JP2009299545A - Electronic control system and control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electronic control system capable of quickly and surely detecting an abnormality in an air-fuel ratio sensor, and a control method. <P>SOLUTION: The electronic control system 4 for determining an abnormality in the air-fuel ratio sensor 123 includes a storage unit for storing an anomaly determination threshold, and a control unit. The control unit performs the following processing that, fuel lean change difference value calculation processing which calculate a difference between a change to a fuel lean state estimated on the basis of an injected fuel quantity and an intake airflow and a change to a fuel lean state determined by signals from an air-fuel ratio sensor when a change from a fuel rich state to a fuel lean state is detected; fuel rich change difference value calculation processing which calculates the difference as in the fuel lean change calculation processing when a change from a fuel lean state to a fuel rich state is detected; and air fuel ratio sensor abnormality determination processing which determines the air fuel sensor to be abnormal when the difference in absolute values of the differences calculated in the respective processing is equal to or higher than the anomaly determination threshold. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electronic control device and a control method for determining abnormality of an air-fuel ratio sensor provided in an exhaust pipe of an internal combustion engine.

  Various self-diagnosis mechanisms for preventing serious accidents and environmental destruction are mounted on vehicle control devices.

  As one of such self-diagnosis mechanisms, a self-diagnosis mechanism that performs failure diagnosis of an air-fuel ratio sensor that detects the air-fuel ratio of exhaust gas is mounted on a vehicle control device.

  If an abnormality occurs in the air-fuel ratio sensor during the control of the internal combustion engine, the electronic control unit that controls the internal combustion engine cannot properly execute the air-fuel ratio control, and there is a possibility that the vehicle traveling may be hindered. This is because when an abnormality occurs in the sensor, it is necessary to quickly detect this.

  As an apparatus for diagnosing an air-fuel ratio sensor failure, for example, in Patent Document 1, an air-fuel ratio sensor is installed in an exhaust passage of an internal combustion engine, and an output current of the air-fuel ratio sensor is caused to flow through a detection resistor. In the air-fuel ratio sensor system that detects the air-fuel ratio of the exhaust gas by detecting the voltage difference between the two, the presence or absence of abnormality in the air-fuel ratio sensor system is diagnosed based on the voltages (reference voltage and detection voltage) across the detection resistor An air-fuel ratio sensor system abnormality diagnosis device for an internal combustion engine provided with abnormality diagnosis means is disclosed.

This air-fuel ratio sensor system abnormality diagnosis device diagnoses a rich abnormality in which the air-fuel ratio detection value shifts to the rich side when the reference voltage is higher than the detection voltage outside the normal range, and when the reference voltage is lower than the detection voltage outside the normal range When the air / fuel ratio detection value is deviated to the lean side, the sensor element of the air / fuel ratio sensor is short-circuited when the reference voltage is within the normal range and the detection voltage is stuck near the power supply voltage for a predetermined time. If the state where the reference voltage is within the normal range and the detected voltage is the same for a predetermined time, the sensor element is diagnosed as being disconnected.
Japanese Patent Laid-Open No. 11-107830

  By the way, there are cases where laws and regulations are made to make the above-described self-diagnosis mechanism effective. For example, according to OBD2 legislation by CARB (California Standby Resources Conservation Agency), all vehicle vehicles sold in the United States are designed to prevent engine emissions from being released due to engine abnormalities. It is obliged to install an ECU (Electronic Control Unit) having a function of monitoring whether a system related to gas generation is operating normally.

  And due to the increasing importance of traffic accident countermeasures and environmental countermeasures in recent years, laws and regulations such as OBD2 are being strengthened. As a result, an apparatus for diagnosing abnormality of the air-fuel ratio sensor is also required to be equipped with a mechanism capable of further improving accuracy and executing more types of diagnosis contents than the present in order to quickly and reliably detect engine abnormality. It has been.

  In view of the above-described conventional problems, an object of the present invention is to provide an electronic control device and a control method that can detect an abnormality of an air-fuel ratio sensor quickly and reliably.

  In order to achieve the above-described object, the electronic control device according to the present invention is characterized in that an electronic control device for determining an abnormality of an air-fuel ratio sensor provided in an exhaust pipe of an internal combustion engine is provided. A storage unit that stores an abnormality determination threshold value for controlling the internal combustion engine, and the air-fuel ratio determined by a signal from the air-fuel ratio sensor changes from a state that indicates fuel rich to a state that indicates fuel lean. Fuel lean reversal detection processing to be detected and fuel rich that detects when the air-fuel ratio determined by a signal from the air-fuel ratio sensor changes from a state showing fuel lean to a state showing fuel rich when controlling the internal combustion engine When the change from the fuel rich state to the fuel lean state is detected by executing the reverse detection process and the fuel lean reverse detection process, the fuel amount injected by the fuel injection unit and the intake air amount detection are detected. Fuel lean change value difference for calculating the difference between the change value to the fuel lean state estimated based on the intake air amount determined by the signal from the unit and the change value to the fuel lean state determined by the signal from the air-fuel ratio sensor Intake amount determined by the fuel amount injected by the fuel injection unit and a signal from the intake amount detection unit when a change from the fuel lean state to the fuel rich state is detected by executing the calculation process and the fuel rich inversion detection process A fuel rich change value difference calculation process for calculating a difference between a change value to the fuel rich state estimated based on the value and a change value to the fuel rich state determined by a signal from the air-fuel ratio sensor, and a fuel lean change value difference The storage unit stores the difference between the absolute value of the difference calculated by executing the calculation process and the absolute value of the difference calculated by executing the fuel rich change value difference calculation process. In the case of that the abnormality determination threshold value or more, in that it comprises a control unit for the air-fuel ratio sensor to perform the air-fuel ratio sensor abnormality determination process determines that abnormality.

  According to the above-described configuration, the control unit makes an abnormality determination of the air-fuel ratio sensor based on the difference between the change value to the fuel lean state and the difference between the change value to the fuel rich state, thereby changing to the fuel lean state. An abnormality is detected when the difference between the change values of the fuel and the difference between the change values to the fuel rich state is greatly different.

  That is, the electronic control unit detects an abnormality detected based on the difference in the change value to the fuel lean state, that is, an abnormality of the air / fuel ratio sensor detected when the air / fuel ratio changes from the fuel rich state to the fuel lean state, In addition to the abnormality detected based on the difference between the change values to the fuel-rich state, that is, the abnormality of the air-fuel ratio sensor detected when the air-fuel ratio changes from the fuel lean state to the fuel-rich state, the difference between both change values It is possible to execute an abnormality determination of the air-fuel ratio sensor when there is a large difference between the two, that is, more types of diagnosis contents can be executed.

