Patent Publication Number: US-9845758-B2

Title: Engine control apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an engine control apparatus, and more particularly to an engine control apparatus installed in a vehicle having an oxygen sensor, an oxygen sensor output value of which varies in accordance with an oxygen concentration of exhaust gas. 
     2. Description of the Related Art 
     An oxygen sensor may be disposed on an exhaust path of a vehicle. Air-fuel ratio feedback control is performed in the vehicle on the basis of an output voltage of the oxygen sensor in order to adjust a fuel injection amount so that an air-fuel ratio of an engine reaches the stoichiometric air-fuel ratio. As a result, a purification performance of a three-way catalyst that purifies exhaust gas can be maintained. 
     The output voltage of the oxygen sensor varies according to an oxygen concentration of the exhaust gas. Further, the output voltage of the oxygen sensor exhibits a characteristic of varying rapidly about the stoichiometric air-fuel ratio. Using this characteristic, a determination can be made from the output voltage value of the oxygen sensor as to whether the air-fuel ratio of the engine is richer or leaner than the stoichiometric air-fuel ratio. A determination result is expressed by binary data based on whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio. Air-fuel ratio feedback based on this binary determination result is implemented widely. 
     In recent years, as exhaust gas regulations become stricter, there is increasing demand for an improvement in the precision of air-fuel ratio feedback control. As described above, the output voltage of the oxygen sensor varies rapidly about the stoichiometric air-fuel ratio. More specifically, when the air-fuel ratio advances to the rich side of the stoichiometric air-fuel ratio, the output voltage of the oxygen sensor increases rapidly initially and then increases gently. When the air-fuel ratio advances to the lean side of the stoichiometric air-fuel ratio, meanwhile, the output voltage of the oxygen sensor decreases rapidly initially and then decreases gently. 
     Further, the characteristic of the oxygen sensor output outside the vicinity of the stoichiometric air-fuel ratio is affected greatly by variation in a sensor element temperature. When an oxygen sensor is used as an air-fuel ratio sensor, it is important to estimate the sensor element temperature of the oxygen sensor. Accordingly, an air-fuel ratio feedback method that includes detection or estimation of the sensor element temperature of the oxygen sensor has been proposed (see JP 4607163 B2, for example). 
     In a system configuration described in JP 4607163 B2, a three-dimensional oxygen sensor map is stored in advance in a memory of a control unit. On the oxygen sensor map, the sensor element temperature of the oxygen sensor is stored in association with an engine rotation speed and a throttle opening. The sensor element temperature of the oxygen sensor is estimated by reading the sensor element temperature from the map in accordance with operating conditions. The oxygen sensor output value is then corrected on the basis of the estimation result of the sensor element temperature. Further, an actual air-fuel ratio (referred to hereafter as the actual air-fuel ratio) is calculated from the corrected oxygen sensor output value. Hence, feedback control is performed on the basis of a deviation between the actual air-fuel ratio and a target air-fuel ratio (the stoichiometric air-fuel ratio). According to JP 4607163 B2, therefore, a large improvement in control precision can be achieved over conventional, widely implemented air-fuel ratio feedback control based on a binary determination result (i.e. whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio). 
     SUMMARY OF THE INVENTION 
     However, with the simplified sensor element temperature estimation method implemented in JP 4607163 B2, various environmental conditions in which a motorcycle is used, such as a temperature condition, an atmospheric pressure condition, and a humidity condition, for example, are not taken into account. During an actual vehicle operation, therefore, an error that adversely affects convergence of the air-fuel ratio may occur in the estimation result of the sensor element temperature. 
     The following two methods, for example, may be considered as methods for improving the precision with which the sensor element temperature of the oxygen sensor is estimated. 
     In a first method, sensor element temperatures under various environmental conditions and operating conditions are recorded in detail in a memory using a large memory and a high-performance CPU. Further, various sensors are mounted on the vehicle side in order to measure the environmental conditions. Hence, during a vehicle operation, the usage environment is measured by the sensors in real time, whereupon an appropriate sensor element temperature of the oxygen sensor is read from the memory. 
     In a second method, a special oxygen sensor with which the sensor element temperature of the oxygen sensor can be measured directly is provided. 
     However, both of these methods are costly, and cannot therefore be applied realistically to an inexpensive system such as that of a motorcycle. 
     This invention has been made to solve the problem described above, and an object thereof is to obtain an engine control apparatus that can make effective use of a characteristic of an oxygen sensor output voltage to enable an air-fuel ratio of an engine to converge on the stoichiometric air-fuel ratio more quickly than with air-fuel ratio control based on a binary determination result, which is currently widely used. 
     Solution to Problem 
     This invention is an engine control apparatus having an oxygen sensor that outputs an oxygen sensor output value corresponding to an oxygen concentration of exhaust gas from an engine, and an air-fuel ratio feedback control unit that performs air-fuel ratio feedback control on the basis of the oxygen sensor output value in order to adjust an amount of fuel injected into the engine, the air-fuel ratio feedback control unit including: an air-fuel ratio region detection unit that detects an air-fuel ratio region, among four or more preset air-fuel ratio regions, to which an air-fuel ratio of the engine belongs on the basis of the oxygen sensor output value; and an air-fuel ratio feedback control correction amount calculation unit that calculates a first feedback control correction amount for use during the air-fuel ratio feedback control in accordance with the air-fuel ratio region detected by the air-fuel ratio region detection unit, wherein the four or more regions include at least a first rich region and a second rich region set on a rich side of a stoichiometric air-fuel ratio in ascending order of a value of the air-fuel ratio, and a first lean region and a second lean region set on a lean side of the stoichiometric air-fuel ratio in descending order of the value of the air-fuel ratio, and the air-fuel ratio region detection unit includes a first determination voltage set at a higher value than a target voltage value that is a voltage value indicating the stoichiometric air-fuel ratio, and a second determination voltage set at a lower value than the target voltage value, compares the oxygen sensor output value respectively with the first determination voltage and the second determination voltage, determines that the air-fuel ratio of the engine is within the first rich region when the oxygen sensor output value equals or exceeds the first determination voltage, determines that the air-fuel ratio of the engine is within the second rich region when the oxygen sensor output value equals or exceeds the target voltage value but is lower than the first determination voltage, determines that the air-fuel ratio of the engine is within the second lean region when the oxygen sensor output value equals or exceeds the second determination voltage but is lower than the target voltage value, and determines that the air-fuel ratio of the engine is within the first lean region when the oxygen sensor output value is lower than the second determination voltage. 
     Advantageous Effects of Invention 
     In the engine control apparatus according to this invention, the air-fuel ratio region to which the air-fuel ratio of the engine belongs, among the four or more divided air-fuel ratio regions, is determined on the basis of the oxygen sensor output value VO 2  of the oxygen sensor, whereupon air-fuel ratio feedback control is performed in accordance with the corresponding air-fuel ratio region. Therefore, the effects of an error occurring during estimation of a sensor element temperature can be suppressed, with the result that convergence of the air-fuel ratio of the engine on the stoichiometric air-fuel ratio can be achieved more quickly than with air-fuel ratio control based on a binary determination result, which is currently used widely. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a configuration of an engine control apparatus according to a first embodiment of this invention, together with an engine; 
         FIG. 2  is a block diagram showing a functional configuration of the engine control apparatus according to the first embodiment of this invention; 
         FIG. 3  is an illustrative view showing an output characteristic of an oxygen sensor and air-fuel ratio regions of the engine used in the first embodiment of this invention; 
         FIG. 4  is a flowchart showing an operation of an air-fuel ratio feedback control unit according to the first embodiment of this invention; 
         FIG. 5A  is an illustrative view showing an example of a proportional gain map used in the first embodiment of this invention; 
         FIG. 5B  is an illustrative view showing another example of the proportional gain map, which is used in a modified example of the first embodiment of this invention; 
         FIG. 6A  is an illustrative view showing an example of an integral gain map used in the first embodiment of this invention; 
         FIG. 6B  is an illustrative view showing another example of the integral gain map, which is used in the modified example of the first embodiment of this invention; 
         FIG. 7  is an illustrative view showing the output characteristic of the oxygen sensor and the air-fuel ratio regions of the engine used in the modified example of the first embodiment of this invention; 
         FIG. 8  is a block diagram showing the functional configuration of the engine control apparatus according to the modified example of the first embodiment of this invention; 
         FIG. 9  is a flowchart showing an operation of the air-fuel ratio feedback control unit according to the modified example of the first embodiment of this invention; 
         FIG. 10A  is an illustrative view showing an example of an oxygen sensor basic temperature map used in the modified example of the first embodiment of this invention; 
         FIG. 10B  is an illustrative view showing another example of the oxygen sensor basic temperature map used in the modified example of the first embodiment of this invention; 
         FIG. 11  is a block diagram showing a functional configuration of an engine control apparatus according to a second embodiment of this invention; 
         FIG. 12  is a flowchart showing an operation of an air-fuel ratio feedback control unit according to the second embodiment of this invention; and 
         FIG. 13  is an illustrative view showing a deterioration condition of an oxygen sensor according to the second embodiment of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of this invention will be described below with reference to the drawings. 
       FIG. 1  is a view showing a configuration of an engine control apparatus according to the first embodiment of this invention when attached to an engine of a vehicle.  FIG. 2  is a block diagram showing a functional configuration of a control unit  1  shown in  FIG. 1 .  FIG. 3  is an illustrative view showing a relationship between an output characteristic of an oxygen sensor and air-fuel ratio regions of the engine, according to the first embodiment. 
     In  FIG. 1 , the control unit  1  constitutes a main portion of the engine control apparatus. The control unit  1  is constituted by a microcomputer having a CPU (not shown) and a memory  30 . The control unit  1  stores programs and maps used to control an overall operation of an engine  19  in the memory  30 . 
     An intake pipe  14  and an exhaust pipe  10  are provided in the engine  19 . The intake pipe  14  introduces intake air A into the engine  19 . The exhaust pipe  10  discharges exhaust gas Ah from the engine  19 . 
     An intake air temperature sensor  2 , a throttle valve  3 , a throttle position sensor  4 , an intake air pressure sensor  5 , and a fuel injection module  8  are provided in the intake pipe  14 . 
