JP2002227690A - Air fuel ratio controller for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To execute a stable sub-feedback control when a catalyst is restored from a saturated adsorption state in a system executing a main/sub-feedback control using an intermediate target value. SOLUTION: Exhaust gas sensors 24, 25 are provided on an upstream side and a downstream side respectively of the catalyst 23, an intermediate target value is set based on the output of the downstream side exhaust gas sensor 25 at a previous computation and a final target value (final downstream side target air fuel ratio), and the correction amount of the upstream side target air fuel ratio is calculated based on the deviation between the output of the present downstream side exhaust gas sensor 25 and the intermediate target value. In this case when the catalyst 23 is restored from the saturated adsorption state, at least one of the updated amount of the intermediate target value, an updated speed, a control gain of the sub-feedback control, a control cycle and a control range is changed, thus limiting the control condition for the sub- feedback control within a range to secure stability when the catalyst 23 is restored from the saturated adsorption state.

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 sensor (linear A) provided upstream and downstream of an exhaust gas purifying catalyst.
/ F sensor) or an air-fuel ratio control device for an internal combustion engine that performs feedback control of the air-fuel ratio of the internal combustion engine by installing an oxygen sensor.

[0002]

2. Description of the Related Art In today's automobiles, a three-way catalyst is installed in an exhaust pipe to purify exhaust gas. However, in order to increase the exhaust gas purification rate of the catalyst, the air-fuel ratio of the exhaust gas must be reduced by using a catalyst purification window. It is necessary to control within (around the stoichiometric air-fuel ratio). Therefore, an exhaust gas sensor (air-fuel ratio sensor or oxygen sensor) is installed on each of the upstream and downstream sides of the catalyst, and the fuel injection amount is adjusted so that the air-fuel ratio of the exhaust gas detected by the upstream exhaust gas sensor becomes the upstream target air-fuel ratio. And performs a sub-feedback control to correct the upstream target air-fuel ratio so that the air-fuel ratio of the exhaust gas detected by the downstream exhaust gas sensor becomes the downstream target air-fuel ratio.

In such a main / sub feedback system, as shown in Japanese Patent No. 2518247, as the deviation between the detected air-fuel ratio of the downstream exhaust gas sensor and the downstream target air-fuel ratio increases, the air-fuel ratio feedback control constant ( It has been proposed to increase the update amount (eg, skip amount).

[0004]

The dynamic characteristics of the catalyst vary depending on the degree of deterioration of the catalyst, the state of adsorbing lean / rich components in the catalyst, and the operating state of the engine. The responsiveness of the sub-feedback control to the change in the dynamic characteristics of the catalyst is not sufficient. Therefore, a response delay of the sub-feedback control with respect to the change in the dynamic characteristics of the catalyst occurs, and the air-fuel ratio on the downstream side of the catalyst (output of the downstream side exhaust gas sensor) becomes unstable, and hunting may occur.

[0005] In order to solve this drawback, the inventors of the present invention, as described in Japanese Patent Application No. 2000-404671, describe the past detected air-fuel ratio of the downstream exhaust gas sensor and the final air-fuel ratio. A sub-feedback control that sets an intermediate target value of the sub feedback control based on the downstream target air-fuel ratio and corrects the upstream target air-fuel ratio based on a deviation between the detected air-fuel ratio of the downstream exhaust gas sensor and the intermediate target value. Is under development for practical use.

In putting this system into practical use,
The following new technical issues have been identified. That is, three-way catalysts generally used for exhaust gas purification are:
Exhaust gas is purified by causing an oxidation / reduction reaction between a rich component (HC, CO, etc.) and a lean component (NOx, oxygen, etc.) in the exhaust gas, or by adsorbing a rich component or a lean component in the exhaust gas on a catalyst. I have to. If the state in which the exhaust gas leans or leans continues, the adsorption amount of the lean or rich component of the catalyst increases, and eventually the adsorption amount of the catalyst becomes saturated. As described above, when the catalyst enters the saturated adsorption state, the air-fuel ratio on the upstream side of the catalyst is controlled in a direction to reduce the adsorption amount of the catalyst by the sub-feedback control, but the catalyst returns from the saturated adsorption state to a state where the adsorption amount is small. During this period, the storage state of the catalyst is unstable, so during this period, if high-response sub-feedback control using the intermediate target value is performed under the same conditions as during normal operation,
The sub-feedback control becomes unstable, overshoot or hunting may occur, and exhaust emission may increase.

The present invention has been made in view of such circumstances, and accordingly, has as its object to provide a system for performing main / sub feedback control using an intermediate target value when the catalyst returns from a saturated adsorption state. Another object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine that can perform stable sub-feedback control and ensure exhaust gas purification performance.

