JP2001234788A - Exhaust emission control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide control oxygen storage quantity in accordance with a deteriorated state of a catalyst in an engine equipped with the catalyst. SOLUTION: This device is furnished with a front A/F sensor 4 to detct an air-fuel ratio of exhaust gas flowing into a catalyst 3a and a rear O2 sensor 5a to detect oxygen concentration of exhaust gas flowing out of the catalyst. The air-fuel ratio of an engine is controlled so that real oxygen storage quantity of the catalyst becomes target quantity in accordance with computed oxygen storage quantity by computing the oxygen storage quantity of the catalyst in accordance with the detected air-fuel ratio. However, the maximum oxygen storage quantity is renewed from the oxygen storage quantity of the catalyst 3a while the air-fuel ratio changes between a previously specified lean judgement value and a rich judgement value by a detection result by the rear O2 sensor 5a, and the target quantity of the oxygen storage quantity is set in accordance with the maximum oxygen storage quantity which is renewed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an engine having a catalyst.

[0002]

2. Description of the Related Art The amount of oxygen absorbed in a three-way catalyst (hereinafter referred to as "oxygen storage amount") is estimated and calculated based on the amount of intake air of an engine and the air-fuel ratio of exhaust flowing into the catalyst. There is known a technique for controlling the air-fuel ratio of an engine so that the oxygen storage amount of a catalyst is constant (Japanese Patent Application Laid-Open No. 9-228873).

In order to maintain the conversion efficiency of NOx, CO, and HC of the three-way catalyst to the maximum, it is necessary to supply exhaust gas generated by combustion at the stoichiometric air-fuel ratio. However, the oxygen storage amount of the catalyst is kept constant. Therefore, when the exhaust gas flowing into the catalyst is shifted to the lean side, oxygen in the exhaust gas is absorbed by the catalyst,
When it is shifted to the rich side, the oxygen absorbed in the catalyst is released, so that the catalyst atmosphere can be substantially maintained at the stoichiometric air-fuel ratio.

By the way, in order to effectively utilize the oxygen storage function of such a catalyst, it is necessary to always control the actual oxygen storage amount to an appropriate amount with respect to the maximum oxygen storage amount of the catalyst. However, since the oxygen storage amount decreases as the catalyst deteriorates, if control is continuously performed to the target amount based on the initial maximum oxygen storage amount of the catalyst, control overshoot occurs as the use time elapses, Exhaust gas purification performance may be impaired.

An object of the present invention is to provide an exhaust gas purifying apparatus for an engine which solves such a problem and keeps the conversion efficiency of the catalyst high by updating the maximum oxygen storage amount according to the state of deterioration of the catalyst. The purpose is.

[0006]

According to a first aspect of the present invention, there is provided a catalyst provided in an engine exhaust pipe, first exhaust characteristic detecting means for detecting characteristics of exhaust gas flowing into the catalyst, and flowing out of the catalyst. Second exhaust characteristic detecting means for detecting characteristics of the exhaust gas, oxygen storage amount calculating means for calculating the oxygen storage amount and the maximum oxygen storage amount of the catalyst using the detected exhaust characteristics, and the calculated oxygen Air-fuel ratio control means for controlling the air-fuel ratio of the engine based on the storage amount so that the oxygen storage amount of the catalyst becomes a target amount determined according to the maximum oxygen storage amount.

According to a second aspect of the present invention, the oxygen storage amount calculating means according to the first aspect of the present invention uses the first exhaust characteristic to calculate a predetermined lean determination value and a rich determination value for the second exhaust characteristic. Calculate the amount of oxygen flowing into the catalyst while changing between
The maximum oxygen storage amount is calculated based on this.

In a third aspect based on the first aspect, a third exhaust characteristic detecting means for detecting a characteristic of exhaust flowing out of the second catalyst is provided, further comprising a second catalyst downstream of the catalyst, After the operation at the lean air-fuel ratio, the air-fuel ratio is controlled using the exhaust characteristics detected by the third exhaust characteristic detecting means.

In a fourth aspect based on the third aspect, an operating state in which fuel cut control such as deceleration is being performed is detected as an operating state at a lean air-fuel ratio.

In a fifth aspect based on the third aspect, the operation at the lean air-fuel ratio is determined based on the exhaust characteristic from the second exhaust characteristic detecting means.

In a sixth aspect of the present invention, the oxygen storage amount calculating means of the first aspect divides the oxygen storage amount of the catalyst into a high-speed component having a high absorption and release rate and a low-speed component having a lower absorption and release rate than the high speed component. It was configured to calculate.

A seventh invention is the third invention, wherein the third invention
When the exhaust characteristic from the second catalyst is leaner than the reference value based on the output from the exhaust characteristic detecting means of
The air-fuel ratio is controlled to be rich until the exhaust characteristic changes to the rich side from the reference value.

According to an eighth aspect of the present invention, there is provided a fuel cell system comprising: a catalyst provided in an exhaust pipe of an engine;
Exhaust characteristic detecting means, second exhaust characteristic detecting means for detecting characteristics of exhaust flowing out of the catalyst, oxygen storage amount calculating means for calculating an oxygen storage amount of the catalyst based on the first exhaust characteristic, Maximum oxygen storage amount calculating means for calculating a maximum oxygen storage amount that can be stored by the catalyst based on the second exhaust characteristic, and target amount calculating means for calculating a target amount of oxygen storage amount for the catalyst according to the maximum oxygen storage amount And air-fuel ratio control means for controlling the air-fuel ratio of the engine based on the oxygen storage amount of the catalyst such that the oxygen storage amount of the catalyst becomes the target amount.

According to a ninth aspect of the present invention, the maximum oxygen storage amount calculating means according to the eighth aspect of the present invention uses the maximum oxygen storage amount calculating means which flows into the catalyst while the second exhaust characteristic changes between a predetermined lean determination value and a rich determination value. The maximum oxygen storage amount is calculated based on the obtained oxygen amount.

According to a tenth aspect, the maximum oxygen storage amount calculating means according to the eighth aspect is characterized in that the second exhaust characteristic is set between a predetermined lean judgment value and a rich judgment value based on the first exhaust characteristic. Is changed, the amount of oxygen flowing into the catalyst is calculated, and the maximum oxygen storage amount is calculated based on the calculated amount.

