Abstract:
A control system and method for optimizing fuel control in an internal combustion engine utilizes a signal from an oxygen sensor disposed in an exhaust downstream of a catalytic converter. An air/fuel mixture introduced into the engine is compensated based on the signal exceeding predetermined enabling thresholds. The predetermined rich or lean condition enable threshold represents an oxygen content so less or greater than desired that the normal closed loop control including regular secondary fuel trim is not sufficient enough to bring the outlet oxygen sensor signal back to the desired window quickly. A reduced or increased amount of fuel is introduced into the engine based on the signal exceeding the predetermined rich or lean enable threshold respectively.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to fuel control systems for gasoline vehicles, and more particularly to engine fuel control systems including an oxygen sensor that is located downstream from a three-way catalytic converter.  
       BACKGROUND OF THE INVENTION  
       [0002]     Three-way catalytic converters reduce exhaust gas emissions in vehicles using an internal combustion engine. The catalytic converter includes a substrate with a coating of catalyst materials that stimulate the oxidation of hydrocarbon and carbon monoxide molecules, and the reduction of nitrogen oxides, in the vehicle exhaust gas. The catalysts operate optimally when the temperature of the catalysts is above a minimum level and when the air/fuel ratio is stoichiometric. Stoichiometry is defined as an ideal air/fuel ratio, which is 14.7 to 1 for gasoline.  
         [0003]     Fuel delivery is managed by an engine control system using either open loop or closed loop feedback control. Open loop control is typically initiated during specific operating conditions such as start up, cold engine operation, heavy load conditions, wide open throttle, and intrusive diagnostic events, etc.  
         [0004]     An engine control system typically employs closed loop control to maintain the air/fuel mixture at or close to the ideal stoichiometric air/fuel ratio. Closed loop fuel control commands a desired fuel delivery based on the oxygen content in the exhaust. The oxygen level in the exhaust is determined by oxygen sensors that are located both upstream and downstream from the catalytic converter. A three-way catalytic converter and the upstream and downstream oxygen sensors are used in gasoline vehicles for emission reduction. The upstream (inlet oxygen sensor) and downstream oxygen sensor (outlet oxygen sensor) are also used for catalytic converter efficiency monitoring.  
         [0005]     Primary closed loop fuel control using an oxygen sensor upstream from a catalytic converter has been widely used, driven by fuel economy and emission reduction. The fundamental idea is to try to maintain catalytic converter inlet oxygen sensor signals toggling around a reference voltage to provide engine combustion at or close to stoichiometric air/fuel ratios.  
         [0006]     Secondary fuel trim using an oxygen sensor after a catalytic converter is also widely used and is mainly driven by trying to meet increasingly stringent emission regulations. The outlet oxygen sensor signal correlates to air/fuel ratios or rich/lean conditions in the catalyst-out gas flow. A three-way catalytic converter has the capacity to store or release oxygen, and thus can maintain good catalyst efficiency despite small or short duration fueling errors from the ideal stoichiometric air/fuel ratio. However, large or long duration fueling errors from stoichiometric, will make emissions break through the catalytic converter, which can be observed by the outlet oxygen sensor signals going to very low or very high voltages. Secondary fuel trim works to maintain the catalytic converter outlet oxygen sensor signal within a window identified as providing optimal catalytic converter efficiency.  
         [0007]     Given the primary closed loop fuel control and secondary fuel trim as designed, there still exists several normal maneuvers as well as intrusive diagnostic tests that could saturate the converter and lead to increased emissions break through. The intrusive diagnostic tests for performance monitoring of the catalytic converters, outlet oxygen sensors and secondary air injection are a few examples of diagnostic tests that can leave the converter in a lower efficiency state. The invention is to minimize these negative impacts and reduce catalyst-out emissions by quickly taking fuel control actions that force outlet oxygen signals back toward the normal or desired zone to get optimum catalyst efficiency. The methodology is to add four new thresholds in addition to the existing target window for the outlet oxygen sensor signal to further optimize engine fuel control strategy.  
       SUMMARY OF THE INVENTION  
       [0008]     A control system and method for optimizing fuel control in an internal combustion engine utilizes a signal from an oxygen sensor disposed in an exhaust downstream of a catalytic converter. Control determines whether the signal exceeds predetermined thresholds. An air/fuel mixture introduced into the engine is compensated based on the signal exceeding the predetermined thresholds.  
         [0009]     To enable rich fuel compensation for a lean downstream condition, control compares the downstream oxygen sensor signal to a predetermined lean condition enable threshold that represents an oxygen content so greater than desired that the normal closed loop control including regular secondary fuel trim is not sufficient enough to bring the outlet oxygen sensor signal back to the desired window quickly. An increased amount of fuel is introduced into the engine based on the signal exceeding (dropping lower than) the predetermined lean condition enable threshold. Similarly, to enable lean fuel compensation for a rich downstream condition, control compares the signal to a predetermined rich condition enable threshold that represents an oxygen content so less than desired that the normal closed loop control including regular secondary fuel trim is not sufficient enough to bring the outlet oxygen sensor signal back to the desired window quickly. A reduced amount of fuel is introduced into the engine based on the signal exceeding the predetermined rich condition enable threshold.  
