JP2009281306A - Air-fuel ratio detection device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To quickly detect air-fuel ratio variation of an internal combustion engine by conducting air-fuel ratio determination based on an output differential value. <P>SOLUTION: In an air-fuel ratio detection device, an O2 sensor 30 generates different electromotive force depending on if the air-fuel ratio of an engine 10 is rich or lean. An ECU 40 carries out detection of the air-fuel ratio using the electromotive force of the O2 sensor 10. In one embodiment, specifically, the ECU 40 determines the differential value of the electromotive force as an output differential value, and detects if the air-fuel ratio is rich or lean based on the determined output differential value. Compared with a case of carrying out detection of the air-fuel ratio based on the electromotive force, the air-fuel ratio variation can be thus quickly detected. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio detection apparatus for an internal combustion engine, and more particularly to an air-fuel ratio detection apparatus for an internal combustion engine that performs air-fuel ratio detection using an electromotive force value of an oxygen sensor.

Conventionally, in order to improve the exhaust emission of an internal combustion engine, an oxygen sensor is provided in the exhaust pipe of the internal combustion engine, and the air-fuel ratio of the exhaust gas is feedback (F / B) controlled based on the electromotive force value of the oxygen sensor. An air-fuel ratio control system is known. Specifically, as the air-fuel ratio F / B control, for example, by comparing an electromotive force value of an oxygen sensor with a determination value, it is determined whether the air-fuel ratio of the exhaust gas is richer or leaner than the target air-fuel ratio. At the time of reversal between lean and rich, first, the air-fuel ratio feedback correction coefficient is largely changed by skip control, and then the air-fuel ratio feedback correction coefficient is changed stepwise by integration operation during the period until the next reversal.
Japanese Patent Laid-Open No. 2001-3788

  By the way, in the oxygen sensor, it is conceivable that the response from when the exhaust gas atmosphere changes until the electromotive force value reaches the determination value is delayed. For this reason, in the configuration in which the air-fuel ratio determination is performed based on the electromotive force value of the oxygen sensor as in Patent Document 1, the air-fuel ratio converges to the target air-fuel ratio (for example, the theoretical air-fuel ratio) due to the response delay of the oxygen sensor. May be delayed. In such a case, there is a concern that the air-fuel ratio deviates from the target air-fuel ratio and exhaust emission deteriorates. In particular, when the deviation between the actual air-fuel ratio and the target air-fuel ratio is large, or when the air-fuel ratio is unstable (for example, when shifting to air-fuel ratio feedback control, during vehicle transient operation, when returning from fuel cut, etc. ), It is considered highly necessary to quickly converge the air-fuel ratio to the target air-fuel ratio in order to suppress exhaust emission deterioration.

  The present invention has been made to solve the above-described problems, and has as its main object to provide an air-fuel ratio detection device for an internal combustion engine that can quickly detect a change in the air-fuel ratio of the internal combustion engine.

  The present invention employs the following means in order to solve the above problems.

  The invention according to claim 1 is applied to an oxygen sensor that generates different electromotive force depending on whether the air-fuel ratio of the internal combustion engine is rich or lean, and performs air-fuel ratio detection using the electromotive force value of the oxygen sensor. In an air-fuel ratio detection apparatus for an internal combustion engine, a calculation unit that calculates a differential value of the electromotive force value as an output differential value, and an air-fuel ratio that detects whether the air-fuel ratio is rich or lean based on the output differential value calculated by the calculation unit. And a fuel ratio detecting means.

  According to the present invention, in order to determine the rich / lean of the air / fuel ratio based on the output differential value of the oxygen sensor, the reversal of the rich / lean of the air / fuel ratio is detected when the sensor output changes in the upward or downward direction. can do. That is, in the conventional configuration in which the air-fuel ratio detection is performed based on the comparison result between the electromotive force value of the oxygen sensor and the determination value, when the actual air-fuel ratio changes, the sensor electromotive force value changes to a predetermined value. Since it takes time to reach this judgment value, a response delay occurs until the air-fuel ratio judgment is made. In the present invention, since the air-fuel ratio judgment is performed based on the output differential value, a change in the air-fuel ratio can be detected quickly. Can do. Here, the output differential value may be calculated from, for example, the amount of change per unit time in the electromotive force value of the oxygen sensor.

  The air-fuel ratio detection based on the output differential value can quickly detect the air-fuel ratio change, whereas the air-fuel ratio detection based on the sensor electromotive force value reliably detects the rich / lean reversal due to the air-fuel ratio change. It is considered possible. It is considered that which one of quickness and certainty of air-fuel ratio detection is required depends on the operating state of the internal combustion engine. In view of this point, the invention according to claim 2 is based on the electromotive force value, the first detecting means for detecting whether the air-fuel ratio is rich or lean based on the output differential value, and the electromotive force value. The second detection means for detecting whether the air-fuel ratio is rich or lean, and the air-fuel ratio detection by the first detection means and the air-fuel ratio detection by the second detection means are switched based on the operating state of the internal combustion engine. Switching means for performing fuel ratio detection. According to this configuration, whether the air-fuel ratio detection is performed based on the output differential value or the electromotive force value is switched according to the operating state of the internal combustion engine. It can be set as appropriate according to the operating state of the internal combustion engine. Here, it is desirable that the operating state of the internal combustion engine is, for example, the rotational speed of the internal combustion engine, the throttle opening, the air-fuel ratio, the elapsed time since the start of predetermined air-fuel ratio control, or the like.

