JPH1047141A - Judging device for deterioration of catalyst for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the aggravation of exhaust gas during judging deterioration by accurately detecting the minute deterioration of a catalyst with controlled cost-up, and carrying out the feedback control of an air-fuel ratio at the same time as the deterioration judgement of the catalyst. SOLUTION: In the case where dither is added into the feed back control of an air-fuel ratio, in the catalyst of normal level, when a sensor output on the downstream of the catalyst is 1.0Hz in the fluctuation frequency of air-fuel ratio fluctuation on the upstream of the catalyst, the sensor output begins to vary toward a lean side, and in the case where the fluctuation frequency is less than 0.5Hz and varys largely, in the catalyst of deterioration level, a sensor output on the downstream of the catalyst begins to vary toward a lean side under such conditions as the variation frequency of 3.0Hz, and fluctuation width of &p1.0(A/F), and in the case where the sensor output is less than 2.0Hz, the sensor output on downstream side largely varys toward the lean side. Thus even the slight deterioration of the catalyst can be accurately judged by paying attention to the above a large fluctuation to very the setting of the fluctuation frequency of the air-fuel ratio fluctuation on the upstream of the catalyst (in the above example, setting up as 2.0Hz).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst deterioration judging device for judging deterioration of an exhaust gas purifying catalyst disposed in an exhaust system of an engine.

[0002]

2. Description of the Related Art Conventionally, as a method for judging the deterioration of a catalyst in an engine exhaust system, an air-fuel ratio sensor is provided upstream of the catalyst, and an air-fuel ratio sensor is also provided downstream. The air-fuel ratio of the engine is feedback-controlled based on the output.At this time, the target air-fuel ratio is changed so that the output of the upstream sensor is inverted at a fixed cycle and repeats the rich side and the lean side, and the fluctuation of the output of the downstream sensor is reduced. Measure and, for example, compare the number of reversals of the upstream sensor output with the number of reversals of the downstream sensor output.If the ratio of the number of reversals of the downstream sensor output to the number of reversals of the upstream sensor output exceeds a predetermined level, the catalyst deteriorates. Is determined. Japanese Unexamined Patent Publication No. 7-259613 is an example of a method for determining the deterioration of a catalyst by this method.

[0003] exhaust gas purifying catalyst occludes excess oxygen when the air-fuel ratio lean, there is a so-called O ₂ storage effect of releasing the oxygen when the air-fuel ratio rich, the oxygen released by the O ₂ storage effect HC (hydrocarbon)
And purifies CO (carbon monoxide). When the catalyst is not degraded, the O ₂ storage effect is large and the oxygen storage capacity is sufficient. The fluctuation in the amount is absorbed, the fluctuation in the air-fuel ratio on the catalyst is reduced, and the output of the downstream sensor does not fluctuate while sticking to the rich side. And
When the catalyst is deteriorated and the O ₂ storage effect is reduced and the purification performance is reduced, the fluctuation in the supplied oxygen amount cannot be absorbed, and the output of the downstream sensor also fluctuates and periodically reverses. The above-described conventional method for determining catalyst deterioration utilizes this phenomenon.

[0004] Further, the control for changing the target air-fuel ratio of the feedback control at a constant cycle around the stoichiometric air-fuel ratio is as follows.
This is called dither control, and as shown in JP-A-64-56936, the purification of the catalyst can be promoted by causing an oxygen component to be periodically present in the exhaust gas.

[0005] Separately, Japanese Unexamined Patent Publication No. Hei.
Japanese Patent Publication No. 38 discloses that the catalyst deterioration is determined based on the time from when the fuel supply is restarted after the deceleration fuel cut to when the output of the oxygen concentration sensor downstream of the catalyst reaches a predetermined level on the rich side. Have been.