According to an eleventh aspect of the present invention, the oxygen storage amount calculating means of the eighth aspect divides the oxygen storage amount of the catalyst into a high speed component having a high absorption and release speed and a low speed component having a lower absorption and release speed than the high speed component. It was configured to calculate.

According to a twelfth aspect, the oxygen storage amount calculating means of the eleventh aspect is configured to reset the high-speed component and the low-speed component to their minimum capacities when the second exhaust characteristic becomes rich. .

According to a thirteenth aspect, the oxygen storage amount calculating means of the eleventh or twelfth aspect is configured to reset the high-speed component to its maximum capacity when the second exhaust characteristic becomes lean.

[0019]

[Function / Effect] In order to effectively use the oxygen storage function of the catalyst, for example, if the air-fuel ratio control characteristic can be equally biased between the lean side and the rich side around the stoichiometric air-fuel ratio, the target It is appropriate to control the oxygen storage amount by setting it to one half of the maximum oxygen storage amount of the catalyst. However, if the target amount is fixed, the actual oxygen storage amount shifts to the excess side as the maximum oxygen storage amount decreases due to the deterioration of the catalyst, and as a result, the inside of the catalyst tends to lean and the NOx emission amount increases. May increase.

On the other hand, according to the present invention, the maximum oxygen storage amount of the catalyst is calculated and the target amount of the oxygen storage amount is set based on the calculation result. Even if
An appropriate target oxygen storage amount can always be set correspondingly, that is, exhaust purification performance can be maximized regardless of the state of deterioration of the catalyst.

As described in the second, ninth, and tenth aspects, the maximum oxygen storage amount is determined while the air-fuel ratio of the exhaust gas from the catalyst changes between a predetermined lean determination value and a rich determination value. It can be calculated based on the amount of oxygen flowing into the catalyst.

On the other hand, when a second catalyst is provided downstream of the above-mentioned catalyst in order to cope with the deterioration of the catalyst or a demand for stricter exhaust treatment performance, the air-fuel ratio control according to the exhaust characteristics of the first catalyst is usually performed. By controlling the oxygen storage amount of the catalyst by the above, it is possible to maintain the oxygen storage amount of the second catalyst at an appropriate amount. However, if the operation is continued in a state in which the fuel supply is cut off due to a lean air-fuel ratio larger than the stoichiometric air-fuel ratio or fuel cut control during deceleration, a large amount of oxygen is supplied to the exhaust pipe, so that each catalyst has The oxygen storage volume quickly reaches its maximum. From this state, the first catalyst can be promptly returned to the target oxygen storage amount by the air-fuel ratio control, but the second catalyst remains in the lean state. In particular, in the case of a deteriorated catalyst, since the maximum oxygen storage amount is reduced, the inside of the catalyst tends to be in a lean state, and the NOx purification performance is reduced.

On the other hand, in the third invention, third exhaust characteristic detecting means is provided on the outlet side of the second catalyst, and after lean operation, the exhaust characteristic detected by the third exhaust characteristic detecting means is used. Since the air-fuel ratio is controlled such that the oxygen storage amount of each catalyst on the upstream side thereof becomes a required amount, it is possible to maintain the oxygen storage amount of each catalyst within an appropriate range and maintain good exhaust gas purification performance. . As the air-fuel ratio control at this time, for example, as shown in the seventh invention, when it is detected that the exhaust characteristic from the second catalyst is leaner than the reference value, the air-fuel ratio becomes richer than the reference value. The air-fuel ratio is controlled to be rich until it changes to the side.

As the operating state at the lean air-fuel ratio, a fuel cut control state at the time of deceleration or the like is typical as shown in the fourth invention. Since the start and end can be detected from the signal of the engine controller, With the end of the fuel cut, the above control after the lean air-fuel ratio can be started. The fuel cut control is not limited to deceleration.For example, when high load or high speed operation is continued, the fuel cut control may be executed for all or some of the cylinders for engine protection. The exhaust gas flowing into the catalyst is in a lean state with excess oxygen. The lean operating condition that causes the catalyst to have an excessive oxygen storage amount is the fifth operating condition.
As described above, the detection may be directly performed from the detection result of the third exhaust characteristic detecting means.

As shown in the sixth and eleventh inventions,
The configuration is such that the oxygen storage amount of the catalyst is divided into a high-speed component with a fast absorption and release rate and a low-speed component with a slower absorption and release rate than the high-speed component to calculate the actual oxygen storage amount according to the characteristics of the catalyst. The calculation can be performed more accurately, and therefore, the actual oxygen storage amount can be controlled more accurately.

Further, according to the twelfth and thirteenth aspects,
When the downstream portion of the catalyst becomes rich or lean, the high-speed component or the low-speed component is reset, and the calculation error accumulated up to that time can be eliminated, so that the calculation accuracy of the oxygen storage amount can be further improved.

[0027]

FIG. 1 shows a schematic configuration of an exhaust gas purifying apparatus to which the present invention is applied. The exhaust gas purifying apparatus of a spark ignition type engine 1 has three catalysts 3a to 3a provided in an exhaust pipe 2.
3c and a front A located on the inlet side of the first catalyst 3a.
/ F sensor 4 and a first rear O, also located on the exit side
_{The controller includes a second} sensor 5a, a second rear O ₂ sensor 5b located on the outlet side of the second catalyst 3b, and a controller 6. Each of the catalysts 3a, 3b, 3c is the first catalyst of the present invention.
, The second catalyst, and the third catalyst. Further, the front A / F sensor 4, the first rear O2 sensor 5a, and the second rear O2 sensor 5b are respectively a first exhaust characteristic detecting means, a second exhaust characteristic detecting means, and a third exhaust characteristic of the present invention. It corresponds to a detecting means.

An intake pipe 7 of the engine 1 has an electronically controlled throttle valve 8 which can be controlled independently of the accelerator operation of the driver, and an air flow meter 9 which detects the intake air amount adjusted by the throttle valve 8. Is provided. Further, the engine 1 is provided with a crank angle sensor 12 for detecting the rotation speed. The throttle valve 8
May open and close directly in conjunction with the accelerator operation.

