JP2001234789A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To constantly and precisely control catalyst oxygen storage quantity to a target value by correcting output variation due to deterioration of an air-fuel ratio sensor in the upstream of a catalyst. SOLUTION: An air-fuel ratio is controlled so that oxygen storage quantity coincides with a target value by setting a catalyst 3 having oxygen storage capacity in an engine exhaust passage and assuming the oxygen storage quantity in accordance with output of an air-fuel ratio sensor 4 in the upstream of the catalyst 3. Output of the upstream air-fuel ratio sensor 4 is corrected in accordance with the output of the downstream air-fuel ratio sensor 5 when the output of the air-fuel ratio sensor 5 in the downstream comes to be on the lean side or the rich side for more than specified time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine.

[0002]

2. Description of the Related Art HC, CO, NOx in exhaust gas of an internal combustion engine
In order to simultaneously purify the catalyst with a three-way catalyst, it is necessary to set the catalyst atmosphere to a stoichiometric air-fuel ratio (hereinafter referred to as stoichiometric). Have.

[0003] If you give a leaner exhaust than stoichiometric,
Until the catalyst takes in the oxygen in the exhaust gas and the oxygen storage amount is saturated, the catalyst atmosphere can be kept stoichiometric. When exhaust gas richer than stoichiometric gas is given, the oxygen held by the catalyst is released, and the catalyst atmosphere is maintained at stoichiometric until all of the retained oxygen is released. In this way, the catalyst compensates for the excess or deficiency of oxygen caused by the temporary air-fuel ratio shift, and the catalyst atmosphere can be kept substantially stoichiometric.

In this case, if the air-fuel ratio is controlled so that the oxygen storage amount of the catalyst always becomes a target value, for example, about half of the maximum storage amount, the capacity of intake and release of the catalyst is equalized, and the stoichiometric air-fuel ratio is reduced. The ability to absorb both fluctuations on the rich and lean sides can be increased, and the exhaust gas purification efficiency can be kept at the best.

[0005] Therefore, based on the output of the air-fuel ratio sensor, the oxygen excess / deficiency amount (converted from the air-fuel ratio) of the exhaust gas flowing into the catalyst is integrated to obtain the oxygen storage amount of the catalyst. Japanese Patent Application Laid-Open Nos. 5-195842 and 7-259602 have proposed proposals for feedback control of the air-fuel ratio so as to coincide with the above.

[0006]

The air-fuel ratio sensor installed on the upstream side of the catalyst is liable to deteriorate with time due to exposure to a high exhaust gas temperature and the like. (A shift to the rich side or the lean side) may occur in the detection of.

If an error occurs in the detected air-fuel ratio, it becomes impossible to accurately calculate the oxygen storage amount of the catalyst based on the output of the air-fuel ratio sensor. As a result, the oxygen storage amount of the catalyst is controlled to a target value. And the exhaust gas purification efficiency is reduced.

SUMMARY OF THE INVENTION In order to solve such a problem, an object of the present invention is to correct output fluctuations due to deterioration of an air-fuel ratio sensor on the upstream side of a catalyst, and to always accurately control the oxygen storage amount to a target value.

[0009]

According to a first aspect of the present invention, there is provided a catalyst having an oxygen storage capacity for taking in or releasing oxygen in exhaust gas in accordance with an exhaust air-fuel ratio, and a method for setting the oxygen storage amount to a target value. In an apparatus for controlling an air-fuel ratio, a means for estimating an oxygen storage amount based on an output of an air-fuel ratio sensor on the upstream side of a catalyst and controlling the air-fuel ratio so that the oxygen storage amount matches a target value, Means for determining whether or not the output of the air-fuel ratio sensor is on the lean side or rich side for a certain time or more, and based on the output of the downstream air-fuel ratio sensor when the downstream air-fuel ratio is on the lean side or rich side for a certain time or more. A correcting means for correcting the output of the upstream air-fuel ratio sensor is provided.

[0010] In a second aspect based on the first aspect, the correcting means shifts the correction value of the output of the upstream air-fuel ratio sensor by a fixed amount to the lean side when the downstream air-fuel ratio sensor is the lean side. To be a positive value, and a negative value for shifting to a rich side by a fixed amount on the rich side.

