JP7132804B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

TECHNICAL FIELD The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly to technology for controlling the oxygen storage amount of an exhaust gas purification catalyst.

In Patent Document 1, when the output of an oxygen concentration sensor downstream of the catalyst is reversed to lean output, the target air-fuel ratio upstream of the catalyst is set to a rich target value, and when the output is reversed to rich output, the above target air-fuel ratio is set. A lean target value , an integrated value of the estimated oxygen desorption value calculated based on the difference between the rich target value and the stoichiometric air-fuel ratio, and an estimated oxygen storage calculated based on the difference between the lean target value and the stoichiometric air-fuel ratio An air-fuel ratio control device for an internal combustion engine is disclosed that calculates a theoretical air-fuel ratio equal to an integrated value of values as a true theoretical air-fuel ratio.

JP-A-2005-098205

By the way, when the exhaust air-fuel ratio downstream of the exhaust gas purification catalyst turns lean, the target air-fuel ratio is set to a rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio, and the exhaust air-fuel ratio downstream of the exhaust gas purification catalyst turns rich. In an air-fuel ratio control device that sets the target air-fuel ratio to a lean target air-fuel ratio that is leaner than the stoichiometric air-fuel ratio when If it is too slow, the exhaust air-fuel ratio in the downstream of the exhaust gas purifying catalyst is reversed before the oxygen storage amount reaches the desired amount, causing the oxygen storage amount to deviate from the target amount and reduce the exhaust performance.

SUMMARY OF THE INVENTION The present invention has been made in view of the conventional circumstances, and its object is to provide an air-fuel ratio control device for an internal combustion engine that can suppress deterioration of exhaust gas performance due to the reaction rate of oxygen in an exhaust gas purification catalyst. That's what it is.

According to one aspect of the present invention, the air-fuel ratio control device for an internal combustion engine sets the target air-fuel ratio to a lean target air-fuel ratio that is leaner than the stoichiometric air-fuel ratio when the output of the post-catalyst sensor is reversed to rich. a target air-fuel ratio setting unit for setting the target air-fuel ratio to a rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio when the output of the post-catalyst sensor is reversed to lean; and an oxygen storage amount in the exhaust gas purification catalyst is estimated. and an oxygen storage amount estimating unit that resets the estimated amount of the oxygen storage amount to a predetermined minimum amount when the output of the post-catalyst sensor is reversed to rich, and the output of the post-catalyst sensor is reversed to lean. resetting the estimated amount of oxygen storage amount to a predetermined maximum amount, and updating the estimated amount of oxygen storage amount based on the output of the pre- catalyst sensor using the minimum amount or the maximum amount as an initial value. and correcting the rich target air-fuel ratio based on the difference between the estimated amount when the output of the post-catalyst sensor is reversed to rich and the target amount of the oxygen storage amount, and correcting the rich target air-fuel ratio, a target air-fuel ratio correcting unit for correcting the lean target air-fuel ratio based on the deviation between the estimated amount and the target amount when the output of is reversed to lean; and an air-fuel ratio controller for generating an air-fuel ratio control signal based on the fuel ratio.

According to the present invention, it is possible to correct the deviation of the oxygen storage amount due to the reaction speed of oxygen in the exhaust gas purification catalyst, thereby suppressing the deterioration of the exhaust gas performance.

1 is a system configuration diagram of a vehicle internal combustion engine; FIG. FIG. 4 is a diagram showing the correlation between the conversion rate of a three-way catalyst and the air-fuel ratio; 4 is a time chart showing basic characteristics of target air-fuel ratio setting control; 4 is a time chart showing the behavior of the oxygen storage amount when the adsorption speed of the exhaust gas purifying catalyst slows down; 4 is a flowchart showing target air-fuel ratio setting processing in air-fuel ratio feedback control; 4 is a flowchart showing target air-fuel ratio setting processing in air-fuel ratio feedback control; 4 is a flowchart showing target air-fuel ratio setting processing in air-fuel ratio feedback control; 4 is a time chart for explaining an estimated error amount ADE during adsorption and an estimated error amount DEE during desorption. FIG. 5 is a diagram showing a correlation between an estimated adsorption error amount ADE and an adsorption speed correction value ADH; 4 is a time chart showing correction processing of the target air-fuel ratio TAF when the adsorption speed is slow; FIG. 4 is a diagram showing the correlation between the desorption estimated error amount DEE and the desorption rate correction value DEH. 4 is a flow chart showing processing for changing the target amount TST, catalyst deterioration diagnosis, and response deterioration diagnosis of the air-fuel ratio sensor. 4 is a flow chart showing processing for changing the target amount TST, catalyst deterioration diagnosis, and response deterioration diagnosis of the air-fuel ratio sensor. 4 is a flow chart showing processing for changing the target amount TST, catalyst deterioration diagnosis, and response deterioration diagnosis of the air-fuel ratio sensor. FIG. 4 is a diagram showing diagnostic characteristics based on an adsorption speed correction value ADH and a desorption speed correction value DEH; FIG. 5 is a diagram for explaining catalyst deterioration diagnosis based on an estimated error amount ADE during adsorption and an estimated error amount DEE during desorption. 4 is a time chart for explaining changes in the target amount TST when the adsorption speed and desorption speed are slow; 4 is a time chart for explaining calculation processing of an average value ADHav of adsorption speed correction values ADH and an average value ADEav of estimated error amounts ADE during adsorption. FIG. 10 is a flowchart showing target air-fuel ratio setting processing in enrichment control after fuel cut; FIG. FIG. 4 is a time chart showing execution timing of enrichment control after fuel cut; FIG.

An embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing one aspect of an internal combustion engine 11 for a vehicle, and the internal combustion engine 11 is a gasoline engine.
In FIG. 1, the intake air of an internal combustion engine 11 passes through an air flow meter 12, an electronically controlled throttle valve 13, and a collector 14 in this order, and then is sucked into a combustion chamber 17 via an intake pipe 15 and an intake valve 16 provided in each cylinder. be done.

The fuel injection valve 21 (fuel injection device) is installed in the intake pipe 15 of each cylinder and injects fuel into the intake pipe 15 .
The internal combustion engine 11 also includes an ignition device 24 having an ignition coil 22 and an ignition plug 23 for each cylinder.

The air-fuel mixture in the combustion chamber 17 is ignited and burned by the spark generated by the spark plug 23, and the exhaust gas generated in the combustion chamber 17 by combustion is discharged to the exhaust pipe 26 provided in each cylinder through the exhaust valve 25. be done.
The internal combustion engine 11 is arranged immediately below the collecting portion of the exhaust pipe 26, and is arranged in a first catalyst device 31 containing a three-way catalyst as an exhaust gas purification catalyst, and an exhaust duct 32 downstream of the first catalyst device 31, and a second catalyst device 33 containing a three-way catalyst as an exhaust gas purification catalyst.

A three-way catalyst is a device that simultaneously purifies hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in exhaust gas by oxidation and reduction, and has oxygen storage capacity.
FIG. 2 is a diagram showing the correlation between the conversion rate of the three-way catalyst and the air-fuel ratio. It is a catalyst that can simultaneously purify hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx).

The internal combustion engine 11 also includes an air-fuel ratio sensor 34 arranged upstream of the first catalyst device 31 and configured to output a detection signal RABF corresponding to the exhaust air-fuel ratio based on the oxygen concentration of the exhaust gas upstream of the first catalyst device 31; and an oxygen sensor 35 arranged downstream of the first catalyst device 31 and outputting a detection signal VO2R indicating whether the exhaust air-fuel ratio is rich or lean with respect to the stoichiometric air-fuel ratio based on the oxygen concentration of the exhaust gas downstream of the first catalyst device 31 .
That is, the air-fuel ratio sensor 34 is a pre-catalyst sensor that detects the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst of the first catalyst device 31, and the oxygen sensor 35 flows out from the exhaust gas purification catalyst of the first catalyst device 31. It is a post-catalyst sensor that detects the air-fuel ratio of the exhaust gas.