  As described above, according to the present invention, it is possible to provide an electronic control device capable of detecting an abnormality of an air-fuel ratio sensor quickly and reliably.

  Hereinafter, an electronic control device and a control method according to the present invention will be described. First, an internal combustion engine equipped with an air-fuel ratio sensor to be detected by an electronic control device will be described.

  In this embodiment, the internal combustion engine is an engine in which one cycle is composed of four strokes, and is composed of four cylinders of # 1, # 2, # 3, and # 4, each of which is 1 / cylinder. It is configured to perform different driving for every four cycles. In addition, the engine 1 of the vehicle shown in FIG. 1 described below shows one extracted from the above-described four cylinders.

  As shown in FIG. 1, the engine 1 includes an internal combustion section 10, an intake section 11 that communicates with the internal combustion section 10, and an exhaust section 12.

  The internal combustion unit 10 includes a piston 102 that reciprocates up and down with a pressure when an air-fuel mixture (a mixture of atomized fuel and air) burns in a combustion chamber 101a inside a cylinder 101, and an internal combustion chamber 101a. An intake valve 103 that opens and closes the intake port of the air-fuel mixture, an exhaust valve 104 that opens and closes the exhaust port to discharge combustion gas after the air-fuel mixture burns, and a crankshaft 105 that changes the reciprocating motion of the piston 102 into a rotational motion A connecting rod 106 connecting the piston 102 and the crankshaft 105, an ignition plug 107 for igniting the air-fuel mixture sucked into the combustion chamber 101a, and an optimal air-fuel ratio (a mixture ratio of air and fuel in the air-fuel mixture) In order to determine the fuel injection timing, the angle position (crank angle) of the crankshaft 105 is detected. The link angle sensor 108 is configured to include a like temperature sensor 109 composed of a thermistor or the like for detecting the temperature of coolant of the engine 1.

  The intake section 11 is for sending fuel and air necessary for burning fuel to the internal combustion section 10. The intake section 11 serves to inject fuel into the intake pipe 111 and the intake valve 103 that serve as a passage for air or fuel. An injector 112 as a fuel injection valve, an air filter 114 for purifying air sucked from the air inlet 113, an air flow meter 115 for detecting the amount of sucked air, and a throttle valve 116 for controlling the amount of air sucked The throttle valve driving mechanism 117 that drives the throttle valve 116 based on the depression amount of the accelerator pedal detected by the accelerator opening sensor, and the throttle opening that detects the opening degree of the throttle valve 116 driven by the throttle valve driving mechanism 117. A degree sensor 118 and the like are provided.

  The exhaust unit 12 is for exhausting the gas burned in the internal combustion unit 10, and is composed of an exhaust pipe 121 serving as a passage for the exhausted gas, and a catalyst 122 configured by a three-way catalyst or the like and purifying the exhausted gas. An air-fuel ratio sensor 123 that is installed upstream of the catalyst 122 and outputs a limit current substantially proportional to the oxygen concentration to detect the air-fuel ratio, and an oxygen sensor 124 that is installed downstream of the catalyst 122 and detects the oxygen concentration. Etc. are provided.

  As shown in FIG. 2, the air-fuel ratio sensor 123 protrudes toward the inside of the exhaust pipe 121, has a U-shaped cross section, and has a small number of small holes 32 communicating with the inside and outside of the cover on its peripheral wall. 30, a sensor main body 34 that generates a limit current corresponding to an oxygen concentration when the air-fuel ratio indicates fuel lean, or an unburned gas concentration such as CO or HC when the air-fuel ratio indicates fuel rich, and a heater 36 etc. are comprised.

  In the sensor body 34, an exhaust gas side electrode layer 34b is fixed to the outer surface of the bottomed cylindrical solid electrolyte layer 34a, an atmosphere side electrode layer 34c is fixed to the inner surface, and diffused outside the exhaust gas side electrode layer 34b. The resistance layer 34d is plasma sprayed.

The solid electrolyte layer 34a is an oxygen ion conductive oxide sintered material in which a stabilizer such as CaO, MgO, Y ₂ O ₃ , Yb ₂ O ₃ is mixed with ZrO ₂ , HfO ₂ , ThO ₂ , Bi ₂ O ₂ or the like. The diffusion resistance layer 34d is made of a heat resistant inorganic material such as alumina, magnesia, siliceous, spinel, mullite.

  Both the exhaust gas side electrode layer 34b and the atmosphere side electrode layer 34c are made of a noble metal having high catalytic activity such as platinum, and the surface thereof is subjected to porous chemical plating or the like.

  The heater 36 is accommodated inside the atmosphere-side electrode layer 34c, and the sensor main body 34 is heated by the generated heat.

  The sensor main body 34 is determined by the area of the exhaust gas side electrode layer 34b, the thickness of the diffusion resistance layer 34d, the porosity, and the average pore diameter in accordance with the oxygen concentration in a state where the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. Generate limit current. In the state where the air-fuel ratio is richer than the stoichiometric air-fuel ratio, the concentration of carbon monoxide, which is an unburned gas, changes almost linearly with respect to the air-fuel ratio, and a limit current corresponding to the unburned gas concentration is generated. To do. The limit current shows a linear characteristic corresponding to the oxygen concentration, but is activated at a temperature of about 600 ° C. or higher, and the sensor body 34 is controlled to the activation temperature range by heating by the heater 36.

  In the air-fuel ratio sensor 123, as shown in FIG. 3A, the inflow current I to the solid electrolyte layer 34a and the applied voltage V have a linear characteristic, and the linear portion L parallel to the applied voltage axis has a limit current. The increase / decrease in the limit current corresponds to the increase / decrease in the air-fuel ratio. That is, as shown in FIG. 3B, the limit current increases as the air-fuel ratio becomes fuel lean, that is, increases, and the limit current decreases as the air-fuel ratio becomes fuel rich, that is, decreases.

  As shown in FIG. 4, the electronic control unit 4 includes a CPU 41, a ROM 42 and / or an EEPROM 43 in which a control program executed by the CPU 41 is stored, a RAM 44 used as a working area, and analog signals from various sensors. And an input interface circuit 45 for inputting a digital signal, an output interface circuit 46 for outputting a signal calculated as a result of the CPU 41 based on the input signal to various actuators, and the like.