[0013] In a third aspect based on the first aspect, the correction means is configured to correct the output of the upstream air-fuel ratio sensor to a lean side when the downstream air-fuel ratio sensor is on the lean side as the correction value of the output of the upstream side air-fuel ratio sensor. The positive value shifts according to the sensor output, and the negative value shifts to the rich side according to the sensor output when the sensor is on the rich side.

In a fourth aspect based on the first aspect, the correction means is configured to correct the output of the upstream air-fuel ratio sensor by a fixed amount up to a predetermined range when the downstream air-fuel ratio sensor is on the lean side. Above that, a positive value that shifts to the lean side in accordance with the sensor output value, a rich value that is a fixed amount up to a predetermined range, and a negative value that shifts to the rich side in accordance with the sensor output above that value. To do.

In a fifth aspect based on the second to fourth aspects, the correction means determines an abnormality of the upstream air-fuel ratio sensor when an absolute value obtained by integrating the correction values is equal to or larger than a predetermined value.

In a sixth aspect based on the first to fifth aspects, the oxygen storage amount is estimated by calculating the oxygen storage amount separately for a high-speed component having a high absorption rate and a low-speed component having a lower absorption rate than the high-speed component. .

[0015]

In the first aspect of the present invention, the oxygen storage amount of the catalyst is always controlled to the target value. Therefore, even if the air-fuel ratio on the upstream side of the catalyst slightly fluctuates, the oxygen storage capacity of the catalyst is not changed. The air-fuel ratio on the downstream side of the catalyst is maintained at a stoichiometric ratio. However, if there is an error in the output of the upstream air-fuel ratio sensor, the oxygen storage amount of the catalyst deviates from the target value. For example, if the output of the air-fuel ratio sensor on the upstream side is apparently shifted to the rich side from the normal state,
It is determined that the oxygen storage amount is insufficient, and the air-fuel ratio is controlled to the lean side. As this state continues, the oxygen storage amount of the catalyst saturates, and the air-fuel ratio on the downstream side goes from stoichiometric to lean. When such a state continues for a certain period of time or more, it is considered that there is a change (output shift) in the output of the upstream air-fuel ratio sensor, and the output of the upstream air-fuel ratio sensor corresponds to the rich side or the lean side. The correction is performed, and the corrected output is fed back to the air-fuel ratio control. Thereby, the oxygen storage amount of the catalyst is corrected toward the target value. Therefore, even if there is an output fluctuation due to deterioration of the upstream air-fuel ratio sensor, the catalyst oxygen storage amount can be accurately controlled to the target value.

In the second aspect, the output of the upstream air-fuel ratio sensor is corrected by a fixed amount based on the downstream air-fuel ratio.
A sharp change in the air-fuel ratio before and after the correction can be avoided.

In the third aspect, the output of the upstream air-fuel ratio sensor is corrected according to the magnitude of the output value of the downstream air-fuel ratio sensor, so that the convergence control of the oxygen storage amount to the target value can be performed early. it can.

According to the fourth aspect, an appropriate correction can be made according to the characteristics of the upstream air-fuel ratio sensor, that is, unnecessary correction when the sensor output is not substantially shifted is avoided, and When the shift amount is large, it is possible to quickly return to the normal oxygen storage amount.

In the fifth aspect of the invention, the output of the upstream air-fuel ratio sensor is largely corrected, and when the integrated value reaches a predetermined value, it becomes difficult to maintain the accuracy and stability of the control, which adversely affects the exhaust performance. At this time, it is possible to urge repair or replacement by determining and reporting the sensor abnormality.

In the sixth aspect of the present invention, the oxygen storage characteristics of the catalyst are divided into characteristics of being rapidly absorbed / released by the noble metal of the catalyst and characteristics of being slowly absorbed / released by the oxygen storage material such as ceria of the catalyst. Therefore, by calculating the oxygen storage amount separately for high-speed and low-speed components according to this characteristic, it is possible to accurately calculate the actual storage amount according to the characteristics of the catalyst, and thus to accurately set the actual oxygen storage amount to the target value. It becomes possible to control.