  When the air-fuel ratio does not converge to a predetermined range including the target value (for example, a region near the stoichiometric range), it is required to quickly detect the rich / lean reversal due to the air-fuel ratio change. Is converging within a predetermined range including the target value, it is considered that it is required to reliably detect the inversion. In view of this, the invention according to claim 3 is applied to an air-fuel ratio control system that performs air-fuel ratio feedback control so that the air-fuel ratio detected by the air-fuel ratio detection means matches a target value, and the switching means is The air-fuel ratio is detected by the first detection means when the air-fuel ratio has not converged to the predetermined range including the target value, and the air-fuel ratio is detected by the second detection means when the air-fuel ratio has converged to the predetermined range. To implement. According to this configuration, when the air-fuel ratio is greatly deviated from the target value, the air-fuel ratio is detected using the output differential value, so that the air-fuel ratio can be quickly converged to the target value, and the air-fuel ratio is the target value. Since the air-fuel ratio is detected by using the electromotive force value after the convergence to, the air-fuel ratio change can be detected reliably.

  In particular, in the configuration in which the exhaust passage of the internal combustion engine is provided with a three-way catalyst as an exhaust purification catalyst, the air-fuel ratio determination based on the sensor electromotive force value is performed when the air-fuel ratio has converged to the theoretical air-fuel ratio as the target value. By doing so, the rich and lean repetitions are ensured after convergence to the stoichiometric air-fuel ratio, and a certain length is secured as the repetition period, so the catalyst performance (oxygen storage function) of the three-way catalyst is improved. It can be sufficiently exerted, which is preferable in terms of improving exhaust emission.

  Here, when the air-fuel ratio does not converge to a predetermined range including the target value, for example, during the transient operation of the internal combustion engine or at the beginning of the air-fuel ratio feedback control (for example, the air-fuel ratio feedback accompanying the passage of the start period of the internal combustion engine) When switching to control or when returning from a fuel cut). Further, in order to detect that the air-fuel ratio does not converge to the predetermined range, for example, it may be performed directly based on the detected air-fuel ratio, or the rotational speed change amount of the internal combustion engine, the throttle opening change amount, etc. May be detected based on a parameter indicating the operating state of the internal combustion engine, or indirectly based on an elapsed time since the start of air-fuel ratio feedback control.

  At the beginning of the air-fuel ratio feedback control, it is conceivable that the air-fuel ratio deviates from an appropriate value for air-fuel ratio control, for example. In such a case, in order to quickly return the air-fuel ratio to its proper value, it is desirable to detect the air-fuel ratio change promptly. In view of this point, the invention according to claim 4 is applied to an air-fuel ratio control system that performs air-fuel ratio feedback control so that the air-fuel ratio detected by the air-fuel ratio detection means matches a target value. The air-fuel ratio is detected by the first detecting means at a predetermined time after the start of the execution of the air-fuel ratio feedback control, and the air-fuel ratio is detected by the second detecting means after the predetermined time has elapsed. According to this configuration, since the air-fuel ratio detection is performed using the output differential value of the oxygen sensor at the beginning of the air-fuel ratio feedback control, the deviation between the detected air-fuel ratio and the target value is quickly eliminated, and the air-fuel ratio is adjusted appropriately. Can be a value.

  In the oxygen sensor, when the followability to the air-fuel ratio change becomes good, it is possible to increase the integral amount of the air-fuel ratio feedback correction amount and to reduce the skip amount. In view of this, the invention according to claim 5 provides an air-fuel ratio feedback correction amount for making the air-fuel ratio detected by the air-fuel ratio detecting means coincide with a target value, a predetermined integral amount in a direction in which the air-fuel ratio is directed toward the target value. Air-fuel ratio control that increases or decreases each time and increases or decreases the air-fuel ratio feedback correction amount by a predetermined skip amount in the opposite direction when it is detected that the air-fuel ratio has reached the target value. Applicable to the air-fuel ratio control system to be implemented. Further, when the air-fuel ratio detection is performed by the first detection means, the integral amount is increased and the skip amount is decreased as compared with the case where the air-fuel ratio detection is performed by the second detection means. According to this configuration, when the air-fuel ratio detection is performed using the output differential value of the oxygen sensor, the air-fuel ratio feedback correction amount can be set to an appropriate value in accordance with the air-fuel ratio change. In particular, when the air-fuel ratio is detected using the output differential value when the air-fuel ratio has not converged to the target value, the skip amount is limited to a small value, so that a rapid change in the fuel injection amount is avoided. As a result, it is possible to avoid overshooting the air-fuel ratio to the rich side or the lean side from the target value.

  In a configuration in which a detection threshold value for detecting rich / lean of the air-fuel ratio is provided and the air-fuel ratio is determined based on a comparison result between the output differential value of the oxygen sensor and the detection threshold value, It is considered that the determination speed and the determination reliability differ depending on the size. Further, it is considered that whether quick determination or certainty is required depends on the operating state of the internal combustion engine. In view of this point, the invention according to claim 6 sets a detection threshold value when performing rich detection or lean detection of the air-fuel ratio based on the output differential value based on the operating state of the internal combustion engine. . The air-fuel ratio detection means performs air-fuel ratio determination based on a comparison result between the detection threshold set by the threshold setting means and the output differential value calculated by the calculation means. In this way, since the detection threshold is set according to the operating state of the internal combustion engine, which of the priority of the quick determination of the air-fuel ratio and the certainty of the determination result is given according to the operating state of the internal combustion engine. It can be set appropriately.

  Since the frequency of fuel injection increases as the rotational speed of the internal combustion engine increases, it is desirable to detect the air-fuel ratio change as soon as possible. In view of this, the invention according to claim 7 is configured such that the detection threshold value is a positive threshold value used when the electromotive force value increases and a negative side value used when the electromotive force value decreases. As the threshold value, the positive threshold value and the negative threshold value are made closer to zero as the rotational speed of the internal combustion engine increases. According to this configuration, the higher the rotational speed of the internal combustion engine, the quicker the air-fuel ratio determination becomes. Therefore, even when the frequency of fuel injection is high, the air-fuel ratio change can be detected quickly.

