JP2003301717A - Catalytic deterioration determining method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalytic deterioration determining method for accurately determining whether or not a first catalyst equipped in an exhaust passage and a second catalyst on the downstream side are respectively deteriorated. <P>SOLUTION: This catalytic deterioration determining method respectively controls the air-fuel ration in the first and second lean air-fuel ratios in the time t1 to t2 and the time t2 to t3, and makes an oxygen storage quantity of the first and second catalysts reach a maximum quantity of oxygen storage, and next, controls the air-fuel ratio in the first rich air-fuel ratio up to the time t4 when first catalyst downstream air-fuel ratio sensor output Voxs1 changes to a rich state, and estimates a maximum quantity of oxygen storage CSCmax of the first catalyst. Next, the method controls the air fuel ratio to the second rich air-fuel ratio leaner than the first rich air-fuel ratio up to the time t5 when second catalyst downstream air-fuel ratio sensor output Voxs2 changes to a rich state, and estimates a maximum oxygen storage quantity CUFmax of the second catalyst, and determines whether or not the respective corresponding catalysts are deteriorated on the basis of the maximum oxygen storage quantities CSCmax, and CUFmax of the first and second catalysts. <P>COPYRIGHT: (C)2004,JPO

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 method for judging whether a catalyst arranged in an exhaust passage of an internal combustion engine has deteriorated.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas of an internal combustion engine (in this specification, simply referred to as "catalyst")
Sometimes it is said. ) Is disposed in the exhaust passage of the engine. This catalyst has an O ₂ storage function (oxygen storage function, oxygen storage function) for storing (storing) oxygen, and when the air-fuel ratio of the inflowing gas is rich, the stored oxygen causes HC In addition to oxidizing unburned components such as CO, when the air-fuel ratio of the inflowing gas is lean, nitrogen oxides (NOx) are reduced and oxygen taken from the NOx is stored inside. As a result, the three-way catalyst can purify unburned components and nitrogen oxides even when the air-fuel ratio of the engine deviates from the stoichiometric air-fuel ratio. Therefore, the greater the maximum value of the amount of oxygen that can be stored in the three-way catalyst (hereinafter, referred to as “oxygen storage amount”) (hereinafter, referred to as “maximum oxygen storage amount”), the greater the purification of the three-way catalyst. The ability is high.

On the other hand, the catalyst deteriorates due to poisoning by lead or sulfur contained in the fuel or heat applied to the catalyst, and the maximum oxygen storage amount decreases as the deterioration of the catalyst progresses. Therefore, if the maximum oxygen storage amount of the catalyst can be estimated, it can be determined whether the catalyst has deteriorated based on the estimated maximum oxygen storage amount.

The catalyst deterioration detecting device of Japanese Patent Laid-Open No. 5-133264 is based on such knowledge, and the air-fuel ratio of the engine is changed from a predetermined rich air-fuel ratio to a lean air-fuel ratio (or the air-fuel ratio thereof). The maximum oxygen storage amount of the catalyst is estimated based on the change in the output of the air-fuel ratio sensor located downstream of the catalyst at that time, and the maximum oxygen storage amount of the catalyst is estimated based on the estimated maximum oxygen storage amount. The deterioration degree of is detected.

More specifically, the above disclosed apparatus controls the air-fuel ratio upstream of the catalyst to a predetermined rich air-fuel ratio to make the oxygen storage amount of the catalyst "0", and thereafter The air-fuel ratio is controlled to a predetermined lean air-fuel ratio, and the time until the oxygen storage amount of the catalyst exceeds the maximum oxygen storage amount and the output of the air-fuel ratio sensor downstream of the catalyst changes to lean The maximum oxygen storage amount is estimated by multiplying by the amount of oxygen flowing in per hour. Alternatively, the air-fuel ratio upstream of the catalyst is controlled to a predetermined lean air-fuel ratio to set the oxygen storage amount to the maximum oxygen storage amount, and then the air-fuel ratio of the catalyst is controlled to a predetermined rich air-fuel ratio, and the oxygen By multiplying the time until the stored amount becomes "0" and the output of the air-fuel ratio sensor downstream of the catalyst changes to rich, and the amount of oxygen released (consumed) per unit time in the same catalyst, Estimate the maximum oxygen storage amount. That is, this device obtains the maximum oxygen storage amount by using at least the output change of the air-fuel ratio sensor downstream of the catalyst and the predetermined rich air-fuel ratio or the predetermined lean air-fuel ratio.

[0006]

By the way, in order to purify the exhaust gas immediately after the start of the internal combustion engine and to further improve the exhaust gas purification performance after the complete warm-up, it is called a start converter in the exhaust passage of the engine. The first catalyst having a relatively small capacity and the second catalyst having a relatively large capacity, which is called an under floor converter, are arranged in the exhaust passage downstream of the first catalyst. There is. In this case, the first catalyst is arranged closer to the exhaust port of the engine than the second catalyst, and the exhaust gas having a higher temperature flows in, so that the first catalyst is warmed up within a short period of time from the start and has a good exhaust purification function. Exert. On the other hand, the second catalyst takes a longer time to warm up than the first catalyst, but has a large capacity, and therefore exhibits an excellent exhaust gas purification function after once warmed up.

However, since the conventional technique disclosed in the above publication has a structure in which only one air-fuel ratio sensor is provided downstream of the catalyst, the same air-fuel ratio sensor is provided between the first catalyst and the second catalyst. When it is provided, there is a problem that the maximum oxygen storage amount of the second catalyst cannot be estimated and the deterioration of the second catalyst cannot be determined. Further, when the air-fuel ratio sensor is arranged downstream of the second catalyst, it is theoretically possible to judge the catalyst deterioration by considering the first catalyst and the second catalyst as one catalyst. It is difficult to identify when the catalyst has deteriorated.

Therefore, one of the objects of the present invention is to provide a catalyst deterioration judging method capable of judging whether each of the first catalyst and the second catalyst has deteriorated. Another object of the present invention is a catalyst deterioration determination method (catalyst deterioration degree detection method) that does not deteriorate emission as much as possible even during catalyst deterioration determination (when detecting the catalyst deterioration degree and measuring the maximum oxygen storage amount). To provide.

[0009]

SUMMARY OF THE INVENTION In order to achieve the above object, in the catalyst deterioration determining method of the present invention, a first catalyst downstream air-fuel ratio sensor is arranged downstream of a first catalyst arranged in an exhaust passage of an internal combustion engine. Assuming that the second catalyst downstream air-fuel ratio sensor is arranged downstream of the second catalyst arranged downstream of the first catalyst downstream air-fuel ratio sensor, first, the first catalyst stores oxygen inside. While storing oxygen to the limit
The air-fuel ratio upstream of the first catalyst is controlled to be leaner than the stoichiometric air-fuel ratio so that the second catalyst can store oxygen up to the limit where oxygen can be stored therein.

Next, the air-fuel ratio upstream of the first catalyst is made richer than the theoretical air-fuel ratio until the output of the first catalyst downstream air-fuel ratio sensor shows that the output is richer than the theoretical air-fuel ratio. The maximum oxygen storage amount of the first catalyst is estimated by utilizing the fact that the first rich air-fuel ratio is controlled and the air-fuel ratio upstream of the first catalyst is controlled to the first rich air-fuel ratio. That is, the maximum oxygen storage amount of the first catalyst is estimated based on the amount of oxygen released (consumed) by the gas having the first rich air-fuel ratio.

Thereafter, the air-fuel ratio upstream of the first catalyst is made richer than the stoichiometric air-fuel ratio until the output of the second catalyst downstream air-fuel ratio sensor shows that the output is richer than the stoichiometric air-fuel ratio. The maximum oxygen storage amount of the second catalyst is estimated by utilizing the fact that the second rich air-fuel ratio is controlled and the air-fuel ratio upstream of the first catalyst is controlled to the second rich air-fuel ratio. That is, the maximum oxygen storage amount of the second catalyst is estimated based on the amount of oxygen released (consumed) by the gas having the second rich air-fuel ratio.

Then, it is determined whether or not the first catalyst is deteriorated based on the estimated maximum oxygen storage amount of the first catalyst, and the same is determined based on the estimated maximum oxygen storage amount of the second catalyst. It is determined whether the second catalyst has deteriorated.

According to this, the time when the oxygen stored in the first catalyst is exhausted can be reliably detected by the change in the output of the first catalyst downstream air-fuel ratio sensor, so that the maximum oxygen storage amount of the first catalyst can be determined. It is possible to make an accurate estimation and reliably determine that the first catalyst has deteriorated. In addition, the time when the oxygen stored in the second catalyst is exhausted
Since it is possible to reliably detect the change in the output of the catalyst downstream air-fuel ratio sensor, it is possible to accurately estimate the maximum oxygen storage amount of the second catalyst and to reliably determine that the second catalyst has deteriorated.

By the way, according to the above-mentioned catalyst deterioration determination method, after it is shown that the output of the first catalyst downstream air-fuel ratio sensor is richer than the theoretical air-fuel ratio due to the air-fuel mixture of the first rich air-fuel ratio. Also, the air-fuel mixture having the second rich air-fuel ratio is supplied to the first catalyst, and thus the second catalyst. The supply of the air-fuel mixture having the second rich air-fuel ratio is an air-fuel ratio in which the oxygen storage amount of the second catalyst becomes "0" and the output of the second catalyst downstream air-fuel ratio sensor is richer than the stoichiometric air-fuel ratio. A time point indicating that (hereinafter, this time point is also simply referred to as “at the time of the second catalyst downstream air-fuel ratio sensor output rich inversion”).
Will continue until.

In this case, since the maximum oxygen storage amount of the second catalyst is estimated at the time of the second catalyst downstream air-fuel ratio sensor output rich inversion, the air-fuel ratio upstream of the first catalyst is no longer richer than the theoretical air-fuel ratio. There is no need to keep it. Further, immediately after this rich inversion of the second catalyst downstream air-fuel ratio sensor output, if the air-fuel ratio of the first catalyst upstream is maintained at an air-fuel ratio richer than the stoichiometric air-fuel ratio, the oxygen storage amount of the first catalyst and the second catalyst is increased. Is 0, so a large amount of unburned CO and HC will be emitted. Therefore, after the second catalyst downstream air-fuel ratio sensor output rich reversal, when it is necessary to set the air-fuel ratio upstream of the first catalyst to the stoichiometric air-fuel ratio or to estimate the maximum oxygen storage amount again, It is preferable to set the air-fuel ratio upstream of the first catalyst to an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

However, at the time of the second catalyst downstream air-fuel ratio sensor output rich reversal, the second port is removed from the exhaust port of the internal combustion engine.
Since the exhaust passage to the catalyst downstream air-fuel ratio sensor and the space formed by the catalyst are filled with the gas of the second rich air-fuel ratio, the second rich air-fuel ratio is a rich air-fuel ratio which is far from the theoretical air-fuel ratio. Then, the amount of unburned CO and HC contained in the filled gas is large, and as a result, the first amount after the second catalyst downstream air-fuel ratio sensor output rich inversion is reversed.
Even if the air-fuel ratio upstream of the catalyst is set to the stoichiometric air-fuel ratio or an air-fuel ratio leaner than the stoichiometric air-fuel ratio, a large amount of unburned CO and HC is discharged into the atmosphere immediately after the inversion of the second catalyst downstream air-fuel ratio sensor output rich. I will end up.

On the other hand, if the first rich air-fuel ratio is set in advance to a rich air-fuel ratio close to the stoichiometric air-fuel ratio and the first rich air-fuel ratio and the second rich air-fuel ratio are set to the same air-fuel ratio, the second catalyst Although it is possible to reduce the amount of unburned CO and HC discharged into the atmosphere immediately after the downstream air-fuel ratio sensor output rich reversal, this is the first time after the air-fuel ratio upstream of the first catalyst is controlled to the first rich air-fuel ratio. It is said that the time until the time when the output of the 1-catalyst downstream air-fuel ratio sensor is an air-fuel ratio richer than the stoichiometric air-fuel ratio (that is, the period for calculating the maximum oxygen storage amount of the first catalyst) becomes long. There's a problem.

Therefore, in the above catalyst deterioration determination method of the present invention, the second rich air-fuel ratio is leaner than the first rich air-fuel ratio (that is, the air-fuel ratio richer than the theoretical air-fuel ratio). However, it is preferable to set the air-fuel ratio closer to the stoichiometric air-fuel ratio than the first rich air-fuel ratio).

According to this, at the time when the output of the second catalyst downstream air-fuel ratio sensor shows that the air-fuel ratio is richer than the stoichiometric air-fuel ratio, the gas filled in the exhaust passage and the space of the catalyst described above. Since the amount of unburned CO and HC in the inside becomes small, the amount of unburned CO and HC discharged immediately after the second catalyst downstream air-fuel ratio sensor output rich inversion can be reduced, and the first rich air-fuel ratio becomes the second Since the air-fuel ratio is on the rich side of the rich air-fuel ratio, the maximum oxygen storage amount calculation period of the first catalyst (hence, the maximum oxygen storage amount calculation period of the first catalyst and the second catalyst) can be relatively shortened.

In another catalyst deterioration determination method of the present invention, a first catalyst downstream air-fuel ratio sensor is arranged downstream of the first catalyst disposed in the exhaust passage of the internal combustion engine, and the first catalyst downstream air-fuel ratio is arranged. Assuming that the second catalyst downstream air-fuel ratio sensor is arranged downstream of the second catalyst arranged downstream of the sensor, first, the first catalyst completely releases the oxygen stored therein. The air-fuel ratio upstream of the first catalyst is controlled to be richer than the stoichiometric air-fuel ratio so that the second catalyst completely releases the oxygen stored therein.

