JP3783510B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.
[0002]
[Prior art]
To simultaneously purify HC, CO, and NOx in the exhaust gas of an internal combustion engine with a three-way catalyst, it is necessary to make the catalyst atmosphere a stoichiometric air-fuel ratio (hereinafter referred to as stoichiometric), and the purification efficiency when it slightly deviates from stoichiometry is reduced. In order not to do so, the catalyst has oxygen storage capability.
[0003]
If exhaust that is leaner than stoichiometric is given, the catalyst takes in oxygen in the exhaust, and the catalyst atmosphere can be maintained stoichiometric until this oxygen storage amount is saturated. Further, when exhaust richer than stoichiometric is given, oxygen retained by the catalyst is released, and the catalyst atmosphere is maintained stoichiometric until all of the retained oxygen is released. In this way, the catalyst compensates for the excess or deficiency of oxygen resulting from a temporary air-fuel ratio shift, and the catalyst atmosphere can be kept substantially stoichiometric.
[0004]
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 catalyst intake and release capacities are equalized, and the rich from the air-fuel ratio stoichiometry, Absorption capacity is enhanced against fluctuations on either side of the lean, and the exhaust purification efficiency can be kept at its best.
[0005]
For this reason, the oxygen storage amount of the catalyst is obtained by integrating the oxygen excess / deficiency amount (converted from the air / fuel ratio) of the exhaust gas flowing into the catalyst based on the output of the air / fuel ratio sensor, and this oxygen storage amount matches the target value. As described above, proposals for feedback control of the air-fuel ratio have been made by Japanese Patent Laid-Open Nos. 5-195842 and 7-259602.
[0006]
[Problems to be Solved by the Invention]
Air-fuel ratio sensors installed on the upstream side of the catalyst are subject to deterioration over time due to exposure to high exhaust temperatures, and there is an error (shift) in air-fuel ratio detection due to variations in quality during sensor manufacture. May occur.
[0007]
If an error occurs in the detected air-fuel ratio, the oxygen storage amount of the catalyst cannot be accurately calculated based on the output of the air-fuel ratio sensor, and as a result, the oxygen storage amount of the catalyst cannot be controlled to the target value. Exhaust gas purification efficiency decreases.
[0008]
In order to solve such a problem, an object of the present invention is to correct an output fluctuation due to deterioration of an air-fuel ratio sensor on the upstream side of a catalyst, and to always control an oxygen storage amount to a target value accurately.
[0009]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a catalyst having an oxygen storage capability for taking in or releasing oxygen in exhaust gas in accordance with an exhaust air / fuel ratio, and an apparatus 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 the downstream air-fuel ratio sensor output is on the lean side The amount of oxygen storage is reduced at the time of, and the target air-fuel ratio is changed so as to increase the amount of oxygen storage at the rich side, and after a predetermined time has elapsed since the change of the target air-fuel ratio by the changing means, If the downstream air-fuel ratio is on the same side as before the target air-fuel ratio change despite the change in the target air-fuel ratio, the upstream air-fuel ratio sensor is based on the rich side or lean side of the air-fuel ratio. And a correction means for correcting the output of. In a second aspect based on the first aspect, the predetermined time is a time for the downstream air-fuel ratio to respond to the change in the target air-fuel ratio.
[0010]
According to a third aspect of the present invention, in the first or second aspect , when the output of the downstream air-fuel ratio sensor is on the lean side, the changing means sets the target air-fuel ratio to the rich side by a certain value, and when the output of the downstream air-fuel ratio sensor is also on the rich side. Change by a certain value to the lean side.
[0011]
According to a fourth aspect of the present invention, in any one of the first to third aspects, the changing means is a lean side when the downstream side air-fuel ratio sensor is on the lean side as a correction value for the output of the upstream side air-fuel ratio sensor. Is a positive value that shifts by a certain amount, and when it is on the rich side, it is a negative value that shifts by a certain amount to the rich side.