  According to an eighth aspect of the present invention, an air-fuel ratio feedback correction amount for making the air-fuel ratio detected by the air-fuel ratio detecting means coincide with a target value is increased by a predetermined integral amount in a direction in which the air-fuel ratio is directed toward the target value. Or, when it is detected that the air-fuel ratio has reached the target value, the air-fuel ratio control is performed to increase or decrease the air-fuel ratio feedback correction amount by a predetermined skip amount in the opposite direction. Applies to air-fuel ratio control system. Further, as the detection threshold is closer to zero, the integration amount is increased and the skip amount is decreased. As described above, when the followability of the oxygen sensor to the air-fuel ratio change is improved, it is possible to increase the integration amount of the air-fuel ratio feedback correction amount and to reduce the skip amount. Therefore, according to the configuration of claim 8, the closer the detection threshold is to zero, the more the air-fuel ratio feedback correction amount can be set to an appropriate value in accordance with the change in the air-fuel ratio. In particular, when the air-fuel ratio is detected using the output differential value when the air-fuel ratio has not converged to the target value, the skip amount is limited to a small value, so that a rapid change in the fuel injection amount is avoided. As a result, it is possible to avoid overshooting the air-fuel ratio to the rich side or the lean side from the target value.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments of the invention will be described with reference to the drawings. In the present embodiment, an engine control system is constructed for a single-cylinder gasoline engine for a motorcycle. In this control system, an electronic control unit (hereinafter referred to as ECU) is used as a center to control the fuel injection amount, control the ignition timing, and the like. FIG. 1 shows an overall schematic configuration diagram of the engine control system.

  In FIG. 1, an engine 10 is provided with an air cleaner 12 at the most upstream portion of an intake pipe 11 (intake passage). A throttle valve 14 whose opening degree is adjusted by a throttle actuator 13 such as a DC motor is provided on the downstream side of the air cleaner 12. The opening of the throttle valve 14 (throttle opening) is detected by a throttle opening sensor built in the throttle actuator 13. An intake pipe pressure sensor 15 for detecting the intake pipe pressure is provided on the downstream side of the throttle valve 14, and an electromagnetically driven fuel injection valve that injects fuel in the vicinity of the intake port on the downstream side thereof. 16 is attached.

  An intake valve 17 and an exhaust valve 18 are provided at the intake port and the exhaust port of the engine 10, respectively. Then, the air / fuel mixture is introduced into the combustion chamber 19 by the opening operation of the intake valve 17, and the exhaust gas after combustion is discharged to the exhaust pipe 21 (exhaust passage) by the opening operation of the exhaust valve 18.

  A spark plug 22 is attached to the cylinder head of the engine 10. A high voltage is applied to the spark plug 22 at a desired ignition timing through an ignition device 23 made of an ignition coil or the like. By applying this high voltage, a spark discharge is generated between the opposing electrodes of each spark plug 22, and the air-fuel mixture introduced into the combustion chamber 19 is ignited and used for combustion.

  The exhaust pipe 21 is provided with a three-way catalyst 24 for purifying CO, HC, NOx and the like in the exhaust gas. On the upstream side of the three-way catalyst 24, an O2 sensor 30 for detecting the air-fuel ratio (oxygen concentration) of the air-fuel mixture with exhaust gas as a detection target is provided.

  The O2 sensor 30 has a cup-type sensor element. FIG. 2 shows a sectional view of the sensor element 31. Actually, the sensor element 31 is configured such that the entire element is accommodated in a housing or an element cover.

  In the sensor element 31, the solid electrolyte layer 32 is formed in a cup shape in cross section, an exhaust gas side electrode layer 33 is provided on the outer surface thereof, and an atmosphere side electrode layer 34 is provided on the inner surface thereof. The solid electrolyte layer 32 is made of an oxygen ion conductive oxide sintered body in which CaO, MgO, Y2O3 or the like is dissolved in ZrO2, HfO2, ThO2, BiO2 or the like as a stabilizer. Each of the electrode layers 33 and 34 is made of a noble metal having high catalytic activity such as platinum, and the surface thereof is subjected to porous chemical plating. An internal space surrounded by the solid electrolyte layer 32 is an atmospheric chamber 35, and a heater 36 is accommodated in the atmospheric chamber 35. The heater 36 has a heat generation capacity sufficient to activate the sensor element 31, and the entire sensor element is heated by the heat generation energy.

  In the sensor element 31, the outer side (electrode layer 33 side) of the solid electrolyte layer 32 is an exhaust gas atmosphere, and the inner side (electrode layer 34 side) is an air atmosphere. An electromotive force is generated between the electrode layers 33 and 34 in accordance with the difference. FIG. 3 is an electromotive force characteristic diagram showing the relationship between the air-fuel ratio of exhaust gas and the electromotive force of the sensor element 31. As shown in FIG. 3, the sensor element 31 has characteristics that generate different electromotive forces depending on whether the air-fuel ratio is rich or lean, and the electromotive force changes suddenly near the stoichiometric air-fuel ratio (stoichiometric). Specifically, the sensor electromotive force when the fuel is rich is about 0.9V, and the sensor electromotive force when the fuel is lean is about 0V.

  Returning to the explanation of FIG. 1, the present system is provided with a crank angle sensor 25 for outputting a crank angle signal at every predetermined crank angle of the engine, a cooling water temperature sensor 26 for detecting the cooling water temperature on the engine side, and the like. Yes.