         [0010]     Once rich fuel compensation for a lean downstream condition has been enabled, control compares the signal to a predetermined lean condition disable threshold that represents an oxygen content greater than a desired level, but appropriate for providing normal closed loop fueling once again. Fuel delivery is returned to normal closed loop operation based on the signal satisfying or rising above the lean condition disable threshold. Similarly, once lean fuel compensation for a rich downstream condition has been enabled, control compares the signal to a predetermined rich condition disable threshold that represents an oxygen content less than a desired level, but appropriate for providing normal closed loop fueling once again. Fuel delivery is returned to normal closed loop operation based on the signal satisfying or dropping lower than the rich condition disable threshold.  
         [0011]     According to other features, this control is only used while meeting closed loop conditions with normally functioning oxygen sensors and while not in an intrusive diagnostic test or any other open loop fuel.  
         [0012]     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  
       [0013]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0014]      FIG. 1  is a functional block diagram of an engine control system according to the present invention for a vehicle;  
         [0015]      FIG. 2  illustrates outlet oxygen sensor thresholds according to the present invention;  
         [0016]      FIG. 3  is a flow diagram illustrating steps for optimizing fuel control; and  
         [0017]      FIG. 4  is a flow diagram illustrating steps for implementing the fuel correction of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0019]     Referring to  FIG. 1 , an exemplary engine control system  8  is shown. A throttle  10  and a fuel system  12  control the air/fuel mixture that is delivered to an engine  14  through an intake  16 . An ignition system  18  ignites the fuel and air mixture in the engine  14 . Exhaust gas that is created by the combustion of the air/fuel mixture is expelled through an exhaust manifold  20 . A catalytic converter  22  receives the exhaust gas from the exhaust manifold  20  and reduces the emissions levels of the exhaust gas.  
         [0020]     A controller  30  communicates with various components of the engine control system  8 , including but not limited to a throttle position sensor  32  (TPS), the fuel system  12 , the ignition system  18 , and a mass airflow sensor  36  (MAF). The controller  30  receives a throttle position signal from the TPS  32  and a mass air flow signal from the MAF  36 . The throttle position signal and the mass air flow signal are used to determine air flow into the engine  14 . The air flow data is then used to calculate the corresponding fuel to be delivered by the fuel system  12  to the engine  14 . The controller  30  further communicates with the ignition system  18  to determine ignition spark timing. Oxygen sensors  46  and  48  are disposed in the exhaust  20  upstream and downstream, respectively, of the catalytic converter  22 . The oxygen sensors  46  and  48  output signals to the controller  30  that represent the oxygen content before and after the catalytic converter  22  in the exhaust  20 .  
         [0021]     The controller  30  may receive additional feedback from other components in the engine control system  8 , including but not limited to coolant temperature from a coolant temperature sensor  50  and engine speed from the engine speed sensor  34  (RPM). These and other variables may affect the overall performance and behavior of the engine control system  8 . The controller  30  utilizes data gathered from the various engine components to monitor and optimize engine performance.  
         [0022]     With continued reference to  FIG. 1  and further reference to  FIG. 2 , the controller  30  according to the present invention establishes a plurality of thresholds during closed loop fuel control to maintain optimum catalyst efficiency. The thresholds are defined as rich condition enable, rich condition disable, lean condition enable and lean condition disable. The control method is active only when normal closed loop fuel control conditions are met. The control method is inactive during other open loop fuel control or intrusive diagnostic modes. The controller  30  receives the oxygen signal generated by the downstream oxygen sensor  48 . Based on the signal from the oxygen sensor  48 , the controller  30  determines whether the established enable thresholds have been exceeded. For example, if the downstream oxygen sensor  48  generates a voltage signal below a predetermined threshold  70 , control will enter an open loop rich control strategy. The open loop rich control strategy includes delivering an increased amount of fuel to the engine  14 . An increased amount of fuel is delivered to return the downstream oxygen sensor  48  to a desired window  68 . For example, increasing the amount of fuel delivered to the engine  14  may include increasing the fuel injection duration. The desired window  68  is defined as a predetermined optimum range of oxygen measured by the downstream oxygen sensor  48 . The open loop rich control continues until accumulated airflow reaches its predetermined value or until a lean condition disable threshold  72  is met.  