Each of the catalysts 3a to 3c has a three-way catalyst function,
When the inflowing exhaust gas is the combustion exhaust gas at the stoichiometric air-fuel ratio, N
Purifies Ox, HC and CO with maximum efficiency. Each of the catalyst carriers is covered with an oxygen storage material such as ceria, and has a function of absorbing or releasing oxygen according to the oxygen concentration of the inflowing exhaust gas (hereinafter, referred to as “oxygen storage function”). A single-function three-way catalyst is used as the first catalyst 3a, and HC trap catalysts with a three-way catalyst function are used as the second or third catalysts 3b and 3c. You can also.

Here, the oxygen storage amount of each of the catalysts 3a to 3c is determined by the absorption / absorption of each noble metal (Pt, Rh, Pd, etc.).
High-speed component HO2 released and absorbed by oxygen storage material /
It can be divided into the released slow component LO2. The low-speed component LO2 can absorb / release more oxygen than the high-speed component HO2, but the absorption / release speed is high-speed component HO2.
It has the characteristic that it is slower than 2.

Further, the high speed component HO2 and the low speed component L
O2:-When oxygen is absorbed, oxygen is absorbed in preference to the high-speed component HO2, and when the high-speed component HO2 reaches the maximum capacity HO2MAX and becomes unable to absorb oxygen, oxygen starts to be absorbed by the low-speed component LO2 .

When releasing oxygen, if the ratio of the low-speed component LO2 to the high-speed component HO2 (LO2 / HO2) is less than a predetermined value, that is, if the high-speed component is relatively large, oxygen is released preferentially from the high-speed component HO2. , Low-speed component LO2 against high-speed component HO2
If the ratio of the low-speed component LO2 to the high-speed component HO2 does not change, the ratio of the high-speed component HO2 and the low-speed component LO
Oxygen is released from both of the two. It has the characteristic.

A front A / A provided upstream of the catalyst 3a
The F sensor 4 linearly detects the air-fuel ratio of the exhaust gas flowing into the catalyst 3a, and a rear O ₂ provided downstream of the catalysts 3a and 3b.
The sensors 5a and 5b detect the concentration of oxygen in the exhaust gas discharged from the respective outlets in reverse to the stoichiometric air-fuel ratio. As the rear O ₂ sensors 5a and 5b, those capable of linearly detecting the air-fuel ratio as in the front A / F sensor 4 may be applied.

The engine 1 is provided with a cooling water temperature sensor 10 for detecting the temperature of the cooling water. The detected cooling water temperature is used for judging the operation state of the engine 1 and the catalyst of the catalyst 3a. Also used to estimate temperature.

The controller 6 is a microprocessor, R
AM, ROM, I / O interface, etc.
The oxygen storage amounts (high-speed component HO2 and low-speed component LO2) of the catalyst 3a are calculated based on the outputs of the air flow meter 9, the front A / F sensor 4, and the cooling water temperature sensor 10.

When the calculated high speed component HO2 of the oxygen storage amount is larger than a predetermined amount (for example, half of the maximum capacity HO2MAX of the high speed component), the controller 6 sets the engine 1
The high-speed component HO2 is decreased by shifting the air-fuel ratio to the rich side, and conversely, when the air-fuel ratio is smaller than the predetermined amount, the air-fuel ratio is shifted to the lean side to increase the high-speed component HO2, thereby increasing the high-speed component HO2 of the oxygen storage amount. Is kept constant.

Further, a difference occurs between the calculated oxygen storage amount and the actual oxygen storage amount due to the calculation error. However, the controller 6 resets the oxygen storage amount at a predetermined timing based on the oxygen concentration downstream of the catalyst 3a. To correct the deviation from the actual oxygen storage amount.

More specifically, when the first rear O ₂ sensor 5a makes a lean determination, it is determined that at least the high speed component HO2 is at a maximum, and the high speed component HO2 is reset to the maximum capacity. When the first rear O ₂ sensor 5a makes a rich determination, oxygen is not released not only from the high-speed component HO2 but also from the low-speed component LO2.
Reset HO2 and fast component LO2 to minimum capacity.

Next, the basic air-fuel ratio control performed by the controller 6 to keep the oxygen storage amount of the first catalyst 3a constant will be described in detail with reference to FIGS. Here, first, the calculation of the oxygen storage amount will be described, and then, the resetting of the oxygen storage amount and the air-fuel ratio control of the engine 1 based on the oxygen storage amount will be described.

FIG. 2 shows the contents of a routine for calculating the oxygen storage amount of the catalyst 3a, which is executed by the controller 6 at predetermined intervals.

According to this, first, typically, outputs of the cooling water temperature sensor 10, the crank angle sensor 12, and the air flow meter 9 are read as various operating condition parameters of the engine, and the temperature TCAT of the catalyst 3a is estimated based on them. (Steps S1 and S2). Then, it is determined whether or not the catalyst 3a has been activated by comparing the estimated catalyst temperature TCAT with the catalyst activation temperature TACTo (Step S).
3).

As a result, when it is determined that the catalyst activation temperature TACTo has been reached, the process proceeds to step S4 to calculate the oxygen storage amount of the catalyst 3a. Catalyst activation temperature TA
When it is determined that CTo has not been reached, the catalyst 3a does not perform the oxygen absorbing / releasing action, and terminates the processing.

In step S4, a subroutine (FIG. 3) for calculating the oxygen excess / deficiency amount O2IN is executed, and the catalyst 3a
The oxygen excess / deficiency amount O2IN in the exhaust gas flowing into the exhaust gas is calculated.
A subroutine (FIG. 4) for calculating A is executed,
The oxygen release rate A of the high-speed component is calculated.

Further, in step S6, a subroutine (FIG. 5) for calculating the high speed component HO2 of the oxygen storage amount is executed, and the high speed component HO2 and the high speed component are calculated based on the oxygen excess / deficiency amount O2IN and the oxygen release rate A of the high speed component. OVERFLOW is calculated for an overflow that is not absorbed by HO2 but overflows in the low-speed component LO2.

In step S7, it is determined based on the overflow amount OVERFLOW calculated in step S6 whether or not the oxygen excess / deficiency amount O2IN in the exhaust gas flowing into the catalyst 3a has been completely absorbed by the high speed component HO2. If the oxygen excess / deficiency amount O2IN has been completely absorbed by the high-speed component (OVERFLOW = 0), the process ends. If not, the process proceeds to step S8 to calculate the low-speed component LO2 (FIG. 6). )
Is executed, and the overflow O overflowing from the high-speed component HO2 is performed.
The low speed component LO2 is calculated based on VERFLOW.