Next, the air-fuel ratio upstream of the first catalyst is made leaner than the stoichiometric air-fuel ratio until the output of the first catalyst downstream air-fuel ratio sensor shows that the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The maximum oxygen storage amount of the first catalyst is estimated by controlling the air-fuel ratio to the first lean air-fuel ratio and controlling the air-fuel ratio upstream of the first catalyst to the first lean air-fuel ratio. That is, the maximum oxygen storage amount of the first catalyst is estimated based on the oxygen amount in the gas having the first lean air-fuel ratio.

Thereafter, the air-fuel ratio upstream of the first catalyst is made leaner than the stoichiometric air-fuel ratio until the time when the output of the second catalyst downstream air-fuel ratio sensor indicates that the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The maximum oxygen storage amount of the second catalyst is estimated by utilizing the fact that the second lean air-fuel ratio is controlled and the air-fuel ratio upstream of the first catalyst is controlled to the second lean air-fuel ratio. That is, the maximum oxygen storage amount of the second catalyst is estimated based on the oxygen amount in the gas having the second lean air-fuel ratio.

Then, it is determined whether or not the first catalyst is deteriorated based on the estimated maximum oxygen storage amount of the first catalyst, and the same is determined based on the estimated maximum oxygen storage amount of the second catalyst. It is determined whether the second catalyst has deteriorated.

According to this, since the time when the oxygen storage amount of the first catalyst reaches the maximum oxygen storage amount can be reliably detected by the output change of the first catalyst downstream air-fuel ratio sensor, the maximum oxygen storage amount of the first catalyst can be detected. Can be accurately estimated and the deterioration of the first catalyst can be reliably determined. In addition, since the time when the oxygen storage amount of the second catalyst reaches the maximum oxygen storage amount can be reliably detected by the output change of the second catalyst downstream air-fuel ratio sensor, the maximum oxygen storage amount of the second catalyst can be accurately estimated, It can be reliably determined that the second catalyst has deteriorated.

By the way, according to the above catalyst deterioration determination method, after it is shown that the output of the first catalyst downstream air-fuel ratio sensor is leaner than the stoichiometric air-fuel ratio due to the air-fuel mixture of the first lean air-fuel ratio. Also, the air-fuel mixture having the second lean air-fuel ratio is supplied to the first catalyst, and thus the second catalyst. The supply of the air-fuel mixture having the second lean air-fuel ratio is performed at an air-fuel ratio in which the oxygen storage amount of the second catalyst reaches the maximum oxygen storage amount and the output of the second catalyst downstream air-fuel ratio sensor is leaner than the stoichiometric air-fuel ratio. The process is continued until a time point indicating that there is such a time point (hereinafter, this time point will also be simply referred to as "the time when the lean output of the second catalyst downstream air-fuel ratio sensor is reversed").

In this case, since the maximum oxygen storage amount of the second catalyst is estimated at the time of lean reversal of the second catalyst downstream air-fuel ratio sensor output, the air-fuel ratio upstream of the first catalyst is no longer leaner than the stoichiometric air-fuel ratio. There is no need to keep it. Further, immediately after the lean reversal of the second catalyst downstream air-fuel ratio sensor output, if the air-fuel ratio of the first catalyst upstream is maintained at an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the oxygen storage amounts of the first catalyst and the second catalyst are increased. Is the maximum oxygen storage amount, so nitrogen oxide NO
A large amount of x is discharged. Therefore, after the lean reversal of the second catalyst downstream air-fuel ratio sensor output, if the air-fuel ratio upstream of the first catalyst is set to the stoichiometric air-fuel ratio, or if it is necessary to estimate the maximum oxygen storage amount again, It is preferable to set the air-fuel ratio upstream of the first catalyst to an air-fuel ratio that is richer than the stoichiometric air-fuel ratio.

However, at the time of lean reversal of the second catalyst downstream air-fuel ratio sensor output, the second port from the exhaust port of the internal combustion engine
Since the exhaust passage to the catalyst downstream air-fuel ratio sensor and the space formed by the catalyst are filled with the gas having the second lean air-fuel ratio, the second lean air-fuel ratio is a lean air-fuel ratio that is largely different from the theoretical air-fuel ratio. Then, the amount of nitrogen oxide NOx contained in the filled gas is large, and as a result, a large amount of nitrogen oxide NOx is exhausted into the atmosphere immediately after the lean reversal of the second catalyst downstream air-fuel ratio sensor output. .

On the other hand, if the first lean air-fuel ratio is set in advance to a lean air-fuel ratio close to the stoichiometric air-fuel ratio and the first lean air-fuel ratio and the second lean air-fuel ratio are set to the same air-fuel ratio, the second catalyst The amount of nitrogen oxides NOx discharged into the atmosphere immediately after the lean inversion of the downstream air-fuel ratio sensor output can be reduced. However, with this, the first catalyst is controlled after the air-fuel ratio upstream of the first catalyst is set to the first lean air-fuel ratio. There is a problem that the time until the downstream air-fuel ratio sensor output indicates that the air-fuel ratio is leaner than the stoichiometric air-fuel ratio (that is, the period for calculating the maximum oxygen storage amount of the first catalyst) becomes long. is there.

Therefore, in the catalyst deterioration determination method of the present invention, the second lean air-fuel ratio is richer than the first lean air-fuel ratio (that is, the air-fuel ratio leaner than the stoichiometric air-fuel ratio. However, it is preferable to set the air-fuel ratio closer to the stoichiometric air-fuel ratio than the first lean air-fuel ratio).

According to this, at the time when the output of the second catalyst downstream air-fuel ratio sensor shows that the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the gas filled in the exhaust passage and the space of the catalyst described above. Since the amount of nitrogen oxide NOx in the inside becomes small, the emission amount of nitrogen oxide NOx immediately after the lean inversion of the second catalyst downstream air-fuel ratio sensor output can be reduced, and the first lean air-fuel ratio becomes the second lean air-fuel ratio. Since the air-fuel ratio is leaner than that, the calculation period of the maximum oxygen storage amount of the first catalyst (hence, the maximum oxygen storage amount calculation period of the first catalyst and the second catalyst) can be relatively shortened.

In the present invention, whether the sum of the value related to the maximum oxygen storage amount of the first catalyst and the value related to the maximum oxygen storage amount of the second catalyst estimated as described above is less than or equal to a predetermined deterioration determination reference value. Based on whether or not the first catalyst and the second catalyst are regarded as one catalyst device, it may be determined whether or not the catalyst device is deteriorated.

That is, according to the present invention, the first catalyst arranged in the exhaust passage of the internal combustion engine, and the first catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the first catalyst. A second catalyst disposed in the exhaust passage downstream of the first catalyst downstream air-fuel ratio sensor, and a second catalyst downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the second catalyst. A method for determining catalyst deterioration of an internal combustion engine, comprising:
Theoretical air-fuel ratio upstream of the first catalyst is set so that the first catalyst stores oxygen to the limit where it can store oxygen and the second catalyst stores oxygen to the limit where it can store oxygen inside. A time point at which the air-fuel ratio is controlled to be leaner than the air-fuel ratio, and thereafter, the air-fuel ratio of the first catalyst upstream is shown to indicate that the output of the first catalyst downstream air-fuel ratio sensor is an air-fuel ratio richer than the theoretical air-fuel ratio. Up to the stoichiometric air-fuel ratio, the first rich air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio, and the fact that the air-fuel ratio upstream of the first catalyst is controlled to the first rich air-fuel ratio is used to maximize the maximum oxygen of the first catalyst. The storage amount is estimated, and then the air-fuel ratio of the first catalyst upstream is calculated from the stoichiometric air-fuel ratio until the time when the output of the second catalyst downstream air-fuel ratio sensor shows an air-fuel ratio richer than the stoichiometric air-fuel ratio. Control to a rich second rich air-fuel ratio The maximum oxygen storage amount of the second catalyst is estimated by utilizing that the air-fuel ratio upstream of the first catalyst is controlled to the second rich air-fuel ratio, and the estimated maximum oxygen storage amount of the first catalyst is set to the estimated maximum oxygen storage amount. Deterioration determination method for determining whether or not the catalyst device including the first catalyst and the second catalyst is deteriorated based on a value corresponding to the estimated maximum oxygen storage amount of the second catalyst Can be provided.

Similarly, according to the present invention, the first catalyst arranged in the exhaust passage of the internal combustion engine and the first catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the first catalyst. And a second catalyst disposed in the exhaust passage downstream of the first catalyst downstream air-fuel ratio sensor, and a second catalyst downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the second catalyst. A method for determining catalyst deterioration of an internal combustion engine, comprising:
The air-fuel ratio upstream of the first catalyst is set to the stoichiometric air-fuel ratio so that the first catalyst completely releases the oxygen stored therein and the second catalyst completely releases the oxygen stored therein. Control to a richer air-fuel ratio, then the air-fuel ratio of the first catalyst upstream, until the time when the output of the first catalyst downstream air-fuel ratio sensor is an air-fuel ratio leaner than the theoretical air-fuel ratio, By controlling the first lean air-fuel ratio leaner than the theoretical air-fuel ratio and controlling the air-fuel ratio upstream of the first catalyst to the first lean air-fuel ratio, the maximum oxygen storage amount of the first catalyst is utilized. And then the air-fuel ratio of the first catalyst upstream is set to be leaner than the stoichiometric air-fuel ratio until the output of the second catalyst downstream air-fuel ratio sensor indicates that the output is leaner than the stoichiometric air-fuel ratio. The second lean air-fuel ratio is controlled to Utilizing the fact that the upstream air-fuel ratio is controlled to the second lean air-fuel ratio, the maximum oxygen storage amount of the second catalyst is estimated, and a value corresponding to the estimated maximum oxygen storage amount of the first catalyst is obtained. A catalyst deterioration determination method for determining whether or not the catalyst device including the first catalyst and the second catalyst is deteriorated based on the estimated value corresponding to the maximum oxygen storage amount of the second catalyst can be provided. .

Further, according to the present invention, the first catalyst arranged in the exhaust passage of the internal combustion engine, and the first catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the first catalyst. A second catalyst disposed in the exhaust passage downstream of the first catalyst downstream air-fuel ratio sensor, and a second catalyst downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the second catalyst. A method for determining catalyst deterioration of an internal combustion engine, comprising:
Theoretical air-fuel ratio upstream of the first catalyst is set so that the first catalyst stores oxygen to the limit where it can store oxygen and the second catalyst stores oxygen to the limit where it can store oxygen inside. When the air-fuel ratio is controlled to be leaner than the air-fuel ratio, and then the air-fuel ratio of the first catalyst upstream is shown to indicate that the output of the first catalyst downstream air-fuel ratio sensor is richer than the theoretical air-fuel ratio. Up to the stoichiometric air-fuel ratio, the first rich air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio, and the fact that the air-fuel ratio upstream of the first catalyst is controlled to the first rich air-fuel ratio is utilized to obtain the maximum oxygen of the first catalyst The storage amount is estimated as the first catalyst first maximum oxygen storage amount, and then the air-fuel ratio of the first catalyst upstream is the air-fuel ratio at which the output of the second catalyst downstream air-fuel ratio sensor is richer than the theoretical air-fuel ratio. Until the point
Control to the second rich air-fuel ratio, which is richer than the theoretical air-fuel ratio,
Using the fact that the air-fuel ratio upstream of the first catalyst is controlled to the second rich air-fuel ratio, the maximum oxygen storage amount of the second catalyst is estimated as the second catalyst first maximum oxygen storage amount, and thereafter, The first lean air-fuel ratio is leaner than the stoichiometric air-fuel ratio until the time when the output of the first catalyst downstream air-fuel ratio sensor indicates that the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The maximum oxygen storage amount of the first catalyst is estimated as the first catalyst second maximum oxygen storage amount by utilizing the fact that the air-fuel ratio upstream of the first catalyst is controlled to the first lean air-fuel ratio. Then, the air-fuel ratio upstream of the first catalyst is set to a value leaner than the stoichiometric air-fuel ratio until the time when the output of the second catalyst downstream air-fuel ratio sensor indicates that the output is leaner than the stoichiometric air-fuel ratio. The air-fuel ratio is controlled to 2 lean and the upstream of the first catalyst is controlled. Ratio is estimated as the maximum oxygen storage amount of the second catalyst second maximum oxygen storage amount of the second catalyst by utilizing the fact that is controlled to the second lean air-fuel ratio, (1)
It is determined whether the first catalyst is deteriorated based on the first catalyst first maximum oxygen storage amount and the first catalyst second maximum oxygen storage amount, and the second catalyst first maximum oxygen storage amount is determined. A catalyst deterioration determining method for determining whether the second catalyst is deteriorated based on the amount and the second catalyst second maximum oxygen storage amount, or (2) the first catalyst first maximum oxygen storage amount, A catalyst device comprising the first catalyst and the second catalyst based on the first catalyst second maximum oxygen storage amount, the second catalyst first maximum oxygen storage amount, and the second catalyst second maximum oxygen storage amount. It is also possible to provide a catalyst deterioration judging method for judging whether or not the catalyst has deteriorated.