  As is well known, the ECU 40 is mainly composed of a microcomputer (hereinafter referred to as a microcomputer) 41 composed of a CPU, ROM, RAM, and the like, and executes various control programs stored in the ROM, so that the engine operation state can be changed each time. In response, various controls of the engine 10 are performed. Specifically, the microcomputer 41 of the ECU 40 inputs various detection signals from the various sensors described above, calculates the fuel injection amount, ignition timing, etc. based on these various detection signals and the like, and calculates the fuel injection valve 16 and the ignition. The drive of the device 23 is controlled.

  The ECU 40 is provided with a sensor control circuit 42 that detects the electromotive force of the O2 sensor 30. As control of the fuel injection amount, the microcomputer 41 of the ECU 40 uses the electromotive force value detected by the sensor control circuit 42 to perform air-fuel ratio control so that the actual air-fuel ratio matches the target air-fuel ratio (for example, the theoretical air-fuel ratio). To do. As the air-fuel ratio control, the microcomputer 41 basically performs stoichiometric combustion control in which the air-fuel ratio is feedback-controlled in a region near the stoichiometric range (region near the stoichiometric range). Note that feedback control is stopped under certain conditions such as when the engine is started, when idling, when fuel is cut, or when the coolant temperature is low.For example, if the engine is starting or idling, the air-fuel ratio is higher than the stoichiometric ratio. Open control is performed so that the rich side is reached. In addition to air-fuel ratio control, the sensor control circuit 42 controls the heater energization of the O2 sensor 30 to keep the sensor element in an active state.

  The sensor control circuit 42 basically uses the characteristic that the electromotive force value of the O2 sensor 30 changes suddenly in the vicinity of the theoretical air-fuel ratio (see FIG. 3), and basically, the detected electromotive force value and the vicinity of the middle of the electromotive force fluctuation. Is compared with a preset reference voltage value Vth (for example, a theoretical air-fuel ratio value of 0.45 V), and fuel rich / lean is determined, and the result is output to the microcomputer 41 as a binary determination signal. Specifically, the air-fuel ratio determination (air-fuel ratio normal determination process) based on the electromotive force value is inputted with an electromotive force value Vo2 which is an output value of the O2 sensor 30, and the electromotive force value Vo2 is compared with the reference voltage value Vth. To do. If the electromotive force value Vo2 is greater than the reference voltage value Vth, it is determined to be lean, and if it is equal to or less than the reference voltage value Vth, it is determined to be rich. In the determination of rich / lean, the rich / lean may be switched when the electromotive force value Vo2 exceeds the reference voltage value Vth or remains below the reference voltage value Vth for a predetermined time.

  Further, the microcomputer 41 performs air-fuel ratio feedback correction based on the rich / lean determination result. The air-fuel ratio feedback correction will be described with reference to the time chart of FIG. In FIG. 4, (a) shows the transition of the sensor electromotive force value Vo2, (b) shows the result of the air-fuel ratio determination, and (c) shows the transition of the air-fuel ratio feedback correction coefficient.

  In FIG. 4, when the air-fuel ratio is reversed from lean determination to rich determination, the air-fuel ratio feedback correction coefficient is maintained while the rich determination is continued after performing the skip control for reducing the air-fuel ratio feedback correction coefficient by the skip amount Ks. Is integrated in a step-by-step manner. As a result, the fuel injection amount is corrected to decrease, and as a result, the air-fuel ratio shifts to the lean side. In addition, when the air-fuel ratio is reversed from the rich determination to the lean determination, after performing the skip control for increasing the air-fuel ratio feedback correction coefficient by the skip amount Ks, the air-fuel ratio feedback correction coefficient is set while the lean determination is continued. Integral control is performed to increase the integration amount Ki step by step. As a result, the fuel injection amount is increased and corrected, and as a result, the air-fuel ratio shifts to the rich side. Note that the skip amount Ks and the integral amount Ki are calculated each time by the ECU 40 according to the engine operating state.

  By the way, when air-fuel ratio detection is performed based on the electromotive force value of the O2 sensor 30, it takes time until the O2 sensor 30 generates an electromotive force corresponding to the oxygen partial pressure even if the rich / lean in the exhaust gas atmosphere is actually reversed. Therefore, it is considered that a response delay occurs until the electromotive force value reaches the reference voltage value Vth after the gas atmosphere changes, and as a result, a time delay occurs until rich / lean is determined. In particular, when the actual air-fuel ratio is significantly different from the target air-fuel ratio (stoichiometric) or when the air-fuel ratio is unstable, it is highly necessary to quickly converge the air-fuel ratio to the target air-fuel ratio. Further, the deviation of the air-fuel ratio from the stoichiometric air-fuel ratio is continued, and as a result, the exhaust emission may be deteriorated.

  In this regard, the inventor of the present application paid attention to the amount of change (output differential value) per unit time in the electromotive force value of the O2 sensor 30 when detecting the rich / lean change of the air-fuel ratio by the O2 sensor 30. That is, it has been found that, based on the output differential value, it is possible to quickly detect the rich / lean inversion of the air-fuel ratio as compared with the case based on the electromotive force value. It has also been found that the air-fuel ratio can be converged more quickly to the target air-fuel ratio by performing the air-fuel ratio F / B control using the result of the air-fuel ratio determination based on the output differential value.

