JP3536764B2 - Engine exhaust purification device - Google Patents

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 gas flowing into the catalyst. A technique for controlling the air-fuel ratio of an engine so that the pressure becomes constant is known (JP-A-9-228873).
issue).

[0003] In order to maintain the conversion efficiency of NOx, CO and HC of the three-way catalyst to the maximum, it is necessary to set the catalyst atmosphere to the stoichiometric air-fuel ratio, but by keeping the oxygen storage amount of the catalyst constant, When the exhaust gas flowing into the catalyst is shifted to the lean side, oxygen in the exhaust gas is absorbed by the catalyst, and when the exhaust gas is shifted to the rich side, the oxygen absorbed by the catalyst is released. At the stoichiometric air-fuel ratio.

[0004] Therefore, in an exhaust gas purifying apparatus that performs such control, an accurate calculation of the oxygen storage amount is required in order to keep the conversion efficiency of the catalyst high.

[0005]

[Problems to be solved by the invention]
The conventional calculation method has a problem that it is difficult to accurately calculate the oxygen storage amount of the catalyst.

[0006] This is because the actual oxygen storage characteristics are divided into the characteristics of being rapidly absorbed / released by the noble metal of the catalyst and the characteristics of being slowly absorbed / released by the oxygen storage material such as ceria of the catalyst. Regardless, conventionally, this point is not considered, and the oxygen storage amount is represented by one parameter.

Therefore, the present applicant has proposed a technique for improving the calculation accuracy of the oxygen storage amount by calculating the oxygen storage amount separately for the high-speed component and the low-speed component in accordance with the actual characteristics. No. 2000-34046). Further, since it is considered that the modification of the catalyst atmosphere is mainly performed by a high-speed component having a high oxygen absorption and release rate,
For example, a technique has been proposed in which the conversion efficiency of the catalyst is kept high by controlling the air-fuel ratio of the engine so that the high-speed component becomes half of its maximum capacity.

However, when the air-fuel ratio of the exhaust gas flowing into the catalyst fluctuates, the air-fuel ratio in the catalyst atmosphere is set to a target value,
For example, there is a problem that the ability to maintain the stoichiometric air-fuel ratio differs between the rich side and the lean side.

That is, since the oxygen absorption speed of the low-speed component is low on the oxygen absorption side, when a lean spike occurs in the air-fuel ratio due to disturbance or the like, the maximum amount of oxygen that can be absorbed is half the maximum capacity of the high-speed component. Only up to On the other hand, since the oxygen release side releases oxygen at a relatively high speed from the low-speed component, the amount of oxygen that can be released when a rich spike occurs in the air-fuel ratio is obtained by adding the low-speed component to the high-speed component Amount.

Therefore, the ability to maintain the catalyst atmosphere at the target air-fuel ratio when the exhaust gas shifts to the lean side is lower than the ability to maintain the catalyst atmosphere at the target air-fuel ratio when the exhaust gas shifts to the rich side. Even if the same deviation as that on the rich side occurs on the lean side, the catalyst may not be able to absorb it. In order to ensure stable conversion efficiency, it is necessary to maintain the catalyst atmosphere at the target air-fuel ratio even if the air-fuel ratio of the exhaust gas flowing into the catalyst shifts to the lean side.

The present invention has been made in view of the above technical problems, and has as its object to secure stable catalyst conversion efficiency in an exhaust gas purification device that controls an air-fuel ratio based on the oxygen storage amount of a catalyst. I do.

[0012]

According to a first aspect of the present invention, there is provided an exhaust gas purifying apparatus for an engine, comprising: a catalyst provided in an exhaust pipe; a means for detecting characteristics of exhaust gas flowing into the catalyst; Oxygen storage amount calculating means for calculating the oxygen storage amount of the catalyst into a high-speed component having a high absorption rate and a low-speed component having a lower absorption speed than the high-speed component. Means for setting a target value, and air-fuel ratio control means for controlling the air-fuel ratio of the engine based on the calculated high-speed component such that the high-speed component of the oxygen storage amount of the catalyst becomes the target value. To do.

According to a second aspect of the present invention, in the first aspect, the means for setting the target value of the high-speed component sets the target value of the high-speed component to be smaller as the calculated low-speed component increases. is there.