Further, according to the present invention, a first catalyst arranged in the exhaust passage of the internal combustion engine, and a first catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the first catalyst. A second catalyst disposed in the exhaust passage downstream of the first catalyst downstream air-fuel ratio sensor, and a second catalyst downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the second catalyst. A method for determining catalyst deterioration of an internal combustion engine, comprising:
The air-fuel ratio upstream of the first catalyst is set to the stoichiometric air-fuel ratio so that the first catalyst completely releases the oxygen stored therein and the second catalyst completely releases the oxygen stored therein. Control to a richer air-fuel ratio, then the air-fuel ratio of the first catalyst upstream, until the time when the output of the first catalyst downstream air-fuel ratio sensor is an air-fuel ratio leaner than the theoretical air-fuel ratio, By controlling the first lean air-fuel ratio leaner than the theoretical air-fuel ratio and controlling the air-fuel ratio upstream of the first catalyst to the first lean air-fuel ratio, the maximum oxygen storage amount of the first catalyst is utilized. Is estimated as the first catalyst first maximum oxygen storage amount, and then the air-fuel ratio of the first catalyst upstream is determined so that the output of the second catalyst downstream air-fuel ratio sensor is an air-fuel ratio leaner than the stoichiometric air-fuel ratio. Up to the point in time, the second that is leaner than the stoichiometric air-fuel ratio The maximum oxygen storage amount of the second catalyst is controlled to the second catalyst first maximum oxygen by utilizing the fact that the lean air-fuel ratio is controlled and the air-fuel ratio upstream of the first catalyst is controlled to the second lean air-fuel ratio. Estimated as the storage amount, and then the first
The air-fuel ratio upstream of the catalyst is controlled to the first rich air-fuel ratio richer than the theoretical air-fuel ratio until the time when the output of the first catalyst downstream air-fuel ratio sensor shows that the air-fuel ratio is richer than the theoretical air-fuel ratio. , The maximum oxygen storage amount of the first catalyst is estimated as the first catalyst second maximum oxygen storage amount by utilizing the fact that the air-fuel ratio upstream of the first catalyst is controlled to the first rich air-fuel ratio, and then , A second rich air richer than the stoichiometric air-fuel ratio until the time point when the output of the second catalyst downstream air-fuel ratio sensor is an air-fuel ratio richer than the stoichiometric air-fuel ratio. The maximum oxygen storage amount of the second catalyst is set to the second catalyst second maximum oxygen storage amount by utilizing the fact that the air-fuel ratio upstream of the first catalyst is controlled to the second rich air-fuel ratio by controlling the fuel ratio to the second catalyst. Estimate (1) the first catalyst the first maximum oxygen storage It is determined whether the first catalyst is deteriorated based on the first catalyst second maximum oxygen storage amount and the second catalyst first maximum oxygen storage amount and the second catalyst second maximum oxygen. A catalyst deterioration determination method for determining whether or not the second catalyst is deteriorated based on the stored amount, or (2) the first catalyst first maximum oxygen storage amount, the first catalyst second maximum oxygen storage amount , The second catalyst first maximum oxygen storage amount, and the second catalyst second maximum oxygen storage amount, it is determined whether the catalyst device including the first catalyst and the second catalyst is deteriorated. A catalyst deterioration determination method can also be provided.

In each of these cases, the second rich air-fuel ratio is leaner than the first rich air-fuel ratio, and the second lean air-fuel ratio is leaner than the first lean air-fuel ratio. A rich air-fuel ratio is suitable for suppressing emission, but is not limited to this.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a catalyst deterioration determining method according to the present invention will be described below with reference to the drawings.
FIG. 1 shows a schematic configuration of a system in which an air-fuel ratio control device (catalyst deterioration determination device) for implementing a catalyst deterioration determination method according to an embodiment of the present invention is applied to a spark ignition type multi-cylinder (4-cylinder) internal combustion engine 10. ing.

The internal combustion engine 10 includes a cylinder block,
Cylinder block lower case, cylinder block portion 20 including an oil pan, and cylinder block portion 20
A cylinder head portion 30 fixed on the upper side, an intake system 40 for supplying a gasoline mixture to the cylinder block portion 20, and an exhaust system 50 for releasing exhaust gas from the cylinder block portion 20 to the outside. Contains.

The cylinder block portion 20 includes the cylinder 2
1, piston 22, connecting rod 23, and crankshaft 2
Includes 4. The piston 22 reciprocates in the cylinder 21, and the reciprocating motion of the piston 22 is transmitted to the crankshaft 24 via the connecting rod 23, whereby the crankshaft 2
4 is designed to rotate. The heads of the cylinder 21 and the piston 22 form a combustion chamber 25 together with the cylinder head portion 30.

The cylinder head portion 30 includes an intake port 31 communicating with the combustion chamber 25, an intake valve 32 that opens and closes the intake port 31, an intake camshaft that drives the intake valve 32, and the phase angle of the intake camshaft is continuous. Variable intake timing device 33, actuator 33a of variable intake timing device 33, and combustion chamber 2
5, an exhaust port 34 that communicates with the exhaust port 5, an exhaust valve 35 that opens and closes the exhaust port 34, an exhaust camshaft 36 that drives the exhaust valve 35, a spark plug 37, and an igniter 38 that includes an ignition coil that generates a high voltage applied to the spark plug 37. And an injector (fuel injection means) 39 for injecting fuel into the intake port 31.

The intake system 40 includes an intake pipe 41 including an intake manifold which communicates with the intake port 31 and forms an intake passage together with the intake port 31, an air filter 42 provided at an end of the intake pipe 41, and an inside of the intake pipe 41. There, a throttle valve 43 that makes the opening cross-sectional area of the intake passage variable, a throttle valve actuator 43a composed of a DC motor that constitutes throttle valve driving means, a swirl control valve (hereinafter referred to as "SCV") 44, and D
The SCV actuator 44a including a C motor is provided.

The exhaust system 50 includes an exhaust manifold 51 communicating with the exhaust port 34, and an exhaust pipe (exhaust pipe) connected to the exhaust manifold 51.
52, an upstream first catalyst (also referred to as an upstream three-way catalyst or a start converter) 53 disposed (interposed) in the exhaust pipe 52, and an exhaust pipe 52 downstream of the first catalyst 53. Installed (interposed) second catalyst (downstream side three-way catalyst, or because it is arranged below the floor of the vehicle, it is also called an under floor converter) 54.
Is equipped with. The exhaust port 34, the exhaust manifold 51, and the exhaust pipe 52 form an exhaust passage.

On the other hand, this system includes a heat ray type air flow meter 61, a throttle position sensor 62, a cam position sensor 63, a crank position sensor 64,
The water temperature sensor 65, an air-fuel ratio sensor 66 (hereinafter referred to as the “upstream-stream air-fuel ratio sensor”) disposed in the exhaust passage upstream of the first catalyst 53, and downstream of the first catalyst 53 and second.
Air-fuel ratio sensor 6 disposed in the exhaust passage upstream of the catalyst 54
7 (hereinafter referred to as "first catalyst downstream air-fuel ratio sensor 67"), an air-fuel ratio sensor 68 (hereinafter referred to as "second catalyst downstream air-fuel ratio sensor 6") disposed in the exhaust passage downstream of the second catalyst 54.
8 ”. ), And an accelerator opening sensor 69.

The hot-wire type air flow meter 61 includes the intake pipe 4
A voltage Vg is output according to the mass flow rate of the intake air flowing in the unit 1. The relationship between the output Vg of the air flow meter 61 and the measured intake air amount (flow rate) AFM is as shown in FIG. The throttle position sensor 62 detects the opening of the throttle valve 43 and outputs a signal representing the throttle valve opening TA. The cam position sensor 63 detects that the intake camshaft is rotated by 90 ° (that is, the crankshaft 24 is rotated by 1).
A signal (G2) with one pulse for every 80 ° rotation
Signal). The crank position sensor 64 has a narrow pulse every time the crankshaft 24 rotates by 10 °, and the crankshaft 24 has a 36 pulse width.
A signal having a wide pulse is output every time it rotates by 0 °. This signal indicates the engine speed NE
Represents The water temperature sensor 65 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW.

As shown in FIG. 3, the most upstream air-fuel ratio sensor 66 outputs a current corresponding to the air-fuel ratio A / F and outputs a voltage vabyfs corresponding to this current. Figure 3
As is apparent from the above, the most upstream air-fuel ratio sensor 66 can accurately detect the air-fuel ratio A / F over a wide range. First catalyst downstream air-fuel ratio sensor 67, and second
As shown in FIG. 4, the catalyst downstream air-fuel ratio sensor 68 outputs the voltages Voxs1 and Voxs2 that suddenly change in the theoretical air-fuel ratio. More specifically, the first and second catalyst downstream air-fuel ratio sensors 67, 68 have a value of about 0.1 (V) when the air-fuel ratio is leaner than the theoretical air-fuel ratio, and the air-fuel ratio is lower than the theoretical air-fuel ratio. A voltage of about 0.9 (V) is output when the air-fuel ratio is rich, and a voltage of about 0.5 (V) is output when the air-fuel ratio is the stoichiometric air-fuel ratio. Accelerator position sensor 69
Detects the operation amount of the accelerator pedal 81 operated by the driver and outputs a signal representing the operation amount Accp of the accelerator pedal 81.

Furthermore, the system comprises an electric control unit 70. The electric control device 70 includes a CPU 71 connected to each other via a bus, a ROM 72 in which routines (programs) executed by the CPU 71, tables (lookup tables, maps), constants, etc. are stored in advance, and the CPU 71 temporarily stores data as necessary. The microcomputer 73 includes a RAM 73 for storing data in a memory, a backup RAM 74 for storing data while the power is on and holding the stored data even while the power is off, and an interface 75 including an AD converter.
The interface 75 is connected to the sensors 61 to 69, supplies signals from the sensors 61 to 69 to the CPU 71, and in response to an instruction from the CPU 71, the actuator 33a of the variable intake timing device 33, the igniter 38, the injector 39, and the throttle. A drive signal is sent to the valve actuator 43a and the SCV actuator 44a.

(Principle of Deterioration of Catalyst) By the way,
The three-way catalyst such as the second catalysts 53 and 54 has a function of oxidizing unburned components (HC, CO) and reducing nitrogen oxides (NOx) at the same time when the air-fuel ratio is substantially the stoichiometric air-fuel ratio. Furthermore, the three-way catalyst has an oxygen storage function, and even if the air-fuel ratio deviates from the stoichiometric air-fuel ratio to a certain extent by this oxygen storage function, HC, CO, and NOx can be purified. That is, when the air-fuel ratio of the engine becomes lean and a large amount of NOx is contained in the gas flowing into the three-way catalyst, the three-way catalyst deprives NOx of oxygen molecules to reduce NOx and purifies NOx. Stores oxygen. In addition, the air-fuel ratio of the engine becomes rich and H
When a large amount of C and CO is contained, the three-way catalyst gives oxygen stored therein to oxidize them, thereby purifying HC and CO.

Therefore, in order for the three-way catalyst to efficiently purify a large amount of HC and CO that continuously flow in, the three-way catalyst must store a large amount of oxygen, and conversely, it must be continuously stored. In order to efficiently purify a large amount of NOx flowing into the
The three-way catalyst would have to be in a condition to store sufficient oxygen.

As is clear from the above, the purifying ability of the three-way catalyst depends on the maximum oxygen amount (maximum oxygen storage amount) that can be stored (stored) in the three-way catalyst. However, the three-way catalyst deteriorates due to poisoning by lead or sulfur contained in the fuel or heat applied to the catalyst, so that the maximum oxygen storage amount gradually decreases. In other words, the first and second catalysts 5
If the maximum oxygen storage amount of each of the catalysts 3 and 54 can be estimated, it can be determined whether each of the first and second catalysts 53 and 54 is deteriorated. Further, based on the combination of these determination results, it is also possible to determine whether or not the first and second catalysts 53 and 54 are regarded as one catalyst device and the same catalyst device is deteriorated.

Based on such knowledge, the catalyst deterioration determining apparatus of this embodiment estimates the maximum oxygen storage amount of each of the first and second catalysts 53 and 54 by the method shown in the time chart of FIG. That is, first, as shown in FIG. 5A, at time t1, the air-fuel ratio of the gas upstream of the first catalyst 53 (actually, the air-fuel ratio of the air-fuel mixture taken by the engine, The "first catalyst upstream air-fuel ratio" may be referred to)) is controlled to a predetermined first lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio.

As a result, a gas having a lean air-fuel ratio flows into the first catalyst 53, so that as shown in FIG.
The oxygen storage amount of the first catalyst 53 gradually increases and reaches the maximum oxygen storage amount CSCmax at time t2. As a result, at time t2, a gas containing oxygen (a gas with a lean air-fuel ratio) begins to flow out from the first catalyst 53, and as shown in FIG. 5B, the output of the first catalyst downstream air-fuel ratio sensor 67. Voxs1 changes from a value indicating rich to a value indicating lean. This time t
The operation between 1 and t2 is called the operation in the first mode (Mode = 1).

At time t2, when the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 changes from a value indicating rich to a value indicating lean, this device makes the first catalyst upstream air-fuel ratio leaner than the stoichiometric air-fuel ratio. However, the second lean air-fuel ratio is controlled to be richer than the first lean air-fuel ratio (see FIG. 5A). As a result, a gas having a lean air-fuel ratio flows into the first catalyst 53, and the oxygen storage amount of the first catalyst 53 is maximized, so that the first catalyst 53 cannot store oxygen. Therefore, the gas containing oxygen continues to flow out from the first catalyst 53.

As a result, as shown in FIG. 5 (E), the oxygen storage amount of the second catalyst 54 gradually increases after time t2 and reaches the maximum oxygen storage amount CUFmax at time t3. As a result, at time t3, the gas containing oxygen starts to flow out from the second catalyst 54, and as shown in FIG. 5D, the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a value indicating rich. The value changes to indicate lean. In addition, from time t2
The operation during t3 is referred to as the operation in the second mode (Mode = 2).

As described above, the first and second modes (Mode =
In 1, Mode = 2), the first catalyst 53 stores oxygen to the limit where it can store oxygen, and the second catalyst 5
The air-fuel ratio upstream of the first catalyst 53 is controlled to be leaner than the stoichiometric air-fuel ratio so that 4 can store oxygen to the limit.