     The intake air temperature sensor  2  measures a temperature (an intake air temperature) Ta of the intake air A flowing through the intake pipe  14 . 
     The throttle valve  3  is driven to open and close by a throttle actuator  4 A. The throttle valve  3  adjusts an intake air amount of the intake air A. 
     The throttle position sensor  4  measures an opening θ of the throttle valve  3 . 
     The intake air pressure sensor  5  measures an intake air pressure Pa downstream of the throttle valve  3 . 
     The fuel injection module  8  includes an injector for injecting fuel into the engine  19 . 
     An engine temperature sensor  6 , a crank angle sensor  7 , and a spark plug  9 A are provided in the engine  19 . 
     The engine temperature sensor  6  measures a wall surface temperature (an engine temperature) Tw of the engine  19 . 
     The crank angle sensor  7  outputs an engine rotation speed Ne, and a crank angle signal SGT (a pulse) corresponding to a crank position. The spark plug  9 A is driven by an ignition coil  9 . 
     Note that the engine control apparatus according to the first embodiment can also be established by a system not including sensors such as the throttle position sensor  4 , the intake air temperature sensor  2 , and the engine temperature sensor  6 . 
     An oxygen sensor  11  and a three-way catalytic converter (referred to simply hereafter as a “three-way catalyst”)  12  are provided in the exhaust pipe  10 . 
     The oxygen sensor  11  functions as an air-fuel ratio sensor. The oxygen sensor  11  outputs an oxygen sensor output value VO 2  indicating a voltage value that corresponds to an oxygen concentration of the exhaust gas Ah discharged from the engine  19 . In this embodiment, the oxygen sensor  11  is constituted by a configuration in which a platinum electrode is provided on each surface of a test tube-shaped zirconia element. Further, to protect the platinum electrodes, outer sides of the platinum electrodes are coated with ceramic using a property of the zirconia element. Here, the property of the zirconia element is that when an oxygen concentration difference exists between an inner surface and an outer surface at a high temperature, electromotive force is generated. 
     The three-way catalyst  12  purifies the exhaust gas Ah. 
     As shown in  FIG. 3 , the oxygen sensor output value VO 2  from the oxygen sensor  11  varies in accordance with the oxygen concentration of the exhaust gas Ah. 
     In  FIG. 3 , the abscissa shows the air-fuel ratio and the ordinate shows the oxygen sensor output value VO 2 . 
     On the abscissa, an air-fuel ratio of 14.7 is the stoichiometric air-fuel ratio. Further, on the ordinate, a voltage value of 0.45 V is a voltage value indicating the stoichiometric air-fuel ratio. In other words, when the value of the oxygen sensor output value VO 2  is 0.45 V, the air-fuel ratio is known to correspond to the stoichiometric air-fuel ratio. 
     In  FIG. 3 , a solid line  300  shows an output characteristic of the oxygen sensor output value VO 2  in a case where a sensor element temperature of the oxygen sensor  11  is at a reference temperature Tst. Here, the reference temperature Tst is 700° C., for example. A dotted line  301  shows the output characteristic of the oxygen sensor output value VO 2  in a case where the sensor element temperature of the oxygen sensor  11  is at a lower temperature than the reference temperature Tst. Here, the lower temperature is 500° C., for example. 
     A dot-dot-dash line  302  shows the output characteristic of the oxygen sensor output value VO 2  in a case where the sensor element temperature of the oxygen sensor  11  is at a higher temperature than the reference temperature Tst. Here, the higher temperature is 900° C., for example. 
     As shown by the dotted line  301  in  FIG. 3 , when the sensor element temperature is 500° C., a voltage value indicating an air-fuel ratio of 13.5 is 0.80 V. 
     As shown by the solid line  300 , when the sensor element temperature is 700° C., the voltage value indicating an air-fuel ratio of 13.5 is 0.75 V. 
     As shown by the dotted line  301 , when the sensor element temperature is 500° C., a voltage value indicating an air-fuel ratio of 14.0 is 0.70 V. Further, as shown by the dot-dot-dash line  302 , when the sensor element temperature is 900° C., the voltage value indicating an air-fuel ratio of 13.5 is likewise 0.70 V. 
     In this embodiment, a first determination voltage is set at 0.70 V on the basis of a case in which the sensor element temperature is 900° C. 
     As shown by the dotted line  301  in  FIG. 3 , when the sensor element temperature is 500° C., a voltage value indicating an air-fuel ratio of 15.2 is 0.20 V. Further, as shown by the dot-dot-dash line  302 , when the sensor element temperature is 900° C., a voltage value indicating an air-fuel ratio of 15.5 is 0.20 V. 
     As shown by the solid line  300 , when the sensor element temperature is 700° C., the voltage value indicating an air-fuel ratio of 15.5 is 0.15 V. 
     As shown by the dotted line  301 , when the sensor element temperature is 500° C., the voltage value indicating an air-fuel ratio of 15.5 is 0.10 V. 
     In this embodiment, a second determination voltage is set at 0.20 V on the basis of a case in which the sensor element temperature is 900° C. 
     Note that in  FIG. 3 , an air-fuel ratio indicated by a voltage value of 0.80 V at 500° C., an air-fuel ratio indicated by a voltage value of 0.75 V at 700° C., and an air-fuel ratio indicated by a voltage value of 0.70 V at 900° C. are all 13.5. 
     Further, an air-fuel ratio indicated by a voltage value of 0.70 V at 500° C. is 14.0. 
     Furthermore, an air-fuel ratio indicated by a voltage value of 0.10 V at 500° C., an air-fuel ratio indicated by a voltage value of 0.15 V at 700° C., and an air-fuel ratio indicated by a voltage value of 0.20 V at 900° C. are all 15.5. 
     As shown in  FIG. 3 , the oxygen sensor output value VO 2  [V] varies rapidly in the vicinity of the stoichiometric air-fuel ratio (=14.7). On a rich side of the stoichiometric air-fuel ratio, the electromotive force of the oxygen sensor  11  increases. On a lean side of the stoichiometric air-fuel ratio, meanwhile, the electromotive force of the oxygen sensor  11  decreases. Hence, the oxygen sensor output value VO 2  [V] increases on the rich side and decreases on the lean side. 
     As described above, the sensor element of the oxygen sensor  11  exhibits a temperature characteristic. According to this characteristic, when the sensor element temperature is high, as shown by the dot-dot-dash line  302 , an amount by which the oxygen sensor output value VO 2  varies about the stoichiometric air-fuel ratio tends to decrease. When the sensor element temperature is low, as shown by the dotted line  301 , on the other hand, the amount by which the oxygen sensor output value VO 2  varies about the stoichiometric air-fuel ratio tends to increase. 
     Detection signals from the oxygen sensor  11  and the sensors  4 ,  6 ,  7  are input into the control unit  1  as operating condition information indicating operating conditions of the engine  19 . The operating condition information includes at least one of the oxygen sensor output value VO 2 , the throttle opening θ, an engine temperature Tw, and the engine rotation speed Ne. If required, the intake air temperature Ta, the intake air pressure Pa, and the crank angle signal SGT may also be input from the sensors  2 ,  5 ,  7 . On the basis of the operating condition information, the control unit  1  outputs drive signals to various actuators such as the throttle actuator  4 A and the ignition coil  9 . 
     Further, a display device  13  is provided in the control unit  1 . The display device  13  displays a control condition of the engine  19 , warning information, and so on to a driver of the vehicle. 
     The control unit  1  calculates an appropriate fuel injection timing and an appropriate fuel injection amount in relation to the intake pipe  14  on the basis of the operating condition information and the oxygen sensor output value VO 2  from the oxygen sensor  11 , and outputs a drive signal to the fuel injection module  8 . 
     Further, the control unit  1  calculates an appropriate ignition timing on the basis of the operating condition information, and outputs an ignition signal to the ignition coil  9 . The ignition coil  9  applies a high voltage required for spark discharge to the spark plug  9 A on the basis of the ignition signal. As a result, an air-fuel mixture in a combustion chamber of the engine  19  undergoes explosive combustion. 
     The exhaust gas Ah from the engine  19  is discharged into the atmosphere from the exhaust pipe  10 . The three-way catalyst  12  for purifying the exhaust gas is provided in the exhaust pipe  10 . The three-way catalyst  12  is an effective device for reducing a plurality of harmful components contained in the exhaust gas Ah simultaneously. The three-way catalyst  12  performs an HC or CO oxidation reaction and a NO x  reduction reaction simultaneously. 
     In the engine control apparatus according to the first embodiment, the oxygen sensor output value VO 2  is not simply replaced with a specific air-fuel ratio. In the engine control apparatus according to the first embodiment, the air-fuel ratio is classified into four or more magnitude regions on the basis of the oxygen sensor output value VO 2 . Further, a gain used during air-fuel ratio feedback control is determined in accordance with the region in which the air-fuel ratio has been classified. A feedback control correction amount that is appropriate for the air-fuel ratio region of the engine  19  is then calculated using the gain obtained in this manner. 
     In accordance with the characteristic of the oxygen sensor  11 , as shown in  FIG. 3 , when the air-fuel ratio is rich or lean, the oxygen sensor output value VO 2  varies by a small amount in response to variation in the air-fuel ratio. When the air-fuel ratio is in the vicinity of the stoichiometric air-fuel ratio, however, the oxygen sensor output value VO 2  varies by a large amount in response to variation in the air-fuel ratio. In other words, the oxygen sensor output value VO 2  varies about a specific air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio. The specific air-fuel ratio depends on the characteristics of the oxygen sensor  11 , for example, but may be an air-fuel ratio of 13.5 on the rich side and 15.5 on the lean side, for example. Hence, in the first embodiment, attention will be focused on the fact that the rich side and the lean side can respectively be divided into two regions. 
     In this embodiment, therefore, as shown in  FIG. 3 , the air-fuel ratio is classified into the following four regions. Here, 14.7 is the stoichiometric air-fuel ratio. 
     In a first rich region R 1 , the air-fuel ratio is smaller than 13.5. In a second rich region R 2 , the air-fuel ratio is no smaller than 13.5 and smaller than 14.7. 
     In a second lean region R 3 , the air-fuel ratio is no smaller than 14.7 and smaller than 15.5. 