[0006]

The normal level of the purification rate of the catalyst is, for example, 83.3% (HC emission: 0.250 g /
Mile), the failure level is set to HC emission 1.
75.0% equivalent to 5 times (HC emission: 0.375 g / m
ile), since the difference between the normal level and the failure level is large, it is difficult to determine the deterioration of the catalyst by monitoring the output of the downstream sensor under normal air-fuel ratio feedback control as in the past. Was not. However, the setting of the normal level of the purification rate of the catalyst is set to, for example, 97.3.
% (HC emission: 0.040 g / mile)
The failure level was 96.0, which is also equivalent to 1.5 times the HC emission.
% (HC emission: 0.060 g / mile), the difference between the normal level and the failure level is as small as 1.3% (0.020 g / mile HC emission). It is difficult to detect a small difference in a normal air-fuel ratio feedback control state. The method described in JP-A-2-136538 is not suitable for detecting such minute deterioration.

Therefore, for example, the catalyst device is made up of two containers, an inlet side and an outlet side, and a catalyst that easily deteriorates is arranged at the inlet side, and the deterioration determination is made by observing the state of deterioration of the more easily deteriorated catalyst. By taking advantage of the fact that the difference in purification rate due to deterioration of the catalyst is larger in cold conditions than in warm conditions, the degree of startup when the catalyst temperature rises and activates after cold start A method of visually judging the deterioration state has been considered. However, the method of arranging easily deteriorating catalysts on the inlet side not only involves a significant cost increase due to the change of the current system, but also the purification performance is not sufficient because the catalyst on the inlet side cannot be increased in terms of detectability. In addition, there is a problem that the detection cannot be performed in a high load region where the gas volume is large, so that the effectiveness of the detection is reduced. In addition, the method of detecting deterioration in the cold region has many variations, and even if it can be overcome, a temperature sensor for determining the temperature condition is required, and the downstream of the catalyst is faster than the catalyst in the cold region. etc. it is necessary to power up the heater to energize the O ₂ sensor, in which also involves a significant cost.

Therefore, it is an object of the present invention to accurately detect minute catalyst deterioration while suppressing an increase in cost.

[0009]

A conventional method for judging deterioration of a catalyst by monitoring a change in sensor output downstream of the catalyst in a normal air-fuel ratio feedback control state or a state in which normal dither control is added to the air-fuel ratio feedback control. Then, as described above, it is difficult to determine minute deterioration. However, even in the case of minute deterioration, if the fluctuation period (or fluctuation width) of the air-fuel ratio fluctuation by dither control is made larger and larger than the fluctuation period of the air-fuel ratio in the state of the normal air-fuel ratio feedback control as described above. When the change in oxygen on the catalyst becomes larger than the oxygen storage capacity of the catalyst, the sensor output downstream of the catalyst largely fluctuates to the lean side. The present invention has found this fact and has paid attention to it, so that instead of performing the catalyst deterioration determination in the state of the air-fuel ratio fluctuation due to the conventional dither control at the time of the normal air-fuel ratio feedback control, the sensor of the deterioration determination sensor is used. A fluctuation cycle (or fluctuation width) at which a large fluctuation appears in the output is obtained in advance, and when determining the deterioration of the catalyst, dither control is performed so that the air-fuel ratio fluctuation at the predetermined fluctuation cycle (or fluctuation width) is obtained. Thus, it is possible to determine a minute deterioration.

Specifically, when the catalyst deterioration is determined, the air-fuel ratio is periodically changed (with a change in the amount of oxygen stored in the catalyst), and the state quantity related to the oxygen storage capacity of the catalyst is preferably changed. The state quantity is detected by the output of an oxygen concentration sensor disposed downstream, and based on the change characteristic of the state quantity according to the change of the characteristic value of the air-fuel ratio fluctuation, the state quantity is set to a certain level (preferably, the downstream side corresponding to an excess air ratio of 1). The characteristic value (at least one of the fluctuation width and the fluctuation period) of the air-fuel ratio fluctuation by the dither control when the fluctuation exceeds the sensor output) is set as the fluctuation characteristic value at the time of the judgment, and the judgment according to the certain level is made. A threshold value is set, and the deterioration of the catalyst is determined by comparing the detected value of the state quantity when the variation due to the variation characteristic value at the determination time is given with the determination threshold value.

