JP2003049685A - Exhaust emission control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a catalyst oxygen storage quantity to a target value continuously precisely by compensating an output variation due to degradation in an air-fuel ratio sensor upstream of a catalyst more appropriately according to an output characteristic of a downstream oxygen sensor. SOLUTION: An oxygen storage quantity is estimated according to an air-fuel ratio sensor output upstream of the catalyst with oxygen storage capacity, and an air-fuel ratio is controlled to bring the estimate coincident with the target value. The air-fuel ratio is charged to decrease and increase the oxygen storage quantity respectively when an oxygen sensor output downstream of the catalyst is on the lean side and on the rich side, and if the downstream air-fuel ratio nevertheless remains for a fixed time in an air-fuel ratio deviation state to the same side as before the change, the air-fuel ratio sensor output is corrected by a given level. The correction is, however, prohibited if a variation speed DRO2 of the oxygen sensor output toward the stoichiometric point is large in the air-fuel ratio deviation state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust purification system for an engine, and more particularly to an improvement of an exhaust purification system provided with a catalyst having an oxygen storage function in an exhaust system.

[0002]

2. Description of the Related Art The amount of oxygen absorbed by a three-way catalyst (hereinafter referred to as "oxygen storage amount") is the intake air amount of the engine and the air-fuel ratio (or oxygen concentration) of the exhaust flowing into the catalyst. The same shall apply hereinafter.)
It is known that the air-fuel ratio of the engine is controlled so that the oxygen storage amount of the catalyst becomes constant (Japanese Patent Laid-Open No. 9-2288).
No. 73).

In order to maintain the maximum conversion efficiency of NOx, CO, and HC of the three-way catalyst, it is necessary to make the catalyst atmosphere a stoichiometric air-fuel ratio, but the oxygen storage amount of the catalyst is an appropriate amount, for example, the maximum oxygen storage amount. By keeping it at about half of that, even if the exhaust gas flowing into the catalyst shifts to the lean side, the oxygen in the exhaust gas is absorbed by the catalyst, or even if it shifts to the rich side, the oxygen absorbed by the catalyst is released. Therefore, the catalyst atmosphere can be kept substantially near the stoichiometric air-fuel ratio.

By the way, the air-fuel ratio sensor provided on the upstream side of the catalyst sometimes deteriorates with time due to exposure to high temperature exhaust gas, and there is a variation in quality at the time of manufacturing the sensor. May cause an error (shift). When an error occurs in the air-fuel ratio detection in this way, it becomes impossible to accurately calculate the oxygen storage amount of the catalyst 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 emission performance deteriorates.

As a countermeasure against this, the present applicant has proposed that the occurrence of the shift of the upstream side air-fuel ratio sensor is determined based on the air-fuel ratio information from the sensor provided on the downstream side of the catalyst. For example, if the output of the upstream air-fuel ratio sensor is shifted to the rich side of the actual air-fuel ratio,
Even if the actual air-fuel ratio is stoichiometric, the apparent air-fuel ratio by the sensor will be on the rich side, so the actual result of air-fuel ratio control will shift to the lean side. If this deviation is detected by the downstream air-fuel ratio sensor, it is possible to correct the output of the upstream air-fuel ratio sensor to the lean side and eliminate the deviation from the actual air-fuel ratio.

The present invention is a further improvement of the exhaust gas purification device which corrects the shift of the upstream side air-fuel ratio sensor in this way, and pays attention to the hysteresis characteristic of the air-fuel ratio sensor to improve the control accuracy. It is intended to provide an improved exhaust emission control device.

[0007]

According to a first aspect of the present invention, there is provided a catalyst provided in an exhaust pipe of an engine, an upstream air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas flowing into the catalyst, and an air-fuel ratio downstream of the catalyst. A downstream air-fuel ratio sensor for detecting the fuel ratio, an oxygen storage amount calculation means for calculating the oxygen storage amount of the catalyst based on the detected air-fuel ratio on the upstream side, and an oxygen storage amount of the catalyst based on the calculated oxygen storage amount. Air-fuel ratio control means for controlling the air-fuel ratio of the engine so as to be a fixed amount, and the air-fuel ratio switching that switches the target air-fuel ratio so that the oxygen storage amount is decreased when the downstream side air-fuel ratio is lean and increased when it is rich. And the downstream side air-fuel ratio detection value is leaner than a predetermined lean side determination threshold value or richer than a predetermined rich side determination threshold value after switching the target air-fuel ratio. An air-fuel ratio correction means for performing an air-fuel ratio correction in a direction for correcting the air-fuel ratio deviation in a fuel ratio deviation state, and a downstream side air-fuel ratio sensor in the judgment threshold value direction in the air-fuel ratio deviation state And a correction prohibiting means for prohibiting the air-fuel ratio correction when the changing speed of the output value becomes larger than the reference value.

According to a second aspect of the present invention, the air-fuel ratio correction means is configured to correct the air-fuel ratio deviation when the duration of the air-fuel ratio deviation state exceeds a reference value. To do.