     In a first lean region R 4 , the air-fuel ratio is equal to or larger than 15.5. 
     As shown in  FIG. 3 , when the sensor element temperature of the oxygen sensor  11  is high, the oxygen sensor output value VO 2  shifts to the rich side. When the sensor element temperature of the oxygen sensor  11  is 500 degrees, as shown by the dotted line  301  in  FIG. 3 , for example, the voltage value at which the oxygen sensor output value VO 2  varies rapidly relative to the air-fuel ratio is 0.80 V. When the sensor element temperature of the oxygen sensor  11  is 900 degrees, as shown by the dot-dot-dash line  302  in  FIG. 3 , on the other hand, the voltage value at which the oxygen sensor output value VO 2  varies rapidly relative to the air-fuel ratio is 0.7 V. 
     When the sensor element temperature varies while the oxygen sensor output value VO 2  remains constant, the air-fuel ratio shifts steadily in a rich direction as the sensor element temperature increases. When the oxygen sensor output value VO 2  is 0.7 V, for example, the air-fuel ratio at a sensor element temperature of 500 degrees is 14.0, as shown by the dotted line  301  in  FIG. 3 , whereas the air-fuel ratio at a sensor element temperature of 900 degrees is 13.5, as shown by the dot-dot-dash line  302  in  FIG. 3 . 
     Hence, on the basis of the characteristic of the oxygen sensor output value VO 2  at a maximum value (900° C., for example) of the sensor element temperature that can be envisaged from the usage environment of the vehicle, the voltage values at which the oxygen sensor output value VO 2  varies rapidly are set as the first determination voltage (0.70 V, for example) and the second determination voltage (0.20 V, for example). The air-fuel ratio is then classified into the four regions in accordance with the oxygen sensor output value VO 2  using the voltage value (0.45 V, for example) indicating the stoichiometric air-fuel ratio, the first determination voltage, and the second determination voltage. As a result, as shown in  FIG. 3 , air-fuel ratios included in the first rich region R 1  are richer than at least 13.5, regardless of the sensor element temperature. Further, air-fuel ratios included in the second rich region R 2  are between the stoichiometric air-fuel ratio (an air-fuel ratio of 14.7) and 13.5. Likewise on the lean side, from the relationship between the second determination voltage and the air-fuel ratio at the respective sensor element temperatures, air-fuel ratios included in the first lean region R 4  are leaner than at least 15.5, while air-fuel ratios included in the second lean region R 3  are between the stoichiometric air-fuel ratio (an air-fuel ratio of 14.7) and 15.5. 
     By classifying the air-fuel ratio into four regions in accordance with the oxygen sensor output value VO 2 , an air-fuel ratio feedback gain can be calculated for each air-fuel ratio region. For example, absolute values of the air-fuel ratio feedback gains of the first rich region R 1  and the first lean region R 4  can be made larger than absolute values of the air-fuel ratio feedback gains of the second rich region R 2  and the second lean region R 3 . 
     In the engine control apparatus according to the first embodiment, as described above, the air-fuel ratio condition of the engine  19  is classified into at least four regions, namely the first rich region R 1 , the second rich region R 2 , the first lean region R 4 , and the second lean region R 3 , in accordance with the oxygen sensor output value VO 2 . 
     The control unit  1  compares the oxygen sensor output value VO 2  with the first determination voltage. The control unit  1  then makes following determinations. 
     (1) When the oxygen sensor output value VO 2  equals or exceeds the first determination voltage, the air-fuel ratio is determined to be in the first rich region R 1 . 
     (2) When the oxygen sensor output value VO 2  equals or exceeds the voltage value indicating the stoichiometric air-fuel ratio but is lower than the first determination value, the air-fuel ratio is determined to be in the second rich region R 2 . 
     (3) When the oxygen sensor output value VO 2  is lower than the second determination value, the air-fuel ratio is determined to be in the first lean region R 4 . 
     (4) When the oxygen sensor output value VO 2  is lower than the voltage value indicating the stoichiometric air-fuel ratio but equals or exceeds the second determination value, the air-fuel ratio is determined to be in the second lean region R 3 . 
     Next, using  FIG. 2 , an interior configuration of the control unit  1  of the engine control apparatus according to the first embodiment will be described. 
     In  FIG. 2 , a sensor group  15  includes the respective sensors  2  and  4  to  7  shown in  FIG. 1 . The operating condition information from the sensor group  15  includes at least one of the engine rotation speed Ne, the throttle opening θ, and the engine temperature Tw. The operating condition information is input into the control unit  1 . If necessary, the operating condition information may also include the intake air temperature Ta, the intake air pressure Pa, and the crank angle signal SGT. The oxygen sensor  11  inputs the oxygen sensor output value VO 2  into the control unit  1 . 
     The control unit  1  includes, in addition to an ignition timing control unit (not shown) that controls an ignition timing, an air-fuel ratio feedback control unit  20  shown in  FIG. 2 . The ignition timing control unit is not a main feature of this invention, and will not therefore be described specifically in this embodiment. The control unit  1  adjusts an amount of fuel injected into the engine  19  on the basis of the operating condition information from the sensor group  15  and the oxygen sensor output value VO 2  from the oxygen sensor  11 . The control unit  1  exchanges various information with the memory  30 , which includes a non-volatile memory  27 . 
     The air-fuel ratio feedback control unit  20  provided in the control unit  1  performs air-fuel ratio feedback control such that the oxygen sensor output value VO 2  matches a voltage value (a target voltage≅0.45 V) VO 2   t  indicating the stoichiometric air-fuel ratio. 
     The air-fuel ratio feedback control unit  20  includes an air-fuel ratio region detection unit  21 , a proportional gain calculation unit  22 , an integral gain calculation unit  23 , a control correction amount calculation unit  24 , and a fuel injection driving unit  25 . 
     The air-fuel ratio region detection unit  21  determines the air-fuel ratio of the engine  19  from the oxygen sensor output value VO 2 . More specifically, the air-fuel ratio region detection unit  21  determines the region to which the current air-fuel ratio of the engine  19  belongs, among the four or more divided regions, on the basis of the oxygen sensor output value VO 2  from the oxygen sensor  11 , the target voltage VO 2   t , and the first and second determination voltages. The four or more regions are set by dividing the rich side of the stoichiometric air-fuel ratio into at least two regions and dividing the lean side of the stoichiometric air-fuel ratio into at least two regions. The two or more regions on the rich side include a region in which the oxygen sensor output value VO 2  increases gently and a region in which the oxygen sensor output value VO 2  increases rapidly. On the rich side, a rate of change (an incline on a graph) of the oxygen sensor output value VO 2  varies rapidly from a certain air-fuel ratio value (13.5 in  FIG. 3 ), and therefore the rich side is divided into two regions about this air-fuel ratio value. Similarly, the two or more regions on the lean side include a region in which the oxygen sensor output value VO 2  decreases gently and a region in which the oxygen sensor output value VO 2  decreases rapidly. In this embodiment, for example, the rich side includes the first region R 1  and the second rich region R 2 , while the lean side includes the first lean region R 4  and the second lean region R 3 . On the lean side, the rate of change (the incline on a graph) of the oxygen sensor output value VO 2  varies rapidly from a certain air-fuel ratio value (15.5 in  FIG. 3 ), and therefore the lean side is divided into two regions about this air-fuel ratio value. 
     A proportional gain switch unit (not shown) and a proportional gain map (see  FIG. 5A ) are provided in the proportional gain calculation unit  22 . The proportional gain calculation unit  22  calculates a proportional gain Gp 1  corresponding to a proportional term of the air-fuel ratio feedback control. The proportional gain Gp 1  is set for each of the air-fuel ratio regions of the engine  19 , and stored in advance on the proportional gain map. The proportional gain calculation unit  22  obtains the corresponding proportional gain Gp 1  from the proportional gain map on the basis of the air-fuel ratio region of the engine  19 , determined by the air-fuel ratio region detection unit  21 . The proportional gain calculation unit  22  uses the proportional gain switch unit to update the current proportional gain to the proportional gain obtained from the map. The proportional gain calculation unit  22  is also capable of correcting the proportional gain Gp on the basis of the rotation speed Ne of the engine  19 , the throttle opening θ, and the intake air pressure Pa using the information from the sensor group  15 . 
     An integral gain switch unit (not shown) and an integral gain map (see  FIG. 6A ) are provided in the integral gain calculation unit  23 . The integral gain calculation unit  23  calculates an integral gain Gi corresponding to an integral term of the air-fuel ratio feedback control. The integral gain Gi is stored in advance on the integral gain map for each of the air-fuel ratio regions of the engine  19 . The integral gain calculation unit  23  obtains the corresponding integral gain Gi from the integral gain map on the basis of the air-fuel ratio region of the engine  19 , determined by the air-fuel ratio region detection unit  21 . The integral gain calculation unit  23  uses the integral gain switch unit to update the current integral gain to the obtained integral gain. The integral gain calculation unit  23  is also capable of correcting the integral gain Gi on the basis of the rotation speed Ne of the engine  19 , the throttle opening θ, and the intake air pressure Pa using the information from the sensor group  15 . 
     The control correction amount calculation unit  24  calculates an air-fuel ratio feedback control correction amount Kfb on the basis of at least one of the proportional gain Gp and the integral gain Gi using preset calculation formulae (Equations (1) and (2) to be described below, for example). As a result, the oxygen sensor output value VO 2  undergoes air-fuel ratio feedback control so as to match the voltage value (the target voltage≅0.45 V) VO 2   t  indicating the stoichiometric air-fuel ratio. 
     The fuel injection driving unit  25  drives the fuel injection module  8  on the basis of the air-fuel ratio feedback control correction amount Kfb. 
     An operation of the air-fuel ratio feedback control unit  20  will be described in detail below with reference to  FIGS. 1 to 3 , a flowchart shown in  FIG. 4 , and illustrative views shown in  FIGS. 5 and 6 . 