  On the other hand, when the air-fuel ratio is in the vicinity of the stoichiometry, certainty is required rather than quickness in detecting the air-fuel ratio change. In other words, the three-way catalyst 24 has a function (O2 storage function) for purifying NOx by storing oxygen in a lean atmosphere and then purging NOx by releasing the stored oxygen in a rich atmosphere (O2 storage function). In addition, there is a characteristic that the catalyst performance is exhibited under a situation where the rich / lean air-fuel ratio is repeated. At this time, in order to sufficiently exhibit the catalyst performance, it is desirable that the rich determination and the lean determination are performed reliably and a certain length is secured as the repetition cycle.

  Therefore, in the present embodiment, the air-fuel ratio control is performed based on the output differential value when the air-fuel ratio deviates from the stoichiometric vicinity region, such as during transition from open control to F / B control or during transient operation of the engine 10. After the fuel ratio is determined and then converges to the vicinity of the air / fuel ratio stoichiometry, the process proceeds to the air / fuel ratio determination based on the sensor electromotive force value.

  Hereinafter, first, the air-fuel ratio determination (air-fuel ratio early determination process) based on the output differential value of the O2 sensor 30 will be described. FIG. 5 is a flowchart showing the procedure of the early air-fuel ratio determination process. This process is executed at predetermined intervals by the microcomputer 41 of the ECU 40. In FIG. 5, first, in step S11, a smoothing process is performed as a noise removal process for the output value of the O 2 sensor 30. As the annealing process, for example, a low-pass filter (LPF) process may be performed to remove noise superimposed on the sensor detection signal, or a moving average process may be performed.

  In the subsequent step S12, an output differential value Vo2 * is calculated. In this embodiment, the previous value of the smoothed value is stored, and the output differential value Vo2 * is calculated by subtracting the previous value from the current value of the smoothed value. Subsequently, in step S13, it is determined whether the previous detected air-fuel ratio is rich determination or lean determination. If the previous detected air-fuel ratio is rich, the process proceeds to step S14 to determine whether the output differential value Vo2 * is equal to or less than the lean determination value Vle. The lean determination value Vle is a detection threshold value used for air-fuel ratio detection when the electromotive force value Vo2 decreases, and is set as a negative value near zero. When the output differential value Vo2 * is larger than the lean determination value Vle, the process proceeds to step S15, and the rich determination is maintained as the determination result of the detected air-fuel ratio. On the other hand, if the output differential value Vo2 * is equal to or less than the lean determination value Vle, the process proceeds to step S16, and the detected air-fuel ratio is switched from the rich determination to the lean determination.

  If the previous detected air-fuel ratio is a lean determination, a negative determination is made in step S13, and the process proceeds to step S17 to determine whether or not the output differential value Vo2 * is equal to or greater than the rich determination value Vri. The rich determination value Vri is a detection threshold value used for air-fuel ratio detection when the electromotive force value Vo2 increases, and is set as a positive value near zero. If the output differential value Vo2 * is less than the rich determination value Vri, the process proceeds to step S18, and the lean determination is maintained as the determination result of the detected air-fuel ratio. On the other hand, if the output differential value Vo2 * is greater than or equal to the rich determination value Vri, the process proceeds to step S19, and the detected air-fuel ratio is switched from lean determination to rich determination.

  Next, the difference between the air-fuel ratio determination result based on the sensor electromotive force value Vo2 and the air-fuel ratio determination result based on the output differential value Vo2 * will be described with reference to FIG. 6A shows the change in the air-fuel ratio of the exhaust gas, FIG. 6B shows the change in the air-fuel ratio determination result based on the electromotive force value Vo2, and FIG. 6C shows the change in the air-fuel ratio based on the output differential value Vo2 *. Shows the transition of judgment results.

  Consider the case where the air-fuel ratio of the exhaust gas is switched from the lean atmosphere to the rich atmosphere in FIG. At this time, when the air-fuel ratio determination is performed based on the electromotive force value Vo2, the lean determination is changed to the rich determination at time t3 when the electromotive force value Vo2 reaches the voltage determination value Vth, as shown in FIG. 6B. Switch. That is, a response delay from time t1 to time t3 occurs from when the exhaust gas atmosphere changes until the ECU 40 determines that the air-fuel ratio has changed. In the configuration in which the result of air-fuel ratio determination is switched on condition that the predetermined time TA is maintained after the sensor electromotive force value Vo2 reaches the voltage determination value Vth, at time t4 when the predetermined time TA has elapsed from time t3. Switching from lean determination to rich determination causes a response delay from time t1 to time t4.

  On the other hand, when the air-fuel ratio determination is performed based on the output differential value Vo2 *, as shown in FIG. 6C, when the exhaust gas atmosphere changes from lean to rich and the sensor electromotive force value Vo2 gradually increases. The output differential value Vo2 * changes from zero to a positive value. Then, at time t2 when the output differential value Vo2 * reaches the rich determination value Vri, the lean determination is switched to the rich determination. That is, according to the air-fuel ratio early determination process, since the air-fuel ratio change is detected based on the rate of change of the electromotive force value Vo2, at time t2, which is earlier than time t3 when the sensor electromotive force value Vo2 reaches the voltage determination value Vth, It is detected that the air-fuel ratio has switched from lean to rich. Therefore, only a response delay from time t1 to time t2 occurs between the time when the exhaust gas atmosphere changes and the time when the ECU 40 determines a change in the air / fuel ratio, and the determination delay of the air / fuel ratio is suppressed.

  Furthermore, the case where the air-fuel ratio F / B control is performed using the result of the air-fuel ratio determination of the air-fuel ratio early determination process will be described in detail with reference to the time chart of FIG. 7A shows the transition of the electromotive force value Vo2, FIG. 7B shows the transition of the output differential value Vo2 *, FIG. 7C shows the transition of the air-fuel ratio determination result, (d ) Shows the transition of the air-fuel ratio F / B correction coefficient. In addition, the dashed-dotted line in FIG.7 (c) shows the result of the air fuel ratio determination based on the electromotive force value Vo2 (refer FIG.4 (b)).