[0012]
According to a fifth aspect of the invention, in any one of the first to third aspects, the changing means is a lean side when the downstream side air-fuel ratio sensor is on the lean side as a correction value for the output of the upstream side air-fuel ratio sensor. The positive value is shifted by an amount corresponding to the sensor output, and the negative value is shifted to the rich side by an amount corresponding to the sensor output when it is on the rich side.
[0013]
In a sixth aspect based on the fourth or fifth aspect , the changing means determines that the upstream air-fuel ratio sensor is abnormal when the absolute value obtained by integrating the correction values reaches a predetermined value.
[0014]
According to a seventh invention, in the first to sixth inventions, the oxygen storage amount is estimated by dividing the oxygen storage amount into a high speed component having a high absorption speed and a low speed component having an absorption speed slower than the high speed component.
[0015]
[Action, effect]
In the first invention, the oxygen storage amount of the catalyst is controlled so as to become the target value. 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. . When the output of the downstream air-fuel ratio sensor is on the lean side, for example, the target air-fuel ratio is corrected to the rich side so as to reduce the oxygen storage amount. If the oxygen storage amount decreases due to the correction to the rich side, the downstream air-fuel ratio should change from the lean side to the stoichiometric or rich side. However, after this correction , when the output of the downstream air-fuel ratio sensor indicates the lean side even though the predetermined time has elapsed , the actual air-fuel ratio has not changed to the rich side. This is because the upstream air-fuel ratio sensor does not output accurately according to the air-fuel ratio, that is, the upstream air-fuel ratio sensor apparently shifts to the rich side from the actual state, so in practice Even if it is not so rich, it will be considered that the rich side has been corrected enough. The actual air-fuel ratio controlled by this does not change so much on the rich side. Therefore, when the correction to the rich side is applied based on the output of the downstream side air-fuel ratio sensor and sufficient correction is not performed even though a predetermined time has elapsed after the correction , the upstream side air-fuel ratio sensor The output is shifted by a predetermined amount to the lean side, and the corrected value is fed back to the air-fuel ratio control. Thus, 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 of the invention, the corrected predetermined time is set as the time for the downstream air-fuel ratio to respond to the change in the target air-fuel ratio, so that it is possible to reliably determine the output error of the upstream air-fuel ratio sensor. It becomes.
[0016]
In the third aspect of the invention, when the downstream air-fuel ratio is other than stoichiometric, the target air-fuel ratio is changed by a certain amount to the rich side or the lean side, so that there is no significant fluctuation of the air-fuel ratio and the stability of engine control is reduced. Can be maintained.
[0017]
In the fourth aspect of the invention, since the output of the upstream air-fuel ratio sensor is corrected by a fixed amount according to the downstream air-fuel ratio, a sudden change in the air-fuel ratio before and after correction can be avoided.
[0018]
In the fifth aspect of the invention, 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 oxygen storage amount can be controlled to converge to the target value at an early stage.
[0019]
In the sixth aspect of the invention, there are many corrections for the output of the upstream side air-fuel ratio sensor, and when the integrated value reaches a predetermined value, it becomes difficult to maintain the accuracy and stability of the control, which may affect the exhaust performance. Therefore, at this time, the sensor abnormality is judged and notified to prompt repair or replacement.
[0020]
In the seventh invention, the oxygen storage characteristic for the catalyst is divided into a characteristic that is absorbed / released at a high speed by the noble metal of the catalyst and a characteristic that is absorbed / released at a low speed by an oxygen storage material such as ceria of the catalyst. By calculating the oxygen storage amount separately for high speed and low speed components according to this characteristic, the actual storage amount corresponding to the characteristics of the catalyst can be calculated accurately, and therefore the actual oxygen storage amount is accurately controlled to the target value. It becomes possible.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0022]
FIG. 1 shows a schematic configuration of an exhaust emission control 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 upstream, and an air-fuel ratio sensor (oxygen) downstream. Sensor) 5 is installed, and a controller 6 for controlling the air-fuel ratio of the fuel supplied to the engine 1 based on the output of these sensors is provided.