     In  FIG. 4 , first, in step S 101 , the air-fuel ratio feedback control unit  20  reads the operating condition information indicating the operating conditions of the engine  19  from the various sensors. In other words, the air-fuel ratio feedback control unit  20  reads the operating condition information from the oxygen sensor  11  and the sensor group  15  connected to the control unit  1 . The sensor group  15  includes the intake air temperature sensor  2 , the throttle position sensor  4 , the intake air pressure sensor  5 , the engine temperature sensor  6 , and the crank angle sensor  7 . However, the operating condition information does not have to include all of the operating condition information from these sensors. 
     Next, in step S 102 , the air-fuel ratio feedback control unit  20  determines on the basis of the operating condition information of the engine whether or not an air-fuel ratio feedback control condition is established. The air-fuel ratio feedback control condition is established when, for example, “the oxygen sensor  11  is activated”, “the oxygen sensor  11  is not broken”, “a fuel cut is not underway”, and so on. A determination as to whether or not the oxygen sensor  11  is activated can be made by comparing the oxygen sensor output value with a preset activation determination threshold. Note, however, that the activation determination threshold differs according to the type of the oxygen sensor and an oxygen sensor input circuit of the control unit. Further, depending on the type of the oxygen sensor and the oxygen sensor input circuit of the control unit, the oxygen sensor  11  may be determined to be activated either when the oxygen sensor output value is higher than the threshold or when the oxygen sensor output value is lower than the threshold. Hence, a specific determination threshold at which to determine that “the oxygen sensor  11  is activated” is set appropriately in accordance with the type of the oxygen sensor and the oxygen sensor input circuit of the control unit. 
     When, as a result of the determination of step S 102 , the air-fuel ratio feedback control condition is established, the processing advances to step S 103 . When the air-fuel ratio feedback control condition is not established, on the other hand, the processing advances to step S 108 . 
     In step S 108 , the air-fuel ratio feedback control correction amount Kfb is set at 1.0 and a sum SGi of the integral gain is set at 0. The processing then returns to step S 101 , whereupon the routine is repeated. 
     In step S 103 , meanwhile, the air-fuel ratio feedback control unit  20  uses the air-fuel ratio region detection unit  21  to determine, on the basis of the oxygen sensor output value VO 2 , the air-fuel ratio region to which the air-fuel ratio of the engine  19  belongs, among the four or more air-fuel ratio regions. 
     More specifically, as shown in  FIG. 3 , the air-fuel ratio regions of the engine are determined on the basis of the relationship of the oxygen sensor output value VO 2  to the first determination voltage, the second determination voltage, and the target voltage VO 2   t . Note that here, the target voltage VO 2   t  is the voltage value indicating the stoichiometric air-fuel ratio. The target voltage VO 2   t  is 0.45 V, for example. 
     The first determination voltage and the second determination voltage are recorded in the memory  30  of the control unit  1  in advance. The first determination voltage and the second determination voltage are determined by determining, through experiment, the voltage values of the subject oxygen sensor  11  at which the rate of change in the oxygen sensor output value VO 2  varies rapidly relative to the air-fuel ratio when the sensor element temperature of the oxygen sensor is high. A temperature variation range of the sensor element temperature of the oxygen sensor  11  may be envisaged from the actual usage environment and operating conditions of the engine  19 . The first determination voltage is higher than the target voltage VO 2   t , and the second determination voltage is lower than the target voltage VO 2   t . Here, the first determination voltage is set at 0.70 V, and the second determination voltage is set at 0.20 V. 
     As indicated by an air-fuel ratio determination voltage updating unit  21   c  according to a modified example of the first embodiment shown in  FIG. 8 , to be described below, the first determination voltage and the second determination voltage may be updated to optimum determination voltages for the sensor element temperature on the basis of an estimation result of the sensor element temperature of the oxygen sensor  11 . 
     As described above, the air-fuel ratio region detection unit  21  determines the air-fuel ratio region to which the current air-fuel ratio of the engine  19  belongs as follows. 
     When the oxygen sensor output value VO 2  the first determination voltage, the air-fuel ratio region of the engine  19  is determined to be the first rich region R 1 . 
     When the target voltage VO 2   t  the oxygen sensor output value VO 2 &lt;the first determination voltage, the air-fuel ratio region of the engine  19  is determined to be the second rich region R 2 . When the oxygen sensor output value VO 2 &lt;the second determination voltage, the air-fuel ratio region of the engine  19  is determined to be the first lean region R 4 . 
     When the second determination voltage≦the oxygen sensor output value VO 2 &lt;the target voltage VO 2   t , the air-fuel ratio region of the engine  19  is determined to be the second lean region R 3 . 
     Between the first rich region R 1  and the second rich region R 2 , the air-fuel ratio is smaller in the first rich region R 1  than in the second rich region R 2 . Further, between the first lean region R 4  and the second lean region R 3 , the air-fuel ratio is larger in the first lean region R 4  than in the second lean region R 3 . In other words, the respective air-fuel ratio regions indicate the degree of richness and the degree of leanness. 
     Note that when the air-fuel ratio is classified into more than four regions, a third determination voltage, a fourth determination voltage, and so on may be added. 
     In step S 104 , the air-fuel ratio feedback control unit  20  uses the proportional gain calculation unit  22  to calculate the proportional gain Gp 1 . Next, the air-fuel ratio feedback control unit  20  uses the integral gain calculation unit  23  to calculate the integral gain Gi 1  in step S 105 . In the air-fuel ratio feedback control according to the first embodiment, proportional/integral (PI) feedback having a proportional gain and an integral gain is used in each of the air-fuel ratio regions of the engine  19  determined in step S 103  to cause the oxygen sensor output value VO 2  to converge on the target voltage VO 2   t.    
     A method employed in step S 104  to determine the proportional gain will now be described. 
     The proportional gain of feedback control is typically used to correct an output value in proportion with a deviation between a target value and a current value of a control subject. In the air-fuel ratio feedback control according to the first embodiment, however, the proportional gain Gp 1  is calculated from the proportional gain map shown in  FIG. 5A  using the air-fuel ratio region of the engine  19  as an axis. On the proportional gain map, the proportional gain Gp 1  is set in advance for each air-fuel ratio region of the engine  19 . 
     More specifically, when the air-fuel ratio region of the engine  19  is the first rich region R 1  or the first lean region R 4 , the oxygen sensor output value VO 2  deviates greatly from the target voltage VO 2   t . Therefore, given that the air-fuel ratio deviation is large, an absolute value of the proportional gain in the first rich region R 1  and the first lean region R 4  is larger than the absolute value of the proportional gain in the second rich region R 2  and the second lean region R 3 . 
     When the air-fuel ratio region of the engine  19  is the second rich region R 2  or the second lean region R 3 , the oxygen sensor output value VO 2  is close to the target voltage VO 2   t . Therefore, given that the air-fuel ratio deviation is small, the absolute value of the proportional gain in the second rich region R 2  and the second lean region R 3  is smaller than the absolute value of the proportional gain in the first rich region R 1  and the first lean region R 4 . The proportional gain Gp 1  is set on the proportional gain map in this manner for each of the air-fuel ratio regions of the engine  19 . The proportional gain Gp 1  is set through experiment by envisaging the deviation between the oxygen sensor output value VO 2  and the target voltage VO 2   t  in each air-fuel ratio region of the engine  19 . 
     The proportional gain map generated in this manner is stored in advance in the memory  30 . The proportional gain calculation unit  22  obtains the corresponding proportional gain Gp 1  from the proportional gain map on the basis of the air-fuel ratio region determined in step S 103 . 
     Note that the exhaust gas reaches the oxygen sensor  11  at different times depending on the rotation speed and the load of the engine  19 . Therefore, a correction may be applied to the proportional gain in accordance with the rotation speed and the load of the engine  19  so that the proportional gain is corrected to an optimum proportional gain in accordance with the operating conditions of the engine  19 . 
     A method employed in step S 105  to determine the integral gain will now be described. 
     The integral gain of feedback control is typically used to correct the output value in proportion with an integrated deviation between the target value and the current value of the control subject. In the air-fuel ratio feedback control according to the first embodiment, however, the integral gain Gi 1  is calculated from an integral gain map shown in  FIG. 6A  using the air-fuel ratio region of the engine  19  as an axis. On the integral gain map, the integral gain Gi 1  is set in advance for each air-fuel ratio region of the engine  19 . 
     Note that the method of setting the integral gain Gi 1  is identical to the method of setting the proportional gain Gp 1 , described above, and therefore an absolute value of the integral gain in the first rich region R 1  and the first lean region R 4  is set to be larger than the absolute value of the integral gain in the second rich region R 2  and the second lean region R 3 . 
     Further, similarly to the proportional gain, the integral gain may be corrected in accordance with the operating conditions of the engine  19 . 
     Note that here, the proportional gain calculation unit  22 , the integral gain calculation unit  23 , and the control correction amount calculation unit  24  together constitute an air-fuel ratio feedback control correction amount calculation unit that calculates a first feedback control correction amount for setting the gain of the air-fuel ratio feedback control in accordance with the air-fuel ratio region of the engine  19 , detected by the air-fuel ratio region detection unit  21 . 
     Here, the first feedback control correction amount is the air-fuel ratio feedback control correction amount Kfb calculated from the proportional gain Gp 1  and the integral gain Gi 1 . 
     In step S 106 , a sum SGi (n) of the integral gain Gi 1  is calculated using the integral gain Gi 1  obtained in step S 105  in accordance with Equation (1), shown below, whereupon the processing advances to step S 107 . Note that the integral gain Gi 1  is inserted into Gi in Equation (1).
 
 SGi ( n )= SGi ( n− 1)+ Gi   (1)
 
     Here, SGi (n) is the current sum of the integral gain Gi 1 , and SGi (n−1) is the previous sum of the integral gain Gi 1 . 
     The operation performed in step S 106  corresponds to an integration operation performed on the deviation of the control subject during typical feedback control, where the integral gain itself serves as the deviation of the control subject. This method is in wide general use as a simplified feedback control method suitable for feedback-controlling the air-fuel ratio of a motorcycle. 
     Next, in step S 107 , the air-fuel ratio feedback control correction amount Kfb is calculated using the sum SGi (n) of the integral gain Gi 1 , determined in step S 106 , in accordance with Equation (2), shown below, whereupon the processing returns to step S 101  so that the routine can be repeated. Note that Gp 1  obtained in step S 105  is input into Gp in Equation (2).