At this time, the fluctuation characteristic value at the time of judgment is preferably set on condition that the state quantity shows a change exceeding a certain level over a predetermined period, so that the reliability of the deterioration judgment is improved. Increase.

Further, when the amount of exhaust gas changes and the gas volume per unit cycle changes, the change in oxygen on the catalyst due to the air-fuel ratio fluctuation exceeds the oxygen storage capacity of the catalyst, causing fluctuations in the sensor output downstream of the catalyst. The limit and the magnitude of the variation vary, and therefore, the variation characteristic value at the time of determination and the determination threshold value should be corrected according to the exhaust gas amount. Air volume)
It is necessary to reduce the fluctuation width of the air-fuel ratio fluctuation at the time of the deterioration judgment, reduce the fluctuation period, or reduce both the fluctuation width and the fluctuation period, and increase the determination threshold value toward the lean side in accordance with the increase in the deterioration. Good.

Since the limit at which the sensor output downstream of the catalyst fluctuates and the magnitude of the fluctuation also vary with the catalyst temperature, the fluctuation characteristic value at the time of judgment and the judgment threshold value should be corrected in accordance with the catalyst temperature. Good. Specifically, when the catalyst temperature rises, the purification performance of the catalyst improves, and it becomes difficult for the sensor output downstream of the catalyst to fluctuate. Therefore, the fluctuation width of the air-fuel ratio fluctuation at the time of the deterioration determination according to the catalyst temperature rises. , The fluctuation period is increased, or both the fluctuation width and the fluctuation period are increased, and the determination threshold value is preferably decreased.

[0014] Further, at the time of transition when the catalyst temperature greatly changes, the oxygen storage capacity of the catalyst is unstable and erroneous determination of deterioration is likely to occur. Therefore, it is preferable to prohibit the deterioration determination for a predetermined period after the transition.

It is preferable to arrange a linear air-fuel ratio sensor upstream of the catalyst and perform air-fuel ratio feedback control based on the output of the linear air-fuel ratio sensor. In this case, the air-fuel ratio detected by the linear air-fuel ratio sensor is preferably used. The air-fuel ratio feedback control is performed simultaneously with the deterioration judgment by changing the target air-fuel ratio with a fluctuation width or a fluctuation cycle corresponding to the fluctuation characteristic value at the time of judgment so that the rich side and the lean side are repeated with the stoichiometric air-fuel ratio in between. Exhaust gas deterioration during the deterioration determination can be prevented.

The air-fuel ratio is feedback-controlled so that the air-fuel ratio detected by the linear air-fuel ratio sensor is made to coincide with a preset target air-fuel ratio which fluctuates at a predetermined cycle and a predetermined amplitude on a rich side and a lean side with respect to a stoichiometric air-fuel ratio. In the catalyst deterioration determination device of the engine, the fluctuation cycle of the upstream air-fuel ratio fluctuation that causes a large fluctuation in the sensor output downstream of the catalyst in the dither control state is the reverse of the rich / lean in the normal air-fuel ratio feedback control. Is substantially longer than the cycle of the rich / lean inversion in the steady running state, for example, the cycle when the frequency is about 16 Hz.

[0017]

FIG. 1 shows an overall engine system to which the present invention is applied. In the figure, reference numeral 1 denotes a spark ignition type engine. In the intake passage 2 of the engine 1, an air cleaner 3, a vane type air flow meter 4 for detecting the amount of intake air, a throttle valve 5, a surge tank 5 for absorbing the pulsation of intake air, and an intake vacuum A boost sensor 7 for detecting pressure (boost) and a fuel injection valve 8 are sequentially arranged. Then, the air-fuel mixture is supplied to the combustion chamber 10 via the intake valve 9. Further, exhaust gas of the engine 1 is discharged to an exhaust passage 12 via an exhaust valve 11. A catalytic converter 13 for purifying exhaust gas is disposed in the exhaust passage 12, and a linear O ₂ sensor 14 whose sensor output changes linearly according to the oxygen concentration in the exhaust gas is provided upstream of the catalytic converter 13. However, on the downstream side of the catalytic converter 13, there is provided a λO ₂ sensor 15 whose output changes suddenly when λ (excess air ratio) = 1.