[0023]
The intake pipe 7 of the engine 1 is provided with a throttle valve 8 and an air flow meter 9 for measuring the intake air amount adjusted by the throttle valve 8.
[0024]
The catalyst 3 is a so-called three-way catalyst, and purifies NOx, HC, and CO with maximum efficiency when the catalyst atmosphere has 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 of holding or releasing oxygen (oxygen storage function) in accordance with the air-fuel ratio of the inflowing exhaust gas.
[0025]
The air-fuel ratio sensor 4 provided upstream of the catalyst 3 has a linear output characteristic corresponding to the air-fuel ratio of the exhaust, and the air-fuel ratio sensor 5 on the downstream side detects the oxygen concentration of the exhaust.
[0026]
Further, the engine 1 is provided with a temperature sensor 10 for detecting the temperature of the cooling water, and is used for determining the activated state of the catalyst 3 as well as the operating state of the engine 1.
[0027]
The controller 6 includes a microprocessor, a RAM, an AOM, an I / O interface, and the like. The controller 6 calculates the oxygen storage amount of the catalyst 3 based on the output of the air flow meter 9 and the upstream air-fuel ratio sensor 4, and this storage amount is the target. The air-fuel ratio is feedback controlled so as to reach a value. When the calculated oxygen storage amount is smaller than the target value, the target air-fuel ratio is made leaner to increase the holding amount, and conversely when it is larger than the target value, the oxygen storage amount is decreased to reduce the oxygen storage amount. To match. Further, although there is a deviation between the oxygen storage amount calculated by the calculation error and the actual oxygen storage amount, based on the oxygen concentration detected by the downstream air-fuel ratio sensor 5, for example, when the engine fuel is cut, After shifting to fuel cut, the oxygen storage amount calculated at a predetermined timing is reset to correct the deviation.
[0028]
A method for calculating the oxygen storage amount of the catalyst 3 will be described later, but is also described in detail in Japanese Patent Application No. 10-295110 by the applicant of the present invention, and only the principle will be briefly described here.
[0029]
The excess oxygen ratio, which is the ratio of excess or deficiency of oxygen in the exhaust gas, is calculated from the exhaust air / fuel ratio upstream of the catalyst. The oxygen excess rate is zero when stoichiometric, and is positive on the lean side and negative on the rich side.
[0030]
The amount of oxygen absorbed or released by the catalyst 3 is known from the oxygen excess rate and the amount of intake air at that time, and the amount of oxygen stored in the catalyst 3 can be estimated by integrating this. When the air-fuel ratio on the downstream side is on the lean side, the oxygen storage amount of the catalyst 3 has reached the saturation retention amount, and no more oxygen amount can be retained, and it flows downstream as it is. When the air-fuel ratio becomes richer than stoichiometric from this state, the retained oxygen amount decreases from the maximum value in accordance with the oxygen deficiency. When the downstream air-fuel ratio 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 operating 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 experiments or the like, for example, a half of the retention amount is set as a target value, and the air-fuel ratio is controlled so that the oxygen storage amount matches this target value.
[0031]
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 it near the stoichiometric condition. In this operating state, the oxygen storage amount matches the target value. Provides a value corresponding to a deviation from the target value of the oxygen storage amount as a correction value for the known λ control for making the air-fuel ratio stoichiometric, while satisfying the required combustion characteristics of the engine 1. The oxygen storage amount can be converged to the target value.
[0032]
In the present invention, it is determined whether the output of the upstream air-fuel ratio sensor 4 for calculating the oxygen storage amount is normal, and if the output is shifted (varied) due to sensor deterioration or the like, the output is changed accordingly. The air-fuel ratio sensor output is corrected to prevent fluctuation of the oxygen storage amount from the target value.