  The electronic control unit 4 inputs detection values from the crank angle sensor 108, the water temperature sensor 109, the air flow meter 115, the throttle opening sensor 118, the air-fuel ratio sensor 123, etc., via the input interface circuit 45, and the CPU 41 Then, a control signal for controlling the engine 1 is calculated from these detected values. For example, the fuel injection timing and the engine speed are calculated based on the detection value of the crank angle sensor 108, and the throttle opening is calculated based on the detection value of the throttle opening sensor 118. The electronic control unit 4 controls the fuel injection amount of the engine 1 by outputting a control signal to the injector 112 and the like via the output interface circuit 46.

  Further, the electronic control unit 4 performs abnormality determination, that is, failure diagnosis of the air-fuel ratio sensor 123 provided in the internal combustion engine, that is, the exhaust pipe 121 of the engine 1, when a predetermined traveling condition is satisfied.

  The predetermined traveling condition is that the vehicle state is stable based on a signal from the vehicle state detection unit. That is, the control unit of the electronic control device 4 described later determines that the vehicle state is stable based on a signal from the vehicle state detection unit based on a signal from the vehicle state detection unit (that is, predetermined traveling). (When it is determined that the condition is satisfied).

  The predetermined traveling conditions include, for example, that the air-fuel ratio sensor 123 has shifted to an active state, that the engine speed is within a predetermined speed range, that the vehicle speed is within a predetermined speed range, and a fuel supply system It is included that the evaporated fuel generated in the above is not purged into the intake pipe 111. Here, the vehicle state detection unit includes an air-fuel ratio sensor 123, a crank angle sensor 108 that detects a crank angle for calculating the engine speed, and the like.

  In other words, it is preferable that the air-fuel ratio sensor 123 is in the active state and satisfies the condition that the fuel injection amount does not fluctuate during the failure diagnosis. As will be described later, the electronic control unit 4 performs a fault diagnosis of the air-fuel ratio sensor 123 by measuring the step response when the fuel injection amount is changed, so that the fuel injection amount is constant during the step response measurement. This is because failure diagnosis cannot be performed properly if the fuel injection amount changes during step response measurement.

  This will be described in detail below. As described above, the output voltage of an oxygen concentration sensor such as the air-fuel ratio sensor 123 is generally highly temperature dependent, and the element temperature is set to a predetermined activation temperature (for example, approximately 600 ° C. or higher). Therefore, the electronic control unit 4 can appropriately perform the failure diagnosis of the air-fuel ratio sensor 123 by determining that the air-fuel ratio sensor 123 is maintained at the activation temperature as one of the predetermined traveling conditions.

  Since the electronic control unit 4 performs control so that the fuel injection amount or the like varies according to the fluctuation when the engine speed fluctuates, the air-fuel ratio in the exhaust pipe 121 fluctuates. The failure diagnosis of the sensor 123 cannot be properly performed. Therefore, the electronic control unit 4 can properly perform the failure diagnosis of the air-fuel ratio sensor 123 by setting one of the predetermined traveling conditions that the engine speed is within the predetermined speed range. Similarly, when the vehicle speed fluctuates, the engine speed fluctuates, so that one of the predetermined traveling conditions is that the vehicle speed is within a predetermined speed range, so that the electronic control unit 4 performs failure diagnosis of the air-fuel ratio sensor 123. Can be implemented properly.

  The fuel tank storing the fuel of the engine 1 is fueled by high-temperature fuel or the like sent from the fuel tank to the injector 112 but sent back to the fuel tank without being injected into the internal combustion unit 10. It is known that steam (fuel vapor) is generated.

  When the fuel vapor is generated, the pressure in the fuel tank rises, so it is necessary to discharge the accumulated fuel vapor to the outside. However, if the fuel vapor is discharged to the outside as it is, a strange odor is generated, which may cause pollution.

  Therefore, a canister for collecting the fuel vapor is provided in communication with the intake pipe 111, the fuel vapor is collected in the canister, and the fuel vapor is sent to the intake pipe 111 when the engine is driven to burn the fuel vapor. Thus, a so-called purge is performed in the vehicle to remove the fuel vapor from the fuel tank without releasing it to the outside.

  That is, when the purge is executed, the air-fuel ratio in the exhaust pipe 121 fluctuates due to the fuel vapor sent into the intake pipe 111, and the electronic control unit 4 cannot properly perform the failure diagnosis of the air-fuel ratio sensor 123. End up. Therefore, one of the predetermined traveling conditions is that the evaporated fuel generated in the fuel supply system is not purged into the intake pipe 111, so that the electronic control unit 4 properly performs the failure diagnosis of the air-fuel ratio sensor 123. it can.

  Hereinafter, failure diagnosis of the air-fuel ratio sensor 123 by the electronic control unit 6 will be described. The electronic control unit 6 includes a lean-side step response when the fuel injection amount is changed to a predetermined lean-side air-fuel ratio while the air-fuel ratio sensor 123 indicates fuel rich, and a state where the air-fuel ratio sensor 123 indicates fuel lean. To measure the rich side step response when the fuel injection amount is changed to the predetermined rich side air-fuel ratio, and calculate the first-order lag system parameters from the measured values, respectively, and the lean side step response and the rich side step response respectively. A control unit that performs failure diagnosis of the air-fuel ratio sensor 123 based on the parameter value and a storage unit that stores failure diagnosis information by the control unit are provided.

  Here, the control unit is the CPU 41, and the above-described functions are realized by the CPU 41 executing a control program stored in the ROM 42. The storage unit is the RAM 44 and / or the EEPROM 43.

  An example of the lean side step response is shown in FIG. 5A, and an example of the rich side step response is shown in FIG. The step response is a characteristic of the air-fuel ratio with respect to time, and is a response characteristic of the actual air-fuel ratio with respect to a step input based on an instantaneous change in the calculated air-fuel ratio.

  The control unit calculates the calculated air-fuel ratio by dividing the air amount detected by the air flow meter 115 by the fuel amount calculated based on the energization time to the injector 112. Instead of calculating the calculated air-fuel ratio based on the detection value of the air flow meter 115, the control unit is installed on the downstream side of the throttle valve and is based on the detection value of the intake pipe pressure sensor 119 that detects the intake pipe pressure. The calculated air-fuel ratio may be calculated by calculating the air amount.

  Further, the control unit calculates the actual air-fuel ratio based on the limit current value input from the air-fuel ratio sensor 123 and the characteristics shown in FIG.