[0021]

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

FIG. 1 shows a schematic configuration of an exhaust gas purifying apparatus to which the present invention is applied. A catalyst 3 is provided in an exhaust pipe 2 of an engine 1, a linear air-fuel ratio sensor 4 is provided upstream thereof, and an air-fuel ratio is provided downstream thereof. A sensor (oxygen sensor) 5 is provided, and a controller 6 for controlling the air-fuel ratio of fuel supplied to the engine 1 based on the output of the sensor is provided.

The intake pipe 7 of the engine 1 has a throttle valve 8
And an air flow meter 9 for measuring the intake air amount adjusted by the throttle valve 8.

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 coated with an oxygen storage material such as ceria, and has a function (oxygen storage function) of retaining or releasing oxygen according to the air-fuel ratio of the inflowing exhaust gas.

The air-fuel ratio sensor 4 provided upstream of the catalyst 3 has a linear output characteristic according to the air-fuel ratio of the exhaust gas, and the downstream air-fuel ratio sensor 5 detects the oxygen concentration of the exhaust gas.

The engine 1 is provided with a temperature sensor 10 for detecting the temperature of cooling water, and is used for determining the operating state of the engine 1 and the activation state of the catalyst 3 and the like.

The controller 6 is a microprocessor, R
It is composed of an AM, an AOM, an I / O interface, etc., and calculates the oxygen storage amount of the catalyst 3 based on the outputs of the air flow meter 9 and the upstream air-fuel ratio sensor 4 so that the storage amount becomes a target value. Feedback control of the air-fuel ratio is performed. When the calculated oxygen storage amount is smaller than the target value, the target air-fuel ratio is set to the lean side to increase the holding amount, and when the calculated oxygen storage amount is larger than the target value, to the rich side to reduce the oxygen storage amount. To match. Further, a difference occurs between the calculated oxygen storage amount and the actual oxygen storage amount due to the calculation error. However, based on the oxygen concentration detected by the downstream air-fuel ratio sensor 5, for example, at the time of fuel cut of the engine, The oxygen storage amount calculated at a predetermined timing after the shift to the fuel cut is reset to correct the deviation.

The method of calculating the oxygen storage amount of the catalyst 3 will be described later.
No. 95110 and the like, and only the principle is briefly described here.

The excess oxygen ratio, which is the ratio of excess or deficient oxygen in the exhaust gas, can be obtained by converting from the exhaust air-fuel ratio upstream of the catalyst. The oxygen excess rate is positive on the lean side and negative on the rich side, with zero at stoichiometry.

The amount of oxygen absorbed or released by the catalyst 3 is known from the excess oxygen rate and the amount of intake air at that time, and the oxygen storage amount of the catalyst 3 can be estimated by integrating the amounts. When the air-fuel ratio on the downstream side is lean, the oxygen storage amount of the catalyst 3 has reached the saturation holding amount, and no more oxygen amount can be held, and the catalyst 3 flows downstream as it is. When the air-fuel ratio becomes richer than the stoichiometric state from this state, the retained oxygen amount decreases from the maximum value according to the oxygen deficiency. When the air-fuel ratio on the downstream side is rich, the oxygen storage amount is zero, and when the air-fuel ratio becomes lean from that state, the oxygen storage amount of the catalyst 3 increases according to the excess oxygen amount at that time. To go. In this way, the oxygen storage amount of the catalyst 3 can be obtained by calculation based on a certain operation state, and by integrating this, the current oxygen storage amount can be obtained. The maximum oxygen storage amount of the catalyst 3 is confirmed in advance by an experiment or the like, and for example, a half of the storage amount is set as a target value, and the air-fuel ratio is controlled so that the oxygen storage amount matches this target value.

However, the air-fuel ratio of the engine has a required value depending on the operating conditions, and when the catalyst 3 functions as a three-way catalyst, it is necessary to control the vicinity of stoichiometry. In order to match the required combustion characteristics of the engine 1, a value corresponding to the deviation from the target value of the oxygen storage amount is given as a correction value to the known λ control for stoichiometric air-fuel ratio. Is satisfied, and the oxygen storage amount converges to the target value.

