JP5407971B2 - Abnormality diagnosis device - Google Patents

Description

  The present invention relates to an abnormality diagnosis apparatus that performs abnormality diagnosis of a catalyst in an internal combustion engine and abnormality diagnosis of a catalyst downstream sensor.

  In an internal combustion engine mounted on a vehicle such as an automobile, an exhaust purification catalyst is provided in the exhaust passage, and NOx, HC, and CO in the exhaust flowing through the exhaust passage are purified by the catalyst. In addition, in order to effectively purify these three components in the exhaust gas, the catalyst has an oxygen storage function, and the stoichiometric air-fuel ratio control is performed to control the air-fuel ratio of the air-fuel mixture in the combustion chamber of the internal combustion engine to the stoichiometric air-fuel ratio. Is called.

  Here, the oxygen storage function of the catalyst means that the oxygen in the exhaust is occluded in the catalyst or the oxygen occluded in the catalyst is desorbed from the catalyst according to the oxygen concentration in the exhaust passing through the catalyst. It is a function to release into the exhaust. Specifically, the oxygen concentration in the exhaust gas is richer than the value when the air-fuel ratio in the combustion chamber is set to the stoichiometric air-fuel ratio when the air-fuel ratio is burned, that is, the air-fuel mixture in the combustion chamber is the stoichiometric air-fuel ratio. In a state where combustion is performed at a leaner air-fuel ratio, oxygen in the exhaust gas passing through the catalyst is occluded in the catalyst by the oxygen storage function of the catalyst described above. On the other hand, the oxygen concentration in the exhaust gas is less than the value when the air-fuel ratio in the combustion chamber is set to the stoichiometric air-fuel ratio, ie, the air-fuel ratio in the combustion chamber is less than the stoichiometric air-fuel ratio. In the state where combustion is performed at a rich air-fuel ratio, oxygen stored in the catalyst is desorbed from the catalyst and released into the exhaust gas by the above-described oxygen storage function of the catalyst.

  Further, in the above stoichiometric air-fuel ratio control, the oxygen concentration in the exhaust gas is controlled so that the oxygen concentration in the exhaust gas matches the value when the air-fuel mixture in the combustion chamber is burned with the stoichiometric air-fuel ratio being the stoichiometric air-fuel ratio. The fuel injection amount of the internal combustion engine is adjusted according to the concentration. In such theoretical air-fuel ratio control, a catalyst upstream sensor that is provided upstream of the catalyst in the exhaust passage and outputs a signal based on the oxygen concentration in the exhaust, and a catalyst upstream sensor that is provided downstream of the catalyst in the exhaust passage and controls the oxygen concentration in the exhaust. And a catalyst downstream sensor that outputs a signal based thereon.

  Specifically, it is output from the catalyst upstream sensor so that the oxygen concentration in the exhaust gas upstream of the catalyst matches the value when the air-fuel ratio in the combustion chamber is made the stoichiometric air-fuel ratio and the air-fuel ratio is combusted. The fuel injection amount of the internal combustion engine is adjusted according to the signal. As a result, the air-fuel ratio of the air-fuel mixture in the combustion chamber of the internal combustion engine is controlled to converge to the stoichiometric air-fuel ratio while fluctuating between rich and lean. However, only by adjusting the fuel injection amount in accordance with the signal output from the catalyst upstream sensor, due to the product variation of the sensor, as described above, it fluctuates between rich and lean so as to converge to the theoretical air-fuel ratio. There is a possibility that the center of fluctuation of the air-fuel ratio of the internal combustion engine that deviates from the stoichiometric air-fuel ratio. In order to correct such a deviation, the catalyst is adjusted so that the fluctuation center of the air-fuel ratio of the internal combustion engine that fluctuates between rich and lean by adjusting the fuel injection amount according to the signal from the catalyst upstream sensor matches the stoichiometric air-fuel ratio. Adjustment of the fuel injection amount of the internal combustion engine according to the signal output from the downstream sensor is also performed.

  As described above, by providing the catalyst with an oxygen storage function and performing the theoretical air-fuel ratio control, it is possible to effectively purify three components such as NOx, HC, and CO in the exhaust gas. Specifically, during the execution of the stoichiometric air-fuel ratio control, when the air-fuel ratio of the air-fuel mixture in the combustion chamber fluctuates and becomes lean, the oxygen concentration in the exhaust gas passing through the catalyst changes the air-fuel ratio of the air-fuel mixture in the combustion chamber to the stoichiometric air-fuel ratio. In this state, the value becomes higher than the value when the air-fuel mixture is burned, so that oxygen in the exhaust gas passing through the catalyst is occluded by the catalyst and NOx in the exhaust gas is reduced. On the other hand, if the air-fuel ratio of the air-fuel mixture in the combustion chamber fluctuates and becomes rich during the execution of the stoichiometric air-fuel ratio control, the oxygen concentration in the exhaust gas passing through the catalyst becomes the stoichiometric air-fuel ratio. Since the value is smaller than the value when the air-fuel mixture is burned in the state, oxygen stored in the catalyst is desorbed from the catalyst and HC and CO in the exhaust gas are oxidized. Therefore, when the air-fuel ratio of the air-fuel mixture in the combustion chamber fluctuates between rich and lean during the execution of the stoichiometric air-fuel ratio control, as described above, NOx, HC, CO in the exhaust gas These three components are effectively purified.

  By the way, in order to maintain the exhaust gas purification in the internal combustion engine in an appropriate state, it is important to take prompt measures when the catalyst deteriorates or when the responsiveness of the downstream sensor of the catalyst is abnormal. become. In order to realize such a quick countermeasure, for example, as described below, it is determined whether or not the catalyst has deteriorated or whether or not the catalyst downstream sensor is abnormal.

  In a catalyst, an oxygen storage function falls with the deterioration. In order to cope with this, a maximum value of the amount of oxygen stored in the catalyst (hereinafter referred to as oxygen storage amount) is obtained, and the presence or absence of deterioration of the catalyst is determined based on the oxygen storage amount.

  Specifically, active air-fuel ratio control for forcibly changing the air-fuel ratio of the internal combustion engine between rich and lean is executed, and the air-fuel ratio of the internal combustion engine changes between rich and lean during the control. In this case, the oxygen storage amount is obtained. In the fuel cut control for stopping the fuel injection in the internal combustion engine according to the driving state of the vehicle, when the air-fuel ratio of the engine changes from the rich side to the lean side when the fuel injection during the control stops, the oxygen It is also possible to determine the amount of occlusion. Obtaining the oxygen storage amount during the active air-fuel ratio control and the fuel cut control is performed in more detail as follows. That is, when the air-fuel ratio of the internal combustion engine changes during active air-fuel ratio control or fuel cut control as described above, a signal corresponding to the change in the air-fuel ratio occurs in the signal of the catalyst upstream sensor before the catalyst downstream sensor. The amount of oxygen stored in the catalyst or the amount of oxygen desorbed from the catalyst is calculated during the period until the signal corresponding to the change in the air-fuel ratio occurs. The amount of oxygen calculated at the end of the period is the maximum value of the oxygen storage amount, that is, the amount of oxygen stored in the catalyst. Based on the oxygen storage amount thus obtained, it is determined whether or not the oxygen storage function of the catalyst has deteriorated, in other words, whether or not the catalyst has deteriorated.

  On the other hand, in the catalyst downstream sensor, the responsiveness of the change in the output signal with respect to the change in the oxygen concentration in the exhaust is reduced due to the abnormality. In order to cope with this, the responsiveness of the catalyst downstream sensor is obtained, and it is determined based on the responsiveness whether the catalyst downstream sensor is abnormal.