Here, the catalyst temperature TCAT is set to the engine 1
Although the temperature is estimated from the cooling water temperature, the engine load, the engine rotation speed, etc., the temperature sensor 11 may be attached to the catalyst 3a as shown in FIG. 1 to directly measure the temperature of the catalyst 3a.

When the catalyst temperature TCAT is lower than the activation temperature TACTo in step S3, the calculation of the oxygen storage amount is not performed. However, step S3 is eliminated, and the effect of the catalyst temperature TCAT is reduced to the oxygen release rate of the high-speed component. It may be reflected in A or the oxygen absorption / release rate B of the low-speed component described later.

Next, steps S4 to S6 and step S4
The subroutine executed in step 8 will be described.

FIG. 3 shows the contents of a subroutine for calculating the oxygen excess / deficiency amount O2IN of the exhaust gas flowing into the catalyst 3a. In this subroutine, the oxygen excess / deficiency O2IN of the exhaust gas flowing into the catalyst 3a is calculated based on the air-fuel ratio upstream of the catalyst 3a and the intake air amount of the engine 1.

According to this, first, the output of the front A / F sensor and the output of the air flow meter are read (step S11).

In step S12, the read output of the front A / F sensor is converted into an air-fuel ratio using a predetermined conversion table, and the excess / deficient oxygen concentration of the exhaust gas flowing into the catalyst 3a is calculated. Here, the excess / deficiency oxygen concentration is a relative concentration based on the oxygen concentration at the stoichiometric air-fuel ratio, and the exhaust gas has a stoichiometric air-fuel ratio of zero, a rich negative value, and a lean value of a positive value.

In step S13, the output of the air flow meter is converted into an intake air amount using a predetermined conversion table, and in step S14, the intake air amount calculated in step S13 is multiplied by the excess / deficiency oxygen concentration calculated in step S12, and the catalyst 3a Calculates the excess or deficiency oxygen amount O2IN of the exhaust gas flowing into the system.

Since the excess / deficient oxygen concentration has the above characteristics, the excess / deficient oxygen amount O2IN takes a value of zero when the exhaust gas flowing into the catalyst 3a has the stoichiometric air-fuel ratio, a negative value when the exhaust gas is rich, and a positive value when the exhaust gas is lean.

FIG. 4 shows the contents of a subroutine for calculating the oxygen release rate A of the high-speed component of the oxygen storage amount. In this subroutine, since the oxygen release speed from the high speed component HO2 is affected by the low speed component LO2, the oxygen release rate A of the high speed component is calculated according to the low speed component LO2.

According to this, first, at step S21, it is determined whether or not the ratio LO2 / HO2 of the low-speed component to the high-speed component is larger than a predetermined value AR.

If it is determined that the ratio LO2 / HO2 is smaller than the predetermined value AR, that is, if the high speed component HO2 is relatively larger than the low speed component LO2, the process proceeds to step S22, where the high speed component HO2 Is preferentially released, and the oxygen release rate A of the high-speed component is set to 1.0.

On the other hand, when it is determined that the ratio LO2 / HO2 is larger than the predetermined value AR, the high speed component HO2 and the low speed component HO2 are controlled so that the ratio of the low speed component LO2 to the high speed component HO2 does not change.
Since oxygen is released from LO2, the process proceeds to step S23 to calculate a value such that the ratio LO2 / HO2 does not change as the oxygen release rate A of the high-speed component.

FIG. 5 shows the contents of a subroutine for calculating the high speed component HO2 of the oxygen storage amount. In this subroutine, the high speed component HO2 is calculated based on the oxygen oxygen excess / deficiency O2IN of the exhaust gas flowing into the catalyst 3a and the oxygen release rate A of the high speed component.

According to this, first, in step S31, it is determined whether the high speed component HO2 is in a state of absorbing oxygen or in a state of releasing oxygen based on the value of the oxygen excess / deficiency amount O2IN.

As a result, when the air-fuel ratio of the exhaust gas flowing into the catalyst 3a is lean and the oxygen excess / deficiency O2IN is greater than zero, it is determined that the high-speed component HO2 is in a state of absorbing oxygen, and step S32 is performed. Then, the following equation (1), HO2 = HO2z + O2IN (1) HO2z: The high-speed component HO2 is calculated from the previous value of the high-speed component HO2.

On the other hand, if the oxygen excess / deficiency amount O2IN is a value equal to or less than zero and it is determined that the high-speed component is in a state of releasing oxygen, the process proceeds to step S33, and the following equation (2): HO2 = HO2z + O2IN × A: (2) A: The high-speed component HO2 is calculated from the oxygen release rate of the high-speed component HO2.

When the high speed component HO2 is calculated in this way, in steps S34 and S35, the value does not exceed the maximum capacity HO2MAX of the high speed component, or the minimum capacity HO2MIN.
(= 0) is determined.

If the high speed component HO2 is larger than the maximum capacity HO2MAX, the process proceeds to step S36, where the high speed component HO2 is
Overflow amount (excess amount) O overflowing without being absorbed by HO2
VERFLOW is calculated by the following equation (3), OVERFLOW = HO2-HO2MAX ... (3), and the high-speed component HO2 has the maximum capacity HO2MA.
Limited to X.

If the high speed component HO2 is less than the minimum capacity HO2MIN, the process proceeds to step S37, where the high speed component HO2 is
Overflow amount (insufficient amount) OVE overflowing without being absorbed by 2
RFLOW is calculated by the following equation (4), OVERFLOW = HO2-HO2MIN ... (4), and the high-speed component HO2 has the minimum capacity HO2MI
Limited to N. In this case, since 0 is given as the minimum capacity HO2MIN, the amount of oxygen deficient when all the high-speed components HO2 are released is calculated as a negative overflow amount.

When the high-speed component HO2 is between the maximum capacity HO2MAX and the minimum capacity HO2MIN, the oxygen excess / deficiency O2IN of the exhaust gas flowing into the catalyst 3 is all absorbed by the high-speed component HO2. Is set to zero.

Here, the overflow OVERFLOW that overflows from the high-speed component HO2 when the high-speed component HO2 is equal to or more than the maximum capacity HO2MAX or equal to or less than the minimum capacity HO2MIN is absorbed or released by the low-speed component LO2.

FIG. 6 shows a low-speed component of the oxygen storage amount.
Shows the contents of the subroutine for calculating LO2. In this subroutine, the low speed component LO2 is calculated based on the overflow OVERFLOW overflowing from the high speed component HO2.