  The control unit injects a control signal for rapidly increasing or decreasing the fuel injection amount so that the air-fuel ratio becomes a lean-side air-fuel ratio (for example, 14.7) or a rich-side air-fuel ratio (for example, 14.5) at a predetermined timing. To 112. A characteristic of the calculated air-fuel ratio calculated based on the fuel injection amount corresponding to the control signal with respect to time becomes a step input.

  For example, as shown in FIG. 6A, the control unit changes the air-fuel ratio to the lean-side air-fuel ratio (14.7) at the timing when the rich-side air-fuel ratio (14.5) continues for a predetermined time t. Then, a control signal for changing the fuel injection amount is output to the injector 112. Then, the actual air-fuel ratio that is the air-fuel ratio in the exhaust pipe 121 is changed from the rich-side air-fuel ratio (14.5) to the lean-side air-fuel ratio via the theoretical air-fuel ratio (14.6) by the fuel injection amount corresponding to the control signal. After changing to (14.7), the control unit measures the change input from the air-fuel ratio sensor 123 as a lean step response.

  Similarly, the control unit changes the fuel injection amount so that the air-fuel ratio becomes the rich-side air-fuel ratio (14.5) at the timing when the state of the lean-side air-fuel ratio (14.7) continues for the predetermined time t. A control signal is output to the injector 112. Then, the actual air-fuel ratio changes from the lean air-fuel ratio (14.7) to the rich air-fuel ratio (14.5) via the stoichiometric air-fuel ratio (14.6) by the fuel injection amount corresponding to the control signal, The control unit measures the change input from the air-fuel ratio sensor 123 as a rich step response.

  That is, the portion in FIG. 6A where the air-fuel ratio is larger than the stoichiometric air-fuel ratio (14.6) corresponds to FIG. 5A (an example of lean side step response), and the air-fuel ratio is the stoichiometric air-fuel ratio (14.6). The smaller part corresponds to FIG. 5B (example of rich side step response).

  The predetermined timing is not limited to the timing at which the rich-side or lean-side air-fuel ratio state continues for the predetermined time t, and may be, for example, the timing at which the air-fuel ratio becomes the stoichiometric air-fuel ratio.

  The control unit calculates a predetermined parameter from the step input and the step response. The parameters include dead time, time constant, and gain, and the control unit performs failure diagnosis of the air-fuel ratio sensor 123 based on whether or not each parameter value falls within a predetermined allowable range.

  The control unit performs failure diagnosis of the air-fuel ratio sensor 123 based on whether or not the difference value of each parameter falls within a predetermined allowable range, which will be described later. In the following description, the description will be based on the lean side step response shown in FIG. 5A, but the rich side step response shown in FIG. 5B is the same as the lean side step response.

  From the above description, the step response refers to a state in which the output value of the air-fuel ratio sensor 123 per predetermined time changes.

  The dead time D is the time from the moment when the step input changes to the rich air-fuel ratio to the moment when the step response changes from the lean air-fuel ratio to the stoichiometric air-fuel ratio corresponding to the change.

  The time constant T is the convergence of the step response from the instant T1 when the step response has changed from the lean air-fuel ratio to the stoichiometric air-fuel ratio, after the instant T1 (after the rise of the step response). This is the time until time T2 that is assumed to have been reached.

  The inclination is calculated by the control unit as follows. The output characteristic of the air-fuel ratio sensor 123 is statistically indicated that the change in the output value of the air-fuel ratio sensor 123 in the initial fixed time after the change from the fuel-lean state to the fuel-rich state and from the fuel-rich state to the fuel-lean state is proportional. I know. The inclination is calculated based on the change in the output value of the air-fuel ratio sensor 123 in the initial fixed time indicating the proportionality.

  The convergence value is a value obtained by preliminarily obtaining a value at which the step response converges.

  The gain K is a value obtained by dividing the step input convergence value (“a” in FIG. 5A) from the convergence value of the step response (“b” in FIG. 5A).

  According to the above-described configuration, the control unit performs failure diagnosis of the air-fuel ratio sensor 123 based on a predetermined parameter, and thus, without performing complicated measurement such as measuring the rising characteristics of the step response at regular intervals. Failure diagnosis of the air-fuel ratio sensor 123 can be performed appropriately.

  Then, the control unit performs a failure diagnosis based on whether the dead time D is within a predetermined range, whether the time constant T is within the predetermined range, and whether the gain K is within the predetermined range. For example, the control unit diagnoses that the dead time of the air-fuel ratio sensor 123 is abnormal when the dead time D is equal to or longer than a predetermined time (for example, 4 (msec)), and the time constant T is within a predetermined range (for example, 0.1 to 0). 4 (msec)), it is diagnosed that the response of the air-fuel ratio sensor 123 is abnormal, and when the gain K is a predetermined value (for example, 1.2) or more, the output expansion abnormality of the air-fuel ratio sensor 123 is abnormal. When the gain K is a predetermined value (for example, 0.8) or less, it is diagnosed that the output reduction of the air-fuel ratio sensor 123 is abnormal.

  In addition, the diagnosis of the dead time abnormality and the responsiveness decrease abnormality is executed separately for each of the rich side step response and the lean side step response, and the diagnosis of the output side expansion response and the output side reduction response is performed separately for the rich side step response and the lean side step. Although it performs based on the average value of a response, it is not restricted to this. For example, the diagnosis of the dead time abnormality and the responsiveness deterioration abnormality may be executed based on the average value of the rich side step response and the lean side step response, and the diagnosis of the output expansion abnormality and the output reduction abnormality is made as the rich side step response. Each of the lean side step responses may be executed individually.

  The diagnosis results by the control unit described above (absence of dead time abnormality, responsiveness deterioration abnormality, output expansion abnormality, output reduction abnormality, and the occurrence time of these abnormalities, etc.) are stored in the storage unit as failure diagnosis information. The

  According to the above-described configuration, the control unit performs failure diagnosis of the air-fuel ratio sensor based on the parameters of the lean-side step response and the rich-side step response, thereby changing the air-fuel ratio from the rich-side air-fuel ratio to the lean-side air-fuel ratio. The abnormality of the air-fuel ratio sensor 123 when the air-fuel ratio changes and the abnormality of the air-fuel ratio sensor 123 when the air-fuel ratio changes from the lean-side air-fuel ratio to the rich-side air-fuel ratio can be detected as separate abnormalities.

