JP2000213396A - Air-fuel ratio controller of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To continue air-fuel ratio control without deteriorating drivability and emission and without causing torque shock when the target air-fuel ratio is drastically changed or just after open loop control is shifted to feedback control. SOLUTION: A CPU 31 in an ECU 30 sets the target air-fuel ratio, corrects the set target air-fuel ratio according to the lean or rich degree, and feedback- controls the actual air-fuel ratio of exhaust gas discharged from an engine 1 to the target air-fuel ratio. When it is judged that the target air-fuel ratio is largely changed in shifting from theoretical air-fuel ratio feedback control to lean feedback control, the CPU 31 temporarily interrupts feedback control. When the deviation between the actual air-fuel ratio and the target air-fuel ratio is not more than a specified value, the feedback control is restarted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art As this kind of prior art, Japanese Patent Publication No.
In “Air-fuel ratio control method of internal combustion engine” of JP 8080,
When the control mode is switched from the open loop control to the feedback control, and when the control is switched between the lean control and the stoichiometric air-fuel ratio control, the feedback control is started after a predetermined time has elapsed after changing the fuel injection amount. I have to. In this case, the feedback control is started after the exhaust gas from the fuel for feedback control reaches the exhaust system, and the air-fuel ratio is overcorrected due to the detection of the exhaust gas from the fuel for open loop control. Will be resolved.

Further, when the control mode is switched between the feedback control of the stoichiometric air-fuel ratio control and the feedback control of the lean control, the control is switched after a predetermined time elapses after changing the fuel injection amount. I have. In this case, for example, the lean control is started after the exhaust gas from the fuel for the lean control reaches the exhaust system, or the stoichiometric air-fuel ratio control is started after the exhaust gas from the fuel for the stoichiometric air-fuel ratio control reaches the exhaust system. Once started, the problem that the air-fuel ratio is overcorrected when switching between lean control and stoichiometric air-fuel ratio control during feedback control is eliminated.

However, according to the above publication, if the actual air-fuel ratio greatly deviates from the target air-fuel ratio before the feedback control is started after the control mode is switched, for example, when switching from the stoichiometric air-fuel ratio control to the lean control, In this case, when the actual air-fuel ratio changes to the lean side from the target air-fuel ratio, drivability deteriorates. Also, when the actual air-fuel ratio changes to a richer side than the target air-fuel ratio when switching from the lean control to the stoichiometric air-fuel ratio control,
Causes poor emission. Further, as described above, when the deviation between the actual air-fuel ratio and the target air-fuel ratio occurs and the large deviation remains even at the start of feedback, the correction amount of the air-fuel ratio at the start of feedback increases, causing a sudden change in the air-fuel ratio. Torque shock occurs.

In addition, when the air-fuel ratio control shifts from the open loop control to the feedback control, for example, at the time of starting the engine, the air-fuel ratio sharply changes due to the start of the feedback control. appear. Therefore, there is a need for improvement.

A more specific description will be given with reference to a time chart of FIG. Here, when the air-fuel ratio feedback control is performed, the feedback correction value FAF set according to the deviation between the actual air-fuel ratio and the target air-fuel ratio at that time.
Also, the fuel injection amount TAU is determined based on a lean correction value FLEAN set according to the lean degree of the target air-fuel ratio (TAU = basic injection amount / FAF.
FLEAN).

At time t11 in FIG. 14, the control mode is switched from the stoichiometric air-fuel ratio feedback control to the lean air-fuel ratio feedback control, and the lean correction value FLEAN (<1.0) is set according to the target air-fuel ratio at that time. )
Is set. At this time t11, the lean feedback control using the target air-fuel ratio as the lean control value has not been started yet, and the time t12 at which the exhaust gas due to the injected fuel reaches the exhaust system at the time t1 (for example, at the time when 720 ° CA elapses from the time t1) ), Lean feedback control is started.

However, in FIG. 14, if the fuel injection amount is overcorrected by the lean correction value FLEAN,
As shown in the figure, the actual air-fuel ratio greatly deviates from the target air-fuel ratio, causing problems such as deterioration of drivability. When lean feedback control is started at time t12,
Since the feedback correction value FAF is large, torque shock is caused.

On the other hand, in the "air-fuel ratio control device for a lean burn engine" disclosed in Japanese Patent No. 2645550, a first-order lag time constant that approximates a response delay of an air-fuel ratio feedback system including a lean air-fuel ratio sensor is set, and a target air-fuel ratio is set. Is changed, a value obtained by performing a weighted averaging process on the target value with the first-order lag time constant is newly set as a correction target value. However, the apparatus disclosed in the above publication has a disadvantage that the calculation load increases when performing the first-order delay processing. In addition, the disclosure only discloses a method of setting a correction target value when the target air-fuel ratio changes during feedback control, and does not disclose a control method when shifting from open loop control to feedback control. Therefore, when shifting to the feedback control, a torque shock occurs due to a sudden change in the air-fuel ratio.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has an object to be performed when the target air-fuel ratio has changed significantly or when open-loop control has been shifted to feedback control. It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine that can continue air-fuel ratio control without causing problems such as deterioration of drivability, emission and torque shock immediately thereafter.

[0011]

According to the first aspect of the present invention, it is determined that the target air-fuel ratio has changed significantly (determining means), and when it is determined that the target air-fuel ratio has changed significantly,
The feedback control is temporarily interrupted, and the feedback control is restarted when the deviation between the actual air-fuel ratio and the target air-fuel ratio becomes equal to or less than a predetermined value (control changing means).

According to the first aspect of the present invention, when the target air-fuel ratio changes significantly, correction is performed according to the degree of lean or rich of the target air-fuel ratio, and the actual air-fuel ratio moves according to the correction amount. However, at this time, the feedback control is temporarily interrupted, and the feedback control is restarted when the deviation between the actual air-fuel ratio and the target air-fuel ratio becomes equal to or smaller than a predetermined value. By interrupting the feedback control, hunting of the control in the transition period is prevented. In addition, the feedback control is restarted on condition that the deviation between the actual air-fuel ratio and the target air-fuel ratio is equal to or less than a predetermined value, thereby solving the problem that the air-fuel ratio is overcorrected when the target air-fuel ratio changes. . as a result,
Drivability even when the target air-fuel ratio changes significantly,
The air-fuel ratio control can be continued without deterioration of the emission or torque shock.

According to the second aspect of the present invention, the control change means temporarily suspends the feedback control, and thereafter, when the deviation between the actual air-fuel ratio and the target air-fuel ratio becomes a predetermined value or less, or when the feedback control is performed. The feedback control is restarted at an earlier timing when a predetermined time has elapsed after the interruption of the control.

In this case, even if the timing at which the deviation between the actual air-fuel ratio and the target air-fuel ratio falls below a predetermined value is unexpectedly prolonged, the feedback control is restarted when a predetermined time has elapsed since the suspension of the feedback control. . Therefore, the inconvenience that the restart of the feedback control is delayed unintended can be avoided.

Here, the amount of change in the target air-fuel ratio varies from time to time, and the greater the amount of change, the longer the time required for the actual air-fuel ratio to approach the target air-fuel ratio. Therefore, as described in claim 3, it is desirable to set the predetermined time of the control changing means according to the change amount of the target air-fuel ratio.