According to this, in step S41, the low speed component LO2 is calculated by the following equation (5): LO2 = LO2z + OVERFLOW × B (5) LO2z: Previous value of low speed component LO2 B: Oxygen absorption / release rate of low speed component Is done. Where the oxygen absorption and release rate B of the slow component
Is set to a positive value of 1 or less, but in fact it has different characteristics for absorption and release, and the actual absorption and release rate is affected by the catalyst temperature TCAT, low-speed component LO2, etc. And the emission rate may be set separately. In this case, when the overflow amount OVERFLOW is positive, oxygen is excessive. At this time, the oxygen absorption B is set to a larger value as the catalyst temperature TCAT is higher and the lower speed component LO2 is smaller. Also, the overflow amount OVER
When FLOW is negative, oxygen is deficient. At this time, the oxygen release rate B is set to be larger, for example, as the catalyst temperature TCAT is higher and as the low-speed component LO2 is larger.

In steps S42 and S43, the high speed component HO
As in the calculation of 2, the calculated low speed component LO2 does not exceed its maximum capacity LO2MAX, or the minimum capacity LO2MIN
(= 0) is determined.

As a result, if the maximum capacity LO2MAX is exceeded, the process proceeds to step S44, where the oxygen excess / deficiency O2OUT overflowing from the low-speed component LO2 is calculated by the following equation (6), O2OUT = LO2-LO2MAX (6). The low-speed component LO2 is limited to the maximum capacity LO2MAX. The oxygen excess / deficiency amount O2OUT directly flows out downstream of the catalyst 3a.

On the other hand, if it is less than the minimum capacity, the process proceeds to step S45, where the low-speed component LO2 is limited to the minimum capacity LO2MIN.

Next, the reset of the oxygen storage amount performed by the controller 6 will be described. By executing the reset of the oxygen storage amount, the calculation error accumulated so far is eliminated, and the calculation accuracy of the oxygen storage amount can be improved.

FIG. 7 shows the contents of the reset condition determination routine. This routine determines whether or not the reset condition of the oxygen storage amount (high-speed component HO2 and low-speed component LO2) is satisfied from the oxygen concentration downstream of the catalyst 3a, and sets the flag Frich and the flag Flean.

According to this, first, the output of the rear O ₂ sensor 5a for detecting the oxygen concentration downstream of the catalyst 3a is read (step S51). Then, the output of the rear O ₂ sensor is compared with the lean determination threshold value and the rich determination threshold value (steps S52 and S53).

If the result of the comparison indicates that the rear O ₂ sensor output is lower than the lean determination threshold value, the flow advances to step S 54 to set “1” in the flag Flean indicating that the lean reset condition for the oxygen storage amount has been satisfied. Is done. Also,
If the rear O ₂ sensor output is higher than the rich determination threshold, the process proceeds to step S55, and “1” indicating that the rich reset condition of the oxygen storage amount is satisfied is set in the flag Frich.

If the rear O ₂ sensor output is between the lean determination threshold and the rich determination threshold, step S56
The flag Flean and Frich are set to “0” indicating that the lean reset condition and the rich reset condition are not satisfied.

The weighted average value may be used as the output of the rear O ₂ sensor. Although the output of the first rear O ₂ sensor 5a is used here, the reset condition may be determined based on the output of the second rear O ₂ sensor 5b after a lean condition described later.

FIG. 8 shows the contents of a routine for resetting the oxygen storage amount.

According to this, in steps S61 and S62, it is determined whether the lean reset condition or the rich reset condition is satisfied based on the change in the values of the flags Flean and Frich.

Then, the flag Flean is changed from "0" to "1".
When it is determined that the lean reset condition is satisfied, the process proceeds to step S63, and the high speed component HO2 of the oxygen storage amount is reset to the maximum capacity HO2MAX. At this time, the low-speed component LO2 is not reset. On the other hand, the flag F
If rich is changed from “0” to “1” and it is determined that the rich reset condition is satisfied, the process proceeds to step S64,
The high speed component HO2 and the low speed component LO2 of the oxygen storage amount are reset to the minimum capacities HO2MIN and LO2MIN, respectively.

The reason why the reset is performed under such conditions is that the oxygen absorption rate of the low-speed component LO2 is low, so that when the high-speed component HO2 reaches the maximum capacity, the oxygen is not transferred downstream of the catalyst even if the low-speed component LO2 does not reach the maximum capacity. This is because at least the high speed component HO2 is considered to have the maximum capacity when the downstream of the catalyst becomes lean.

When the downstream of the catalyst becomes rich,
This is because no oxygen is released from the low-speed component LO2 that slowly releases oxygen, and it is considered that both the high-speed component HO2 and the low-speed component LO2 hardly hold oxygen and have a minimum capacity.

Further, the air-fuel ratio control (oxygen storage amount constant control) performed by the controller 6 will be described.

FIG. 9 shows the contents of the routine for calculating the target air-fuel ratio from the oxygen storage amount.

According to this, first, the high-speed component HO2 of the current oxygen storage amount is read (step S71),
Deviation DHO2 between current high-speed component HO2 and target value TGHO2 of high-speed component
(= The oxygen excess / deficiency required by the catalyst 3a) is calculated (step S72). The target value TGHO2 of the high-speed component is
For example, it is set to one half of the maximum capacity HO2MAX of the high-speed component.

In step S73, the calculated deviation DHO2 is converted into a value corresponding to the air-fuel ratio, and the target air-fuel ratio of the engine 1 is set.

Therefore, according to this routine, when the high-speed component HO2 of the oxygen storage amount is less than the target amount, the target air-fuel ratio of the engine 1 is set to the lean side,
The oxygen storage amount (high-speed component HO2) is increased.
On the other hand, when the high-speed component HO2 exceeds the target amount, the target air-fuel ratio of the engine 1 is set to the rich side,
The oxygen storage amount (high-speed component HO2) is reduced.

Next, the overall operation of the above control will be described.

In the exhaust gas purifying apparatus according to the present invention, when the engine 1 is started, the calculation of the oxygen storage amount of the catalyst 3a is started, and the oxygen storage amount of the catalyst 3a is maintained in order to keep the conversion efficiency of the catalyst 3a at a maximum. The air-fuel ratio control of the engine 1 is performed so that the amount becomes constant.