  Specifically, when the air-fuel ratio of the internal combustion engine changes between rich and lean, when the output signal of the catalyst downstream sensor changes corresponding to the change, the response of the change in the output signal is measured. . Based on the measured responsiveness of the catalyst downstream sensor, it is determined whether or not an abnormality related to responsiveness has occurred in the sensor. Here, examples of the situation in which the air-fuel ratio of the internal combustion engine changes between the rich side and the lean side include situations such as the above-described active air-fuel ratio control and fuel cut control. Incidentally, measuring the responsiveness of the catalyst downstream sensor when the air-fuel ratio changes during fuel cut control is disclosed in Patent Document 1, and when the air-fuel ratio changes during active air-fuel ratio control, the catalyst downstream sensor is disclosed. It is disclosed in Patent Document 2 to measure the responsiveness. There is a large difference in the amount of change in the air-fuel ratio that changes during control between the measurement of the response of the catalyst downstream sensor during active air-fuel ratio control and the measurement of the response of the catalyst downstream sensor during fuel cut control. For example, the measurement conditions for the response of the catalyst downstream sensor are greatly different. Therefore, the responsiveness of the catalyst downstream sensor is measured under different measurement conditions, such as during active air-fuel ratio control and during fuel cut control, and whether there is an abnormality in the sensor based on the measured responsiveness of the catalyst downstream sensor. By determining the above, the result of the determination can be made more accurate.

JP 2003-343339 (paragraphs [0040] to [0043]) JP 2003-247451 (paragraphs [0025] to [0030], FIGS. 3 and 4)

  As described above, the responsiveness of the catalyst downstream sensor is measured under different measurement conditions such as during active air-fuel ratio control and during fuel cut control, and abnormalities of the sensor are determined based on the measured responsiveness of the catalyst downstream sensor. If the presence or absence is determined, the result of the determination can be made more accurate.

  However, in this case, in addition to obtaining the oxygen storage amount used to determine whether the catalyst has deteriorated, the response of the catalyst downstream sensor during active air-fuel ratio control and the catalyst downstream sensor during fuel cut control Each of the responsiveness must be measured. In other words, in order to complete the determination of the presence or absence of catalyst degradation and the determination of the presence or absence of abnormality of the catalyst downstream sensor, the oxygen storage amount of the catalyst, the response of the catalyst downstream sensor during active air-fuel ratio control, and the fuel cut control It is necessary to acquire three parameters such as the response of the catalyst downstream sensor.

  Therefore, in determining whether the catalyst has deteriorated and whether the catalyst downstream sensor is abnormal, it is necessary to acquire the above three parameters in order to accurately determine whether the catalyst downstream sensor is abnormal. Therefore, it takes time to acquire these parameters. As a result, it takes much time to complete the determination of the presence or absence of catalyst deterioration and the determination of the presence or absence of abnormality of the catalyst downstream sensor.

  The present invention has been made in view of such circumstances, and its purpose is to determine whether or not there is an abnormality in the catalyst downstream sensor when determining whether or not the catalyst has deteriorated and whether or not there is an abnormality in the catalyst downstream sensor. It is an object of the present invention to provide an abnormality diagnosing apparatus capable of suppressing the time taken to complete the determination while accurately determining the above.

  In order to achieve the above object, according to the first aspect of the present invention, the presence or absence of deterioration of the catalyst is determined based on the oxygen storage amount of the exhaust purification catalyst provided in the exhaust passage of the internal combustion engine, and the exhaust gas is exhausted. The presence / absence of abnormality of the sensor is determined based on the responsiveness of the catalyst downstream sensor provided downstream of the catalyst in the passage and outputting a signal based on the oxygen concentration in the exhaust gas.

  The oxygen storage amount used for determining whether or not the catalyst has deteriorated is obtained by the first calculation means as follows. That is, when the air-fuel ratio of the internal combustion engine changes between rich and lean, a signal corresponding to the change in the air-fuel ratio occurs in the signal of the catalyst upstream sensor provided upstream of the catalyst in the exhaust passage. During the period until the change corresponding to the change in the air-fuel ratio occurs in the signal of the downstream sensor, the amount of oxygen stored in the catalyst or the amount of oxygen desorbed from the catalyst is calculated. The amount of oxygen calculated in this way is used as the oxygen storage amount. Note that the situation in which the air-fuel ratio of the internal combustion engine changes between rich and lean as described above is, for example, during active air-fuel ratio control in which the air-fuel ratio of the internal combustion engine is forcibly changed between rich and lean. Another example is when the fuel injection is stopped in the fuel cut control for stopping the fuel injection in the internal combustion engine and the air-fuel ratio of the engine changes from the rich side to the lean side.

  The responsiveness of the sensor used for the presence or absence of abnormality of the catalyst downstream sensor is determined by the second calculation means as follows. That is, the oxygen storage amount obtained by the first calculation means during active air-fuel ratio control, the oxygen storage amount obtained by the first calculation means during fuel cut control, and the active air-fuel ratio control and the fuel cut control Based on the correlation of the responsiveness of the catalyst downstream sensor with respect to the inside, the responsiveness of the catalyst downstream sensor under either of the active air-fuel ratio control and the fuel cut control is obtained. Here, the oxygen storage amount obtained by the first calculating means during the active air-fuel ratio control reflects the responsiveness of the catalyst downstream sensor during the active air-fuel ratio control. Further, the oxygen storage amount obtained by the first calculation means during the fuel cut control reflects the response of the catalyst downstream sensor during the fuel cut control. This is because when the air-fuel ratio of the internal combustion engine changes between the rich side and the lean side as the response of the catalyst downstream sensor decreases, the change in the output signal of the catalyst downstream sensor corresponding to the change is delayed. This is because the period for calculating the oxygen storage amount becomes longer and the required oxygen storage amount tends to increase. However, such an increasing tendency differs between the oxygen storage amount required during active air-fuel ratio control and the oxygen storage amount required during fuel cut control. This is because the responsiveness of the catalyst downstream sensor is different from that at the time of active air-fuel ratio control and fuel cut control due to the large difference in the change range of the air-fuel ratio of the internal combustion engine between active air-fuel ratio control and fuel cut control This is because it differs greatly from time to time.

  As can be understood from the responsiveness of the catalyst downstream sensor obtained by the second calculating means, the oxygen storage amount reflecting the responsiveness of the catalyst downstream sensor during active air-fuel ratio control, and during the fuel cut control of the sensor. The oxygen storage amount reflecting the responsiveness is taken into account. Therefore, by determining whether the sensor is abnormal based on the responsiveness of the downstream sensor of the catalyst obtained by the second calculating means, the result of the determination is such as during active air-fuel ratio control and during fuel cut control. It is accurate based on the response of the catalyst downstream sensor under three different situations. On the other hand, the determination of the presence or absence of catalyst deterioration by the determination means is performed based on at least one of the oxygen storage amounts obtained by the first calculation means. As can be seen from the above, when judging whether or not the catalyst downstream sensor is abnormal and determining whether or not the catalyst downstream sensor is abnormal, it is necessary to determine whether or not the catalyst downstream sensor is abnormal. There are only two parameters to be acquired, that is, the oxygen storage amount of the catalyst required during the active air-fuel ratio control and the oxygen storage amount of the catalyst required during the fuel cut. For this reason, it can be suppressed that it takes time to acquire these parameters, and in turn, it can be prevented that it takes a long time to complete the determination of the presence or absence of catalyst deterioration and the determination of the presence or absence of abnormality of the catalyst downstream sensor. become able to.

  According to the second aspect of the present invention, as the response of the catalyst downstream sensor, the signal per unit amount of the signal output from the catalyst downstream sensor when the air-fuel ratio of the internal combustion engine changes by active air-fuel ratio control or fuel cut control. The response time required for the change is obtained by the second calculating means. And when the response time calculated | required in this way is more than a threshold value, it judges that a catalyst downstream sensor is abnormal by a judgment means. Further, when the response time is less than the threshold value, the determination unit determines that the catalyst downstream sensor is normal. As described above, it is possible to accurately determine whether or not the catalyst downstream sensor is abnormal.