In the present invention, the controller 6 further determines whether or not the output of the upstream air-fuel ratio sensor 4 for calculating the oxygen storage amount is normal, and if the output is shifted (fluctuates) due to sensor deterioration or the like. If so, the output of the air-fuel ratio sensor 4 is corrected in accordance with this to prevent fluctuations in the oxygen storage amount from the target value. Catalyst 3
Since the oxygen storage amount is always controlled to the target value, even if the air-fuel ratio on the upstream side of the catalyst fluctuates slightly,
Due to the oxygen storage capacity of the catalyst, the air-fuel ratio downstream of the catalyst is maintained at a stoichiometric ratio. However, if there is an error in the output of the air-fuel ratio sensor 4 on the upstream side, the oxygen storage amount of the catalyst deviates from the target value. For example, if the output of the air-fuel ratio sensor on the upstream side is apparently shifted to the rich side from the normal state, it is determined that the oxygen storage amount is insufficient, and the air-fuel ratio is controlled to the lean side. . As this state continues, the oxygen storage amount of the catalyst saturates, and the air-fuel ratio on the downstream side shifts from stoichiometric to lean. When such a state continues for a certain period of time or more, it is considered that there is a change (output shift) in the output of the upstream air-fuel ratio sensor 4, and the upstream air-fuel ratio sensor 4 corresponds to rich or lean at that time. Output is corrected.

This control will be described in detail with reference to the flowchart of FIG.

This flow is repeatedly executed by the controller 6 at regular intervals in an operating state where the basic air-fuel ratio is stoichiometric.

In step S1, the air-fuel ratio is controlled based on the output of the air-fuel ratio sensor 4 on the upstream side of the catalyst so that the oxygen storage amount of the catalyst 3 becomes a target value. The target air-fuel ratio is determined based on a comparison between the calculated value of the oxygen storage amount and the target value, and the amount of fuel supplied to the engine 1 is controlled so as to reach the target air-fuel ratio.

Next, in step S2, it is determined from the output of the air-fuel ratio sensor 5 on the downstream side whether or not the air-fuel ratio is stoichiometric.
In the case of the stoichiometry, the control operation ends. Normally, the exhaust air-fuel ratio downstream of the catalyst is stoichiometric due to the oxygen storage capacity of the catalyst 3. However, the downstream air-fuel ratio such as when the oxygen storage amount of the catalyst 3 becomes saturated or when all the oxygen is released. Fluctuates from stoichiometry.

If it is determined that the vehicle is not stoichiometric, the process proceeds to step S3, and the time from when the vehicle becomes rich or lean is measured. In step S4, it is determined whether or not the measurement time has reached a certain time (for example, 30 seconds). If the certain time has elapsed, it is determined in step S5 that the output of the upstream air-fuel ratio sensor 4 has shifted. In step S5, a shift amount with respect to the output of the upstream air-fuel ratio sensor 4 is calculated, and the calculated shift amount is fed back to the air-fuel ratio control.

The calculation of the shift amount is performed as follows.

If the output of the air-fuel ratio sensor 4 on the upstream side is apparently shifted to the rich side (deviation from the normal value) from the actual air-fuel ratio, the target value is set based on this sensor output. Even if the feedback control is performed, the actual oxygen storage amount becomes larger than the target value. By continuing this, the oxygen storage amount of the catalyst 3 eventually saturates, and the air-fuel ratio on the downstream side goes from stoichiometric to lean.
Therefore, in this case, the output of the air-fuel ratio sensor 4 on the upstream side is corrected toward the lean side by a fixed amount, and this is fed back to the control of the air-fuel ratio.

Conversely, if the output of the air-fuel ratio sensor 4 on the upstream side is apparently shifted to the lean side from the actual air-fuel ratio, the actual oxygen storage amount becomes smaller than the target value and eventually becomes zero. The air-fuel ratio on the downstream side changes from stoichiometric to rich. In this case, the upstream air-fuel ratio sensor 4
May be shifted to the lean side, and in order to correct this, the output of the upstream air-fuel ratio sensor 4 is corrected to the rich side by a fixed amount. The correction result is stored as a learning value of the air-fuel ratio control, and when there are several corrections, the correction results are sequentially added.