The detection signal VO2R output by the oxygen sensor 35 is a voltage signal, and the oxygen sensor 35 outputs a high-level voltage signal when the exhaust air-fuel ratio downstream of the first catalyst device 31 is richer than the stoichiometric air-fuel ratio. and outputs a low-level voltage signal when the exhaust air-fuel ratio downstream of the first catalyst device 31 is leaner than the stoichiometric air-fuel ratio.
The internal combustion engine 11 also includes an exhaust gas recirculation device 43 having an exhaust gas recirculation pipe 41 that communicates the exhaust pipe 26 with the collector 14 and an exhaust gas recirculation control valve 42 that adjusts the opening area of the exhaust gas recirculation pipe 41 .

The control device 51 applied to the internal combustion engine 11 includes a computer having a microprocessor and a memory, and processes detection signals from various sensors to perform fuel injection by the fuel injection valve 21 and opening of the electronically controlled throttle valve 13. It has a function of controlling the degree of ignition by the ignition plug 23, the opening degree of the exhaust gas recirculation control valve 42, etc. as software.
The control device 51 acquires the detection signal RABF output by the air-fuel ratio sensor 34 and the detection signal VO2R output by the oxygen sensor 35, and outputs a signal indicating the intake air flow rate QA of the internal combustion engine 11 output by the air flow meter 12. , a signal indicating the rotation angle position POS of the crankshaft 53 output by the crank angle sensor 52, a signal indicating the cooling water temperature TW of the internal combustion engine 11 output by the water temperature sensor 54, and an accelerator pedal 56 output by the accelerator opening sensor 55. A signal or the like indicating the depression amount (accelerator opening degree ACC) is acquired.

The control device 51 calculates the engine rotation speed NE based on information on the rotational angular position POS of the crankshaft 53, and obtains the engine load based on the intake air flow rate QA and the engine rotation speed NE.
Then, the control device 51 calculates the ignition timing and the target EGR amount based on the engine operating conditions such as the engine load, the engine speed NE, the cooling water temperature TW (engine temperature), and the ignition signal to the ignition coil 22 based on the ignition timing. , and outputs an opening degree control signal to the exhaust gas recirculation control valve 42 based on the target EGR amount.

The control device 51 also calculates a target opening of the electronically controlled throttle valve 13 from the accelerator opening ACC and the like, and drives and controls the throttle motor of the electronically controlled throttle valve 13 based on this target opening.
Furthermore, the control device 51 calculates a fuel injection pulse width TI (ms) proportional to the amount of fuel to be injected from the fuel injection valve 21 in one combustion cycle, and an injection timing, and injects the fuel injection pulse width TI at the injection timing. A pulse signal (air-fuel ratio control signal) is output to the fuel injection valve 21 to control the air-fuel ratio of the internal combustion engine 11 .
Thus, the control device 51 functions as an air-fuel ratio control section that generates an air-fuel ratio control signal and outputs it to the fuel injection valve 21 .

In controlling the fuel injection amount (air-fuel ratio), the control device 51 controls the fuel injection pulse width TI (fuel injection amount) based on the deviation between the actual air-fuel ratio detected by the air-fuel ratio sensor 34 and the target air-fuel ratio. Implement feedback control. That is, the control device 51 is an air-fuel ratio control device that controls the air-fuel ratio of the internal combustion engine.
In such air-fuel ratio feedback control, the control device 51 shifts the target air-fuel ratio TAF to be richer or leaner than the stoichiometric air-fuel ratio by proportional action when the output of the oxygen sensor 35 is reversed, and then shifts the target air-fuel ratio TAF to be richer or leaner than the stoichiometric air-fuel ratio by integral action and differential action. The target air-fuel ratio TAF is brought closer to the stoichiometric air-fuel ratio to invert the output of the oxygen sensor 35 again, and the oxygen storage amount ST of the first catalyst device 31 is set to the maximum oxygen storage amount STMAX (oxygen storage amount in saturated state). Control to the target amount TST which is half.

FIG. 3 is a time chart for explaining the basic characteristics of the target air-fuel ratio TAF setting process performed by the control device 51. The target air-fuel ratio TAF, the oxygen storage amount ST (target value for control), , the correlation of the detected signal VO2R of .
When the detection signal VO2R of the oxygen sensor 35 crosses the rich determination threshold value RTH and changes in the rich direction, the control device 51 sets the target air-fuel ratio TAF to the target amount of the engine speed NE, the intake air flow rate QA, and the oxygen storage amount ST. The target air-fuel ratio TAF is set to a lean target air-fuel ratio that is leaner than the stoichiometric air-fuel ratio by shifting in the lean direction by the lean control P (proportional portion) obtained from TST.

After that, the control device 51 calculates the engine rotation speed NE, the intake air flow rate QA, the target amount TST, the lean control I component (integral component) and the lean control D component (differential component) obtained from the air-fuel ratio detected by the air-fuel ratio sensor 34. , the target air-fuel ratio TAF is gradually enriched and returned to the stoichiometric air-fuel ratio.
After the target air-fuel ratio TAF returns from the lean target air-fuel ratio to the stoichiometric air-fuel ratio, the control device 51 maintains the target air-fuel ratio TAF at the stoichiometric air-fuel ratio until the output of the oxygen sensor 35 is reversed to lean.

Here, the controller 51 assumes that the oxygen storage amount ST has reached the minimum oxygen storage amount STMIN, which is a predetermined amount corresponding to an empty state, when the output of the oxygen sensor 35 is inverted to rich. The lean control P, lean control I, and lean control D are set by control model calculation based on the engine operating conditions and the target amount TST so that the amount STMIN can quickly converge to the target amount TST.
Since the target air-fuel ratio TAF is the lean target air-fuel ratio, the oxygen storage amount ST in the first catalyst device 31 increases (oxygen adsorption), and the detection signal VO2R of the oxygen sensor 35 crosses the lean determination threshold value LTH. It changes in the lean direction (in other words, the output of the oxygen sensor 35 is reversed to lean).

Then, when the output of the oxygen sensor 35 is turned lean, the control device 51 adjusts the target air-fuel ratio TAF by the rich control P obtained from the engine speed NE, the intake air flow rate QA, and the target amount TST of the oxygen storage amount ST. The target air-fuel ratio TAF is set to a rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio.
After that, the control device 51 calculates the target air-fuel ratio based on the engine speed NE, the intake air flow rate QA, the target amount TST of the oxygen storage amount ST, and the rich control I minute and the rich control D minute obtained from the air-fuel ratio detected by the air-fuel ratio sensor 34. The fuel ratio TAF is gradually made lean and returned to the stoichiometric air-fuel ratio.

After the target air-fuel ratio TAF returns from the rich target air-fuel ratio to the stoichiometric air-fuel ratio, the control device 51 maintains the target air-fuel ratio TAF at the stoichiometric air-fuel ratio until the output of the oxygen sensor 35 is reversed to rich.
Here, the control device 51 assumes that the oxygen storage amount ST has reached the maximum oxygen storage amount STMAX, which is a predetermined amount corresponding to a saturated state, when the output of the oxygen sensor 35 is turned lean. Rich control P, rich control I, and rich control D are set by control model calculation based on the engine operating conditions and target amount TST so that the amount STMAX can quickly converge to the target amount TST.

By setting a rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio as the target air-fuel ratio TAF, the decrease (oxygen desorption) of the oxygen storage amount ST in the first catalyst device 31 progresses, and the detection signal VO2R of the oxygen sensor 35 increases. The rich judgment threshold value RTH is crossed and changes in the rich direction (in other words, the output of the oxygen sensor 35 is inverted to rich), and the control device 51 again sets the target air-fuel ratio TAF to the lean target air-fuel ratio. .
By repeating the control of the target air-fuel ratio TAF, the control device 51 controls the oxygen storage amount ST of the first catalyst device 31 to the target amount TST, and the first catalyst device 31 stores hydrocarbons (HC) and carbon monoxide. (CO) and nitrogen oxides (NOx) can be purified at high conversion rates.