[0033]
For this reason, the output of the air-fuel ratio sensor 5 on the downstream side of the catalyst 3 that is not easily affected by high heat is used. When the air-fuel ratio on the downstream side of the catalyst is rich, the oxygen storage amount is increased. Adjusts the target air-fuel ratio in a direction to reduce the oxygen storage amount, and when the air-fuel ratio downstream of the catalyst is on the same side without returning to the stoichiometry, the output of the upstream air-fuel ratio sensor 4 shifts. As a result, the output of the air-fuel ratio sensor 4 is corrected accordingly.
[0034]
The details of this control will be described in detail with reference to the flowchart of FIG.
[0035]
This flow is repeatedly executed at regular intervals by the controller 6 in an operation state in which the basic air-fuel ratio becomes stoichiometric.
[0036]
In step S1, the air-fuel ratio is controlled so that the oxygen storage amount of the catalyst 3 becomes a target value based on the output of the air-fuel ratio sensor 4 on the upstream side of the catalyst. 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 fuel supply amount to the engine 1 is controlled so as to be the target air-fuel ratio.
[0037]
Next, in step S2, it is determined whether the air-fuel ratio is stoichiometric or not from the output of the downstream air-fuel ratio sensor 5, and when it is stoichiometric, the control operation is terminated. Normally, the exhaust air / fuel ratio downstream of the catalyst becomes stoichiometric due to the oxygen storage capacity of the catalyst 3, but the downstream air / fuel ratio becomes low when the oxygen storage amount of the catalyst 3 becomes saturated or when all the oxygen is released. Varies from stoichiometric.
[0038]
If it is determined that it is not stoichiometric, the process proceeds to step S3, and the target air-fuel ratio of the air-fuel ratio control is changed by a predetermined amount. That is, when the detected air-fuel ratio is on the lean side, the target air-fuel ratio is set to the rich side by a predetermined amount. Conversely, when the detected air-fuel ratio is on the rich side, the target air-fuel ratio is set to the lean side by a predetermined amount. For this reason, the air-fuel ratio on the downstream side of the catalyst changes toward the stoichiometry.
[0039]
In step S4, it is determined whether the output of the downstream air-fuel ratio sensor 5 is on the same side or reversed with the change of the target air-fuel ratio. If they are on the same side, that is, if they remain on the lean side or rich side despite changing the target air-fuel ratio, it is determined that there is a deviation in the output of the upstream side air-fuel ratio sensor 4. In step S5, the shift amount with respect to the output of the upstream air-fuel ratio sensor 4 is calculated, and this is fed back to the air-fuel ratio control.
[0040]
The shift amount is calculated as follows.
[0041]
When the detected downstream air-fuel ratio is on the lean side, and therefore the downstream air-fuel ratio is still on the lean side even though the target air-fuel ratio is changed to the rich side, it is also in FIG. In addition, the oxygen storage amount of the catalyst 3 is almost saturated. This is because the actual air-fuel ratio does not become so rich even on the rich side. If there is a shift in the output of the upstream air-fuel ratio sensor 4 (deviation from the normal value), the actual air-fuel ratio will not be so rich even if feedback control is performed so that it becomes the target value based on this sensor output. This is because the air-fuel ratio sensor 4 apparently outputs a richer output than the actual air-fuel ratio. Therefore, in this case, the output of the upstream air-fuel ratio sensor 4 is corrected to the lean side by a certain amount, and this is fed back to the control of the air-fuel ratio.
[0042]
In contrast, as shown in FIG. 3B, the detected downstream air-fuel ratio is on the rich side, and the downstream air-fuel ratio is still rich despite the target air-fuel ratio being changed to the lean side. On the other hand, in contrast to the above, it is conceivable that the output of the upstream air-fuel ratio sensor 4 is apparently shifted to the lean side from the actual air-fuel ratio, and in order to correct this, the upstream side The output of the air-fuel ratio sensor 4 is corrected to the rich side by a certain amount.
[0043]
The correction amount is not a fixed amount, but can be changed according to the magnitude of the absolute value of the output of the downstream air-fuel ratio sensor 5. In this case, the oxygen storage amount is converged to the target value early after correction.