  As described above, the air-fuel ratio early determination process has a smaller air-fuel ratio determination delay than the air-fuel ratio normal determination process (see FIG. 6). Therefore, in the air-fuel ratio early determination process, as shown in FIG. 7 (d), the skip amount Ks is made smaller than the skip amount Ks and the integral amount Ki in the air-fuel ratio normal determination process, and the integral amount Ki is set to be smaller. It is getting bigger.

  Next, a description will be given of the execution timing when the air-fuel ratio determination is based on the electromotive force value Vo2 and when the air-fuel ratio determination is based on the output differential value Vo2 * when performing the air-fuel ratio F / B control. As described above, in this embodiment, when the air-fuel ratio is out of the stoichiometric vicinity region or unstable, the air-fuel ratio determination is performed based on the output differential value Vo2 *, and the air-fuel ratio is in the stoichiometric vicinity region. In the case of convergence, the air-fuel ratio determination is performed based on the electromotive force value Vo2. Then, air-fuel ratio F / B control is performed using the result of the air-fuel ratio determination.

  FIG. 8 is a flowchart showing an example of a processing procedure of air-fuel ratio control in the present embodiment. This process is executed every predetermined time by the microcomputer 41 of the ECU 40. First, in step S21 in FIG. 8, it is determined whether or not an execution condition (air-fuel ratio F / B condition) of the air-fuel ratio F / B control is satisfied. The air-fuel ratio F / B condition includes, for example, that the coolant temperature detected by the coolant temperature sensor 26 is equal to or higher than a predetermined temperature, that it is not in a high rotation / high load state, and that the O2 sensor 30 is in an active state. .

  If the air-fuel ratio F / B condition is satisfied, the process proceeds to step S22, and whether the condition for executing the air-fuel ratio determination based on the output differential value Vo2 * (early determination execution condition) is satisfied. In order to determine whether or not, the following execution condition determination processing is executed.

  FIG. 9 is a flowchart illustrating a processing procedure of the execution condition determination process. In FIG. 9, first, in step S31, it is determined whether or not a predetermined time has elapsed since the start of the air-fuel ratio F / B control. If the predetermined time has not elapsed, the process proceeds to step S32, and it is determined that the early determination execution condition is satisfied. Immediately before the start of the air-fuel ratio F / B control, for example, when the air-fuel ratio open control on the rich side is performed, the actual air-fuel ratio is on the rich side with respect to the stoichiometric, so the air-fuel ratio is changed from the rich side to the stoichiometric vicinity region. There is a high need for rapid convergence. Therefore, at the beginning of air-fuel ratio F / B control, air-fuel ratio detection is performed based on the output differential value Vo2 *.

  On the other hand, if a predetermined time has elapsed since the start of the air-fuel ratio F / B, the process proceeds to step S33, and it is determined whether the current engine operating state is a steady state or a transient state. Regarding the engine operating state, in this embodiment, steady operation is performed when both the increase / decrease change amount of the engine speed is equal to or less than a predetermined value and the increase / decrease change amount of the throttle opening is equal to or less than the predetermined opening degree are satisfied. If it does not satisfy any of the above conditions, it is determined as a transient operation state. Note that the engine operating state may be determined using, for example, a map that uses the engine speed change amount and the throttle opening change amount as parameters instead of determining whether the engine is in a steady operation state or a transient operation state. .

  If the engine operating state is a steady state, the process proceeds to step S34, and the early determination execution condition is not satisfied. That is, if the engine 10 is in a steady operation state, it is considered that the air-fuel ratio has converged in the stoichiometric vicinity region, so the air-fuel ratio determination is performed based on the sensor electromotive force value Vo2. On the other hand, in the case of the transient operation state, the process proceeds to step S32, and it is assumed that the early determination execution condition is satisfied. That is, when the engine 10 is in a transient operation state, it is considered that the air-fuel ratio becomes unstable due to a sudden change in the intake air amount and the like and deviates from the stoichiometric vicinity region of the air-fuel ratio. In order to converge to the air-fuel ratio, the air-fuel ratio determination is performed based on the output differential value Vo2 *.

  Returning to the description of FIG. 8, in step S23, it is determined whether or not the early determination execution condition is satisfied. If the early determination execution condition is not satisfied, the process proceeds to step S24, and the air-fuel ratio normal determination process is performed. Carry out air-fuel ratio determination. On the other hand, when the early determination execution condition is satisfied, the process proceeds to step S25, and the air-fuel ratio determination is performed by the air-fuel ratio early determination process (see FIG. 5). After performing the air-fuel ratio determination, the process proceeds to step S26, and air-fuel ratio F / B control is performed using the determination result.

  FIG. 10 shows a time chart showing the transition of air-fuel ratio control in this system. As shown in FIG. 10, in the air-fuel ratio early determination process, the air-fuel ratio is switched to the stoichiometric combustion control during the rich-side air-fuel ratio control (in the case of A in the figure), or during vehicle acceleration or deceleration. Is carried out when is unstable (B in the figure). When the engine 10 shifts from the transient operation state to the steady operation state, the air-fuel ratio can be converged in the vicinity of the stoichiometric range, and the routine is switched to the air-fuel ratio normal determination process.

  According to the embodiment described in detail above, the following excellent effects can be obtained.

  Since the air-fuel ratio determination is performed based on the output differential value Vo2 * of the O2 sensor 30, the rich / lean reversal of the air-fuel ratio can be detected when the sensor output changes in the upward or downward direction. . Therefore, the change in the air-fuel ratio can be detected more quickly than in the case where the air-fuel ratio determination is performed based on the electromotive force value Vo2 of the O2 sensor 30.