The intake passage 2 is provided with a bypass passage 16 for supplying bypass air bypassing the throttle valve 5 during idling operation. In the middle of the bypass passage 16, the bypass air amount is adjusted. Duty solenoid valve 1 for controlling engine speed during idling
7 are provided.

The engine 1 is provided with a spark plug 18 for each cylinder. The igniter 19 generates a high voltage required for ignition, and the igniter 19 is linked to a crankshaft (not shown). A distributor 20 for distributing the generated high voltage to the spark plug 18 of each cylinder is provided. The distributor 20 is provided with an engine rotation sensor 21 that generates a pulse signal according to the rotation of the crankshaft. The throttle valve 12 is provided with an idle switch 22 for detecting that the throttle is fully closed. Further, an intake air temperature sensor 23 for detecting the temperature of the intake air is provided, and a water temperature sensor 24 for detecting the temperature of the engine cooling water is provided.

The engine 1 has a control unit 25 composed of a microcomputer. The control unit 25 includes an engine rotation sensor 21, an air flow sensor 4, an intake air temperature sensor 23, an idle switch 22, a water temperature sensor 24, a linear O ₂ sensor 14 and a λO ₂ sensor 15, and a vehicle speed sensor 2.
6, various signals output from the catalyst temperature sensor 27 and the like are input. The control unit 25 controls the fuel injection valve 8 based on these signals, and controls the duty solenoid valve 17 in the bypass passage 16.

In the control of the fuel injection valve 8, a basic injection amount is calculated based on the engine speed and the intake air amount, and various corrections such as a water temperature correction are added thereto. Further, in an air-fuel ratio feedback region set in advance according to the engine operating state, air-fuel ratio feedback correction is added to the fuel injection amount so that the air-fuel ratio detected by the linear O ₂ sensor 14 upstream of the catalyst matches the target air-fuel ratio. Then, the fuel injection valve 8 is driven by the injection pulse corresponding to the set final injection amount, and the air-fuel ratio is controlled. At this time, dither control for changing the target air-fuel ratio of the air-fuel ratio feedback control between the rich side and the lean side at a constant cycle of about 16 Hz with the frequency centering on the stoichiometric air-fuel ratio is added.

After the engine is warmed up, the control unit 25 operates when the fluctuations in the engine speed and the load are small and the catalyst is in an appropriate temperature range in a deterioration determination region predetermined by the vehicle speed, the engine speed and the load. Next, control of catalyst deterioration determination is performed.

In the control of the catalyst deterioration determination, while the air-fuel ratio feedback control by the linear O ₂ sensor 14 upstream of the catalyst is being executed, the fluctuation of the air-fuel ratio detected by the linear O ₂ sensor 14 is the same as the normal air-fuel ratio feedback control. By changing the dither control so that the fluctuation width (amplitude) and the fluctuation period are larger than those of the dither control, the output of the λO ₂ sensor 15 downstream of the catalyst is changed to the lean side even if the catalyst is slightly deteriorated. To do.