At time t3, when the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a value indicating rich to a value indicating lean, the present apparatus makes the first catalyst upstream air-fuel ratio richer than the theoretical air-fuel ratio. Control to the first rich air-fuel ratio. As a result, a gas having a rich air-fuel ratio flows into the first catalyst 53, so that oxygen in the first catalyst 53 is consumed for oxidizing unburned HC and CO flowing into the first catalyst 53. As a result, the oxygen storage amount of the first catalyst 53 is the maximum oxygen storage amount CSC.
It decreases from max. Then, at time t4, the oxygen storage amount of the first catalyst 53 becomes “0”, so that the gas of the rich air-fuel ratio starts to flow out from the first catalyst 53, and the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 becomes The value indicating lean changes to the value indicating rich. The operation between the times t3 and t4 is called the operation in the third mode (Mode = 3).

In the present apparatus, during the time t3 to t4, the maximum oxygen storage amount C of the first catalyst 53 is as follows.
SCmax is estimated as the maximum oxygen storage amount CSCmax3.
That is, from the time t3 when the first catalyst upstream air-fuel ratio is set to the first rich air-fuel ratio, the output of the first catalyst downstream air-fuel ratio sensor 67
Until time t4 when Voxs1 changes to a value indicating rich,
Change Δ of oxygen storage amount based on the following Equation 1 and Equation 2
O2 is calculated and integrated, and the integrated value at the same time t4 is calculated as the maximum oxygen storage amount CSCmax3.

[0057]

[Equation 1] ΔO2 = 0.23 ・ mfr ・ (stoich-abyfs)

[0058]

[Equation 2] CSCmax3 = ΣΔO2 (section t = t3 to t4)

In Expression 1, the value "0.23" is the weight ratio of oxygen contained in the atmosphere. mfr is the total amount of the fuel injection amount Fi within a predetermined time (calculation period tsample), and stoich is the theoretical air-fuel ratio (for example, 14.7).
abyfs is the most upstream air-fuel ratio sensor 6 at a predetermined time tsample
6 is the air-fuel ratio A / F detected by No. 6. Note that abyfs
May be an average value of the air-fuel ratio A / F detected by the most upstream air-fuel ratio sensor 66 within the predetermined time tsample.

As shown in the equation 1, the predetermined time tsampl
In the total injection amount mfr in e, the detected air-fuel ratio A /
By multiplying the deviation of stoichiometric air-fuel ratio of F (stoich-abyfs), the air shortage amount at the same predetermined time tsample is obtained, and by multiplying this air shortage amount by the oxygen weight ratio, The change amount of oxygen storage amount (consumption amount of stored oxygen) ΔO2 is calculated. And the number 2
As shown in, the oxygen storage amount change amount ΔO2 is calculated from time t3 to t3.
By integrating over 4, the oxygen consumption amount from the state where the first catalyst 53 stores the maximum amount of oxygen to the time when all the oxygen is consumed, that is, the maximum oxygen storage amount CSCmax3 is estimated and calculated. As described above, in the present embodiment, the maximum oxygen storage amount CSCmax3 is estimated by utilizing the fact that the first catalyst upstream side air-fuel ratio is controlled to the first rich air-fuel ratio.

In this embodiment, from time t3.
Since the first catalyst upstream air-fuel ratio is the constant first rich air-fuel ratio during t4, the time from time t3 to t4 is Δt3, the first
Assuming that the air-fuel ratio is abyfR1 and the fuel supply amount per unit time during that period is mfr3, from Equation 1 and Equation 2 above,
The maximum oxygen storage amount CSCmax3 is 0.23 · mfr3 · (st
It can also be easily obtained as oich-abyfR1) · Δt3.

At time t4, when the output of the first catalyst downstream air-fuel ratio sensor 68 changes from a value indicating lean to a value indicating rich, the present apparatus changes the first catalyst upstream air-fuel ratio to an air richer than the stoichiometric air-fuel ratio. The second rich air-fuel ratio, which is a fuel ratio and is leaner than the first rich air-fuel ratio, is controlled. At this time, since the oxygen storage amount of the first catalyst 53 is “0”, the gas having a rich air-fuel ratio flows into the second catalyst 54. As a result, the oxygen stored in the second catalyst 54 is consumed for the oxidation of unburned HC and CO flowing into the second catalyst 54,
The oxygen storage amount of the second catalyst 54 is the maximum oxygen storage amount CUFmax.
To decrease from.

Then, at time t5, the second catalyst 54
Since the oxygen storage amount of is 0, the rich air-fuel ratio gas starts to flow out from the second catalyst 54, and the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a lean value to a rich value. To do. That is, during the time t3 to t4, the first catalyst 53 completely releases the oxygen stored inside, and during the time t4 to t5, the second catalyst 54 stores the oxygen stored inside. Time t3 to t so as to release completely
The air-fuel ratio upstream of the first catalyst 53 during 5 is controlled to an air-fuel ratio richer than the stoichiometric air-fuel ratio. In addition, from time t4 to t5
The operation during the period is called the operation in the fourth mode (Mode = 4).

The present apparatus performs the calculations shown by the following equations 3 and 4 similar to the case where the maximum oxygen storage amount CSCmax3 is obtained between the times t4 and t5, whereby the maximum oxygen of the second catalyst 54 is The storage amount CUFmax is calculated and estimated as the maximum oxygen storage amount CUFmax4. Even in this case, since the first catalyst upstream air-fuel ratio is the constant second rich air-fuel ratio from time t4 to t5, the time from time t4 to t5 is Δt4, the second air-fuel ratio is abyfR2, and during that time. If the fuel supply amount per unit time is mfr4, the maximum oxygen storage amount CUFmax4 is 0.23 · mfr4 · (stoich
-AbyfR2) · Δt4 can be easily obtained. As described above, in the present embodiment, the maximum oxygen storage amount CUFmax4 is estimated by utilizing the fact that the first catalyst upstream side air-fuel ratio is controlled to the second rich air-fuel ratio.

[0065]

[Equation 3] ΔO2 = 0.23 ・ mfr ・ (stoich-abyfs)

[0066]

[Formula 4] CUFmax4 = ΣΔO2 (section t = t4 to t5)

At time t5, when the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a value indicating lean to a value indicating rich, the present apparatus makes the first catalyst upstream air-fuel ratio leaner than the stoichiometric air-fuel ratio. Control to the first lean air-fuel ratio. As a result, a gas having a lean air-fuel ratio flows into the first catalyst 53. Further, at the time t5, the first catalyst 53
The oxygen storage amount of is 0. Therefore, time t5
After that, the oxygen storage amount in the first catalyst 53 continues to increase from “0”, and reaches the maximum oxygen storage amount CSCmax at time t6. As a result, at time t6, the gas containing oxygen starts to flow out from the first catalyst 53, and the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 changes from a value indicating rich to a value indicating lean. The operation between the times t5 and t6 is called the operation in the fifth mode (Mode = 5).

The present apparatus estimates the maximum oxygen storage amount CSCmax of the first catalyst 53 as the maximum oxygen storage amount CSCmax5 in the following manner even during the time t5 to t6. That is, the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67
Indicates that the oxygen storage amount of the first catalyst 53 has reached the maximum oxygen storage amount CSCmax at the time t6 when becomes a value indicating the lean air-fuel ratio. The change amount ΔO2 of the oxygen storage amount is calculated based on the equation 6, and this is integrated, and the integrated value at the same time t6 is estimated and calculated as the maximum oxygen storage amount CSCmax5.

[0069]

[Equation 5] ΔO2 = 0.23 · mfr · (abyfs − stoich)

[0070]

[Equation 6] CSCmax5 = ΣΔO2 (section t = t5 to t6)

As shown in the equation 5, the predetermined time tsampl
By multiplying the total amount mfr of the injection amount in e by the deviation (abyfs-stoich) of the air-fuel ratio A / F from the theoretical air-fuel ratio,
The excess amount of air at the same predetermined time tsample is calculated,
By multiplying the excess amount of air by the weight ratio of oxygen, the oxygen storage amount change amount (stored oxygen amount) ΔO2 at the same predetermined time tsample can be obtained. Then, as shown in Equation 6,
By integrating the oxygen storage amount change amount ΔO2 over the time t5 to t6, the oxygen amount from the state where the oxygen storage amount of the first catalyst 53 is “0” to the maximum oxygen storage, that is, the maximum oxygen The storage amount CSCmax5 is estimated and calculated.

Even in this case, times t5 to t
Since the first catalyst upstream air-fuel ratio is a constant first lean air-fuel ratio between 6 and 6, the time from time t5 to t6 is Δt5, the first lean air-fuel ratio is abyfL1, and the fuel supply amount per unit time during that time is mfr5. If so, the maximum oxygen storage amount CSCma
x5 is 0.23 · mfr5 · (abyfL1 − stoich) · Δt
It can be easily calculated as 5. As described above, in the present embodiment, the maximum oxygen storage amount CSC is utilized by utilizing the fact that the first catalyst upstream side air-fuel ratio is controlled to the first lean air-fuel ratio.
Estimate max5.

At time t6, when the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 changes from a value indicating rich to a value indicating lean, this device makes the first catalyst upstream air-fuel ratio leaner than the stoichiometric air-fuel ratio. The air-fuel ratio is controlled to the second lean air-fuel ratio which is richer than the first lean air-fuel ratio. In this case, the oxygen storage amount of the first catalyst 53 is the maximum oxygen storage amount CS
It has reached Cmax. Therefore, a gas having a lean air-fuel ratio flows out from the first catalyst 53, and this gas flows into the second catalyst 54. On the other hand, at the time t6, the oxygen storage amount of the second catalyst 54 is “0”. Therefore, after the time t6, the oxygen storage amount in the second catalyst 54 continues to increase from “0”, and at the time t7, the maximum oxygen storage amount C
Reach UFmax. As a result, at time t7, the gas containing oxygen starts to flow out from the second catalyst 54, and the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a value indicating rich to a value indicating lean. The operation between the times t6 and t7 is called the operation in the sixth mode (Mode = 6).

The present apparatus estimates the maximum oxygen storage amount CUFmax of the second catalyst 54 as the maximum oxygen storage amount CUFmax6 in the following manner even during the time t6 to t7. That is, the change amount ΔO2 of the oxygen storage amount is calculated and integrated based on the following equations 7 and 8, and the integrated value at the same time t7 is calculated as the maximum oxygen storage amount CUFmax6.

[0075]

[Equation 7] ΔO2 = 0.23 · mfr · (abyfs − stoich)

[0076]

[Equation 8] CUFmax6 = ΣΔO2 (section t = t6 to t7)

Even in this case, times t6 to t
Since the first catalyst upstream air-fuel ratio is a constant second lean air-fuel ratio between 7 and 7, the time from time t6 to t7 is Δt6, the second lean air-fuel ratio is abyfL2, and the fuel supply amount per unit time during that time is mfr6. If so, the maximum oxygen storage amount CUFma
x6 is 0.23 · mfr6 · (abyfL2 − stoich) · Δt
It can be easily calculated as 6. As described above, in the present embodiment, the maximum oxygen storage amount CUF is utilized by utilizing the fact that the first catalyst upstream side air-fuel ratio is controlled to the second lean air-fuel ratio.
Estimate max6.

Then, this device returns the air-fuel ratio of the air-fuel mixture sucked into the engine to the stoichiometric air-fuel ratio at time t7, and
After the same time t7, it is determined as follows whether or not the first and second catalysts 53 and 54 are deteriorated.

First, regarding the first catalyst 53,
Whether the maximum oxygen storage amount CSCmax3 of the catalyst 53 is smaller than the first catalyst deterioration determination reference value CSCRdn, the first catalyst 5
Whether the maximum oxygen storage amount CSCmax5 of 3 is smaller than the first catalyst deterioration determination reference value CSCRup, and the average maximum oxygen storage amount of the first catalyst 53 which is an average value of the maximum oxygen storage amount CSCmax3 and the maximum oxygen storage amount CSCmax5. Quantity CSCmax
It is determined whether (= (CSCmax3 + CSCmax5) / 2) is smaller than the first catalyst deterioration determination reference value CSCRave.

The maximum oxygen storage amount CSCmax3 is smaller than the first catalyst deterioration determination reference value CSCRdn, the maximum oxygen storage amount CSCmax5 is smaller than the first catalyst deterioration determination reference value CSCRup, and the maximum oxygen storage amount is When either of CSCmax is smaller than the first catalyst deterioration determination reference value CSCRave, it is determined that the first catalyst 53 is deteriorated. In this case, the first catalyst 53 may be determined to be deteriorated when any two combinations of the above three conditions are established, and the first catalyst 53 may be determined to be deteriorated only when all the three conditions are satisfied. It may be configured to determine that one catalyst 53 has deteriorated.

Next, regarding the second catalyst 54,
Whether or not the maximum oxygen storage amount CUFmax4 of the catalyst 54 is smaller than the second catalyst deterioration determination reference value CUFRdn, the second catalyst 5
Whether the maximum oxygen storage amount CUFmax6 of 4 is smaller than the second catalyst deterioration determination reference value CUFRup, and the average maximum of the second catalyst 54 which is the average value of the maximum oxygen storage amount CUFmax4 and the second maximum oxygen storage amount CUFmax6. Oxygen storage capacity CUFma
It is determined whether x (= (CUFmax4 + CUFmax6) / 2) is smaller than the second catalyst deterioration determination reference value CUFRave.

The maximum oxygen storage amount CUFmax4 is smaller than the second catalyst deterioration determination reference value CUFRdn, the maximum oxygen storage amount CUFmax6 is smaller than the second catalyst deterioration determination reference value CUFRup, and the average maximum oxygen storage amount is When either the amount CUFmax is smaller than the second catalyst deterioration determination reference value CUF Rave, the second catalyst 5
It is determined that 4 is deteriorated. Also in this case, it may be configured to determine that the second catalyst 54 has deteriorated when any two combinations of the above three conditions are satisfied, and when all the three conditions are satisfied. Only the second catalyst 54 may be determined to be deteriorated.