The correction amount can be changed according to the magnitude of the absolute value of the output of the downstream air-fuel ratio sensor 5 instead of being a constant value. In this case, the oxygen storage amount can be made to converge to the target value within a short time after the correction.

On the other hand, in step S6, an abnormality of the upstream air-fuel ratio sensor 4 is determined from the integrated value of the shift amount with respect to the sensor output.

In this case, the correction amount of the air-fuel ratio sensor 4 on the upstream side is integrated, and when the absolute value of the integrated amount reaches a predetermined limit value, it is determined that the air-fuel ratio sensor 4 is abnormal. When the deterioration of the air-fuel ratio sensor 4 progresses and the integrated correction amount of the sensor output reaches a certain limit, stable air-fuel ratio control becomes difficult, which may adversely affect exhaust performance. An abnormality is determined and reported to prompt early repair and replacement.

The shift amount of the output is calculated as a positive fixed value when the downstream air-fuel ratio sensor 5 indicates the lean side, and as a negative fixed value when the downstream air-fuel ratio sensor 5 indicates the rich side. When the absolute value of the correction amount reaches a preset limit value, it is determined that the correction is abnormal.

Next, the overall operation will be described.

By controlling the oxygen storage amount of the catalyst 3 to a target value, for example, about half of the maximum storage amount, the catalyst 3 can efficiently purify NOx, HC and CO. The oxygen storage amount is calculated based on the output of the upstream air-fuel ratio sensor 4. When the oxygen storage amount decreases below the target value, the air-fuel ratio is controlled to the lean side, and the storage amount increases. Controlled and reduce the amount of storage.

Therefore, in a normal state in which the catalyst 3 functions properly, the air-fuel ratio on the downstream side of the catalyst 3 becomes stoichiometric, and does not become lean or rich.

However, when the upstream side air-fuel ratio sensor 4 deteriorates with time and the sensor output shifts from a normal state, the air-fuel ratio is detected on the lean side or the rich side more than actually. Then, even if the oxygen storage amount is calculated based on the output of the air-fuel ratio sensor 4, an accurate holding amount cannot be obtained, and the oxygen storage amount of the catalyst 3 may be saturated or may be completely discharged. .

In this case, the air-fuel ratio on the downstream side of the catalyst varies from stoichiometric to rich or lean. It is assumed that there is such a change in the air-fuel ratio, and the downstream air-fuel ratio is now on the lean side.

In this state, it means that the upstream air-fuel ratio sensor 4 is apparently shifted to a richer side than the actual one. Therefore, the sensor output is shifted to the lean side by a fixed amount by the correction. Air-fuel ratio sensor 4
Is relatively shifted to the lean side, by feeding back this, the actual air-fuel ratio is appropriately corrected to the rich side, and the target air-fuel ratio is achieved.

The above is performed in the same manner even when the output of the air-fuel ratio sensor 4 is shifted in the opposite direction. In this case, the direction of correction is reversed.

By performing such control, even if the output of the upstream air-fuel ratio sensor 4 shifts, the oxygen storage amount can be made to converge to the target value.

As shown in FIGS. 3A and 3B, when the output of the air-fuel ratio sensor 4 on the upstream side is largely deviated, even if the output of the air-fuel ratio sensor is corrected once, the downstream air-fuel ratio is not changed. Several corrections are required until the stoichiometry is achieved.

As described above, when the correction in the same direction is repeated many times, there is a high possibility that the sensor deterioration has greatly progressed. Therefore, if the absolute value of the correction amount of the sensor output reaches the limit value, An abnormality of the air-fuel ratio sensor 4 is determined, and the user is prompted to replace the air-fuel ratio sensor 4 with a new one rather than to continue the correction.

Since a single correction amount of the output of the air-fuel ratio sensor 4 on the upstream side is fixed, a large change in the air-fuel ratio due to the correction can be avoided, and the combustion state of the engine 1 can be stabilized. On the other hand, the correction amount of the output of the air-fuel ratio sensor 4 is
If the size is determined according to the output of the downstream air-fuel ratio sensor 5 at that time, the return of the oxygen storage amount to the target value by the correction is accelerated, and the purification efficiency of the catalyst 3 can be normalized at an early stage.