[0014]

Therefore, in the exhaust gas purifying apparatus according to the present invention, the oxygen storage amount of the catalyst is calculated based on the characteristics of the exhaust gas flowing into the catalyst (for example, the exhaust air-fuel ratio). The characteristics of oxygen storage are divided into the characteristics that are absorbed / released at high speed by noble metals and the characteristics that are absorbed / released at low speed by oxygen storage material such as ceria of the catalyst. Is calculated separately. The air-fuel ratio of the engine is controlled such that the high-speed component of the oxygen storage amount becomes the target value based on the calculated high-speed component, so that the conversion efficiency of the catalyst is maintained at a high level.

When the inflowing exhaust gas fluctuates to the lean side, the catalyst atmosphere is corrected mainly by the high-speed component because the oxygen absorption speed of the low-speed component is lower than that of the high-speed component. The correction of the catalyst atmosphere in the case of (1) is performed by both the high-speed component and the low-speed component since oxygen is released from the low-speed component at a relatively high speed.

As a result, the amount of oxygen absorbed and the amount of oxygen released that can be used for correcting the catalyst atmosphere are not equal, and the ability to maintain the catalyst atmosphere at the target air-fuel ratio fluctuates toward the rich side when it fluctuates toward the lean side. It will be lower than the case.

However, according to the present invention, the target value of the high-speed component is set according to the low-speed component in consideration of the influence of the low-speed component on the ability to maintain the catalyst atmosphere at the target air-fuel ratio. For example, the target value of the high-speed component is set to a smaller value as the low-speed component increases.

With this, the ability to maintain the catalyst atmosphere at the target air-fuel ratio can be similarly provided between the rich side and the lean side. Therefore, if the air-fuel ratio is within a predetermined range, either the rich side or the lean side is provided. Even if it deviates, it can be absorbed and the conversion efficiency of the catalyst can be stabilized.

[0019]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

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 includes a catalyst 3 provided in an exhaust pipe 2 and a front A / F.
A sensor 4, a rear O ₂ sensor 5, and a controller 6 are provided.

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 an intake air amount adjusted by the throttle valve 8. Is provided. The throttle valve may be one that opens and closes in direct association with the accelerator operation. Further, the engine 1 is provided with a crank angle sensor 12 for detecting the engine speed.

The catalyst 3 is a so-called three-way catalyst, and purifies NOx, HC and CO with maximum efficiency when the catalyst atmosphere is at a stoichiometric air-fuel ratio. The catalyst 3 has a catalyst carrier covered with an oxygen storage material such as ceria, and has a function of absorbing or releasing oxygen according to the air-fuel ratio of the inflowing exhaust gas (hereinafter, “oxygen storage function”).

Here, the oxygen storage amount of the catalyst 3 is determined by the high-speed component HO2 absorbed / released by the noble metal (Pt, Rh, Pd, etc.) of the catalyst 3 and the low-speed component absorbed / released by the oxygen storage material of the catalyst 3 It can be divided into 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.

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 from the high-speed component HO2 with priority. However, when the ratio of the low speed component LO2 to the high speed component HO2 is equal to or greater than a predetermined value, oxygen is released from both the high speed component HO2 and the low speed component LO2 so that the ratio of the low speed component LO2 to the high speed component HO2 does not change. It has the characteristic of.

A front A / F provided upstream of the catalyst 3
The sensor 4 linearly detects the air-fuel ratio of the exhaust gas flowing into the catalyst 3, and the rear O ₂ sensor 5 provided downstream of the catalyst 3 detects the oxygen concentration downstream of the catalyst 3 in reverse to the stoichiometric air-fuel ratio. . Here, an inexpensive O ₂ sensor is provided downstream of the catalyst 3, but an A / F sensor that can linearly detect the air-fuel ratio may be provided.

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 operating state of the engine 1 and for detecting the temperature of the catalyst 3. 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 3 are calculated based on the outputs of the air flow meter 9, the front A / F sensor 4, and the coolant temperature sensor 10.

When the calculated high-speed component HO2 of the oxygen storage amount is larger than the target value TGHO2, the controller 6 shifts the air-fuel ratio of the engine 1 to the rich side to reduce the high-speed component HO2. When the value is smaller than the value TGHO2, the air-fuel ratio is shifted to the lean side to increase the high-speed component HO2, and the high-speed component HO2 of the oxygen storage amount becomes the target value TGHO2.
So that

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 3. To correct the deviation from the actual oxygen storage amount.