Further, the present apparatus has the first and second catalysts 53, 54.
Is regarded as one catalyst device, it is determined whether or not the catalyst device is deteriorated by determining whether the following formula 9 is satisfied.

[0084]

[Equation 9] CSCmax + CUFmax> CRave

CSCmax on the left side of the equation 9 is C
It may be replaced with either SCmax3 or CSCmax5, and CUFmax on the left side of the same number 9 is CUFmax4 or CUFm.
It may be replaced with any of ax6. CRave on the right side is the first
When the catalyst 53 and the second catalyst 54 are regarded as one catalyst device, it is a reference value of the maximum oxygen storage amount (a deterioration determination reference value of the entire catalyst) for determining deterioration of the catalyst device. The above is the outline of the catalyst deterioration determination method by the present apparatus.

<Actual Operation> Next, regarding the actual operation of the air-fuel ratio control device (and the catalyst deterioration determination device) configured as described above, a routine (program) executed by the CPU 71 of the electric control device 70 is executed. This will be described with reference to FIGS. 6 to 17 shown by the flowcharts.

(Normal Air-Fuel Ratio Control) The CPU 71 is shown in FIG.
The routine for calculating the final fuel injection amount Fi and instructing the fuel injection shown in FIG. 2 is performed by setting the crank angle of each cylinder to a predetermined crank angle before each intake top dead center (for example, BTDC 90 ° C
Each time it becomes A), it is repeatedly executed.
Therefore, when the crank angle of any cylinder reaches the predetermined crank angle, the CPU 71 starts the process from step 600 and proceeds to step 605, where the air flow meter 61
Based on the intake air amount AFM measured by and the engine rotation speed NE, the basic fuel injection amount Fbase for making the air-fuel ratio of the engine the stoichiometric air-fuel ratio is obtained from the map.

Next, the CPU 71 proceeds to step 610 and sets a value obtained by adding a later-described air-fuel ratio feedback correction amount DFi to the value obtained by multiplying the basic fuel injection amount Fbase by the coefficient K as the final fuel injection amount Fi. The value of the coefficient K is normally "1.00", and as will be described later, when the air-fuel ratio is forcibly changed to make catalyst deterioration determination, "1.
It is set to a predetermined value other than "00". Then the CPU 71
Advances to step 615, and in step 615, the injector 39 is instructed to inject the fuel of the final fuel injection amount Fi. After that, the CPU 71 proceeds to step 620, and sets a value obtained by adding the final fuel injection amount Fi to the total fuel injection amount mfr at that time to the new fuel injection amount integrated value mfr. This fuel injection amount integrated value mfr is
It is used when calculating the oxygen storage amount described later. afterwards,
The CPU 71 proceeds to step 695 to end this routine once. As described above, the feedback-corrected fuel of the final fuel injection amount Fi is injected into the cylinder that is approaching the intake stroke.

Next, the air-fuel ratio feedback correction amount D
The calculation of Fi will be described. The CPU 71 repeatedly executes the routine shown in FIG. 7 every elapse of a predetermined time. Therefore, when the predetermined timing comes, the CPU 71 starts the process from step 700 and proceeds to step 705 to determine whether or not the feedback control condition is satisfied. The air-fuel ratio feedback control conditions are, for example, that the engine cooling water temperature THW is equal to or higher than the first predetermined temperature, the intake air amount (load) per revolution of the engine is equal to or lower than a predetermined value, and the most upstream air-fuel ratio sensor 66 is normal. And the value of the later-described catalyst deterioration determination execution flag HAN is "0". As will be described later, the catalyst deterioration determination execution flag HAN indicates that when the value is "1", the air-fuel ratio control for forcibly changing the air-fuel ratio for catalyst deterioration determination is being executed. When the value is "0", it indicates that the air-fuel ratio control for determining the catalyst deterioration is not executed.

Continuing with the description assuming that the air-fuel ratio feedback control condition is satisfied, the CPU 71 makes a “Yes” determination at step 705 to determine step 710.
The output of the most upstream air-fuel ratio sensor 66 at the present time vabyfs
And the sub feedback control amount vafsfb described later (va
Byfs + vafsfb) is converted based on the map shown in FIG. 3 to obtain the upstream control air-fuel ratio abyfs of the first catalyst 53 at the present time.

Next, the CPU 71 proceeds to step 715 and sets the in-cylinder intake air amount Mc (k−N) which is the intake air amount of the cylinder which has reached the intake stroke N strokes (N intake strokes) before the present time. The obtained upstream side control air-fuel ratio abyfs
By dividing by, the in-cylinder fuel supply amount Fc (k−N) N strokes before is calculated. The value N varies depending on the displacement of the internal combustion engine, the distance from the combustion chamber 25 to the most upstream air-fuel ratio sensor 66, and the like.

Thus, in order to obtain the in-cylinder fuel supply amount Fc (k−N) N strokes before the present time, the in-cylinder intake air amount Mc (k−N) before the N stroke before the present time is controlled upstream. The reason for dividing by the air-fuel ratio abyfs is that it takes a time equivalent to N strokes for the air-fuel mixture burned in the combustion chamber 25 to reach the most upstream air-fuel ratio sensor 66. The in-cylinder intake air amount Mc is the output A of the air flow meter 61 at that time for each intake stroke of each cylinder.
It is determined based on FM and the engine speed NE (for example, it is determined by dividing the output AFM of the air flow meter 61 subjected to the first-order delay processing by the engine speed NE), and corresponds to each intake stroke. Are stored in the RAM 73.

Next, the CPU 71 proceeds to step 720, and the cylinder intake air amount Mc N strokes before the present time Mc.
By dividing (k−N) by the target air-fuel ratio abyfr (k−N) (theoretical air-fuel ratio in this example) at the time N strokes before the present time, the target in-cylinder fuel supply amount N strokes before the current time. Determine Fcr (k−N). Then, the CPU 71 proceeds to step 725 and proceeds from the target in-cylinder fuel supply amount Fcr (k−N) to the in-cylinder fuel supply amount Fc (k−
A value obtained by subtracting N) is set as the in-cylinder fuel supply amount deviation DFc. That is, the in-cylinder fuel supply amount deviation DFc is an amount representing the excess or deficiency of the fuel supplied into the cylinder at the time before N strokes. Next, the CPU 71 proceeds to step 730,
Air-fuel ratio feedback correction amount DFi
Ask for.

[0094]

[Equation 10] DFi = (Gp · DFc + Gi · SDFc) · KFB

In the above equation 10, Gp is a preset proportional gain, and Gi is a preset integral gain. The coefficient KFB of the equation 10 is the engine speed N
It is preferable to be variable depending on E, the cylinder intake air amount Mc, etc., but here it is set to "1". Also, the value S
DFc is an integrated value of the in-cylinder fuel supply amount deviation DFc, and is updated in the next step 735. That is, the CPU 71
Is a new integrated value of the in-cylinder fuel supply amount deviation obtained by adding the in-cylinder fuel supply amount deviation DFc obtained in step 725 to the integrated value SDFc of the in-cylinder fuel supply amount deviation DFc at step 735. Find SDFc, Step 7
This routine is once ended at 95.

From the above, the air-fuel ratio feedback correction amount DFi is obtained by the proportional-plus-integral control, and this air-fuel ratio feedback correction amount DFi is described in step 61 of FIG.
Since the fuel injection amount is reflected by 0 and step 615, the excess or deficiency of the fuel supply amount before N strokes is compensated, and the average value of the air-fuel ratio is made to substantially match the target air-fuel ratio abyfr.

On the other hand, at the time of determination in step 705,
If the air-fuel ratio feedback control condition is not satisfied, CP
U71 makes a “No” determination at step 705 to proceed to step 740, where the air-fuel ratio feedback correction amount DFi
Is set to "0", and the routine proceeds to step 795 to end this routine once. As described above, when the air-fuel ratio feedback control condition is not satisfied (including during catalyst deterioration determination execution), the air-fuel ratio feedback correction amount DFi is set to “0”.
As a result, the air-fuel ratio (basic fuel injection amount Fbase) is not corrected.

Next, the air-fuel ratio feedback control based on the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 will be described. Note that such control is also called sub feedback control. By this sub feedback control, the sub feedback control amount vafsfb is calculated.

The CPU 71 controls the sub feedback control amount.
In order to obtain vafsfb, the routine shown in FIG. 8 is executed every time a predetermined time elapses. Therefore, when the predetermined timing comes, the CPU 71 starts the process from step 800 and proceeds to step 805 to determine whether or not the sub feedback control condition is satisfied. The sub feedback control conditions include, for example, the cooling water temperature TH of the engine in addition to the air-fuel ratio feedback control conditions in step 705 described above.
It is established when W is equal to or higher than the second predetermined temperature which is higher than the first predetermined temperature, and when the first catalyst downstream air-fuel ratio sensor 67 is normal.

Continuing the description assuming that the sub feedback control condition is satisfied, the CPU 71 makes a “Yes” determination at step 805 to proceed to step 810, at which a predetermined target value Voxref for the first catalyst at the present time is reached. The output deviation amount DVoxs is obtained by subtracting the output Voxs1 of the downstream air-fuel ratio sensor 67. The target value Voxsref is determined so that the purification efficiency of the first catalyst 53 is good (best), and is set here to a value corresponding to the theoretical air-fuel ratio. Next, the CPU 71 proceeds to step 815 and obtains the sub feedback control amount vafsfb based on the following Expression 11.

[0101]

[Equation 11] vafsfb = Kp · DVoxs + Ki · SDVoxs

In Equation 11, Kp is a preset proportional gain, and Ki is a preset integral gain. SDVoxs is an integrated value of the output deviation amount DVoxs and is a value updated in the next step 820.
That is, when the CPU 71 proceeds to step 820, it adds the output deviation amount DVoxs obtained in step 810 to the integrated value SDVoxs of the output deviation amount at that time point to obtain a new integrated value SDVoxs of the output deviation amount, and thereafter. , Proceeds to step 895 to end the present routine tentatively.

In this way, the sub feedback control amount vafsfb is obtained, and this value is added to the actual output of the most upstream air-fuel ratio sensor 66 in step 710 of FIG. 7 described above, and the sum (vabyfs + vafsfb) Is converted to the upstream side control air-fuel ratio abyfs on the basis of the map shown in FIG. In other words, the output of the first catalyst downstream air-fuel ratio sensor 67
The upstream control air-fuel ratio abyfs calculated based on Voxs1 is
An air-fuel ratio different from the air-fuel ratio actually detected by the most upstream air-fuel ratio sensor 66 by an amount corresponding to the sub feedback control amount vafsfb is obtained.

As a result, the above-mentioned step 715 of FIG. 7 is performed.
Since the in-cylinder fuel supply amount Fc (k−N) calculated in step 1 changes in accordance with the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67, the air-fuel ratio feedback correction amount DFi is set to the first by the steps 725 and 730. Catalyst downstream air-fuel ratio sensor 67
The output can be changed according to the output Voxs1. This allows
The air-fuel ratio of the engine is controlled so that the air-fuel ratio on the downstream side of the first catalyst 53 matches the target value Voxsref.

For example, since the average air-fuel ratio of the engine is lean, the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67
Indicates a value corresponding to an air-fuel ratio that is leaner than the theoretical air-fuel ratio, the output deviation amount DV obtained in step 810
Since oxs has a positive value, the sub feedback control amount vafsfb obtained in step 815 has a positive value. Therefore, abyfs obtained in step 710 is obtained as a value (larger value) leaner than the air-fuel ratio actually detected by the most upstream air-fuel ratio sensor 66. For this reason,
In-cylinder fuel supply amount Fc (k
-N) is a small value, and the in-cylinder fuel supply amount deviation DFc is obtained as a large value, so the air-fuel ratio feedback correction amount DFi is a large positive value. As a result, FIG.
Final fuel injection amount Fi obtained in step 610 of
Becomes larger than the basic fuel injection amount Fbase, and is controlled so that the air-fuel ratio of the engine becomes rich.

On the contrary, since the average air-fuel ratio of the engine is rich, the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 is increased.
Indicates a value corresponding to the rich air-fuel ratio rather than the stoichiometric air-fuel ratio, the output deviation amount DVoxs obtained in step 810.
Has a negative value, the sub feedback control amount vafsfb obtained in step 815 has a negative value. Therefore, abyfs obtained in step 710 is obtained as a richer value (smaller value) than the air-fuel ratio actually detected by the most upstream air-fuel ratio sensor 66. Therefore, the in-cylinder fuel supply amount Fc (k-
N) is a large value, and the in-cylinder fuel supply amount deviation DFc is obtained as a negative value, so the air-fuel ratio feedback correction amount DFi becomes a negative value. As a result, the final fuel injection amount Fi obtained in step 710 of FIG. 7 becomes smaller than the basic fuel injection amount Fbase, and the air-fuel ratio of the engine is controlled to be lean.

(Air-fuel ratio control for determining catalyst deterioration) Next, the air-fuel ratio control for determining catalyst deterioration will be described. The CPU 71 executes each routine shown by the flowcharts of FIGS. 9 to 15 every time a predetermined time elapses.

Therefore, when the predetermined timing comes, the CP
U71 starts the process from step 900 of FIG. 9, proceeds to step 905, and determines whether or not the value of the catalyst deterioration determination execution flag HAN is “0”. Now, assuming that the air-fuel ratio control for catalyst deterioration determination is not performed and the catalyst deterioration determination condition is not satisfied, the value of the catalyst deterioration determination executing flag HAN becomes “0”. There is. Therefore, the CPU 71 sends “Yes” at step 905.
s ”and proceeds to step 910, where the value of the coefficient K used in step 610 of FIG. 6 described above is 1.0.
Set to 0.