  According to the third aspect of the present invention, when determining whether the catalyst has deteriorated or not by the determining means, among the oxygen storage amounts determined by the first calculating means, the downstream of the catalyst determined by the second calculating means. Correction based on the responsiveness is added to the oxygen storage amount corresponding to the responsiveness of the sensor. Then, it is determined whether or not the catalyst has deteriorated based on the corrected oxygen storage amount. Here, the oxygen storage amount of the catalyst obtained by the first calculation means is affected by the response of the catalyst downstream sensor. This is because when the air-fuel ratio of the internal combustion engine changes between the rich side and the lean side as the response of the catalyst downstream sensor decreases, the change in the output signal of the catalyst downstream sensor corresponding to the change is delayed. This is because the period for calculating the oxygen storage amount becomes longer and the required oxygen storage amount tends to increase. Therefore, if it is determined whether or not the catalyst has deteriorated based on the oxygen storage amount affected by the response of the catalyst downstream sensor as described above, the determination result may be inaccurate. In this respect, in the invention according to claim 3, since the presence or absence of deterioration of the catalyst is determined based on the oxygen storage amount after correction according to the response of the catalyst downstream sensor, the result of the determination is not as described above. It is suppressed that it becomes exact.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic which shows the whole engine with which the catalyst deterioration detection apparatus of this embodiment is applied. The graph which shows the change of the output signal of the air fuel ratio sensor with respect to the change of the oxygen concentration in exhaust gas. The graph which shows the change of the output signal of an oxygen sensor with respect to the change of the oxygen concentration in exhaust_gas | exhaustion. (A) to (d) are time charts showing changes in the air-fuel ratio of the engine, changes in the output signal of the air-fuel ratio sensor, changes in the required oxygen storage amount, and changes in the output signal of the oxygen sensor in active air-fuel ratio control. . (A)-(d) is a time chart which shows the change of the air fuel ratio of an engine in fuel cut control, the change of the output signal of an air fuel ratio sensor, the change of the calculated | required oxygen storage amount, and the change of the output signal of an oxygen sensor. The flowchart which shows the execution procedure of judgment of the presence or absence of abnormality of an oxygen sensor, and the judgment of the presence or absence of deterioration of a three-way catalyst. The flowchart which shows the execution procedure of judgment of the presence or absence of abnormality of an oxygen sensor, and the judgment of the presence or absence of deterioration of a three-way catalyst.

Hereinafter, an embodiment in which the present invention is applied to an automobile engine will be described with reference to FIGS.
In the engine 1 shown in FIG. 1, a throttle valve 13 is provided in an intake passage 3 connected to the combustion chamber 2 so as to be openable and closable, and air is taken into the combustion chamber 2 through the intake passage 3 and a fuel injection valve. The fuel injected from 4 is supplied to the combustion chamber 2 through the intake passage 3. The mixture of air and fuel supplied to the combustion chamber 2 is ignited by the spark plug 5 and burned. Then, when the air-fuel mixture burns in the combustion chamber 2, the piston 6 reciprocates and the crankshaft 7 that is the output shaft of the engine 1 rotates.

  On the other hand, the air-fuel mixture burned in the combustion chamber 2 is sent out from the combustion chamber 2 to the exhaust passage 8 as exhaust. Exhaust gas that passes through the exhaust passage 8 is discharged to the outside after purifying harmful components such as HC, CO, and NOx in the exhaust gas by the three-way catalyst of the catalytic converter 16 provided in the exhaust passage 8. This three-way catalyst has an oxygen storage function in order to effectively remove the above three components in the exhaust gas. The three-way catalyst has this oxygen storage function, and the stoichiometric air-fuel ratio is controlled to the stoichiometric air-fuel ratio so that the oxygen concentration in the catalyst atmosphere converges to the value at the time of combustion of the air-fuel mixture at the stoichiometric air-fuel ratio. By performing the fuel ratio control, it is possible to effectively purify the three components such as NOx, HC, and CO in the exhaust gas with the three-way catalyst.

  Further, in the exhaust passage 8, an air-fuel ratio sensor 17 is provided upstream of the catalytic converter 16 as a catalyst upstream sensor that outputs a signal based on the oxygen concentration in the exhaust, and the oxygen concentration in the exhaust downstream of the catalytic converter 16. An oxygen sensor 18 is provided as a catalyst downstream sensor that outputs a signal based on the above.

As shown in FIG. 2, the air-fuel ratio sensor 17 outputs a linear signal corresponding to the oxygen concentration in the exhaust gas upstream of the catalyst.
That is, the output signal VAF of the air-fuel ratio sensor 17 decreases as the oxygen concentration in the exhaust upstream of the catalyst decreases, and when the air-fuel mixture is burned at the stoichiometric air-fuel ratio, the oxygen concentration X in the exhaust at that time For example, “0A” is set. Accordingly, the output signal VAF of the air-fuel ratio sensor 17 becomes smaller than “0A” as the oxygen concentration in the exhaust gas upstream of the catalyst becomes thinner due to the combustion of the air-fuel mixture richer than the stoichiometric air-fuel ratio (rich combustion). Become. Further, the output signal VAF of the air-fuel ratio sensor 17 becomes larger than “0A” as the oxygen concentration in the exhaust gas upstream of the catalyst becomes higher due to the combustion of the air-fuel mixture leaner than the stoichiometric air-fuel ratio (lean combustion). Become.

As shown in FIG. 3, the oxygen sensor 18 outputs a rich signal or a lean signal according to the oxygen concentration in the exhaust gas downstream of the catalyst.
That is, the output signal VO of the oxygen sensor 18 is, for example, “0.5 v” when the oxygen concentration in the exhaust gas downstream of the catalyst is a value (oxygen concentration X) when the air-fuel mixture is burned at the stoichiometric air-fuel ratio. Is output. When the oxygen concentration in the exhaust gas downstream of the catalyst becomes higher than the oxygen concentration X described above due to the lean combustion, a value smaller than “0.5 v” is output from the oxygen sensor 18 as a lean signal. Is done. Regarding the lean signal, when the oxygen concentration in the exhaust gas downstream of the catalyst becomes larger than the oxygen concentration X, a rapid change from “0.5 V” to a decrease side with respect to the increase in oxygen concentration near the oxygen concentration X occurs. On the other hand, when the distance from the vicinity of the oxygen concentration X is increased, the change toward the decrease side with respect to the increase in the oxygen concentration becomes moderate.

  Further, when the oxygen concentration in the exhaust gas downstream of the catalyst becomes lower than the above-described oxygen concentration X due to the rich combustion being performed, a value larger than “0.5 v” is output as a rich signal from the oxygen sensor 18. Is done. Regarding the rich signal, when the oxygen concentration in the exhaust gas downstream of the catalyst becomes smaller than the oxygen concentration X, a rapid change from “0.5 V” to an increase side with respect to the decrease in the oxygen concentration in the vicinity of the oxygen concentration X. On the other hand, when the distance from the vicinity of the oxygen concentration X is increased, the change toward the increasing side with respect to the decrease in the oxygen concentration becomes gradual.

Next, the electrical configuration of the catalyst deterioration detection apparatus in the present embodiment will be described with reference to FIG.
The air-fuel ratio control device includes an electronic control device 21 that executes various controls relating to the engine 1. The electronic control unit 21 is a CPU that executes various arithmetic processes related to the above control, a ROM that stores programs and data necessary for the control, a RAM that temporarily stores the arithmetic results of the CPU, etc. The input / output port for inputting / outputting is provided.

In addition to the air-fuel ratio sensor 17 and the oxygen sensor 18, the following various sensors are connected to the input port of the electronic control unit 21.
An accelerator position sensor 28 that detects the amount of depression (accelerator depression amount) of the accelerator pedal 27 that is depressed by the driver of the automobile.

A throttle position sensor 30 that detects the opening (throttle opening) of the throttle valve 13 provided in the intake passage 3.
An air flow meter 32 for detecting the amount of air taken into the combustion chamber 2 through the intake passage 3;

An intake pressure sensor 33 that detects a pressure (intake pressure) downstream of the throttle valve 13 in the intake passage 3.
A crank position sensor 34 that outputs a signal corresponding to the rotation of the crankshaft 7 and is used for calculation of the engine rotation speed or the like.

The output port of the electronic control device 21 is connected to the fuel injection valve 4, the spark plug 5, the drive circuit for the throttle valve 13, and the like.
The electronic control unit 21 grasps the engine operating state such as the engine speed and the engine load (the amount of air taken into the combustion chamber 2 per cycle of the engine 1) based on the detection signals input from the various sensors. To do. The engine speed is obtained based on a detection signal from the crank position sensor 34. The engine load is calculated from the intake air amount of the engine 1 obtained based on detection signals from the accelerator position sensor 28, the throttle position sensor 30, the air flow meter 32, and the like, and the engine rotation speed. The electronic control unit 21 outputs command signals to various drive circuits connected to the output port according to the engine operating state such as the engine load and the engine speed. Thus, fuel injection amount control, ignition timing control, intake air amount control, and the like in the engine 1 are performed through the electronic control unit 21.

  The above theoretical air-fuel ratio control for effectively purifying the exhaust of the engine 1 with the three-way catalyst of the catalytic converter 16 is based on the output signal VAF of the air-fuel ratio sensor 17 and the output signal VO from the oxygen sensor 18. Realized by adjusting. Specifically, the output signal VAF so that the output signal VAF of the air-fuel ratio sensor 17 coincides with the value when the air-fuel mixture in the combustion chamber 2 of the engine 1 is burned at the stoichiometric air-fuel ratio (in this example, “0A”). The fuel injection amount of the engine 1 is increased or decreased based on the above. Thus, the air-fuel ratio of the air-fuel mixture in the combustion chamber 2 of the engine 1 is controlled so as to converge to the stoichiometric air-fuel ratio while fluctuating between rich and lean. However, only by adjusting the fuel injection amount in accordance with the output signal VAF of the air-fuel ratio sensor 17, the rich and lean levels are converged to the stoichiometric air-fuel ratio as described above due to product variations of the air-fuel ratio sensor 17. There is a possibility that the center of fluctuation of the air-fuel ratio of the engine 1 that fluctuates in time will deviate from the theoretical air-fuel ratio. In order to correct such a deviation, the fluctuation center of the air-fuel ratio of the engine 1 that fluctuates between rich and lean by adjusting the fuel injection amount in accordance with the output signal VAF of the air-fuel ratio sensor 17 matches the stoichiometric air-fuel ratio. The fuel injection amount of the engine 1 is also adjusted according to the signal output from the oxygen sensor 18.

  By the way, in order to maintain the exhaust purification in the engine 1 in an appropriate state, when the three-way catalyst in the catalytic converter 16 is deteriorated or when the responsiveness of the oxygen sensor 18 is abnormal, countermeasures are taken. It is important to take it promptly. In order to realize such prompt measures, it is determined whether or not the three-way catalyst is deteriorated or whether or not the oxygen sensor 18 is abnormal.

  The determination of the presence or absence of deterioration of the three-way catalyst involves determining the maximum amount of oxygen stored in the catalyst (hereinafter referred to as oxygen storage amount) and comparing the calculated oxygen storage amount with a predetermined threshold value. Based on. Specifically, if the obtained oxygen storage amount is less than the above threshold value, it can be considered that the oxygen storage function of the three-way catalyst is deteriorated, so that it is determined that the catalyst is deteriorated. On the other hand, if the obtained oxygen storage amount is equal to or greater than the above threshold value, it can be considered that the oxygen storage function of the three-way catalyst has not deteriorated, and therefore it is determined that the catalyst is not deteriorated (normal).

  When obtaining the oxygen storage amount used for determining whether or not the three-way catalyst has deteriorated, active air-fuel ratio control is executed in which the air-fuel ratio of the engine 1 is forcibly changed between rich and lean at predetermined timings. Is done. When the air-fuel ratio of the engine 1 changes between rich and lean during active air-fuel ratio control, the oxygen storage amount of the three-way catalyst is obtained. Further, in the fuel cut control for stopping the fuel injection in the engine 1 according to the driving state of the automobile, the oxygen storage is performed when the air-fuel ratio of the engine 1 changes from the rich side to the lean side when the fuel injection during the control stops. It is also possible to determine the quantity. The determination of the oxygen storage amount during the active air-fuel ratio control and the fuel cut control is performed in more detail as follows. That is, when the air-fuel ratio of the engine 1 changes during the active air-fuel ratio control or the fuel cut control as described above, the oxygen sensor after the change corresponding to the change of the air-fuel ratio occurs in the signal of the air-fuel ratio sensor 17. The amount of oxygen stored in the three-way catalyst or the amount of oxygen desorbed from the catalyst is calculated during a period until a change corresponding to the change in the air-fuel ratio occurs in the 18 signal. The amount of oxygen calculated at the end of the period is the maximum value of the oxygen storage amount, that is, the amount of oxygen stored in the catalyst.

  The determination of the presence or absence of abnormality of the oxygen sensor 18 is performed based on the responsiveness of the change in the output signal VO to the change in the oxygen concentration in the exhaust gas, and the comparison between the obtained responsiveness and a predetermined threshold value. Can be considered. Specifically, if the obtained responsiveness is equal to or greater than the threshold value, it can be considered that an abnormality that reduces the responsiveness of the oxygen sensor 18 has occurred, and therefore it is determined that the oxygen sensor 18 is abnormal. On the other hand, if the obtained responsiveness is less than the threshold value, it can be considered that the responsiveness of the oxygen sensor 18 has not deteriorated, so it is determined that the oxygen sensor 18 is not abnormal (normal).

  Regarding the responsiveness used for determining whether or not the oxygen sensor 18 is abnormal, when the air-fuel ratio of the engine 1 changes between the rich side and the lean side during the above-described active air-fuel ratio control or fuel cut control. It is conceivable to make a measurement. The responsiveness measurement during the active air-fuel ratio control and the responsiveness measurement during the fuel cut control have a large difference in the amount of change in the air-fuel ratio that changes during the control. The response measurement conditions are greatly different. For this reason, the responsiveness of the oxygen sensor 18 is measured under different measurement conditions such as during active air-fuel ratio control and during fuel cut control, and the presence or absence of abnormality of the sensor 18 is determined based on the measured responsiveness. Then, the result of the determination can be made more accurate.

  As described above, the responsiveness of the oxygen sensor 18 is measured under different measurement conditions such as during active air-fuel ratio control and during fuel cut control, and based on the measured responsiveness of the oxygen sensor 18. If the presence or absence of the abnormality is determined, the result of the determination can be made more accurate.

  However, in this case, in addition to obtaining the oxygen storage amount used to determine whether or not the three-way catalyst has deteriorated, the responsiveness of the oxygen sensor 18 during active air-fuel ratio control and the fuel cut control during Each of the responsiveness of the oxygen sensor 18 must be measured. In other words, in order to complete the determination of the presence or absence of deterioration of the three-way catalyst and the determination of whether or not the oxygen sensor 18 is abnormal, the oxygen storage amount of the three-way catalyst and the responsiveness of the oxygen sensor 18 during active air-fuel ratio control are described. And three parameters such as the responsiveness of the oxygen sensor 18 during fuel cut control must be acquired.

  Therefore, in determining whether or not the three-way catalyst has deteriorated and whether or not the oxygen sensor 18 is abnormal, it is necessary to acquire the above three parameters in order to accurately determine whether or not the oxygen sensor 18 is abnormal. Therefore, it takes time to acquire these parameters. As a result, it takes much time to complete the determination of whether the three-way catalyst has deteriorated and the determination of whether the oxygen sensor 18 has an abnormality.

  In order to cope with such a situation, in the present embodiment, the responsiveness of the sensor 18 used for determining whether there is an abnormality in the oxygen sensor 18 is obtained as follows. That is, the oxygen storage amount C1max obtained during the active air-fuel ratio control, the oxygen storage amount C2max obtained during the fuel cut control, and the responsiveness of the oxygen sensor 18 during the active air-fuel ratio control and during the fuel cut control. Based on this correlation, the responsiveness of the oxygen sensor 18 under either the active air-fuel ratio control or the fuel cut control is required.

  Here, the oxygen storage amount C1max obtained during the active air-fuel ratio control reflects the responsiveness of the oxygen sensor 18 during the active air-fuel ratio control. Further, the oxygen storage amount C2max obtained during the fuel cut control reflects the responsiveness of the oxygen sensor 18 during the fuel cut control. This is because, as the responsiveness of the oxygen sensor 18 decreases, when the air-fuel ratio of the engine 1 changes between the rich side and the lean side, the change in the output signal VO of the oxygen sensor 18 corresponding to the change is delayed. This is because the period for calculating the oxygen storage amounts C1max and C2max becomes longer and the required oxygen storage amounts C1max and C2max tend to increase. However, such an increasing tendency is different between the oxygen storage amount C1max obtained during the active air-fuel ratio control and the oxygen storage amount C2max obtained during the fuel cut control. This is because, for example, the variation range of the air-fuel ratio of the engine 1 is greatly different between the active air-fuel ratio control and the fuel cut control, and the responsiveness of the oxygen sensor 18 is different from that during the active air-fuel ratio control and the fuel cut control. This is because it differs greatly from time to time.

  As described above, the responsiveness of the oxygen sensor 18 obtained as described above can be understood from the method of obtaining the oxygen storage amount C1max reflecting the responsiveness of the oxygen sensor 18 during active air-fuel ratio control, and the fuel cut control of the sensor 18. The oxygen occlusion amount C2max reflecting the responsiveness inside is taken into account. Therefore, by determining the presence or absence of abnormality of the oxygen sensor 18 based on the responsiveness of the obtained oxygen sensor 18, the result of the determination is based on two different situations such as during active air-fuel ratio control and during fuel cut control. This is an accurate one based on the responsiveness of the oxygen sensor 18. On the other hand, whether or not the three-way catalyst has deteriorated is determined based on at least one of the obtained oxygen storage amounts C1max and C2max.

  As can be seen from the above, when determining whether or not the three-way catalyst has deteriorated and whether or not the oxygen sensor 18 is abnormal, the determination is made when it is desired to accurately determine whether or not the oxygen sensor 18 is abnormal. Therefore, the following two parameters should be obtained by performing active air-fuel ratio control and fuel cut control. That is, these parameters are suppressed to two, namely, the oxygen storage amount C1max of the three-way catalyst obtained during active air-fuel ratio control and the oxygen storage amount C2max of the three-way catalyst obtained during fuel cut. For this reason, it can suppress that it takes time to acquire these parameters, and therefore it takes a lot of time to complete the determination of the presence or absence of the deterioration of the three-way catalyst and the determination of the presence or absence of abnormality of the oxygen sensor 18. Can be suppressed.

Here, the calculation of the oxygen storage amount C1max during the active air-fuel ratio control and the calculation of the oxygen storage amount C2max during the fuel cut control will be individually described in detail.
[Calculation of oxygen storage amount C1max]
Regarding the active air-fuel ratio control, the calculation of the oxygen storage amount C1max of the three-way catalyst has not been completed once after the start of the engine 1, and the engine is operated within the predetermined engine operating range for calculating the oxygen storage amount. The process is started when all the various execution conditions such as 1 is in steady operation and the temperature of the three-way catalyst is in the activation temperature range are satisfied. In addition, when any one of the above-mentioned various execution conditions is not satisfied during execution of the active air-fuel ratio control, or when calculation and measurement of various values that are the purpose of execution of the control are completed The same control being executed is stopped.

  In the active air-fuel ratio control, when the air-fuel ratio of the engine 1 is forcibly switched between rich and lean at timing t1 in FIG. 4A (in this example, switching from rich to lean), the change is handled. Then, the output signal VAF of the air-fuel ratio sensor 17 changes as shown in FIG. 4B, the output signal VAF of the air-fuel ratio sensor 17 is a value corresponding to the oxygen concentration in the exhaust when the air-fuel mixture is burned at the stoichiometric air-fuel ratio in the combustion chamber 2 of the engine 1. This is the timing. After timing t2 in the figure, the exhaust gas having a high oxygen concentration passes through the three-way catalyst in response to the change of the air-fuel ratio to the lean side. However, since the oxygen in the exhaust gas is stored in the three-way catalyst, the oxygen concentration in the exhaust gas downstream of the catalyst remains thin while the storage is being performed. Therefore, the solid line in FIG. As shown, the output signal VO of the oxygen sensor 18 does not change corresponding to the change of the air-fuel ratio to the lean side. Then, when oxygen cannot be stored in the three-way catalyst and exhaust gas having a high oxygen concentration flows downstream of the catalyst, a change corresponding to the change of the air-fuel ratio to the lean side occurs in the output signal VO of the oxygen sensor 18. . Regarding the determination that the change corresponding to the change of the air-fuel ratio to the lean side has occurred in the output signal VO of the oxygen sensor 18, the output signal VO reaches the lean determination value HL for determining that. It is possible to do based on what has been done. When the output signal VO reaches the lean determination value HL as described above (t3), the air-fuel ratio of the engine 1 is forcibly switched from lean to rich.

  During a period from the time when the air-fuel ratio change from rich to lean occurs in the output signal VAF of the air-fuel ratio sensor 17 to the time when the output signal VO of the oxygen sensor 18 changes corresponding to the change in air-fuel ratio (t2 to t3) ), The total amount of oxygen stored in the catalyst represents the maximum value of oxygen stored in the three-way catalyst (oxygen storage amount). This oxygen storage amount is obtained as follows during the period (t2 to t3). That is, during the period (t2 to t3 in FIG. 4C), the oxygen storage amount ΔOSC is calculated based on the following equation (1) as the amount of oxygen stored in the three-way catalyst every minute time.

ΔOSC = (ΔA / F) · Q · K (1)
ΔOSC: Oxygen storage capacity per minute
ΔA / F: Air-fuel ratio difference
Q: Fuel injection amount
K: Oxygen ratio The air-fuel ratio difference ΔA / F in the equation (1) represents the absolute value of the value obtained by subtracting the theoretical air-fuel ratio from the air-fuel ratio obtained from the output signal VAF of the air-fuel ratio sensor 17. Further, the fuel injection amount Q in the equation (1) is the fuel injection amount of the engine 1 that causes the air-fuel ratio obtained based on the output signal VAF of the air-fuel ratio sensor 17, that is, the fuel injected from the fuel injection valve 4. Represents the amount. Furthermore, the oxygen ratio K in the formula (1) represents the ratio of oxygen contained in the air. Note that. Here, for example, a fixed value of “0.23” is used as the oxygen ratio K. The oxygen storage amount ΔOSC calculated for each minute time based on the above formula (1) is integrated over the period (t2 to t3), and the value obtained by the integration is the oxygen stored in the three-way catalyst. As a quantity. For this reason, the value obtained by the integration at the end of the period (t2 to t3) is the maximum amount of oxygen that can be stored in the three-way catalyst (oxygen storage amount C1max).