Specifically, when the rear O ₂ sensor 5 makes a lean determination, it is determined that at least the high-speed component HO2 is the maximum, and the high-speed component HO2 is reset to the maximum capacity. When the rear O ₂ sensor 5 makes a rich determination,
Since oxygen is not released from the low-speed component LO2 as well as the high-speed component HO2, the low-speed component HO2 and the high-speed component LO2 are reset to the minimum capacity.

Hereinafter, the control performed by the controller 6 will be described in detail.

Here, the calculation of the oxygen storage amount will be described first, 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 3, which is executed by the controller 6 at predetermined intervals.

According to this, first, as the various operating parameters of the engine 1, typically, the cooling water temperature sensor 10,
The outputs of the crank angle sensor 12 and the air flow meter 9 are read, and the temperature TCAT of the catalyst 3 is estimated based on them (steps S1 and S2). Then, it is determined whether or not the catalyst 3 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 flow proceeds to step S4 to calculate the oxygen storage amount of the catalyst 3. Catalyst activation temperature TACT
If it is determined that o does not reach o, the catalyst 3 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 O2IN is executed to calculate the oxygen excess / deficiency O2IN in the exhaust gas flowing into the catalyst 3, and in step S5, the oxygen storage amount is increased. Oxygen release rate A of component
Is executed, and 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. An overflow OVERFLOW is calculated which overflows into the low-speed component LO2 without being absorbed by HO2.

In step S7, it is determined based on the overflow amount OVERFLOW calculated in step S6 whether or not the oxygen excess / deficiency O2IN in the exhaust gas flowing into the catalyst 3 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, where a subroutine for calculating the low-speed component LO2 (FIG. 6) )
Is executed, and the low-speed component LO2 is calculated based on the overflow OVERFLOW overflowing from the high-speed component HO2.

In this case, the catalyst temperature TCAT is set to the engine 1
Although the temperature is estimated from the cooling water temperature, the engine load, and the engine speed, the temperature sensor 11 may be attached to the catalyst 3 as shown in FIG.

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

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 3 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 positive value.

In step S13, the output of the air flow meter is converted into an intake air amount using a predetermined conversion table. In step S14, the catalyst 3 is obtained by multiplying the intake air amount calculated in step S13 by the excess / deficient oxygen concentration calculated in step S12. Calculate 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 3 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 rate of the high-speed component HO2 is affected by the low-speed component LO2, the low-speed component L
The oxygen release rate A of the high-speed component is calculated according to O2.

According to this, first, in 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, if 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 also released from LO2, the process proceeds to step S23 to calculate a value such that the ratio LO2 / HO2 does not change as the high-speed component oxygen release rate A.

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 H is determined based on the oxygen-oxygen excess / deficiency amount O2IN of the exhaust gas flowing into the catalyst 3 and the oxygen release rate A of the high-speed component.
The operation of O2 is performed.

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

As a result, if it is determined that the oxygen excess / deficiency amount O2IN is larger than zero and the high-speed component HO2 is in a state of absorbing oxygen, the process proceeds to step S32, and 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: The high-speed component HO2 is calculated from the oxygen release rate of the high-speed component HO2.

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

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
The overflow amount (excess amount) OVERFLOW overflowing without being absorbed by HO2 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 (insufficient amount) that overflows without being absorbed by 2 O
VERFLOW 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. Here, since zero is given as the minimum capacity HO2MIN, the insufficient oxygen amount in a state where 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 volume HO2MAX and the minimum volume 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 component OVERFLOW overflowing 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: the previous value of the low-speed component LO2 B: oxygen absorption of the low-speed component It is calculated by the emission rate. Where the oxygen absorption and release rate of the slow component B
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 separately set and variably set. In that case, when the overflow amount OVERFLOW is positive, oxygen is excessive, and the oxygen absorption rate B at this time is
For example, a higher value is set as the catalyst temperature TCAT is higher and the lower speed component LO2 is lower. Further, when the overflow amount OVERFLOW is negative, oxygen is insufficient, and the oxygen release rate B at this time is set to a larger value, for example, as the catalyst temperature TCAT is higher and 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
It is determined whether (= 0) or less.