 
 Kfb= 1.0+ Gp+SGi ( n )  (2)
 
     After calculating the air-fuel ratio feedback control correction amount Kfb in the manner described above, the air-fuel ratio feedback control correction amount Kfb is input into the fuel injection driving unit  25 . 
     According to the first embodiment, as described above, the air-fuel ratio of the engine  19  is classified as one of four or more regions on the basis of the oxygen sensor output value VO 2  from the oxygen sensor  11 , whereupon optimum proportional and integral gains for the air-fuel ratio feedback control are selected in accordance with the air-fuel ratio region. As a result, convergence on the target voltage can be achieved more quickly than with the binary-based (i.e. based on whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio) air-fuel ratio feedback control that is currently used widely. Further, in the first embodiment, the first and second determination voltages used to classify the air-fuel ratio of the engine  19  are set in consideration of whether the sensor element temperature of the oxygen sensor  11  is high or low. Hence, there is no need to estimate the sensor element temperature of the oxygen sensor  11  while the engine  19  is operative, and therefore the air-fuel ratio feedback control can be implemented even by an inexpensive, low-performance CPU. 
     Although there is no need to estimate the sensor element temperature of the oxygen sensor  11  in the first embodiment, as described above, when it is possible to estimate the sensor element temperature, the oxygen sensor output value VO 2  can be used even more effectively. Advantages of estimating the sensor element temperature will be described below. 
     The first determination voltage and second determination voltage of the first embodiment are determined on the basis of the voltage values at which the rate of change in the oxygen sensor output value VO 2  varies rapidly relative to the air-fuel ratio when the sensor element temperature of the oxygen sensor  11  is high within the range of the actual usage environment of the engine  19 . When the sensor element temperature is not estimated, the second rich region R 2  and the second lean region R 3  among the air-fuel ratio regions of the engine  19  are in actuality limited to air-fuel ratio regions that also take into account a case in which the sensor element temperature of the oxygen sensor  11  is low. More specifically, as shown in  FIG. 3 , in a case where the first determination voltage is 0.7 V, for example, the air-fuel ratio corresponds to 13.5 when the sensor element temperature is high, but corresponds to 14.0 when the sensor element temperature is low. Therefore, the air-fuel ratios indicating the second rich region R 2  of the air-fuel ratio regions of the engine  19  are limited to a range extending from the stoichiometric air-fuel ratio (=14.7) to 14.0. Likewise on the lean side, the air-fuel ratios indicating the second lean region R 3  are limited to a narrow range. In this case also, the proportional gain and the integral gain of the air-fuel ratio feedback control can be set at optimum values, and therefore convergence of the oxygen sensor voltage on the target voltage can be achieved quickly. However, when the air-fuel ratio ranges of the second rich region R 2  and the second lean region R 3  are narrow, the air-fuel ratio of the engine  19  is frequently determined to be within the first rich region R 1  or the first lean region R 4  during an actual engine operation. As a result, it may be difficult to set the proportional gain and the integral gain of the air-fuel ratio feedback control at large values in the first rich region R 1  and the first lean region R 4 . 
     Therefore, the sensor element temperature is estimated, whereupon the first determination voltage and the second determination voltage are updated in accordance with the sensor element temperature. In so doing, the respective air-fuel ratio ranges of the second rich region R 2  and the second lean region R 3  among the air-fuel ratio regions of the engine  19  can be widened. 
     More specifically, as shown in  FIG. 7 , when the sensor element temperature of the oxygen sensor  11  is high, the first determination voltage is set at 0.7V (corresponding to an air-fuel ratio of 13.5), for example, and when the sensor element temperature is low, the first determination voltage is updated to 0.80 V, i.e. the voltage value corresponding to an air-fuel ratio of 13.5. As a result, the air-fuel ratio indicated by the first determination voltage remains at 13.5 at all times, regardless of the sensor element temperature. In this case, the air-fuel ratio range of the second rich region R 2  is widened to a range extending from the stoichiometric air-fuel ratio (=14.7) to 13.5, and therefore the air-fuel ratio feedback gains of the first rich region R 1  can be set at larger values than those of the first embodiment, described above. As a result, convergence on the target voltage can be improved. 
     However, when an inexpensive CPU such as that used in a motorcycle is employed, it is difficult to estimate the sensor element temperature of the oxygen sensor  11  accurately. Therefore, when the first determination voltage and second determination voltage are updated on the basis of the estimation result of the sensor element temperature and an error occurs in the estimation result of the sensor element temperature, the convergence performance of the air-fuel ratio feedback control may deteriorate. 
     To reduce this risk, it is effective to implement air-fuel ratio feedback control based on the air-fuel ratio region of the engine  19  only when the operating conditions of the engine  19  indicate a transient operation, and to implement binary-based air-fuel ratio feedback control, i.e. feedback control based on whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio, when the operating conditions of the engine  19  indicate a steady state operation. 
     In a case where the air-fuel ratio of the engine  19  is very rich or very lean, intake air may be introduced rapidly into the engine  19  when the vehicle accelerates, leading to a deficiency in the fuel injection amount. When the vehicle decelerates, meanwhile, the amount of intake air may decrease, causing the fuel injection amount to become excessive. Hence, the large air-fuel ratio feedback gains obtained when the air-fuel ratio region of the engine  19  is classified as the first rich region R 1  or the first lean region R 4  are effective for improving convergence on the target voltage of the oxygen sensor  11 . By employing air-fuel ratio feedback control based on the air-fuel ratio region of the engine  19  only when the engine  19  is in a transient operating condition, situations in which the convergence performance of the air-fuel ratio feedback control deteriorates can be limited even when an estimation error occurs in the sensor element temperature such that the gains of the air-fuel ratio feedback control are not optimal. 
     This modified example of the first embodiment of the invention will be described below with reference to  FIGS. 7 to 10 . 
     An overall configuration of the engine control apparatus according to this modified example is as shown in  FIG. 1 . The control unit  1  employs a configuration shown in  FIG. 8  rather than the configuration shown in  FIG. 2 . 
       FIG. 2  and  FIG. 8  differ from each other in that in  FIG. 8 , an engine transient operating condition detection unit  21   a , an oxygen sensor element temperature estimation unit  21   b , and an air-fuel ratio determination voltage updating unit  21   c  are added to the configuration shown in  FIG. 2 .  FIG. 8  also differs from  FIG. 2  in that processing for switching the proportional gain and the integral gain in accordance with the transient operating condition of the engine  19  is added to  FIG. 8 . 
     The engine transient operating condition detection unit  21   a  determines whether the engine  19  is in the transient operating condition or the steady state operating condition on the basis of at least one of the engine rotation speed Ne, the throttle opening θ, and the intake air pressure Pa. 
     The oxygen sensor element temperature estimation unit  21   b  estimates the sensor element temperature of the oxygen sensor  11  on the basis of the engine rotation speed Ne and the throttle opening θ. 
     The air-fuel ratio determination voltage updating unit  21   c  determines on the basis of the estimated temperature of the sensor element of the oxygen sensor  11  whether or not a determination voltage updating condition is established, and when the determination voltage updating condition is established, updates the first determination voltage and second determination voltage for determining the air-fuel ratio region of the engine  19 . Note that when the estimated temperature of the sensor element is higher than a threshold, the first determination voltage and second determination voltage are reduced below current values, and when the estimated temperature of the sensor element is equal to or lower than the threshold, the first determination voltage and second determination voltage are increased above the current values. 
     All other configurations are identical to  FIG. 2 , and will not therefore be described here. 
       FIG. 9  is a flowchart showing calculation processing performed by the air-fuel ratio feedback control unit according to this modified example.  FIG. 9  differs from the flowchart shown in  FIG. 4 , described above, in the addition of a step for detecting the transient operating condition of the engine  19  (S 103   a ), a step for estimating the sensor element temperature of the oxygen sensor  11  (S 103   b ), a step for updating the first determination voltage and second determination voltage (S 103   c ), and processing for switching the proportional gain and the integral gain in accordance with the transient operating condition of the engine  19  (S 104   a , S 105   a ). Here, only these additional steps will be described. 
     Note that when step numbers in the flowchart of  FIG. 9  are identical to the step numbers in the flowchart in  FIG. 4 , identical operations are performed in those steps. 
     In step S 103   a , the engine transient operating condition detection unit  21   a  determines, on the basis of signals from the sensor group  15 , whether or not the engine  19  is in the transient operating condition, which corresponds to an acceleration operating condition or a deceleration operating condition. The determination as to whether or not the engine  19  is in the transient operating condition is made by determining whether or not one or a combination of two or more of the following three conditions is established: (1) an amount of variation in the engine rotation speed Ne equals or exceeds a threshold; (2) an amount of variation in the throttle opening θ equals or exceeds a threshold; and (3) an amount of variation in the intake air pressure Pa equals or exceeds a threshold. In other words, when the determination is made using one of the three conditions, the condition is selected from the three conditions in advance, and when the condition is established, the engine  19  is determined to be in the transient operating condition. Alternatively, the engine  19  is determined to be in the transient operating condition when any one of the three conditions is established. When the determination is made using a combination of two or more of the conditions, the two or more conditions are selected in advance from the three conditions, and when all of the two or more conditions are established, the engine  19  is determined to be in the transient operating condition. Alternatively, the engine  19  is determined to be in the transient operating condition when any two or more of the three conditions are established. 
     In step S 103   b , the oxygen sensor element temperature estimation unit  21   b  estimates/calculates a sensor element temperature Toe of the oxygen sensor  11  using an oxygen sensor basic map based on the engine rotation speed Ne and the throttle opening θ.  FIG. 10A  shows an example of the oxygen sensor basic map. The oxygen sensor basic map is a three-dimensional map having the engine rotation speed Ne and the throttle opening θ as axes. Values of the sensor element temperature Toe are set in advance on the oxygen sensor basic map in association with the engine rotation speed Ne and the throttle opening θ.
         Note that the oxygen sensor basic map is not limited to the example shown in  FIG. 10A , and as shown in  FIG. 10B , the intake air pressure Pa may be used instead of the throttle opening θ.       

     Instead of estimating/calculating the sensor element temperature Toe from the map shown in  FIG. 10A  or  FIG. 10B , a temperature sensor may be attached to the oxygen sensor  11  so that the sensor element temperature is measured directly. When the sensor element temperature is measured directly, the processing skips step S 103   b  and advances to step S 103   c.    
