Abstract:
An air/fuel control method for an engine including a NO x  sensor in operative relationship to a catalytic converter. The method comprises the steps of providing a base fuel signal related to a quantity of air inducted into the engine and generating a bias signal for biasing the base fuel signal towards a leaner air/fuel ratio. The output of the NO x  sensor is monitored to detect a predetermined exhaust gas NO x  value representing a predefined NO x  conversion efficiency. The base fuel signal is then modified as a function of the bias signal corresponding to the predetermined exhaust gas NO x  value to maintain the catalytic converter within a desired efficiency range. In one aspect of the invention, the process of detecting the edge of the NO x  conversion efficiency window is executed at predetermined time periods measured by the distance the vehicle traveled, or the elapsed time since last base fuel value modification.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an air/fuel ratio control system for an internal combustion engine coupled to a catalytic converter. 
     2. Description of the Related Art 
     Three-way catalytic converters (TWC) are commonly used to remove pollutants such as NO x , HC, and CO components in the exhaust gas of an internal combustion engine. NO x  is removed from the exhaust gas by reduction using the CO, HC and H 2  in the exhaust gas. There is also typically enough O 2  present to oxidize the CO and HC. Generally, however, the catalyst used in such converters is able to remove the pollutants from the exhaust gas simultaneously only when the air/fuel ratio of the exhaust gas is kept in a narrow range near the stoichiometric air/fuel ratio. 
     FIG. 1 shows the conversion efficiency of a typical TWC as a function of measured exhaust gas air/fuel ratio. As can be seen in FIG. 1, TWCs require that the air/fuel ratio of the engine be held in a relatively narrow range, such as window  10 , to assure high conversion efficiencies. Therefore, in order to reduce undesirable emissions within the exhaust gas, it is important to keep the air/fuel ratio of the engine in the region where the TWC has high efficiency. Typically, this is near the stoichiometric air/fuel ratio or at a predetermined offset near stoichiometric. Component drifting or aging can result in an altered air/fuel ratio and, hence, less than optimum efficiency of the TWC. 
     SUMMARY OF THE INVENTION 
     An object of the invention herein is to provide a method of locating the peak TWC efficiency window. Another object is to maintain engine air/fuel operation within the peak efficiency window of a catalytic converter. 
     An air/fuel control method for an engine including a NO x  sensor in operative relationship to a catalytic converter. The method comprises the steps of providing a base fuel signal related to a quantity of air inducted into the engine and generating a bias signal for biasing the base fuel signal towards a leaner air/fuel ratio. The output of the NO x  sensor is monitored to detect a predetermined exhaust gas NO x  value representing a predefined NO x  conversion efficiency. The base fuel signal is then modified as a function of the bias signal corresponding to the predetermined exhaust gas NO x  value to maintain the catalytic converter within a desired efficiency range. In one aspect of the invention, the process of detecting the edge of the NO x  conversion efficiency window is executed at predetermined time periods measured by the distance the vehicle has traveled, or the elapsed time since last base fuel value modification. 
     One advantage of the present invention is that it suppresses fluctuation in the air/fuel ratio. Another advantage is that it improves the efficiency of the catalytic converter. 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference should now be had to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. In the drawings: 
     FIG. 1 is a graph of the conversion efficiency of a typical TWC as a function of measured exhaust gas air/fuel ratio. 
     FIG. 2 is a block diagram of an engine system where the present invention may be advantageously used. 
     FIG. 3 is a logic flow diagram representing one method of controlling the air/fuel ratio feedback control system of FIG.  2 . 
     FIG. 4 is a logic flow diagram of one embodiment of the fuel correction term routine for use in the system of FIG.  3 . 
     FIG. 5 is a logic flow diagram of one embodiment of the catalyst window locator routine according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An example of an engine and associated control system incorporating a NO x  sensor for fuel control will now be discussed. Fuel delivery system  11 , shown in FIG. 2 of an automotive internal combustion engine  13  is controlled by controller  15 , such as an EEC or PCM. In general terms, controller  15  controls engine air/fuel ratio in response to a feedback variable derived from the output of an upstream exhaust gas oxygen sensor  54 . Feedback from a second downstream rear exhaust gas oxygen sensor  55  is used to bias the feedback variable to provide improved air/fuel control. Concurrently, as described herein with reference to FIG. 4, controller  15  provides air/fuel bias in response to the output of the NO x  sensor  100 . Sensor  100  is a NO x  sensor having an output corresponding to the air/fuel ratio of the engine  13 . As described later herein, the air/fuel biasing forces engine air/fuel operation to be within the peak efficiency window of the three-way (HC, CO, NO x  ) catalytic converter  52 . 