The present invention provides an air-fuel ratio control device for selectively performing stoichiometric air-fuel ratio feedback control using a stoichiometric air-fuel ratio as a target air-fuel ratio and lean feedback control using a predetermined lean value as a target air-fuel ratio. The present invention is applied to an air-fuel ratio control device that selectively performs lean feedback control for setting the lean value of the target air-fuel ratio as a target air-fuel ratio and rich purge control for releasing stored NOx of a lean NOx catalyst provided in an engine exhaust system. In the invention according to the fourth aspect, the determination unit is configured to execute
When the control is switched from one of the two feedback controls to the other, it is determined that the target air-fuel ratio has significantly changed. In the invention described in claim 5, the determination means determines that the target air-fuel ratio has significantly changed when the execution / non-execution of the rich purge is switched.

Even when the control mode is switched as in claims 4 and 5, according to the configuration of the invention described above, drivability, emission deterioration and torque shock when the target air-fuel ratio changes can be suppressed.

On the other hand, according to the invention of claim 6, when the control mode is switched at the beginning, for example, when shifting from the open loop control to the feedback control, or when the target air-fuel ratio of the feedback control is largely changed, etc. The target air-fuel ratio is changed from the actual air-fuel ratio to gradually approach the target air-fuel ratio suitable for the control mode, and when the deviation between the changed value and the target air-fuel ratio suitable for the control mode becomes equal to or less than a predetermined value. Thereafter, the target air-fuel ratio is changed to a value suitable for the control mode (mode switching control means).

According to the sixth aspect of the present invention, when the control mode is switched, the control with the new target air-fuel ratio is not immediately started regardless of the deviation between the actual air-fuel ratio and the target air-fuel ratio. , The target air-fuel ratio is gradually changed.
Therefore, the conventional problem that a torque shock occurs due to a sudden change in the air-fuel ratio when switching the control mode is solved. Further, since the feedback control is switched on condition that the deviation of the air-fuel ratio is equal to or less than a predetermined value, the control mode can be switched smoothly without causing a problem such as torque shock.

In order to realize the sixth aspect of the present invention, in the seventh aspect of the present invention, the first target setting means sets a target air-fuel ratio corresponding to a control mode at each time as a first target value. Further, the second target setting means of the mode switching control means initializes the actual air-fuel ratio as the second target value and gradually changes the second target value so as to approach the first target value. The control switching means starts a new feedback control using the second target value as a target air-fuel ratio, and when the deviation between the first and second target values becomes equal to or less than a predetermined value, sets the target air-fuel ratio of the feedback control to the first value. The second target value is switched to the first target value.

According to this configuration, when the control mode is switched, at the beginning, the target air-fuel ratio consisting of the second target value is gradually changed from the actual air-fuel ratio, and the second target value is changed by the second target value. When the target value is sufficiently approached, the target air-fuel ratio is switched from the second target value to the first target value at that time. In such a case, the conventional problem that a torque shock occurs due to a sudden change in the air-fuel ratio when the control mode is switched is solved.

In the invention described in claim 8, the mode switching control means is provided when the deviation between the first and second target values becomes equal to or less than a predetermined value, or when a predetermined time has elapsed since the switching of the control mode. The target air-fuel ratio of the feedback control is switched from the second target value to the first target value at an earlier timing among the time points.

In this case, even if the timing at which the deviation between the first and second target values falls below the predetermined value is unexpectedly prolonged,
The target air-fuel ratio is changed when a predetermined time has elapsed from the switching of the control mode (second target value → first target value). Therefore, it is possible to avoid a disadvantage that the start of the original feedback control based on the first target value is delayed contrary to the intention.

Here, the deviation between the first and second target values immediately after the switching of the control mode differs from time to time, and the greater the deviation, the longer the time required for the deviation to converge. Therefore, as described in claim 9, it is desirable to set the predetermined time of the mode switching control means in accordance with the deviation between the first and second target values immediately after the switching of the control mode.

According to the tenth aspect of the present invention, when the control mode is switched, the initial target air-fuel ratio approaches the target air-fuel ratio corresponding to the control mode from the rich side or the lean side. The threshold value for determining the deviation of the target air-fuel ratio is set variably.

The fact that the threshold value for judging the deviation of the target air-fuel ratio is large means that the feedback control is started relatively early after the switching of the control mode, and conversely, that the threshold value is small. This means that the feedback control is started relatively late after the switching of the control mode. In this case, it is preferable to set the threshold value in accordance with the optimal time to start the feedback control.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) A first embodiment of the present invention will be described below with reference to the drawings. In the air-fuel ratio control system according to the present embodiment,
The target air-fuel ratio of the feedback control is set to the stoichiometric air-fuel ratio or leaner than the stoichiometric air-fuel ratio, and the combustion of the vehicle internal combustion engine is controlled based on the target air-fuel ratio. As a main configuration of the system, an air-fuel ratio sensor (A
/ F sensor or O2 sensor), and an electronic control unit (hereinafter, referred to as an ECU) mainly composed of a microcomputer takes in the detection result of the air-fuel ratio sensor, and performs the air-fuel ratio feedback control based on the detection result. . Hereinafter, the detailed configuration will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an air-fuel ratio control system according to the present embodiment. As shown in FIG.
The internal combustion engine is configured as a four-cylinder four-cycle spark ignition engine (hereinafter, referred to as engine) 1. The intake air is supplied from upstream to an air cleaner 2, an intake pipe 3, a throttle valve 4, a surge tank 5, and an intake manifold 6.
And is mixed with fuel injected from the fuel injection valve 7 for each cylinder in the intake manifold 6. Then, the mixture is supplied to each cylinder as a mixture having a predetermined air-fuel ratio.

A high voltage supplied from an ignition circuit 9 is distributed and supplied to a spark plug 8 provided in each cylinder of the engine 1 through a distributor 10, and the ignition plug 8 controls the mixture of each cylinder at a predetermined timing. To ignite. Exhaust gas discharged from each cylinder after combustion passes through an exhaust manifold 11 and an exhaust pipe 12 and passes through a lean NOx catalyst (hereinafter simply referred to as a NOx catalyst) 13 provided in the exhaust pipe 12 and composed of a so-called NOx storage reduction catalyst. After passing, it is released to the atmosphere. The NOx catalyst 13 stores NOx in exhaust gas during combustion at a lean air-fuel ratio, and reduces and releases the stored NOx as rich components (CO, HC, etc.) during combustion at a rich air-fuel ratio.

The intake pipe 3 is provided with an intake air temperature sensor 21 and an intake air pressure sensor 22. The intake air temperature sensor 21 detects the temperature of the intake air (intake air temperature Tam), and the intake pressure sensor 22 detects the intake air downstream of the throttle valve 4. The pipe negative pressure (intake pressure PM) is detected. A throttle sensor 23 for detecting the opening degree (throttle opening degree TH) of the throttle valve 4
The throttle sensor 23 outputs an analog signal corresponding to the throttle opening TH. The throttle sensor 23 also has a built-in idle switch, and outputs a detection signal indicating that the throttle valve 4 is almost fully closed.

A water temperature sensor 24 is provided on a cylinder block of the engine 1.
The temperature of the cooling water circulating in the inside (cooling water temperature Thw) is detected. The distributor 10 is provided with a rotation speed sensor 25 for detecting the rotation speed of the engine 1 (engine rotation speed Ne). The rotation speed sensor 25 is provided at equal intervals every two rotations of the engine 1, that is, every 720 ° CA. Output pulse signals.

Further, in the exhaust pipe 12, the NOx catalyst 1
An A / F sensor 26 of a limiting current type is disposed on the upstream side of the A / F 3, and the A / F sensor 26 is proportional to the oxygen concentration of the exhaust gas discharged from the engine 1 (or the CO concentration in the unburned gas). And outputs a wide range and linear air-fuel ratio signal. An O2 sensor 27 is provided downstream of the NOx catalyst 13, and the sensor 27 outputs a different electromotive force signal depending on whether the exhaust gas has a rich or lean air-fuel ratio.

On the other hand, the ECU 30 comprises a CPU 31, a ROM
32, a RAM 33, a backup RAM 34, etc., which are configured as a logical operation circuit, and connected via a bus 37 to an input port 35 for inputting a detection signal of each sensor and an output port 36 for outputting a control signal to each actuator. Have been. The ECU 30 inputs detection signals (the intake temperature Tam, the intake pressure PM, the throttle opening TH, the cooling water temperature Thw, the engine speed Ne, the air-fuel ratio signal, etc.) of the various sensors through the input port 35. And
Based on these values, the fuel injection amount TAU and the ignition timing I
The control signals such as g are calculated, and the control signals are output to the fuel injection valve 7 and the ignition circuit 9 via the output port 36, respectively.

Next, the operation of the air-fuel ratio control system configured as described above will be described. FIG. 2 is a flowchart showing a fuel injection control process, which is executed by the CPU 31 for each fuel injection of each cylinder (in this embodiment, for every 180 ° CA). In the present embodiment, basically: open-loop control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio regardless of the actual air-fuel ratio λr detected by the A / F sensor 26; and setting the stoichiometric air-fuel ratio to the target air-fuel ratio λTG. Stoichiometric air-fuel ratio feedback control (hereinafter, referred to as stoichiometric air-fuel ratio F / B control) for controlling the actual air-fuel ratio λr to the target air-fuel ratio λTG;-setting the target air-fuel ratio λTG in the lean air-fuel ratio range, Each control mode such as lean feedback control (hereinafter referred to as lean F / B control) for controlling to the target air-fuel ratio λTG is selectively executed according to the engine operating state.

In the lean F / B control, a so-called rich purge is performed in which the rich combustion is temporarily performed during the lean combustion to release the NOx stored in the NOx catalyst 13. For example, when the NOx adsorption amount of the NOx catalyst 13 exceeds a predetermined level, or when a predetermined time (predetermined number of times of lean combustion) has elapsed since the last rich purge, a predetermined rich purge condition is satisfied, and only for an extremely short time. Rich combustion is performed. As a result, the lean combustion and the rich combustion are alternately performed at a predetermined cycle.

In FIG. 2, first, at step 101,
Reads sensor detection results (engine speed Ne, intake pressure PM, cooling water temperature Thw, etc.) representing the engine operating state,
In the following step 102, a basic injection amount Tp according to the engine speed Ne and the intake pressure PM at each time is calculated using a basic injection amount map stored in the ROM 32 in advance.

In step 103, it is determined whether a known air-fuel ratio F / B condition is satisfied. here,
The air-fuel ratio F / B conditions include that the cooling water temperature Thw is equal to or higher than a predetermined temperature, that the cooling water temperature is not high and that the load is not high, that the A / F sensor 26 is active, and the like. Step 1
If 03 is NO (if the F / B condition is not satisfied), open loop control is performed, and the routine proceeds to step 104, where the lean correction value FLEAN and the rich correction value FRIC are set.
H and the feedback correction value FAF are both "1.0"
And

If step 103 is YES (if the F / B condition is satisfied), the stoichiometric air-fuel ratio F / B control or lean F / B control is executed, and step 200 is executed.
And the setting process of the target air-fuel ratio λTG is performed. In step 300, a lean correction value FLEAN and a rich correction value FRICH are set. Next step 40
At 0, a feedback correction value FAF is set.

By the way, FLEAN and FRICH are respectively
The target air-fuel ratio λTG is set in accordance with the deviation amount (the degree of lean or rich) from the stoichiometric air-fuel ratio, and basically, the larger the deviation amount, the larger the value. The FAF is set in accordance with the deviation between the actual air-fuel ratio λr (measured value of the A / F sensor 26) and the target air-fuel ratio λTG at that time. However, the details of each of the processes in steps 200, 300, and 400 will be described later.

After setting the values of FLEAN, FRICH, and FAF, in step 105, final values are set based on the basic injection amount Tp, various correction coefficients FALL (various correction coefficients for water temperature, air conditioner load, etc.) and correction values FAF, FLEAN, and FRICH. Is calculated.

TAU = Tp / FALL / FAF / FLE
After calculating the fuel injection amount TAU, a control signal corresponding to the TAU value is output to the fuel injection valve 7, and the process is temporarily terminated.

Next, the air-fuel ratio F / F will be described with reference to the flowcharts of FIGS.
A procedure for setting the target air-fuel ratio during the B control will be described. In the processing shown in FIGS. 3 and 4, the "target air-fuel ratio [lambda] TG" is set, and in addition, the F / B start counter C is set.
1 and the target air-fuel ratio λTGS used only immediately after the operation of the target air-fuel ratio change counter C2 or the start of the F / B control.
T ”is set. Here, the F / B start counter C1
Is operated only immediately after switching from the open loop control to the air-fuel ratio F / B control, and it is determined based on the counter value that the F / B has just been started. Further, the target air-fuel ratio change counter C2 is used to set the target air-fuel ratio λ when switching from the stoichiometric air-fuel ratio F / B control to the lean F / B control, for example.
The operation is performed only when the TG has been largely changed, and it is determined based on the count value that the TG has been changed. However, the initial values of both the counters C1 and C2 are “0”.

First, at step 201 in FIG. 3, a target air-fuel ratio λTG is set according to the engine operating state (eg, engine speed Ne, intake pressure PM) at that time using a setting map (not shown). Here, the target air-fuel ratio λTG is set so as to correspond to the control mode at that time. For example, if the engine operating state is in the execution range of the stoichiometric air-fuel ratio F / B control, A / F = 14.7 as the λTG value. The value is λ if the engine operating state is in the region where the lean F / B control is performed.
A value corresponding to A / F = 20 to 23 is set as the TG value.

Thereafter, in step 202, it is determined whether or not the air-fuel ratio F / B condition was satisfied at the time of the previous processing. If it is immediately after switching from the open-loop control to the air-fuel ratio F / B control (immediately after the start of F / B), step 202 is NO and the process proceeds to step 240. In step 240, the target air-fuel ratio λT immediately after the start of F / B
Set GST. In addition, even when the result of step 202 is YES, if the F / B start counter C1 is not “0” in the subsequent step 203, the process proceeds to step 240. F /
The B start counter C1 is operated in the processing of step 240 described later, and the fact that C1> 0 is satisfied in step 240
Indicates that the process is performed with priority.

Here, the process of step 240 will be described with reference to FIG. In FIG. 5, in step 241, F /
It is determined whether or not the B start counter C1 is "0".
Since C1 = 0 (initial value) at the beginning, step 242 is executed.
To set a predetermined value KC1 in the counter C1. The predetermined value KC1 is set, for example, according to the relationship in FIG. In FIG. 8, a predetermined value KC1 is set according to a deviation between the present actual air-fuel ratio λr detected by the A / F sensor 26 and the target air-fuel ratio λTG set in the step 201,
The larger the deviation of the air-fuel ratio (| λr−λTG |), the greater the KC1
Is set to a large value. However, the predetermined value KC1 may be a predetermined fixed value.

Thereafter, in step 243, the current actual air-fuel ratio λr is set as the target air-fuel ratio λTGST immediately after the start of the F / B, and then this processing ends. That is, the actual air-fuel ratio λr becomes the initial value of the target air-fuel ratio λTGST.

When C1 = KC1 is set, thereafter,
Step 241 is NO, and the process proceeds to step 244 to decrement the F / B start counter C1 by "1". In step 245, the current λTGST, λ
The TGs are compared to determine whether or not λTGST <λTG, that is, whether or not λTGST is richer than λTG.

If step 245 is YES (λTGS
If T <λTG), the routine proceeds to step 246, where a predetermined value Δλ1 is added to the current target air-fuel ratio λTGST. If step 245 is NO (λTGST ≧ λTG
), The routine proceeds to step 247, where a predetermined value Δλ2 is subtracted from the current target air-fuel ratio λTGST. Step 24
According to the processing of 6,247, the target air-fuel ratio λTGST immediately after the start of F / B is gradually increased or decreased so as to approach the target air-fuel ratio λTG.

Here, it is desirable that the predetermined values Δλ1 and Δλ2 for changing the target air-fuel ratio λTGST satisfy Δλ1 <Δλ2. Thereby, when λTGST approaches λTG from the lean side (for example, F / B from fuel cut)
At the start), the speed of change becomes larger than when approaching from the rich side (for example, at the time of starting F / B from the increased start).

Thereafter, in step 248, λTGS
It is determined whether or not the absolute value of the deviation between T and λTG is equal to or smaller than a predetermined value KAF1 (for example, 0.3). That is, it is determined whether or not | λTGST−λTG | ≦ KAF1 is satisfied.

Then, | λTGST−λTG |> KAF
If it is 1, this processing is terminated as it is, and | λTGST-
If .lambda.TG | .ltoreq.KAF1, the routine proceeds to step 249, where the F / B start counter C1 is cleared to "0", and then the present processing is terminated. With the processing of C1 = 0, a series of processing of FIG. 5 is terminated. That is, in the next processing, step 203 in FIG. 3 becomes YES.

In step 248, the predetermined value KAF1 may be changed depending on whether the target air-fuel ratio λTGST immediately after the start of F / B approaches the target air-fuel ratio λTG from the lean side or approaches the rich side. For example, when the target air-fuel ratio λTGST approaches from the lean side, the predetermined value KAF1 is made larger than when approaching from the rich side. The fact that the predetermined value KAF1 is large means that λTGST, λT
Even if the deviation of G is relatively large, it means that the affirmative determination is immediately made in step 248.

On the other hand, in FIG.
If both are YES, that is, if step 240 (the process of FIG. 5) is performed immediately after the start of the F / B control and the air-fuel ratio F / B control is continued thereafter, step 20
Proceeding to 4, the target air-fuel ratio change counter C2 is decremented by "1". However, the lower limit of the target air-fuel ratio change counter C2 is guarded by the initial value (0), and is maintained at "0" before a predetermined value KC2 described later is set.

Thereafter, at step 205, the lean F / B
It is determined whether or not the condition is satisfied. And step 2
If 05 is YES, it is determined whether the lean F / B condition has been satisfied last time (step 206), and step 2 is performed.
If 05 is NO, it is determined whether or not the lean F / B condition was not satisfied last time (step 207). Note that the lean F / B condition is a condition that is more limited than the air-fuel ratio F / B condition in step 202. Basically, the lean F / B control is performed if the lean F / B condition is satisfied, and the stoichiometric air-fuel ratio F / B control is performed if the lean F / B condition is not satisfied.

If the lean F / B condition has changed from "not satisfied to established" in the previous and current times (YES in step 205 and NO in step 206), or the lean F / B condition has been "established in the previous and current times" (“NO” in steps 205 and 207), step 20
8 and the target air-fuel ratio change counter C2 stores the predetermined value KC2
Set. The predetermined value KC2 is set, for example, according to the relationship in FIG. In FIG. 9, the predetermined value KC2 is set according to the deviation between the current actual air-fuel ratio λr and the target air-fuel ratio λTG,
The larger the deviation of the air-fuel ratio (| λr-λTG |), the greater the KC2
Is set to a large value. However, the predetermined value KC2 may be a predetermined fixed value.

When the lean F / B condition is satisfied the previous time and the present time (steps 205 and 206 are both YE
S), or when the lean F / B condition is not satisfied in the previous and current times (NO in step 205, Y in step 207)
ES), step 208 is skipped, and the process proceeds directly to step 209.

Thereafter, in step 209 of FIG. 4, it is determined whether or not the rich purge condition is satisfied. Here, for example, when the NOx adsorption amount of the NOx catalyst 13 exceeds a predetermined level, or when a predetermined time (predetermined number of times of combustion) has elapsed since the last rich purge, the rich purge condition is satisfied, and the rich air-fuel ratio is determined. F / B control is performed.

Only when the rich purge condition is satisfied, in step 210, a rich control value (for example, a value corresponding to A / F = 13) is set as the target air-fuel ratio λTG. Thereafter, in step 211, it is determined whether or not the rich purge condition was satisfied last time. Step 209 is N
In the case of O, in step 212, it is determined whether or not the rich purge condition was not satisfied last time.

If the rich purge condition has changed from "not satisfied to established" in the previous and current times (YES in step 209, NO in step 211), or the rich purge condition has changed from "established to unsatisfied" in the previous and current times. (Steps 209 and 212 are both NO), step 21
3 and the target air-fuel ratio change counter C2 stores a predetermined value KC2
Set. The predetermined value KC2 is set according to, for example, the relationship in FIG.

When the rich purge condition is satisfied last time and this time (steps 209 and 211 are both YE
S) or when the rich purge condition is not satisfied in the previous and current times (NO in step 209, Y in step 212)
ES), the step 213 is skipped, and the process proceeds to the subsequent step 214 as it is.

At step 214, it is determined whether or not the target air-fuel ratio change counter C2 is larger than "0".
If it is 0, this processing is terminated as it is. When the target air-fuel ratio change counter C2 is held at the initial value (= 0) or when the value is decremented to C2 = 0 in step 204, the determination in step 214 is negative and the process ends.

If C2> 0, step 215
Then, it is determined whether or not the absolute value of the deviation between the current actual air-fuel ratio λr and the target air-fuel ratio λTG is less than a predetermined value KAF2 (for example, 0.3). That is, it is determined whether or not | λr−λTG | ≦ KAF2 is satisfied. And | λr-λT
If G |> KAF2, the present process is terminated as it is.
If | λr−λTG | ≦ KAF2, the target air-fuel ratio change counter C2 is cleared to “0” in step 216, and then the present process ends.

Next, the procedure for setting the lean correction value FLEAN and the rich correction value FRICH will be described with reference to the flowchart of FIG. 6 showing the processing of step 300 of FIG. 2 in detail.

In FIG. 6, in step 301, it is determined whether or not the lean F / B condition is satisfied.
In 2, it is determined whether the rich purge condition is satisfied. When the lean F / B condition is not satisfied (step 3
If 01 is NO), proceed to steps 303 and 304,
Both the lean correction value FLEAN and the rich correction value FRICH are set to “1.0”.

If the lean F / B condition is satisfied and the rich purge condition is not satisfied (YES in step 301 and NO in step 302), step 30 is executed.
5 to set a lean correction value FLEAN (<1.0) in accordance with the target air-fuel ratio λTG at that time, and then go to step 3
In 06, the rich correction value FRICH is set to “1.0”.

If the lean F / B condition and the rich purge condition are both satisfied (YES in steps 301 and 302), the flow advances to step 307 to set the lean correction value FLEAN to "1.0", and then to the next step. At 308, the rich correction value F is set according to the target air-fuel ratio λTG at that time.
Set RICH (> 1.0).

Next, the procedure for setting the feedback correction value FAF will be described with reference to the flowchart of FIG. 7 which shows the processing of step 400 of FIG. 2 in detail. Step 401
Then, it is determined whether or not the F / B start counter C1 is greater than "0". In step 402, it is determined whether or not the target air-fuel ratio change counter C12 is greater than "0".

If C1> 0, step 403 is executed.
And the target air-fuel ratio λTGST immediately after the start of F / B
The feedback correction value FAF is set based on the (set value in FIG. 5). If C1 = 0 and C2> 0, the routine proceeds to step 404, where the feedback correction value FAF is set to "1.0". If C2 = 0 and C1 = 0, the routine proceeds to step 405, where a feedback correction value FAF is set based on the target air-fuel ratio λTGST.

In steps 403 and 405, a feedback correction value FAF is set based on the deviation between the actual air-fuel ratio λr and the target air-fuel ratio λTG at that time. In the present embodiment, the air-fuel ratio F / B control based on the modern control theory is performed. In the F / B control, the actual air-fuel ratio λr
A feedback correction value FAF for making (the output of the A / F sensor 26) equal to the target air-fuel ratio λTG is calculated using the following equation. The setting procedure of the FAF value is disclosed in detail in Japanese Patent Application Laid-Open No. 1-110853.

FAF = K1..lambda.r + K2.FAF1 +.
. + Kn + 1 FAFn + ZI ZI = ZI1 + Ka. (. Lambda.TG-.lambda.r) In the above formula, K1 to Kn + 1 represent F / B constants and ZI
Represents an integral term, and Ka represents an integral constant. The subscripts 1 to n + 1 are variables indicating the number of times of control since the start of sampling.

Next, the above-described processing by the CPU 31 will be described more specifically with reference to the time charts of FIGS. 10 and 11. FIG. 10 shows the operation immediately after the start of the F / B, and FIG. 11 shows the operation when the rich purge (rich F / B control) is performed during the lean F / B control.

In FIG. 10, at time t1, the air-fuel ratio F / B
The timing at which the condition is satisfied is shown. Before t1, open loop control is performed, and after t1, the stoichiometric air-fuel ratio F /
B control is performed. Incidentally, at the beginning of the engine start, the actual air-fuel ratio λ
r is a rich value.

At time t1, the air-fuel ratio F / B condition
Unsatisfied, this time = satisfied (step 202 in FIG. 3)
Is NO), a predetermined value KC1 is set in the F / B start counter C1, and the actual air-fuel ratio λr at that time is set as the target air-fuel ratio λTGST immediately after the start of F / B (steps 242 and 243 in FIG. 5). . After time t1, the stoichiometric air-fuel ratio F
Since the / B control is performed, the stoichiometric air-fuel ratio is set as the target air-fuel ratio λTG.

After time t1, the F / B start counter C1 is gradually decremented, and the target air-fuel ratio λTGST immediately after the start of F / B is gradually increased so as to approach the target air-fuel ratio λTG (see FIG. 5). Steps 244 and 246). Then, at time t2 when | λTGST−λTG | ≦ KAF1
Then, the F / B start counter C1 is cleared to "0" (step 249 in FIG. 5).

The feedback correction value FAF is calculated at time t1
Target air-fuel ratio λT immediately after the start of F / B during the period from to t2
It is set according to GST (step 403 in FIG. 7), and after time t2, set according to the target air-fuel ratio λTG (the stoichiometric air-fuel ratio) in the normal control (step 405 in FIG. 7). Note that the lean correction value FLEAN and the rich correction value FRIC
H is maintained at “1.0” throughout FIG.

According to FIG. 10, at the beginning of the F / B control, the control by λTG is not started immediately regardless of the deviation of λr and λTG, but λTGST used immediately after the start of F / B is changed from λr. Since it is gradually changed to the λTG side, the conventional problem that a torque shock occurs due to a sudden change in the air-fuel ratio when shifting to the F / B control is solved.

In FIG. 10, | λTGST-λTG | ≦ K
At time t2 when AF1 is reached, the target air-fuel ratio of the F / B control is set to λ
Switching from TGST to λTG, but temporarily | λTGS
If the KC1 time elapses and C1 becomes 0 before T−λTG | ≦ KAF1, the target air-fuel ratio of the F / B control is switched from λTGST to λTG at that time.

Incidentally, the fuel cut is performed and the air-fuel ratio F
Even when the / B control is interrupted or the air-fuel ratio F / B control is interrupted by operating the engine in a high load range, the F / B after the interruption is performed in the same manner as at times t1 to t2 in FIG. When the control is resumed, the target air-fuel ratio λ immediately after the start of F / B
TGST is set, and F / B is set based on the λTGST.
Control is performed. When the deviation between λTGST and λTG becomes sufficiently small, the target air-fuel ratio is changed from λTGST to λTGST.
Switch to TG.

On the other hand, in FIG.
Indicates a period during which the rich purge is temporarily performed during the lean F / B control. Before time t3, the feedback correction value FAF and the lean correction value FLEAN are respectively set according to the target air-fuel ratio λTG at that time (however, the rich correction value FRICH is 1.0).

At time t3, the rich purge condition
Since it is not established, this time is established, the rich control value is set as the target air-fuel ratio λTG, and the predetermined value KC2 is set in the target air-fuel ratio change counter C2 (steps 210 and 213 in FIG. 4).

At time t3, with the start of the rich purge, the lean correction value LEAN is changed from a value less than 1.0 to "1.0", and the rich correction value FRICH is changed to a value up to that time. The value is changed from “1.0” to a value exceeding 1.0.

After time t3, the target air-fuel ratio change counter C
2 is gradually subtracted, and the actual air-fuel ratio λr approaches the target air-fuel ratio λTG (rich control value). And | λr-
At time t4 when λTG | ≦ KAF2, the target air-fuel ratio change counter C2 is cleared to “0” (step 216 in FIG. 4).

During the period from time t3 to time t4, the feedback correction value FAF becomes "1.0" (step 4 in FIG. 7).
04), the air-fuel ratio F / B control is temporarily interrupted. Then, when C2 = 0 at time t4, thereafter, the feedback correction value FAF is set according to the target air-fuel ratio λTG (rich control value) (step 405 in FIG. 7), and rich F / B control is started. .

Thereafter, at time t5, the rich purge condition becomes the previous = satisfied and the present = unsatisfied, so that the predetermined value KC2 is set again in the target air-fuel ratio change counter C2 (FIG. 4).
Step 213). After time t5, the control mode is switched to lean F / B control, so that target air-fuel ratio λTG
Returns to the lean control value. At time t5, the lean correction value LEAN is changed from “1.0” to a value less than 1.0, and the rich correction value FRICH is changed from “1.0” to “1. "0".

After time t5, the target air-fuel ratio change counter C
2 is gradually subtracted, and the actual air-fuel ratio λr approaches the target air-fuel ratio λTG. And | λr−λTG | ≦ KAF
At time t6 when the target air-fuel ratio changes to 2, the target air-fuel ratio change counter C2 is cleared to "0" (step 216 in FIG. 4).

During the period from time t5 to t6, time t3 to t6
4, the feedback correction value FAF is “1.
0 "(step 404 in FIG. 7), and the air-fuel ratio F / B control is temporarily interrupted. Then, at time t6, C2 = 0
, The feedback correction value FAF is thereafter set according to the target air-fuel ratio λTG (step 40 in FIG. 7).
5) Lean F / B control is started.

As described above, the hunting of the control during the transition period is prevented by temporarily suspending the F / B control. When the deviation between λr and λTG becomes equal to or smaller than a predetermined value, the F / B control is restarted, so that the target air-fuel ratio λTG
The problem that the air-fuel ratio is overcorrected at the time of the change is solved.

In FIG. 11, | λr−λTG | ≦ KAF2
F / B control is resumed at times t4 and t6 when the time CC2 elapses and C2 = 0 before | λr−λTG | ≦ KAF2, at which point the F / B control is resumed. It will be restarted.

Although illustration is omitted, when the execution condition is changed between the lean F / B condition and other conditions during the execution of the air-fuel ratio F / B control (step 206 or 207 in FIG. 3 is executed). NO), ie, when the control mode is switched between the stoichiometric air-fuel ratio F / B control and the lean F / B control, the times t3 to t4 or t5 to t6 in FIG.
Similarly to the above, immediately after the control switching, the F / B control is temporarily interrupted. When the deviation between λr and λTG becomes equal to or smaller than a predetermined value, the F / B control is restarted.

In this embodiment, the determination means of the present invention is constituted by steps 205 to 207, 209, 211 and 212 of FIGS. 3 and 4, and steps 214 to 21 of FIG.
6 and FIG. 7 constitute a control changing means. The first target setting means is constituted by step 201 of FIG. 3, and the second target setting means is constituted by steps 243, 245 to 247 of FIG.
The target setting means is constituted, and the control switching means is constituted by the processing of FIG. 5 and FIG. At this time, the target air-fuel ratio λTG in the normal state corresponds to the “first target value”, and the target air-fuel ratio λTGST immediately after the start of the F / B corresponds to the “second target value”.

According to the present embodiment described in detail above, the following effects can be obtained. (A) The stoichiometric air-fuel ratio F / B control⇔When the target air-fuel ratio λTG greatly changes, for example, at the time of switching between lean F / B control and execution / non-execution of rich purge, the F / B is temporarily changed. The control is interrupted, and the actual air-fuel ratio λr and the target air-fuel ratio λTG
The F / B control is restarted when the deviation from the predetermined value becomes equal to or less than a predetermined value KAF2. According to this configuration, problems such as hunting of control in the transition period and overcorrection of the air-fuel ratio are solved. At this time, after switching the control mode, there is no problem that the actual air-fuel ratio λr greatly deviates from the target air-fuel ratio λTG before the start of the F / B control. As a result, even when the target air-fuel ratio changes, the disturbance of the air-fuel ratio is suppressed, and the air-fuel ratio control can be continued without deteriorating drivability, emission, or torque shock.

(B) After interruption of F / B control, λr, λTG
At the time when the deviation becomes smaller than or equal to a predetermined value KAF2, or F /
The F / B control is restarted at an earlier timing when the predetermined time KC2 has elapsed from the suspension of the B control. In this case, even if the convergence of the deviation is unexpectedly prolonged,
The F / B control is restarted when the KC2 time has elapsed.
Therefore, the inconvenience that the restart of the F / B control is delayed unexpectedly can be avoided.

(C) When shifting from the open-loop control to the F / B control, the target air-fuel ratio λTGST is initially set at the beginning of the F / B control, and the target air-fuel ratio λr immediately after the start is set to the target air-fuel ratio λr corresponding to the current control mode. The air-fuel ratio is changed so as to gradually approach the air-fuel ratio λTG, and when the deviation between λTGST and λTG becomes equal to or less than a predetermined value KAF1, the target air-fuel ratio is thereafter changed to λTG.
Switching from TGST to λTG was performed. In this case, the conventional problem that a torque shock occurs due to a sudden change in the air-fuel ratio at the beginning of the F / B control is solved, and the switching to the F / B control is smoothly performed.

(D) The target air-fuel ratio of the F / B control at an earlier timing when the difference between λTGST and λTG becomes equal to or less than a predetermined value KAF1 or when a predetermined time KC1 has elapsed since the start of the F / B control. Was switched from λTGST to λTG. In this case, even if the convergence of the deviation is unexpectedly prolonged, the target air-fuel ratio is changed when the KC1 time has elapsed (λTGST → λTG). Therefore, the inconvenience that the start of the original F / B control based on the target air-fuel ratio λTG is delayed unexpectedly can be avoided.

(E) Since the predetermined values KC1 and KC2 to be set in the counters C1 and C2 are set according to the deviations of λr and λTG, if the deviations are different, different predetermined values KC1 and KC2 are given at each time. , Fine control can be realized.

(F) At the start of F / B control, a threshold value is set according to whether the target air-fuel ratio λTGST immediately after the start of F / B approaches the target air-fuel ratio λTG from the rich side or the lean side. Since the (predetermined value KAF1) is set variably, the start of the F / B control is the optimal time.

Next, second and third embodiments of the present invention will be described. However, in the configurations of the following embodiments, the same components as those of the above-described first embodiment are denoted by the same reference numerals in the drawings, and the description is simplified. The following description focuses on differences from the first embodiment.

(Second Embodiment) FIG. 12 is a flowchart showing a part of the process of setting the target air-fuel ratio λTG. This process is carried out by replacing steps 205 to 213 in FIGS. .

In step 501 of FIG. 12, the change amount λDV is calculated from the current value λTGi and the previous value λTGi-1 of the target air-fuel ratio (λDV = | λTGi-λTGi-1).
|). In the subsequent step 502, it is determined whether or not the change amount λDV of the target air-fuel ratio is larger than a predetermined value KDV, and on the condition that λDV> KDV, step 50 is performed.
In step 3, a predetermined value KC2 is set in a target air-fuel ratio change counter C2. Here, the predetermined value KC2 may be set according to the relationship of FIG. 9 or may be a fixed value.

According to the above processing, when the target air-fuel ratio λTG is largely changed, C2 = KC2 is set, and accordingly, the F / B control is interrupted at first (assuming that FAF = 1.0), The F / B control is restarted at the earlier timing of whether the deviation between the fuel ratio λr and the target air-fuel ratio λTG becomes equal to or less than a predetermined value, or the target air-fuel ratio change counter C2 is gradually reduced to C2 = 0. Is done.

At this time, as described in the first embodiment, when the control mode is switched between the stoichiometric air-fuel ratio F / B control and the lean F / B control, or when the rich purge is executed / not executed. When the user presses the key, the step 502 in FIG.
Is YES, and processing such as interruption of F / B control is performed. In the present embodiment, step 502 corresponds to the determining means of the present invention.

As described above, according to the second embodiment, problems such as control hunting and air-fuel ratio overcorrection during the transition period are eliminated, as in the above-described first embodiment. As a result, even when the target air-fuel ratio changes, the disturbance of the air-fuel ratio is suppressed, and the air-fuel ratio control can be continued without deteriorating drivability, emission, or torque shock.

According to the present embodiment, A / F = about 2
Normal lean F / B where 0-23 is the target air-fuel ratio λTG
When shifting from the control to the super-lean F / B control in which A / F = about 45 to 50 is set as the target air-fuel ratio λTG, the control can be smoothly switched.

(Third Embodiment) In each of the above-described embodiments, when the open-loop control is switched to the air-fuel ratio F / B control, the stoichiometric air-fuel ratio F / B control and the lean F / B control Is switched, when the rich purge is switched on / off, and when the amount of change in the target air-fuel ratio λTG exceeds a predetermined value.
KC2 is set, and its counters C1 and C2 are gradually subtracted, but this configuration is changed. For example, counter C1,
Flags F1 and F2 are used in place of C2, F1 = 1 immediately after the start of F / B, and F2 = 1 when the target air-fuel ratio λTG is greatly changed.

In practice, step 202 in FIG.
When it becomes O, the flag F1 is set to "1", and in the subsequent period of F1 = 1, the target air-fuel ratio λTGST is set as shown in FIG. 5 and | λTGST-λTG
When | ≦ KAF1, the flag F1 is cleared. In the process of FIG. 13 in which a part of FIG. 7 is changed, during the period of F1 = 1, the FAF value is set according to the target air-fuel ratio λTGST (step 601 → 403), and F1 =
After that, the FAF is adjusted according to the target air-fuel ratio λTG.
A value is set (steps 601 → 602 → 405).

On the other hand, 206, 207 and 2 in FIGS.
If any of 11 and 212 is NO, or if step 502 in FIG. 12 is YES, the flag F2 is set to “1”, and in the subsequent period of F2 = 1, as in step 215 in FIG. It is determined whether or not | λr−λTG | ≦ KAF2, and if the step is YES, the flag F2 is cleared. Then, in the process of FIG.
In the period of F2 = 1, the F / B control is interrupted by setting FAF = 1.0 (step 602 → 404). When F2 = 0, thereafter, the FAF value is set according to the target air-fuel ratio λTG (step 602 → 405).

As described above, according to the third embodiment, the disturbance of the air-fuel ratio is suppressed even when the target air-fuel ratio changes, similarly to the above-described embodiments, which leads to deterioration of drivability, emission, and torque shock. Thus, the air-fuel ratio control can be continued without any need. Although the control switching timing can be obtained only by the deviation of the target air-fuel ratio, the counters C1, C2
Setting processing, counter processing, and the like can be omitted, and the configuration can be simplified.

The embodiments of the present invention can be embodied in the following forms other than the above. In the first embodiment, only at the time of transition from open loop control to F / B control, (a) the actual air-fuel ratio λr is initially set as the target air-fuel ratio λTGST (second target value) and the control mode ΛTG so as to approach the target air-fuel ratio λTG (first target value)
Change ST gradually. (B) New feedback control is started with λTGST as the target air-fuel ratio. (C) When the deviation between λTGST and λTG becomes equal to or smaller than a predetermined value, the target air-fuel ratio is switched from λTGST to λTG. Although the series of processes shown in FIGS. 5 and 7 have been performed, this process may be performed when the target air-fuel ratio λTG is changed during the execution of the F / B control.

That is, when the stoichiometric air-fuel ratio F / B control is switched to the lean F / B control, or when the execution of the rich purge is switched, the F / B control is temporarily interrupted. Instead of the above-described process (the process of FIGS. 3 and 4) of restarting the F / B control when the deviation of λTG becomes equal to or less than the predetermined value KAF2,
The above processes (a) to (c) are performed.

Also, as in the second embodiment, when the change amount λDV (= | λTGi−λTGi−1 |) of the target air-fuel ratio is larger than the predetermined value KDV, the above-mentioned (A) is similarly performed.
(C) may be performed. In any of these configurations, as described above, the disturbance of the air-fuel ratio is suppressed even when the target air-fuel ratio changes, and the air-fuel ratio control can be continued without deteriorating drivability, emission, or torque shock.

In the above embodiment, for example, at the time of transition from open loop control to F / B control, the actual air-fuel ratio λ
r is initially set as the target air-fuel ratio λTGST (second target value). At this time, the initial value of λTGST is not necessarily the actual air-fuel ratio λr itself, and the actual air-fuel ratio λr and the target air-fuel ratio λTG at that time are not necessarily required. May be set as appropriate according to. For example, a value obtained by approaching the actual air-fuel ratio λr by a predetermined value α to the target air-fuel ratio λTG suitable for the control mode is initially set as the target air-fuel ratio λTGST (second target value). The predetermined value α may be determined according to the deviation between λr and λTG. Actually, if the actual air-fuel ratio λr is richer than the target air-fuel ratio λTG, “λr + α” is set as the initial value of λTGST. If the actual air-fuel ratio λr is leaner than the target air-fuel ratio λTG, “Λr−α” is set as the initial value of λTGST. In this case, the actual air-fuel ratio λr quickly converges to the target air-fuel ratio λTG, and the start of the F / B control can be hastened.

In the first embodiment, the rich F / B control is performed at the time of the rich purge. However, this may be performed by open loop control. In this case, when switching from the rich purge to the lean F / B control, the F / B control may be started at the beginning by setting the target air-fuel ratio λTGST immediately after the start of the F / B according to the procedure of FIG.

In the second embodiment described above, the processing of FIG. 12 is executed instead of steps 205 to 213 of FIGS. 3 and 4, but this is changed. For example, FIGS.
12 may be performed in combination with the processing of FIG. 12, and in this case, the processing of FIG. 12 may be performed following steps 205 to 213 of FIGS.

