Abstract:
A method of determining a goal voltage for a fuel/air sensor of an engine electronic fuel injection system includes the steps of determining a goal fuel/air sensor voltage, superimposing a wave form forcing function to the fuel/air sensor voltage for providing a goal fuel/air sensor voltage having a wave form pattern and controlling the engine to operate according to the goal fuel/air sensor voltage. The wave form forcing function provides the required fuel/air perturbations that are required to retain proper oxygen storage of the catalyst to maintain high three-way conversion efficiency.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electronic fuel injection systems for internal combustion engines in automotive vehicles and, more particularly, to a method of feedback control for an electronic fuel injection system in an internal combustion engine for an automotive vehicle. 
     BACKGROUND 
     Modern automotive vehicles have an exhaust system which includes a three-way catalyst to simultaneously reduce HC, CO and NO x  emissions from an internal combustion engine in the vehicle if the fuel/air ratio of the feed gas to the engine is maintained within a narrow window. To accomplish this, automotive vehicles have used an O 2  sensor located upstream of the catalyst for fuel/air feedback control. 
     With the current  0   2  sensor for feedback control, a voltage output signal of the O 2  sensor is compared to a calibratible voltage threshold to determine if the fuel/air ratio is rich or lean. When the voltage output signal is determined to switch from lean to rich (for example, to go from below to above the O 2  sensor switch point calibration), an O 2  controller kicks lean and begins to ramp lean until the O 2  sensor voltage output signal changes from rich to lean. Then, the O 2  controller kicks rich and begins to ramp rich until the O 2  sensor voltage output signal changes again from lean to rich. 
     While the use of the current O 2  sensor has worked well, the O 2  sensor is subject to both short and long term errors that affect fuel/air control. The short term errors are due to shifts in the O 2  sensor voltage output signal based on exhaust gas temperature and composition. The long term errors are due to aging of the sensor as a result of sustained high exhaust gas temperatures and to potentially poisonous exhaust emissions. These factors can lead to a slowed O 2  sensor response and a shift in the voltage of the output signal relative to the fuel/air ratio with time. 
     SUMMARY OF THE INVENTION 
     It is, therefore, one object of the present invention to provide an upstream fuel/air sensor and a downstream O 2  sensor for fuel/air feedback control to improve catalyst efficiency and reduce exhaust emissions. 
     It is another object of the present invention to provide a method of electronic fuel injection feedback control based on the use of a fuel/air sensor upstream of the catalyst and an O 2  sensor downstream of the catalyst. (Although the primary object of the present invention is exploiting the use of an upstream fuel/air sensor, the scope of the invention can also include other sensors such as a typical upstream oxygen sensor.) 
     To achieve the foregoing objects, the present invention provides a method of determining a goal voltage for a fuel/air sensor of an engine control system comprising the steps of determining an optimal fuel/air ratio for the current vehicle operating conditions; determining a fuel/air sensor target voltage corresponding with said optimal fuel/air ratio; applying a wave form forcing function to said fuel/air sensor voltage for providing a goal fuel/air sensor voltage having a wave form pattern; and controlling said engine to operate according to said goal fuel/air sensor voltage. 
     In order to obtain a high level of catalyst efficiency through the fuel/air control via a fuel/air sensor, a wave form pattern goal voltage is utilized according to the present invention. Catalysts require fuel/air perturbations to retain proper oxygen storage to maintain high efficiency. Fuel/air sensor output signals are relatively flat as opposed to the characteristics of an oxygen sensor signal, which is normally vertical at stoichiometric. Thus, a swing through the stoichiometric fuel/air mixture level becomes more difficult with a fuel/air sensor when locking on to a goal voltage. The use of a wave form pattern goal voltage insures that there are periodic fuel/air perturbations to apply a forcing function to cause the fuel/air ratio to go from rich to lean periodically. 
     The wave form forcing function can be a sine wave, a square wave, or V wave, or other wave forms. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     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 schematic diagram of an electronic fuel injection system, according to the present invention, illustrated in operational relationship with an internal combustion engine and exhaust system of an automotive vehicle; and 
     FIG. 2 is a flowchart of a method of feedback control, according to the present invention, for the electronic fuel injection system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an electronic fuel injection system  10 , according to the present invention, is illustrated in operational relationship with an internal combustion engine  12  in an exhaust system  14  of an automotive vehicle (not shown). The exhaust system  14  includes an exhaust manifold  16  connected to the engine  12  and a catalyst  18  such as a catalytic converter connected by an upstream conduit  20  to the exhaust manifold  16 . The exhaust system  14  also includes a downstream conduit  22  connected to the catalyst  18  and extending downstream to a muffler (not shown). The engine  12  includes an intake manifold  24  connected thereto and a throttle body  26  connected to the intake manifold  24 . The engine  12  includes an air filter  28  connected by a conduit  29  to the throttle body  26 . It should be appreciated that the engine  12  and exhaust system  14  are conventional and known in the art. 
     The electronic fuel injection system  10  includes an engine controller  30  having fuel injector outputs  31  connected to corresponding fuel injectors  32  of the engine  12  which meter an amount of fuel to the cylinders (not shown) of the engine  12 . The electronic fuel injection system  10  also includes a throttle position sensor  34  connected to the throttle body  26  and the engine controller  30  to sense an angular position of the throttle plate (not shown) in the throttle body  26 . The electronic fuel injection system includes a manifold absolute pressure (MAP) sensor and/or mass airflow sensor (MAF)  36  connected to the intake manifold  24  and the engine controller  30  to sense MAP and/or MAF. The electronic fuel injection system  10  also includes a coolant temperature sensor  38  connected to the engine  12  and the engine controller  30  to sense a temperature of the engine  12 . The electronic fuel injection system  10  further includes an upstream fuel/air sensor  40  connected to the upstream conduit  20  of the exhaust system  14  and a downstream O 2  sensor  42  connected to the downstream conduit  22  of the exhaust system  14 . The front fuel/air sensor  40  and the rear O 2  sensor  42  are connected to the engine controller  30  to sense the uncatalized fuel/air and the fully catalized O 2  levels, respectively, in the exhaust gas from the engine  12 . 
     It should be appreciated that the engine controller  30  and sensors  34 ,  36 ,  38  and  42  are conventional and known in the art. Less known is the fuel/air sensor  40  which is a wide range fuel/air sensor. This sensor enables measurement of all ranges of the fuel/air mixture, but it can also detect the stoichiometric point precisely. The output of a wide range fuel/air sensor is an oxygen pumping current that is proportional to the amount of oxygen in the exhaust gas on the lean side (range) and the amount of oxygen required for complete combustion in the exhaust gas on the rich side (range). At stoichiometric, when the oxygen partial pressure of the exhaust gas and that in the detecting cavity is the same, oxygen pumping is not accomplished and the pumping current is always equal to zero. 
     The fuel/air sensor  40  is described in SAE paper number 920234, which is herein incorporated by reference. 
     Referring to FIG. 2, a method of feedback control, according to the present invention, is illustrated for the electronic fuel injection system  10 . The methodology begins in diamond  50  and determines whether predetermined conditions have been met for feedback from the front fuel/air sensor  40 , such as whether the throttle angle and MAP are within predetermined ranges as sensed by the sensors  34  and  36 , respectively. If not, the methodology advances to bubble  52  and performs open loop control of the fuel injection system  10 . Alternatively, if the front fuel/air sensor conditions have been met, the methodology advances to diamond  54  and determines whether predetermined conditions have been met for feedback from the rear O 2  sensor  42 , such as whether the throttle angle and MAP are within predetermined ranges. If not, the methodology advances to block  56  and uses a previously adapted front fuel/air sensor switching voltage threshold (Vt) which is an initial value Vo based on either a previous front sensor switching voltage threshold or a RAM location from a front sensor switching target voltage adaptive matrix stored in memory of the engine controller  30 . The methodology then advances to block  58  and adds a wave form forcing function to the switch voltage threshold (Vt). The wave form pattern goal voltage is utilized in order to obtain a high level of catalyst efficiency. Catalysts require fuel/air perturbations to retain proper oxygen storage to maintain high efficiency. The wave form can be a sine wave, square wave, V-wave, or other wave form. Bubble  59  then uses the front sensor switching voltage threshold (Vt) with the superimposed wave form for controlling the electronic fuel injection system  10  to be described. 
     In diamond  54 , if the predetermined conditions have been met for feedback from the rear O 2  sensor  42 , the methodology advances to block  60 . In block  60 , the methodology reads and filters the voltage output signal from the rear O 2  sensor  42 . The methodology then advances to block  62  and reads a rear O 2  target voltage and calculates a rear O 2  voltage error. The engine controller  30  reads the rear O 2  target voltage based on the engine operating conditions and is obtained from a matrix of RPM and MAP. The engine controller  30  calculates the rear O 2  voltage error by subtracting the actual voltage of the output signal from the rear O 2  sensor  42  of block  60  from the rear O 2  target voltage. The rear O 2  voltage error (target voltage-actual voltage) is passed through a linear PI (proportional integral) control routine to produce the front sensor switching voltage threshold (Vt) changes. The methodology advances to block  64  and calculates the proportional and integral PI terms based on the rear O 2  signal as follows: 
     