According to a third aspect of the invention, the air-fuel ratio correction of the first or second aspect of the invention is performed by correcting the output value of the upstream side air-fuel ratio sensor according to the air-fuel ratio deviation direction. .

A fourth aspect of the invention is to perform the air-fuel ratio correction of the first or second aspect of the invention by correcting the air-fuel ratio control target value in the air-fuel ratio control means in accordance with the air-fuel ratio deviation direction. And

A fifth invention is the same as the third invention,
The output correction value of the upstream air-fuel ratio sensor is a positive value that shifts to the lean side by a fixed amount when the output of the downstream side air-fuel ratio sensor is lean, and a negative value that shifts to the rich side by a fixed amount when the output is rich. Shall be set to the value of.

A sixth invention is based on the third invention.
The output correction value of the upstream side air-fuel ratio sensor is a positive value that shifts to the lean side by an amount according to the sensor output when the output of the downstream side air-fuel ratio sensor is lean side, and to the rich side when it is rich side. It should be set to a negative value that shifts by an amount according to the sensor output.

In a seventh aspect based on the fifth or sixth aspect, it is determined that the upstream side air-fuel ratio sensor is abnormal when the absolute value obtained by integrating the correction values reaches a predetermined value. Constitute.

According to an eighth aspect of the present invention, the oxygen storage amount calculating means of the first aspect is operated by dividing the oxygen storage amount into a high-speed component having a fast absorption rate and a low-speed component having a higher absorption rate than the high-speed component. It is configured so as to be estimated.

A ninth aspect of the present invention is a catalyst provided in an exhaust pipe of an engine, an upstream air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas flowing into the catalyst, and a downstream side for detecting an air-fuel ratio downstream of the catalyst. An air-fuel ratio sensor, and air-fuel ratio control means for controlling the air-fuel ratio of the engine based on the detected air-fuel ratio on the upstream side,
When the downstream side air-fuel ratio is lean, it is on the rich side, and when it is rich, air-fuel ratio switching means for switching the target air-fuel ratio to lean side, and after the target air-fuel ratio is switched, the downstream side air-fuel ratio detection value is a predetermined lean value. When the air-fuel ratio deviation state is leaner than the side determination threshold value or richer than the predetermined rich side determination threshold value, the air-fuel ratio is corrected in the direction of correcting the air-fuel ratio deviation. Correction means and correction prohibiting means for prohibiting the air-fuel ratio correction when the rate of change of the output value of the downstream side air-fuel ratio sensor toward the determination threshold value in the air-fuel ratio deviation state becomes larger than a reference value. With.

[0016]

[Operation and effect] In each of the following inventions,
By the air-fuel ratio switching means, the target air-fuel ratio is switched to the rich direction so that the oxygen storage amount decreases when the air-fuel ratio downstream of the catalyst becomes lean, and the oxygen storage amount decreases when the air-fuel ratio downstream of the catalyst becomes rich. The target air-fuel ratio is switched to the lean direction so as to increase.

At this time, if the upstream side air-fuel ratio sensor shifts, for example, in the rich direction, the apparent air-fuel ratio becomes rich even if the actual air-fuel ratio is stoichiometric, so the calculated value of the oxygen storage amount is oxygen release. The air-fuel ratio is biased toward the lean direction because an error occurs in the direction and the oxygen storage amount is always insufficient, and as a result, the air-fuel ratio is switched to the rich direction by the air-fuel ratio switching means described above. Instead, the air-fuel ratio deviation state in the lean region where the output of the downstream side air-fuel ratio sensor exceeds the lean side threshold value can continue. On the contrary, when the output of the upstream air-fuel ratio sensor shifts in the lean direction, the calculated value of the oxygen storage amount has an error in the oxygen absorption direction and always indicates that the oxygen storage amount is large. Therefore, the air-fuel ratio is biased toward the rich side, and as a result, the output of the downstream side air-fuel ratio sensor exceeds the rich-side threshold regardless of the lean-direction switching of the air-fuel ratio by the air-fuel ratio switching means described above. The air-fuel ratio excursion condition in 1 can continue.

When such an air-fuel ratio deviation state continues, it is judged from the output of the downstream side air-fuel ratio sensor and the air-fuel ratio correction means corrects the deviation, that is, the upstream side air-fuel ratio. Air-fuel ratio correction is performed in the rich direction when the sensor shifts in the rich direction, and in the lean direction when the shift occurs in the lean direction. However, even if the air-fuel ratio deviation state is entered, when the air-fuel ratio downstream of the catalyst is about to return to the stoichiometric direction due to the switching of the air-fuel ratio described above, correction is performed rather than causing the air-fuel ratio to fluctuate in the opposite direction. There is a possibility that there is a possibility that the air-fuel ratio will not be corrected. That is, if the output of the downstream side air-fuel ratio sensor is monitored in the air-fuel ratio deviation state and the correction is not performed when the output becomes equivalent to stoichiometry, the stability of the air-fuel ratio control is impaired by unnecessary correction. The fear can be avoided.