Here, the PID control amount for the target air-fuel ratio TAF (lean control P minute, lean control I minute, lean control D minute, rich control P minute, rich control I minute, rich control D minute) is obtained by control model calculation. If the reaction speed (adsorption speed, desorption speed) of oxygen in the first catalyst device 31 used as a reference when determining the characteristics differs from the actual reaction speed, the oxygen storage amount ST shifts from the target amount TST, causing exhaust gas Decreased performance.
FIG. 4 is a time chart showing the correlation between the target air-fuel ratio TAF, the oxygen storage amount ST, and the detection signal VO2R of the oxygen sensor 35 when the oxygen adsorption speed in the first catalyst device 31 is slower than the reference.

If the oxygen adsorption speed in the first catalyst device 31 is slower than the appropriate level of the PID control amount, the inflowing oxygen is reduced to the first The catalytic device 31 is unable to absorb the gas and discharges it, and as a result, the detection signal VO2R of the oxygen sensor 35 is reversed to lean.
As a result, the control device 51 shifts the target air-fuel ratio TAF to the rich target air-fuel ratio and desorbs oxygen from the first catalyst device 31. As a result, the oxygen storage amount ST of the first catalyst device 31 reaches the target It becomes less than the amount TST, and the exhaust gas performance is deteriorated.

On the other hand, if the desorption speed of oxygen in the first catalyst device 31 is slower than the appropriate level of the PID control amount, the output of the oxygen sensor 35 is reached before the oxygen storage amount ST becomes empty or reaches the target amount TST. becomes rich, the target air-fuel ratio is switched to the lean target air-fuel ratio, the oxygen storage amount ST of the first catalyst device 31 becomes larger than the target amount TST, and exhaust gas performance is lowered.
Therefore, the control device 51 has a function ( target air-fuel ratio correction unit).

5 to 7 are flowcharts showing the procedure for setting the target air-fuel ratio TAF by the control device 51, which is started when the control device 51 is turned on by the engine switch.
First, in step S101, the control device 51 determines whether or not the air-fuel ratio feedback control based on the output of the air-fuel ratio sensor 34 can be started. move on.
For example, when the load and rotational speed of the internal combustion engine 11 are in a high output region where the air-fuel ratio is richer than the stoichiometric air-fuel ratio, the control device 51 waits in step S101.

In step S102, the control device 51 determines whether or not the oxygen sensor 35 is active based on, for example, whether or not the detection signal VO2R has changed from lean output to rich output.
If the oxygen sensor 35 is activated, the control device 51 proceeds to step S103 and determines whether or not the conditions for setting the target air-fuel ratio TAF for controlling the oxygen storage amount ST of the first catalyst device 31 are met. do.

For example, during a fuel cut and within a predetermined period for enriching the air-fuel ratio after the fuel cut, the control device 51 determines that the control condition for the oxygen storage amount ST is not established, and waits in step S103.
As will be described later, the control device 51 controls the air-fuel ratio of the internal combustion engine 11 to be richer than the stoichiometric air-fuel ratio for a predetermined period after the fuel cut. A process (recovery rich control section) for quickly desorbing oxygen is performed.

Then, when controlling the oxygen storage amount ST of the first catalyst device 31, the control device 51 proceeds to step S104, and calculates the average value ADHav of the adsorption speed correction values ADH stored in the memory in the previous operating state of the internal combustion engine 11. , the average value DEHav of the desorption rate correction value DEH, the target amount TST, and the maximum oxygen storage amount STMAX are read.
Next, the control device 51 proceeds to step S105 to determine whether the detection signal VO2R of the oxygen sensor 35 has exceeded the rich determination threshold RTH from the state below the rich determination threshold RTH. Determine whether or not

Then, when the detection signal VO2R of the oxygen sensor 35 exceeds the rich determination threshold value RTH from the state below the rich determination threshold value RTH, the control device 51 advances to step S107 to calculate the estimated oxygen storage amount EST (estimated amount of the oxygen storage amount ST). is reset to the minimum oxygen storage amount STMIN.
In the next step S108 (oxygen storage amount estimator), the control device 51 sets the minimum oxygen storage amount STMIN as an initial value, the intake air flow rate QA (air amount, exhaust flow rate), and the catalyst detected by the air-fuel ratio sensor 34. An update calculation of the estimated oxygen storage amount EST based on the upstream exhaust air-fuel ratio is started.

In step S109, the control device 51 calculates a basic lean control P to be used in controlling the target air-fuel ratio TAF by control model calculation based on the engine speed NE, the intake air flow rate QA, and the target amount TST.
Then, in the next step S110 (target air-fuel ratio setting section, target air-fuel ratio correction section), the control device 51 adjusts the basic lean control P obtained in step S109 to the average value ADHav (ADHav≧1.0) of the adsorption speed correction value ADH. ), the target air-fuel ratio TAF is shifted in the lean direction based on the lean control P obtained by dividing by ), thereby switching the target air-fuel ratio TAF to a lean target air-fuel ratio that is leaner than the stoichiometric air-fuel ratio.

By setting the lean target air-fuel ratio, the control device 51 causes lean exhaust gas having a high oxygen concentration to flow into the first catalyst device 31 to increase the oxygen storage amount ST of the first catalyst device 31 .
Calculation of the adsorption speed correction value ADH and the average value ADHav will be described later in detail.

Next, the control device 51 proceeds to step S111, in which control model calculation based on the engine speed NE, the intake air flow rate QA, the target amount TST, and the exhaust air-fuel ratio upstream of the catalyst detected by the air-fuel ratio sensor 34 is performed. A basic lean control I component and a basic lean control D component used in controlling the fuel ratio TAF are calculated.
Then, in the next step S112 (target air-fuel ratio setting unit, target air-fuel ratio correction unit), the control device 51 calculates the average of the adsorption speed correction value ADH for the basic lean control I component and the basic lean control D component obtained in step S111. The lean control I minute and lean control D minute are obtained by dividing by the value ADHav. ) is brought closer to the stoichiometric air-fuel ratio.

The adsorption speed correction value ADH is a correction term for correcting the lean target air-fuel ratio when the adsorption speed is slower than the reference.
The adsorption speed correction value ADH is a value of 1.0 or more, and when the average value ADHav of the adsorption speed correction values ADH is 1.0, the control device 51 performs basic lean control P, basic lean control I, and basic lean control D. is set as the lean control P minute, the lean control I minute, and the lean control D minute used for the control of the target air-fuel ratio TAF.

On the other hand, as the average value ADHav of the adsorption speed correction value ADH becomes larger than 1.0, the control device 51 changes the lean control P, lean control I, and lean control D to smaller values than the basic values. Decrease the lean level of the fuel ratio.
That is, when the adsorption speed is slower than the reference, the control device 51 corrects the lean control P to reduce the shift amount of the target air-fuel ratio TAF in the lean direction, and then performs lean control I and lean control D. The speed of returning the target air-fuel ratio TAF to the stoichiometric air-fuel ratio is slowed down by reducing and correcting the air-fuel ratio to secure the total amount of oxygen flowing into the first catalyst device 31, and the oxygen that the first catalyst device 31 cannot absorb is reduced. Inflow into the first catalyst device 31 is suppressed.

In step S113, the control device 51 determines whether or not the detection signal VO2R of the oxygen sensor 35 again exceeds the rich determination threshold value RTH from the state below the rich determination threshold value RTH.
Then, when the detection signal VO2R of the oxygen sensor 35 again exceeds the rich determination threshold value RTH from the state below the rich determination threshold value RTH, the control device 51 returns to step S107.

On the other hand, when the detection signal VO2R of the oxygen sensor 35 has not exceeded the rich determination threshold value RTH from the state of being equal to or less than the rich determination threshold value RTH, the control device 51 proceeds to step S114.
In step S114, the control device 51 determines whether or not the detection signal VO2R of the oxygen sensor 35 has fallen below the lean determination threshold value LTH, that is, whether or not the output of the oxygen sensor 35 has turned lean.

If the detection signal VO2R of the oxygen sensor 35 is equal to or greater than the lean determination threshold value LTH, the control device 51 returns to step S111 and performs the processes of steps S111 and S112 to increase the average value ADHav of the adsorption speed correction value ADH. The target air-fuel ratio TAF is brought closer to the stoichiometric air-fuel ratio by the lean control I component and the lean control D component corrected based on the above.
In this manner, the control device 51 repeats the processing of steps S111 and S112 to gradually bring the target air-fuel ratio TAF, which is lean-shifted by the lean control P, closer to the stoichiometric air-fuel ratio.