As shown in FIG. 4, the correction amount of the upstream air-fuel ratio sensor 4 is not a fixed amount, but a value corresponding to the magnitude of the output of the downstream air-fuel ratio sensor 5. That is, when the change amount to the rich side or the lean side is large, the correction amount can be set to increase accordingly.

In this way, when the shift amount of the upstream air-fuel ratio sensor 4 is large, the correction amount is increased and the catalytic oxygen storage amount can be returned to the normal state promptly.

Further, as shown in FIG. 5, the output characteristic of the upstream air-fuel ratio sensor 4 may be such that the downstream air-fuel ratio swings to the rich side or the lean side even if the sensor output is not necessarily shifted. Moreover, the swing range on the lean side is larger than that on the rich side. For this reason, even if the downstream air-fuel ratio swings to the rich side or the lean side, a fixed amount of correction is performed up to a predetermined range, and if it exceeds the range, the correction amount is increased according to the downstream air-fuel ratio. It may be.

In this way, an appropriate correction can be made according to the characteristics of the air-fuel ratio sensor 4 on the upstream side, that is, unnecessary correction when the sensor is not shifted is avoided, and the shift amount is large. Sometimes, it is possible to quickly return to the normal oxygen storage amount.

Next, a specific example of calculating the oxygen storage amount of the catalyst 3 based on the output of the upstream air-fuel ratio sensor 4 will be described with reference to FIGS. The oxygen storage characteristics of the catalyst 3 are divided into characteristics of being rapidly absorbed / released by the noble metal of the catalyst and characteristics of being slowly absorbed / released by an oxygen storage material such as ceria of the catalyst. Therefore, by calculating the oxygen storage amount separately for high-speed and low-speed components in accordance with this characteristic, the actual storage amount according to the characteristics of the catalyst can be accurately calculated.

FIG. 6 is a flowchart for calculating the oxygen storage amount of the high-speed component, and FIG. 7 is a flowchart for calculating the low-speed component similarly.

Referring to FIG. 6, in this subroutine, the high-speed component HO2 is calculated 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.

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, if the air-fuel ratio of the exhaust gas flowing into the catalyst 3 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 equal to or less than zero and it is determined that the high-speed component is 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.

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

If the high speed component HO2 is equal to or greater than the maximum amount HO2MAX, the process proceeds to step S36, where the high speed component HO2 is determined.
Overflow amount (excess amount) OVE overflowing without being absorbed by 2
RFLOW is calculated by the following equation (3), OVERFLOW = HO2-HO2MAX ... (3), and the high-speed component HO2 is the maximum amount HO2MAX
Is limited to

If the high speed component HO2 is less than the minimum amount HO2MIN, the process proceeds to step S37, where the high speed component HO2
Overflow (insufficient amount) OVER overflowing without being absorbed
FLOW is calculated by the following equation (4), OVERFLOW = HO2-HO2MIN ... (4), and the high-speed component HO2 is the minimum amount HO2MIN
Is limited to Here, since 0 is given as the minimum amount HO2MIN, the insufficient oxygen amount in a state where all the high-speed components HO2 are released is calculated as the negative overflow amount.

When the high speed component HO2 is between the maximum amount HO2MAX and the minimum amount HO2MIN, the oxygen excess / deficiency O2IN of the exhaust gas flowing into the catalyst 3 is all absorbed by the high speed component HO2.
Zero is set in the overflow amount OVERFLOW.

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 amount HO2MAX or equal to or less than the minimum amount HO2MIN is absorbed or released by the low-speed component LO2.

FIG. 7 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 case of the calculation of 2, the calculated low speed component LO2 does not exceed its maximum amount LO2MAX, or the minimum amount LO2MIN (=
0) It is determined whether it is less than or equal to.

As a result, if the maximum amount LO2MAX is exceeded, the process proceeds to step S44, where the oxygen excess / deficiency amount 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 amount 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 amount, the process proceeds to step S45, where the low speed component LO2 is limited to the minimum amount LO2MIN.