Open loop control and stoichiometric air-fuel ratio F / B
The present invention may be applied to a device that performs only control (a device that does not perform lean F / B control). In this case, the process of FIG. 5 described above is performed when switching from the open loop control to the stoichiometric air-fuel ratio F / B control. However, when the rich purge is performed in accordance with the amount of the harmful component adsorbed during the stoichiometric air-fuel ratio F / B control in order to recover the ability of the catalyst to adsorb harmful components (CO, HC, NOx) (for example, Japanese Patent Application Laid-Open Nos. 6-74072 and 8
Execution of rich purge /
Processing such as temporary suspension of F / B control in response to non-execution and return of F / B control in accordance with the air-fuel ratio deviation (FIGS. 3, 4, and 7)
May be appropriately performed. As a result, problems such as deterioration of emission and drivability are solved.

As one mode, the control immediately after the start of F / B (the processing of FIG. 5) is omitted, and the control when the amount of change in the target air-fuel ratio λTG is large (steps 202 and 202 in FIGS. 3 and 4).
(Processes other than 203 and 240) may be used.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an outline of an air-fuel ratio control device for an internal combustion engine according to an embodiment.

FIG. 2 is a flowchart showing a fuel injection control process.

FIG. 3 is a flowchart showing a process for setting a target air-fuel ratio.

FIG. 4 is a flowchart showing a setting process of a target air-fuel ratio, following FIG. 3;

FIG. 5 is a flowchart showing a process for setting a target air-fuel ratio immediately after the start of F / B.

FIG. 6 is a flowchart showing a setting process of FLEAN and FRICH.

FIG. 7 is a flowchart showing FAF setting processing.

FIG. 8 is a relationship diagram for setting a predetermined value KC1.

FIG. 9 is a relationship diagram for setting a predetermined value KC2.

FIG. 10 is a time chart more specifically showing the operation immediately after the start of F / B.

FIG. 11 is a time chart more specifically showing an operation at the time of switching the F / B control.

FIG. 12 is a flowchart illustrating a part of a target air-fuel ratio setting process according to the second embodiment;

FIG. 13 is a flowchart illustrating FAF setting processing according to the third embodiment.

FIG. 14 is a time chart for explaining a problem of the related art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 12 ... Exhaust pipe, 13 ... NO
x catalyst, 26 ... A / F sensor, 30 ... ECU, 31 ... judging means, control change means, mode switching control means, first target setting means, second target setting means, CPU constituting control switching means.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3G301 HA01 JA04 JA06 JA07 JA21 KA11 LB02 MA01 MA13 NA06 NA08 NB11 NC08 ND02 ND14 ND16 NE00 NE13 NE14 NE15 NE19 NE23 PA07Z PA10Z PA11Z PD02A PD02Z PE01Z PE03Z PE08Z

Claims (10)

[Claims] A target air-fuel ratio is set, a correction is made in accordance with the degree of leanness or richness of the target air-fuel ratio, and an actual air-fuel ratio of exhaust gas discharged from an internal combustion engine is feedback-controlled to a target air-fuel ratio. An air-fuel ratio control device for an internal combustion engine, wherein: a determining means for determining that the target air-fuel ratio has changed significantly; and when determining that the target air-fuel ratio has changed significantly, the feedback control is temporarily interrupted and the actual air An air-fuel ratio control device for an internal combustion engine, comprising: control change means for restarting feedback control when a deviation between a fuel ratio and a target air-fuel ratio becomes equal to or less than a predetermined value. 2. The method according to claim 1, wherein the control change means temporarily suspends the feedback control, and thereafter, when the deviation between the actual air-fuel ratio and the target air-fuel ratio becomes equal to or less than a predetermined value, or when a predetermined time has elapsed since the suspension of the feedback control. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the feedback control is restarted at an earlier timing among the time points. 3. The air-fuel ratio control device for an internal combustion engine according to claim 2, wherein a predetermined time of said control changing means is set according to a change amount of a target air-fuel ratio. 4. An air-fuel ratio control device that selectively performs stoichiometric air-fuel ratio feedback control with a stoichiometric air-fuel ratio as a target air-fuel ratio and lean feedback control with a predetermined lean value as a target air-fuel ratio. 4. The air-fuel ratio control for an internal combustion engine according to claim 1, wherein when the control is switched from one of the two feedback controls to the other, the target air-fuel ratio is determined to have changed significantly. apparatus. 5. An air-fuel ratio for selectively performing a lean feedback control for setting a predetermined lean value as a target air-fuel ratio and a rich purge control for releasing stored NOx of a lean NOx catalyst provided in an engine exhaust system. 4. The control device according to claim 1, wherein the determination unit determines that the target air-fuel ratio has significantly changed when the execution / non-execution of the rich purge is switched. 5. Fuel ratio control device. 6. An air-fuel ratio feedback control corresponding to a deviation between an actual air-fuel ratio of exhaust gas discharged from an internal combustion engine and a target air-fuel ratio and an open-loop control not corresponding to an air-fuel ratio deviation are selectively performed. In the air-fuel ratio control device of the internal combustion engine, when switching from the open-loop control to the feedback control, or when the target air-fuel ratio of the feedback control changes greatly, such as when switching the control mode, initially the target air-fuel ratio, From the actual air-fuel ratio side, a change is made so as to gradually approach a target air-fuel ratio suitable for the control mode. An air-fuel ratio control device for an internal combustion engine, comprising: mode switching control means for changing an air-fuel ratio to a value suitable for a control mode. 7. The air-fuel ratio control device for an internal combustion engine according to claim 6, further comprising: first target setting means for setting, as a first target value, a target air-fuel ratio corresponding to a current control mode; Switching control means for initially setting the actual air-fuel ratio as a second target value, and for gradually changing a second target value so as to approach the first target value; A new feedback control is started using the target value of the target air-fuel ratio as a target air-fuel ratio, and when the deviation between the first and second target values becomes equal to or less than a predetermined value, the target air-fuel ratio of the feedback control is changed from the second target value to the first air-fuel ratio. An air-fuel ratio control device for an internal combustion engine, comprising: control switching means for switching to a target value. 8. The air-fuel ratio control device for an internal combustion engine according to claim 7, wherein the mode switching control means is configured to determine when the deviation between the first and second target values becomes equal to or less than a predetermined value, or when the control mode is changed. An air-fuel ratio control device for an internal combustion engine that switches the target air-fuel ratio of feedback control from a second target value to a first target value at an earlier timing when a predetermined time has elapsed from the switching of the control. 9. The air-fuel ratio control apparatus for an internal combustion engine according to claim 8, wherein a predetermined time of said mode switching control means is set in accordance with a deviation between the first and second target values immediately after switching of the control mode. An air-fuel ratio control device for an internal combustion engine. 10. The air-fuel ratio control apparatus for an internal combustion engine according to claim 6, wherein when switching the control mode, the initial target air-fuel ratio is set to a target air-fuel ratio corresponding to the control mode. Depending on whether you are approaching from the rich side or the lean side
An air-fuel ratio control device for an internal combustion engine that variably sets a threshold value for determining a deviation of a target air-fuel ratio.