On the other hand, when the control device 51 determines in step S114 that the detection signal VO2R of the oxygen sensor 35 has fallen below the lean determination threshold value LTH, the process proceeds to step S115.
In step S115, the control device 51 sets the estimated oxygen storage amount EST at the time when the output of the oxygen sensor 35 is turned lean to the lean reversal oxygen storage amount ESTL.

Then, in the next step S116, the control device 51 obtains the deviation between the lean inversion oxygen storage amount ESTL obtained in step S115 and the target amount TST as the adsorption estimated error amount ADE, and stores it in the memory.
Note that the control device 51 stores the past plural times of the adsorption estimated error amount ADE obtained in step S116.

After determining the adsorption estimated error amount ADE, the control device 51 proceeds to step S117, calculates the adsorption speed correction value ADH based on the adsorption estimated error amount ADE obtained in step S116, and stores it in the memory.
Note that the controller 51 stores a plurality of past suction speed correction values ADH obtained in step S117.

FIG. 8 is a time chart showing the error represented by the estimated error amount ADE during adsorption and calculation timing.
When the output of the oxygen sensor 35 exceeds the rich determination threshold value RTH (time t1) and the target air-fuel ratio TAF is set to the lean target air-fuel ratio, oxygen adsorption in the first catalyst device 31 progresses, and the estimated oxygen storage amount EST becomes Increase and change.

Then, when the output of the oxygen sensor 35 falls below the lean determination threshold value LTH (time t2), the control device 51 sets the difference between the estimated oxygen storage amount EST and the target amount TST at that time as an estimated error amount ADE during adsorption. Ask.
Here, if the oxygen adsorption speed in the first catalyst device 31 is slow, the first catalyst device 31 cannot fully adsorb the inflowing oxygen, and the timing at which the output of the oxygen sensor 35 inverts to lean is hastened. Therefore, the slower the adsorption speed of oxygen in the first catalyst device 31, the larger the estimated error amount ADE during adsorption.

FIG. 9 is a graph showing the correlation between the adsorption estimated error amount ADE and the adsorption speed correction value ADH.
When the estimated error amount ADE during adsorption is zero, the adsorption speed correction value ADH (ADH≧1.0) is set to 1.0 at which the PID control amount is not substantially corrected. The value ADH is set to a larger value, that is, a value that corrects the PID control amount to a smaller value.

That is, the control device 51 corrects the PID control amount to be smaller as the oxygen adsorption speed in the first catalyst device 31 is slower, thereby reducing the amount of shift of the target air-fuel ratio TAF in the lean direction by the lean control P, In addition, the speed of returning the target air-fuel ratio TAF to the stoichiometric air-fuel ratio is slowed down by the lean control I and the lean control D.
As a result, the exhaust gas air-fuel ratio is set to be excessively lean with respect to the adsorption speed, and oxygen that cannot be adsorbed by the first catalyst device 31 is prevented from flowing into the first catalyst device 31. The lean target air-fuel ratio is continued until the oxygen storage amount ST reaches the desired amount at the adsorption speed, and the first catalyst device 31 is caused to adsorb the desired amount of oxygen.

After obtaining the suction speed correction value ADH in step S117, the control device 51 proceeds to step S118 and counts up the calculation counter of the suction speed correction value ADH.
As will be described later, the control device 51 obtains the moving average of the adsorption speed correction values ADH as the average value ADHav of the adsorption speed correction values ADH, and calculates the PID control amount (lean control P, lean It is used for correcting the control (I component, lean control D component).

FIG. 10 shows the oxygen adsorption speed in the first catalyst device 31 slower than the reference, the estimated error amount ADE during adsorption occurs, and the PID control amount is corrected by the adsorption speed correction value ADH (average value ADHav). 4 is a time chart showing the behavior of the output of sensor 35, estimated oxygen storage amount EST, and target air-fuel ratio TAF;
At time t1 in FIG. 10, the target air-fuel ratio TAF is lean-shifted by the basic lean control P minutes, and then the target air-fuel ratio TAF is gradually returned to the stoichiometric air-fuel ratio by the basic lean control I minutes and the basic lean control D minutes. do.

Here, since the adsorption speed of oxygen in the first catalyst device 31 is slower than the reference, the adsorption estimated error amount ADE occurs at the timing when the output of the oxygen sensor 35 is turned lean at time t2.
At time t3, when the output of the oxygen sensor 35 is turned rich, the basic lean control P is corrected by the adsorption speed correction value ADH based on the adsorption estimated error amount ADE, and the target air-fuel ratio TAF is made lean by the lean control P. limited to small.

In addition, the lean control I minute and the lean control D minute used in the process of returning the target air-fuel ratio TAF (lean target air-fuel ratio) to the stoichiometric air-fuel ratio after time t3 are also corrected to be smaller than the basic values by the adsorption speed correction value ADH. Therefore, the speed at which the target air-fuel ratio TAF is returned to the stoichiometric air-fuel ratio is corrected slower than the basic speed.
As a result, when oxygen is adsorbed by the first catalyst device 31, the lean level of the target air-fuel ratio TAF is suppressed to prevent the oxygen that the first catalyst device 31 cannot absorb from flowing into the first catalyst device 31. In addition, since the first catalyst device 31 can adsorb a predetermined amount of oxygen while securing a period for adsorbing oxygen, the estimated error amount ADE during adsorption is reduced.
Therefore, even if the oxygen adsorption speed in the first catalyst device 31 is slower than the standard, the oxygen storage amount ST of the first catalyst device 31 is prevented from decreasing below the target amount TST, and the exhaust gas performance is prevented from deteriorating. can be suppressed.

The control device 51 determines whether or not the target amount determination flag Ft is 1 in step S131. As will be described later, the target amount determination flag Ft is set to 1 when the counter for the number of calculations of the adsorption speed correction value ADH counted up in step S118 reaches a predetermined value, and the target amount based on the adsorption speed correction value ADH is set. This flag is set to 0 when the TST change processing and diagnosis processing are completed.
The control device 51 waits while the target amount determination flag Ft is 1, that is, until various processing based on the data of the adsorption speed correction value ADH is completed, and when the target amount determination flag Ft is reset to 0, step The process proceeds from S131 to step S119.

On the other hand, when the control device 51 determines in step S105 that the detection signal VO2R of the oxygen sensor 35 has crossed the rich determination threshold value RTH and has not changed in the rich direction, the process proceeds to step S106.
In step S106, the control device 51 determines whether the detection signal VO2R of the oxygen sensor 35 has changed from being equal to or greater than the lean determination threshold value LTH (lean determination threshold value LTH<rich determination threshold value RTH) to below the lean determination threshold value LTH. , determines whether the output of the oxygen sensor 35 has turned lean.
If the control device 51 determines that the detection signal VO2R of the oxygen sensor 35 has crossed the lean determination threshold value LTH and has not changed in the lean direction, the process returns to step S105.

On the other hand, when the control device 51 determines that the detection signal VO2R of the oxygen sensor 35 crosses the lean determination threshold value LTH and changes in the lean direction, the process proceeds to step S119.
It should be noted that the control device 51 proceeds from step S131 to step S119 also when the detection signal VO2R of the oxygen sensor 35 falls below the lean determination threshold value LTH from the state of the lean determination threshold value LTH or more.

In step S119, the control device 51 resets the estimated oxygen storage amount EST to the maximum oxygen storage amount STMAX, which is the oxygen storage amount ST in the saturated state.
In the next step S120, the control device 51 sets the maximum oxygen storage amount STMAX as an initial value, the intake air flow rate QA (air amount), and the estimated oxygen storage amount based on the exhaust air-fuel ratio upstream of the catalyst detected by the air-fuel ratio sensor 34. Start the update calculation of the quantity EST.