  Further, the control unit performs a failure diagnosis of the air-fuel ratio sensor 123 based on the difference values of the parameters of the lean side step response and the rich side step response, and the storage unit stores failure diagnosis information by the control unit.

  Even when the failure diagnosis of the air-fuel ratio sensor 123 is performed based on the difference value of the parameter, the parameter includes the dead time D, the time constant T, and the gain K as described above.

  Then, the control unit performs failure diagnosis of the air-fuel ratio sensor 123 based on whether or not the difference value for each corresponding parameter falls within a predetermined allowable range.

  That is, if the absolute value of the difference between the dead time D in the lean side step response and the dead time D in the rich side step response is equal to or greater than a predetermined value, the air-fuel ratio sensor 123 is diagnosed as having a dead time asymmetric abnormality, and the lean side step When the absolute value of the difference between the time constant T in the response and the time constant T in the rich step response is greater than or equal to a predetermined value, the air-fuel ratio sensor 123 is diagnosed as having an asymmetric response abnormality, and the gain K in the lean step response When the absolute value of the gain K difference in the rich side step response is equal to or greater than a predetermined value, it is diagnosed that the gain of the air-fuel ratio sensor 123 is abnormal.

  The diagnosis results by the control unit described above (the presence / absence of dead time asymmetric abnormality, responsive asymmetry abnormality, gain asymmetry abnormality, and the occurrence time of these abnormalities, etc.) are also provided as failure diagnosis information, similar to the above-described diagnosis results. Stored in the storage unit.

  The control unit determines that the air-fuel ratio sensor 123 is in failure when there is at least one of the abnormalities described above.

  In other words, the control unit, when the parameters of the lean side step response and the rich side step response fall within the predetermined allowable range (all of the dead time abnormality, the responsiveness reduction abnormality, the output expansion abnormality, and the output reduction abnormality have occurred). Even when the difference value for each corresponding parameter deviates from the predetermined allowable range (when any of the dead time asymmetry abnormality, responsiveness asymmetry abnormality, or gain asymmetry abnormality occurs). Diagnoses that the air-fuel ratio sensor 123 is malfunctioning.

  According to the above configuration, the control unit can diagnose an abnormality of the air-fuel ratio sensor 123 that is determined to be normal in the measurement based only on the lean-side step response or the rich-side step response. 123 abnormality can be detected more reliably.

  The controller repeats the measurement of the lean side step response and the rich side step response a plurality of times (for example, 5 times) alternately at the time of one failure diagnosis, and performs the failure diagnosis based on the average value of the parameters calculated respectively. Also good.

  This will be described in detail below. When the predetermined traveling condition described above is satisfied, the control unit outputs a control signal that alternately repeats rapid increase and decrease of the fuel injection amount to the injector 112 as a step input. For example, the control unit outputs to the injector 112 a pulse signal in which the same waveform as shown in FIG. 6A is repeated a predetermined number of times (for example, 5 times).

  Each time the control unit changes the pulse value of the control signal (change from the rich air-fuel ratio to the lean air-fuel ratio and change from the lean air-fuel ratio to the rich air-fuel ratio), Side step response and rich side step response are measured.

  The control unit calculates the average value of each parameter by dividing the total value of each parameter calculated based on the step response measured each time of the predetermined number of pulse signals by the predetermined number of times. For example, when the dead times calculated in five measurements are D1, D2, D3, D4, and D5, the average value of the dead times is calculated as (D1 + D2 + D3 + D4 + D5) / 5.

  The control unit performs failure diagnosis of the air-fuel ratio sensor 123 based on the calculated average value of each parameter and whether or not the difference of the average value for each parameter falls within a predetermined allowable range.

  According to the above-described configuration, the control unit calculates the parameter a plurality of times and performs failure diagnosis based on the average value. Therefore, even when some of the parameters calculated a plurality of times are accompanied by an error due to the influence of noise or the like. Thus, failure diagnosis of the air-fuel ratio sensor 123 can be performed appropriately.

  As a result of the failure diagnosis, the control unit stores data for identifying whether the air-fuel ratio sensor 123 is normal or failed, and the monitoring frequency in the storage unit.

  In other words, the control unit stores the data for identifying whether the air-fuel ratio sensor 123 is normal or abnormal, or the monitoring frequency in the storage unit by executing the abnormality determination process.

  Data identifying whether the air-fuel ratio sensor 123 is normal or malfunctioning is information indicating the presence or absence of various abnormalities in the above-described failure diagnosis information.

  The monitoring frequency is normal based on the number of times counted when a predetermined driving condition is satisfied (one trip from IG-ON to IG-OFF is one driving) and a failure diagnosis performed in that case. This is the ratio of the number of monitoring times counted when it is determined as abnormal, and is represented by, for example, a value obtained by dividing the number of monitoring times by the number of traveling times.

  The number of times of travel is incremented when a predetermined travel condition is satisfied, and the number of times of monitoring is incremented when determination of normality or abnormality of the air-fuel ratio sensor 123 is completed as a result of failure diagnosis by the electronic control unit 4. That is, as a result of the failure diagnosis, when normal or abnormal cannot be determined, the monitoring number is maintained as it is.

  In the RateBase monitoring method of OBD2, it is necessary to continuously store the monitoring frequency in relation to the IPT (In-use Performance Tracking) of the service 09. According to the above configuration, the electronic control unit 4 You can comply with the provisions of the law.

  When the calculated value of the parameter indicates a value that cannot be diagnosed, the control unit stores the cumulative ratio of the number of times of diagnosis impossible with respect to the number of times of diagnosis in the storage unit and performs fault diagnosis again until the cumulative ratio exceeds a predetermined ratio.

  This will be described in detail below. The undiagnosable value is a value that each parameter cannot actually take, and is, for example, a negative value, a dead time D greater than the period of the pulse signal, a value of the time constant T, or the like.

  The number of times of diagnosis execution is the number of times that failure diagnosis has been performed. Here, in the present embodiment, since the control unit performs a failure diagnosis when a predetermined traveling condition is satisfied, the number of times of diagnosis is equal to the number of times of traveling described above. However, if the control unit does not always perform a failure diagnosis even if a predetermined traveling condition is satisfied, the number of times of diagnosis and the number of traveling may be different values.

  The number of undiagnosable times is counted when failure diagnosis by the control unit is not possible. Specifically, the value is incremented when at least one of the parameters calculated by the control unit is an undiagnosable value.