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

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 resetting 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 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 3, and sets the flag Frich and the flag Flean.

According to this, first, the output of the rear O ₂ sensor 5 for detecting the oxygen concentration downstream of the catalyst 3 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). As a result of the comparison, the rear O ₂ sensor output is lower than the lean determination threshold value. In this case, the process proceeds to step S54,
ean is set to “1” indicating that the lean reset condition of the oxygen storage amount has been satisfied. If the output of the rear O ₂ sensor has exceeded the rich determination threshold, the process proceeds to step S55, and “1” indicating that the rich reset condition of the oxygen storage amount has been satisfied is set in the flag Frich.

If the rear O ₂ sensor output is between the lean judgment threshold and the rich judgment 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.

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 Fri
When ch is changed from “0” to “1” and it is determined that the rich reset condition is satisfied, the process proceeds to step S64, where 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. Is done.

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, even if the low-speed component LO2 does not reach the maximum capacity, the oxygen is downstream of the catalyst. 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.

FIG. 9 shows how the high-speed component HO2 changes when the oxygen storage amount constant control is performed. In this case, at the time t _1, the output of the rear O ₂ sensor 5 is lean reset condition becomes smaller than a lean determining threshold is satisfied, the high speed component HO2 is reset to the maximum capacity HO2MAX. However, at this time, the low-speed component LO2 is not always maximized, so that the low-speed component LO2 is not reset.

At times t ₂ and t ₃ , the output of the rear O ₂ sensor 5 becomes larger than the rich determination threshold value, and the rich reset condition is satisfied.
Is reset to the minimum capacity (= 0). At this time, the low speed component LO2 is also reset to the minimum capacity (not shown).

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

Further, the air-fuel ratio control performed by the controller 6 will be described.

FIG. 10 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 and the low speed component LO2 of the current oxygen storage amount are read, and the target value TGHO2 of the high speed component corresponding to the low speed component LO2 is set with reference to a predetermined table. (Steps S71, S72).

Here, the target value TGHO2 of the high-speed component is
Makes the air-fuel ratio fluctuation that can be absorbed equal between the rich side and the lean side, and if the air-fuel ratio deviates within a predetermined range, the catalyst atmosphere is corrected to the stoichiometric air-fuel ratio regardless of whether it deviates to the rich side or the lean side. A smaller value is set as the low-speed component LO2 increases.

Specifically, the target value TGHO2 of the high-speed component is
The following equation (7), TGHO2 = HO2CMAX / TAGHOS (7) HO2CMAX: Maximum oxygen storage amount of a fast component TAGHOS: Calculated based on a correction table reference value. Here, the reference value TAGHOS is obtained by referring to the table shown in FIG. 11, and has a characteristic that the reference value TAGHOS increases as the current low-speed component LO2 increases. Therefore, as the low-speed component LO2 increases, the target value TGHO2 of the high-speed component decreases. Typically, when a fuel cut is performed when the vehicle is decelerating,
According to the flowchart of the above, the low-speed component LO2 is calculated to be a large value, and accordingly, the target value TGHO2 of the high-speed component is corrected to a small value.

Then, a difference DHO2 between the current high-speed component HO2 and the target value TGHO2 of the high-speed component (= the oxygen excess / deficiency required by the catalyst 3) is calculated (step S73), and the calculated difference DHO2 corresponds to the air-fuel ratio. The target air-fuel ratio of the engine 1 is set (step S74).

Therefore, according to this routine, the target value TGHO2 of the high speed component of the oxygen storage amount is set according to the low speed component LO2, and the target value TGH increases as the low speed component LO2 increases.
O2 is set to a small value.

When the high speed component HO2 is less than the target value TGHO2, the target air-fuel ratio of the engine 1 is set to the lean side to increase the high speed component HO2, and when the high speed component HO2 exceeds the target value TGHO2, Is set to the rich side to reduce the high-speed component HO2.

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 3 is started. The controller 6 estimates and calculates the oxygen storage amount of the catalyst 3 based on the air-fuel ratio of the exhaust gas flowing into the catalyst 3 and the intake air amount of the engine 1. The oxygen storage amount depends on the actual characteristics of the high speed component HO2 and the low speed component LO2. Is calculated separately.