     A method of estimating/calculating the sensor element temperature Toe in step S 103   b  will now be described. 
     First, an estimated basic sensor temperature Toeb serving as a basic value of the sensor element temperature Toe is calculated on the basis of the engine rotation speed Ne and the throttle opening θ from an oxygen sensor basic temperature map ( FIG. 10A ) having the engine rotation speed Ne and the throttle opening θ as axes. 
     The oxygen sensor basic temperature map shown in  FIG. 10A  is obtained by attaching a temperature sensor capable of measuring the sensor element temperature directly to the oxygen sensor  11  during calibration of the vehicle prior to shipment, and measuring the sensor element temperature of the oxygen sensor  11  accurately at each engine load by experiment. 
     Note that a temperature sensor is attached to the oxygen sensor  11  only during calibration of the vehicle, and in the case of a mass-produced vehicle, a temperature sensor is not typically attached during estimation/calculation of the sensor element temperature Toe (step S 103   b ). 
     Further, in the case of a vehicle to which an O2 heater (not shown) is attached in order to activate the oxygen sensor  11 , a correction is performed in accordance with the effect of heat generated by the O2 heater on the sensor element temperature Toe and disturbance variation such as exhaust gas temperature variation caused by variation in the ignition timing of the engine  19  and the air-fuel ratio. 
     For example, as specific content of the correction, a voltage applied to the O2 heater is controlled by PWM control following the elapse of a fixed time after the oxygen sensor  11  is activated so as to prevent the amount of heat generated by the O2 heater from varying due to variation in a power supply voltage of an in-vehicle battery (not shown). In so doing, the amount of heat generated by the O2 heater can be kept constant regardless of variation in the power supply voltage, and as a result, the amount of heat generated by the O2 heater can be reproduced when creating the oxygen sensor basic temperature map of  FIG. 10A . 
     Further, to deal with an increase in the exhaust gas temperature occurring when the ignition timing varies from an advanced side to a retarded side, a correction map (not shown) having the ignition timing as an axis may be prepared so that the estimated/calculated sensor element temperature Toe can be corrected when the ignition timing varies to the retarded side of the ignition timing during creation of the oxygen sensor basic temperature map of  FIG. 10 . 
     Similarly, in view of the fact that the exhaust gas temperature decreases when the air-fuel ratio is on the rich side and increases when the air-fuel ratio is on the lean side, a correction map (not shown) having the air-fuel ratio as an axis may be prepared so that the estimated/calculated sensor element temperature Toe can be corrected in accordance with variation in the air-fuel ratio. 
     Furthermore, the exhaust gas temperature increases when the intake air temperature Ta of the engine  19  increases, for example, and therefore the estimated value of the sensor element temperature Toe is corrected by comparing the intake air temperature Ta with the intake air temperature Ta during creation of the oxygen sensor basic temperature map of  FIG. 10 . 
     By implementing the correction processing described above on the estimated basic sensor temperature Toeb, the final sensor element temperature (the estimated value thereof) Toe is calculated. 
     Further, when the estimated value of the sensor element temperature Toe varies greatly in response to variation in the operating conditions, the fuel injection amount from the fuel injection module  8  is also affected, and therefore rapid variation in the sensor element temperature Toe is undesirable. Hence, filter calculation processing is implemented on the calculated sensor element temperature Toe, as shown below in Equation (3), in order to calculate a filter calculation-processed sensor element temperature Toef.
 
 Toef ( n )= Toe+Cf ×( Toe−Toef ( n− 1))/ R   (3)
 
     In Equation (3), Toef (n) denotes the newest filter calculation-processed oxygen sensor element temperature, and Toef (n−1) denotes the previous value thereof. Further, Cf is a filter factor having a resolution R. 
     Hence, in step S 103   b , the sensor element temperature Toef subjected to filter processing (smoothing calculation) using Equation (3) is set as the final sensor element temperature Toe. 
     In step S 103   c , the air-fuel ratio determination voltage updating unit  21   c  corrects and updates the determination voltages set in relation to the oxygen sensor output VO 2  in order to determine the air-fuel ratio regions of the engine in the following step S 103 . Here, it is assumed that the air-fuel ratio of the engine  19  is classified into four regions, and therefore the determination voltages to be corrected and updated are the first determination voltage that differentiates the first rich region R 1  from the second rich region R 2  and the second determination voltage that differentiates the first lean region R 4  from the second lean region R 3 . When the air-fuel ratio of the engine  19  is classified into more than four regions, the number of determination voltages may be increased and then corrected and updated using a similar method. The target voltage indicating the stoichiometric air-fuel ratio (=14.7) is used to differentiate the rich side from the lean side. 
     The determination voltages set in the control unit  1  are brought closer to the actual oxygen sensor output characteristic by implementing following processing. 
     First, when the amount of variation in the sensor element temperature of the oxygen sensor  11  remains at or above a threshold continuously for at least a set time, a determination voltage update condition is determined to be established. 
     When the determination voltage update condition is established, the first determination voltage and second determination voltage used to detect the air-fuel ratio of the engine  19  are updated on the basis of the oxygen sensor element temperature using Equation (4), shown below.
 
first determination voltage( n )=first determination voltage( n− 1)×Cof  (4)
 
     A correction coefficient Cof is determined in accordance with the oxygen sensor element temperature so as to be smaller when the sensor element temperature is higher than a reference sensor element temperature Tst and larger when the sensor element temperature is lower than the reference sensor element temperature Tst. 
     Further, the sensor element temperature and the characteristic of the oxygen sensor may be measured by experiment, and on the basis of the results, determination voltages may be prepared in advance for each sensor element temperature. 
     A similar correction to that of Equation (4) is implemented likewise on the second determination voltage, although in the case of the second determination voltage, Cof is increased when the sensor element temperature is higher than the reference sensor element temperature Tst and reduced when the sensor element temperature is lower than the reference sensor element temperature Tst. 
     In step S 104   a , proportional gains Gp 1 , Gp 2  are determined, and in step S 105   a , integral gains Gi 1 , Gi 2  are determined. 
     A method employed in step S 104   a  to determine the proportional gains is as follows. 
     First, the proportional gain Gp 1  based on the air-fuel ratio region of the engine  19  is calculated from a map of the air-fuel ratio region of the engine  19  and the proportional gain Gp 1 , such as that shown in  FIG. 5A . In other words, when the air-fuel ratio of the engine  19  belongs to the first rich region R 1 , a value of −0.01 is derived as Gp 1 . 
     Next, the proportional gain Gp 2  is determined. The proportional gain Gp 2  is a proportional gain based on a determination result indicating whether the oxygen sensor output value VO 2  is a higher voltage or a lower voltage than (i.e. on the rich side or the lean side of) the stoichiometric air-fuel ratio (the target voltage) VO 2   t . The proportional gain Gp 2  is determined according to whether the air-fuel ratio of the engine  19  is rich or lean. Hence, although the proportional gain Gp 2  takes various values depending on operating conditions such as the engine rotation speed and the throttle opening, the value thereof is not affected by the magnitude of the air-fuel ratio of the engine  19 . 
     The proportional gain Gp 2  is calculated from a second proportional gain map such as that shown in  FIG. 5B  on the basis of the determination result indicating whether the air-fuel ratio of the engine  19  is rich or lean. A value of the proportional gain Gp 2  when the air-fuel ratio is lean and a value of the proportional gain Gp 2  when the air-fuel ratio is rich are stored respectively on the second proportional gain map shown in  FIG. 5B . In other words, when the oxygen sensor output value VO 2  is a high voltage (on the rich side), a value of −0.005 is derived as Gp 2 . 
     A method employed in step S 105   a  to determine the integral gains is as follows. 
     First, the integral gain Gi 1  based on the air-fuel ratio region of the engine  19  is calculated from a map of the air-fuel ratio region of the engine  19  and the integral gain Gi 1 , such as that shown in  FIG. 6A . In other words, when the air-fuel ratio of the engine  19  belongs to the first rich region R 1 , a value of −0.001 is derived as Gi 1 . 
     Next, the integral gain Gi 2  is determined. The integral gain Gi 2  is an integral gain based on a determination result indicating whether the oxygen sensor output value VO 2  is a higher voltage or a lower voltage than (i.e. on the rich side or the lean side of) the stoichiometric air-fuel ratio (the target voltage) VO 2   t . The integral gain Gi 2  is determined according to whether the air-fuel ratio of the engine  19  is rich or lean. Hence, although the integral gain Gi 2  takes various values depending on operating conditions such as the engine rotation speed and the throttle opening, the value thereof is not affected by the magnitude of the air-fuel ratio of the engine  19 . 
     The integral gain Gi 2  is calculated from a second integral gain map such as that shown in  FIG. 6B  on the basis of the determination result indicating whether the air-fuel ratio of the engine  19  is rich or lean. A value of the integral gain Gi 2  when the air-fuel ratio is lean and a value of the integral gain Gi 2  when the air-fuel ratio is rich are stored respectively on the second integral gain map shown in  FIG. 6B . In other words, when the oxygen sensor output value VO 2  is a high voltage (on the rich side), a value of −0.0005 is derived as Gi 2 . 
     In step S 106   a , the result obtained by the engine operating condition detection unit in step S 103   a  is referenced. When the engine  19  is in the transient operating condition, the processing advances to step S 106   b , and when the engine  19  is not in the transient operating condition, the processing advances to step S 106   c.    
     The final proportional gain Gp and the final integral gain Gi are then calculated in either step S 106   b  or step S 106   c . More specifically, when the engine  19  is in the transient operating condition, the final proportional gain Gp and the final integral gain Gi are calculated respectively as Gp=Gp 1  (the proportional gain based on the air-fuel ratio region of the engine) and Gi=Gi 1  (the integral gain based on the air-fuel ratio region of the engine) in step S 106   b , whereupon the processing advances to step S 106 . When the engine  19  is not in the transient operating condition, the final proportional gain Gp and the final integral gain Gi are calculated respectively as Gp=Gp 2  and Gi=Gi 2  in step S 106   c , whereupon the processing advances to step S 106 . 