     Continuing with FIG. 2, engine  13  includes fuel injectors  18 , which are in fluid communication with fuel rail  22  to inject fuel into the cylinders (not shown) of engine  13 , and temperature sensor  132  for sensing temperature of engine  13 . Fuel delivery system  11  has fuel rail  22 , fuel rail pressure sensor  33  connected to fuel rail  22 , fuel line  40  coupled to fuel rail  22  via coupling  41 , and fuel delivery means  42 , which is housed within fuel tank  44 , to selectively deliver fuel to fuel rail  22  via fuel line  40 . 
     Engine  13  also includes exhaust manifold  48  coupled to exhaust ports of the engine (not shown). TWC  52  is coupled to exhaust manifold  48 . An exhaust gas oxygen sensor  54  (i.e., a wide range exhaust gas oxygen sensor) is positioned upstream of the catalytic converter  52  in exhaust manifold  48 . An additional EGO sensor  55  is located downstream of the catalyst  52 . Engine  13  further includes intake manifold  56  coupled to intake ports of the engine (not shown). Intake manifold  56  is also coupled to throttle body  58  having throttle plate  60  therein. 
     Controller  15  is shown as a conventional microcontroller including: a CPU  114 , random access memory  116  (RAM), computer storage medium (ROM)  118  having a computer readable code encoded therein, which is an electronically programmable chip in this example, and input/output (I/O) bus  120 . Controller  15  controls engine  13  by receiving various inputs through I/O bus  120  such as fuel pressure in fuel delivery system  11 , as sensed by pressure sensor  33 ; relative exhaust air/fuel ratio as sensed by exhaust gas oxygen sensors  54  and  55 ; temperature of engine  13  as sensed by temperature sensor  132 ; measurement of inducted mass airflow (MAF) from mass airflow sensor  158 ; speed of engine (RPM) from engine speed sensor  160 ; relative exhaust gas NO x  concentration from NO x  sensor  100 ; and various other sensors  156 . 
     Controller  15  also generates various outputs through I/O bus  120  to actuate the various components of the engine control system. Such components include fuel injectors  18  and fuel delivery means  42 . It should be noted that the fuel may be liquid fuel, in which case fuel delivery means  42  is an electronic fuel pump and the delivery of fuel is in proportion to the pulse width of signal FPW from controller  15 . 
     Fuel delivery control means  42 , upon demand from engine  13  and under control of controller  15 , pumps fuel from fuel tank  44  through fuel line  40 , and into pressure fuel rail  22  for distribution to the fuel injectors during conventional operation. Controller  15  controls fuel injectors  18  to maintain a desired air/fuel ratio in response to exhaust gas oxygen sensor  54 . EGO sensor  54  provides a signal to the controller  15  which converts the signal into a two-state signal (EGOs). A high voltage state of signal EGOs indicates exhaust gases are rich of a reference air/fuel ratio and a low voltage state of the converted signal indicates exhaust gases are lean of the reference air/fuel ratio. Typically, the reference air/fuel ratio or switch point of EGO sensor  54  should be at stoichiometry, and stoichiometry should correspond to the peak efficiency window of the average catalytic converter. However, due to manufacturing processes and component aging, the switch point of EGO sensor  54  may not be at stoichiometry. To correct for this, a correction term or offset is applied to the switch voltage of the EGO sensor  54 . Further, the peak efficiency window of TWC  52  may not be at stoichiometry. Therefore, it may be desirable to offset the switch voltage of the EGO sensor(s) to maintain the TWC  52  in the peak efficiency window. 
     There are many methods of controlling engine air/fuel ratio with the use of one or more EGO sensors. One example of a method of controlling the air/fuel ratio of the engine  13  with the exhaust gas oxygen sensors  54  and  55  will now be discussed with respect to FIG.  3 . Referring now to FIG. 3, a flowchart of a routine performed by controller  15  to control the fuel pulse width signal (FPW) is shown. Fuel pulse width signal (FPW) is the signal sent by controller  15  to fuel injectors  18  to deliver the desired quantity of fuel to engine  13 . A determination is first made whether closed-loop air/fuel control is to be commenced (step  204 ) by monitoring engine operating conditions such as temperature. When closed-loop control commences, the desired fuel signal FD is calculated as a function of MAF, the desired air/fuel ratio term Afd, a feedback correction term Fpi, and a fuel correction term (FC) as shown in step  206 . In step  208 , the signal FD is converted to fuel pulse width signal FPW representing a time to actuate fuel injectors  18 . In step  210 , signal EGO is read from sensor  54  and subsequently processed in a proportional plus integral controller in step  212 , to achieve the desired air/fuel ratio. 