The controller 6 estimates and calculates the oxygen storage amount of the catalyst 3a based on the air-fuel ratio of the exhaust gas flowing into the catalyst 3a and the intake air amount of the engine 1. At this time, the calculation of the oxygen storage amount is based on the high speed component HO2 and the low speed component HO2. Perform separately for LO2.

More specifically, the controller 6 performs the calculation by assuming that the high-speed component HO2 is preferentially absorbed during oxygen absorption, and that the low-speed component LO2 starts to be absorbed when the high-speed component HO2 cannot be completely absorbed. When oxygen is released, the low-speed component LO
If the ratio of 2 to the high-speed component HO2 (LO2 / HO2) is below a certain ratio AR, oxygen is released preferentially from the high-speed component HO2,
When the ratio LO2 / HO2 reaches a certain ratio, the oxygen storage amount is calculated assuming that oxygen is released from both the low speed component LO2 and the high speed component HO2 so as to maintain the ratio LO2 / HO2.

When the calculated high speed component HO2 of the oxygen storage amount is larger than the target value, the controller 6
Controls the air-fuel ratio of the engine 1 to the rich side to increase the high-speed component HO
2 is decreased, and when it is smaller than the target value, the air-fuel ratio is controlled to the lean side to increase the high-speed component HO2.

As a result, the high-speed component HO of the oxygen storage amount
Since 2 is maintained at the target value, even if the air-fuel ratio of the exhaust gas flowing into the catalyst 3 deviates from the stoichiometric air-fuel ratio, oxygen is immediately absorbed or released from the high-response high-speed component HO2, and the catalyst atmosphere is reduced. The correction is made in the direction of the stoichiometric air-fuel ratio, and the conversion efficiency of the catalyst 3 is kept at the maximum.

Further, when the calculation error accumulates, the calculated oxygen storage amount deviates from the actual oxygen storage amount. However, at the timing when the downstream of the catalyst 3 becomes rich or lean, the oxygen storage amount (high-speed component HO2 and low-speed component HO2) L
O2) is reset, and the difference between the calculated value and the actual oxygen storage amount is corrected.

FIG. 10 shows how the high-speed component HO2 changes when the oxygen storage amount constant control is performed. In this case, at times t ₂ and t ₃ , the rear O ₂ sensor 5
Since the output of a becomes equal to or higher than the rich determination threshold value and the rich reset condition is satisfied, the high-speed component of the oxygen storage amount
HO2 is reset to the minimum capacity (= 0). At this time, the low speed component LO2 is also reset to the minimum capacity (not shown).

At time t ₁ , the output of the rear O ₂ sensor 5a becomes equal to or less than the lean determination threshold value, and the lean reset condition is satisfied. Therefore, the high speed component HO2 is reset to the maximum capacity HO2MAX. However, at this time, the low-speed component LO2 is not necessarily at its maximum, so that the low-speed component LO2 is not reset.

As described above, the oxygen storage amount is reset at the timing when the exhaust gas downstream of the catalyst 3a becomes rich or lean, and the deviation from the actual oxygen storage amount is corrected. Is further improved, the accuracy of air-fuel ratio control for keeping the oxygen storage amount constant is also increased, and the conversion efficiency of the catalyst can be kept high.

The above is an example of the air-fuel ratio control based on the present invention. In the present invention, the catalysts 3a to 3
By learning and correcting the maximum oxygen storage amounts HO2MAX and 2HO2MAX of b, the oxygen storage amount of each catalyst is optimally controlled according to the catalyst deterioration state. Hereinafter, this point will be described with reference to FIGS.

FIGS. 11 and 13 show the first and second catalysts 3a, 3a.
Control Content of Embodiment for Controlling Oxygen Storage Volume of 3b
FIG. 12 shows an air-fuel ratio obtained by this control.
It is a time chart which shows a state of a change. This process
Periodically executed in synchronization with the air-fuel ratio control described above
The rear O used in the reset condition determination routine of FIG. _Two
Selection of sensor output and use in air-fuel ratio control routine of FIG.
To set the target oxygen storage amount (TGHO2)
have.

In this process, first, it is determined whether or not the fuel cut control at the time of deceleration or the like has been performed (step S81). The presence or absence of the fuel cut control is determined by monitoring the signal of the fuel control system as described above or by independently detecting the fuel cut condition. As an example of a control condition for the fuel cut control during deceleration, the start condition is that the vehicle speed, the engine speed is equal to or higher than the reference value, the accelerator pedal is released, and the transmission is not neutral, The fuel cut thus started is terminated and recovery is performed when the engine rotation speed becomes equal to or lower than the lower limit reference value or when the accelerator pedal is depressed. Therefore, by monitoring such conditions,
Fuel cut and recovery during deceleration can be determined.

In this step, it is determined whether or not recovery after fuel cut has started, and the flag FFCR is set to 1 at the time of detection of recovery start (step S8).
1, S84). The flag FFCR indicates that the transient rich air-fuel ratio control after the recovery is being executed, and is reset to 0 when this control ends (step S87). When the flag FFCR is 0, the output of the first rear O ₂ sensor 5a is employed as the output of the rear O ₂ sensor used for the reset determination (FIG. 7), and the maximum value of the first catalyst 3a is set as the target oxygen storage amount TGHO2. Oxygen storage volume H
One half of O2MAX is set (steps S82, S8
3). As a result, the oxygen storage amount of the first catalyst 3a is maintained at about half of the maximum amount by the above-described air-fuel ratio control, and stable exhaust purification performance is exhibited. During this time, the air-fuel ratio of the exhaust gas from the upstream side is stable near the stoichiometric air-fuel ratio of the downstream-side catalysts 3b and 3c.
The maximum oxygen storage amount HO2MAX is updated according to the state of deterioration of the catalyst by a learning process described later.

On the other hand, during the air-fuel ratio control after the recovery (FFCR = 1), first, it is determined whether or not the calculated oxygen storage amount is the target amount. In step S85, the flag FFCR is reset to 0, and the process returns to normal processing (steps S85 and S87). Here, if the oxygen storage amount is not the target amount, the second rear O ₂ sensor 5b is used as the output of the rear O ₂ sensor used for reset determination.
And the maximum oxygen storage amounts HO2MAX and 2HO2M of the first catalyst 3a and the second catalyst 3b, respectively.
One half of the sum of AX is set as the target oxygen storage amount TGHO2 (step S86). Thereby, each catalyst 3a,
Since the air-fuel ratio control is performed so that the oxygen storage amount of the catalyst 3b becomes about one half of each, the catalysts 3a and 3b having the excessive oxygen storage amount in the lean atmosphere by the fuel cut are subjected to the oxygen storage. The storage amount quickly returns to an appropriate amount to restore the expected exhaust purification performance.