Next, the CPU 71 determines in step 915 whether or not the catalyst deterioration determination condition is satisfied. The catalyst deterioration determination condition is that the cooling water temperature THW is equal to or higher than a predetermined temperature, the vehicle speed obtained by a vehicle speed sensor (not shown) is equal to or higher than a predetermined high vehicle speed, and the change amount of the throttle valve opening TA per unit time is a predetermined amount. The following is true when the engine is in steady operation. Further, the catalyst deterioration determination condition is that a predetermined time or more has passed since the last catalyst deterioration determination, that the vehicle has been operated for a predetermined distance or more since the last catalyst deterioration determination, and the internal combustion engine 10 has been operated for a predetermined time since the last catalyst deterioration determination. Any one of the above operations or one or more may be added to the catalyst deterioration determination condition. At this stage, as described above, the catalyst deterioration determination condition is not satisfied, so the CPU 7
1 is determined to be “No” in Step 915, and Step 9 is performed.
The program then proceeds to 95 to end this routine once.

Next, like the time t1 in FIG. 5 described above, the air-fuel ratio control for determining the catalyst deterioration is not performed at that time, but the description will be continued assuming that the catalyst deterioration determination condition is satisfied. In this case, the CPU 71 makes a step 90
In step 5, the determination is “Yes” and the process proceeds to step 910. In step 910, the value of the coefficient K is set to 1.00.
Next, since the catalyst deterioration condition is satisfied, the CPU 71 makes a “Yes” determination at step 915 to proceed to step 920, at which step 920 sets the value of the catalyst deterioration determination execution flag HAN to “1”. Set.

Then, the CPU 71 proceeds to step 925, sets the value of Mode to "1" for shifting to the first mode, and sets the value of the coefficient K to 0.98 in the following step 930, The routine proceeds to 995 to end this routine once. As a result, the above-mentioned air-fuel ratio feedback control condition is no longer satisfied, so the CPU 71 executes the process shown in FIG.
Step 705, it is judged as “No” and Step 74
0, the air-fuel ratio feedback correction amount DF
The value of i is set to 0. As a result, step 6 in FIG.
By executing Step 10, a value obtained by multiplying the basic fuel injection amount Fbase by 0.98 is calculated as the final fuel injection amount Fi, and fuel of this final fuel injection amount Fi is injected, so the air-fuel ratio of the engine is the theoretical air-fuel ratio. The lean air-fuel ratio is controlled to be leaner than the above.

Thereafter, the CPU 71 repeatedly executes the processing of the routine of FIG. 9 from step 900, but since the value of the catalyst deterioration determination in-progress flag HAN is "1", it is judged "No" in step 905. Upon making a determination, the process immediately proceeds to step 995 to end this routine once.

On the other hand, the CPU 71 repeatedly executes the first mode control routine shown in FIG. 10 every elapse of a predetermined time. Therefore, at a predetermined timing, the CPU 7
1 starts the process from step 1000 and then starts step 10
In step 05, it is determined whether the Mode value is "1". In this case, the processing of step 925 in FIG.
Since the value of Mode is “1”, the CPU 71 makes a “Yes” determination at step 1005 to determine at step 101
Going to 0, it is determined whether the output Voxs1 of the first catalyst downstream air-fuel ratio sensor has changed from a value showing an air-fuel ratio richer than the theoretical air-fuel ratio to a value showing an air-fuel ratio leaner than the theoretical air-fuel ratio. . At this time, since the air-fuel ratio of the engine has been changed to the first lean air-fuel ratio, the first catalyst downstream air-fuel ratio sensor output Voxs1 shows an air-fuel ratio richer than the stoichiometric air-fuel ratio. Therefore, the CPU 71 returns to “N
It is determined to be “o”, and this routine is once ended in step 1095.

After that, the CPU 71 causes the step 10 in FIG.
00 to 1010 are repeatedly executed. Further, since the air-fuel ratio is maintained at the first lean air-fuel ratio, the first catalyst downstream air-fuel ratio sensor output Voxs1 shows a lean value from a lean value after a predetermined time, as at time t2 in FIG. Changes to. This causes the CPU 71 to execute the step 1010.
When the process proceeds to step 1010, it is determined “Yes” in the same step 1010, the process proceeds to step 1015, the value of Mode is set to “2”, and the value of the coefficient K is set to 0.99 in the following step 1020. After that, this routine is once ended in step 1095.

As a result, by executing step 610 of FIG. 6, a value obtained by multiplying the basic fuel injection amount Fbase by 0.99 is calculated as the final fuel injection amount Fi, and the fuel of this final fuel injection amount Fi is injected. Therefore, the air-fuel ratio of the engine is leaner than the stoichiometric air-fuel ratio and is controlled to the second lean air-fuel ratio which is richer than the first lean air-fuel ratio.

When the second mode (Mode = 2) is reached, the CPU 71 thereafter executes the same mode control and sets the mode to the third mode.
The mode is sequentially switched to the fourth, fifth, and sixth modes, and control according to each mode is executed. In brief, in the second mode of which the routine is shown in the flowchart of FIG. 11, it is determined in step 1105 whether or not the value of Mode is “2”, and if the value of Mode is “2”. For example, step 1105 to step 1110
In step 1110, whether the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 has changed from a value indicating an air-fuel ratio richer than the theoretical air-fuel ratio to a value indicating an air-fuel ratio leaner than the theoretical air-fuel ratio. To monitor.

Then, as shown at time t3 in FIG.
When the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a value indicating an air-fuel ratio richer than the stoichiometric air-fuel ratio to a value indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio, step 1115
The value of Mode is set to "3" to shift to the third mode by proceeding to step 3, and the value of the coefficient K is set to 1.02 in the following step 1120.
Set to. As a result, the air-fuel ratio of the engine is controlled to the first rich air-fuel ratio which is richer than the stoichiometric air-fuel ratio.

Similarly, in the third mode whose flowchart is shown in FIG.
At 205, it is determined whether or not the value of Mode is “3”, and Mo
If the value of de is "3", the process proceeds from step 1205 to step 1210, and in step 1210, the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 shows a leaner air-fuel ratio than the stoichiometric air-fuel ratio. It is monitored whether or not the value has changed to a value indicating an air-fuel ratio richer than the fuel ratio.

Then, as shown at time t4 in FIG.
If the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 changes from a value that is leaner than the stoichiometric air-fuel ratio to a value that is richer than the stoichiometric air-fuel ratio, step 1210
From step 1215 to move to the fourth mode
The value of Mode is set to "4", and the value of the coefficient K is set to 1.01 in the following step 1220. As a result, the air-fuel ratio of the engine is controlled to the second rich air-fuel ratio which is richer than the stoichiometric air-fuel ratio but leaner than the first rich air-fuel ratio.

Similarly, in the fourth mode whose routine is shown in the flowchart of FIG. 13, step 1
At 305, it is determined whether the value of Mode is “4”, and Mo
If the value of de is "4", the process proceeds from step 1305 to step 1310, and in step 1310, the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 shows an air-fuel ratio leaner than the theoretical air-fuel ratio from the theoretical air-fuel ratio. It is monitored whether or not the value has changed to a value indicating an air-fuel ratio richer than the fuel ratio.

Then, as shown at time t5 in FIG.
When the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a value showing an air-fuel ratio leaner than the stoichiometric air-fuel ratio to a value showing an air-fuel ratio richer than the stoichiometric air-fuel ratio, step 1310
To step 1315 to shift to the fifth mode
The value of Mode is set to "5", and the value of the coefficient K is set to 0.98 in the following step 1320. As a result, the air-fuel ratio of the engine is controlled to the first lean air-fuel ratio.

Similarly, in the fifth mode whose routine is shown in the flowchart of FIG. 14, step 1
At 405, it is determined whether the value of Mode is "5", and Mo
If the value of de is “5”, the process proceeds from step 1405 to step 1410, and in step 1410, the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 shows an air-fuel ratio richer than the theoretical air-fuel ratio from the theoretical air-fuel ratio. It is monitored whether the air-fuel ratio is leaner than the fuel ratio.

Then, as shown at time t6 in FIG.
When the output Voxs1 of the first catalyst downstream air-fuel ratio sensor 67 changes from a value showing an air-fuel ratio richer than the stoichiometric air-fuel ratio to a value showing an air-fuel ratio leaner than the stoichiometric air-fuel ratio, step 1410
To step 1415 to shift to the sixth mode
The value of Mode is set to "6", and the value of the coefficient K is set to 0.99 in the following step 1420. As a result, the air-fuel ratio of the engine is controlled to the second lean air-fuel ratio.

In the sixth mode, the routine of which is shown in the flow chart of FIG.
In 05, it is determined whether or not the value of Mode is "6", and Mode
If the value of is 6 then steps 1505 to 1
Proceeding to 510, at step 1510, it is determined whether or not the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 has changed from a value showing an air-fuel ratio richer than the stoichiometric air-fuel ratio to a value showing a leaner air-fuel ratio than the stoichiometric air-fuel ratio. Monitor.

Then, as shown at time t7 in FIG.
If the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 changes from a value showing an air-fuel ratio richer than the stoichiometric air-fuel ratio to a value showing an air-fuel ratio leaner than the stoichiometric air-fuel ratio, step 1510 is executed.
To Step 1515, the value of Mode is reset to "0", and the value of the catalyst deterioration determination execution flag HAN is set to "0" at Step 1520, and then Step 159
The routine proceeds to step 5 to end this routine once. This gives C
The PU 71 executes step 90 when executing the routine of FIG.
Since it is determined to be “Yes” in Step 5 and the process proceeds to Step 910, the value of the coefficient K is returned to 1.00. If the other air-fuel ratio feedback control conditions and the other sub-feedback control conditions are satisfied, the CPU 71 determines in step 7
Since it is determined to be “Yes” at 05 and step 805, the air-fuel ratio feedback control and the sub feedback control are restarted.

As described above, when the catalyst deterioration determination condition is satisfied, the air-fuel ratio of the engine becomes the first lean air-fuel ratio and the second air-fuel ratio.
Lean air-fuel ratio, first rich air-fuel ratio, second rich air-fuel ratio,
The first lean air-fuel ratio and the second lean air-fuel ratio are forcibly controlled in this order.

(Estimation of Oxygen Storage Quantity and Catalyst Deterioration Judgment) Next, the operation of estimating the maximum oxygen storage quantity for determining the catalyst deterioration and determining the catalyst deterioration based on the estimated maximum oxygen storage quantity will be described. To do. CPU 71 is shown in FIG.
The routines shown by the flowcharts 6 and 17 are executed each time a predetermined time elapses.

Therefore, at a predetermined timing, CP
U71 starts the process from step 1600 of FIG.
In step 1605, the oxygen storage amount change amount ΔO2 is calculated by the following equation 12.

[0129]

[Equation 12] ΔO2 = 0.23 · mfr · (abyfs − stoich)

Next, the CPU 71 proceeds to step 1610 and determines whether or not the value of Mode is "3". If the value of Mode is "3", "Yes" in the same step 1610.
And proceeds to step 1615. And CPU7
In step 1615, a value obtained by adding the absolute value of the oxygen storage amount change amount ΔO2 to the oxygen storage amount OSA3 in the third mode at that time is set as a new oxygen storage amount OSA3 in step 1615.
After that, it proceeds to step 1650. It should be noted that the reason why the absolute value of the oxygen storage amount change amount ΔO2 is added is that, as is clear from the comparison between the above equation 1 and the above equation 12, according to the equation 12, the oxygen storage amount change amount ΔO2 in the third mode is Is calculated as a negative value.

Such a procedure (steps 1600 to 16)
15) is repeatedly executed as long as the value of Mode is "3". As a result, the oxygen storage amount OSA3 of the first catalyst 53 is calculated in the third mode (Mode = 3) in which the air-fuel ratio upstream of the first catalyst 53 is the first rich air-fuel ratio. When the determination in step 1610 is “No”, the CPU 71 directly proceeds from step 1610 to step 1620.

If the CPU 71 proceeds to step 1620, it determines whether or not the value of Mode is "4", and if the value of Mode is "4", "Yes" at step 1620.
And proceeds to step 1625. And CPU7
In step 1625, a value obtained by adding the absolute value of the oxygen storage amount change amount ΔO2 to the fourth mode oxygen storage amount OSA4 at that time is set as a new oxygen storage amount OSA4.
After that, it proceeds to step 1650. It should be noted that the absolute value of the oxygen storage amount change amount ΔO2 is added, as is clear from the comparison between the above equation 3 and the above equation 12, according to the equation 12, the oxygen storage amount change amount ΔO2 in the fourth mode is shown. Is calculated as a negative value.

Such measures (steps 1600, 16
05, 1610, 1620, 1625) is repeatedly executed as long as the value of Mode is “4”. As a result, the first
In the fourth mode (Mode = 4) in which the air-fuel ratio upstream of the catalyst 53 is the second rich air-fuel ratio, the oxygen storage amount OSA4 of the second catalyst 54 is calculated. Note that step 1620
If it is determined to be “No” in the determination in
1 directly proceeds from step 1620 to step 1630.

Similarly, when the CPU 71 proceeds to step 1630, it determines whether the value of Mode is "5",
If the value of Mode is "5", the process proceeds to step 1635, and a value obtained by adding the oxygen storage amount change amount ΔO2 to the oxygen storage amount OSA5 of the fifth mode at that time is a new oxygen storage amount OSA5.
, And then proceeds to step 1650.

Such measures (steps 1600, 16
05, 1610, 1620, 1630, 1635) is M
It is repeatedly executed as long as the value of ode is "5". As a result, in the fifth mode (Mode = 5) in which the air-fuel ratio upstream of the first catalyst 53 is set to the first lean air-fuel ratio, the first catalyst 53
The oxygen storage amount OSA5 of is calculated. If the result of the determination in step 1630 is “No”, the CPU 71 proceeds from step 1630 to step 16
Go directly to 40.