[0008]

In order to achieve the above object, an air-fuel ratio control apparatus for an internal combustion engine according to the first aspect of the present invention includes a past air-fuel ratio detected by a downstream exhaust gas sensor and a final downstream target air-fuel ratio. In the sub-feedback control for setting an intermediate target value of the sub-feedback control based on the fuel ratio and correcting the upstream target air-fuel ratio based on the deviation between the detected air-fuel ratio of the downstream exhaust gas sensor and the intermediate target value. When the catalyst returns from the saturated adsorption state, the return control means changes at least one of the update amount of the intermediate target value, the update speed, the control gain of the sub feedback control, the control cycle, and the control range. The return control is executed for a predetermined period. With this configuration, when the catalyst returns from the saturated adsorption state, even if the storage state of the catalyst is unstable, the control condition of the sub-feedback control is limited to a range where stability can be ensured, and stable Sub-feedback control can be performed, and exhaust gas purification performance can be ensured.

In this case, a value obtained by multiplying the difference between the past detected air-fuel ratio of the downstream exhaust gas sensor and the final downstream target air-fuel ratio by the damping rate, and the final downstream target An intermediate target value may be obtained by adding the air-fuel ratio and the attenuation rate may be changed when the catalyst returns from the saturated adsorption state. With this configuration, the intermediate target value can be set by a simple calculation process, and the control condition of the sub-feedback control when the catalyst returns from the saturated adsorption state can be changed by a simple calculation process.

In addition, the value calculated by the proportional integral operation with respect to the deviation between the detected air-fuel ratio of the downstream exhaust gas sensor and the intermediate target value is limited within a predetermined control range, so that the upstream target A correction amount of the air-fuel ratio is obtained, and a gain (control gain) and / or a control range (an upper guard value and a lower guard value) of the proportional integration operation are changed according to a parameter related to an operating state of the internal combustion engine or a state of the catalyst. You may do it. In this way, the change in the dynamic characteristics of the catalyst can be reflected in the correction amount of the upstream target air-fuel ratio with good response, and the control condition of the sub-feedback control when the catalyst returns from the saturated adsorption state can be changed. , Can be performed by simple arithmetic processing.

Further, the longer the time that the air-fuel ratio of the exhaust gas is rich or lean continues, the longer the adsorption amount of the catalyst increases, and the longer it takes to return the catalyst to the state of low adsorption amount. In consideration of this, the period in which the return control is executed may be set according to the time during which the rich or lean state in which the catalyst enters the saturated adsorption state continues. With this configuration, the period during which the return control is executed can be set to an appropriate time according to the actual adsorption amount of the catalyst, and the return control can be performed without excess or deficiency.

Alternatively, noting that the air-fuel ratio of the exhaust gas flowing out of the catalyst changes depending on the amount of catalyst adsorbed,
As described above, the timing of ending the return control and returning to the normal control may be determined based on the detected air-fuel ratio of the downstream exhaust gas sensor that detects the air-fuel ratio of the exhaust gas flowing out of the catalyst. With this configuration, it is possible to end the return control and return to the normal control while confirming the timing at which the catalyst returns to the state in which the amount of adsorption is small based on the air-fuel ratio on the downstream side of the catalyst detected by the downstream side exhaust gas sensor. It is possible to return from the return control to the normal control at an optimum timing.

Further, it may be determined that the catalyst is in a saturated adsorption state during execution of the fuel cut. That is, when executing the fuel cut, the intake air flows into the exhaust system without burning as it is, so that the oxygen concentration (lean component concentration) in the exhaust gas flowing inside the catalyst increases remarkably, and the lean component adsorption amount of the catalyst sharply increases. And the catalyst becomes saturated adsorption in a relatively short time. Therefore, even if it is determined that the catalyst is in the saturated adsorption state during the fuel cut,
This is a valid decision.

[0014]

[Embodiment (1)] An embodiment of the present invention will be described below with reference to the drawings. First, a schematic configuration of the entire engine control system will be described with reference to FIG. An air cleaner 13 is provided at the most upstream portion of an intake pipe 12 of an engine 11 which is an internal combustion engine, and an air flow meter 14 for detecting an intake air amount is provided downstream of the air cleaner 13. This air flow meter 1
Downstream of 4, a throttle valve 15 is provided.

Further, a surge tank 17 is provided downstream of the throttle valve 15.
Further, an intake manifold 19 for introducing air into each cylinder of the engine 11 is provided. A fuel injection valve 20 for injecting fuel is attached near each intake port of the intake manifold 19 of each cylinder. An ignition plug 21 is attached to a cylinder head of the engine 11 for each cylinder.

On the other hand, a catalyst 23 such as a three-way catalyst for purifying CO, HC, NOx and the like in exhaust gas is provided in the exhaust pipe 22 of the engine 11. Exhaust gas sensors 24 and 25 for detecting an exhaust gas air-fuel ratio or rich / lean are installed upstream and downstream of the catalyst 23, respectively. In the present embodiment, an air-fuel ratio sensor (linear A / F sensor) that outputs a linear air-fuel ratio signal according to the exhaust gas air-fuel ratio is used as the upstream exhaust gas sensor 24, and the downstream exhaust gas sensor 25 is an exhaust gas air-fuel sensor. An oxygen sensor whose output voltage is inverted depending on whether the fuel ratio is rich or lean with respect to the stoichiometric air-fuel ratio is used. Therefore, the downstream side exhaust gas sensor 25
Generates an output voltage of about 0.1 V when the air-fuel ratio is lean, and generates an output voltage of about 0.9 V when the air-fuel ratio is rich. The cylinder block of the engine 11 is provided with a water temperature sensor 26 for detecting a cooling water temperature and a rotation speed sensor 27 for detecting an engine rotation speed.

An engine control circuit (hereinafter referred to as “ECU”) 28 includes a ROM 29, a RAM 30, a CPU 31,
Backup RAM backed up by battery 32
The microcomputer is mainly composed of a microcomputer including an input port 33, an input port 34, an output port 35, and the like. The output signal of the rotation speed sensor 27 is input to the input port 34, and the output signals of the air flow meter 14, the upstream and downstream exhaust gas sensors 24 and 25, and the water temperature sensor 26 are respectively supplied to the A / D converter 36. Is entered via The output port 35 is connected to the fuel injection valve 20, the ignition plug 21 and the like via a drive circuit 39.

The ECU 28 executes the fuel injection control program and the ignition control program stored in the ROM 29 by the CPU 3.
1, the fuel injection valve 20 and the spark plug 21
By executing the air-fuel ratio control program, the air-fuel ratio (fuel injection amount) is feedback-controlled so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio.

Hereinafter, the air-fuel ratio feedback control system of the embodiment (1) will be described with reference to FIGS. Here, FIG. 2 is a block diagram showing the function of the air-fuel ratio control means 40 realized by the arithmetic processing function of the CPU 31, and FIG. 3 is a block diagram showing the function of the entire air-fuel ratio feedback control system.

The air-fuel ratio control means 40 comprises a fuel injection amount feedback control unit 41 and a target air-fuel ratio calculation unit 42. The target air-fuel ratio calculation unit 42 includes a load target air-fuel ratio calculation unit 43 and a target air-fuel ratio correction unit. 44.

The fuel injection amount feedback control unit 41
The fuel injection time Tinj of the fuel injection valve 20 is calculated so that the detected air-fuel ratio AF of the upstream side exhaust gas sensor 24 converges on the upstream side target air-fuel ratio AFref. This fuel injection time Tinj
Is calculated by the optimal regulator constructed for the linear equation of the model to be controlled. The fuel injection amount feedback control unit 41 plays a role corresponding to an air-fuel ratio feedback control unit described in claims.

On the other hand, the load target air-fuel ratio calculation unit 43
The load target air-fuel ratio AFbase according to the intake air amount (or the intake pipe pressure) and the engine speed is calculated from the function formula or map stored in M29. This load target air-fuel ratio AF
The function formula or map for calculating the base is such that when the output O2out (detected air-fuel ratio) of the downstream exhaust gas sensor 25 is constantly substantially equal to the final target value O2targ (final downstream target air-fuel ratio), If the side target air-fuel ratio AFref is maintained at the load target air-fuel ratio AFbase, the output O2out of the downstream side exhaust gas sensor 25 is set in advance by a test or the like so as to be maintained near the final target value O2targ.

The target air-fuel ratio corrector 44 calculates a correction amount AFcomp of the upstream target air-fuel ratio AFref based on the output O2out of the downstream exhaust gas sensor 25 by using an intermediate target value O2midtarg described later. Then, this correction amount AFco
By adding mp to the load target air-fuel ratio AFbase, an upstream target air-fuel ratio AFref is obtained, and this upstream target air-fuel ratio AFref is input to the fuel injection amount feedback control unit 41. AFref = AFbase + AFcomp Note that instead of the above equation, the upstream target air-fuel ratio AFre is calculated by the following equation.
f may be calculated. AFref = (1 + AFcomp) × AFbase

In this case, the target air-fuel ratio calculation unit 42 (the load target air-fuel ratio calculation unit 43 and the target air-fuel ratio correction unit 44) plays a role corresponding to the sub-feedback control means in the claims.

Next, a method of calculating the correction amount AFcomp of the upstream target air-fuel ratio AFref by setting the intermediate target value O2midtarg in the target air-fuel ratio correction unit 44 will be described with reference to FIG.

The control object is a system including a fuel injection amount feedback control unit 41, a fuel injection valve 20, an engine 11, a catalyst 23, a downstream exhaust gas sensor 25, and the like. The target air-fuel ratio correction unit 44 includes a time delay element (1 / z) 45, an intermediate target value calculation unit 46, an attenuation rate setting unit 47, and a correction amount calculation unit 48.
The time delay element 45 inputs the output O2out (i-1) of the downstream exhaust gas sensor 25 at the time of the previous calculation to the intermediate target value calculation unit 46.

On the other hand, the intermediate target value calculating section 46 plays a role corresponding to the intermediate target value setting means in the claims, and outputs the output O2out of the downstream side exhaust gas sensor 25 at the time of the previous calculation.
Based on (i-1) and the final target value O2targ (i) (final downstream target air-fuel ratio), an intermediate target value O2midtarg (i) is calculated using the following equation (1). Thus, the intermediate target value O2midtarg (i) is set between the output O2out (i-1) of the downstream exhaust gas sensor 25 at the time of the previous calculation and the final target value O2targ (i). O2midtarg (i) = O2targ (i) + Kdec × {O2out (i-1) -O2targ (i)} (1)

In the above equation, O2targ (i) is the current final target value, and O2out (i-1) is the output of the downstream exhaust gas sensor 25 at the time of the previous calculation. Kdec is an attenuation rate, which is set by the control condition setting unit 47 within the range of 0 <Kdec <1. The attenuation rate Kdec is switched between a return control when the catalyst 23 returns from the saturated adsorption state and a normal control. The damping rate Kdec during the return control is set to a value larger than the damping rate Kdec during the normal control, and the update amount of the intermediate target value during the return control is set to be smaller than that in the normal control. The damping rate Kdec in each control mode may be a fixed value for simplicity of the calculation process, but the engine operating state, the state of the catalyst 23, the downstream exhaust gas sensor 25
May be set by a map or a mathematical expression in accordance with the output characteristics of the above.

As described above, using the attenuation rate Kdec set by the control condition setting unit 47, the intermediate target value calculation unit 46
After calculating the intermediate target value O2midtarg (i), the correction amount AFcomp (i) of the upstream target air-fuel ratio AFref is calculated using the intermediate target value O2midtarg (i) by the following equation. AFcomp (i) = Fsat ｛K1 × (O2midtarg (i) −O2o
ut (i)) + K2 × {(O2midtarg (i) −O2out (i))} = Fsat (K1 × ΔO2 (i) + K2 × ΔΔO2 (i)) where ΔO2 (i) = O2midtarg (i) −O2out ( i)

In the above equation, Fsat is a saturation function having a characteristic as shown in FIG. 4, and the correction amount AFcomp (i) is K1 ×
The calculated value of ΔO2 (i) + K2 × Σ (ΔO2 (i)) is obtained by performing guard processing in a predetermined control range (upper guard value and lower guard value). In the above equation, K1 is a proportional gain, and K2 is an integral gain. K1 × ΔO2 (i) is a proportional term, and increases as the deviation ΔO2 (i) between the intermediate target value O2midtarg (i) and the output O2out (i) of the downstream side exhaust gas sensor 25 increases. K2 × ΣΔO2 (i) is an integral term, and increases as the integrated value of the deviation ΔO2 (i) between the intermediate target value O2midtarg (i) and the output O2out (i) of the downstream exhaust gas sensor 25 increases. The correction amount AFcomp (i) is obtained by performing a guard process on a value obtained by adding the proportional term and the integral term in a predetermined control range (upper guard value and lower guard value).

In the present embodiment, the proportional / integral gains K1,
K2 and the control range (upper guard value and lower guard value) are switched by the control condition setting unit 47 between the return control when the catalyst 23 returns from the saturated adsorption state and the normal control, similarly to the above-described damping rate Kdec. Can be The proportional / integral gains K1 and K2 during the return control are set to values smaller than those in the normal control, and the width of the control range (upper guard value and lower guard value) during the return control is It is set to be narrower than that. The proportional / integral gains K1 and K2 and the control range (upper guard value and lower guard value) in each control mode may be fixed values for simplification of the calculation process. It may be set by a map or a mathematical expression according to the output characteristics of the downstream side exhaust gas sensor 25 and the like. The control condition setting unit 47 plays a role corresponding to a return control unit described in the claims.

The calculation of the correction amount AFcomp (i) by the target air-fuel ratio correction unit 44 described above is executed in accordance with the respective programs shown in FIGS. Hereinafter, the processing contents of each program will be described.

The sub-feedback control program of FIG. 5 is executed every predetermined time or every predetermined crank angle. When the program is started, first, in step 100, it is determined whether or not the air-fuel ratio of the air-fuel mixture supplied to the engine 11 is in a rich region (for example, λ <0.98). The program proceeds to 101, where a rich time counter Crich for counting the time of staying in the rich region is incremented by, for example, two.
To reset the rich time counter Crich to 0. The value of the rich time counter Crich is information for estimating the rich component adsorption amount (rich side saturated adsorption degree) of the catalyst 23. For example, during power increase, λ <0.
Since it is 98, the rich time counter Crich keeps counting up by two at a predetermined cycle.

Thereafter, the routine proceeds to step 103, where the air-fuel ratio of the air-fuel mixture supplied to the engine 11 is in a lean region (for example, λ>
1.02), and if it is a lean region,
Proceeding to step 104, the lean time counter Clean that counts the time during which the vehicle stays in the lean area is incremented by, for example, two. If it is not the lean area, the processing proceeds to step 105, where the lean time counter Clean is reset to zero. The value of the lean time counter Clean serves as information for estimating the lean component adsorption amount (lean side saturated adsorption degree) of the catalyst 23. For example, during execution of the fuel cut, since λ> 1.02, the lean time counter Clean continues to be incremented by 2 in a predetermined cycle.

Thereafter, the program proceeds to a step 106, wherein it is determined whether or not the sub-feedback control is being executed. If the sub-feedback control is not being executed, the present program is terminated. Proceeding to step 107, the sub-feedback condition setting program of FIG. 6 is executed, and the control conditions of the sub-feedback control are set as follows.

When the sub-feedback condition setting program shown in FIG. 6 is started, first, in step 111, whether the output of the downstream side exhaust gas sensor 25 is, for example, 0.75 V or more is determined based on whether or not the exhaust gas flowing out of the catalyst 23 is empty. It is determined whether or not the fuel ratio is rich beyond a predetermined value. If not, it is determined that the catalyst 23 is not in the rich side saturated adsorption state, and the routine proceeds to step 112, where the rich time counter Crich is set to 0. Reset and proceed to the next step 113. The output of the downstream exhaust gas sensor 25 is 0.75 V
In the following cases, the process proceeds to step 113.

In this step 113, it is determined whether or not the value of the rich time counter Crich is larger than 0. If the value of the rich time counter Crich is larger than 0, the saturated adsorption state on the rich side of the catalyst 23 is determined. Execute the return control for returning from. During this return control, at step 114, the rich time counter Crich is decremented by one, and the routine proceeds to step 115, where the attenuation rate Kdec is set to the attenuation rate Kdecrich at the time of the rich-side return control.

Further, in the next step 116, the proportional / integral gains K1 and K2 are set to the proportional / integral gains K1rich and K2rich for the return control on the rich side, and in the next step 117, the upper limit guard value and the lower limit guard are set. The upper guard value Kuprich and the lower guard value K at the time of return control on the rich side
Set to udrich. Thereafter, the routine proceeds to step 129, where a correction amount calculation program shown in FIG. 8 described later is executed to calculate a correction amount AFcomp (i) of the upstream target air-fuel ratio AFref.

On the other hand, if it is determined in step 113 that the value of the rich time counter Crich is 0, the process proceeds to step 119, in which whether the output of the downstream exhaust gas sensor 25 is, for example, 0.2 V or less is determined. It is determined whether or not the air-fuel ratio of the exhaust gas flowing out of the catalyst 23 is lean or more than a predetermined value. , Resets the lean time counter Clean to 0, and proceeds to the next step 121. In addition, the downstream side exhaust gas sensor 2
Also when the output of No. 5 is 0.2 V or more, the process proceeds to step 121.

In this step 121, it is determined whether or not the value of the lean time counter Clean is greater than 0. If the value of the lean time counter Clean is greater than 0, the saturated adsorption state on the lean side of the catalyst 23 is determined. Execute the return control for returning from. During this return control, at step 122, the lean time counter Clean is decremented by one, and the routine proceeds to step 123, where the attenuation rate Kdec is set to the attenuation rate Kdeclean at the time of the lean-side return control.

Further, in the next step 124, the proportional / integral gains K1 and K2 are set to the proportional / integral gains K1lean and K2lean for the return control on the lean side, and in the next step 125, the upper limit guard value and the lower limit guard The upper guard value Kuplean and the lower guard value K at the time of the lean-side return control
Set to udlean. Thereafter, the routine proceeds to step 129, where a correction amount calculation program shown in FIG. 8 described later is executed to calculate a correction amount AFcomp (i) of the upstream target air-fuel ratio AFref.

On the other hand, if it is determined in step 121 that the value of the lean time counter Clean is 0, the catalyst 23
Is not in the saturated adsorption state, and normal control is executed. During the normal control, the damping rate Kdec is set to the damping rate Kdecnomal in the normal control in step 126 of FIG.

Further, in the next step 127, the proportional / integral gains K1 and K2 are changed to the proportional / integral gains K1n in the normal control.
omal, K2nomal and the next step 128
Then, the upper and lower guard values are set to the upper and lower guard values Kupnomal and Kudnomal during normal control. Thereafter, the routine proceeds to step 129, where a correction amount calculation program of FIG. 8 described below is executed to calculate a correction amount AFcomp (i) of the upstream target air-fuel ratio AFref.

When the correction amount calculation program shown in FIG. 8 is started, first, in step 201, the current output O2out (i) of the downstream side exhaust gas sensor 25 is read, and the next step 20 is executed.
In step 2, the attenuation factor Kdec, the proportional / integral gains K1, K2, the upper limit guard value, and the lower limit guard value set by the above-described sub-feedback condition setting program in FIGS. 6 and 7 are read.

Thereafter, the routine proceeds to step 203, where the attenuation rate K
Using dec, the intermediate target value O2midtarg based on the output O2out (i-1) of the downstream exhaust gas sensor 25 at the previous calculation and the final target value O2targ (i) (final downstream target air-fuel ratio).
(i) is calculated using the above equation (1). Thereby, the output O2out (i-1) of the downstream exhaust gas sensor 25 at the time of the previous calculation is obtained.
And the final target value O2targ (i), the intermediate target value O2midtarg
(i) is set.

Thereafter, the routine proceeds to step 204, where the intermediate target value O2midtarg (i) and the output O2o of the downstream side exhaust gas sensor 25 are output.
The deviation ΔO2 (i) from ut (i) is calculated. ΔO2 (i) = O2midtarg (i)-O2out (i)

Then, in the next step 205, the integrated value ΣΔO2 (i-1) of the deviation ΔO2 up to the previous time is added to the current deviation ΔO2
(i) is integrated and the integrated value of the deviation ΔO2 up to this time ΣΔO2
Find (i). ΣΔO2 (i) = ΣΔO2 (i-1) + ΔO2 (i)

Thereafter, the routine proceeds to step 206, where a correction amount AFcomp (i) of the upstream target air-fuel ratio AFref is calculated by the following equation. AFcomp (i) = Fsat (K1 × ΔO2 (i) + K2 × ΣΔO
2 (i)) Thus, the correction amount AFco of the upstream target air-fuel ratio AFref
mp (i) has a proportional term (K1 × ΔO2 (i)) and an integral term (K2 × Σ
ΔO2 (i)) is obtained by performing guard processing on the value obtained by adding the upper guard value and the lower guard value.

Then, in the next step 207, the current Δ
O2 (i) and ΣΔO2 (i) are replaced by the previous ΔO2 (i-1) and ΣΔ
This is stored as O2 (i-1) and the program ends.

During the operation of the engine, the load target air-fuel ratio A according to the intake air amount (or the intake pipe pressure) and the engine speed.
By calculating the Fbase, and adding the correction amount AFcomp calculated by the correction amount calculation program of FIG. 8 to the load target air-fuel ratio AFbase, an upstream target air-fuel ratio AFref is obtained, and the detected air-fuel ratio AF of the upstream exhaust gas sensor 24 is calculated. The fuel injection time Tinj (fuel injection amount) is calculated so that the target value converges to the upstream target air-fuel ratio AFref.

An example of the execution of the main / sub feedback control of the present embodiment described above will be described with reference to the time charts of FIGS. FIG. 9 shows a control example after returning from the fuel cut. During the fuel cut, the sub-feedback execution condition is not satisfied, and the calculation of the damping rate Kdec, the proportional / integral gains K1, K2, the upper guard value, and the lower guard value is stopped. When the fuel cut is executed, the lean component adsorption amount of the catalyst 23 becomes saturated. Therefore, immediately after the fuel cut is restored, the return control for returning the catalyst 23 from the lean saturated adsorption state is executed. In the lean-side return control, the attenuation rate Kdec, the proportional / integral gains K1, K2, the upper guard value, and the lower guard value are set to the values at the time of the lean-side return control. Thus, the update amount of the intermediate target value is set to be smaller than that in the case of the normal control, and the correction amount AFcomp of the upstream target air-fuel ratio AFref by the sub feedback control is set to be smaller than that in the case of the normal control. During this return control, the attenuation rate Kdec, the proportional / integral gains K1, K2, the upper guard value and the lower guard value are changed according to the elapsed time of the return control (that is, the catalyst 23).
(Depending on the degree of recovery), and gradually approaches the value during normal control.

In this way, after returning from the fuel cut,
When the catalyst 23 returns from the lean side saturated adsorption state to the state where the amount of adsorption is small, even if the storage state of the catalyst 23 is unstable, the control condition of the sub-feedback control is limited to a range where stability can be ensured. As a result, stable sub-feedback control can be performed, and exhaust gas purification performance after returning from fuel cut can be ensured.

The execution time of the return control is set in accordance with the fuel cut execution time counted by the lean time counter Clean, and after the set time has elapsed, the return control is terminated and the control shifts to the normal control. Further, even before the set time has elapsed, the output of the downstream exhaust gas sensor 25 is, for example, 0.5.
When the voltage becomes 2 V or more, it is determined that the catalyst 23 has returned to the state in which the lean component adsorption amount is small, and the return control is ended to return to the normal control.

In this normal control, the damping rate Kdec is proportional to
The integral gains K1 and K2, the upper guard value and the lower guard value are each switched to the value at the time of the normal control to increase the update amount of the intermediate target value as compared with the case of the return control, and the upstream target air-fuel ratio by the sub feedback control. The correction amount AFcomp of AFref is made larger than in the case of the return control. As a result, during normal control, high-response sub-feedback control that follows the change in the dynamic characteristics of the catalyst 23 with good response is executed, and the exhaust gas purification performance is maximized.

FIG. 10 shows an example of control after returning from the power increase. During the power increase, the sub-feedback execution condition is not satisfied, and the calculation of the damping rate Kdec, the proportional / integral gains K1, K2, the upper guard value, and the lower guard value is stopped. When the power increase is performed, the catalyst 2
Since the rich component adsorption amount of No. 3 becomes saturated, the return control for returning the catalyst 23 from the rich side saturated adsorption state is executed immediately after the power increase return. In the return control on the rich side, the attenuation rate Kdec, the proportional / integral gain K1
, K2, the upper limit guard value and the lower limit guard value are set to the values at the time of the return control on the rich side. As a result, the update amount of the intermediate target value is made smaller than in the case of the normal control, and the upstream target air-fuel ratio A
The correction amount AFcomp of Fref is made smaller than in the case of the normal control.

By doing so, after returning from the power increase,
When the catalyst 23 returns from the saturated adsorption state on the rich side,
Even if the storage state of the catalyst 23 is unstable, the control condition of the sub-feedback control can be limited to a range where stability can be ensured, and stable sub-feedback control can be performed. Purification performance can be ensured.

The execution time of the return control is set in accordance with the power increase execution time counted by the rich time counter Crich. After the elapse of the set time, the return control ends and the control shifts to the normal control. Even before the set time has elapsed, the output of the downstream side exhaust gas sensor 25 is, for example, 0.75
At the time point when the voltage becomes V or less, it is determined that the catalyst 23 has returned to the state in which the rich component adsorption amount is small, and the return control is ended to return to the normal control.

In the above-described embodiment, the control conditions for the return control on the lean side and the rich side are set to different conditions in accordance with the characteristics of the catalyst 23, the output characteristics of the downstream exhaust gas sensor 25, and the like. In order to simplify the arithmetic processing, the control conditions for the return control on the lean side and the rich side may be set to the same condition.

In the above embodiment, the damping rate Kdec, the proportional / integral gains K1, K2, and the control range (upper guard value and lower guard value) are all changed to the values at the time of the return control at the time of the return control. Alternatively, only some of these may be changed.

In the above embodiment, the update amount of the intermediate target value O2midtarg (i) is changed by changing the attenuation rate Kdec between the return control and the normal control. The update amount of the intermediate target value O2midtarg (i) may be changed. Alternatively, the update cycle (update speed) of the intermediate target value O2midtarg (i) between the return control and the normal control
May be changed.

The intermediate target value O2midtarg (i) is calculated by a two-dimensional map using the output O2out (i-1) of the downstream exhaust gas sensor 25 and the final target value O2targ (i) at the time of the previous calculation as parameters. You may do it. In this case, the intermediate target value calculation map for the return control and the intermediate target value calculation map for the normal control may be set by experiments or simulations.

The control cycle of the sub-feedback control (calculation cycle of the correction amount AFcomp (i)) may be changed between the return control and the normal control. Further, in the above-described embodiment, whether or not the catalyst 23 is in the saturated adsorption state is determined based on whether or not the output voltage of the downstream side exhaust gas sensor 25 has become, for example, 0.75 V or more or 0.2 V or less. Alternatively, it may be determined that the catalyst 23 is in the saturated adsorption state when executing the fuel cut. Alternatively, it may be determined whether the catalyst 23 has entered the saturated adsorption state by determining whether the output voltage of the downstream side exhaust gas sensor 25 is equal to or higher than a predetermined rich voltage or equal to or lower than a predetermined lean voltage for a predetermined time or longer. You may do it.

In the above embodiment, the intermediate target value O2m
When calculating idtarg (i), the output O2out (i-1) of the downstream exhaust gas sensor 25 at the time of the previous calculation was used, but the output O2out (in) of the downstream exhaust gas sensor 25 before the predetermined number of calculations was used. Is also good.

The downstream exhaust gas sensor 25 may use an air-fuel ratio sensor (linear A / F sensor) instead of the oxygen sensor. The upstream exhaust gas sensor 24 may use an air-fuel ratio sensor (linear A / F sensor). An oxygen sensor may be used instead of the (F sensor).

In addition, according to the present invention, the intermediate target value O2midtarg
It goes without saying that the present invention can be implemented with various changes, for example, the calculation formula of (i) and the calculation formula of the correction amount AFcomp (i) may be appropriately changed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an entire engine control system showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a function of an air-fuel ratio control unit realized by an arithmetic processing function of a CPU of an ECU;

FIG. 3 is a functional block diagram showing functions of the entire air-fuel ratio feedback control system.

FIG. 4 is a diagram illustrating a saturation function for calculating a correction amount AFcomp (i).

FIG. 5 is a flowchart showing a flow of processing of a sub-feedback control program;

FIG. 6 is a flowchart (part 1) illustrating a processing flow of a sub-feedback condition setting program;

FIG. 7 is a flowchart showing a processing flow of a sub-feedback condition setting program (part 2);

FIG. 8 is a flowchart showing the flow of processing of a correction amount calculation program.

FIG. 9 is a time chart showing a control example after returning from fuel cut;

FIG. 10 is a time chart showing a control example after returning from a power increase.

[Explanation of symbols]

11 ... engine (internal combustion engine), 20 ... fuel injection valve, 22
... exhaust pipe, 23 ... catalyst, 24 ... upstream exhaust gas sensor, 2
5 downstream exhaust gas sensor, 28 ECU (air-fuel ratio feedback control means, sub feedback control means, intermediate target value setting means), 31 CPU, 40 air-fuel ratio control means, 41 fuel injection amount feedback control unit (air Fuel ratio feedback control means), 42 ... target air-fuel ratio calculation unit (sub feedback control means), 43 ... load target air-fuel ratio calculation unit, 44 ... target air-fuel ratio correction unit, 45 ... time delay element (1 / z), 46 ... Intermediate target value calculation section (intermediate target value setting means), 47 ... control condition setting section (return control means), 47
... A correction amount calculation unit.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 45/00368 F02D 45/00 368G (72) Inventor Koichi Shimizu 1-1-1, Showa-cho, Kariya-shi, Aichi Prefecture In Denso Co., Ltd. (72) Inventor Hisashi Iida 1-1-1 Showa-cho, Kariya-shi, Aichi F-term in Denso Co., Ltd. (Reference) 3G084 BA09 BA13 DA10 DA12 EA07 EB09 EB13 EB14 EB15 EB18 EB20 FA20 FA30 FA33 3G091 AA02 AA17 AA28 AB03 BA01 BA14 BA15 BA19 CB02 DA01 DA02 DA03 DA05 DA10 DB04 DB05 DB06 DB07 DB08 DB10 DC01 DC03 EA01 EA05 EA30 FA05 FA07 FA11 FB10 FB11 FB12 HA36 HA37 HA42 3G301 HA01 JA21 KA26 MA01 MA12 MA24 NA03 NE04 NE02 ND05 PD17 PE01Z PE08Z

Claims (6)

[Claims] 1. An upstream exhaust gas sensor and a downstream exhaust gas sensor for detecting an air-fuel ratio or rich / lean of exhaust gas on an upstream side and a downstream side of an exhaust gas purifying catalyst, respectively. Air-fuel ratio feedback control means for feedback-controlling the fuel injection amount so as to attain the upstream target air-fuel ratio; and an intermediate target value based on the past detected air-fuel ratio of the downstream exhaust gas sensor and the final downstream target air-fuel ratio. And a sub-feedback control means for performing sub-feedback control for correcting the upstream target air-fuel ratio based on the detected air-fuel ratio of the downstream exhaust gas sensor and the intermediate target value. In the air-fuel ratio control device for an internal combustion engine, when the catalyst returns from the saturated adsorption state, the update amount, the update speed, and the Control gain feedback controller, the control cycle, the air-fuel ratio control apparatus for an internal combustion engine, characterized in that a return control for changing at least one and a recovery control means for executing a predetermined period of the control range. 2. An intermediate target value setting means, comprising: a value obtained by multiplying a deviation between a past detected air-fuel ratio of the downstream exhaust gas sensor and a final downstream target air-fuel ratio by an attenuation rate; 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the intermediate target value is obtained by adding a target air-fuel ratio, and the return control means changes the attenuation rate during the return control. 3. 3. The sub-feedback control means limits a value calculated by a proportional integral operation to a deviation between an air-fuel ratio detected by the downstream exhaust gas sensor and the intermediate target value to a predetermined control range to thereby control the upstream-side exhaust gas sensor. The internal combustion engine according to claim 1 or 2, wherein a correction amount of the side target air-fuel ratio is obtained, and the return control means changes a gain of the proportional integration operation and / or the control range during the return control. Air-fuel ratio control device. 4. The return control means sets a period during which the return control is performed in accordance with a time during which a rich or lean state in which the catalyst enters a saturated adsorption state continues. An air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 3. 5. The method according to claim 1, wherein said return control means determines a timing of terminating said return control and returning to normal control based on a detected air-fuel ratio of said downstream side exhaust gas sensor. An air-fuel ratio control device for an internal combustion engine according to any one of the above. 6. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the return control means determines that the catalyst has reached a saturated adsorption state when executing a fuel cut.