     Hereafter, the air-fuel ratio feedback control correction amount Kfb calculated from the proportional gain Gp 2  and the integral gain Gi 2  will be referred to as a second feedback control correction amount. 
     Note that the second feedback control correction amount is determined by inserting Gi 2  and Gp 2  respectively as Gi and Gp in Equations (1) and (2), shown above. 
     Hence, according to this modified example of the first embodiment of this invention, by estimating the sensor element temperature of the oxygen sensor  11 , the air-fuel ratio region of the engine  19  can be selected in accordance with the sensor element temperature, enabling an improvement in the convergence performance when the air-fuel ratio of the engine  19  is much richer or much leaner than the target voltage, for example when the air-fuel ratio belongs to the first rich region R 1  or the first lean region R 4 . Further, by implementing the air-fuel ratio feedback control based on the air-fuel ratio region of the engine  19  only when the engine  19  is in the transient operating condition, adverse effects generated when an error occurs during estimation of the sensor element temperature of the oxygen sensor  11  can be suppressed. 
     In the first embodiment, as described above, the engine control apparatus includes the oxygen sensor  11  that outputs the oxygen sensor output value corresponding to the operating condition information of the engine  19  and the oxygen concentration of the exhaust gas, and the air-fuel ratio feedback control unit  20  that performs air-fuel ratio feedback control on the basis of the oxygen sensor output value VO 2  in order to adjust the amount of fuel injected into the engine  19 . 
     The air-fuel ratio feedback control unit  20  includes the air-fuel ratio region detection unit  21  that detects the air-fuel ratio region, among the four or more preset air-fuel ratio regions, to which the air-fuel ratio of the engine  19  belongs on the basis of the oxygen sensor output value VO 2 , and the air-fuel ratio feedback control correction amount calculation units  22   a ,  23   a ,  24  that calculate the first feedback control correction amount Kfb for use during the air-fuel ratio feedback control in accordance with the air-fuel ratio region detected by the air-fuel ratio region detection unit  21 . 
     Note that the four or more regions include at least the first rich region R 1  and the second rich region R 2 , which are set on the rich side of the stoichiometric air-fuel ratio in ascending order of the air-fuel ratio value, and the first lean region R 4  and the second lean region R 3 , which are set on the lean side of the stoichiometric air-fuel ratio in descending order of the air-fuel ratio value. The air-fuel ratio region detection unit  21  includes the first determination voltage, which is set at a higher value than a target voltage value that is a voltage value indicating the stoichiometric air-fuel ratio, and the second determination voltage, which is set at a lower value than the target voltage value. The air-fuel ratio region detection unit  21  compares the oxygen sensor output value VO 2  respectively with the first determination voltage and the second determination voltage. As a result of the determination, the air-fuel ratio region detection unit  21  determines that the air-fuel ratio of the engine  19  is within the first rich region R 1  when the oxygen sensor output value VO 2  equals or exceeds the first determination voltage, determines that the air-fuel ratio of the engine  19  is within the second rich region R 2  when the oxygen sensor output value VO 2  equals or exceeds the target voltage value but is lower than the first determination voltage, determines that the air-fuel ratio of the engine  19  is within the second lean region R 3  when the oxygen sensor output value VO 2  equals or exceeds the second determination voltage but is lower than the target voltage value, and determines that the air-fuel ratio of the engine  19  is within the first lean region R 4  when the oxygen sensor output value VO 2  is lower than the second determination voltage. 
     Hence, in the first embodiment, the air-fuel ratio of the engine  19  is classified into four or more regions on the basis of the oxygen sensor output value VO 2  of the oxygen sensor  11 , whereupon air-fuel ratio feedback control is implemented on the basis of the classification result. Therefore, when an error occurs during estimation of the sensor element temperature, the effect of the error can be reduced. Moreover, according to the first embodiment, convergence of the air-fuel ratio can be achieved more quickly than with binary air-fuel ratio feedback control based on the richness or leanness of the air-fuel ratio, which is in wide general use. Furthermore, there is no need to employ a large memory or a high-performance CPU, and no need to provide a sensor to measure the sensor element temperature of the oxygen sensor  11  directly. As a result, costs can be suppressed. 
     Moreover, according to the modified example of the first embodiment, the air-fuel ratio feedback control unit  20  further includes the oxygen sensor element temperature estimation unit  21   b  that estimates the temperature of the sensor element constituting the oxygen sensor  11 , and the air-fuel ratio determination voltage updating unit  21   c  that corrects at least one of the first determination voltage and the second determination voltage on the basis of the sensor element temperature estimated by the oxygen sensor element temperature estimation unit  21   b.    
     When the estimated temperature of the sensor element is higher than a reference value, the air-fuel ratio determination voltage updating unit  21   c  updates at least one of the first determination voltage and the second determination voltage such that the first determination voltage is reduced below the current value and the second determination voltage is increased above the current value, and when the estimated temperature of the sensor element is lower than the reference value, the air-fuel ratio determination voltage updating unit  21   c  updates at least one of the first determination voltage and the second determination voltage such that the first determination voltage is increased above the current value and the second determination voltage is reduced below the current value. Hence, the first determination voltage and the second determination voltage are corrected and updated on the basis of the sensor element temperature, and therefore, when the air-fuel ratio of the engine  19  is classified into four or more regions on the basis of the oxygen sensor output value VO 2  of the oxygen sensor  11 , the air-fuel ratio can be classified more accurately. As a result, convergence of the air-fuel ratio can be achieved even more quickly. 
     The air-fuel ratio feedback control unit  20  further includes the engine transient operating condition detection unit  21   a  that determines whether or not the engine  19  is in the transient operating condition on the basis of the operating conditions of the engine detected by the sensor group  15 . 
     The air-fuel ratio feedback control correction amount calculation units  22   a ,  23   a ,  24  determine whether or not the oxygen sensor output value equals or exceeds the target voltage value, calculate the second feedback control correction amount for use during air-fuel ratio feedback control corresponding to the determination result, output the first feedback control correction amount as the final feedback control correction amount when the transient operating condition detection unit  21   a  determines that the engine is in the transient operating condition, and output the second feedback control correction amount as the final feedback control correction amount when the transient operating condition detection unit  21   a  determines that the engine is not in the transient operating condition. 
     By implementing air-fuel ratio feedback control based on the air-fuel ratio region of the engine  19  only when the engine  19  is in the transient operating condition in this manner, adverse effects generated when an error occurs during estimation of the sensor element temperature of the oxygen sensor  11  can be suppressed. 
     Second Embodiment 
     Although not mentioned specifically in the first embodiment, the output value of the oxygen sensor  11  may vary due to manufacturing irregularities in and deterioration of the oxygen sensor  11 . In this case, the determination voltages set in advance in order to classify the air-fuel ratio of the engine  19  may not align with the characteristic of the oxygen sensor  11  during actual use. Hence, the determination voltages are preferably updated in response to manufacturing irregularities in and deterioration of the oxygen sensor  11 . 
     An overall configuration of an engine control apparatus according to the second embodiment of this invention is as shown in  FIG. 1 . The control unit  1  employs a configuration shown in  FIG. 11  in place of the configuration shown in  FIG. 8 . 
       FIG. 8  and  FIG. 11  differ from each other in that in  FIG. 11 , a sensor deterioration detection unit  26  and the non-volatile memory  27  are added to the configuration shown in  FIG. 8 . Further,  FIG. 11  differs from  FIG. 8  in that the air-fuel ratio determination voltage updating unit  21   c  updates the determination voltages in consideration of manufacturing irregularities in and deterioration of the oxygen sensor  11 . 
     The sensor deterioration detection unit  26  detects sensor deterioration of the oxygen sensor  11 . A detection method will be described below. 
     The non-volatile memory  27  stores the deterioration detection result obtained by the sensor deterioration detection unit  26  even after a power supply of the control unit  1  has been switched OFF. The non-volatile memory  27  is provided in the memory  30  of the control unit  1 . 
     All other configurations are identical to  FIG. 8 , and will not therefore be described here. 
       FIG. 12  is a flowchart showing calculation processing performed by the air-fuel ratio feedback control unit  20  according to the second embodiment of this invention. In  FIG. 12 , processing (see S 101   a , S 103   c   1 , S 103   d , S 105   b ) for dealing with manufacturing irregularities in and deterioration of the oxygen sensor has been added to the flowchart of  FIG. 9 . Only the additional steps will be described here. When step numbers in the flowchart of  FIG. 12  are identical to the step numbers in  FIG. 9 , identical operations are performed in those steps. 
     In step S 101   a , the air-fuel ratio feedback control unit  20  reads deterioration information relating to the oxygen sensor  11 , which is written to the non-volatile memory  27 , in order to obtain information indicating that the oxygen sensor  11  has already been determined to have deteriorated. 
     In step S 103   c   1 , the air-fuel ratio feedback control unit  20  uses the air-fuel ratio determination voltage updating unit  21   c  to correct and update the determination voltages set in relation to the oxygen sensor output value VO 2 . In the first embodiment, the determination voltages are corrected and updated in response to variation in the sensor element temperature. In the second embodiment, a case in which the determination voltages are corrected and updated when the oxygen sensor output value VO 2  varies due to manufacturing irregularities in and deterioration of the oxygen sensor  11  will be described. 
     Step S 103   c   1  focuses on a maximum value and a minimum value of the oxygen sensor output value VO 2  in a case where the engine  19  is not determined to be in the transient operating condition in step S 103   a , i.e. during a steady state operation. When the oxygen sensor  11  includes manufacturing irregularities or deteriorates, the maximum value and the minimum value vary. Therefore, when the maximum value and the minimum value vary, the air-fuel ratio determination voltage updating unit  21   c  updates and corrects the determination voltages on the assumption that the oxygen sensor  11  includes manufacturing irregularities or has deteriorated. When air-fuel ratio feedback is implemented during a steady state operation, the actual value of the air-fuel ratio of the engine  19  remains stable within a fixed range centering on the stoichiometric air-fuel ratio. The oxygen sensor output value VO 2  also varies within a fixed range. More specifically, as indicated by a graph of a “normal oxygen sensor” in  FIG. 13 , the oxygen sensor output value VO 2  varies within a range of 0 to 1 V, centering on approximately 0.45 V. 