However, as shown in FIG. 8, there is hysteresis in the signal of the air-fuel ratio sensor, and an output showing stoichiometry (λ = 1) at the time of changing from lean to rich and at the time of changing from rich to lean. Since the values are different, if it is decided to determine the stoichiometric equivalent by the signal value, the error between the signal value due to hysteresis and the actual air-fuel ratio will cause excess or deficiency of the air-fuel ratio correction, and the accuracy of the air-fuel ratio control will deteriorate. Resulting in. Further, as shown as the second aspect of the invention, when the air-fuel ratio correction is configured to be performed when the air-fuel ratio deviation state has passed a certain time, the air-fuel ratio correction can be surely performed with the passage of time. On the other hand, due to an error between the air-fuel ratio sensor output and the actual air-fuel ratio, the sensor output shows an air-fuel ratio deviation state even though the actual air-fuel ratio has already become stoichiometric, so the time elapses. As a result, the air-fuel ratio correction is performed, which may cause unnecessary air-fuel ratio fluctuation.

On the other hand, according to the present invention, even if the air-fuel ratio deviation state continues, the output of the downstream side air-fuel ratio sensor changes in the judgment threshold value direction (stoichiometric direction) at a speed larger than the reference value. Since the correction of the air-fuel ratio is prohibited while the air-fuel ratio is being corrected, the unnecessary air-fuel ratio correction described above can be avoided and more appropriate air-fuel ratio control can be performed.

The correction of the air-fuel ratio is performed by correcting the output value of the upstream side air-fuel ratio sensor according to the direction of deviation of the air-fuel ratio as shown in the third invention, or as shown in the fourth invention. Thus, it can be performed by correcting the air-fuel ratio control target value in the air-fuel ratio control means according to the air-fuel ratio deviation direction.

According to the fifth aspect of the present invention, the output of the upstream air-fuel ratio sensor is corrected by a fixed amount according to the downstream air-fuel ratio, so that a rapid change in the air-fuel ratio before and after the correction can be avoided.

According to the sixth aspect of the invention, the output of the upstream side air-fuel ratio sensor is corrected according to the magnitude of the output value of the downstream side air-fuel ratio sensor, so that the oxygen storage amount converges to the target value early. Can be made.

According to the seventh aspect of the invention, the output of the upstream side air-fuel ratio sensor is often corrected, and if the integrated value becomes large, it becomes difficult to maintain the control accuracy and stability, which adversely affects the exhaust performance. However, since the abnormality is determined when the integrated value of the correction amount exceeds the predetermined value, a sensor abnormality warning can be given to prompt repair or replacement.

On the other hand, the oxygen storage characteristic of the catalyst is divided into a characteristic of being rapidly absorbed / desorbed by the precious metal of the catalyst and a characteristic of being slowly absorbed / desorbed by an oxygen storage agent such as ceria of the catalyst. As shown in the invention, by calculating the oxygen storage amount by dividing it into a high speed component and a low speed component according to this characteristic, the oxygen storage amount according to the characteristics of the catalyst can be calculated more accurately. It is possible to accurately control the amount to the target value.

According to the ninth aspect, more appropriate air-fuel ratio control can be performed while avoiding unnecessary air-fuel ratio correction.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic structure of an exhaust emission control device to which the present invention is applied. A catalyst 3 is provided in an exhaust pipe 2 of an engine 1.
Is provided, the upstream side thereof is provided with a linear air-fuel ratio sensor 4 as an upstream side air-fuel ratio sensor, and the downstream thereof is provided with an oxygen sensor (hereinafter referred to as “O2 sensor”) 5 as a downstream side air-fuel ratio sensor. Based on engine 1
A controller 6 for controlling the air-fuel ratio of the fuel supplied to The air-fuel ratio sensor 4 has a linear output characteristic according to the oxygen concentration in the exhaust gas, and the downstream O2 sensor 5 has a characteristic in which the output changes more sensitively depending on the presence or absence of oxygen.

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 an oxygen storage agent such as ceria held in the catalyst carrier, and has a function of holding or releasing oxygen (oxygen storage function) according to the air-fuel ratio of the inflowing exhaust gas.

A throttle valve 8 is provided in the intake pipe 7 of the engine 1.
And an air flow meter 9 for measuring the amount of intake air adjusted by the throttle valve 8. A temperature sensor 10 that detects the temperature of the cooling water is attached to the engine 1 and is used to determine the operating state of the engine 1 and the activation state of the catalyst 3 and the like.

The controller 6 is configured as a microcomputer including a CPU and its peripheral devices, calculates the oxygen storage amount of the catalyst 3 based on the outputs of the air flow meter 9 and the air-fuel ratio sensor 4, and this storage amount is the target. The air-fuel ratio is feedback-controlled so that it becomes a value. When the calculated oxygen storage amount is less than the target value, the target air-fuel ratio is set to the lean side to increase the holding amount. Conversely, when it is greater than the target value, it is set to the rich side to reduce the oxygen storage amount. To match. Further, although there is a difference between the calculated oxygen storage amount and the actual oxygen storage amount due to a calculation error, based on the oxygen concentration detected by the downstream O2 sensor 5, for example, at the time of fuel cut of the engine, After shifting to the fuel cut, the oxygen storage amount calculated at a predetermined timing is reset to correct the deviation.

The outline of the method for calculating the oxygen storage amount of the catalyst 3 is as follows. That is, the excess oxygen ratio, which is the ratio of excess or deficiency of oxygen in the exhaust gas, can be found by converting from the exhaust air-fuel ratio upstream of the catalyst. The oxygen excess rate has a positive value on the lean side and a negative value on the rich side, with zero at stoichiometry. From the oxygen excess rate and the intake air amount at that time, the amount of oxygen absorbed or released by the catalyst 3 is known, and the oxygen storage amount of the catalyst 3 can be estimated by accumulating this.

When the air-fuel ratio on the downstream side is lean, when the oxygen storage amount of the catalyst 3 has reached the saturated holding amount, it is not possible to hold the oxygen amount more than that, and the oxygen flows directly to the downstream side. From this state, when the air-fuel ratio becomes richer than stoichiometric, the retained oxygen amount decreases from the maximum value according to the oxygen shortage amount. When the air-fuel ratio on the downstream side is rich, it means that the oxygen storage amount is zero, and when the air-fuel ratio goes to the lean side from that state, the oxygen storage amount of the catalyst 3 increases in accordance with the oxygen excess amount at that time. To go. In this way, the oxygen storage amount of the catalyst 3 can be calculated by using the operating state as a reference, and by accumulating the oxygen storage amount, 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, and for example, a half of the retained amount is set as a target value, and the air-fuel ratio is controlled so that the oxygen storage amount matches this target value. is there.

However, the air-fuel ratio of the engine has a required value depending on the operating conditions, and when the catalyst 3 is made to function as a three-way catalyst, it is necessary to control it near stoichiometry, and in this operating state the oxygen storage amount is set to the target value. In order to make them coincide with each other, by adding a value corresponding to the deviation of the oxygen storage amount from the target value as a correction value to the known λ control for making the air-fuel ratio stoichiometric, the required combustion characteristics of the engine 1 are obtained. While satisfying, the oxygen storage amount can be made to converge to the target value.

In this embodiment, it is determined 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 due to sensor deterioration or the like, the output is responded accordingly. The output of the air-fuel ratio sensor is corrected to prevent the oxygen storage amount from changing from the target value.

For this reason, the output of the O2 sensor 5 on the downstream side of the catalyst 3 which is not easily affected by high heat is utilized, and when the air-fuel ratio on the downstream side of the catalyst is rich, the oxygen storage amount is increased and the lean side is increased. In this case, the target air-fuel ratio is adjusted to reduce the amount of oxygen storage, and nevertheless the air-fuel ratio on the downstream side of the catalyst does not return to stoichiometric and is biased to the same side. Assuming that the output of the air-fuel ratio sensor 4 is shifted, the output of the air-fuel ratio sensor 4 is corrected (shift correction) accordingly. However, even before the elapse of the time, the output of the air-fuel ratio sensor 4 is not corrected when the output of the O2 sensor 5 greatly changes in the stoichiometric direction.

The control contents 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 becomes stoichiometric. In the drawings and the following description, the numbers with the reference symbol S represent processing step numbers.

First, in 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 reaches a target value. The target air-fuel ratio is determined based on the 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 reach this target air-fuel ratio.

Next, at S2, it is judged from the output of the downstream side air-fuel ratio sensor 5 whether the air-fuel ratio is stoichiometric, and if it is stoichiometric, the control operation is ended. Normally, the exhaust air-fuel ratio downstream of the catalyst becomes stoichiometric due to the oxygen storage capacity of the catalyst 3, but when the oxygen storage amount of the catalyst 3 becomes saturated, or when all the oxygen is released,
The downstream air-fuel ratio fluctuates from stoichiometry.

If it is determined that the stoichiometry is not stoichiometric, S3
Then, 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 lean, the target air-fuel ratio is set to the rich side by a predetermined amount, and conversely, when the detected air-fuel ratio is rich, the target air-fuel ratio is set to the lean side by a predetermined amount. Therefore, the air-fuel ratio on the downstream side of the catalyst changes toward stoichiometry.

At S4, it is judged whether the output of the downstream side air-fuel ratio sensor 5 is on the same side or reversed with the change of the target air-fuel ratio. When they are on the same side, that is, when the target side air-fuel ratio remains lean or rich despite the change, the output of the upstream side air-fuel ratio sensor 4 is judged to be misaligned. , S5, the shift amount for the output of the upstream side air-fuel ratio sensor 4 is calculated, and this is fed back to the air-fuel ratio control.

The calculation of this shift amount is performed as follows, for example. When the detected downstream side air-fuel ratio is on the lean side and therefore the target side air-fuel ratio is changed to the rich side, but the downstream side air-fuel ratio is still on the lean side, FIG.
As shown in (A), 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 air-fuel ratio sensor 4 on the upstream side, the actual air-fuel ratio will not become so rich even if feedback control is performed based on this sensor output so as to reach the target value.
This is because the air-fuel ratio sensor 4 is apparently richer than the actual air-fuel ratio. Therefore, in this case, the output of the upstream side air-fuel ratio sensor 4 is corrected to the lean side by a fixed amount, and this is fed back to the control of the air-fuel ratio.

On the other hand, as shown in FIG.
Even if the detected downstream air-fuel ratio is on the rich side and the target air-fuel ratio is changed to the lean side, but the downstream air-fuel ratio is still on the rich side, conversely to the above, the upstream air-fuel ratio is It is conceivable that the output of the sensor 4 apparently shifts to the lean side relative to the actual air-fuel ratio, and in order to correct this, the output of the upstream side air-fuel ratio sensor 4 is shifted to the rich side by a certain amount. Is corrected. It should be noted that this correction amount is not a fixed amount but may be changed according to the magnitude of the absolute value of the output of the O2 sensor 5. In this case, the oxygen storage amount can be quickly converged to the target value after correction.

As described above, in S5, basically, the correction amount for the shift of the output of the upstream side air-fuel ratio sensor 4 is calculated and fed back to the air-fuel ratio control to increase the oxygen storage amount toward the target value or Reduce. However, as will be described in detail later, in the present invention, the correction is prohibited according to the output change speed of the O2 sensor 5.

On the other hand, in S6, the abnormality of the upstream side air-fuel ratio sensor 4 is judged from the integrated value of the shift amount with respect to this sensor output. This is to accumulate the correction amount of the air-fuel ratio sensor 4 on the upstream side, and determine that the air-fuel ratio sensor 4 is abnormal when the absolute value of the integrated amount reaches a predetermined limit value. When the deterioration of the air-fuel ratio sensor 4 progresses and the accumulated correction amount of the sensor output reaches a certain limit, stable air-fuel ratio control becomes difficult and exhaust performance may be adversely affected. However, by informing the user, prompt repair and replacement can be promoted.

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

Next, the overall operation will be described. The oxygen storage amount of the catalyst 3 is a target value, for example, is controlled to about one half of the maximum storage amount, so that the catalyst 3 can efficiently purify NOx, HC, and CO. The oxygen storage amount is calculated based on the output of the air-fuel ratio sensor 4 on the upstream side. When this amount decreases below the target value, the air-fuel ratio is controlled to the lean side, and the storage amount increases, and conversely, when it increases above the target value, the rich side increases. Controlled to reduce the amount of storage.
Therefore, in a normal state where the catalyst 3 functions properly, the catalyst 3
The air-fuel ratio on the downstream side of the engine becomes stoichiometric and does not become lean or rich.

However, when the air-fuel ratio sensor 4 deteriorates with time and the sensor output shifts from the normal state, the air-fuel ratio is actually detected to the lean side or the rich side. Then, even if the oxygen storage amount is calculated on the basis of 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 all of it may be released. .

In this case, the air-fuel ratio on the downstream side of the catalyst fluctuates from stoichiometric to rich or lean. If there is such a change in the air-fuel ratio and the downstream side air-fuel ratio is now on the lean side, the target air-fuel ratio is changed to the rich side by a predetermined amount. By changing the target 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 stoichiometric. However, when the catalyst downstream side air-fuel ratio is still on the lean side in spite of the change of the target air-fuel ratio, it is assumed that the output shift due to deterioration of the upstream side air-fuel ratio sensor 4 is large as described above. As in (A), the sensor output is corrected by a fixed amount.

Even though the target air-fuel ratio is changed to the rich side, it does not become so rich, which means that the upstream side air-fuel ratio sensor 4 is apparently shifted to the rich side rather than the actual side. In this correction, the sensor output of the upstream air-fuel ratio sensor 4 is shifted to the lean side by a fixed 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 corrected to the rich side by feeding back this, and the target air-fuel ratio is obtained.

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 as shown in FIG. 3 (B), and at this time, the correction is performed. The directions are reversed.

By performing such control, the oxygen storage amount can be converged to the target value even if the output of the upstream side air-fuel ratio sensor 4 is shifted.

However, the above-described correction operation may be continuously performed depending on the deviation of the output of the air-fuel ratio sensor 4. Further, when the correction in the same direction is repeated many times, the sensor deterioration greatly progresses. There is also the possibility of leaving. Therefore, when the absolute value of the correction amount of the sensor output with respect to deterioration reaches a limit value, it is determined that the sensor is abnormal, and it is recommended to replace the sensor with a new one rather than to continue the correction.

Next, referring to FIGS. 4 and 5, the air-fuel ratio sensor 4
A specific example of calculating the oxygen storage amount of the catalyst 3 based on the output of The oxygen storage characteristic for the catalyst 3 is divided into a characteristic of being rapidly absorbed / released by the precious metal of the catalyst and a characteristic 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 the high speed component and the low speed component according to this characteristic, the actual storage amount according to the characteristic of the catalyst can be accurately calculated.

FIG. 4 is a flowchart for calculating the oxygen storage amount of the high speed component, and FIG. 5 is a flowchart for similarly calculating the low speed component.

In FIG. 4, in this subroutine, the high speed component HO2 is calculated based on the oxygen excess / deficiency 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 S31, it is judged based on the value of the oxygen excess / deficiency O2IN whether the high-speed component HO2 is in the state of absorbing oxygen or in the state of releasing oxygen.

As a result, when the air-fuel ratio of the exhaust gas flowing into the catalyst 3 is lean and the oxygen excess / deficiency O2IN is larger than zero, it is judged that the high speed component HO2 is in the state of absorbing oxygen, and the routine proceeds to S32. Next, 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 O2IN is a value of zero or less and it is determined that the high-speed component is in the state of releasing oxygen, the process proceeds to S33, and the following equation (2), 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.

When the high speed component HO2 is calculated in this manner, the value is the maximum amount HO2MAX of the high speed component in S34 and S35.
It is determined whether or not the value exceeds HO2MIN (= 0) or less.

If the high-speed component HO2 is equal to or larger than the maximum amount HO2MAX, the process proceeds to S36, and the overflow amount (excess amount) OVERFLOW that overflows without being absorbed by the high-speed component HO2 is given by the following equation (3), OVERFLOW = HO2 − HO2MAX… (3), and the high-speed component HO2 is the maximum amount HO2MAX.
Limited to.

If the high-speed component HO2 is less than the minimum amount HO2MIN, the process proceeds to S37, and the overflow amount (deficiency amount) OVERFLOW that overflows without being absorbed by the high-speed component HO2 is given by the following equation (4), OVERFLOW = HO2 − HO2MIN… (4), and the high-speed component HO2 is the minimum amount HO2MIN
Limited to. Since 0 is given as the minimum amount HO2MIN here, the oxygen amount deficient in the state where all the high-speed component HO2 is released is calculated as a negative overflow amount.

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

Here, the overflow amount OVERFLOW that overflows from the high speed component HO2 when the high speed component HO2 becomes the maximum amount HO2MAX or more or the minimum amount HO2MIN or less is absorbed or released by the low speed component LO2.

FIG. 5 shows the low-speed component of the oxygen storage amount.
The contents of the subroutine for calculating LO2 are shown below. In this subroutine, the low speed component LO2 is calculated based on the overflow amount OVERFLOW overflowing from the high speed component HO2.

According to this, in 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 It Here, the oxygen absorption / release rate B of the low-speed component
Is set to a positive value of 1 or less, but it actually has different characteristics for absorption and release, and the actual absorption / desorption rate is affected by the catalyst temperature TCAT, low-speed component LO2, etc. The release rate and the release rate may be set separately. In that case, when the overflow amount OVERFLOW is positive, oxygen is excessive, and the oxygen absorption rate 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. In addition, overflow OV
When ERFLOW is negative, oxygen is insufficient, and the oxygen release rate B at this time is set larger as the catalyst temperature TCAT is higher and the low speed component LO2 is larger.

At S42 and S43, the calculated low-speed component LO2 is the maximum amount LO2MA, as in the calculation of the high-speed component HO2.
It is determined whether it does not exceed X or is less than or equal to the minimum amount LO2MIN (= 0).

As a result, when the maximum amount LO2MAX is exceeded, the routine proceeds to S44, where the oxygen excess / deficiency O overflowing from the low speed component LO2 is exceeded.
2OUT is calculated 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 to the downstream of the catalyst 3 as it is.

On the other hand, if it is less than the minimum amount, S4
5, the low speed component LO2 is limited to the minimum amount LO2MIN.

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.

Next, the above-described air-fuel ratio correction (shift correction of the air-fuel ratio sensor 4) and correction prohibition processing will be described with reference to the flowchart shown in FIG. In this processing, basically, when the rich or lean air-fuel ratio deviation state in which the catalyst downstream exceeds the determination threshold value continues for the reference time, a shift amount for compensating the output of the air-fuel ratio sensor 4 is provided, but the reference time elapses. Even before, when the output of the rear O2 sensor 5 is changing toward the stoichiometry at a predetermined speed, the correction by the shift amount is prohibited. Hereinafter, description will be made step by step.

First, engine start is detected from the state of the starter switch, and when the starter switch is turned on, the timer value TMHOS which is the reference for the elapsed time of the air-fuel ratio deviation state is reset to 0 as an initialization process, and then the rear O2
The output of the sensor 5 is read (S11-S13). When the starter switch is not on, the engine is already in operation, and therefore the TMHOS reset process is not performed and the rear O2 sensor output is read.

Next, from the output of the rear O2 sensor 5, whether the downstream side of the catalyst 3 is in an air-fuel ratio deviation state (during rich determination) exceeding a predetermined rich determination threshold value or a predetermined lean determination threshold value is set. It is determined whether or not the air-fuel ratio deviation state exceeds the value (lean determination is being performed) (S14, S21). If neither is the case, this indicates that the air-fuel ratio is in the vicinity of stoichiometry, so the timer value TMHOS is reset and the processing of this time is ended (S125).

When it is determined in S14 that the rich determination is being performed, the timer value TMHOS is added next, and the decrease margin DRO2 of the output of the rear O2 sensor 5 per unit time is calculated (S15, S16). . The D
RO2 is the changing direction of the sensor output, that is, the changing speed toward stoichiometry, and a value obtained by taking the difference between the sensor values detected in the processing cycle of this routine can be used.

Next, this DRO2 is compared with the reduction speed stoichiometric determination value RO2DWNST (S17). Where D
If RO2 ≧ RO2DWNST, it is considered that the air-fuel ratio deviation state in the rich region continues, and then the timer value TMH
It is determined whether the OS has exceeded the reference value (S18). T
When MHOS exceeds the reference value, a process of integrating the shift amount for correction with respect to the air-fuel ratio sensor 4 for the air-fuel ratio correction is performed, and then the timer value TMHOS is reset to end the present process (S19, S20). However, in the determination in S7, DRO2 <RO2DWNST
If this is the case, this indicates that the rear O2 sensor output is decreasing toward the stoichiometry at a speed equal to or higher than the determination value. Therefore, the timer value TMHOS is set without performing the shift correction process in S19 to S20. Reset and finish this process.

When it is determined in S21 that the lean determination is being performed, the timer value TMHOS is added, and then the rise margin DRO2 of the output of the rear O2 sensor 5 per unit time is calculated (S22, S23). in this case,
The change speed in the increasing direction of the sensor output is substituted into DRO2.

Next, this DRO2 is compared with the rising speed stoichiometric determination value RO2UPST (S24). DR here
If O2 ≦ RO2UPST, it is considered that the air-fuel ratio deviation state in the lean region continues, and then the timer value TMHOS
Is determined to exceed the reference value (S18). TMH
When the OS exceeds the reference value, a process for accumulating the correction shift amount for the air-fuel ratio sensor 4 for the air-fuel ratio correction is performed, and then the timer value TMHOS is reset and the present process is terminated (S19, S20). However, S
In the determination in 24, if DRO2> RO2DWNST, this indicates that the rear O2 sensor output is increasing toward the stoichiometry at a speed equal to or higher than the determination value. Therefore, the shift correction process in S19 to S20 is performed. Is not performed, the timer value TMHOS is reset and the processing of this time is ended.

By repeating the above-described processing, the air-fuel ratio control for properly maintaining the output value of the air-fuel ratio sensor 3 and maintaining the oxygen storage amount at an appropriate amount can be performed more accurately. FIG. 7 is a timing chart showing a change state of the sensor output and the like when such control is executed in comparison with a case where the correction prohibition process is not performed. This is a diagram showing a state in which the output of the rear O2 sensor 5 decreases in the stoichiometric direction from the state during rich determination, but as shown by the broken line in the figure, when the correction prohibition process is not performed, it is a real space. Even when the fuel ratio is about to converge to the stoichiometric range, shift correction is performed after the elapse of TMHOS for a certain period of time, resulting in overcorrection and instability of control such as inversion of the air-fuel ratio in the lean direction.

On the other hand, according to the control of the present invention, O
2 Sensor output drop margin DRO2 is the reference value (RO2DWN
(ST) When the vehicle speed decreases to a stoichiometric speed larger than ST), the shift correction timer value TMHOS is reset so that the shift correction is not performed. Therefore, the shift correction amount of the air-fuel ratio sensor 4 is set. It is possible to perform a more appropriate air-fuel ratio control while avoiding the problem that the value becomes excessive.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of an engine to which the present invention is applied.

FIG. 2 is a flowchart showing a basic control operation in the embodiment.

FIG. 3 is an explanatory diagram of a relationship between an oxygen storage amount and a target air-fuel ratio.

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).

FIG. 6 is a flowchart regarding processing for prohibiting correction of an air-fuel ratio sensor output in the embodiment.

7 is a timing chart showing an execution result of the processing of FIG.

FIG. 8 is an explanatory diagram of a hysteresis characteristic of the oxygen sensor.