     When open-loop control is used, the signal FD is calculated by adding MAF to the desired air/fuel ratio term Afd less any fuel correction (FC) as shown in step  214 . 
     Referring now to FIG. 4, there is shown one embodiment of a logic routine for generating the fuel correction term of steps  206  and  214  of FIG.  3 . Continuing with FIG. 4, it is determined whether the engine is operating under closed-loop fuel control in step  300 . If so, the downstream or rear EGO sensor  55  output is read in step  302 . In step  304 , the fuel correction (FC) term is generated. This is accomplished by performing proportional-integral-differential control on the EGO sensor output voltage. The error term used by the controller is the EGO output voltage, less any calibration offset, plus any correction derived from the NO x  sensor output as described below. Alternatively, integral only control could be used to generate the FC term. This FC term is then output to the primary air/fuel control scheme such as that shown in FIG.  3 . 
     A logic routine will now be described with particular reference to FIG. 5 for biasing the air/fuel control through the variable FC so that engine air/fuel operation is maintained within the peak efficiency window of converter  52 . 
     In step  400 , the routine determines if the engine is operating under closed-loop air/fuel control. Further, in step  402 , it is determined whether the engine is operating under steady state conditions. If these conditions are satisfied, a timer is started in step  404 . The timer is used to dictate how often the NO x  window detection routine as described below is executed. This routine is only periodically executed because it is an intrusive test. The timer may relate to time or distance that the vehicle is operating under closed-loop, steady-state conditions. The timer is compared against a predetermined value (VALUE 1) in step  406  which, again, may be minutes or miles since the last routine execution. 
     Instead of, or in addition to, the timer, the NO x  window detection routine may be executed at each start-up, for example, after warmed-up conditions are satisfied. 
     The first time the NO x  window detection routine is executed in step  408 , the base FC value is stored in step  410 . As discussed, with respect to FIGS. 3 and 4, FC is the fuel correction term which is used by the primary air/fuel ratio control scheme. 
     The routine then continues to step  412 , where the FC control output is incremented at a predetermined rate (RAMP RATE) from the base FC value to a desired ramp value (OFFSET). The FC value is incremented to bias the air/fuel control towards a leaner air/fuel ratio. The ramp rate is set such that the system delay between the change in air/fuel ratio and the detection of the change by the downstream EGO and NO x  sensors correlates. In other words, if the FC value is incremented too quickly, it is difficult to correlate the NO x  window edge with the FC value responsible for reaching the edge of the window. 
     After each FC value increment, the NO x  sensor output is monitored to determine whether the edge of the efficiency window has been reached. This is accomplished by comparing the NO x  sensor output to a predetermined value corresponding to the desired efficiency defining the edge of the window. The routine is continuously executed until the window edge has been reached. 
     Once the edge of the NO x  efficiency window has been detected, the change in FC value necessary to reach the window edge is determined in step  416 . This value (ΔFC) is then used to correct the downstream EGO control set voltage to maintain the air/fuel ratio within a range such that the NO x  conversion efficiency is maximized. This is accomplished by a calibrateable lookup table wherein the number of increments to reach the window edge is correlated to the EGO voltage switch point to maximize TWC efficiency. The resulting NO x  sensor TWC window correction term is then used as described above to generate the FC term which, in turn, is used by the primary air/fuel control scheme. 
     Alternatively, the NO x  sensor TWC window correction term could be applied directly to the primary air/fuel control scheme as the FC value used to modify the base fuel signal. 
     In step  420 , the timer is reset and the routine is ended or continued as desired. 
     From the foregoing, it can be seen that there has been brought to the art a new and improved air/fuel ratio control scheme which maintains the air/fuel ratio such that the catalytic converter operates near peak efficiency. While the invention has been described in connection with one or more embodiments, it should be understood that it is not limited to those embodiments. On the contrary, the invention covers all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims.