WO2013157048A1 - Catalyst anomaly diagnosis device - Google Patents

Abstract

Provided is a device for diagnosing anomalies in the catalyst arranged in the exhaust gas passage of an internal combustion engine. A post-catalyst sensor for detecting the exhaust gas air-fuel ratio downstream of the catalyst is provided, said sensor having hysteresis characteristics near stoichiometry. Simultaneously with the post-catalyst sensor output reaching a prescribed determination value, active air-fuel ratio control is performed for switching the air-fuel ratio upstream of the catalyst from a lean air-fuel ratio to a rich air-fuel ratio, or vice versa, and during said control, the amount of oxygen absorbed and released by the catalyst is cumulatively measured. During the control in which the air-fuel ratio is set to the lean air-fuel ratio or the rich air-fuel ratio, cumulative oxygen measurement ends when the output of the post-catalyst sensor assumes a value indicating that the exhaust gas air-fuel ratio downstream of the catalyst has started to change from stoichiometry towards said lean air-fuel ratio or rich air-fuel ratio.

Description

Catalyst abnormality diagnosis device

The present invention relates to an abnormality diagnosis of a catalyst, and more particularly to an apparatus for diagnosing an abnormality of a catalyst disposed in an exhaust passage of an internal combustion engine.

For example, in an internal combustion engine for automobiles, a catalyst for purifying exhaust gas is installed in the exhaust system. Some of these catalysts have an oxygen storage capacity (O ₂ storage capacity). When the air-fuel ratio of the exhaust gas flowing into the catalyst becomes larger than the stoichiometric air-fuel ratio (stoichiometric), that is, when the engine becomes lean, the catalyst having oxygen storage capacity occludes excess oxygen present in the exhaust gas. When the fuel ratio becomes smaller than stoichiometric, that is, when it becomes rich, the stored oxygen is released. For example, in a gasoline engine, air-fuel ratio control is performed so that the exhaust gas flowing into the catalyst is in the vicinity of the stoichiometric. However, if a three-way catalyst having an oxygen storage capacity is used, the actual air-fuel ratio slightly deviates from the stoichiometric depending on the operating conditions. However, such an air-fuel ratio shift can be absorbed by the oxygen storage / release action of the three-way catalyst.

On the other hand, when the catalyst deteriorates, the purification rate of the catalyst decreases. There is a correlation between the deterioration degree of the catalyst and the reduction degree of the oxygen storage capacity. Therefore, it is possible to detect deterioration or abnormality of the catalyst by detecting a decrease in oxygen storage capacity. In general, active air-fuel ratio control is performed to alternately and richly control the air-fuel ratio upstream of the catalyst, and the amount of oxygen absorbed and released by the catalyst during the lean control and rich control is measured. A method of diagnosing catalyst abnormality based on the amount (so-called Cmax method) is employed (see, for example, Patent Document 1).

By the way, in this Cmax method, a post-catalyst sensor for detecting the exhaust air / fuel ratio downstream of the catalyst is provided, and at the same time as the output of the post-catalyst sensor is reversed, the lean control and the rich control are switched, and the measurement of the oxygen amount is finished. I am doing so.

However, when measuring the amount of oxygen, there is a problem of measurement error that the amount of oxygen that is not actually absorbed and released is also measured. In particular, in the case of the conventional Cmax method, in the case of an abnormal catalyst, the error rate immediately before reversal of the sensor output after the catalyst increases compared to the case of a normal catalyst, and the tendency that the measured value becomes larger than the true value increases. This can lead to misdiagnosis of an abnormal catalyst as normal. In addition, the difference in the measured oxygen amount between the normal catalyst and the abnormal catalyst cannot be increased, and particularly in the case of a catalyst in which these differences are originally small, there is a possibility that sufficient diagnostic accuracy cannot be ensured.

Therefore, the present invention was created in view of the above circumstances, and one object of the present invention is to provide a catalyst abnormality diagnosis device that can reduce measurement errors, improve diagnosis accuracy, and suppress erroneous diagnosis. is there.

JP 2008-8158 A

According to one aspect of the invention,
An apparatus for diagnosing abnormality of a catalyst disposed in an exhaust passage of an internal combustion engine,
This is a post-catalyst sensor that detects the exhaust air / fuel ratio downstream of the catalyst, and the output changes suddenly at the stoichiometric boundary, and when the exhaust air / fuel ratio changes from the rich side to the lean side with respect to the stoichiometry, and changes from the lean side to the rich side A post-catalyst sensor having a hysteresis characteristic in which the output characteristic near the stoichiometry differs depending on
Active air-fuel ratio control for executing active air-fuel ratio control for switching the air-fuel ratio upstream of the catalyst from one of the lean air-fuel ratio and the rich air-fuel ratio in synchronization with the output of the post-catalyst sensor reaching a predetermined determination value Means,
Measuring means for integrating and measuring the amount of oxygen stored or released by the catalyst during execution of the active air-fuel ratio control;
Determination means for determining whether the catalyst is normal or abnormal based on the amount of oxygen measured by the measurement means;
With
During the control in which the air-fuel ratio on the upstream side of the catalyst is set to one of the lean air-fuel ratio and the rich air-fuel ratio, the output of the post-catalyst sensor changes the exhaust air-fuel ratio on the downstream side of the catalyst from the stoichiometric direction toward the one. Thus, there is provided a catalyst abnormality diagnosis device characterized in that the integrated measurement of the oxygen amount is terminated when a value indicating that it has started.

Preferably, the determination value includes a lean determination value that defines the switching timing from the lean air-fuel ratio to the rich air-fuel ratio, and a rich determination value that defines the switching timing from the rich air-fuel ratio to the lean air-fuel ratio,
During the lean control in which the air-fuel ratio on the upstream side of the catalyst is set to the lean air-fuel ratio, the measuring means is configured so that the output of the post-catalyst sensor reaches the lean side from the predetermined stoichiometric determination value before reaching the lean determination value. When the change starts, the integrated measurement of the oxygen amount is terminated.

Preferably, the stoichiometric determination value is a value corresponding to stoichiometry on a hysteresis characteristic line when the post-catalyst sensor output changes from the rich side to the lean side.

The stoichiometric determination value may be a richer value than the rich determination value.

The stoichiometric determination value may be a value equal to the rich determination value.

The stoichiometric determination value may be a value leaner than the rich determination value, and may be a richer value than a stoichiometric equivalent value on a single characteristic line of the post-catalyst sensor output.

Preferably, the determination value includes a lean determination value that defines the switching timing from the lean air-fuel ratio to the rich air-fuel ratio, and a rich determination value that defines the switching timing from the rich air-fuel ratio to the lean air-fuel ratio,
During the rich control in which the air-fuel ratio on the upstream side of the catalyst is set to the rich air-fuel ratio, the measurement means is configured to output the post-catalyst sensor before reaching the rich determination value and from the predetermined stoichiometric determination value toward the rich side. When the change starts, the integrated measurement of the oxygen amount is terminated.

Preferably, the stoichiometric rich determination value is a value corresponding to the stoichiometric value on the hysteresis characteristic line when the post-catalyst sensor output changes from the lean side to the rich side.

The stoichiometric determination value may be a leaner value than the lean determination value.

The stoichiometric determination value may be a value equal to the lean determination value.

The stoichiometric determination value may be a value on the rich side with respect to the lean determination value, and may be a value on the lean side with respect to a stoichiometric equivalent value on a single characteristic line of the post-catalyst sensor output.

Preferably, the active air-fuel ratio control means changes the determination value according to the intake air amount.

Preferably, the active air-fuel ratio control means switches the air-fuel ratio after a predetermined delay time has elapsed since the output of the post-catalyst sensor reaches the determination value, and changes the delay time according to the intake air amount.

According to the present invention, an excellent effect of reducing measurement errors and improving diagnostic accuracy and suppressing misdiagnosis is exhibited.

It is the schematic which shows the structure of embodiment of this invention. It is a schematic sectional drawing which shows the structure of a catalyst. It is a time chart of active air fuel ratio control in a basic method. It is a time chart which shows the measuring method of the oxygen storage capacity in a basic method. It is a graph which shows the output characteristic of a pre-catalyst sensor and a post-catalyst sensor. It is a time chart which shows the test result at the time of rich control, and is a case of a normal catalyst. It is a time chart which shows the test result at the time of rich control, and is a case of an abnormal catalyst. It is a fragmentary sectional view of a post-catalyst sensor. It is a time chart which shows the measuring method of the oxygen storage capacity in this embodiment. The figure which added the example which changed the sulfur concentration of a fuel and the oxygen storage capacity measuring method to the example of FIG. 9 is shown. It is a figure for showing the influence which the responsiveness variation of the post-catalyst sensor has on the measured value. It is a graph which shows the relationship between the value of the post-catalyst sensor output used as the integration end timing, and the final integrated value of the stored oxygen amount. It is a figure for demonstrating the change method of the lean determination value and rich determination value according to the amount of intake air. It is a figure for demonstrating a delay process. It is a figure which shows the relationship between FIG. 15A and FIG. 15B. It is a flowchart which shows the abnormality diagnosis process of this embodiment. It is a flowchart which shows the abnormality diagnosis process of this embodiment. The map for calculating delay time is shown.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing the configuration of the present embodiment. As shown in the figure, an engine 1 that is an internal combustion engine burns a mixture of fuel and air in a combustion chamber 3 formed in a cylinder block 2 and reciprocates a piston 4 in the combustion chamber 3 to drive power. Is generated. The engine 1 of the present embodiment is a multi-cylinder engine for automobiles (only one cylinder is shown), and is a spark ignition type internal combustion engine, more specifically, a gasoline engine.

The cylinder head of the engine 1 is provided with an intake valve Vi for opening and closing the intake port and an exhaust valve Ve for opening and closing the exhaust port for each cylinder. Each intake valve Vi and each exhaust valve Ve are opened and closed by a camshaft (not shown). A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder.

The intake port of each cylinder is connected to a surge tank 8 which is an intake manifold through an intake manifold. An intake pipe 13 that forms an intake manifold passage is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. The intake pipe 13 includes an air flow meter 5 for detecting an air amount per unit time flowing into the engine, that is, an intake air amount Ga (g / s), and an electronically controlled throttle valve 10 in order from the upstream side. Is provided. An intake passage is formed by the intake port, the intake manifold, the surge tank 8 and the intake pipe 13.

An injector for injecting fuel into the intake passage, particularly the intake port, that is, a fuel injection valve 12 is provided for each cylinder. The fuel injected from the injector 12 is mixed with intake air to form an air-fuel mixture. The air-fuel mixture is sucked into the combustion chamber 3 when the intake valve Vi is opened, compressed by the piston 4, and ignited and burned by the spark plug 7. It is done.

On the other hand, the exhaust port of each cylinder is connected to an exhaust pipe 6 forming an exhaust collecting passage through an exhaust manifold. An exhaust passage is formed by the exhaust port, the exhaust manifold, and the exhaust pipe 6. The exhaust pipe 6 is provided with a catalyst composed of a three-way catalyst having oxygen storage capacity, that is, an upstream catalyst 11 and a downstream catalyst 19 in series on the upstream side and the downstream side. For example, the upstream catalyst 11 is disposed immediately after the exhaust manifold, and the downstream catalyst 19 is disposed under the floor of the vehicle.

On the upstream side and downstream side of the upstream catalyst 11, air-fuel ratio sensors that detect the air-fuel ratio of the exhaust gas based on the oxygen concentration, that is, the pre-catalyst sensor 17 and the post-catalyst sensor 18, are provided. As shown in FIG. 5, the pre-catalyst sensor 17 is composed of a wide-range air-fuel ratio sensor, can continuously detect the air-fuel ratio over a relatively wide range, and outputs a signal having a value proportional to the air-fuel ratio. On the other hand, the post-catalyst sensor 18 is composed of an oxygen sensor (O ₂ sensor) and has a characteristic (Z characteristic) in which the output value changes suddenly with the theoretical air-fuel ratio as a boundary.

The above-described spark plug 7, throttle valve 10, injector 12 and the like are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 14 that detects the crank angle of the engine 1, and an accelerator opening that detects the accelerator opening, as shown in the figure. The degree sensor 15 and other various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 controls the ignition plug 7, the injector 12, the throttle valve 10, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, fuel injection amount, fuel injection timing, throttle opening. Control the degree etc.

The upstream catalyst 11 and the downstream catalyst 19 simultaneously convert NOx, HC, and CO at high efficiency when the air-fuel ratio A / F of the exhaust gas flowing into the upstream catalyst 11 and the downstream catalyst 19 is the stoichiometric air-fuel ratio (stoichiometric, for example, A / Fs = 14.6). Purify. Therefore, in accordance with this characteristic, the ECU 20 controls the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 (specifically, so that the air-fuel ratio of the exhaust gas upstream of the upstream catalyst 11 matches the stoichiometric condition during normal operation of the engine. The amount of fuel injected from the injector 12 is feedback-controlled based on the outputs of the pre-catalyst sensor 17 and the post-catalyst sensor 18.

Here, the upstream catalyst 11 to be subjected to abnormality diagnosis will be described in more detail. The downstream catalyst 19 is configured in the same manner as the upstream catalyst 11. As shown in FIG. 2, in the catalyst 11, a coating material 31 is coated on the surface of a carrier base material (not shown), and a large number of particulate catalyst components 32 are supported on the coating material 31 in a dispersed manner. The catalyst 11 is exposed inside. The catalyst component 32 is mainly composed of a noble metal such as Pt or Pd, and serves as an active point when reacting exhaust gas components such as NOx, HC and CO. On the other hand, the coating material 31 plays a role of a promoter that promotes a reaction at the interface between the exhaust gas and the catalyst component 32 and includes an oxygen storage component capable of absorbing and releasing oxygen according to the air-fuel ratio of the atmospheric gas. The oxygen storage component is made of, for example, cerium dioxide CeO ₂ or zirconia. Note that “absorption” or “adsorption” may be used in the same meaning as “occlusion”.

For example, if the atmospheric gas in the catalyst is leaner than the stoichiometric air-fuel ratio, the oxygen storage component present around the catalyst component 32 absorbs oxygen from the atmospheric gas, and as a result, NOx is reduced and purified. On the other hand, when the atmospheric gas in the catalyst is richer than the stoichiometric air-fuel ratio, oxygen stored in the oxygen storage component is released, and the released oxygen oxidizes and purifies HC and CO.

This oxygen absorption / release action can absorb this variation even if the actual air-fuel ratio varies somewhat with respect to stoichiometry during normal stoichiometric air-fuel ratio control.

By the way, in the new catalyst 11, as described above, a large number of catalyst components 32 are evenly distributed, and the contact probability between the exhaust gas and the catalyst component 32 is kept high. However, when the catalyst 11 deteriorates, some of the catalyst components 32 are lost, and some of the catalyst components 32 are baked and solidified by exhaust heat (see broken lines in the figure). If it becomes like this, the contact probability of exhaust gas and the catalyst component 32 will fall, and it will become the cause of reducing a purification rate. In addition to this, the amount of the coating material 31 existing around the catalyst component 32, that is, the amount of the oxygen storage component decreases, and the oxygen storage capacity itself decreases.

Thus, there is a correlation between the degree of deterioration of the catalyst 11 and the degree of decrease in the oxygen storage capacity of the catalyst 11. Therefore, in the present embodiment, the deterioration degree of the upstream catalyst 11 is detected by detecting the oxygen storage capacity of the upstream catalyst 11 that has a particularly large influence on the emission, and the abnormality of the upstream catalyst 11 is diagnosed. Here, the oxygen storage capacity of the catalyst 11 is represented by the amount of oxygen storage capacity (OSC; O ₂ Storage Capacity, unit is g), which is the amount of oxygen that the current catalyst 11 can store or release.

[Basic method of abnormality diagnosis]
The abnormality diagnosis of this embodiment is based on the following method based on the Cmax method described above. In the abnormality diagnosis, the active air-fuel ratio control is executed by the ECU 20. That is, the ECU 20 controls the air-fuel ratio on the upstream side of the catalyst, specifically, the air-fuel ratio of the air-fuel mixture in the combustion chamber 3 alternately and richly and lean, with the stoichiometric A / Fs being the central air-fuel ratio as a boundary. As a result, the air-fuel ratio of the exhaust gas supplied to the catalyst 11 is also controlled to be rich and lean alternately.

In addition, active air-fuel ratio control and diagnosis are executed only when predetermined preconditions are satisfied. This precondition will be described later.

Hereinafter, a method for measuring the oxygen storage capacity of the upstream catalyst 11 will be described with reference to FIGS. 3 and 4.

3A, the broken line indicates the target air-fuel ratio A / Ft, and the solid line indicates the output of the pre-catalyst sensor 17 (however, the converted value to the pre-catalyst air-fuel ratio A / Ff). In FIG. 3B, the solid line indicates the output of the post-catalyst sensor 18 (however, the output voltage Vr).

As shown in the figure, before the time t1, lean control for switching the air-fuel ratio to lean is executed. At this time, the target air-fuel ratio A / Ft is set to a predetermined lean air-fuel ratio A / Fl (for example, 15.1), and the catalyst 11 is supplied with a lean gas having an air-fuel ratio equal to the target air-fuel ratio A / Ft. At this time, the catalyst 11 continues to occlude oxygen. However, when the oxygen is occluded until it is saturated, that is, full, it can no longer occlude oxygen. As a result, the lean gas passes through the catalyst 11 and flows out downstream of the catalyst 11. When this happens, the output of the post-catalyst sensor 18 changes to the lean side, and the target air-fuel ratio A / Ft becomes the predetermined rich air-fuel ratio at the time t1 when the output voltage Vr reaches a predetermined lean determination value VL (for example, 0.2 V). It is switched to A / Fr (for example, 14.1). As a result, the air-fuel ratio control is switched from lean control to rich control, and rich gas having an air-fuel ratio equal to the target air-fuel ratio A / Ft is supplied.

When the rich gas is supplied, the catalyst 11 continues to release the stored oxygen. When the stored oxygen is eventually released from the catalyst 11, the catalyst 11 cannot release oxygen at that time, and the rich gas passes through the catalyst 11 and flows out downstream of the catalyst 11. When this happens, the output of the post-catalyst sensor 18 changes to the rich side, and at the time t2 when the output voltage Vr reaches a predetermined rich determination value VR (for example, 0.6 V), the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / It is switched to Fl. As a result, the air-fuel ratio control is switched from rich control to lean control, and a lean gas having an air-fuel ratio equal to the target air-fuel ratio A / Ft is supplied.

When the catalyst 11 again stores oxygen until it is full and the output voltage Vr of the post-catalyst sensor 18 reaches the lean determination value VL, the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio A / Fr at time t3. Rich control is started.

Thus, every time the output of the post-catalyst sensor 18 is reversed, the lean control and the rich control are alternately and repeatedly executed. A set of adjacent lean control and rich control is defined as one cycle of active air-fuel ratio control. Active air-fuel ratio control is executed in a predetermined N cycle (N is an integer of 2 or more).

Here, the lean determination value VL defines the switching timing from lean control to rich control. As shown in FIG. 5, the lean determination value VL is set to a value smaller (lean side) than the stoichiometric equivalent value Vst of the post-catalyst sensor output.

Similarly, the rich determination value VR defines the switching timing from rich control to lean control. As shown in FIG. 5, the rich determination value VR is set in advance to a value that is larger (rich side) than the stoichiometric equivalent value Vst of the post-catalyst sensor output.

During execution of this active air-fuel ratio control, the oxygen storage capacity OSC of the catalyst 11 is measured by the following method.

The larger the oxygen storage capacity of the catalyst 11, the longer the time during which oxygen can be stored or released. That is, when the catalyst has not deteriorated, the inversion period of the post-catalyst sensor output Vr (for example, the time from t1 to t2) becomes longer, and the inversion period becomes shorter as the deterioration of the catalyst proceeds.

Therefore, using this, the oxygen storage capacity OSC is measured as follows. As shown in FIG. 4, immediately after the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio A / Fr at time t1, the pre-catalyst air-fuel ratio A / Ff as an actual value is slightly delayed with the rich air-fuel ratio A / Ff. Switch to Fr. From the time t11 when the pre-catalyst air-fuel ratio A / Ff reaches the stoichiometric A / Fs to the time t2 when the post-catalyst sensor output Vr is next reversed, the oxygen storage capacity for each predetermined calculation cycle is obtained by the following equation (1). dOSC is sequentially calculated, and the oxygen storage capacity dOSC is sequentially accumulated from time t11 to time t2. Thus, the oxygen storage capacity OSC as the final integrated value during the rich control, that is, the amount of released oxygen indicated by OSCb in FIG. 4 is measured.

Q is the fuel injection amount. When the air-fuel ratio difference ΔA / F is multiplied by the fuel injection amount Q, the air amount that is insufficient or excessive with respect to the stoichiometry can be calculated. σ is a constant representing the proportion of oxygen contained in air (about 0.23).

Similarly, during the lean control, the oxygen storage capacity, that is, the stored oxygen amount indicated by OSCa in FIG. 4 is measured. Each time the rich control and the lean control are alternately performed, the released oxygen amount and the stored oxygen amount are alternately measured.

Thus, when a plurality of measured values of the released oxygen amount and the occluded oxygen amount are obtained in each manner, the normality / abnormality determination of the catalyst is performed by the following method.

First, the ECU 20 calculates an average value OSCav of the measured values of the released oxygen amount and the stored oxygen amount. The average value OSCav is compared with a predetermined abnormality determination value α. The ECU 20 determines that the catalyst 11 is normal when the average value OSCav is greater than the abnormality determination value α, and determines that the catalyst 11 is abnormal when the average value OSCav is less than or equal to the abnormality determination value α. When it is determined that the catalyst is abnormal, it is preferable to activate a warning device (not shown) such as a check lamp in order to notify the user of the fact.

[Abnormality diagnosis method of this embodiment]
Next, the abnormality diagnosis method of this embodiment will be described. “Oxygen storage capacity OSC” and “oxygen amount” are terms that encompass “amount of stored oxygen OSCa” and “amount of released oxygen OSCb”.

As described above, when measuring the oxygen storage capacity OSC, there is a problem of measurement error in which the amount of oxygen that is not actually absorbed and released is also measured. In particular, in the case of the conventional Cmax method, in the case of an abnormal catalyst, the error rate immediately before reversal of the sensor output after the catalyst increases compared to the case of a normal catalyst, and the tendency that the measured value becomes larger than the true value increases. In this case, there is a possibility that an abnormal catalyst is actually misdiagnosed as normal.

This point will be described in detail with reference to FIGS. FIG. 6 shows the case of a normal catalyst, and FIG. 7 shows the case of an abnormal catalyst. Both figures show the test results when switching from lean control to rich control. However, even when the post-catalyst sensor output Vr is reversed (that is, even when the rich determination value VR is reached), switching to lean control is not performed.

In both figures, (A) shows the target air-fuel ratio A / Ft, the pre-catalyst air-fuel ratio A / Ff (line a) detected by the pre-catalyst sensor 17, and the post-catalyst air-fuel ratio A / Fr (line b). Indicates. Here, an air-fuel ratio sensor similar to the pre-catalyst sensor 17 is installed for testing on the downstream side of the catalyst, and the air-fuel ratio detected by the test air-fuel ratio sensor is set as the post-catalyst air-fuel ratio A / Fr.

(B) shows the post-catalyst sensor output Vr, and (C) shows the integrated value of the released oxygen amount OSCb. The post-catalyst sensor output Vr can vary within the range of 0 to 1 (V). The rich determination value VR of the post-catalyst sensor output Vr is 0.6 (V).

First, the case of the normal catalyst in FIG. 6 will be described. From the time t1 when the pre-catalyst air-fuel ratio A / Ff decreases and reaches the stoichiometric (= 14.6), until the time t3 when the post-catalyst sensor output Vr moves to the rich side and reaches the rich determination value VR, the released oxygen The amount OSCb is integrated. The final integrated value of the released oxygen amount OSCb at the time t3 can be expressed by the area of the region c shown in (A). This region c is a region sandwiched between the stoichiometry (14.6) and the pre-catalyst air-fuel ratio A / Ff from time t1 to time t3.

On the other hand, the post-catalyst air-fuel ratio A / Fr is slightly richer than stoichiometric during this period t1 to t3. The area of the region d sandwiched between the stoichiometry and the post-catalyst air-fuel ratio A / Fr is the portion of the rich gas that could not be actually processed by the catalyst, in other words, the amount of oxygen that could not be released from the catalyst (OSCe for convenience) Represents. The area of the region d corresponds to an error in the total released oxygen amount OSCb at time t3.

The value obtained by subtracting the area (OSCe) of the region d from the area (OSCb) of the region c represents the amount of oxygen actually released from the catalyst. Thus, the measured released oxygen amount OSCb includes the actually released oxygen amount OSCe.

In the apparatus configuration of the present embodiment shown in FIG. 1, since there is no air-fuel ratio sensor that can detect the absolute value of the post-catalyst air-fuel ratio A / Fr, the error itself cannot be measured alone. For convenience, the region c sandwiched between the stoichiometry and the pre-catalyst air-fuel ratio A / Ff is measured as the released oxygen amount OSCb.

By the way, paying attention to the post-catalyst air-fuel ratio A / Fr and the post-catalyst sensor output Vr, the post-catalyst air-fuel ratio A / Fr starts to decrease to the rich side at time t2 between time t1 and time t3, and the post-catalyst sensor output The rate of increase or change rate of Vr to the rich side has begun to increase. This is considered to mean that the release of oxygen from the catalyst is substantially completed at time t2, and thereafter oxygen remaining in the catalyst is released relatively slowly. Alternatively, it is considered that the main oxygen release of the catalyst is completed at the time t2, and then the secondary residual oxygen is released.

However, even during the period from time t2 to time t3, there is a difference between the post-catalyst air-fuel ratio A / Fr and the pre-catalyst air-fuel ratio A / Ff, and oxygen is actually released and rich gas is processed. Therefore, it is considered that the proportion of the error is relatively small in the released oxygen amount OSCb measured during the period t2 to t3. In the case of a normal catalyst, since the value of the total released oxygen amount measured in the entire period t1 to t3 is large, the ratio of the error in the period t2 to t3 in the total released oxygen amount is relatively small. Conceivable.

(C) schematically shows the amount of oxygen OSCe corresponding to the error. Of the total released oxygen amount OSCb at time t3, the proportion of the oxygen amount OSCe corresponding to the error is relatively small.

In contrast, in the case of the abnormal catalyst shown in FIG. 7, during the period from time t2 to time t3, there is no difference between the post-catalyst air-fuel ratio A / Fr and the pre-catalyst air-fuel ratio A / Ff. There is almost no difference. This means that the catalyst does not substantially release oxygen. However, even during this period t2 to t3, the difference between the stoichiometric ratio and the pre-catalyst air-fuel ratio A / Ff is integrated, and the released oxygen amount OSCb is measured as if the catalyst is releasing oxygen.

Therefore, it is considered that the ratio of the error is very large in the released oxygen amount OSCb measured in the period t2 to t3. In the case of an abnormal catalyst, since the value of the total released oxygen amount measured during the entire period t1 to t3 is relatively small, the error amount in the period t2 to t3 accounts for a large proportion of the total released oxygen amount. it is conceivable that.

(C) schematically shows the amount of oxygen OSCe corresponding to the error. Of the total released oxygen amount OSCb at time t3, the proportion of the oxygen amount OSCe corresponding to the error is large.

Thus, with the basic method, in the case of an abnormal catalyst, the error rate immediately before reversing the sensor output after the catalyst increases compared to the case of a normal catalyst, and the increase rate of the measured value relative to the true value also increases. In this case, there is a possibility that an abnormal catalyst is actually misdiagnosed as normal.

Also, the difference in the measured oxygen amount between the normal catalyst and the abnormal catalyst cannot be enlarged, and there is a possibility that sufficient diagnostic accuracy cannot be ensured particularly in the case of a catalyst where these differences are originally small. In recent years, there is a tendency to reduce the amount of noble metal in the catalyst, and in such a catalyst, the difference in the amount of oxygen that can be absorbed and released between the normal and abnormal catalysts is originally small. Therefore, if the error ratio is large, it is not possible to distinguish a subtle difference in oxygen amount between the normal and abnormal catalysts, and there is a possibility that sufficient diagnostic accuracy cannot be ensured.

Of course, the difference in the amount of oxygen that can be absorbed and released is small between the normal catalyst that has deteriorated slightly on the normal side and the abnormal catalyst that is on the abnormal side. Therefore, if the error rate is large, these subtle differences in oxygen amount cannot be distinguished, and there is a possibility that sufficient diagnostic accuracy cannot be ensured.

Incidentally, one of the causes of this problem is that the responsiveness of the post-catalyst sensor 18 is relatively poor.

As shown in FIG. 8, the post-catalyst sensor 18 has a cup-shaped detection element 31 disposed in the exhaust pipe 6, and the detection element 31 is covered with a cover 32 with a hole. An inner surface or inner electrode (not shown) of the detection element 31 is exposed to the atmosphere (air), and an outer surface or outer electrode of the detection element 31 is exposed in the cover 32. Exhaust gas outside the cover 32 enters the cover 32 through the hole 33 of the cover 32. The difference in oxygen partial pressure between the inner and outer surfaces of the detection element 31, in other words, the oxygen partial pressure between the atmospheric gas that is the atmospheric gas on the inner surface of the detection element 31 and the exhaust gas that is the atmospheric gas on the outer surface of the detection element 31. An electromotive force is generated according to the difference. Based on this electromotive force, the air-fuel ratio of the exhaust gas is detected. The smaller the oxygen concentration of the exhaust gas, that is, the richer the air-fuel ratio of the exhaust gas, the larger the electromotive force.

As described above, the post-catalyst sensor 18 generates an electromotive force according to the air-fuel ratio of the ambient gas outside the detection element 31, and rather, passively generates an output corresponding to the air-fuel ratio of the ambient gas. . Therefore, even if the ambient gas outside the cover 32 changes to rich gas, the rich gas enters the cover 32 and is exchanged with the existing gas in the cover 32, and the post-catalyst sensor 18 generates an electromotive force corresponding to the rich gas in the cover. There is a time delay before it occurs. This delay is a response delay, and this response delay is much larger than the response delay of the pre-catalyst sensor 17 composed of the wide-range air-fuel ratio sensor and the test air-fuel ratio sensor installed on the downstream side of the catalyst. This is because the pre-catalyst sensor 17 and the test air-fuel ratio sensor are applied with a predetermined voltage and rather can actively generate an output corresponding to the air-fuel ratio of the atmospheric gas.

Also, one of the causes of the above problem is that the post-catalyst sensor 18 has a hysteresis characteristic.

As shown in FIG. 5, the post-catalyst sensor 18 has a single characteristic as indicated by a solid line qualitatively or statically, but has a hysteresis characteristic as indicated by a one-dot chain line in practice or dynamically. When the exhaust air-fuel ratio changes from the rich side to the lean side with respect to the stoichiometry (line a), and when the exhaust air-fuel ratio changes from the lean side to the rich side (line b), the output characteristics or transient characteristics near the stoichiometry are different. For example, when changing to the lean side, it changes to the lean side later than the single characteristic indicated by the solid line, and vice versa. This hysteresis characteristic also causes a response delay of the post-catalyst sensor output Vr, resulting in a measurement error.

In these examples, the post-catalyst sensor output Vr diagram does not match the post-catalyst air-fuel ratio A / Fr diagram (line b) in the examples of FIGS.

In the example of FIG. 7, if the post-catalyst sensor output Vr instantaneously reaches the rich judgment value VR at time t2 when the rich gas begins to leak from the catalyst, the measurement error due to the response delay is greatly suppressed. However, since this is not the case, the measurement error due to the response delay becomes remarkable.

Note that the examples of FIGS. 6 and 7 are the case of rich control, but the same problem occurs in the case of lean control.

Therefore, in view of the above problems, in the present embodiment, in order to reduce the error rate due to the responsiveness of the post-catalyst sensor 18, the oxygen amount measurement method is changed as follows. First, generally speaking, during the control in which the air-fuel ratio on the upstream side of the catalyst is one of the lean air-fuel ratio and the rich air-fuel ratio, the output of the post-catalyst sensor 18 indicates that the exhaust air-fuel ratio on the downstream side of the catalyst moves from the stoichiometric direction toward the one. When the value reaches a value indicating that it has started to change, the integrated measurement of the oxygen amount is terminated. The air-fuel ratio switching timing of the active air-fuel ratio control is not changed, the oxygen amount measurement end timing is changed, and the integrated measurement of the oxygen amount is ended at a timing earlier than the air-fuel ratio switching timing.

By so doing, the integration of the oxygen amount can be completed at the moment when the one gas starts to leak from the catalyst, and only the substantial oxygen amount occluded or released can be measured. The subsequent measurement of the error during the response delay time of the post-catalyst sensor 18 can be eliminated, and the measurement error due to the response delay of the post-catalyst sensor 18 can be greatly reduced.

As a result, the measurement error can be reduced to improve the diagnostic accuracy, and misdiagnosis can be suppressed. And the difference of the measured value between right and wrong catalysts is expanded substantially, and even if the difference of both originally is delicate, it becomes possible to distinguish the difference correctly.

Hereinafter, the oxygen amount measurement method of the present embodiment will be described in detail with reference to FIG.

(A) shows the pre-catalyst air-fuel ratio A / Ff and target air-fuel ratio A / Ft detected by the pre-catalyst sensor 17, and (B) shows the post-catalyst sensor output Vr. (C) shows the post-catalyst air-fuel ratio A / Fr detected by the test air-fuel ratio sensor for convenience, and (D) shows the integrated value of the stored oxygen amount OSCa.

As described above, the test air-fuel ratio sensor is much more responsive than the post-catalyst sensor 18. Therefore, the post-catalyst air-fuel ratio A / Fr shown in (C) can be considered to accurately indicate the exhaust air-fuel ratio downstream of the catalyst.

At the time t2, as soon as the post-catalyst sensor output Vr reaches the rich determination value VR, the target air-fuel ratio A / Ft is switched from the rich air-fuel ratio to the lean air-fuel ratio, and lean control is started. Here, VR = 0.66 (V).

Then, from the time when the pre-catalyst air-fuel ratio A / Ff becomes stoichiometric at time t3, integrated measurement of the stored oxygen amount OSCa is started. That is, the measurement start timing of the oxygen amount is the same as in the basic method.

Thereafter, while the post-catalyst sensor output Vr is changing to the lean side, at the time t4 when the post-catalyst sensor output Vr first falls below the predetermined stoichiometric determination value VA, the integrated measurement of the stored oxygen amount OSCa is completed. Is done.

Thereafter, the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio at the time t5 when the post-catalyst sensor output Vr further changes to the lean side and reaches the lean determination value VL, and rich control is started. Here, VL = 0.2 (V). As described above, the air-fuel ratio switching timing is the same as in the basic method, and the integration end timing is earlier than the air-fuel ratio switching timing.

Here, paying attention to the post-catalyst air-fuel ratio A / Fr in (C), the post-catalyst air-fuel ratio A / Fr approaches the stoichiometry during the period from t2 to t3, and is maintained substantially stoichiometric during the period from t3 to t4. It has a waveform with a certain shelf in the middle, which approaches the lean air-fuel ratio in the period of t5. The post-catalyst sensor output Vr in (B) also has a waveform similar to this.

In the period when the post-catalyst air-fuel ratio A / Fr is maintained at stoichiometric, it is considered that all excess oxygen in the lean gas is occluded by the catalyst. However, at the time when the post-catalyst air-fuel ratio A / Fr starts to change from stoichiometric to the lean air-fuel ratio, that is, at time t4, it is considered that a part of the excess oxygen in the lean gas is not stored in the catalyst but starts to leak.

Therefore, in principle, the stoichiometric determination value VA is set in accordance with the timing when the leakage starts. That is, the timing at which the post-catalyst sensor output Vr first falls below the stoichiometric determination value VA means the timing at which substantial oxygen occlusion has ended in the catalyst and oxygen begins to leak.

Here, the rich determination value VR and the lean determination value VL, and the stoichiometric determination value VA and the stoichiometric rich determination value VB (details will be described later) are all adapted values set in advance in consideration of the test results, sensor characteristics, and the like. is there. However, while the lean determination value VL is a value indicating that the post-catalyst air-fuel ratio A / Fr is completely leaner than the stoichiometric value, the stoichiometric determination value VA is equal to the post-catalyst air-fuel ratio A / Fr. This value indicates that the stoichi has started to become lean.

Similarly, the rich determination value VR is a value indicating that the post-catalyst air-fuel ratio A / Fr is completely richer than the stoichiometric ratio, while the stoichiometric rich determination value VB is the post-catalyst air-fuel ratio A / Fr. Is a value indicating that has started to become rich from stoichiometric.

As shown in FIG. 9, the stoichiometric determination value VA is a value close to the rich determination value VR. Further, the stoichiometric determination value VB is a value close to the lean determination value VL.

Referring to FIG. 5, stoichiometric determination value VA is most preferably value VA ₁ corresponding to stoichiometry on hysteresis characteristic line a when changing from the rich side to the lean side. In the illustrated example, the value VA ₁ is a value on the rich side with respect to the rich determination value VR.

However, the output characteristics and hysteresis characteristics of the post-catalyst sensor 18 differ depending on the sensor, and may not be as shown in the example of illustration. Further, the above value may not necessarily be a value indicating that the post-catalyst air-fuel ratio A / Fr starts to change from stoichiometric to lean.

Therefore, depending on the situation, the stoichiometric determination value VA may alternatively be a value VA ₂ equal to the rich determination value VR, or stoichiometrically on the single characteristic line (solid line) on the lean side with respect to the rich determination value VR. The value VA ₃ on the richer side than the equivalent value Vst may be used. FIG. 9 shows a case where VR> VA and VA = VA ₃ . In the example of FIG. 9, for example, VA = 0.6 (V).

Note that it may be preferable that VR ≧ VA in terms of control. This is because if VR <VA, the post-catalyst sensor output Vr that has increased to the rich side and exceeded the rich determination value VR may decrease before reaching VA.

From the above description, it will be understood that the same measurement method is adopted when the air-fuel ratio is controlled to the opposite side, that is, the rich air-fuel ratio. That is, as shown in FIG. 9, during the rich control before time t2, while the after-catalyst sensor output Vr is changing to the rich side, the after-catalyst sensor output Vr first reaches the predetermined stoichiometric determination value VB. At the time t1 when the value exceeds, the integrated measurement of the released oxygen amount OSCb is completed.

Thereafter, at the time t2 when the post-catalyst sensor output Vr further changes to the rich side and reaches the rich determination value VR, the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio, and lean control is started.

As shown in (C), the post-catalyst air-fuel ratio A / Fr is almost stoichiometric before time t1, but starts to change toward the rich air-fuel ratio at time t1. It is considered that at this time t1, the processing of the rich gas by the oxygen released from the catalyst is not in time, and part of the rich gas has started to leak from the catalyst.

Therefore, in principle, the stoichiometric determination value VB is set in accordance with the timing when the leakage starts. That is, the timing at which the post-catalyst sensor output Vr first exceeds the stoichiometric rich determination value VB means the timing at which the substantial oxygen release has ended in the catalyst and the rich gas has started to leak.

Referring to FIG. 5, stoichiometric rich determination value VB is most preferably value VB ₁ corresponding to stoichiometry on hysteresis characteristic line b when changing from the lean side to the rich side. In the illustrated example, this value VB ₁ is a value on the lean side of the lean determination value VL.

However, the output characteristics and hysteresis characteristics of the post-catalyst sensor 18 differ depending on the sensor, and may not be as shown in the example of illustration. Further, the above value may not necessarily be a value indicating the time point when the post-catalyst air-fuel ratio A / Fr starts to change from stoichiometric to rich.

Therefore, depending on the situation, the stoichiometric determination value VB may alternatively be a value VB ₂ equal to the lean determination value VL, or stoichiometric on the rich characteristic side of the lean determination value VL and on a single characteristic line (solid line). The value VB ₃ on the lean side of the equivalent value Vst may be used. FIG. 9 shows a case where VL> VB and VB = VB ₁ . In the example of FIG. 9, for example, VB = 0.18 (V).

For control purposes, it may be preferable to satisfy VL ≦ VB. This is because, if VL> VB, the post-catalyst sensor output Vr that has decreased to the lean side and exceeded the lean determination value VL may increase before reaching VB.

Next, additional explanation will be given regarding the operational effects of the present embodiment.

FIG. 10 shows a diagram in which an example in which the method for measuring the sulfur (S) concentration of fuel and the amount of oxygen is changed is added to the example of FIG. The example of FIG. 9 is an example in the case of using a standard fuel having a low sulfur concentration and a predetermined value or less (hereinafter referred to as low S fuel). For this reason, the same diagram as FIG. 9 in FIG. 10 is displayed as “low S”, and the additional diagram is also displayed as “low S” when the low S fuel is used. On the other hand, “high S” is displayed for a diagram in the case where a fuel having a high sulfur concentration and exceeding a predetermined value (hereinafter referred to as high S fuel) is used.

In (D), A1 is the stored oxygen amount measurement value when the integration is terminated at time t4 when the post-catalyst sensor output Vr first falls below the stoichiometric determination value VA according to the present embodiment when low S fuel is used.

A2 is the stored oxygen amount measurement value when the integration is terminated at time t5 when the post-catalyst sensor output Vr reaches the lean determination value VL according to the basic method when using low S fuel. Since the integration is completed at a later timing than in the present embodiment when low S fuel is used, the measured value increases (A1 <A2).

B1 is a measured value of the stored oxygen amount when the integration ends at time t6 when the post-catalyst sensor output Vr first falls below the stoichiometric determination value VA according to the present embodiment when high S fuel is used. Since the integration is completed at an earlier timing than in the present embodiment when using low S fuel, the measured value decreases (B1 <A1).

B2 is the measured value of the stored oxygen amount when the integration ends at time t7 when the post-catalyst sensor output Vr reaches the lean determination value VL according to the basic method when using high S fuel. Compared with the basic method when using low S fuel, the integration is completed at an earlier timing, so the measured value decreases (B2 <A2).

Note that when high S fuel is used, the catalyst is sulfur poisoned (S poison), and the apparent oxygen storage capacity of the catalyst is reduced. However, since such deterioration due to S poisoning is recoverable temporary deterioration, it is necessary to distinguish from permanent deterioration that cannot be recovered due to thermal deterioration or the like, which is a detection target of the present embodiment.

Here, the difference between A1 and B1 in this embodiment is smaller than the difference between A2 and B2 in the basic method. Therefore, it can be said that the change or variation of the measured value with respect to the change or variation in the S concentration of the fuel is less in the present embodiment than in the basic method. Therefore, the present embodiment has an advantage that a relatively stable measurement value can be obtained without being easily influenced by the S concentration of the fuel.

This difference is due to the fact that the integration end timing of this embodiment is earlier than the integration end timing of the basic method. That is, as shown in (B), in the integration end timing of this embodiment, the difference in the integration end timing due to the difference in S concentration is a relatively short time between t6 and t4. On the other hand, at the integration end timing of the basic method, the difference in the integration end timing due to the difference in S concentration becomes a relatively long time between t7 and t5. The reason for this is that the rate of change of the post-catalyst sensor output Vr after the integration end timing of the present embodiment varies depending on the S concentration, and the later the difference in the post-catalyst sensor output Vr increases. Therefore, there is a large difference between the two timings at the integration end timing of the basic method, and a large difference also occurs in the measured values obtained as a result.

Next, FIG. 11 is a diagram for illustrating the influence of the response variation of the post-catalyst sensor 18 on the measured value. In the figure, (A) shows the pre-catalyst air-fuel ratio A / Ff and post-catalyst air-fuel ratio A / Fr, (B) shows the integrated value of the stored oxygen amount OSCa, and (C) shows the post-catalyst sensor output. Vr is shown.

In (A), the area of the portion I sandwiched between the pre-catalyst air-fuel ratio A / Ff and the post-catalyst air-fuel ratio A / Fr represents the true stored oxygen amount OSCa of the catalyst. On the other hand, the area of the portion sandwiched between the pre-catalyst air-fuel ratio A / Ff and the stoichiometry represents the actually stored stored oxygen amount OSCa, and the area of the portion II sandwiched between the post-catalyst air-fuel ratio A / Fr and the stoichiometry. Represents an error in the measured value. (B) shows integrated values I ′ and II ′ corresponding to the portions I and II. As shown in the figure, the ratio of the error to the measured value increases as the later stage of the lean control.

In (C), a indicates the case of a post-catalyst sensor (hereinafter referred to as a reference sensor) whose response is an intermediate reference. On the other hand, b shows the case of a post-catalyst sensor (hereinafter referred to as a high response sensor) whose response is faster than the reference sensor due to secular change, and c is a catalyst whose response is slower than the reference sensor due to secular change. The case of a rear sensor (hereinafter referred to as a low response sensor) is shown.

In the basic method in which the integration is continued until the post-catalyst sensor output Vr reaches the lean determination value VL, the integration end timing is over a relatively long period from t3 to t4 due to the response variation of the post-catalyst sensor. It varies. This is because, as described above, the difference in the post-catalyst sensor output Vr becomes larger due to the difference in the change rate of the post-catalyst sensor. Therefore, as shown in (B), the variation of the final integrated value increases as shown by d.

However, in the present embodiment in which the integration ends at a timing earlier than the basic method, the variation in the integration end timing is reduced to a relatively short period from t1 to t2. This is because the integration ends before the difference in the post-catalyst sensor output Vr increases. Therefore, as shown in (B), the variation of the final integrated value can also be reduced as shown by e. In addition, the proportion of error included in these final integrated values is very small, and a highly accurate final integrated value can be obtained.

FIG. 12 shows the relationship between the post-catalyst sensor output Vr value (horizontal axis), which is the integration end timing, and the final integrated value (vertical axis) of the stored oxygen amount OSCa. As shown in the figure, the final integrated value tends to be larger as the post-catalyst sensor output Vr is decreased toward the lean side.

In the figure, line a is reference data when a normal catalyst, a reference sensor, and low S fuel are used. On the other hand, the line b is lower limit data when the normal catalyst, the high response sensor, and the high S fuel are used, that is, data when the sensor response and the fuel S concentration vary so that the final integrated value becomes the smallest. It is. A fuel having an S concentration of 30 ppm is used as the low S fuel, and a fuel having an S concentration of 200 ppm is used as the high S fuel.

Similarly, line c is reference data when an abnormal catalyst, a reference sensor, and low S fuel are used. On the other hand, the line d is upper limit data when the abnormal catalyst, the low response sensor, and the low S fuel are used, that is, when the sensor response and the fuel S concentration vary so that the final integrated value becomes the largest. It is data.

The difference between the lower limit data b of the normal catalyst and the upper limit data d of the abnormal catalyst when the integration end timing is set to the lean determination value VL as in the basic method is indicated by e. On the other hand, the difference between the lower limit data b of the normal catalyst and the upper limit data d of the abnormal catalyst when the integration end timing is set to the stoichiometric determination value VA as in this embodiment is indicated by f.

In the case of the basic method, since the original final integrated value is large, the ratio or ratio of the difference e with respect to the final integrated value is small. Therefore, in consideration of variations in sensor responsiveness and fuel S concentration, the difference between the final integrated values between normal and abnormal catalysts is substantially reduced, making it difficult to distinguish both.

On the other hand, in the case of this embodiment, the magnitude of the difference f itself is not much different from the difference e of the basic method, but since the original final integrated value is small, the ratio or ratio of the difference f with respect to the final integrated value is large. . Therefore, when taking into account variations in sensor responsiveness and fuel S concentration, the difference between the final integrated values between the normal and abnormal catalysts can be substantially enlarged, and both can be easily identified and the resolution can be improved.

Thus, according to the present embodiment, accurate diagnosis can be executed even when the post-catalyst sensor and the fuel properties vary.

By the way, in the present embodiment, the following processing is executed for further accuracy improvement.

First, the lean determination value VL and the rich determination value VR are changed according to the intake air amount Ga detected by the air flow meter 5.

That is, as shown in FIG. 5, the hysteresis width c, which is the width between the hysteresis characteristic line a when changing to the lean side and the hysteresis characteristic line b when changing to the rich side, is supplied to the post-catalyst sensor 18. It changes in accordance with the flow rate of the gas, and consequently the intake air amount Ga that is the substitute value, and tends to increase as the intake air amount Ga increases. Then, according to the intake air amount Ga, the air-fuel ratio corresponding to the lean determination value VL and the rich determination value VR changes, and the value of the air-fuel ratio for switching the air-fuel ratio in the active air-fuel ratio control changes.

Therefore, the lean determination value VL and the rich determination value VR are changed according to the intake air amount Ga in order to compensate for the change in the air-fuel ratio.

As shown in FIG. 13, for example, when the intake air amount Ga is a predetermined reference value, it is assumed that the hysteresis characteristic line when changing to the lean side is a ₁ and the lean determination value is VL ₁ . When the intake air amount Ga increases than the reference hysteresis characteristic line is changed to a _2. Therefore, the lean determination value is changed to a larger (rich side) VL ₂ so that switching is performed at the same air-fuel ratio at this time as well. Such a change is performed using a map or the like stored in advance in the ECU 20.

It will be understood that when the intake air amount Ga decreases from the reference value, the lean judgment value is changed to a smaller value (on the lean side). Thus, the lean determination value is changed or corrected so that the value of the air-fuel ratio at which the air-fuel ratio is switched is always the value when the intake air amount Ga is the reference value.

Although not shown, the same change is made for the rich determination value VR. When the intake air amount Ga increases from the reference value, the rich determination value is changed to a smaller value, and the intake air amount Ga decreases from the reference value. Sometimes the lean determination value is changed to a larger value, and the rich determination value is changed or corrected so that the value of the air-fuel ratio at which the air-fuel ratio is switched is always the value when the intake air amount Ga is the reference value.

Next, in this embodiment, a delay process for switching the air-fuel ratio is performed later than the timing when the post-catalyst sensor output Vr reaches the lean determination value VL or the rich determination value VR. That is, the air-fuel ratio is switched after a predetermined delay time has elapsed since the post-catalyst sensor output Vr reaches the lean determination value VL or the rich determination value VR. The delay time is changed according to the intake air amount Ga.

Fig. 14 shows an example of delay processing. First, paying attention to the solid line, the post-catalyst sensor output Vr reaches the rich determination value VR at time t1, and the target air-fuel ratio A / Ft changes from the rich air-fuel ratio A / Fr (for example, 14.1) to the lean air-fuel ratio A / Fl ( For example, the control is switched to 15.1) and the lean control is started. Thereafter, the post-catalyst sensor output Vr reaches the lean determination value VL at time t2, the target air-fuel ratio A / Ft is switched from the lean air-fuel ratio A / Fl to the rich air-fuel ratio A / Fr, and rich control is started.

In the latter period of lean control, the lean gas starts to leak from the catalyst around time t11, and the value of the post-catalyst air-fuel ratio A / Fr starts to increase from the stoichiometric value toward the lean air-fuel ratio A / Fl. When the post-catalyst sensor output Vr reaches the lean determination value VL, the post-catalyst air-fuel ratio A / Fr has sufficiently reached the vicinity of the lean air-fuel ratio A / Fl, and it can be considered that the catalyst has completely occluded oxygen. .

However, as indicated by the alternate long and short dash line, when the responsiveness of the post-catalyst sensor 18 becomes faster due to aging, the post-catalyst sensor output Vr reaches the lean determination value VL at an earlier timing, and the air-fuel ratio becomes the rich air-fuel ratio A / Fr. Can be switched. Then, even though the post-catalyst air-fuel ratio A / Fr has not yet reached the vicinity of the lean air-fuel ratio A / F1, that is, oxygen is not completely occluded in the catalyst, the air-fuel ratio switching occurs. The rich control is started before the desired initial state for the rich control is achieved. Therefore, during rich control, an oxygen amount with a smaller value than the original value may be measured.

Therefore, in order to solve this problem, in this embodiment, the air-fuel ratio is switched after a predetermined delay time has elapsed since the post-catalyst sensor output Vr reaches the lean determination value VL or the rich determination value VR. By setting such a delay time, it is possible to create a desired initial state in which oxygen is completely stored in the catalyst or oxygen is completely released from the catalyst before the start of rich control or lean control. While suppressing the influence of the responsiveness variation of the post-catalyst sensor 18, it is possible to improve the measurement accuracy of the oxygen amount.

The delay time is increased as the intake air amount Ga is smaller. This is because the smaller the intake air amount Ga, the lower the gas flow rate with respect to the catalyst, the lowering the oxygen storage rate or release rate, and it takes time to create a complete storage state or complete release state. The change of the delay time is performed using a map or the like stored in advance in the ECU 20.

Note that the delay process can be omitted, and in this case, the air-fuel ratio is switched at the same time when the post-catalyst sensor output Vr reaches the lean determination value VL or the rich determination value VR. In any case, the air-fuel ratio is performed in synchronization with the post-catalyst sensor output Vr reaching the lean determination value VL or the rich determination value VR.

The above description is mainly for the case where the air-fuel ratio is switched to the lean side, that is, the case of lean control, but it will be understood that the same explanation applies to the reverse case, that is, the case of rich control. .

[Abnormality diagnosis processing of this embodiment]
Next, the abnormality diagnosis process of the present embodiment executed by the ECU 20 will be described with reference to FIGS. 15A and 15B.

In step S101, it is determined whether or not the diagnosis permission flag is on. The diagnosis permission flag is a flag that is turned on when a precondition for diagnosis execution is satisfied. The precondition is satisfied when the following conditions are satisfied.
(1) The upstream catalyst 11 is activated.
(2) The pre-catalyst sensor 17 and the post-catalyst sensor 18 are activated.
(3) The engine is in steady operation.
(4) The diagnosis is incomplete during the current trip.

Condition (1) is established when the catalyst temperature Tc of the upstream catalyst 11 is within a predetermined activation temperature range. The catalyst temperature Tc is estimated by the ECU 20 based on the engine operating state, but may be detected directly by a temperature sensor.

Condition (2) is satisfied when the temperatures of the detection elements of the pre-catalyst sensor 17 and the post-catalyst sensor 18 estimated by the ECU 20 are within a predetermined activation temperature range.

Condition (3) is when the engine speed calculated based on the output of the crank angle sensor 14 and the fluctuation range of the intake air amount Ga detected by the air flow meter 5 within a predetermined time are within a predetermined value. Is established.

Regarding condition (4), the trip means the period from one start to stop of the engine. In this embodiment, the diagnosis is executed once per trip, and (4) is established when the diagnosis has not been completed once during the current trip.

If the diagnosis permission flag is not on (if it is off), it enters a standby state. On the other hand, when the diagnosis permission flag is on, the first target air-fuel ratio (A / Ft) of the active air-fuel ratio control is set in steps S102 to S104.

In step S102, it is determined whether the post-catalyst sensor output Vr is equal to or greater than the stoichiometric equivalent value Vst. As shown in FIG. 5, the stoichiometric equivalent value Vst refers to the value of the post-catalyst sensor output Vr corresponding to the stoichiometry on a single characteristic line indicated by a solid line.

If the post-catalyst sensor output Vr is greater than or equal to the stoichiometric equivalent value Vst, the current post-catalyst gas is considered to be substantially rich, and the process proceeds to step S103 where the initial target air-fuel ratio A / Ft is set to the lean air-fuel ratio. The As a result, lean control is executed.

On the other hand, if the post-catalyst sensor output Vr is not equal to or greater than the stoichiometric equivalent value Vst, it is assumed that the current post-catalyst gas is substantially lean, and the process proceeds to step S104 where the initial target air-fuel ratio A / Ft is set to the rich air-fuel ratio. Is done. Thus, rich control is executed.

Thus, the active air-fuel ratio control is started from the air-fuel ratio opposite to the air-fuel ratio of the current post-catalyst gas. However, the first lean control or rich control is a so-called deserted mountain where no oxygen amount measurement is performed. By performing such empty control and starting the measurement after completely storing or releasing oxygen, the initial condition can be made constant and the measurement accuracy can be improved.

Next, in step S105, a lean determination value VL and a rich determination value VR are calculated based on the detected intake air amount Ga. This calculation is performed according to a predetermined map as described above. As the intake air amount Ga increases, a larger lean determination value VL is calculated, and a smaller rich determination value VR is calculated.

Next, in step S106, the delay time D is calculated based on the detected intake air amount Ga. This calculation is performed according to a predetermined map as shown in FIG. 16, and the larger the delay time D is, the smaller the intake air amount Ga is.

Next, in step S107, it is determined whether the current target air-fuel ratio A / Ft is a rich air-fuel ratio, that is, whether rich control is being executed. If the target air-fuel ratio A / Ft is a rich air-fuel ratio, the process proceeds to step S121. If the target air-fuel ratio A / Ft is not a rich air-fuel ratio (in the case of a lean air-fuel ratio), the process proceeds to step S108.

In step S108, it is determined whether or not the post-catalyst sensor output Vr is equal to or less than the lean determination value VL, that is, whether or not the post-catalyst sensor output Vr is reversed to the lean side. If the after-catalyst sensor output Vr is not less than or equal to the lean determination value VL, the standby state is entered.

In step S109, the time from when the post-catalyst sensor output Vr first becomes equal to or less than the lean determination value VL is counted, and whether or not this time is equal to or greater than the delay time D is determined. If no, the process enters a standby state. If yes, the process proceeds to step S110.

In step S110, the target air-fuel ratio A / Ft is set to the rich air-fuel ratio, and rich control is started.

After the rich control is started, it is determined in step S111 whether or not the pre-catalyst air-fuel ratio A / Ff detected by the pre-catalyst sensor 17 is less than the stoichiometric value. If not, the process returns to step S110 to enter a standby state, and if so, the process proceeds to step S112, and the oxygen storage capacity OSC, here, the released oxygen amount OSCb is integrated and measured.

Next, in step S113, whether or not the post-catalyst sensor output Vr that is changing to the rich side exceeds the stoichiometric determination value VB, that is, whether or not the post-catalyst air-fuel ratio starts to change from stoichiometric to the rich air-fuel ratio. To be judged. If not, the process returns to step S110. If it exceeds, the process proceeds to step S114, and the integration of the released oxygen amount OSCb is terminated.

In step S115, it is determined whether or not both the stored oxygen amount OSCa and the released oxygen amount OSCb have been measured. If not measured, the process proceeds to step S121, and if measured, the process proceeds to step S116.

If the measurement has not been completed, in steps S121 to S128, processing in which rich and lean are reversed from steps S108 to S115 is performed.

That is, in step S121, it is determined whether or not the post-catalyst sensor output Vr is greater than or equal to the rich determination value VR, that is, whether or not the post-catalyst sensor output Vr is reversed to the rich side. If not, the process enters a standby state. If the post-catalyst sensor output Vr is equal to or greater than the rich determination value VR, the process proceeds to step S122.

In step S122, the time from when the post-catalyst sensor output Vr first becomes equal to or greater than the rich determination value VR is counted, and it is determined whether or not this time is equal to or greater than the delay time D. If no, the process enters a standby state. If yes, the process proceeds to step S123.

In step S123, the target air-fuel ratio A / Ft is set to a lean air-fuel ratio, and lean control is started.

After starting the lean control, in step S124, it is determined whether or not the pre-catalyst air-fuel ratio A / Ff detected by the pre-catalyst sensor 17 is larger than the stoichiometric value. If not, the process returns to step S123 to enter a standby state, and if so, the process proceeds to step S125, and the oxygen storage capacity OSC, here the stored oxygen amount OSCa, is integrated and measured.

Next, in step S126, it is determined whether or not the post-catalyst sensor output Vr that is changing to the lean side is lower than the stoichiometric determination value VA, that is, whether or not the post-catalyst air-fuel ratio starts to change from stoichiometric to the lean air-fuel ratio. To be judged. If not, the process returns to step S123. If it is less, the process proceeds to step S127, and the accumulation of the stored oxygen amount OSCa is terminated.

In step S127, it is determined whether or not both the stored oxygen amount OSCa and the released oxygen amount OSCb have been measured. If not measured, the process proceeds to step S108, and if measured, the process proceeds to step S116.

In step S116, the value of the oxygen storage capacity OSC is calculated based on the measured stored oxygen amount OSCa and the released oxygen amount OSCb. That is, the value of the oxygen storage capacity OSC is calculated by dividing the sum of the stored oxygen amount OSCa and the released oxygen amount OSCb by 2 and performing simple averaging (OSC = (OSCa + OSCb) / 2).

Next, in step S117, the calculated value of the oxygen storage capacity OSC is compared with a predetermined abnormality determination value α. If OSC> α, the upstream catalyst 11 is determined to be normal in step S119, and the process proceeds to step S120. If OSC ≦ α, the upstream catalyst 11 is determined to be abnormal in step S118, and the process proceeds to step S120.

In step S120, the diagnosis permission flag is turned off, thereby ending the diagnosis process.

As mentioned above, although the embodiment of the present invention has been described in detail, various other embodiments of the present invention are conceivable. For example, the use and type of the internal combustion engine are arbitrary, and may be other than for automobiles, or may be a direct injection type or the like. In the above description, only one of the lean side and the rich side or the occlusion side and the discharge side has been described. However, it will be apparent to those skilled in the art that the description of one of the two can also be understood.

In the example of the diagnosis process shown in FIGS. 15A and 15B, the diagnosis was performed by measuring the stored oxygen amount OSCa and the released oxygen amount OSCb once for simplification. However, of course, in order to improve accuracy, the lean control and the rich control are alternately executed repeatedly, the stored oxygen amount OSCa and the released oxygen amount OSCb are measured a plurality of times, and the average value is calculated. Based on the average value, A diagnosis may be made. Further, the diagnosis may be performed based on only one measurement value of the stored oxygen amount OSCa and the released oxygen amount OSCb.

The present invention includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

Claims (13)

1. An apparatus for diagnosing abnormality of a catalyst disposed in an exhaust passage of an internal combustion engine,
   This is a post-catalyst sensor that detects the exhaust air / fuel ratio downstream of the catalyst, and the output changes suddenly at the stoichiometric boundary, and when the exhaust air / fuel ratio changes from the rich side to the lean side with respect to the stoichiometry, and changes from the lean side to the rich side A post-catalyst sensor having a hysteresis characteristic in which the output characteristic near the stoichiometry differs depending on
   Active air-fuel ratio control for executing active air-fuel ratio control for switching the air-fuel ratio upstream of the catalyst from one of the lean air-fuel ratio and the rich air-fuel ratio in synchronization with the output of the post-catalyst sensor reaching a predetermined determination value Means,
   Measuring means for integrating and measuring the amount of oxygen stored or released by the catalyst during execution of the active air-fuel ratio control;
   Determination means for determining whether the catalyst is normal or abnormal based on the amount of oxygen measured by the measurement means;
   With
   During the control in which the air-fuel ratio on the upstream side of the catalyst is set to one of the lean air-fuel ratio and the rich air-fuel ratio, the output of the post-catalyst sensor changes the exhaust air-fuel ratio on the downstream side of the catalyst from the stoichiometric direction toward the one. The catalyst abnormality diagnosis device is characterized in that the integrated measurement of the oxygen amount is terminated when the value reaches a value indicating that the operation has started.
2. The determination value includes a lean determination value that defines the switching timing from the lean air-fuel ratio to the rich air-fuel ratio, and a rich determination value that defines the switching timing from the rich air-fuel ratio to the lean air-fuel ratio,
   During the lean control in which the air-fuel ratio on the upstream side of the catalyst is set to the lean air-fuel ratio, the measuring means is configured so that the output of the post-catalyst sensor reaches the lean side from the predetermined stoichiometric determination value before reaching the lean determination value. The catalyst abnormality diagnosis device according to claim 1, wherein when the change starts, the integrated measurement of the oxygen amount is terminated.
3. The catalyst abnormality diagnosis device according to claim 2, wherein the stoichiometric determination value is a value corresponding to stoichiometry on a hysteresis characteristic line when the post-catalyst sensor output changes from a rich side to a lean side.
4. The catalyst abnormality diagnosis device according to claim 2 or 3, wherein the stoichiometric determination value is a value on a richer side than the rich determination value.
5. The catalyst abnormality diagnosis device according to claim 2, wherein the stoichiometric determination value is equal to the rich determination value.
6. The stoichiometric determination value is a value that is leaner than the rich determination value, and is a value that is richer than a stoichiometric equivalent value on a single characteristic line of the post-catalyst sensor output. Or the catalyst abnormality diagnosis device according to 3.
7. The determination value includes a lean determination value that defines the switching timing from the lean air-fuel ratio to the rich air-fuel ratio, and a rich determination value that defines the switching timing from the rich air-fuel ratio to the lean air-fuel ratio,
   During the rich control in which the air-fuel ratio on the upstream side of the catalyst is set to the rich air-fuel ratio, the measurement means is configured to output the post-catalyst sensor before reaching the rich determination value and from the predetermined stoichiometric determination value toward the rich side. The catalyst abnormality diagnosis device according to any one of claims 1 to 6, wherein when the change starts, the integrated measurement of the oxygen amount is terminated.
8. The catalyst abnormality diagnosis device according to claim 7, wherein the stoichiometric rich determination value is a value corresponding to stoichiometry on a hysteresis characteristic line when the post-catalyst sensor output changes from a lean side to a rich side.
9. The catalyst abnormality diagnosis device according to claim 7 or 8, wherein the stoichiometric determination value is a value on a lean side with respect to the lean determination value.
10. The catalyst abnormality diagnosis device according to claim 7 or 8, wherein the stoichiometric determination value is equal to the lean determination value.
11. The stoichiometric rich determination value is a value on the rich side of the lean determination value, and is a value on the lean side of a stoichiometric equivalent value on a single characteristic line of the post-catalyst sensor output. Or the catalyst abnormality diagnosis device according to 8.
12. The catalyst abnormality diagnosis device according to any one of claims 1 to 11, wherein the active air-fuel ratio control means changes the determination value according to an intake air amount.
13. The active air-fuel ratio control means switches the air-fuel ratio after a predetermined delay time has elapsed since the output of the post-catalyst sensor reaches the determination value, and changes the delay time according to the intake air amount. The catalyst abnormality diagnosis device according to any one of claims 1 to 12.

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