Thus, the oxygen storage amount for the catalyst 3 is calculated, and the air-fuel ratio is controlled so that the oxygen storage amount matches the target value.

It is apparent that the present invention is not limited to the above-described embodiment, but can be variously modified within the scope of the technical idea.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of the present invention.

FIG. 2 is a flowchart showing a control operation.

3A and 3B are explanatory diagrams showing a relationship between an air-fuel ratio on the downstream side of a catalyst and a correction amount of an upstream-side air-fuel ratio sensor. FIG. 3A shows a case where the air-fuel ratio on the downstream side is lean, and FIG. Is shown.

FIG. 4 is an explanatory diagram showing a relationship between an air-fuel ratio on the downstream side of a catalyst and a correction amount of an upstream-side air-fuel ratio sensor.

FIG. 5 is an explanatory diagram showing correction amount allocation showing the relationship between the air-fuel ratio on the downstream side of the catalyst and the correction amount of the upstream air-fuel ratio sensor.

FIG. 6 is a flowchart for calculating an oxygen storage amount (high-speed component storage amount).

FIG. 7 is a flowchart for calculating an oxygen storage amount (low-speed component storage amount).

[Explanation of symbols]

 Reference Signs List 1 engine 3 catalyst 4 upstream air-fuel ratio sensor 5 downstream air-fuel ratio sensor 6 controller

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 41/22 305 F02D 41/22 305K 45/00 314 45/00 314Z // B01D 53/86 ZAB B01D 53 / 36 ZABB (72) Inventor Persimmon (Zaki) Narusho F-term (reference) 3G084 BA00 BA09 DA10 DA27 DA30 EA07 EA11 EB11 EC03 FA07 FA30 3G091 AB03 BA14 BA15 BA19 BA27 BA32. DA20

Claims (6)

[Claims] 1. A catalyst having an oxygen storage capacity for taking in or releasing oxygen in exhaust gas according to an exhaust air-fuel ratio, and a device for controlling an air-fuel ratio so that the oxygen storage amount becomes a target value. Means for estimating the oxygen storage amount based on the output of the upstream air-fuel ratio sensor and controlling the air-fuel ratio so that the oxygen storage amount matches the target value; and output of the downstream air-fuel ratio sensor for a certain time or more. Means for judging whether the engine has reached the lean side or the rich side, and, when the downstream air-fuel ratio is on the lean side or the rich side for a predetermined time or more, corrects the output of the upstream air-fuel ratio sensor based on the output of the downstream air-fuel ratio sensor. An air-fuel ratio control device for an internal combustion engine, comprising a correction means. 2. The correction means as a correction value of the output of the upstream air-fuel ratio sensor, a positive value which shifts by a fixed amount to the lean side when the downstream air-fuel ratio sensor is lean, and a correction value when the downstream air-fuel ratio sensor is rich. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the negative value is shifted to a rich side by a fixed amount. 3. The correction means as a correction value of the output of the upstream air-fuel ratio sensor, wherein when the downstream air-fuel ratio sensor is lean, a positive value which shifts to lean according to the sensor output value; 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the value of the air-fuel ratio is set to a negative value that shifts to the rich side in accordance with the sensor output. 4. The correction means as a correction value of the output of the upstream air-fuel ratio sensor, when the downstream air-fuel ratio sensor is on the lean side, a fixed amount up to a predetermined range, and when the downstream air-fuel ratio sensor is over, the sensor output value on the lean side is increased. 2. The internal combustion engine according to claim 1, wherein a positive value that shifts according to a predetermined value, a constant value up to a predetermined range when the value is on the rich side, and a negative value that shifts toward the rich side in accordance with the sensor output when the value exceeds the positive value. Engine air-fuel ratio control device. 5. The internal combustion engine according to claim 2, wherein said correction means determines an abnormality in the upstream air-fuel ratio sensor when an absolute value obtained by integrating the correction values is equal to or greater than a predetermined value. Air-fuel ratio control device. 6. The oxygen storage amount according to claim 1, wherein the oxygen storage amount is estimated by separately calculating a high-speed component having a high absorption rate and a low-speed component having a lower absorption rate than the high-speed component. An air-fuel ratio control device for an internal combustion engine.