[0044]
Thus, in step S5, the correction amount for the shift of the output of the upstream air-fuel ratio sensor 4 is calculated, and this is fed back to the air-fuel ratio control, thereby increasing or decreasing the oxygen storage amount toward the target value.
[0045]
On the other hand, in step S6, abnormality determination of the upstream air-fuel ratio sensor 4 is performed from the integrated value of the shift amount with respect to the sensor output.
[0046]
This is because the correction amount of the upstream air-fuel ratio sensor 4 is integrated, and when the absolute value of the integration 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 and may adversely affect the exhaust performance. By notifying them, early repairs and replacements are encouraged.
[0047]
The output shift amount is calculated as a positive fixed value when the downstream air-fuel ratio sensor 5 indicates the lean side, and is calculated as a negative fixed value when the downstream air-fuel ratio sensor 5 indicates the rich side. When the absolute value reaches a preset limit value, it is determined that there is an abnormality.
[0048]
Next, the overall operation will be described.
[0049]
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, and if this decreases below the target value, the air-fuel ratio is controlled to the lean side. Controlled and reduces storage.
[0050]
Therefore, in a normal state where 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.
[0051]
However, when the upstream air-fuel ratio sensor 4 deteriorates with time and the sensor output shifts from the normal state, the air-fuel ratio is detected on the lean side or actually on the rich side. Then, even if the oxygen storage amount is calculated based on the output of the air-fuel ratio sensor 4, an accurate retention amount cannot be obtained, and the oxygen storage amount of the catalyst 3 may be saturated or all may be released. .
[0052]
In this case, the air-fuel ratio on the downstream side of the catalyst varies from stoichiometric to rich or lean. If there is such a change in the air-fuel ratio, and the downstream air-fuel ratio is now on the lean side, the target air-fuel ratio is changed by a predetermined amount to the rich side. Due to the shift of the air-fuel ratio to the rich side, the oxygen storage amount of the catalyst 3 decreases, and the air-fuel ratio on the downstream side of the catalyst returns to stoichiometry. However, when the catalyst downstream air-fuel ratio is still on the lean side despite the change of the target air-fuel ratio, it is assumed that the output shift due to deterioration of the upstream air-fuel ratio sensor 4 is large as described above, as shown in FIG. As in (A), the sensor output is corrected by a certain amount.
[0053]
Since the target air-fuel ratio is not so rich despite the rich air-fuel ratio, it means that the upstream air-fuel ratio sensor 4 is apparently shifted to the rich side rather than the actual. The sensor output of the upstream air-fuel ratio sensor 4 is shifted to the lean side by a certain amount. When the output of the air-fuel ratio sensor 4 is relatively shifted to the lean side by the correction, the actual air-fuel ratio is appropriately corrected to the rich side by feeding back this, and becomes the target air-fuel ratio.
[0054]
The above is performed in the same way even when the output of the air-fuel ratio sensor 4 is shifted in the opposite direction as shown in FIG. 3B. At this time, the correction direction is reversed. become.
[0055]
By performing such control, the oxygen storage amount can be converged to the target value even if the output of the upstream air-fuel ratio sensor 4 is shifted.
[0056]
However, depending on the deviation of the output of the air-fuel ratio sensor 4, the correction operation described above may be performed continuously, and when the correction in the same direction is repeated many times, the sensor deterioration may be greatly advanced. There is also sex. Therefore, when the absolute value of the correction amount of the sensor output with respect to the deterioration reaches the limit value, it is determined that the sensor is abnormal, and it is urged to replace it with a new one rather than continuing the correction.
[0057]
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 characteristic for the catalyst 3 is divided into a characteristic that is absorbed / released at a high speed by the noble metal of the catalyst and a characteristic that is absorbed / released at a low speed by an oxygen storage material such as ceria of the catalyst. Therefore, the actual storage amount corresponding to the catalyst characteristics can be accurately calculated by dividing the oxygen storage amount into high speed and low speed components according to this characteristic.