The relationship between the change in the air-fuel ratio behavior upstream of the catalyst due to the change in the dither control and the sensor output of the λO ₂ sensor 15 downstream of the catalyst is, for example, as shown in FIG. In the example of FIG. 2, the normal level is 0.035 g / mi in terms of HC emission.
le, the deterioration determination level is 0.10 g / HC
Mile catalyst. (A) shows the change in the sensor output downstream of the catalyst when the fluctuation width and the fluctuation frequency of the air-fuel ratio fluctuation upstream of the catalyst are changed with respect to the catalyst at the deterioration determination level (HC emission amount: 0.035 g / mile). Show.
(B) indicates a normal level (discharge amount: 0.035 g /
7 shows changes in the sensor output downstream of the catalyst when the fluctuation width and the fluctuation frequency of the air-fuel ratio fluctuation upstream of the catalyst are changed with respect to the catalyst in the range of FIG. In the case of this example, the fluctuation frequency of the air-fuel ratio fluctuation upstream of the catalyst is 4.0 for the catalyst at the deterioration determination level.
When the fluctuation width is about ± 0.5 Hz, no noticeable fluctuation appears in the sensor output downstream of the catalyst, but the fluctuation frequency of the air-fuel ratio fluctuation upstream of the catalyst is 3.0 Hz and the fluctuation width is ± 1.0 (A / F). )
In the degree, the sensor output downstream of the catalyst has started to fluctuate to the lean side. When the fluctuation frequency is 2.0 Hz or less, the fluctuation width is about ± 1.0, and the sensor output downstream of the catalyst largely fluctuates to the lean side. On the other hand, in the case of a normal level catalyst, the sensor output downstream of the catalyst starts to fluctuate to the lean side at around 1.0 Hz, and the sensor output downstream of the catalyst fluctuates greatly at 0.5 Hz or lower. It is time. Therefore, in this case, the deterioration of the catalyst can be accurately determined by setting the fluctuation frequency of the air-fuel ratio fluctuation upstream of the catalyst to about 2.0 Hz. Further, the fluctuation range of the air-fuel ratio at the time of the deterioration determination may be about ± 1.0, and compatibility with the traveling performance is also possible.

The control unit 25 executes a catalyst deterioration determination as follows according to the routine shown in FIGS.

First, in step S1 of the main routine shown in FIG. 3, whether or not deterioration determination is completed is determined by a flag, and if deterioration determination is completed, the process directly proceeds to step S8 described later. If the deterioration determination is not completed, it is determined in steps S2 to S6 whether or not the condition for catalyst deterioration determination is satisfied. That is, step S2
Then, it is determined whether or not the engine has been warmed up based on whether or not the cooling water temperature is equal to or higher than a setting. Then, if it is after the warm-up, it is then determined in step S3 whether the vehicle speed, the engine speed and the load are within the set ranges. If the vehicle speed, the engine speed and the load are all within the set ranges, step S4 is further performed. It is determined whether the fluctuations in the engine speed and the load are equal to or less than the set values. If the determinations in steps S2 to S4 are all YES, the process proceeds to step S5, in which the catalyst temperature is measured in step S5, and it is determined in step S6 whether the catalyst temperature is within the setting. If it is within the setting, the process proceeds to step S7, and the deterioration determination control is performed by the routine shown in FIG. On the other hand, steps S2 to S4
If any of the determinations in step S6 is NO or step S6
If the determination is NO, normal air-fuel ratio feedback control is performed in step S8. In step S9, the dither counter of the flow in FIG. 4 is reset.

In the routine shown in FIG. 4, first, in step S11, the fluctuation cycle and fluctuation width (amplitude) of the air-fuel ratio fluctuation by dither control are set in accordance with the operating load of the engine and the catalyst temperature using a preset map. I do. This setting is to decrease both the fluctuation cycle and the fluctuation width as the engine operating load (the amount of exhaust gas) increases, and to increase both the fluctuation cycle and the fluctuation width as the catalyst temperature increases. Then, dither control is executed in step S12 according to the thus set fluctuation period and fluctuation width, and it is checked in step S13 whether the number of dithers has reached the set number from the start of dithering. Then, since the dither control is started and is unstable for a while, the process returns without performing anything until the set number of times is reached, and when the set number is reached, the number of dithers for deterioration detection is measured in step S14. In step S15, the rear O ₂ sensor (λ
O ₂ sensor 15) Count the number of inversions of the output. And
In step S16, it is determined whether the dither count has reached the set value. If the dither count has not reached the set value, the process returns. If the dither count has reached the set value, the process proceeds to step S17. Then, it calculates the ratio between the dither count reaches a transition number and the set value of the rear O ₂ sensor output at step S17 as the deterioration detection value. Then, the process proceeds to step S18, and a deterioration determination is made based on whether the calculated deterioration detection value is equal to or larger than a threshold value. If the deterioration detection value is equal to or larger than the threshold value, it means deterioration, and the process proceeds to step S19 to perform processing at the time of judgment of deterioration. To execute the normal judgment process. Then, finally, a deterioration determination completion flag is set in step S21.