Next, a process for updating the maximum oxygen storage amounts HO2MAX and 2HO2MAX in order to appropriately set the target oxygen storage amount in response to catalyst deterioration will be described with reference to the flowchart shown in FIG. In this processing, the exhaust oxygen concentration from the first catalyst 3a is reduced to the first rear O
₍₂₎ Detecting the output OSV of the sensor 5a and integrating the amount of oxygen flowing into the catalyst 3a when the output OSV changes between a predetermined lean determination value THL and a rich determination value THR (see FIG. 14). Is used to calculate the maximum oxygen storage amount.

Specifically, first, the output OV of the rear O ₂ sensor is detected, and a flag Fr- is determined as to whether the output has crossed the rich judgment value THR in the lean direction or whether it has crossed the lean judgment value THL in the rich direction. The determination is made with reference to l and Fl-r (steps S901 and S902). The flag Fr-l,
Fl-r is initially set to 0, and when the rear O ₂ sensor output OSV crosses the rich determination value THR in the lean direction (see FIG. 14A), Fr-l = 1. When the vehicle crosses the lean determination value THL in the rich direction (see FIG. 14B), Fl-r = 1 is set, and the learning value HO2LRN of the maximum oxygen storage amount is initialized ( Steps S903, S904, S905, S90
6). Each of the flags Fr-l and Fl-r is 0 and the rear O
_{When the two-} sensor output OSV does not change so as to cross each of the determination values THR and THL, the previously stored maximum oxygen storage amount HO2MAX is maintained (step S907).
The change in the air-fuel ratio between the rich judgment value THR and the lean judgment value THL is determined by using the air-fuel ratio change generated by the air-fuel ratio control after the lean operation shown in FIG. It may be generated intentionally for calculating the amount.

If the flag Fr-l or Fl-r is 1 as a result of the flag determination in step S902, the process of integrating the amount of oxygen flowing into the catalyst 3a is next performed (step S9).
08). Here, first, when the rear O ₂ sensor output OSV crosses the rich determination value THR in the lean direction (Fr-l =
1) integrates the learning value HO2LRN with the oxygen inflow amount ΔHO2 per unit time (for example, one cycle of the control loop) until the output OSV crosses the lean determination value THL, and adds this to the new maximum oxygen storage. Set as quantity HO2MAX. Or, when the rear O ₂ sensor output OSV crosses the lean determination value THL in the rich direction (Fl−r = 1), the output is then changed to the rich determination value
Oxygen inflow ΔH per unit time before crossing THR
O2 is added to the learning value HO2LRN, and this is set as a new maximum oxygen storage amount HO2MAX (steps S908 and S908).
910). The amount of oxygen flowing into the catalyst 3a can be determined, for example, by the product of the oxygen concentration detected by the front A / F sensor 4 and the intake air amount detected by the air flow meter 9.

After the completion of the integration of the maximum oxygen storage amount HO2MAX, the maximum oxygen storage amount 2HO2MAX of the second catalyst 3b is estimated based on the maximum oxygen storage amount HO2MAX updated based on the learning result, and each flag Fr-l , Fl-r
Is reset to 0, and the current learning process ends (steps S911 and S912). The maximum oxygen storage amount 2HO2MAX of the second catalyst 3b is, for example, as shown in FIG. 15, a characteristic or a table prepared in advance so that the maximum oxygen storage amount HO2MAX of the first catalyst 3a is given as a parameter. Set based on: Note that the characteristics shown in FIG. 15 indicate that if the catalysts 3a and 3b are simultaneously used, the deterioration states of the respective catalysts 3a and 3b do not greatly differ. Therefore, the maximum oxygen storage amount of the second catalyst 3b is: This is based on the fact that it can be considered to decrease at the same rate as the maximum oxygen storage amount of the first catalyst.

On the other hand, if the rear O ₂ sensor output OSV crosses the rich determination value THR in the lean direction due to a sudden change in the operating state or the like, and returns to the rich determination value THR without reaching the lean determination value THL. Or, when the output OSV crosses the lean determination value THL in the rich direction and then returns to the lean determination value THL without reaching the rich determination value THR, the maximum oxygen storage amount can be appropriately calculated. Since it is not possible, the flags Fr-l and Fl-r are reset to 0 and the current processing is terminated (steps S909-S91) irrespective of the accumulated result of the learning value HO2LRN.
2).

As described above, by updating the actual maximum oxygen storage amount HO2MAX or 2HO2MAX in accordance with the state of deterioration of the catalyst, the oxygen storage amount of the catalyst can be optimally controlled based on this, that is, It is possible to avoid overshoot of the oxygen storage amount due to catalyst deterioration and always maintain good exhaust emission performance of the engine.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an exhaust gas purification device according to the present invention.

FIG. 2 is a flowchart showing a routine for calculating an oxygen storage amount of a catalyst.

FIG. 3 is a flowchart showing the contents of a subroutine for calculating an oxygen excess / deficiency amount of exhaust gas flowing into a catalyst.

FIG. 4 is a flowchart showing the contents of a subroutine for calculating the oxygen release rate of a high-speed component.

FIG. 5 is a flowchart showing the contents of a subroutine for calculating a high-speed component of the oxygen storage amount.

FIG. 6 is a flowchart showing the contents of a subroutine for calculating a low speed component of the oxygen storage amount.

FIG. 7 is a flowchart showing the contents of a reset condition determination routine.

FIG. 8 is a flowchart showing the contents of a routine for resetting the oxygen storage amount.

FIG. 9 is a flowchart showing a routine for calculating a target air-fuel ratio from an oxygen storage amount.

FIG. 10 is a time chart showing a state when the oxygen storage amount constant control is performed.

FIG. 11 is a flowchart showing the contents of a processing routine related to air-fuel ratio control after a lean operation.

FIG. 12 is a time chart showing a state when the air-fuel ratio control is performed.