Further, in step S121, the control device 51 calculates the basic rich control P used in controlling the target air-fuel ratio TAF by control model calculation based on the engine speed NE, the intake air flow rate QA, and the target amount TST.
Then, in the next step S122, the control device 51 divides the basic rich control P obtained in step S121 by the average value DEHav (DEHav≧1.0) of the desorption speed correction value DEH to obtain the rich control P. , the target air-fuel ratio TAF is shifted in the rich direction to switch the target air-fuel ratio TAF to a rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio.

By setting the rich target air-fuel ratio, the control device 51 causes rich exhaust gas having a low oxygen concentration and a high concentration of CO and HC to flow into the first catalyst device 31 to desorb oxygen, thereby reducing the oxygen storage amount ST. Let
The desorption speed correction value DEH is used when the basic rich control P is reduced and corrected to desorb oxygen from the first catalyst device 31 when the oxygen desorption speed in the first catalyst device 31 is slower than the reference. is a correction value that reduces the rich level of the rich target air-fuel ratio.
Calculation of the desorption rate correction value DEH and average value DEHav will be described later in detail.

Next, the control device 51 proceeds to step S123 to calculate the target air-fuel ratio by control model calculation based on the engine speed NE, the intake air flow rate QA, the target amount TST, and the exhaust air-fuel ratio upstream of the catalyst detected by the air-fuel ratio sensor 34. A basic rich control I portion and a basic rich control D portion used in controlling the fuel ratio TAF are calculated.
Then, in the next step S124, the control device 51 divides the basic rich control I component and the basic rich control D component obtained in step S123 by the average value DEHav (DEHav≧1.0) of the desorption speed correction value DEH to obtain a rich control value. A control I minute and a rich control D minute are obtained, and the target air-fuel ratio TAF is changed in the lean direction according to the rich control I minute and the rich control D minute to bring the target air-fuel ratio TAF (rich target air-fuel ratio) close to the stoichiometric air-fuel ratio. .

Here, the desorption speed correction value DEH is obtained by reducing and correcting the basic rich control I and the basic rich control D when the oxygen desorption speed of the first catalyst device 31 is slower than the desired value. The speed at which the rich target air-fuel ratio approaches the stoichiometric air-fuel ratio when oxygen is desorbed from the device 31 is slowed down.
In other words, the control device 51 reduces and corrects the basic rich control P amount to reduce the shift amount of the target air-fuel ratio TAF in the rich direction, and then reduces and corrects the basic rich control I amount and the basic rich control D amount. Decrease the speed of returning the target air-fuel ratio TAF to the stoichiometric air-fuel ratio.
As a result, the desorption period is ensured while suppressing the rich level of the exhaust gas flowing into the first catalyst device 31 according to the slow desorption speed, and the output of the oxygen sensor 35 becomes rich when the desorption does not proceed sufficiently. Suppress that it is reversed to

In step S125, the control device 51 determines whether or not the detection signal VO2R of the oxygen sensor 35 has fallen below the lean determination threshold value LTH from the state of being equal to or greater than the lean determination threshold value LTH again.
Then, when the detection signal VO2R of the oxygen sensor 35 again falls below the lean determination threshold LTH from the state of being equal to or greater than the lean determination threshold LTH, the control device 51 returns to step S119.

On the other hand, if the detection signal VO2R of the oxygen sensor 35 has not changed from being equal to or greater than the lean determination threshold LTH to falling below the lean determination threshold LTH, the control device 51 proceeds to step S126.
In step S126, the control device 51 determines whether or not the detection signal VO2R of the oxygen sensor 35 has exceeded the rich determination threshold value RTH, that is, whether or not the output of the oxygen sensor 35 has reversed to rich.

If the detection signal VO2R of the oxygen sensor 35 is equal to or lower than the rich determination threshold value RTH, the control device 51 returns to step S123 and executes the processes of steps S123 and S124 to obtain the average value DEHav of the desorption rate correction value DEH. The target air-fuel ratio TAF is brought closer to the stoichiometric air-fuel ratio by the rich control I component and the rich control D component corrected based on .
In this way, the control device 51 repeats the processing of steps S123 and S124 to gradually bring the target air-fuel ratio TAF (rich target air-fuel ratio) closer to the stoichiometric air-fuel ratio.

On the other hand, when the control device 51 determines in step S126 that the detection signal VO2R of the oxygen sensor 35 exceeds the rich determination threshold value RTH, the process proceeds to step S127.
In step S127, the control device 51 sets the estimated oxygen storage amount EST when the output of the oxygen sensor 35 is reversed to rich as the oxygen storage amount ESTR at the time of rich reversal.

Then, in the next step S128, the control device 51 obtains the deviation between the rich inversion oxygen storage amount ESTR obtained in step S127 and the target amount TST as the desorption estimated error amount DEE, and stores it in the memory.
Note that the control device 51 stores the past plural times of the desorption estimated error amount DEE obtained in step S128.

After obtaining the desorption estimated error amount DEE, the controller 51 proceeds to step S129, and calculates the desorption rate correction value DEH (DEH≧1.0) based on the desorption estimated error amount DEE obtained in step S128. and store it in memory.
FIG. 11 is a graph showing the correlation between the desorption estimated error amount DEE and the desorption rate correction value DEH.

When the desorption estimated error amount DEE is zero, the desorption rate correction value DEH (DEH≧1.0) is set to 1.0 at which the PID control amount is not substantially corrected. The desorption speed correction value DEH is set to a larger value, that is, a value that corrects the PID control amount to a smaller value.
The characteristics of the desorption rate correction value DEH are the same as those of the adsorption rate correction value ADH.
Note that the control device 51 stores the desorption rate correction value DEH obtained in step S129 for a plurality of times in the past.

After obtaining the desorption speed correction value DEH in step S129, the controller 51 proceeds to step S130 and counts up the desorption speed correction value DEH calculation count counter.
As will be described later, the control device 51 obtains the moving average of the desorption speed correction values DEH as the average value DEHav of the desorption speed correction values DEH, and calculates the PID control amount (rich control P component , rich control I portion, rich control D portion).

In step S132, the control device 51 determines whether or not the target amount determination flag Ft is 1, as in step S131.
Then, the control device 51 waits while the target amount determination flag Ft is 1, that is, until various processes based on the data of the desorption speed correction value DEH are completed, and the target amount determination flag Ft is reset to 0. Then, the process proceeds from step S132 to step S107.

12 to 14 are flow charts showing procedures of the control device 51 for changing the target amount TST and diagnosing the first catalyst device 31 and the air-fuel ratio sensor 34. When the control device 51 is turned on by the engine switch, is activated.
First, in step S201, the controller 51 calculates the adsorption speed correction value ADH, the desorption speed correction value DEH, the adsorption speed correction value ADH calculation count counter, and the desorption speed correction value DEH stored in a memory such as an EEPROM. An initial process is performed to clear all of the calculation number counter, the estimated error amount ADE during adsorption, and the estimated error amount DEE during desorption.

Next, in step S202, the control device 51 determines whether or not the air-fuel ratio feedback control based on the output of the air-fuel ratio sensor 34 can be started. move on.
In step S203, the control device 51 determines whether or not the oxygen sensor 35 is active based on, for example, whether or not the detection signal VO2R has changed from lean output to rich output.
Then, if the oxygen sensor 35 is activated, the control device 51 advances to step S204 to determine whether the conditions for setting the target air-fuel ratio TAF for controlling the oxygen storage amount ST of the first catalyst device 31 are met. is determined, and if it is determined that the condition is satisfied, the process proceeds to step S205.

In step S205, the control device 51 determines whether or not the counter for the number of calculations of the adsorption speed correction value ADH has reached a predetermined value. It is determined whether or not the DEH calculation number counter has reached a predetermined value.
Here, if the counter for the number of calculations of the desorption speed correction value DEH has not reached the predetermined value, the control device 51 proceeds to step S233 and determines whether or not the engine switch (key switch, ignition switch) is turned off. If the engine switch is kept on, the process returns from step S233 to step S204.

When the control device 51 determines in step S205 that the number of calculations counter for the suction speed correction value ADH has reached a predetermined value, the process proceeds to step S206 to set the target amount determination flag Ft from 0 to 1.
Note that the target amount determination flag Ft set to 1 in step S206 is the flag determined in steps S131 and S132 of the flow charts of FIGS.