The above is an example of the embodiment of the present invention. Next, other embodiments will be described with reference to flowcharts.

The flowchart shown in FIG. 5 shows a routine of deterioration determination control according to another example of the embodiment.
In this example, the feedback correction amount of the air-fuel ratio set based on the output of the linear O ₂ sensor 14 upstream of the catalyst is corrected based on the output of the λ O ₂ sensor 15 downstream of the catalyst, and the feedback correction amount thus corrected Average value (cfba
v) is detected as a deviation amount of the air-fuel ratio (A / F), and catalyst deterioration determination is performed based on whether the deviation amount, ie, cfbav, is larger than a threshold value. That is,
As described above with reference to FIG. 4, in step S111, the fluctuation cycle and fluctuation width (amplitude) of the air-fuel ratio fluctuation due to the dither control are set according to the engine operating load and the catalyst temperature using a preset map. This setting also reduces both the fluctuation cycle and the fluctuation width as the engine operation load (the amount of exhaust gas) increases, and increases both the fluctuation cycle and the fluctuation width as the catalyst temperature increases. Then, dither control is executed in step S112 according to the fluctuation period and fluctuation width set in this way, and it is checked in step S113 whether the number of dithers has reached the set number from the start of dithering. Then, since the dither control is started and is unstable for a while, the process returns without performing anything until the set number of times is reached, and when the set number is reached, the linear O ₂ sensor 14 upstream of the catalyst is determined in step S114.
The deviation amount of the air-fuel ratio (A / F), that is, the average value (cfbav) of the feedback correction amount, is detected.
Next, catalyst deterioration determination is performed based on whether the detected cfbav is greater than a threshold value (x). that time,
The threshold value (x) decreases as the amount of exhaust gas increases. And when cfbav is larger than the threshold value,
The process proceeds to step S116 for deterioration, and the process at the time of deterioration determination is performed. When cfbav is equal to or less than the threshold value, the process is normal and the process at the time of normal determination is executed in step S117. Then, finally, a deterioration determination completion flag is set in step S118.

When the catalyst deterioration is determined based on the average value (cfbav) of the feedback correction amounts in the routine of FIG. 5, the threshold value (x) for the deterioration determination becomes smaller as the catalyst temperature rises. Good to do.

A flowchart shown in FIG. 6 shows a main routine for determining deterioration according to still another example of the embodiment. In this example, the deterioration determination is prohibited for a predetermined period after the transition in order to prevent the erroneous deterioration determination during the transition when the catalyst temperature greatly changes.
At 01, it is determined by a flag whether or not the deterioration determination is completed. If the deterioration determination is completed, the process directly proceeds to step S209. If the deterioration determination is not completed, the warm-up state is determined in step S202 based on whether the cooling water temperature is equal to or higher than a set value. It is determined whether the engine speed and the load are within the set ranges. If the vehicle speed, the engine speed and the load are all within the set ranges, it is further determined in step S204 whether the fluctuations of the engine speed and the load are equal to or less than the set values. judge. Then, these steps S202 to S20
If the determinations in step 4 are both YES, the process proceeds to step S205, in which the catalyst temperature is rapidly increased, for example, from 500 ° C. to 700 ° C., or conversely, from 700 ° C. to 500 ° C.
In order to determine a transient state including a time of deceleration such as suddenly decreasing, it is determined whether or not a change in the intake air amount is equal to or more than a predetermined value. Then, when the amount of intake air change is equal to or greater than the predetermined, that the above transitional state, and counts up the timer (T) in step S206, to see if the predetermined time period T ₁ has elapsed in step S207, the predetermined time period T Until _{1 has} elapsed, normal feedback control is continued in step S209 (deterioration determination is prohibited), and after a predetermined period has elapsed, the process proceeds to step S208 to perform deterioration determination control. On the other hand, when any of the determinations in steps S202 to S204 is NO, or when the determination in step S207 is NO, step S
At 209, normal air-fuel ratio feedback control is performed. In step S210, the dither counter is reset.