Similarly, when the CPU 71 proceeds to step 1640, it determines whether or not the value of Mode is "6",
If the value of Mode is "6", the process proceeds to step 1645, and a value obtained by adding the oxygen storage amount change amount ΔO2 to the oxygen storage amount OSA6 of the sixth mode at that time is a new oxygen storage amount OSA6.
, And then proceeds to step 1650.

Such a procedure (steps 1600, 16
05, 1610, 1620, 1630, 1640, 164
5) is repeatedly executed as long as the value of Mode is "6". As a result, in the sixth mode (Mode = 6) in which the air-fuel ratio upstream of the first catalyst 53 is the second lean air-fuel ratio,
The oxygen storage amount OSA6 of the catalyst 54 is calculated. If the result of the determination in step 1640 is “No”, the CPU 71 directly proceeds from step 1640 to step 1650.

Then, the CPU 71 proceeds to step 1650.
In step 1650, the total amount mfr of the fuel injection amount Fi is set to “0”, and then step 1695.
Then, the routine ends once by going to.

Further, the CPU 71 is adapted to repeatedly execute the routine for catalyst deterioration determination shown in FIG. 17 every time a predetermined time elapses. Therefore, at a predetermined timing, the CPU 71 starts the process from step 1700 and proceeds to step 1705 to monitor whether or not the value of the catalyst deterioration determination execution flag HAN has changed from “1” to “0”. At this time, when the sixth mode ends and the value of the catalyst deterioration determination execution flag HAN is changed to “0” in step 1520 of FIG. 15 described above, the CPU 7
In step 1705, 1 is determined as “Yes” and the process proceeds to step 1710. In addition, the catalyst deterioration determination execution flag H
If the value of AN has not changed, the CPU 71 proceeds directly from step 1705 to step 1795 to end the present routine tentatively.

Now, assuming that it is immediately after the end of the sixth mode, the value of the catalyst deterioration determination execution flag HAN is "1".
Immediately after the change from "0" to "0", the CPU 71 proceeds from step 1705 to step 1710, and the oxygen storage amounts OSA3, OSA4, OSA5, and O at that time are stored.
SA6 is the maximum oxygen storage amount CSCmax3 (first catalyst first maximum oxygen storage amount), CUFmax4 (second catalyst first maximum oxygen storage amount), CSCmax5 (first catalyst second maximum oxygen storage amount),
And CUFmax6 (second catalyst second maximum oxygen storage amount). Next, the CPU 71 proceeds to step 1715, and at step 1715, the maximum oxygen storage amount CSCmax3.
And the average value of the maximum oxygen storage amount CSCmax5, the first catalyst 53
It is stored as the average maximum oxygen storage amount CSCmax.

Next, the CPU 71 proceeds to step 1720 to determine whether the average maximum oxygen storage amount CSCmax is less than or equal to the first catalyst deterioration determination reference value CSCRave, and the average maximum oxygen storage amount average value CSCmax is the first catalyst deterioration determination. Reference value CS
If CRave or less, the value of the first catalyst deterioration determination result flag XSCR is set to "1" in step 1725,
This indicates that the first catalyst 53 has deteriorated. On the other hand,
When the average maximum oxygen storage amount CSCmax is larger than the first catalyst deterioration determination reference value CSCRave in the determination of step 1720, the CPU 71 proceeds to step 1730 to perform the first
The value of the catalyst deterioration determination result flag XSCR is set to "0", which indicates that the first catalyst 53 has not deteriorated.

Next, the CPU 71 proceeds to step 1735 and proceeds to the maximum oxygen storage amount CUFmax4 and the maximum oxygen storage amount CU.
The average value of Fmax6 is the average maximum oxygen storage amount C of the second catalyst 54.
It is stored as UFmax, and in the subsequent step 1740, the average maximum oxygen storage amount CUFmax is the second catalyst deterioration determination reference value C.
It is determined whether it is less than or equal to UFRave. The average catalyst maximum oxygen storage amount CUFmax is the second catalyst deterioration determination reference value CU.
If it is equal to or less than FRave, the CPU 71 proceeds to step 174.
In step 5, the value of the second catalyst deterioration determination result flag XUFR is set to "1" to indicate that the second catalyst 54 has deteriorated. On the other hand, when it is determined in step 1740 that the average maximum oxygen storage amount CUFmax is larger than the second catalyst deterioration determination reference value CUFRave, the CPU 71 determines in step 1
In step 750, the value of the second catalyst deterioration determination result flag XUFR is set to "0", which indicates that the second catalyst 54 has not deteriorated.

Next, the CPU 71 proceeds to step 1755 and proceeds to step 1755. The average maximum oxygen storage amount CSCmax which is the value relating to the maximum oxygen storage amount of the first catalyst 53 and the average maximum oxygen storage amount which is the value relating to the maximum oxygen storage amount of the second catalyst 54. Quantity CUFmax
And the total catalyst deterioration determination reference value CRave or less. Then, the CPU 71, when the sum is less than or equal to the catalyst overall deterioration determination reference value CRave, the step S176
At 0, the value of the catalyst overall determination result flag XALLR is set to "1".
Is set to indicate that the first catalyst 53 and the second catalyst 54 have deteriorated as a whole. On the other hand, step 175
In the determination of 5, the sum is the catalyst deterioration determination reference value C
When it is larger than Rave, the CPU 71 proceeds to step 1765.
Then, the value of the overall catalyst determination result flag XALLR is set to "0", which indicates that the first catalyst 53 and the second catalyst 54 are not deteriorated as a whole.

Next, the CPU 71 proceeds to step 1770 to set all the values of the oxygen storage amounts OSA3, OSA4, OSA5 and OSA6 to "0", and proceeds to step 1795 to end this routine once. In this way, it is determined whether or not the maximum oxygen storage amounts CSCmax, CUFmax of the first and second catalysts 53, 54, and their sums are smaller than the corresponding reference values, respectively, and when it is smaller than the corresponding reference values. , It is determined that the corresponding catalyst has deteriorated.

As described above, according to the catalyst deterioration judging method of the catalyst deterioration judging device of the present invention, the first catalyst 5
3, the first catalyst downstream air-fuel ratio sensor 67 and the second catalyst downstream air-fuel ratio sensor 68 are provided downstream of the third catalyst 54 and the second catalyst 54, respectively.
Is arranged, it is possible to reliably detect when the oxygen storage amount of each catalyst becomes “0” and when it reaches the maximum oxygen storage amount, and as a result, the first catalyst 53 and the second catalyst The maximum oxygen storage amounts CSCmax and CUFmax of 54 can be accurately obtained. Therefore, the first catalyst 53, and the second
It is possible to independently and accurately determine whether or not the catalyst 54 is deteriorated. Further, when the first catalyst 53 and the second catalyst 54 are viewed as one catalyst device, it is possible to determine whether or not the entire catalyst device has deteriorated.

During the catalyst deterioration determination execution, the first
After the oxygen storage amount of the first catalyst 53 reaches the maximum oxygen storage amount due to the lean air-fuel ratio mixture, the mixture of the second lean air-fuel ratio that is richer than the first lean air-fuel ratio is The two catalysts 53 and 54 are allowed to flow (see FIG. 5).
See times t2 to t3 and t6 to t7. ).

The reason why the second lean air-fuel ratio is made richer than the first lean air-fuel ratio (the side closer to the stoichiometric air-fuel ratio) is as follows. Since the air-fuel mixture having the second lean air-fuel ratio flows into the first and second catalysts 53 and 54, the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 shows the air-fuel ratio richer than the theoretical air-fuel ratio from the theoretical air-fuel ratio. When the air-fuel ratio becomes leaner than the fuel ratio (at times t3 and t7 in FIG. 5).
See. ), Since the space formed by the exhaust passages 51, 52 and the catalysts 53, 54 from the exhaust port 34 of the internal combustion engine 10 to the second catalyst downstream air-fuel ratio sensor 68 is filled with the gas having the second lean air-fuel ratio. , And the second lean air-fuel ratio is a lean air-fuel ratio that is significantly different from the stoichiometric air-fuel ratio (for example, the same air-fuel ratio as the first lean air-fuel ratio), the amount of nitrogen oxide NOx contained in the filled gas. However, a large amount of nitrogen oxide NOx is exhausted into the atmosphere immediately after the lean reversal of the second catalyst downstream air-fuel ratio sensor output. On the other hand, as in the above embodiment, the second
By setting the lean air-fuel ratio to an air-fuel ratio closer to the stoichiometric air-fuel ratio than the first lean air-fuel ratio, the amount of nitrogen oxide NOx contained in the space is relatively changed at the time of lean reversal of the second catalyst downstream air-fuel ratio sensor output. Therefore, a large amount of nitrogen oxide NOx can be avoided thereafter.

Since the first lean air-fuel ratio is set to the leaner side than the second lean air-fuel ratio, the first catalyst 53
It is possible to relatively shorten the period (time t1 to t2 and t5 to t6 in FIG. 5) until the maximum oxygen storage amount of the oxygen storage amount is reached, and the period for calculating the maximum oxygen storage amount ( Times t1 to t7) in FIG. 5 can also be relatively short.

Similarly, during execution of the catalyst deterioration determination, after the oxygen storage amount of the first catalyst 53 becomes "0" due to the air-fuel mixture of the first rich air-fuel ratio, the lean side of the first rich air-fuel ratio is obtained. The air-fuel mixture having the second rich air-fuel ratio of is made to flow into the first and second catalysts 53 and 54 (time t in FIG. 5).
See 4-t5. ).

The reason why the second rich air-fuel ratio is made leaner than the first rich air-fuel ratio (the side closer to the stoichiometric air-fuel ratio) is as follows. When the air-fuel mixture having the second rich air-fuel ratio flows into the first and second catalysts 53 and 54, the output Voxs2 of the second catalyst downstream air-fuel ratio sensor 68 shows a leaner air-fuel ratio than the theoretical air-fuel ratio. When the air-fuel ratio becomes richer than the fuel ratio (see time t5 in FIG. 5), the exhaust passages 51 and 52 from the exhaust port 34 of the internal combustion engine 10 to the second catalyst downstream air-fuel ratio sensor 68 and the catalyst. Since the gas formed by the second rich air-fuel ratio is filled in the space formed by 53 and 54, the second rich air-fuel ratio is significantly different from the stoichiometric air-fuel ratio. If the air-fuel ratio is the same), the amount of unburned CO and HC contained in this filled gas is large, and a large amount of unburned CO and HC is present immediately after the second catalyst downstream air-fuel ratio sensor output rich inversion. Being discharged into the atmosphere Mau. On the other hand, as in the above embodiment, by setting the second rich air-fuel ratio to an air-fuel ratio closer to the stoichiometric air-fuel ratio than the first rich air-fuel ratio, the second catalyst downstream air-fuel ratio sensor output rich reverse Unburned C contained in the space
It is possible to relatively reduce the amounts of O and HC, and therefore it is possible to avoid that a large amount of unburned CO and HC is discharged thereafter.

Since the first rich air-fuel ratio is set to be richer than the second rich air-fuel ratio, the first catalyst 53
It is possible to relatively shorten the period (time t3 to t4 in FIG. 5) until the oxygen storage amount is consumed to reach “0”, and the period for calculating the maximum oxygen storage amount (see FIG. 5). Times t1 to t7) can be relatively shortened.

The present invention is not limited to the above embodiment, and various modifications can be adopted within the scope of the present invention. For example, the maximum oxygen storage amount of each catalyst may be calculated based on the equation describing the reaction in each catalyst. Also. Although the first rich air-fuel ratio, the second rich air-fuel ratio, the first lean air-fuel ratio, and the second lean air-fuel ratio are constant, they may be variable. In that case, the second rich air-fuel ratio is the first
It is preferable that the air-fuel ratio is leaner than the rich air-fuel ratio.
The lean air-fuel ratio is preferably richer than the first lean air-fuel ratio. Further, the first rich air-fuel ratio and the second rich air-fuel ratio
The rich air-fuel ratios may be equal rich air-fuel ratios. The first lean air-fuel ratio and the second lean air-fuel ratio are
It may be the same lean air-fuel ratio.

Further, in the above embodiment, when the catalyst deterioration determination condition is satisfied, the first catalyst downstream air-fuel ratio sensor output Voxs1 and the second catalyst downstream air-fuel ratio sensor output Voxs2 at that time are not used. Although the upstream air-fuel ratio was set to the first lean air-fuel ratio, the oxygen storage amount is changed according to the first catalyst downstream air-fuel ratio sensor output Voxs1 and the second catalyst downstream air-fuel ratio sensor output Voxs2 when the catalyst deterioration determination condition is satisfied. Setting the first catalyst upstream air-fuel ratio initially set for estimation to a different air-fuel ratio is preferable in order to reduce emissions.

Specifically, when the catalyst deterioration determination condition is satisfied, it is shown that both the first catalyst downstream air-fuel ratio sensor output Voxs1 and the second catalyst downstream air-fuel ratio sensor output Voxs2 are rich air-fuel ratios. If so, control of the first catalyst upstream air-fuel ratio is started from the first mode as in the above embodiment. That is, the first catalyst upstream air-fuel ratio is set to the first lean air-fuel ratio.

On the other hand, when the catalyst deterioration determination condition is satisfied,
When the first catalyst downstream air-fuel ratio sensor output Voxs1 is lean and the second catalyst downstream air-fuel ratio sensor output Voxs2 is rich, the second mode in which the first catalyst upstream air-fuel ratio is the second lean air-fuel ratio Start control from.