[Explanation of symbols]

1 engine 3 catalyst 4 Air-fuel ratio sensor 5 O2 sensor (oxygen sensor) 6 controller

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/24 F02D 41/22 305K F02D 41/22 305 45/00 358K 45/00 358 B01D 53/36 103B F-term (reference) 3G084 BA09 DA04 EA04 EA11 EA13 EB12 EB22 EC01 EC03 FA07 FA10 FA20 FA30 3G091 AA17 AA28 AB03 BA14 BA15 BA19 BA27 BA32 CB02 DA01 DA02 DA08 DB06 DB07 DB08 DB10 DB13 DB03 DB08 DB10 DB13 DB03 HA39 3G301 HA01 JA11 JA16 JB09 LB01 MA01 NA06 NA08 NB03 NB15 ND01 NE14 NE17 NE19 NE23 NE26 PA01Z PD09A PD09Z PE08Z 4D048 AA06 AA13 AA18 AB05 AB07 DA01 DA02 DA03 DA05 DA08 DA13 DA20 EA04

Claims (9)

[Claims] 1. A catalyst provided in an exhaust pipe of an engine, an upstream air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas flowing into the catalyst, and a downstream air-fuel ratio sensor for detecting an air-fuel ratio downstream of the catalyst. An oxygen storage amount calculating means for calculating the oxygen storage amount of the catalyst based on the upstream detected air-fuel ratio, and an engine air-fuel ratio so that the oxygen storage amount of the catalyst becomes a predetermined amount based on the calculated oxygen storage amount. Air-fuel ratio control means for controlling, the oxygen storage amount, an air-fuel ratio switching means for switching the target air-fuel ratio so as to decrease when the downstream side air-fuel ratio is lean and increase when the downstream air-fuel ratio is rich, and after the target air-fuel ratio is switched When the downstream side air-fuel ratio detection value is in an air-fuel ratio deviation state that is leaner than a predetermined lean side determination threshold value or richer than a predetermined rich side determination threshold value The air-fuel ratio correction means for correcting the air-fuel ratio deviation in the direction for correcting the air-fuel ratio deviation, and the change speed of the output value of the downstream side air-fuel ratio sensor toward the determination threshold value in the air-fuel ratio deviation state is a reference value. An exhaust emission control device for an engine, comprising: a correction prohibiting means for prohibiting the air-fuel ratio correction when it becomes larger than the above. 2. The air-fuel ratio correction means is configured to perform the air-fuel ratio correction in a direction of correcting the air-fuel ratio deviation when the duration of the air-fuel ratio deviation state exceeds a reference value. The exhaust emission control device for an engine according to claim 1. 3. The engine exhaust gas purification apparatus according to claim 1, wherein the air-fuel ratio correction is performed by correcting an output value of the upstream side air-fuel ratio sensor in accordance with the air-fuel ratio deviation direction. . 4. The engine exhaust according to claim 1, wherein the air-fuel ratio correction is performed by correcting an air-fuel ratio control target value in the air-fuel ratio control means in accordance with the air-fuel ratio deviation direction. Purification device. 5. The output correction value of the upstream side air-fuel ratio sensor is
When the output of the downstream side air-fuel ratio sensor is lean, it is set to a positive value that shifts to the lean side by a fixed amount, and when it is to the rich side, it is set to a negative value that shifts to the rich side by a fixed amount. 3. An engine exhaust gas purification device as described in 3. 6. The output correction value of the upstream side air-fuel ratio sensor is
When the output of the downstream air-fuel ratio sensor is lean, it is a positive value that shifts to the lean side by an amount according to the sensor output, and when it is rich, it is a negative value that shifts to the rich side by an amount according to the sensor output. The engine exhaust gas purification device according to claim 3, wherein the exhaust gas purification device is set to have a value. 7. The method according to claim 5, wherein the upstream side air-fuel ratio sensor is determined to be abnormal when the absolute value obtained by integrating the correction values reaches a predetermined value. Engine exhaust purification device. 8. The oxygen storage amount calculation means is configured to estimate the oxygen storage amount by separately calculating the oxygen storage amount into a high-speed component having a high absorption rate and a low-speed component having an absorption rate slower than the high-speed component. The exhaust gas purification device for an engine according to claim 1 or 2. 9. A catalyst provided in an exhaust pipe of an engine, an upstream air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas flowing into the catalyst, and a downstream air-fuel ratio sensor for detecting an air-fuel ratio downstream of the catalyst. An air-fuel ratio control means for controlling the air-fuel ratio of the engine based on the detected air-fuel ratio on the upstream side, and an air-fuel ratio for switching the target air-fuel ratio to the rich side when the downstream side air-fuel ratio is lean, and to the lean side when the air-fuel ratio is rich. After the switching of the fuel ratio switching means and the target air-fuel ratio, the downstream side air-fuel ratio detection value is leaner than a predetermined lean side determination threshold value or richer than a predetermined rich side determination threshold value. When the air-fuel ratio deviation is detected, the air-fuel ratio correction means for correcting the air-fuel ratio deviation is output, and the output value of the downstream side air-fuel ratio sensor in the judgment threshold value direction in the air-fuel ratio deviation state. The change rate of is the standard value Exhaust purification apparatus for an engine when the became large, characterized in that it comprises a correction inhibiting means for inhibiting the air-fuel ratio correction.