FIG. 13 is a flowchart showing the contents of a processing routine for calculating a maximum oxygen storage amount.

FIG. 14 is an explanatory diagram showing air-fuel ratio conditions when calculating a maximum oxygen storage amount.

FIG. 15 is an explanatory diagram of a characteristic of giving an oxygen storage amount of a second catalyst from a maximum oxygen storage amount of a first catalyst according to a deterioration state.

[Explanation of symbols]

Reference Signs List 1 engine 2 exhaust pipe 3a first catalyst 3b second catalyst 3c third catalyst 4 front A / F sensor (first exhaust characteristic detecting means) 5a first rear O ₂ sensor (second exhaust characteristic detection) Means) 5b Second rear O ₂ sensor (third exhaust characteristic detecting means) 6 Controller 7 Intake pipe 8 Throttle valve 9 Air flow meter 10 Cooling water temperature sensor 11 Temperature sensor 12 Crank angle sensor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F02D 41/12 305 F02D 41/12 305 330J 330 45/00 314Z 45/00 314 358N 358 370B 370 B01D 53/36 103Z F-term (reference) 3G084 BA05 BA09 CA04 CA06 DA08 DA10 DA25 EA05 EA07 EA11 EB08 EB12 EB20 EB24 EB25 EC01 EC03 EC04 FA05 FA06 FA07 FA10 FA13 FA20 FA27 FA30 FA33 FA02 A3AB01A28 BA15 BA19 BA32. KA09 KA16 KA25 KA26 KA27 LA03 MA01 MA24 MA25 NA02 NA 08 NA09 NB00 NB02 NC02 NC08 ND02 ND25 NE13 NE15 NE17 NE19 NE23 PA01Z PB03Z PD02A PD03A PD04A PD09A PD09Z PD12Z PE01Z PE03Z PE08Z PF01Z PF03Z PF07Z 4D048 AA06 AA13 AA18 AB01 DA01 DA02 EA04

Claims (13)

[Claims] 1. A catalyst provided in an engine exhaust pipe, first exhaust characteristic detecting means for detecting characteristics of exhaust flowing into the catalyst, and second exhaust detecting characteristics of exhaust flowing from the catalyst. A characteristic detecting unit, an oxygen storage amount calculating unit that calculates an oxygen storage amount and a maximum oxygen storage amount of the catalyst using the detected exhaust characteristics, and an oxygen storage amount of the catalyst based on the calculated oxygen storage amount. An exhaust gas purification device for an engine, comprising: air-fuel ratio control means for controlling the air-fuel ratio of the engine such that the amount becomes a target amount determined according to the maximum oxygen storage amount. 2. The method according to claim 1, wherein the oxygen storage amount calculating means uses the first exhaust characteristic to determine whether the second exhaust characteristic changes between a predetermined lean determination value and a predetermined rich determination value. The exhaust gas purifying apparatus for an engine according to claim 1, wherein an amount of the inflowing oxygen is calculated, and a maximum oxygen storage amount is calculated based on the calculated amount. 3. A method according to claim 3, further comprising the step of: providing a second catalyst downstream of the catalyst and detecting a characteristic of exhaust gas flowing out of the second catalyst.
2. An exhaust system for an engine according to claim 1, wherein said exhaust characteristic detecting means is provided to control an air-fuel ratio using the exhaust characteristic detected by said third exhaust characteristic detecting means after operation at a lean air-fuel ratio. Purification device. 4. The exhaust gas purifying apparatus for an engine according to claim 3, wherein an operating state in which fuel cut control is performed during deceleration or the like is detected as an operating state at a lean air-fuel ratio. 5. The exhaust gas purifying apparatus for an engine according to claim 3, wherein the operation at the lean air-fuel ratio is determined based on exhaust characteristics from the second exhaust characteristic detecting means. 6. The engine according to claim 1, wherein the oxygen storage amount calculating means calculates the oxygen storage amount of the catalyst by dividing the oxygen storage amount into a high-speed component having a high absorption and release speed and a low-speed component having a lower absorption and release speed than the high-speed component. Exhaust purification equipment. 7. When the exhaust characteristic from the second catalyst becomes leaner than the reference value based on the output from the third exhaust characteristic detecting means, the exhaust characteristic becomes richer than the reference value. 4. The exhaust gas purifying apparatus for an engine according to claim 3, wherein the air-fuel ratio is controlled to be rich until the air-fuel ratio changes. 8. A catalyst provided in an exhaust pipe of the engine, first exhaust characteristic detecting means for detecting characteristics of exhaust flowing into the catalyst, and second exhaust characteristics for detecting characteristics of exhaust flowing out of the catalyst. Detecting means; oxygen storage amount calculating means for calculating the oxygen storage amount of the catalyst based on the first exhaust characteristic; and maximum oxygen calculating the maximum oxygen storage amount that the catalyst can store based on the second exhaust characteristic. Storage amount calculation means; target amount calculation means for calculating a target amount of oxygen storage amount of the catalyst according to the maximum oxygen storage amount; And an air-fuel ratio control means for controlling an air-fuel ratio of the engine. 9. A maximum oxygen storage amount calculating means based on the amount of oxygen flowing into the catalyst while the second exhaust characteristic changes between a predetermined lean determination value and a rich determination value. The exhaust gas purifying apparatus for an engine according to claim 8, wherein the amount is calculated. 10. A maximum oxygen storage amount calculating means, comprising:
While the second exhaust characteristic changes between a predetermined lean determination value and a rich determination value, the amount of oxygen flowing into the catalyst is calculated based on the exhaust characteristic, and the maximum oxygen storage amount is calculated based on the calculated amount. The exhaust gas purifying apparatus for an engine according to claim 8, which calculates the following. 11. The engine according to claim 8, wherein the oxygen storage amount calculation means calculates the oxygen storage amount of the catalyst by dividing the oxygen storage amount into a high-speed component having a high absorption and release speed and a low-speed component having a lower absorption and release speed than the high-speed component. Exhaust purification equipment. 12. The exhaust gas purifying apparatus for an engine according to claim 11, wherein the oxygen storage amount calculating means resets the high-speed component and the low-speed component to their minimum capacity when the second exhaust characteristic is enriched. 13. The exhaust gas purifying apparatus for an engine according to claim 11, wherein the oxygen storage amount calculating means resets the high-speed component to its maximum capacity when the second exhaust characteristic becomes lean.