Specifically, when oxygen is absorbed, the high-speed component HO2
Is preferentially absorbed, and when the high-speed component HO2 cannot be completely absorbed, the calculation is performed assuming that the low-speed component LO2 starts to be absorbed. When releasing oxygen, the low-speed component LO2 and the high-speed component H
High-speed component H when the ratio of O2 (LO2 / HO2) is below a certain ratio AR
It is assumed that oxygen is released preferentially from O2, and when the ratio LO2 / HO2 reaches a certain ratio, the low-speed component L is maintained so that the ratio LO2 / HO2 is maintained.
The calculation is performed on the assumption that oxygen is released from both O2 and the high-speed component HO2.

When the calculated high-speed component HO2 of the oxygen storage amount becomes larger than the target value TGHO2, the controller 6 controls the air-fuel ratio of the engine 1 to the rich side to increase the high-speed component.
When HO2 is decreased and becomes smaller than the target value TGHO2, 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
2 is maintained at the target value TGHO2, and 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 changes to the stoichiometric air-fuel ratio. Can be corrected in the direction.

Further, when the inflowing exhaust gas fluctuates to the lean side, the catalyst atmosphere is corrected mainly because the oxygen absorption rate of the low speed component LO2 is lower than that of the high speed component HO2.
The correction of the catalyst atmosphere when the inflowing exhaust gas fluctuates to the rich side is performed by O2, but oxygen is released from the low-speed component LO2 at a relatively high speed.
And the low speed component LO2.

For this reason, if the target value TGHO2 of the high-speed component is fixed, the deviation of the air-fuel ratio that can be absorbed differs between the lean side and the rich side. Since the value TGHO2 is set to a small value, it is possible to absorb many high-speed components up to the maximum capacity of the high-speed components, and the oxygen absorption amount that can be used for correcting the catalyst atmosphere when the inflowing exhaust fluctuates lean. Can be secured. Further, even if the target value of the high-speed component is set to a small value, at this time, since many low-speed components are secured, even if the exhaust fluctuates richly, the release of oxygen by the low-speed component is obtained, The amount of released oxygen that can be used for modifying the catalyst atmosphere can be secured.

As a result, if the deviation of the exhaust gas flowing into the catalyst 3 from the stoichiometric air-fuel ratio is equal to or less than a predetermined amount, the deviation is absorbed to maintain the catalyst atmosphere at the stoichiometric air-fuel ratio irrespective of either the rich side or the lean side. And the conversion efficiency of the catalyst 3 can be stabilized.

When the low-speed component LO2 is small, the oxygen release amount required to maintain the catalyst atmosphere at the stoichiometric air-fuel ratio cannot be secured if the oxygen storage amount of the high-speed component remains small. Due to the characteristic of 11, the target value of the high-speed component is returned to the original value and corrected to a large value.
Even if it shifts to the rich side, the catalyst atmosphere can be maintained at the stoichiometric air-fuel ratio.

[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 an 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 time chart showing a state when the oxygen storage amount constant control is performed.

FIG. 10 is a flowchart showing a routine for calculating a target air-fuel ratio from an oxygen storage amount.

FIG. 11 is an example of a table used when calculating a target value of a high-speed component based on a low-speed component.

[Explanation of symbols]

Reference Signs List 1 engine 2 exhaust pipe 3 three-way catalyst 4 front A / F sensor 5 rear O ₂ sensor 7 intake pipe 8 throttle valve 9 air flow meter 10 cooling water temperature sensor 11 catalyst temperature sensor

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) F01N 3/24 F01N 3/28 F02D 41/14 310 F02D 45/00 314 B01D 53/86 ZAB B01D 53/94

Claims (2)

(57) [Claims] 1. A catalyst provided in an exhaust pipe, means for detecting characteristics of exhaust gas flowing into the catalyst, and a high-speed component having a high absorption rate of an oxygen storage amount of the catalyst based on the detected exhaust characteristics. Means for calculating the amount of oxygen storage that is divided into a low-speed component and a low-speed component that is slower than the high-speed component; a means for setting a target value of the high-speed component according to the calculated low-speed component; And an air-fuel ratio control means for controlling an air-fuel ratio of the engine so that a high-speed component of the oxygen storage amount of the catalyst becomes a target value. 2. The means for setting a target value of the high-speed component,
2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the target value of the high-speed component is set smaller as the calculated low-speed component increases.