  The cumulative ratio is a ratio between the number of times of diagnosis impossible and the number of times of diagnosis, and is represented by a value obtained by dividing the number of times of diagnosis impossible by the number of times of diagnosis.

  According to the above-described configuration, the control unit compares the calculated value of the parameter with the undiagnosable value, thereby causing the electronic control device 4 to fail in acquiring data from the air-fuel ratio sensor 123, the air flow meter 115, and the like. It is possible to detect a failure in calculating the calculated air-fuel ratio and the actual air-fuel ratio. In addition, even when the calculated value of the parameter indicates an undiagnosable value, the control unit performs failure diagnosis again until the cumulative ratio of the number of undiagnosable times exceeds a predetermined ratio. Even if an abnormality occurs, the air-fuel ratio sensor 123 can be prevented from being immediately diagnosed as abnormal.

  The control unit is configured to perform the failure diagnosis at least once from the engine start to the stop, and resets the cumulative ratio stored in the storage unit when the failure diagnosis is normally completed a predetermined number of times.

  For example, when a predetermined traveling condition is initially satisfied between the engine start and the stop, the control unit performs a predetermined time every time the predetermined traveling condition is satisfied or a predetermined number of times set in advance. A fault diagnosis is performed every time driving conditions are satisfied. Note that the control unit does not perform a failure diagnosis when a predetermined traveling condition is not satisfied between the engine start and the stop.

  In addition, the control unit increments each time the failure diagnosis ends normally (for example, when the calculated parameter is not an undiagnosable value), and sets the number of times that the diagnosis ends normally when the failure diagnosis does not end normally. In addition, when the number of normal diagnosis ends is equal to or greater than a predetermined number (for example, 20 times), the cumulative ratio is reset by resetting the number of times diagnosis is impossible and the number of times diagnosis is performed.

  According to the above-described configuration, in the past, in the failure diagnosis by the control unit, the calculated value of the parameter often indicated an undiagnosable value (hereinafter referred to as “calculation failure”), but now the calculated value of the parameter is diagnosed. When the calculated value of the parameter newly indicates an undiagnosable value when there is hardly any impossible value, the cumulative ratio is prevented from immediately exceeding the predetermined ratio due to the influence of the past calculation failure. be able to.

  Hereinafter, failure diagnosis of the air-fuel ratio sensor 123 by the control unit of the electronic control unit 4 will be described based on the flowchart shown in FIG.

  When a predetermined traveling condition is satisfied and failure diagnosis of the air-fuel ratio sensor 123 is started, the number of traveling times (and the number of times of diagnosis is performed) is incremented (S1).

  The control unit outputs a pulse signal, which repeats the same waveform a predetermined number of times, to the injector 112 as a step input that alternately repeats the rapid increase and decrease of the fuel injection amount. Then, the lean side step response and the rich side step response are measured at each time of the pulse signal, and the parameters of the first-order lag system (dead time D, time constant T, and gain K) are calculated from the measured values. Finally, the average value of each parameter is calculated by dividing the total value of each parameter by a predetermined number of times (S2).

  When the calculated parameter (or the average value of the parameters) indicates an undiagnosable value (S3), the control unit increments the undiagnosable number and calculates a cumulative ratio based on the current number of diagnoses and undiagnosable times. The calculated cumulative ratio is stored in the storage unit (S4).

  When the cumulative ratio is greater than or equal to a predetermined ratio set in advance (S5), it is determined that failure diagnosis is impossible (S6). When the cumulative ratio is smaller than the predetermined ratio (S5), the average value of each parameter is calculated again. (S2). That is, the calculation of the average value of each parameter is attempted until the cumulative ratio is equal to or greater than the predetermined ratio, or until the calculated parameter does not indicate a diagnosis impossible value.

  When the calculated parameter does not indicate an undiagnosable value (S3), the control unit increments the number of normal diagnosis ends, and when the number of normal diagnosis ends is equal to or greater than a predetermined number (S7), the number of times of diagnosis execution, undiagnosable The number of times and the cumulative ratio are reset (S8).

  The control unit determines that the parameter (or parameter average value) calculated in step S2 falls within a predetermined allowable range (S9), and the difference value for each parameter (or each parameter average value) is a predetermined allowable range. (S10), the air-fuel ratio sensor 123 is diagnosed as normal (S11). On the other hand, if it does not fall within the predetermined allowable range in either step S9 or S10, the air-fuel ratio sensor 123 is diagnosed as malfunctioning (S12). The control unit stores these diagnosis results (normal or failure results) in the storage unit.

  Thereafter, the control unit increments the number of monitoring (S13), calculates a monitoring frequency based on the current number of running times and the number of monitoring, and stores the calculated monitoring frequency in the storage unit (S14).

  As described above, the failure diagnosis method according to the present invention is a failure diagnosis method for the air-fuel ratio sensor 123 provided in the exhaust pipe 121 of the internal combustion engine 1 that is executed when a predetermined traveling condition is satisfied. The lean side step response when the fuel injection amount is changed to a predetermined lean side air-fuel ratio while the fuel ratio sensor 123 indicates fuel rich, and the predetermined rich side air fuel ratio when the air fuel ratio sensor 123 indicates fuel lean When the fuel injection amount is changed, the rich side step response is measured, and the parameters of the first-order lag system are calculated from the measured values. Based on the difference values of the parameters of the lean side step response and the rich side step response, respectively. This is a method of performing a failure diagnosis of the air-fuel ratio sensor 123.

  Hereinafter, another embodiment will be described. In the above-described embodiment, the control unit has described the case where the measurement of the step response is alternately measured as the lean side step response and the rich side step response as indicated by the broken line in FIG. 6B, each step response may be continuously measured as indicated by a broken line in FIG. 6B, or each step response is discretely measured as indicated by a broken line in FIG. 6C (for example, (Measured every predetermined number). FIG. 6B shows a case where the lean side step response is continuously measured, and FIG. 6C shows a case where the lean side step response is measured every other time.

  In the above-described embodiment, the control unit stores the cumulative ratio in the storage unit, and the case where it is determined that failure diagnosis is impossible when the cumulative ratio exceeds a predetermined ratio has been described. This is not necessarily the case. For example, the control unit may determine that failure diagnosis is impossible when the calculated value of the parameter continuously indicates a diagnosis impossible value for a predetermined number of times or more.

