods may be used to estimate the catalyst state. One manner would be to intrusively sweep the estimated state from one limit to the other preferably at a light engine loading condition where emissions are lowest. If the state saturates at opposite values to frequency this would be an indication that the estimated capacity is too large and thus the estimated capacity may be reduced. By compensating for the catalyst capacity, the age of the catalyst may also be taken into consideration. As the catalyst gets older, the capacity will be reduced. Thus, by determining the catalyst state having a maximum value, a minimum value, and a target point therebetween, a commanded air-fuel ratio may be determined to drive the catalyst to the target point. The engine is then operated with the commanded air-fuel ratio.