Next, the controller 51 proceeds to step S207 to update the average value ADHav of the adsorption speed correction value ADH.
Specifically, the control device 51 divides the total of all the stored adsorption speed correction values ADH by the number of calculations (the number of samples) to obtain an average, and determines the difference between the sample average value and the average value ADHav up to that point. Let the average value be the latest value of the average value ADHav.
ADHav (latest value) = {ADHav (current value) + (sum of adsorption speed correction value ADH/number of samples)}/2

Next, the controller 51 proceeds to step S208 to calculate and store an average value ADEav of the estimated error amounts ADE during adsorption. The controller 51 divides the sum of all the stored estimated error amounts ADE during adsorption to obtain the average value ADEav, and stores the obtained average value ADEav in the memory.
・ADEav = total sum of estimated error amount ADE during adsorption/number of samples

On the other hand, when the controller 51 determines in step S209 that the counter for the number of calculations of the desorption rate correction value DEH has reached a predetermined value, the process proceeds to step S210 to set the target amount determination flag Ft from 0 to 1.
Next, the controller 51 proceeds to step S211 to update the average value DEHav of the desorption rate correction values DEH.

Specifically, the control device 51 divides the total of all stored desorption rate correction values DEH by the number of calculations (the number of samples) to obtain an average, and calculates the average value of the sample and the average value DEHav up to that point. is the latest value of the average value DEHav.
・DEHav (latest value) = {DEHav (current value) + (sum of adsorption speed correction value DEH/number of samples)}/2

Next, the controller 51 proceeds to step S212 to calculate and store an average value DEEav of the desorption estimated error amount DEE. The controller 51 divides the sum of all the stored desorption estimated error amounts DEE by the number of samples to obtain an average value DEEav, and stores the obtained average value DEEav in the memory.
・DEEav = total sum of estimated error amount DEE during adsorption/number of samples

After obtaining the average value ADEav in step S208 or after obtaining the average value DEEav in step S212, the control device 51 proceeds to step S213.
In step S213, the control device 51 determines whether or not both the calculation counter for the adsorption speed correction value ADH and the calculation counter for the desorption speed correction value DEH have reached predetermined values.

Then, if at least one calculation number counter has not reached the predetermined value, the control device 51 proceeds to step S232 and resets the target amount determination flag Ft to zero.
On the other hand, when both the calculation number counter for the adsorption speed correction value ADH and the calculation number counter for the desorption speed correction value DEH have reached the predetermined values, the control device 51 proceeds from step S213 to step S214.

In step S214, the control device 51 counts up a counter for calculating the average value ADHav of the adsorption speed correction value ADH and the average value DEHav of the desorption speed correction value DEH (hereinafter referred to as the average correction value calculation counter).
Next, in step S215, the control device 51 clears the calculation frequency counter of the adsorption speed correction value ADH and the calculation frequency counter of the desorption speed correction value DEH. All the data of the value ADH and the desorption rate correction value DEH are cleared.
Further, in step S217, the control device 51 clears all the stored data of the estimated adsorption error amount ADE and the desorption estimated error amount DEE.

Next, in step S218, the control device 51 determines whether or not the average correction value calculation count counter has reached a predetermined value. Reset to 0.
On the other hand, if the average correction value calculation frequency counter has reached the predetermined value, the control device 51 proceeds to step S219, where the average value ADHav of the adsorption speed correction values ADH is equal to or greater than the first threshold TH1, and the desorption speed correction It is determined whether or not the average value DEHav of the values DEH is greater than or equal to the second threshold TH2.

FIG. 15 is a diagram showing the correlation between the average value ADHav of the adsorption speed correction values ADH, the average value DEHav of the desorption speed correction values DEH, and the first threshold TH1 and the second threshold TH2.
Region B, where the average value ADHav is equal to or greater than the first threshold TH1 and the average value DEHav is equal to or greater than the second threshold TH2, is a state in which both the desorption speed and the adsorption speed are slow enough to require a change in the target amount TST. be.

When the average value ADHav is equal to or greater than the first threshold TH1 and the average value DEHav is equal to or greater than the second threshold TH2, the control device 51 proceeds to step S220 (catalyst diagnostic unit) to determine the average value of the adsorption estimated error amount ADE. It is determined whether or not both the minimum value of the stored data of ADEav and the minimum value of the stored data of the average value DEEav of the desorption estimated error amount DEE are equal to or greater than the third threshold TH3.
In other words, in step S220, the controller 51 determines that the average value ADEav of the estimated error amounts ADE during adsorption is greater than the first abnormality determination value, and that the average value DEEav of the estimated error amounts DEE during desorption is the second abnormality determination value. Determines whether the state is greater than the value.
FIG. 16 shows the correlation between the stored average value ADEav and average value DEEav and the third threshold TH3.

The third threshold TH3 is a threshold for determining a deterioration failure of the first catalyst device 31, and is the minimum value of the average values ADEav obtained a plurality of times in the past and the minimum value of the average values DEEav obtained a plurality of times in the past. are both equal to or greater than the third threshold TH3, that is, if even the smallest value among the plurality of stored average values ADEav and DEEav is equal to or greater than the third threshold TH3, deterioration failure of the first catalyst device 31 is adapted to estimate
Therefore, when the control device 51 determines in step S220 that both the minimum value of the average value ADEav and the minimum value of the average value DEEav are equal to or greater than the third threshold value TH3, the process proceeds to step S221, and the first catalyst device After determining the occurrence of the deterioration failure of No. 31 and storing the diagnosis result of the deterioration failure, the process proceeds to step S222.

On the other hand, if the deterioration failure condition that both the minimum value of the average value ADEav and the minimum value of the average value DEEav are equal to or greater than the third threshold TH3 is not satisfied, the control device 51 bypasses step S221 and proceeds to step S222. .
The control device 51 updates the target amount TST of the oxygen storage amount ST and the maximum oxygen storage amount STMAX in step S222 (target amount change unit).

The control device 51 updates the target amount TST according to the following formula.
・Target amount TST (updated value) = Target amount TST (current value) -|Maximum value of stored values of estimated error amount ADE during adsorption and estimated error amount DEE during desorption|
Further, the control device 51 updates the maximum oxygen storage amount STMAX according to the following formula.
・Maximum oxygen storage amount STMAX (updated value) = target amount TST (updated value) x 2

That is, due to deterioration of the first catalyst device 31, the maximum oxygen storage amount STMAX decreases, the adsorption speed and the desorption speed decrease, and the estimated error amount ADE during adsorption and the estimated error amount DEE during desorption increase. The target amount TST and the maximum oxygen storage amount STMAX are decreased based on the estimated error amount ADE and the desorption estimated error amount DEE, and updated to the target amount TST and maximum oxygen storage amount STMAX suitable for the state of deterioration.

FIG. 17 shows the behavior of the output of the oxygen sensor 35, the estimated oxygen storage amount EST, and the target air-fuel ratio TAF when both the adsorption speed and the desorption speed become slower than the reference due to deterioration of the first catalyst device 31. show.
When both the adsorption speed and the desorption speed become slower than the standard due to the deterioration of the first catalyst device 31, oxygen that cannot be adsorbed by the first catalyst device 31 is generated, and the lean reversal is accelerated. is generated, and the estimated oxygen storage amount EST is reset to the maximum oxygen storage amount STMAX, which is larger than the actual amount when the output of the oxygen sensor 35 is reversed to lean. , and the desorption estimation error amount DEE is generated.

Then, the control device 51 updates the adsorption speed correction value ADH and the desorption speed correction value DEH so as to reduce the estimated error amount ADE during adsorption and the estimated error amount DEE during desorption, but the target amount TST is reached. It will be in a state where it cannot be done.
Therefore, when both the adsorption speed correction value ADH and the desorption speed correction value DEH are larger than predetermined values, the control device 51 determines that the adsorption speed and the desorption speed are slowed due to deterioration of the first catalytic device 31. Then, the maximum oxygen storage amount STMAX and the target amount TST are lowered so that the target amount TST can be reached.