  In the above-described embodiment, the control unit alternately repeats the measurement of the lean side step response and the rich side step response a predetermined number of times (for example, 5 times) at the time of one failure diagnosis, and is based on the average value of the parameters calculated respectively. The case where failure diagnosis is performed has been described.

  In this case, the control unit determines that the parameter calculation has failed when the parameter calculation value cannot be properly calculated at least once of the predetermined number of times because the parameter calculation value indicates an undiagnosable value. In other words, the configuration may be such that the number of times the diagnosis cannot be performed is incremented, or the failure diagnosis is performed with an average of only parameters that can be appropriately calculated.

  In the above-described embodiment, the control unit has described the case where the failure diagnosis information, the monitoring frequency, the cumulative ratio, and the like are stored in the RAM 44 and / or the EEPROM 43 as the storage unit. , And the cumulative ratio may be temporarily stored in the RAM 44 and transferred from the RAM 44 to the EEPROM 43 when the engine is stopped. As a result, the use of the EEPROM 43 whose data writing / reading speed is generally slower than that of the RAM 44 can be minimized.

  In the above-described embodiment, a case where the control unit performs failure diagnosis (abnormality determination) of the air-fuel ratio sensor 123 based on the first-order lag system parameter calculated based on the lean-side step response and the rich-side step response will be described. However, the abnormality determination may be performed based on parameters other than the first-order lag system parameters.

  An example will be described in detail below. For example, based on the signal from the air-fuel ratio sensor 123, the control unit calculates a time t1 from the moment when the fuel rich state shifts to the fuel lean state until the moment when the fuel lean state shifts again to the fuel rich state. Then, a time t2 from the moment when the fuel lean state shifts to the fuel rich state to the moment when the fuel rich state shifts again to the fuel lean state is calculated.

  Further, the control unit changes the fuel lean state based on the amount of fuel injected from the injector 112 serving as the fuel injection unit and the intake air amount calculated from the signal from the air flow meter 115 serving as the intake air amount detection unit. The estimated time t1 ′ corresponding to the time t1 is estimated, and the estimated time t2 ′ corresponding to the time t2 as the change value to the fuel rich state is estimated.

  In addition, the control unit calculates a difference (t1−t1 ′) between the time t1 and the time t1 ′ and a difference (t2−t2 ′) between the time t2 and the time t2 ′ and each difference (t1−t1 ′). , (T2−t2 ′) absolute value difference t is calculated.

  Then, the control unit estimates that the air-fuel ratio sensor 123 is abnormal when the difference t is equal to or greater than a preset abnormality determination threshold. In addition, when each difference (t1-t1 ') and (t2-t2') are more than the preset abnormality determination threshold value, it may be estimated that the air-fuel ratio sensor 123 is abnormal.

  From the above description, the electronic control unit 4 according to the present invention executes the following processing regardless of whether or not the abnormality determination of the air-fuel ratio sensor 123 is performed based on the parameters of the first-order lag system.

  That is, the electronic control unit 4 is an electronic control unit 4 that performs an abnormality determination of the air-fuel ratio sensor 123 provided in the exhaust pipe 121 of the internal combustion engine 1, and an abnormality determination threshold value for determining an abnormality of the air-fuel ratio sensor 123. And the following five types of processing (fuel lean determination detection processing, fuel rich inversion detection processing, fuel lean change value difference calculation processing, fuel rich change value difference calculation processing, and air-fuel ratio sensor abnormality determination processing ) Is executed.

  The fuel lean reversal detection process is a process for detecting a change in the air-fuel ratio determined by the signal from the air-fuel ratio sensor 123 from the fuel-rich state to the fuel-lean state when controlling the internal combustion engine 1. is there.

  The fuel-rich inversion detection process is a process for detecting when the air-fuel ratio determined by a signal from the air-fuel ratio sensor 123 changes from a state showing fuel lean to a state showing fuel rich when the internal combustion engine 1 is controlled. is there.

  The fuel lean change value difference calculation process is performed by the fuel amount injected by the fuel injection unit 112 and the intake air amount detection unit 115 when a change from the fuel rich state to the fuel lean state is detected by executing the fuel lean inversion detection process. This is a process of calculating the difference between the change value to the fuel lean state estimated based on the intake air amount determined from the signal of and the change value to the fuel lean state determined from the signal from the air-fuel ratio sensor 123.

  In the fuel rich change value difference calculation process, when the change from the fuel lean state to the fuel rich state is detected by executing the fuel rich inversion detection process, the fuel amount injected by the fuel injection unit 112 and the intake air amount detection unit 115 This is a process of calculating the difference between the change value to the fuel rich state estimated based on the intake air amount determined from the signal of and the change value to the fuel rich state determined from the signal from the air-fuel ratio sensor 123.

  The air-fuel ratio sensor abnormality determination process stores the difference between the absolute value of the difference calculated by executing the fuel lean change value difference calculation process and the absolute value of the difference calculated by executing the fuel rich change value difference calculation process. This is a process for determining that the air-fuel ratio sensor 123 is abnormal when it is equal to or greater than the abnormality determination threshold stored in the unit.

  Here, as in the above-described embodiment, when the electronic control unit 4 performs abnormality determination of the air-fuel ratio sensor 123 based on the parameters of the first-order lag system, the change values to the fuel rich and fuel lean are as follows: It is.

  That is, in the abnormality determination process executed by the control unit, the change values to the fuel rich and the fuel lean determined by the signal from the air-fuel ratio sensor 123 are obtained from the fuel amount injected by the fuel injection unit 112 and the intake air amount detection unit 115. Is a parameter indicating a first-order lag in relation to the fuel rich and the change value to the fuel lean estimated based on the intake air amount determined by the signal, and the parameter includes a dead time, a time constant, or a gain.

  As described above, the control method for determining the abnormality of the air-fuel ratio sensor 123 provided in the exhaust pipe 121 of the internal combustion engine 1 according to the present invention has the following five steps (fuel lean inversion detection step, fuel rich inversion detection step). , A fuel lean change value difference calculating step, a fuel rich change value difference calculating step, and an air-fuel ratio sensor abnormality determining step).

  The fuel lean inversion detection step is a step of detecting a change in the air-fuel ratio determined by the signal from the air-fuel ratio sensor 123 from the fuel-rich state to the fuel-lean state when controlling the internal combustion engine 1. is there.

  The fuel rich inversion detection step is a step of detecting when the air-fuel ratio determined by a signal from the air-fuel ratio sensor 123 changes from a state showing fuel lean to a state showing fuel rich when the internal combustion engine 1 is controlled. is there.