  Since the air-fuel ratio determination based on the output differential value Vo2 * and the air-fuel ratio determination based on the electromotive force value Vo2 are switched according to the engine operating state, which one of priority is given to the quickness or certainty of the air-fuel ratio detection, It can be set as appropriate according to the engine operating state.

  During a transient operation of the engine 10 or within a predetermined time from the start of the air-fuel ratio F / B, an air-fuel ratio determination based on the output differential value Vo2 * is performed, and a predetermined time from the steady operation of the engine 10 and the start of the air-fuel ratio F / B. Since the air-fuel ratio determination based on the electromotive force value Vo2 is performed after the elapse of time, the air-fuel ratio detection can be performed in an embodiment suitable for the operating state of the engine 10. That is, during transient operation of the engine 10 or at the beginning of the air-fuel ratio F / B, it is considered that the air-fuel ratio deviates from the vicinity of the stoichiometric region and is unstable. In this embodiment, based on the output differential value Vo2 *. Since air-fuel ratio determination is performed and air-fuel ratio F / B control is performed using the determination result, the air-fuel ratio disturbance can be quickly resolved. On the other hand, when the air-fuel ratio has converged in the vicinity of the stoichiometric range, the air-fuel ratio change is detected based on the electromotive force value Vo2, so that a change in the air-fuel ratio can be reliably detected. In addition, when the air-fuel ratio has converged in the vicinity of the stoichiometric range, the air-fuel ratio determination based on the sensor electromotive force value Vo2 is performed, so that the rich and lean repetitions are reliably performed, and the repetition period is somewhat Since the length is maintained, the catalyst performance (oxygen storage function) of the three-way catalyst 24 can be sufficiently exhibited.

  In the air-fuel ratio feedback correction coefficient, when the air-fuel ratio determination based on the output differential value Vo2 * is performed, the skip amount Ks is increased and the integral amount Ki is set as compared with the case where the air-fuel ratio determination based on the electromotive force value Vo2 is performed. Since the air-fuel ratio is converged to the stoichiometric vicinity region, a sudden change in the fuel injection amount can be avoided, and as a result, the air-fuel ratio greatly overshoots to the rich side or lean side than the stoichiometric vicinity region. Can be avoided.

  Since the air-fuel ratio determination is performed based on the comparison result of the rich determination value Vri, the lean determination value Vle, and the output differential value Vo2 *, erroneous determination can be suppressed.

(Other embodiments)
The present invention is not limited to the description of the above embodiment, and may be implemented as follows, for example.

  In the above embodiment, when the air-fuel ratio determination is performed based on the output differential value Vo2 *, the rich determination value Vri and the lean determination value Vle as detection threshold values used for air-fuel ratio detection are fixed values. The lean determination value Vri and the lean determination value Vle may be made variable according to the engine operating state. According to this configuration, when detecting a change in the air-fuel ratio based on the output differential value Vo2 *, it is set as appropriate according to the engine operating state or the like, which priority should be given to the quickness or certainty of the detection. can do. Here, it is desirable that the engine operating state is an engine speed, a throttle opening, or an elapsed time from the start of air-fuel ratio F / B control. For example, when the determination values Vri and Vle are made variable according to the engine rotation speed, it is desirable that the determination values Vri and Vle be closer to zero as the engine rotation speed is higher. As a result, the higher the engine speed, the better the speed of air-fuel ratio determination. Therefore, even when the frequency of fuel injection is high, it is possible to quickly detect a change in air-fuel ratio. The rich determination value Vri corresponds to the positive threshold value, and the lean determination value Vle corresponds to the negative threshold value.

  In the above embodiment, when the determination values Vri and Vle are made variable, the skip amount Ks is increased and the integration amount is increased as the rich determination value Vri and the lean determination value Vle are closer to zero for the air-fuel ratio feedback correction coefficient. It is good to make Ki small. In this way, when the determination values Vri and Vle are relatively close to zero, the air-fuel ratio feedback correction coefficient can be set to an appropriate value in accordance with the change in the air-fuel ratio. In particular, since the skip amount Ks is limited to a relatively small value, a rapid change in the fuel injection amount can be avoided, and the air-fuel ratio overshoots to the rich side or lean side more than the target value. Can be avoided.

  In the above embodiment, the air-fuel ratio determination based on the output differential value Vo2 * and the air-fuel ratio determination based on the electromotive force value Vo2 are switched according to the engine operating state. The air-fuel ratio determination may be performed based on the value Vo2 *. At this time, the determination values Vri and Vle may be made variable according to the engine operating state in the same manner as described above. In this way, the determination values Vri and Vle are set according to the engine operating state, and accordingly, whether to give priority to the quickness of the air-fuel ratio determination or the certainty of the determination result is appropriately set according to the engine operating state. be able to.

  In the above-described embodiment, the early determination execution condition is that the air-fuel ratio F / B control is within a predetermined time from the start, and the engine 10 is in a transient operation state. The air-fuel ratio may be out of the stoichiometric vicinity region. By doing this, it is possible to directly detect the situation where the air-fuel ratio should converge to the stoichiometric vicinity region, which is suitable for setting the execution period of the air-fuel ratio early determination process. Specifically, for example, it is performed based on the electromotive force value of the O2 sensor 30. In the O2 sensor 30, when the air-fuel ratio is in the stoichiometric vicinity region, the rich signal and the lean signal are repeated at a constant cycle. On the other hand, when the air / fuel ratio is outside the stoichiometric vicinity region, the rich signal or lean signal is continuously output. . Therefore, when the electromotive force value Vo2 of the O2 sensor 30 does not change for a predetermined time, it is determined that the air-fuel ratio is out of the stoichiometric vicinity region.