  The oxygen storage amount C1max obtained as described above is a value reflecting the response of the oxygen sensor 18 during active air-fuel ratio control. Specifically, as a parameter representing such responsiveness, when an output signal VO of the oxygen sensor 18 changes based on a change between rich and lean of the air / fuel ratio of the engine 1 during active air / fuel ratio control (for example, FIG. 4 (d) ts to t3) is a time required for a change per unit amount of the output signal VO (hereinafter referred to as response time Tact). Then, as the response time of the oxygen sensor 18 during the active air-fuel ratio control decreases and the response time Tact increases, the period (t2 to t3) for obtaining the oxygen storage amount C1max becomes longer. C1max becomes a value shifted to the increasing side with respect to the appropriate value.

  During the active air-fuel ratio control, the air-fuel ratio of the engine 1 is forcibly switched between rich and lean at every predetermined timing until the oxygen storage amount C1max is properly obtained. In the above-described example, the oxygen storage amount C1max is obtained after the air-fuel ratio of the engine 1 is switched from rich to lean. However, if this cannot be properly performed, the engine 1 is at timing t3 in FIG. The air-fuel ratio is forcibly switched from lean to rich, and the oxygen storage amount C1max is obtained after the switching.

  That is, when the output signal VAF of the air-fuel ratio sensor 17 changes in response to the change of the air-fuel ratio from lean to rich, the exhaust having a low oxygen concentration passes through the three-way catalyst, but is stored in the three-way catalyst. Since the released oxygen is released and released into the exhaust gas, the oxygen concentration in the exhaust gas downstream of the catalyst remains high while the oxygen is released from the catalyst. When the oxygen stored in the three-way catalyst is exhausted and oxygen cannot be released to the exhaust gas, and when the exhaust gas having a low oxygen concentration flows downstream of the catalyst, the empty signal is output to the output signal VO of the oxygen sensor 18. A change corresponding to a change to the rich side of the fuel ratio occurs. Regarding the determination that the change corresponding to the change of the air-fuel ratio to the rich side has occurred in the output signal VO of the oxygen sensor 18, the output signal VO reaches the rich determination value HR for determining that. It is possible to do based on what has been done. When the output signal VO reaches the rich determination value HR as described above, the air-fuel ratio of the engine 1 is forcibly switched from rich to lean.

  During the period from when the change in the air-fuel ratio from lean to rich occurs in the output signal VAF of the air-fuel ratio sensor 17 until the change corresponding to the change in air-fuel ratio occurs in the output signal VO of the oxygen sensor 18. The total value of the amount of oxygen desorbed from the catalyst represents the maximum amount of oxygen stored in the three-way catalyst (oxygen storage amount C1max). The oxygen storage amount C1max is obtained during the above period using the same method as during the period “t2 to t3” in FIG.

[Calculation of oxygen storage amount C2max]
In the fuel cut control, the fuel injection of the fuel injection valve 4 in the engine 1 is stopped on condition that the accelerator depression amount is “0” and the vehicle speed is equal to or greater than a predetermined value greater than “0”. The autonomous operation of the engine 1 is stopped. On the other hand, when the accelerator depression amount becomes larger than “0” or the vehicle speed becomes less than the predetermined value while the engine 1 is stopped by the fuel cut control, the same engine is discharged through the fuel injection of the fuel injection valve 4. 1 autonomous operation is resumed.

  In the fuel cut control, when the fuel injection in the engine 1 is stopped at the timing t5 in FIG. 5A, the air-fuel ratio of the engine 1 changes from the rich side to the lean side. Specifically, the air-fuel ratio of the air-fuel mixture in the combustion chamber 2 in the engine 1 changes from a state where, for example, the stoichiometric air-fuel ratio becomes leaner as the gas in the combustion chamber 2 becomes substantially air. In response to such a change in the air-fuel ratio of the engine 1 from the rich side to the lean side, the output signal VAF of the air-fuel ratio sensor 17 changes as shown in FIG. 5B, the output signal VAF of the air-fuel ratio sensor 17 corresponds to the oxygen concentration in the exhaust when the air-fuel mixture is burned at the stoichiometric air-fuel ratio in the combustion chamber 2 of the engine 1. It is the timing when it changes from the high oxygen concentration side (upper side in the figure). After timing t6 in the figure, although the exhaust substantially equal to the air passes through the three-way catalyst, the oxygen concentration in the exhaust downstream of the catalyst is kept while oxygen in the exhaust is being stored in the three-way catalyst. Therefore, as shown by a solid line in FIG. 5D, the output signal VO of the oxygen sensor 18 does not change corresponding to the lean change of the air-fuel ratio. Then, when oxygen cannot be stored in the three-way catalyst and exhaust gas having a high oxygen concentration flows downstream of the catalyst, a change corresponding to the change of the air-fuel ratio to the lean side occurs in the output signal VO of the oxygen sensor 18. . Note that the determination that the change corresponding to the change of the air-fuel ratio to the lean side has occurred in the output signal VO of the oxygen sensor 18 is made with respect to the lean determination value HLfc corresponding to the fuel cut control for determining that. This can be done based on the arrival of the output signal VO (t7).

  During a period from the time when the air-fuel ratio changes from the rich side to the lean side in the output signal VAF of the air-fuel ratio sensor 17 until the time when the output signal VO of the oxygen sensor 18 changes corresponding to the change in the air-fuel ratio (t6) ~ T7), the total value of the amount of oxygen stored in the catalyst represents the maximum value (oxygen storage amount C2max) of the amount of oxygen stored in the three-way catalyst. The oxygen storage amount C2max is obtained during the period “t6 to t7” by using the same method as in the period “t2 to t3” in FIG. 4 during the active air-fuel ratio control.

  The oxygen storage amount C2max obtained as described above is a value reflecting the responsiveness of the oxygen sensor 18 during fuel cut control. Specifically, as a parameter representing such responsiveness, when the output signal VO of the oxygen sensor 18 changes based on the change from the rich side to the lean side of the air-fuel ratio of the engine 1 during the fuel cut control (for example, FIG. 5 ( The time required for the change per unit amount of the output signal VO from ts to t7) in d) (hereinafter referred to as response time Tfc) is given. Then, as the responsiveness of the oxygen sensor 18 during the fuel cut control decreases and the response time Tfc becomes longer, the period (t6 to t7) for obtaining the oxygen storage amount C2max becomes longer, so the oxygen storage amount C2max. Becomes a value shifted to the increasing side with respect to the appropriate value.

Next, how to determine the responsiveness of the oxygen sensor 18 using the oxygen storage amount C1max during active air-fuel ratio control and the oxygen storage amount C2max during fuel cut control will be described.
Regarding the oxygen storage amount C1max during active air-fuel ratio control, the true value A of the oxygen storage amount of the three-way catalyst and the oxygen when the air-fuel ratio of the engine 1 is changed between rich and lean during active control. Using the response time Tact, which is a parameter indicating the response of the sensor 18, it can be expressed by the following equation (2).

C1max = A + K1 · Tact (2)
C1max: oxygen storage amount during active air-fuel ratio control
A: True value of oxygen storage capacity
K1: Intake amount coefficient
Tact: Response time As can be seen from the equation (2), the oxygen storage amount C1max obtained during the active air-fuel ratio control is equal to “K1 · in equation (2) with respect to the true value A of the oxygen storage amount of the three-way catalyst. The value is shifted by the amount of the term “Tact”. The deviation of the oxygen storage amount C1max relative to the true value A by the term “K1 · Tact” is caused by the response of the change in the output signal VO of the oxygen sensor 18 to the change in the oxygen concentration in the exhaust during the active air-fuel ratio control. Arises. The intake air amount coefficient K1 in the term “K1 · Tact” corresponds to the fact that the deviation of the oxygen storage amount C1max with respect to the true value A of the oxygen storage amount of the three-way catalyst varies depending on the intake amount of the engine 1. A value for making “Tact” variable, and is calculated based on the intake amount of the engine 1 and the like.