When the catalyst deterioration determination condition is satisfied,
When both the first catalyst downstream air-fuel ratio sensor output Voxs1 and the second catalyst downstream air-fuel ratio sensor output Voxs2 indicate a lean air-fuel ratio, the first catalyst upstream air-fuel ratio is set to the first
The control is started from the third mode in which the rich air-fuel ratio is set. In this case, since the maximum oxygen storage amount estimated in the first third mode and the fourth mode is not accurate, the same maximum oxygen storage amount is not used for the catalyst deterioration determination, and the third oxygen storage amount is again used after the sixth mode ends. It is preferable that the maximum oxygen storage amount is measured in each mode by sequentially executing the mode and the fourth mode, and the maximum oxygen storage amount is used for catalyst deterioration determination. In this case, the maximum oxygen storage amount of the first catalyst 53 obtained in the first fifth mode is the first catalyst first maximum oxygen storage amount, and the maximum oxygen storage amount of the second catalyst 54 obtained in the next sixth mode. Is the second catalyst first maximum oxygen storage amount, the first obtained in the following third mode
The maximum oxygen storage amount of the catalyst 53 corresponds to the first catalyst second maximum oxygen storage amount, and the maximum oxygen storage amount of the second catalyst 54 obtained in the following fourth mode corresponds to the second catalyst second maximum oxygen storage amount.

Whether the first catalyst 53 is deteriorated based on the first catalyst first maximum oxygen storage amount and the first catalyst second maximum oxygen storage amount (for example, based on their average value). It is determined whether the second catalyst 54 is deteriorated based on the second catalyst first maximum oxygen storage amount and the second catalyst second maximum oxygen storage amount (for example, based on their average value). You may judge whether. In addition, based on (the average value of) these four maximum oxygen storage amounts, the first catalyst 53 and the second catalyst 5
It may be determined whether or not the catalyst devices 4 and 4 are deteriorated when they are regarded as one catalyst device.

Furthermore, when the catalyst deterioration determination condition is satisfied,
When the first catalyst downstream air-fuel ratio sensor output Voxs1 is rich and the second catalyst downstream air-fuel ratio sensor output Voxs2 is lean, the fourth mode in which the first catalyst upstream air-fuel ratio is the second rich air-fuel ratio Start control from. in this case,
Since the maximum oxygen storage amount estimated in the first fourth mode is not accurate, the maximum oxygen storage amount is not used for catalyst deterioration determination. It is preferable that the third and fourth modes are executed after the fifth and sixth modes are executed, and the maximum oxygen storage amount estimated in these modes is used for catalyst deterioration determination. In this case (when the catalyst deterioration determination condition is satisfied, it is indicated that the first catalyst downstream air-fuel ratio sensor output Voxs1 is rich and the second catalyst downstream air-fuel ratio sensor output Voxs2 is lean), In order to reduce the NOx emission amount, the control is started from the first rich air-fuel ratio, and from the time when the second catalyst downstream air-fuel ratio sensor output Voxs2 is inverted from the lean value to the rich value, the fifth, sixth, third, The four modes may be sequentially executed to obtain the maximum oxygen storage amount.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an internal combustion engine equipped with an air-fuel ratio control device (catalyst deterioration determination device) that executes a catalyst deterioration determination method according to the present invention.

FIG. 2 is a map showing a relationship between an output voltage of the air flow meter shown in FIG. 1 and a measured intake air amount.

FIG. 3 is a map showing the relationship between the output voltage of the most upstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio.

FIG. 4 is a map showing the relationship between the output voltage of the first catalyst downstream air-fuel ratio sensor and the second catalyst downstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio.

5 shows changes in the air-fuel ratio upstream of the first catalyst, the output of each air-fuel ratio sensor, and the oxygen storage amount of each catalyst when the catalyst deterioration determination device shown in FIG. 1 executes the catalyst deterioration determination. It is a time chart.

FIG. 6 is a flowchart showing a routine for calculating a fuel injection amount executed by the CPU shown in FIG.

7 is a flow chart showing a routine for calculating an air-fuel ratio feedback correction amount executed by the CPU shown in FIG.

8 is a flowchart showing a routine for calculating a sub feedback control amount executed by the CPU shown in FIG.

9 is a flowchart showing a routine executed by the CPU shown in FIG. 1 for determining whether or not to start catalyst deterioration determination.

10 is a flowchart showing a first mode routine executed by the CPU shown in FIG.

11 is a flowchart showing a second mode routine executed by the CPU shown in FIG.

12 is a flowchart showing a third mode routine executed by the CPU shown in FIG.

13 is a flowchart showing a fourth mode routine executed by the CPU shown in FIG.

FIG. 14 is a flowchart showing a fifth mode routine executed by the CPU shown in FIG.

15 is a flowchart showing a sixth mode routine executed by the CPU shown in FIG.

16 is a flowchart showing a routine executed by the CPU shown in FIG. 1 for calculating an oxygen storage amount.

17 is a flowchart showing a routine for determining catalyst deterioration executed by the CPU shown in FIG.

[Explanation of symbols]

10 ... Internal combustion engine, 25 ... Combustion chamber, 39 ... Injector,
52 ... Exhaust pipe (exhaust pipe), 53 ... Three-way catalyst (first catalyst, pre-catalyst), 54 ... Three-way catalyst (second catalyst,
Second-stage catalyst), 66 ... Most upstream air-fuel ratio sensor, 67 ... First catalyst downstream air-fuel ratio sensor, 68 ... Second catalyst downstream air-fuel ratio sensor, 70 ... Electric control device, 71 ... CPU.

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) F02D 41/14 310 F02D 41/14 310F 45/00 314 45/00 314Z 368 368F (72) Inventor Yasuhiro Oi Toyota, Aichi Prefecture City, Toyota-Cho, Toyota Motor Co., Ltd. (72) Inventor Noriho Adachi, Toyota-Cho, Aichi Prefecture Toyota-Cho, Toyota Motor Co., Ltd. (72) Inventor, Koji Ide, Toyota-City, Aichi Toyota-Cho, 1 Incorporated (72) Inventor Daisuke Kobayashi 1 Toyota-cho, Toyota-shi, Aichi Prefecture Toyota Motor Corporation (72) Inventor Shuntaro Okazaki 1-cho, Toyota-cho, Aichi Toyota Motor Corporation (72) Inventor Naoto Kato 1 Toyota Town, Toyota-shi, Aichi F-term in Toyota Motor Corporation (reference) 3G084 BA09 BA24 DA00 DA10 DA27 EA02 EA03 EA11 EC04 FA07 FA10 FA20 FA30 FA33 FA38 FA39 3G091 AA02 AA17 AA23 AA28 AB03 BA14 BA15 BA02 BA32 BA33 BA33 BA33 BA33 CB07 CB08 DA01 DA02 DB04 DB06 DB07 DB08 DB10 DB13 DC01 EA00 EA01 EA05 EA07 EA16 EA30 EA34 EA38 FB10 FB11 FB12 HA03 HA08 HA36 HA37 HA42 3G301 HA01 JA33 JB09 LA03 MA01 NA07 NA09 NB01 PA01 PE01 PE01 PE04 PEZ PE09Z0409

Claims (6)

[Claims] 1. A first catalyst disposed in an exhaust passage of an internal combustion engine, a first catalyst downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the first catalyst, and the first catalyst downstream. An internal combustion engine comprising: a second catalyst arranged in the exhaust passage downstream of the air-fuel ratio sensor; and a second catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the second catalyst. The method for determining catalyst deterioration according to claim 1, wherein the first catalyst stores oxygen up to a limit at which oxygen can be stored therein, and the second catalyst stores oxygen up to a limit at which oxygen can be stored therein. The air-fuel ratio upstream of the first catalyst is controlled to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, and thereafter, the output of the first catalyst downstream air-fuel ratio sensor is richer than the stoichiometric air-fuel ratio. The theoretical air-fuel ratio The maximum oxygen storage amount of the first catalyst is estimated by utilizing the fact that the first rich air-fuel ratio is controlled to be extremely rich and the air-fuel ratio upstream of the first catalyst is controlled to the first rich air-fuel ratio. Then, the air-fuel ratio upstream of the first catalyst is set to the second richer than the stoichiometric air-fuel ratio until the time when the output of the second catalyst downstream air-fuel ratio sensor indicates that the air-fuel ratio is richer than the stoichiometric air-fuel ratio. It is controlled to a rich air-fuel ratio, the maximum oxygen storage amount of the second catalyst is estimated by utilizing that the air-fuel ratio upstream of the first catalyst is controlled to the second rich air-fuel ratio, and the estimated first Based on the maximum oxygen storage capacity of the catalyst
A catalyst deterioration determination method for determining whether or not the catalyst is deteriorated and determining whether or not the second catalyst is deteriorated based on the estimated maximum oxygen storage amount of the second catalyst. 2. The catalyst deterioration determination method according to claim 1, wherein the second rich air-fuel ratio is an air-fuel ratio leaner than the first rich air-fuel ratio. 3. A first catalyst arranged in an exhaust passage of an internal combustion engine, a first catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the first catalyst, and the first catalyst downstream. An internal combustion engine comprising: a second catalyst arranged in the exhaust passage downstream of the air-fuel ratio sensor; and a second catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the second catalyst. The method for determining catalyst deterioration according to claim 1, wherein the first catalyst completely releases the oxygen stored therein and the second catalyst completely releases the oxygen stored therein. The air-fuel ratio upstream of the catalyst is controlled to an air-fuel ratio richer than the stoichiometric air-fuel ratio, and thereafter, the air-fuel ratio upstream of the first catalyst is set to an air-fuel ratio in which the output of the first catalyst downstream air-fuel ratio sensor is leaner than the stoichiometric air-fuel ratio. It is leaner than the theoretical air-fuel ratio until the time when it shows that it is the fuel ratio. A first lean air-fuel ratio is controlled, and the maximum oxygen storage amount of the first catalyst is estimated by utilizing the fact that the air-fuel ratio upstream of the first catalyst is controlled to the first lean air-fuel ratio, and then the The air-fuel ratio upstream of the first catalyst is set to a second lean air-fuel ratio leaner than the stoichiometric air-fuel ratio until the time when the output of the second catalyst downstream air-fuel ratio sensor indicates that the output is leaner than the stoichiometric air-fuel ratio. The maximum oxygen storage amount of the second catalyst is controlled by utilizing the fact that the air-fuel ratio upstream of the first catalyst is controlled to the second lean air-fuel ratio, and the estimated maximum oxygen of the first catalyst is controlled. A catalyst for determining whether or not the first catalyst is deteriorated based on the stored amount, and for determining whether or not the second catalyst is deteriorated based on the estimated maximum oxygen storage amount of the second catalyst Deterioration determination method. 4. The catalyst deterioration determination method according to claim 3, wherein the second lean air-fuel ratio is an air-fuel ratio richer than the first lean air-fuel ratio. 5. A first catalyst arranged in an exhaust passage of an internal combustion engine, a first catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the first catalyst, and the first catalyst downstream. An internal combustion engine comprising: a second catalyst arranged in the exhaust passage downstream of the air-fuel ratio sensor; and a second catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the second catalyst. The method for determining catalyst deterioration according to claim 1, wherein the first catalyst stores oxygen up to a limit at which oxygen can be stored therein, and the second catalyst stores oxygen up to a limit at which oxygen can be stored therein. A step of controlling the air-fuel ratio upstream of the first catalyst to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, and thereafter, changing the air-fuel ratio of the first catalyst upstream from the stoichiometric air-fuel ratio from the theoretical air-fuel ratio. Up to the point that the air-fuel ratio is also rich A step of controlling to a first rich air-fuel ratio which is richer than the stoichiometric air-fuel ratio, and thereafter, an air-fuel ratio where the output of the second catalyst downstream air-fuel ratio sensor is richer than the stoichiometric air-fuel ratio. Until the time point indicating that the second rich air-fuel ratio is richer than the stoichiometric air-fuel ratio and leaner than the first rich air-fuel ratio, the air-fuel ratio upstream of the first catalyst is the first rich air-fuel ratio. The step of estimating the maximum oxygen storage amount of the first catalyst by utilizing the fact that it is controlled to the fuel ratio, and the fact that the air-fuel ratio upstream of the first catalyst is controlled to the second rich air-fuel ratio is utilized. Estimating the maximum oxygen storage amount of the second catalyst according to the present invention. 6. A first catalyst arranged in an exhaust passage of an internal combustion engine, a first catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the first catalyst, and the first catalyst downstream. An internal combustion engine comprising: a second catalyst arranged in the exhaust passage downstream of the air-fuel ratio sensor; and a second catalyst downstream air-fuel ratio sensor arranged in the exhaust passage downstream of the second catalyst. The method for determining catalyst deterioration according to claim 1, wherein the first catalyst completely releases the oxygen stored therein and the second catalyst completely releases the oxygen stored therein. A step of controlling the air-fuel ratio upstream of the catalyst to an air-fuel ratio richer than the stoichiometric air-fuel ratio; and thereafter, the air-fuel ratio of the first catalyst upstream is controlled so that the output of the first catalyst downstream air-fuel ratio sensor is leaner than the stoichiometric air-fuel ratio. The theoretical air-fuel ratio A step of controlling the first lean air-fuel ratio to be extremely lean, and thereafter, the air-fuel ratio of the first catalyst upstream should be such that the output of the second catalyst downstream air-fuel ratio sensor is leaner than the stoichiometric air-fuel ratio. Up to the point of time at which the second lean air-fuel ratio is leaner than the stoichiometric air-fuel ratio and richer than the first lean air-fuel ratio, and the air-fuel ratio upstream of the first catalyst is controlled to the first lean air-fuel ratio. Is used to estimate the maximum oxygen storage amount of the first catalyst, and the air-fuel ratio upstream of the first catalyst is controlled to the second lean air-fuel ratio. (2) A method of estimating catalyst deterioration including the step of estimating the maximum oxygen storage amount of the catalyst.