  Alternatively, the early determination execution condition may include a time of return from the fuel cut. This is because when returning from the fuel cut, since the air-fuel ratio is greatly deviated from the stoichiometric vicinity region, it is highly necessary to quickly converge the air-fuel ratio to the stoichiometric region. For example, it may be determined based on the elapsed time from the stop of the fuel cut that the vehicle is returning from the fuel cut. In the above-described embodiment, the early determination execution condition may be either within a predetermined time from the start of the air-fuel ratio F / B control or the engine operating state is in a transient state.

1 is an overall schematic configuration diagram of an engine control system. Sectional drawing of the sensor element in an O2 sensor. The electromotive force characteristic view which shows the relationship between the air fuel ratio of exhaust gas, and the electromotive force of a sensor element. The time chart explaining air-fuel ratio feedback correction. The flowchart which shows the process sequence of an air fuel ratio early determination process. The time chart explaining the difference between the air fuel ratio determination by an electromotive force value and the air fuel ratio determination by an output differential value. The time chart explaining the air fuel ratio feedback correction | amendment in an air fuel ratio early determination process. The flowchart which shows the process sequence of air fuel ratio control. The flowchart which shows the process sequence of an implementation condition determination process. The time chart which shows transition of air fuel ratio control.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Engine, 16 ... Fuel injection valve, 24 ... Three-way catalyst, 30 ... O2 sensor, 40 ... ECU, 41 ... Microcomputer, 42 ... Sensor control circuit.

Claims (8)

In an air-fuel ratio detection apparatus for an internal combustion engine that is applied to an oxygen sensor that generates different electromotive force depending on whether the air-fuel ratio of the internal combustion engine is rich or lean, and performs air-fuel ratio detection using the electromotive force value of the oxygen sensor,
Calculating means for calculating a differential value of the electromotive force value as an output differential value;
Air-fuel ratio detection means for detecting whether the air-fuel ratio is rich or lean based on the output differential value calculated by the calculation means;
An air-fuel ratio detection apparatus for an internal combustion engine, comprising: The air-fuel ratio detection means includes
First detecting means for detecting whether the air-fuel ratio is rich or lean based on the output differential value;
Second detection means for detecting whether the air-fuel ratio is rich or lean based on the electromotive force value;
Switching means for performing air-fuel ratio detection by switching air-fuel ratio detection by the first detection means and air-fuel ratio detection by the second detection means based on the operating state of the internal combustion engine;
The air-fuel ratio detection apparatus for an internal combustion engine according to claim 1, comprising: Applied to an air-fuel ratio control system that performs air-fuel ratio feedback control so that the air-fuel ratio detected by the air-fuel ratio detection means matches a target value;
The switching means performs air-fuel ratio detection by the first detection means when the air-fuel ratio has not converged to a predetermined range including the target value, and the second detection means when the air-fuel ratio has converged to the predetermined range. The air-fuel ratio detection apparatus for an internal combustion engine according to claim 2, wherein the air-fuel ratio detection by means of is performed. Applied to an air-fuel ratio control system that performs air-fuel ratio feedback control so that the air-fuel ratio detected by the air-fuel ratio detection means matches a target value;
The switching means performs air-fuel ratio detection by the first detection means at a predetermined time after the start of execution of the air-fuel ratio feedback control, and performs air-fuel ratio detection by the second detection means after the predetermined time has elapsed. The air-fuel ratio detection apparatus for an internal combustion engine according to claim 2 or 3, characterized by the above-mentioned. The air-fuel ratio feedback correction amount for making the air-fuel ratio detected by the air-fuel ratio detecting means coincide with the target value is increased or decreased by a predetermined integral amount in the direction toward the target value, and the air-fuel ratio is Applied to an air-fuel ratio control system that performs air-fuel ratio control to increase or decrease the air-fuel ratio feedback correction amount by a predetermined skip amount in the opposite direction when it is detected that the target value has been reached,
When the air-fuel ratio detection is performed by the first detection means, the integral amount is increased and the skip amount is decreased compared to the case where the air-fuel ratio detection is performed by the second detection means. Item 5. The air-fuel ratio detection device for an internal combustion engine according to any one of Items 2 to 4. Threshold value setting means for setting a detection threshold value when performing rich detection or lean detection of the air-fuel ratio based on the output differential value based on the operating state of the internal combustion engine;
The air-fuel ratio detection means performs air-fuel ratio determination based on a comparison result between the detection threshold set by the threshold setting means and the output differential value calculated by the calculation means. Item 6. The air-fuel ratio detection apparatus for an internal combustion engine according to any one of Items 1 to 5. The detection threshold consists of a positive threshold used when the electromotive force value increases and a negative threshold used when the electromotive force value decreases,
The air-fuel ratio of the internal combustion engine according to claim 6, wherein the threshold value setting means brings the positive threshold value and the negative threshold value closer to zero as the rotational speed of the internal combustion engine is higher. Detection device. The air-fuel ratio feedback correction amount for making the air-fuel ratio detected by the air-fuel ratio detecting means coincide with the target value is increased or decreased by a predetermined integral amount in the direction toward the target value, and the air-fuel ratio is Applied to an air-fuel ratio control system that performs air-fuel ratio control to increase or decrease the air-fuel ratio feedback correction amount by a predetermined skip amount in the opposite direction when it is detected that the target value has been reached,
The air-fuel ratio detection apparatus for an internal combustion engine according to claim 6 or 7, wherein the integral amount is increased and the skip amount is decreased as the detection threshold is closer to zero.

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