  The fuel lean change value difference calculating step is executed by the fuel amount injected by the fuel injection unit 112 and the intake air amount detection unit 115 when the change from the fuel rich state to the fuel lean state is detected by executing the fuel lean inversion detection step. This is a step of calculating the difference between the change value to the fuel lean state estimated based on the intake air amount determined from the signal of and the change value to the fuel lean state determined from the signal from the air-fuel ratio sensor 123.

  In the fuel rich change value difference calculating step, when the change from the fuel lean state to the fuel rich state is detected by executing the fuel rich inversion detection step, the fuel amount injected by the fuel injection unit 112 and the intake air amount detection unit 115 This is a step of calculating the difference between the change value to the fuel rich state estimated based on the intake air amount determined by the signal of and the change value to the fuel rich state determined by the signal from the air-fuel ratio sensor 123.

  The air-fuel ratio sensor abnormality determination step stores the difference between the absolute value of the difference calculated by executing the fuel lean change value difference calculating step and the absolute value of the difference calculated by executing the fuel rich change value difference calculating step. This is a step for determining that the air-fuel ratio sensor 123 is abnormal when it is equal to or greater than the abnormality determination threshold stored in the unit.

  In addition, the above-mentioned embodiment is only an example of this invention, and it cannot be overemphasized that the concrete structure of each block etc. can be changed and designed suitably in the range with the effect of this invention.

Illustration of the engine Air-fuel ratio sensor illustration (A) is a characteristic diagram of inflow current-applied voltage of the air-fuel ratio sensor, and (b) is a characteristic diagram of limit current-air-fuel ratio of the air-fuel ratio sensor. Block diagram of electronic control unit (A) is an explanatory diagram of a lean side step response, (b) is an explanatory diagram of a rich side step response (A) is an explanatory diagram of step input and step response when alternately measuring each step response, (b) is an explanatory diagram of step input and step response when continuously measuring each step response, ( c) Explanatory drawing of step input and step response when discretely measuring each step response Flow chart for explaining failure diagnosis of air-fuel ratio sensor by electronic control unit

Explanation of symbols

1: Engine (internal combustion engine)
4: Electronic control unit 112: Injector 121: Exhaust pipe 123: Air-fuel ratio sensor

Claims (5)

An electronic control device that performs abnormality determination of an air-fuel ratio sensor provided in an exhaust pipe of an internal combustion engine,
A storage unit for storing an abnormality determination threshold for determining abnormality of the air-fuel ratio sensor;
A fuel lean reversal detection process for detecting that the air-fuel ratio determined by a signal from the air-fuel ratio sensor changes from a fuel-rich state to a fuel-lean state when controlling the internal combustion engine;
A fuel rich inversion detection process for detecting that the air-fuel ratio determined by a signal from the air-fuel ratio sensor changes from a state indicating fuel lean to a state indicating fuel rich when controlling the internal combustion engine;
Estimated based on the amount of fuel injected by the fuel injection unit and the intake amount determined by the signal from the intake amount detection unit when a change from the fuel rich state to the fuel lean state is detected by executing the fuel lean inversion detection process A fuel lean change value difference calculating process for calculating a difference between the change value to the fuel lean state and the change value to the fuel lean state determined by a signal from the air-fuel ratio sensor;
Estimated based on the amount of fuel injected by the fuel injection unit and the intake amount determined by the signal from the intake amount detection unit when a change from the fuel lean state to the fuel rich state is detected by executing the fuel rich inversion detection process A fuel rich change value difference calculation process for calculating a difference between the changed value to the fuel rich state and the change value to the fuel rich state determined by a signal from the air-fuel ratio sensor;
The difference between the absolute value of the difference calculated by executing the fuel lean change value difference calculation process and the absolute value of the difference calculated by executing the fuel rich change value difference calculation process is equal to or greater than the abnormality determination threshold stored in the storage unit. And an air-fuel ratio sensor abnormality determination process for determining that the air-fuel ratio sensor is abnormal.   In the abnormality determination process executed by the control unit, the change value to the fuel rich and the fuel lean determined by the signal from the air-fuel ratio sensor is determined by the fuel amount injected by the fuel injection unit and the signal from the intake air amount detection unit. 2. The electronic control according to claim 1, wherein the parameter is a parameter indicating a first-order lag in relation to a fuel rich and a fuel lean change value estimated on the basis of an intake air amount, and the parameter includes dead time, a time constant, or a gain apparatus.   3. The control unit according to claim 1, wherein the control unit stores, in the storage unit, data for identifying whether the air-fuel ratio sensor is normal or abnormal, or a monitoring frequency by executing an abnormality determination process. Electronic control unit.   The electronic device according to any one of claims 1 to 3, wherein the control unit executes a series of processes when it is determined that the state of the vehicle is stable based on a signal from a state detection unit of the vehicle. Control device. A control method for determining abnormality of an air-fuel ratio sensor provided in an exhaust pipe of an internal combustion engine,
A fuel lean inversion detection step for detecting that the air-fuel ratio determined by a signal from the air-fuel ratio sensor changes from a state showing fuel rich to a state showing fuel lean when controlling the internal combustion engine;
A fuel rich inversion detection step for detecting that the air-fuel ratio determined by a signal from the air-fuel ratio sensor changes from a state indicating fuel lean to a state indicating fuel rich when controlling the internal combustion engine;
Estimated based on the amount of fuel injected by the fuel injection unit and the intake amount determined by the signal from the intake amount detection unit when a change from the fuel rich state to the fuel lean state is detected by executing the fuel lean inversion detection step A fuel lean change value difference calculating step for calculating a difference between the change value to the fuel lean state and the change value to the fuel lean state determined by a signal from the air-fuel ratio sensor;
Estimated based on the amount of fuel injected by the fuel injection unit and the amount of intake determined by the signal from the intake amount detection unit when a change from the fuel lean state to the fuel rich state is detected by executing the fuel rich inversion detection step A fuel rich change value difference calculating step for calculating a difference between the changed value to the fuel rich state and the change value to the fuel rich state determined by a signal from the air-fuel ratio sensor;
The difference between the absolute value of the difference calculated by executing the fuel lean change value difference calculating step and the absolute value of the difference calculated by executing the fuel rich change value difference calculating step is equal to or greater than the abnormality determination threshold stored in the storage unit. In this case, a control method for executing an air-fuel ratio sensor abnormality determining step for determining that the air-fuel ratio sensor is abnormal.

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