[0058]
FIG. 4 is a flowchart for calculating the oxygen storage amount of the high speed component, and FIG. 5 is a flowchart for calculating the low speed component.
[0059]
In FIG. 4, 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 3 and the oxygen release rate A of the high speed component.
[0060]
According to this, first, in step S31, based on the value of oxygen excess / deficiency O2IN, it is determined whether the high speed component HO2 is in a state of absorbing oxygen or in a state of releasing oxygen.
[0061]
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 the process proceeds to step S32. The following formula (1),
HO2 = HO2z + O2IN (1)
HO2z: The high speed component HO2 is calculated from the previous value of the high speed component HO2.
[0062]
On the other hand, if it is determined that the oxygen excess / deficiency amount O2IN is equal to or less than zero and the high-speed component is in a state of releasing oxygen, the process proceeds to step S33, where
HO2 = HO2z + O2IN x A (2)
A: The high speed component HO2 is calculated from the oxygen release rate of the high speed component HO2.
[0063]
When the high speed component HO2 is calculated in this way, it is determined in steps S34 and S35 whether the value does not exceed the maximum amount HO2MAX of the high speed component or not less than the minimum amount HO2MIN (= 0).
[0064]
If the high-speed component HO2 is greater than or equal to the maximum amount HO2MAX, the process proceeds to step S36, where the overflow amount (excess amount) OVERFLOW that overflows without being absorbed by the high-speed component HO2 is expressed by the following equation (3):
OVERFLOW = HO2-HO2MAX (3)
Further, the high speed component HO2 is limited to the maximum amount HO2MAX.
[0065]
If the high speed component HO2 is less than or equal to the minimum amount HO2MIN, the process proceeds to step S37, and the overflow amount (insufficient amount) OVERFLOW overflowing without being absorbed by the high speed component HO2 is expressed by the following equation (4):
OVERFLOW = HO2-HO2MIN (4)
Further, the high speed component HO2 is limited to the minimum amount HO2MIN. Here, since 0 is given as the minimum amount HO2MIN, the amount of oxygen deficient in the state in which all the high-speed component HO2 is released is calculated as a negative overflow amount.
[0066]
In addition, when the high speed component HO2 is between the maximum amount HO2MAX and the minimum amount HO2MIN, all the oxygen excess / deficiency O2IN of the exhaust gas flowing into the catalyst 3 is absorbed by the high speed component HO2, so that the overflow amount OVERFLOW is zero. Is set.
[0067]
Here, the overflow amount OVERFLOW overflowing from the high speed component HO2 when the high speed component HO2 is greater than or equal to the maximum amount HO2MAX or less than the minimum amount HO2MIN is absorbed or released by the low speed component LO2.
[0068]
FIG. 5 shows the contents of a subroutine for calculating the low speed component LO2 of the oxygen storage amount. In this subroutine, the low speed component LO2 is calculated based on the overflow amount OVERFLOW overflowing from the high speed component HO2.
[0069]
According to this, in step S41, the low speed component LO2 is expressed by the following equation (5),
LO2 = LO2z + OVERFLOW x B (5)
LO2z: Previous value of low-speed component LO2
B: Calculated by the oxygen absorption / release rate of the low speed component. Here, the oxygen absorption / release rate B of the low-speed component is set to a positive value of 1 or less, but actually has different characteristics in absorption and release, and the actual absorption / release rate is the catalyst temperature TCAT, low-speed component Since it is affected by LO2, etc., the absorption rate and the release rate may be set separately. In this case, when the overflow amount OVERFLOW is positive, oxygen is excessive, and the oxygen absorption B at this time is set to a larger value as the catalyst temperature TCAT is higher and the low speed component LO2 is smaller, for example. Further, when the overflow amount OVERFLOW is negative, oxygen is insufficient, and the oxygen release rate B at this time is set to be larger as the catalyst temperature TCAT is higher and the low speed component LO2 is larger, for example.