       
         PI term=Kp*Ve+Σ(Ki*Ve)dt 
       
     
     The proportional term for the PI term is (Kp*Ve) where Kp is a calibration constant for the proportional term and Ve is the rear O 2  voltage error calculated in Block  62 . The integral term for the PI term is essentially the summation of the voltage error over time; example &lt;Σ(Ki*Ve) dt&gt; or &lt;Ki*ΣVe dt&gt; where Ki is a calibration constant for the integral term which may vary with operating conditions and dt is the time factor. It should be appreciated that the PI term is a proportional gain element multiplied by the rear O 2  sensor voltage error, plus an integral gain element multiplied by voltage error. 
     From block  64 , the methodology advances to diamond  66  and determines whether it is time to update the front sensor switching voltage threshold (Vt). If not, the methodology advances to block  58  and then bubble  59  previously described. Alternatively, if it is time to update the front sensor switching threshold (Vt), the methodology advances to block  68  and adds the PI term calculated in block  64  to the current front sensor switching voltage threshold initial value (V o ) as follows: 
     
       
         Vt=Vo+PI Term 
       
     
     The methodology then advances to block  70  and updates the front sensor switching target voltage adaptive matrix for Vo with the newly calculated Vt term. The methodology then advances to block  58  and bubble  59  previously described. 
     After bubble  59 , the methodology compares a voltage output from the front sensor  40  to the front sensor switching voltage threshold (Vt) with the added wave form to determine if the fuel/air ratio of the engine is rich or lean. The methodology then decreases or increases the amount of fuel to the engine  12  by the fuel injectors in response to signals from the engine controller  30  via the fuel injector outputs  32 . 
     Accordingly, the rear O 2  sensor  42  is used to modify the front sensor switching voltage threshold or switch point (instead of using a fixed value for the front sensor over the life of the vehicle). The rear O 2  sensor output voltage is monitored, filtered, and compared to a target voltage to calculate a rear O 2  voltage error. The rear O 2  voltage error is integrated over time and adjustments are made to the front sensor switch point to drive the error in the rear O 2  sensor voltage to zero. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.