         [0023]     Similarly, if the downstream oxygen sensor  48  generates a voltage signal above a predetermined threshold  60 , control will enter an open loop lean control strategy. The open loop lean control strategy includes delivering a reduced amount of fuel to the engine  14 . A reduced amount of fuel is desired to return the downstream oxygen sensor  48  to the desired window  68 . For example, decreasing the amount of fuel delivered to the engine  14  may include decreasing the fuel injection duration. The open loop lean control continues for a predetermined accumulated airflow or until a rich condition disable threshold  62  is met.  
         [0024]     Referring now to  FIG. 3 , steps for optimizing fuel control in an internal combustion engine are shown generally at  100 . Control begins with step  102 . In step  104 , control determines whether any applicable active faults are identified. The applicable active faults are those that may prevent the control from correct performance. Active faults may include component diagnostic trouble codes such as catalytic converter fault codes, oxygen sensor fault codes, cylinder misfire codes, and secondary fuel trim fault codes, etc. If one or more active fault codes are identified in step  104 , control returns in step  124 . If no active faults are identified in step  104 , control determines whether the downstream oxygen sensor  48  is ready to support closed loop operation in step  108 . If the oxygen sensor  48  is not ready to support closed loop operation, control returns in step  124 . If the oxygen sensor  48  is ready, control determines whether other open loop fuel or intrusive diagnostic modes having higher priority are active in step  112 .  
         [0025]     If other open loop fuel or intrusive diagnostic modes are active, control returns in step  124 . If there are no other open loop fuel or intrusive diagnostic modes active, control determines whether the engine  14  is operating correctly in closed loop mode in step  118 . If the closed loop operation conditions are not met, control returns in step  124 . If all the entire enable conditions are true, control runs a correction routine in step  120 .  
         [0026]     With reference now to  FIG. 4 , the correction routine  120  will be described in greater detail. The correction routine  120 , as previously described, implements a brief open loop rich or lean control if the oxygen sensor  48  communicates a signal exceeding the enable conditions  60  or  70  ( FIG. 2 ). The correction routine  120  begins in step  140 . In step  144 , control determines if the oxygen sensor signal  48  has dropped below the lean condition enable threshold  70  or if the signal has exceeded the rich condition enable threshold  60  in step  148 .  
         [0027]     If the oxygen sensor signal  48  is below the lean condition enable threshold  70  in step  144 , then an intrusive air/fuel ratio (AFR) control is implemented in step  150  having a rich fuel mixture of air/fuel (a ratio less than 14.7 to 1 for gasoline). An accumulated engine airflow variable is also set to zero in step  150  upon initiation of intrusive AFR control. In step  152  control determines if the oxygen sensor  48  communicates a signal satisfying the lean condition disable threshold  72 . If the oxygen sensor  48  communicates a signal satisfying the lean condition disable threshold  72 , control returns to normal closed loop control mode in step  156  and control returns in step  158 . If the lean condition disable threshold  72  is not satisfied in step  152 , control determines if intrusive AFR control has been running beyond one or more predetermined applicable criteria in step  160 . The applicable criteria can be an accumulated airflow, lapsed time or other variables. For an example, the accumulated airflow is used for demonstration. If the AFR control has exceeded the predetermined accumulated engine airflow calibration in step  160 , control returns to normal closed loop control mode in step  156  and control returns in step  158 . If the AFR control has not exceeded the calibration, the accumulated engine airflow is incremented in step  164  and control loops to step  152 .  
         [0028]     In step  148 , control determines whether the oxygen sensor  48  communicates a signal exceeding the rich condition enable threshold  60 . If the rich condition enable threshold  60  is not exceeded in step  148 , control returns to normal closed loop control in step  156  and control returns in step  158 .  
         [0029]     If the rich condition enable threshold  60  is exceeded in step  148 , an intrusive air/fuel ratio (AFR) control is implemented in step  170  having a lean fuel mixture of air/fuel (a ratio greater than 14.7 to 1 for gasoline). An accumulated engine airflow variable is also set to zero in step  170  upon initiation of intrusive AFR control. In step  172 , control determines whether the oxygen sensor  48  communicates a signal satisfying the rich condition disable threshold  62 . If the oxygen sensor  48  communicates a signal satisfying the rich condition disable threshold  62 , control returns to normal closed loop control mode in step  156  and control returns in step  158 . If the rich condition disable threshold  62  is not satisfied in step  172 , control determines whether intrusive AFR control has been running beyond one or more predetermined applicable criteria in step  180 . The applicable criteria can be an accumulated airflow, lapsed time or other variables. For an example, the accumulated airflow is used for demonstration. If the AFR control has exceeded the predetermined accumulated engine airflow calibration in step  180 , control returns to normal closed loop control mode in step  156  and control returns in step  158 . If the AFR control has not exceeded the calibration, the accumulated engine airflow is incremented in step  184  and control loops to step  172 .  
         [0030]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.