[0070]
In steps S42 and S43, similarly to the calculation of the high speed component HO2, it is determined whether the calculated low speed component LO2 does not exceed the maximum amount LO2MAX or is not less than the minimum amount LO2MIN (= 0). .
[0071]
As a result, if the maximum amount LO2MAX is exceeded, the process proceeds to step S44, where the oxygen excess / deficiency O2OUT overflowing from the low speed component LO2 is expressed by the following equation (6):
O2OUT = LO2 − LO2MAX (6)
And the low speed component LO2 is limited to the maximum amount LO2MAX. The oxygen excess / deficiency O2OUT flows out downstream of the catalyst 3 as it is.
[0072]
On the other hand, if the amount 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.
[0073]
In this way, the oxygen storage amount for the catalyst 3 is calculated, and the air-fuel ratio is controlled so that this oxygen storage amount matches the target value.
[0074]
The present invention is not limited to the above-described embodiment, and it is obvious that various modifications can be made 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.
FIGS. 3A and 3B illustrate a relationship between an oxygen storage amount and a target air-fuel ratio. FIG. 3A shows a case where the air-fuel ratio on the downstream side of the catalyst is lean, and FIG.
FIG. 4 is a flowchart for calculating an oxygen storage amount (high-speed component storage amount).
FIG. 5 is a flowchart for calculating an oxygen storage amount (low-speed component storage amount).
[Explanation of symbols]
1 Engine 3 Catalyst 4 Upstream air-fuel ratio sensor 5 Downstream air-fuel ratio sensor 6 Controller

Claims (7)

In a catalyst capable of storing or releasing oxygen in the exhaust gas according to the exhaust air-fuel ratio, and an apparatus for controlling the air-fuel ratio so that the oxygen storage amount becomes a target value,
Means for estimating an oxygen storage amount based on an output of an air-fuel ratio sensor upstream of the catalyst, and controlling the air-fuel ratio so that the oxygen storage amount matches a target value;
When the output of the downstream air-fuel ratio sensor is lean, reduce the oxygen storage amount,
Also, means for changing the target air-fuel ratio to increase the oxygen storage amount when on the rich side,
After a predetermined time has elapsed from the change of the target air-fuel ratio by the changing means,
When the downstream air-fuel ratio is on the same side as before the target air-fuel ratio change despite the change in the target air-fuel ratio, the output of the upstream air-fuel ratio sensor is corrected based on the rich side or lean side of the air-fuel ratio. An air-fuel ratio control apparatus for an internal combustion engine, comprising: a correction means. 2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the predetermined time is a time during which a downstream air-fuel ratio responds to a change in the target air-fuel ratio.   3. The change unit according to claim 1, wherein when the output of the downstream side air-fuel ratio sensor is on the lean side, the target air-fuel ratio is changed by a constant value to a rich side, and when the output is also rich, it is changed to a lean value by a constant value. An air-fuel ratio control apparatus for an internal combustion engine.   When the downstream air-fuel ratio sensor is on the lean side, the changing means is a positive value that shifts by a certain amount to the lean side, and when the downstream air-fuel ratio sensor is on the rich side, it is constant on the rich side. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the negative value is shifted by an amount.   The change means, as a correction value for the upstream air-fuel ratio sensor output, is a positive value that shifts to the lean side by an amount corresponding to the sensor output when the downstream air-fuel ratio sensor is on the lean side, and rich when it is on the rich side. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the negative value is shifted to the side by an amount corresponding to the sensor output.   The air-fuel ratio control apparatus for an internal combustion engine according to claim 4 or 5, wherein the changing means determines that the upstream air-fuel ratio sensor has an abnormality when an absolute value obtained by integrating the correction values reaches a predetermined value.   The air-fuel ratio of the internal combustion engine according to any one of claims 1 to 6, wherein the oxygen storage amount is estimated by dividing the oxygen storage amount into a high speed component having a high absorption speed and a low speed component having an absorption speed slower than the high speed component. Control device.

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