  On the other hand, regarding the oxygen storage amount C2max during fuel cut control, the true value A of the oxygen storage amount of the three-way catalyst and the oxygen when the air-fuel ratio of the engine 1 changes from the rich side to the lean side during fuel cut control. Using the response time Tfc, which is a parameter indicating the response of the sensor 18, it can be expressed by the following equation (3).

C2max = A + K2 · Tfc (3)
C2max: Oxygen storage amount during fuel cut control
A: True value of oxygen storage capacity
K2: Intake amount coefficient
Tfc: Response time As can be seen from the equation (3), the oxygen storage amount C2max obtained during the fuel cut control is equal to “K2 · Tfc” in the equation (3) with respect to the true value A of the oxygen storage amount of the three-way catalyst. The value is shifted by the term "." The deviation of the oxygen storage amount C2max from the true value A by the term “K2 · Tfc” is attributed to the response of the change in the output signal VO of the oxygen sensor 18 to the change in the oxygen concentration in the exhaust during the fuel cut control. Arise. The intake air amount coefficient K2 in the term “K2 · Tfc” corresponds to the fact that the deviation of the oxygen storage amount C2max with respect to the true value A of the oxygen storage amount of the three-way catalyst changes depending on the intake amount of the engine 1. The value for making “Tfc” variable and calculated based on the intake air amount of the engine 1 and the like.

  The response time Tact used in the above equation (2) and the response time Tfc used in the above equation (3) indicate the change width of the air / fuel ratio of the engine 1 during the active air / fuel ratio control and during the fuel cut control. There is a correlation expressed by the following equation (4) due to a large difference. Note that “α” in the equation (4) is a proportional constant, which is a fixed value that can be obtained in advance through experiments or the like.

Tact = α · Tfc (4)
Tact: Response time
Tfc: Response time
α: Proportional constant Here, regarding the difference “C1max−C2max” between the oxygen storage amount C1max and the oxygen storage amount C2max, the following expression “C1max−C2max = K1 · Tact” is used using Expression (2) and Expression (3). −K2 · Tfc (5) ”. When the right side of the above equation (4) is substituted into the response time Tact of the equation (5) and transformed, the following equation “Tfc = (C1max−C2max) / (K1 · α) for calculating the response time Tfc is obtained. -K2) (6) "is obtained.

  In the present embodiment, the presence / absence of an abnormality related to the responsiveness of the oxygen sensor 18 is determined based on the response time Tfc obtained using the equation (6). As can be seen from the equation (6), the response time Tfc reflects the oxygen storage amount C1max reflecting the response of the oxygen sensor 18 during the active air-fuel ratio control and the response of the sensor 18 during the fuel cut control. The oxygen storage amount C2max is taken into account. Therefore, by determining whether or not there is an abnormality related to the responsiveness of the oxygen sensor 18 based on the response time Tfc, the result of the determination is oxygen under two different situations such as during active air-fuel ratio control and during fuel cut control. This is accurate based on the responsiveness of the sensor 18.

  Next, an execution procedure for determining whether or not the three-way catalyst has deteriorated and determining whether or not the oxygen sensor 18 has an abnormality will be described with reference to flowcharts of FIGS. 6 and 7 showing an abnormality diagnosis routine. This abnormality diagnosis routine is periodically executed through the electronic control device 21 by, for example, a time interruption every predetermined time.

  In this routine, it is first determined whether or not the calculation of the oxygen storage amount C1max during the active air-fuel ratio control is incomplete (S101). If the determination is affirmative, C1max for calculating the oxygen storage amount C1max is determined. A calculation process (S102) is executed. In the C1max calculation process, for the purpose of calculating the oxygen storage amount C1max, the same control is executed when the execution condition of the active air-fuel ratio control is satisfied. In the active air-fuel ratio control, when the air-fuel ratio of the engine 1 is forcibly switched between rich and lean, the oxygen storage amount C1max is calculated.

  In this routine, it is also determined whether or not the calculation of the oxygen storage amount C2max during the fuel cut control is incomplete (S103). If the determination is affirmative, C2max for calculating the oxygen storage amount C2max is determined. A calculation process (S104) is executed. In the C2max calculation process, when the fuel injection of the engine 1 is stopped and the air-fuel ratio of the engine 1 changes from the rich side to the lean side in the fuel cut control, the oxygen storage amount C2max is calculated.

  When the calculation of both the oxygen storage amount C1max and the oxygen storage amount C2max is completed (S105: YES), the response time is a parameter representing the responsiveness of the oxygen sensor 18 during fuel cut control using the equation (6). Tfc is calculated (S106). Specifically, based on the calculated oxygen storage amounts C1max and C2max and the intake air amount coefficients K1 and K2 obtained based on the intake air amount of the engine 1 when calculating the oxygen storage amounts C1max and C2max, the equation The response time Tfc is calculated using (6). Subsequently, it is determined whether or not the calculated response time Tfc is equal to or greater than a predetermined threshold (S107: FIG. 7). If the determination is affirmative, it is determined that there is an abnormality related to the responsiveness of the oxygen sensor 18 (S108). On the other hand, if the determination is negative, it is determined that there is no abnormality (normal) regarding the responsiveness of the oxygen sensor 18 (S109).

  Thereafter, a correction based on the response time Tfc is applied to the oxygen storage amount C2max corresponding to the response time Tfc of the oxygen storage amount C1max and the oxygen storage amount C2max (S110). Specifically, the oxygen storage amount C2max is corrected to decrease by the amount of the term “K2 · Tfc” obtained by multiplying the response time Tfc by the intake air amount coefficient K2. The corrected oxygen storage amount is hereinafter referred to as a corrected oxygen storage amount A. As can be seen from the above equation (3), the value obtained by subtracting the term “K2 · Tfc” from the oxygen storage amount C2max is the true value A of the oxygen storage amount in the three-way catalyst. Therefore, the corrected oxygen storage amount A is equal to the true value A of the equation (3). In other words, the corrected oxygen storage amount A is a value obtained by removing the influence of the responsiveness of the oxygen sensor 18 from the oxygen storage amount C2max.

  Then, using this corrected oxygen storage amount A, it is determined whether or not the three-way catalyst has deteriorated. Specifically, it is determined whether or not the corrected oxygen storage amount A is less than a predetermined threshold value (S111). If the determination is affirmative, it is determined that the three-way catalyst has deteriorated (abnormal) (S112). On the other hand, if the determination is negative, it is determined that the three-way catalyst has not deteriorated (normal) (S113).

According to the embodiment described in detail above, the following effects can be obtained.
(1) When determining whether or not the three-way catalyst has deteriorated and determining whether or not the oxygen sensor 18 has an abnormality, in order to accurately determine whether or not the oxygen sensor 18 has an abnormality, The parameters to be acquired by performing active air-fuel ratio control and fuel cut control can be suppressed to two of the oxygen storage amount C1max and the oxygen storage amount C2max. For this reason, it can suppress that it takes time to acquire these parameters, and therefore it takes a lot of time to complete the determination of the presence or absence of the deterioration of the three-way catalyst and the determination of the presence or absence of abnormality of the oxygen sensor 18 Can be suppressed.