Next, the control device 51 advances to step S223, and based on the deviation ΔTST (the degree of change of the target amount TST) between the target amount TST before update and the target amount TST after update in the updating process of the target amount TST in step S222, A rich target air-fuel ratio in the air-fuel ratio enrichment control after fuel cut (hereinafter referred to as a rich target air-fuel ratio at first return) is obtained and stored in a memory.
When the maximum oxygen storage amount STMAX (target amount TST) decreases due to deterioration of the first catalyst device 31 (exhaust gas purifying catalyst), the oxygen storage amount ST decreases when resuming fuel injection from a fuel cut. The rich level of the rich target air-fuel ratio required for rapid reduction can be reduced. Therefore, the control device 51 reduces the rich level of the return-time rich target air-fuel ratio as the amount of decrease in the target amount TST increases.

In the next step S224, the controller 51 clears all the stored average value ADHav of the adsorption speed correction values ADH and average value DEHav of the desorption speed correction values DEH.
That is, the control device 51 proceeds to step S224 when the target amount TST is updated. By updating the target amount TST, the adsorption estimated error amount ADE and the desorption estimated error amount DEE are reduced, and the target amount TST is reduced. Since the average value ADHav of the adsorption rate correction values ADH and the average value DEHav of the desorption rate correction values DEH obtained before updating are no longer suitable, the control device 51 calculates the stored average value ADHav of the adsorption rate correction values ADH and All the average values DEHav of the desorption rate correction values DEH are cleared.

On the other hand, in step S219, the controller 51 satisfies the conditions that the average value ADHav of the adsorption rate correction values ADH is equal to or greater than the first threshold TH1 and the average value DEHav of the desorption rate correction values DEH is equal to or greater than the second threshold TH2. If it is determined that it is not, the process proceeds to step S225.
In step S225, the controller 51 determines that the average value ADHav of the adsorption speed correction value ADH (the degree of correction of the lean target air-fuel ratio) is greater than or equal to the first failure determination threshold THM1 (first failure determination threshold THM1>first threshold TH1), Also, whether or not the average value DEHav of the desorption speed correction value DEH (correction degree of the rich target air-fuel ratio) is less than a second normality determination threshold THN2 (second normality determination threshold THN2<second threshold TH2), that is, It is determined whether or not it corresponds to the D area in FIG.

Then, when the average value ADHav of the adsorption speed correction values ADH is equal to or greater than the first failure determination threshold THM1 and the average value DEHav of the desorption speed correction values DEH is less than the second normality determination threshold THN2, the control device 51 Proceeding to step S226, it is determined that the air-fuel ratio sensor 34 has a response failure from lean to rich, and after the determination result of the response failure of the air-fuel ratio sensor 34 is stored in the memory, the process proceeds to step S227.
On the other hand, if the average value ADHav of the adsorption speed correction values ADH is equal to or greater than the first failure determination threshold THM1 and the average value DEHav of the desorption speed correction values DEH is less than the second normal determination threshold THN2, the control Device 51 bypasses step S226 and proceeds to step S227.

In step S227, the control device 51 determines that the average value ADHav of the adsorption speed correction values ADH is less than the first normality determination threshold THN1 (first failure determination threshold THM1<first threshold TH1) and that the desorption speed correction value DEH It is determined whether or not the average value DEHav is greater than or equal to a second failure determination threshold THM2 (second failure determination threshold THM2>second threshold TH2), that is, whether or not it falls under region C in FIG.
Then, when the average value ADHav of the adsorption speed correction values ADH is less than the first normality determination threshold THN1 and the average value DEHav of the desorption speed correction values DEH is equal to or greater than the second failure determination threshold THM2, the control device 51 Proceeding to step S228, a rich→lean response failure of the air-fuel ratio sensor 34 is determined, and the determination result of the response failure of the air-fuel ratio sensor 34 is stored in the memory.
The functions of steps S225 to S228 in the control device 51 correspond to the pre-catalyst sensor diagnosis section.

On the other hand, the control device 51 satisfies the conditions that the average value ADHav of the adsorption rate correction values ADH is less than the first normality determination threshold THN1 and the average value DEHav of the desorption rate correction values DEH is equal to or greater than the second failure determination threshold THM2. If not, the process proceeds to step S229.
For example, if the rich→lean response of the air-fuel ratio sensor 34 is normal and the lean→rich response is delayed, then when the target air-fuel ratio TAF is changed in the rich direction to approach the stoichiometric air-fuel ratio, the air-fuel ratio Due to the delay in the output change of the fuel ratio sensor 34, the response of the estimated oxygen storage amount EST based on the output of the air-fuel ratio sensor 34 is delayed, and the adsorption estimated error amount ADE increases, while the desorption estimated error amount DEE does not increase. .
Therefore, when one of the adsorption speed correction value ADH and the desorption speed correction value DEH is at a normal level and the other is at a failure level, the control device 51 can diagnose a response failure in the air-fuel ratio sensor 34 .

In step S229, the control device 51 sets a rich target air-fuel ratio (hereinafter referred to as a second A return rich target air-fuel ratio) is obtained and stored in a memory.
In other words, the larger the average value ADHav of the adsorption speed correction value ADH, the slower the adsorption speed. Therefore, it is possible to reduce the rich level in the air-fuel ratio enrichment control for quickly returning the oxygen storage amount ST to the vicinity of the target amount TST after the fuel cut.
Therefore, as the average value ADHav of the adsorption speed correction value ADH increases, the control device 51 decreases the rich level of the second return rich target air-fuel ratio to bring it closer to the stoichiometric air-fuel ratio.

After the processing in steps S224, S228, and S229, the control device 51 proceeds to step S230 and clears the average correction value calculation frequency counter.
Further, in step S231, the control device 51 clears the average value ADEav of the estimated error amount ADE during adsorption and the average value DEEav of the estimated error amount DEE during desorption, and then proceeds to step S232 to set the target amount determination flag. Reset Ft to 0.

Next, in step S233, the control device 51 determines whether or not the engine switch is turned off. If the engine switch is turned on, the process returns to step S204. proceed to
In step S234, in preparation for the next operation of the internal combustion engine 11, the control device 51 sets the average value ADHav of the adsorption speed correction value ADH, the average value DEHav of the desorption speed correction value DEH, the target amount TST, and the maximum oxygen storage amount STMAX. is stored in memory.

The control device 51 stores the average value ADHav of the adsorption speed correction value ADH, the average value DEHav of the desorption speed correction value DEH, the target amount TST, and the maximum oxygen storage amount STMAX stored in step S234 when the internal combustion engine 11 is operated next time. It is read in step S104 and used to control the target air-fuel ratio TAF.

FIG. 18 is a time chart for explaining the arithmetic processing of the average value ADHav of the adsorption speed correction value ADH and the average value ADEav of the estimated error amount ADE during adsorption.
The average value DEHav of the desorption rate correction value DEH and the average value DEEav of the estimated error amount DEE during desorption are calculated by calculating the average value ADHav of the adsorption rate correction value ADH and the average value ADEav of the estimated error amount ADE during adsorption. Since it is performed in the same manner as the arithmetic processing, detailed description is omitted.

In the example shown in FIG. 18, at time t1 when the output of the oxygen sensor 35 is lean-reversed, the adsorption estimated error amount ADE is 16 and the lean-reversal oxygen storage amount ESTL is 34, and the control device 51 Based on the adsorption estimated error amount ADE=16, the adsorption speed correction value ADH is set to 1.20, and the number of calculations of the adsorption speed correction value ADH is increased to one.
At time t2 and time t3, which are lean reversal timings thereafter, the control device 51 obtains the estimated error amount ADE during lean reversal and the oxygen storage amount ESTL during lean reversal, and further corrects the adsorption speed based on the estimated error amount ADE during adsorption. Find the value ADH.

Then, at time t3 when the number of calculations of the adsorption estimated error amount ADE and the number of calculations of the adsorption speed correction value ADH reach three times, which is the specified number of times, the control device 51 calculates the three estimated errors including the latest value. An average value ADEav of the amount ADE and an average value ADHav of three adsorption speed correction values ADH including the latest value are calculated.
Here, when the average value ADHav of the adsorption speed correction values ADH and the average value DEHav of the desorption speed correction values DEH obtained separately are included in the region B (see FIG. 15), the control device 51 controls the average value ADEav or The target amount TST is updated based on the average value DEEav, and the maximum oxygen storage amount STMAX is updated based on the target amount TST.