In each of the above-described examples, when the catalyst deterioration is determined, minute deterioration can be determined by changing both the fluctuation cycle and the fluctuation width of the air-fuel ratio fluctuation with respect to the normal air-fuel ratio feedback control. However, any one of the fluctuation cycle and the fluctuation width of the air-fuel ratio fluctuation may be changed.

In each of the above-described examples, the catalyst deterioration is determined based on the number of reversals of the output of the catalyst downstream sensor, or the catalyst deterioration is determined based on the deviation of the air-fuel ratio. The deterioration can also be determined based on the phase difference between the sensor output fluctuations on the upstream and downstream sides of the catalyst, and the deterioration can be determined on the basis of the integral value of the output voltage of the downstream sensor.

In each of the above examples, the λO ₂ sensor is arranged downstream of the catalyst, but the sensor downstream of the catalyst may be a linear O2 sensor.

Further, the present invention not only when the catalyst is first vent (1 container), in the case of 2 the vent (2 container), the degradation determination by using the O ₂ sensor disposed upstream and downstream of the two vent catalyst This may be done.

[0036]

According to the present invention, the variation period or variation width (determination variation characteristic value) of the air-fuel ratio variation that causes a large variation in the sensor output when the catalyst is deteriorated is determined in advance, and is determined in advance when the catalyst degradation is determined. By controlling the variation characteristic value at the time of determination to be an air-fuel ratio variation, minute catalyst deterioration can be accurately detected, and an increase in cost can be suppressed.

Further, according to the present invention, by arranging the linear air-fuel ratio sensor upstream of the catalyst, the air-fuel ratio feedback control is performed simultaneously with the catalyst deterioration determination, so that the exhaust gas deterioration during the deterioration determination can be prevented. .

[Brief description of the drawings]

FIG. 1 is an overall system diagram of an engine to which the present invention is applied.

FIG. 2 is a characteristic diagram of an upstream air-fuel ratio behavior and a downstream sensor output by dither control.

FIG. 3 is a flowchart showing a main routine for determining catalyst deterioration according to the present invention.

FIG. 4 is a flowchart illustrating a subroutine of deterioration determination control.

FIG. 5 is a flowchart illustrating a subroutine of deterioration determination control of another example.

FIG. 6 is a flowchart illustrating a main routine for determining catalyst deterioration in another example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Engine 4 Air flow meter 7 Boost sensor 8 Fuel injection valve 13 Catalytic converter 14 Linear O ₂ sensor 15 λO ₂ sensor 25 Control unit 27 Catalyst temperature sensor

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Gen Suetsugu 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd.