  (2) A parameter representing the responsiveness of the oxygen sensor 18 is required for a change per unit amount of the output signal VO from the oxygen sensor 18 when the air-fuel ratio of the engine 1 changes from the rich side to the lean side by fuel cut control. Response time Tfc is determined. When the response time Tfc thus obtained is equal to or greater than the threshold value, it is determined that the oxygen sensor 18 is abnormal. When the response time Tfc is less than the threshold value, it is determined that the oxygen sensor 18 is normal. From the above, it is possible to accurately determine whether there is an abnormality related to the responsiveness of the oxygen sensor 18.

  (3) When determining whether or not the three-way catalyst has deteriorated, of the obtained oxygen storage amounts C1max and C2max, the oxygen storage amount C2max that corresponds to the response time Tfc obtained by the equation (6) Is corrected based on the responsiveness. Based on the corrected oxygen storage amount (corrected oxygen storage amount A), it is determined whether or not the three-way catalyst has deteriorated. Here, regarding the oxygen storage amount C2max, when the air-fuel ratio of the engine 1 changes from the rich side to the lean side as the responsiveness of the oxygen sensor 18 decreases, the output signal VO of the oxygen sensor 18 corresponding to the change changes. The period for calculating the oxygen storage amount C2max is delayed with a delay in the change, and therefore shows an increasing tendency with respect to the appropriate value (true value A). Therefore, if it is determined whether or not the three-way catalyst has deteriorated based on the oxygen storage amount C2max affected by the responsiveness of the oxygen sensor 18 as described above, the determination result may be inaccurate. is there. However, the determination of whether or not the three-way catalyst has deteriorated is made based on the oxygen storage amount (corrected oxygen storage amount A) corrected according to the response of the oxygen sensor 18 (more precisely, the response time Tfc). , It is suppressed that the result of the determination becomes inaccurate as described above.

In addition, the said embodiment can also be changed as follows, for example.
The response time Tact may be obtained as a parameter representing the responsiveness of the oxygen sensor 18 instead of the response time Tfc, and the presence / absence of an abnormality related to the responsiveness of the oxygen sensor 18 may be determined based on the obtained response time Tact. .

  In this case, the response time Tact is obtained based on the correlation between the oxygen storage amount C1max, the oxygen storage amount C2max, and the responsiveness of the oxygen sensor 18 during active air-fuel ratio control and fuel cut control. Specifically, the response time Tact is calculated using the following equation: “Tact = (C1max−C2max) / (K1− (K2 / α)) (7)”. In this equation (7), the right side of the following equation “Tfc = (1 / α) · Tact” obtained by modifying the equation (4) is substituted into the response time Tfc of the equation (5), and then the equation (7) It is obtained by deforming as in 7).

  The determination of the presence / absence of abnormality of the oxygen sensor 18 based on the response time Tact calculated by the equation (7) is performed based on a comparison between the response time Tact and a predetermined threshold value. That is, it is determined that there is an abnormality in the oxygen sensor 18 based on the calculated response time Tact being equal to or greater than the threshold value, while there is no abnormality in the oxygen sensor 18 based on the response time Tact being less than the threshold value ( Normal).

  Note that the threshold used here is larger than the threshold used in S107 (FIG. 7) of the abnormality diagnosis routine of the above embodiment. This is because the change in the air-fuel ratio of the engine 1 during the active air-fuel ratio control is smaller than the change in the air-fuel ratio of the engine 1 during the fuel cut control, and the response time Tact becomes longer than the response time Tfc under the influence. This is because there is a tendency.

  As described above, when the response time Tact is obtained as a parameter representing the responsiveness of the oxygen sensor 18, the oxygen storage amount corresponding to the response time Tact is used as an oxygen storage amount for determining whether or not the three-way catalyst has deteriorated. It is preferable to use C1max. Specifically, the oxygen storage amount C1max is reduced and corrected by the amount of the term “K1 · Tact” obtained by multiplying the response time Tact by the intake air amount coefficient K1, and the corrected oxygen storage amount A (= true value A ) And the presence or absence of deterioration of the catalyst is determined based on the calculated corrected oxygen storage amount A.

Instead of determining whether the catalyst has deteriorated based on the corrected oxygen storage amount A, the determination may be based on one or both of the oxygen storage amount C1max and the oxygen storage amount C2max.
An air-fuel ratio sensor may be provided instead of the oxygen sensor 18 as a catalyst downstream sensor.

  An oxygen sensor may be provided as a catalyst upstream sensor instead of the air-fuel ratio sensor 17.

  DESCRIPTION OF SYMBOLS 1 ... Engine, 2 ... Combustion chamber, 3 ... Intake passage, 4 ... Fuel injection valve, 5 ... Spark plug, 6 ... Piston, 7 ... Crankshaft, 8 ... Exhaust passage, 13 ... Throttle valve, 16 ... Catalytic converter, 17 ... Air-fuel ratio sensor, 18 ... Oxygen sensor, 21 ... Electronic control device (first calculation means, second calculation means, judgment means), 27 ... Accelerator pedal, 28 ... Accelerator position sensor, 30 ... Throttle position sensor, 32 ... Air flow Meter, 33 ... intake pressure sensor, 34 ... crank position sensor.

Claims (3)

An exhaust purification catalyst provided in the exhaust passage, a catalyst upstream sensor provided upstream of the catalyst in the exhaust passage and outputting a signal based on the oxygen concentration in the exhaust, and provided downstream of the catalyst in the exhaust passage Applied to an internal combustion engine including a catalyst downstream sensor that outputs a signal based on the oxygen concentration in the exhaust gas, and determines whether or not the catalyst has deteriorated based on the oxygen storage amount of the catalyst, and the response of the catalyst downstream sensor In the abnormality diagnosis device that determines the presence or absence of abnormality of the sensor based on
When the air-fuel ratio of the internal combustion engine changes between rich and lean, a change corresponding to the change in the air-fuel ratio occurs in the signal of the catalyst upstream sensor, and then the change in the air-fuel ratio in the signal of the catalyst downstream sensor The amount of oxygen occluded in the catalyst or the amount of oxygen desorbed from the catalyst is calculated during a period until the change corresponding to the first occurs, and the amount of oxygen is defined as the oxygen occlusion amount of the catalyst. A calculation means;
In the fuel cut control for stopping the fuel injection in the internal combustion engine, the oxygen storage amount obtained by the first calculation means during the active air fuel ratio control for forcibly changing the air fuel ratio of the internal combustion engine between rich and lean The oxygen storage amount obtained by the first calculation means when the fuel injection is stopped and the air-fuel ratio of the engine changes to the lean side, and during the active air-fuel ratio control and during the fuel cut control A second calculating means for determining the response of the catalyst downstream sensor under any of the active air-fuel ratio control and the fuel cut control based on the correlation of the response of the catalyst downstream sensor.
The presence / absence of deterioration of the catalyst is determined based on at least one of the oxygen storage amounts determined by the first calculating means, and the sensor is determined based on the responsiveness of the catalyst downstream sensor determined by the second calculating means. A determination means for determining whether or not there is an abnormality,
An abnormality diagnosis apparatus comprising: The response of the catalyst downstream sensor depends on the response time required for a change per unit amount of the signal output from the catalyst downstream sensor when the air-fuel ratio of the internal combustion engine changes by the active air-fuel ratio control or the fuel cut control. Is represented,
The determination unit determines that the catalyst downstream sensor is abnormal when the response time obtained by the second calculation unit is equal to or greater than a predetermined threshold value, and when the response time is less than the threshold value. The abnormality diagnosis device according to claim 1, wherein it is determined that the catalyst downstream sensor is normal. The determination means has the same responsiveness to the oxygen storage amount corresponding to the responsiveness of the catalyst downstream sensor determined by the second calculation means among the oxygen storage amounts determined by the first calculation means. The abnormality diagnosis apparatus according to claim 1, wherein a correction based on the correction is made, and the presence or absence of deterioration of the catalyst is determined based on the oxygen storage amount after the correction.

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