     However, when the output characteristic of the oxygen sensor  11  varies due to manufacturing irregularities in or deterioration of the oxygen sensor  11 , the oxygen sensor output value VO 2  varies relative to the air-fuel ratio. Therefore, by detecting the variation in the oxygen sensor output value VO 2 , a determination can be made as to whether or not the output characteristic of the oxygen sensor  11  has varied. For this purpose, first, an average value of the maximum value of the oxygen sensor output value VO 2  during a steady state operation is determined over a preset fixed period. When the average value differs from an average value (a reference value) of the maximum value stored in the control unit  1 , it can be determined that the output characteristic of the oxygen sensor  11  has varied. In this case, the determination voltages used to define the air-fuel ratio regions of the engine  19  are corrected and updated. Note that an average value of the minimum value of the oxygen sensor output value VO 2  may be determined in addition to the average value of the maximum value. 
     When the engine  19  is not warm, the air-fuel ratio of the engine  19  may be unstable and the sensor element temperature of the oxygen sensor  11  may not have risen sufficiently. As a result, the true sensor output characteristic may not be exhibited. Therefore, step S 103   c   1  is implemented when the engine  19  is sufficiently warm. It is also preferable not to implement step S 103   c   1  when the environmental temperature is extremely low or extremely high. Hence, a condition according to which step S 103   c   1  is implemented only when a sufficient amount of time has elapsed following implementation of the air-fuel ratio feedback control may be added. 
     When the average value of the maximum value of the oxygen sensor output value VO 2  during a steady state operation is lower or higher than the average value (the reference value) stored in the control unit  1 , the first determination voltage is corrected and updated in accordance with Equation (5), shown below.
 
first determination voltage( n )=first determination voltage( n− 1)×Cof a   (5)
 
     In Equation (5), Cofa is a preset correction coefficient. At least two correction coefficients Cofa are prepared, one of which takes a value smaller than 1 and the other of which takes a value larger than 1. When the average value of the maximum value of the oxygen sensor output value VO 2  during a steady state operation is lower than the average value (the reference value) of the maximum value stored in the control unit  1 , the value smaller than 1 is used as the correction coefficient Cofa. As a result, the first determination voltage (n) is reduced below the current value. When the average value of the maximum value of the oxygen sensor output value VO 2  during a steady state operation is higher than the average value of the maximum value stored in the control unit  1 , on the other hand, the value larger than 1 is used as the correction coefficient Cofa. As a result, the first determination voltage (n) is increased above the current value. 
     Further, when the average value of the minimum value is determined together with the average value of the maximum value, the average value of the minimum value may be compared with an average value (a reference value) of the minimum value stored in the control unit  1 , similarly to the average value of the maximum value. The first determination voltage and the second determination voltage may then be updated only when the average value of the maximum value differs from the reference value and the average value of the minimum value differs from the reference value. In this case, the determination values are updated less frequently, but variation in the characteristic of the oxygen sensor  11  can be determined more carefully, and therefore updating errors can be suppressed. 
     Furthermore, the second determination voltage is corrected in addition to the first determination voltage using a similar equation to Equation (5). The correction coefficient Cofa used at this time may be set for each of the first determination voltage and the second determination voltage, or identical values may be used for ease. 
     The first determination voltage, the second determination voltage, and the average values of the maximum value and minimum value of the oxygen sensor output value VO 2 , obtained in this step, are stored in the non-volatile memory  27 . 
     In step S 103   d , the air-fuel ratio feedback control unit  20  uses the sensor deterioration detection unit  26  to detect the presence of deterioration in the oxygen sensor  11 . As described above, it is known that the output value of a “normal oxygen sensor” is typically between 0 and 1 V, and centers on approximately 0.45 V, as shown in  FIG. 13 . However, it is also known that when the oxygen sensor  11  deteriorates, the high voltage side voltage value and the low voltage side voltage value shift. In other words, the high voltage side decreases from 1 V to 0.9 V to 0.8 V and so on to 0.5 V (1 V→0.9 V→0.8 V→ . . . →0.5 V), as indicated by a “deteriorated oxygen sensor  1 ” in  FIG. 13 , and the low voltage side increases from 0 V to 0.1 V to 0.2 V and so on to 0.4 V (0 V→0.1V→0.2 V→ . . . →0.4V), as indicated by a “deteriorated oxygen sensor  2 ” in  FIG. 13 . 
     In step S 103   d , a threshold determination is performed using the average values of the maximum value and the minimum value of the oxygen sensor output value VO 2 , determined in step S 103   a . In other words, the average value of the maximum value and the average value of the minimum value are compared respectively with preset deterioration determination values  1  and  2 . When, as a result of the comparison, the average value of the maximum value of the oxygen sensor  11 &gt;the deterioration determination value  1  or the average value of the minimum value of the oxygen sensor  11 &lt;the deterioration determination value  2 , the oxygen sensor  11  is determined to have deteriorated. 
     Note that the deterioration determination value  1  and the deterioration determination value  2  are determined by experiment on the basis of an amount of harmful exhaust gas discharged when air-fuel ratio feedback travel is performed after shifting the oxygen sensor output value VO 2  respectively to a high voltage side voltage and a low voltage side voltage. In other words, a high voltage side deviation value and a low voltage side deviation value obtained when the amount of discharged exhaust gas exceeds a threshold are determined and set respectively as the deterioration determination value  1  and the deterioration determination value  2 . 
     The deterioration determination values differ according to the type of the engine, but in experiments, the deterioration determination value  1  and the deterioration determination value  2  are often found to be approximately 0.6 to 0.8 V and approximately 0.3 to 0.4 V, respectively. 
     Further, on the illustrative view showing the deterioration condition in  FIG. 13 , the voltage value undergoes a simple shift in response to deterioration of the oxygen sensor  11 , but when a response speed varies due to deterioration, the voltage value may undergo a gradual shift. 
     When the oxygen sensor  11  is determined to have deteriorated in step S 103   d , deterioration information relating to the oxygen sensor  11  is written to the non-volatile memory  27 . 
     In step S 105   b , the deterioration detection result obtained in relation to the oxygen sensor  11  in step S 103   d  is referenced. When deterioration has not been determined, the processing advances to step S 106   a , and when deterioration has been determined, the processing advances to step S 106   c.    
     Hence, according to the second embodiment, when the oxygen sensor output value VO 2  varies due to manufacturing irregularities in or deterioration of the oxygen sensor  11 , the determination voltages can be updated. In so doing, the air-fuel ratio regions of the engine  19  can be divided in accordance with the oxygen sensor output value VO 2 . Thus, optimum proportional and integral gains for the air-fuel ratio feedback control can be selected, and as a result, convergence on the target voltage can be achieved more quickly. Further, when determining deterioration of the oxygen sensor  11 , air-fuel ratio feedback is implemented on the basis of a conventional method that is in general use (i.e. whether the air-fuel ratio is rich or lean), and therefore an effect on control of the air-fuel ratio of the engine  19  can be minimized. 
     According to the second embodiment, as described above, similar effects to the first embodiment are obtained. In addition, according to the second embodiment, the air-fuel ratio feedback control unit  20  includes the transient operating condition detection unit  21   a  that determines whether the engine  19  is in the transient operating condition or the steady state operating condition on the basis of the engine operating conditions detected by the sensor group  15  that detects engine operating conditions including at least one of the engine rotation speed, the throttle opening, and the engine temperature. Furthermore, the air-fuel ratio feedback control unit  20  includes the air-fuel ratio determination voltage updating unit  21   c  that determines the average value of the maximum value or the average value of the minimum value of the oxygen sensor output value VO 2  over a preset period in a state in which the engine  19  is determined to be in the steady state operating condition by the transient operating condition detection unit  21   a , and corrects at least one of the first determination voltage and the second determination voltage when the average value of the maximum value or the average value of the minimum value differs from the reference value set in relation thereto. When the average value of the maximum value or the average value of the minimum value is lower than the reference value set in relation thereto, the air-fuel ratio determination voltage updating unit  21   c  reduces at least one of the first determination voltage and the second determination voltage below the current value, and when the average value of the maximum value or the average value of the minimum value is higher than the reference value set in relation thereto, the air-fuel ratio determination voltage updating unit  21   c  increases at least one of the first determination voltage and the second determination voltage above the current value. 
     The average value of the maximum value and the average value of the minimum value vary when the oxygen sensor  11  deteriorates, and therefore, by comparing the average values with the corresponding reference values, it is possible to determine whether or not the oxygen sensor  11  has deteriorated. Moreover, when the oxygen sensor  11  is determined to have deteriorated, the determination voltages are corrected and updated in accordance with the deterioration, and as a result, the air-fuel ratio of the engine  19  can be classified more accurately. 
     Furthermore, in this embodiment, the air-fuel ratio feedback control correction amount calculation units  22   a ,  23   a ,  24  output the second feedback control correction amount as the final feedback control correction amount when the sensor deterioration detection unit  26  detects deterioration of the oxygen sensor  11 , output the first feedback control correction amount as the final feedback control correction amount when the sensor deterioration detection unit  26  does not detect deterioration of the oxygen sensor  11  and the engine transient operating condition detection unit  21   a  determines that the engine  19  is in the transient operating condition, and output the second feedback control correction amount as the final feedback control correction amount when the sensor deterioration detection unit  26  does not detect deterioration of the oxygen sensor  11  and the engine transient operating condition detection unit  21   a  determines that the engine  19  is not in the transient operating condition. 
     Hence, when deterioration of the oxygen sensor  11  is determined, air-fuel ratio feedback is implemented on the basis of a conventional method that is in general use (i.e. whether the air-fuel ratio is rich or lean), and therefore an effect on control of the air-fuel ratio of the engine  19  can be minimized. Further, similarly to the modified example of the first embodiment, the air-fuel ratio feedback control based on the air-fuel ratio region of the engine  19  is implemented only when the engine  19  is in the transient operating condition, and therefore adverse effects generated when an error occurs during estimation of the sensor element temperature of the oxygen sensor  11  can be suppressed. 
     Note that in the second embodiment, the determination voltages may be corrected and updated in step S 103   c   1  in response to variation in the sensor element temperature, similarly to the first embodiment.