Claims (11)

[Claims] 1. A catalyst deterioration judging device for judging deterioration of an exhaust gas purification catalyst disposed in an exhaust system of an engine, wherein the air-fuel ratio fluctuation periodically varies the air-fuel ratio of an air-fuel mixture supplied to a combustion chamber. Control means; state quantity detection means for detecting a state quantity related to the oxygen storage capacity of the catalyst; and a state quantity change characteristic according to a change in a characteristic value of air-fuel ratio fluctuation by the air-fuel ratio fluctuation control means. Setting the characteristic value of the air-fuel ratio fluctuation when the state quantity shows a change exceeding a certain level as a fluctuation characteristic value at the time of determination;
A determination condition setting means for setting a determination threshold value according to the fixed level; and an output of the state quantity detection means when the air-fuel ratio is varied by the air-fuel ratio variation control means by the determination-time variation characteristic value. An engine catalyst deterioration determining device, comprising: a deterioration determining unit that performs a catalyst deterioration determination by comparing with a determination threshold value. 2. The apparatus according to claim 1, wherein the characteristic value of the air-fuel ratio fluctuation is at least one of a fluctuation width and a fluctuation period. 3. The periodic change in the air-fuel ratio is accompanied by a change in the amount of oxygen stored in the catalyst, and the state quantity is detected by an output of an oxygen concentration sensor disposed downstream of the catalyst. The engine catalyst deterioration determination device according to claim 1 or 2. 4. The catalyst deterioration determination for an engine according to claim 1, wherein the determination-time variation characteristic value is set on condition that the state quantity shows a change exceeding a certain level over a predetermined period. apparatus. 5. The catalyst deterioration determination device according to claim 3, wherein the determination-time variation characteristic value is set based on the characteristic value of the air-fuel ratio variation when the output of the oxygen concentration sensor exceeds an excess air ratio of one. 6. The catalyst deterioration determining device for an engine according to claim 1, wherein at least one of the determination-time variation characteristic value and the determination threshold value is corrected according to an exhaust gas amount. 7. The catalyst degradation determination device for an engine according to claim 1, wherein at least one of the variation characteristic value at the time of determination and the determination threshold value is corrected according to a catalyst temperature. 8. The method according to claim 1, wherein the determination of deterioration is prohibited during a predetermined period after a transition in which the catalyst temperature greatly changes.
8. The device for judging catalyst deterioration of an engine according to 6 or 7. 9. A linear air-fuel ratio sensor capable of linearly detecting an air-fuel ratio by a sensor output that changes linearly according to the oxygen concentration is provided upstream of the catalyst, and the air-fuel ratio detected by the linear air-fuel ratio sensor is set in advance. The air-fuel ratio is feedback-controlled by the air-fuel ratio feedback control means so that the air-fuel ratio coincides with the target air-fuel ratio, and the air-fuel ratio detected by the linear air-fuel ratio sensor repeats a rich side and a lean side with respect to the stoichiometric air-fuel ratio. 4. The air-fuel ratio feedback control is performed simultaneously with the deterioration determination by changing the target air-fuel ratio of the control by the air-fuel ratio feedback control unit by the fuel ratio fluctuation control unit.
9. The apparatus for determining catalyst deterioration of an engine according to 4, 5, 6, 7 or 8. 10. An exhaust system having an air-fuel ratio sensor capable of detecting an air-fuel ratio based on a sensor output that changes in accordance with the oxygen concentration, wherein the air-fuel ratio detected by the air-fuel ratio sensor is defined as a rich side and a lean side with respect to a stoichiometric air-fuel ratio. A catalyst deterioration determination device that determines the deterioration of an exhaust purification catalyst disposed in an exhaust system of an engine that feedback-controls an air-fuel ratio so as to match a preset target air-fuel ratio that fluctuates to a side. An air-fuel ratio variation control means for giving a variation in the air-fuel ratio of the mixture to a variation cycle longer than the variation cycle of the target air-fuel ratio during a steady operation in which the change in the operating state of the engine for the air-fuel ratio feedback control is small; and A state quantity detecting means for detecting a state quantity related to the oxygen storage capacity of the catalyst, and comparing the output of the state quantity detecting means with a predetermined judgment threshold to determine An engine catalyst deterioration determining device, comprising: a deterioration determining means for performing a change determination. 11. The state quantity detecting means is an oxygen concentration sensor disposed downstream of a catalyst.
11. The apparatus for judging catalyst deterioration of an engine according to 6, 7, 8, 9 or 10.