The flowchart of FIG. 19 shows the process of setting the rich target air-fuel ratio (recovery rich target air-fuel ratio changing section) in the air-fuel ratio enrichment control after fuel cut (recovery rich control section).
When the control device 51 is powered on and activated, in step S301, the first return rich target air-fuel ratio based on the updated amount of the target amount TST and the second return time based on the average value ADHav of the adsorption speed correction value ADH. The rich target air-fuel ratio is reset to zero, which is the initial value, and the final return rich target air-fuel ratio is set to the reference rich air-fuel ratio.

Next, the control device 51 proceeds to step S302 and determines whether or not conditions for starting the air-fuel ratio feedback control are satisfied.
Then, when the conditions for starting the air-fuel ratio feedback control are established, the control device 51 determines whether or not the oxygen sensor 35 is activated in step S303, and proceeds to step S304 when the oxygen sensor 35 is activated.

In step S304, the control device 51 determines whether or not a fuel cut to temporarily stop the fuel injection by the fuel injection valve 21, for example, a fuel cut during deceleration operation, is being performed.
The control device 51 waits until the fuel cut is performed, and when it determines that the fuel cut is to be performed, the process proceeds to step S305.

In step S305, the control device 51 determines whether or not the conditions for executing the enrichment control after the fuel cut are satisfied.
Then, if the conditions for executing rich control after fuel cut are satisfied, the control device 51 proceeds to step S306 (recovery rich control section, recovery rich target air-fuel ratio change section), and finally returns. An hour-rich target air-fuel ratio setting process is performed.

Specifically, the control device 51 selects the air-fuel ratio closest to the stoichiometric air-fuel ratio among the first recovery rich target air-fuel ratio, the second recovery rich target air-fuel ratio, and the reference rich air-fuel ratio, in other words, the rich level is set as the final return-time rich target air-fuel ratio.
The control device 51 controls the air-fuel ratio of the internal combustion engine 11 to the final return-time rich target air-fuel ratio for a predetermined period from the end of the fuel cut (the restart of fuel injection).
As a result, it is possible to perform enrichment control in accordance with the increase in the oxygen storage amount ST during fuel cut, thereby suppressing deterioration in fuel efficiency and exhaust gas performance due to excessive enrichment.

Next, in step S307, the control device 51 determines whether or not the enrichment control after the fuel cut has been started. If the enrichment control after the fuel cut has been started, the process proceeds to step S308.
In step S308, the control device 51 determines whether or not the output of the oxygen sensor 35 has changed in the rich direction exceeding a threshold value for determining the release of the enrichment control. Proceed to step S310.

Also when the control device 51 determines in step S307 that the enrichment control after the fuel cut has not started, the process proceeds to step S310.
In step S310, the control device 51 determines whether or not fuel cut has been re-entered. If there is no re-entry, the process returns to step S307, and if fuel cut has been re-entered, the process returns to step S305.
When the control device 51 determines in step S308 that the output of the oxygen sensor 35 exceeds the threshold value for determining the cancellation of the enrichment control and has changed in the rich direction, the process proceeds to step S309, and the enrichment after the fuel cut is performed. Release the control (rich control section at return).

The time chart of FIG. 20 shows the behavior of the output of the oxygen sensor 35, the target air-fuel ratio, and the fuel cut flag when the enrichment control after the fuel cut is performed.
When the fuel cut condition is satisfied, the fuel cut flag is raised to 1 and fuel cut is performed (time t1).
When the fuel cut is performed, air (maximum lean exhaust gas) flows into the first catalyst device 31, the oxygen storage amount ST increases, and the output of the oxygen sensor 35 changes in the lean direction.

When the conditions for resuming the fuel supply are met, the fuel cut flag is lowered to 0 (time t2), and the enrichment control for quickly returning the oxygen storage amount ST of the first catalyst device 31 to the vicinity of the target amount TST is started. be done.
The rich target air-fuel ratio in this enrichment control is set in step S306 described above, and is changed to an air-fuel ratio closer to the stoichiometric air-fuel ratio if the first catalyst device 31 has deteriorated.
When desorption progresses under the enrichment control and the oxygen storage amount ST decreases, the output of the oxygen sensor 35 exceeds the threshold value for determining the cancellation of the enrichment control and changes in the rich direction (time t3). is released.
By changing the rich target air-fuel ratio in the enrichment control as described above, the control device 51 suppresses excessive enrichment of the air-fuel ratio, and quickly increases the oxygen storage amount ST to the target amount after the fuel cut. It can be returned to near TST.

Each of the technical ideas described in the above embodiments can be used in appropriate combination as long as there is no contradiction.
Although the content of the present invention has been specifically described with reference to preferred embodiments, it is obvious that those skilled in the art can make various modifications based on the basic technical idea and teaching of the present invention. is.

In the above embodiment, the control device 51 changes all of the PID control amounts when the adsorption speed or the desorption speed slows down, but changes some of the P, I, and D minutes. can be done.
Further, for example, when the adsorption speed becomes slow, the control device 51 gradually enriches the target air-fuel ratio TAF by lean control I minutes and lean control D minutes from the time when the output of the oxygen sensor 35 is reversed to rich. After that, the target air-fuel ratio TAF is gradually returned to the stoichiometric air-fuel ratio by lean control I and lean control D, thereby changing the lean target air-fuel ratio in a mountain shape. In other words, the control of the target air-fuel ratio TAF is not limited to a control pattern in which when the output of the oxygen sensor 35 is reversed, it is shifted to rich or lean, and then gradually returned to the stoichiometric air-fuel ratio.

In addition, the control device 51 calculates the correlation between the adsorption estimated error amount ADE and the adsorption speed correction value ADH, and the correlation between the desorption estimated error amount DEE and the desorption speed correction value DEH as the adsorption speed correction value ADH, Learning can be performed based on the correction result of the desorption rate correction value DEH.
Further, the control device 51 is a warning device that warns the driver of the vehicle of the occurrence of an abnormality (failure) when diagnosing deterioration failure of the first catalyst device 31 or when diagnosing response deterioration of the air-fuel ratio sensor 34. can be activated.

Further, the control device 51 can stop air-fuel ratio feedback control and switch to feedforward control when diagnosing response deterioration of the air-fuel ratio sensor 34 .
Also, the target amount TST is not limited to half the maximum oxygen storage amount STMAX, and the ratio between the maximum oxygen storage amount STMAX and the target amount TST can be set arbitrarily.

9 and the correlation between the estimated error amount DEE during desorption and the desorption rate correction value DEH in FIG. 11, the estimated error amount ADE, The correction values ADH and DEH increase proportionally to the increase in DEE, but the correlation between the estimated error amounts ADE and DEE and the correction values ADH and DEH can be appropriately set and is not limited to a proportional relationship.
Further, the control device 51 can limit the adsorption speed correction value ADH and the desorption speed correction value DEH to the upper limit values or less.

In addition, the control device 51 can perform average value calculation based on data excluding the maximum value and/or minimum value in the sample data when calculating the average value of various parameters.
Further, the control device 51 can determine deterioration failure of the first catalyst device 31 (exhaust gas purification catalyst) when the adsorption speed correction value ADH and the desorption speed correction value DEH exceed the threshold values.

Reference Signs List 11 Internal combustion engine 21 Fuel injection valve 31 First catalyst device (exhaust gas purifying catalyst) 34 Air-fuel ratio sensor (pre-catalyst sensor) 35 Oxygen sensor (post-catalyst sensor) 51 Control device (air fuel ratio controller)

Claims (7)

an exhaust gas purification catalyst for purifying exhaust gas from an internal combustion engine;
a pre-catalyst sensor that detects the air-fuel ratio of the exhaust gas flowing into the exhaust gas purifying catalyst;
a post-catalyst sensor for detecting the air-fuel ratio of the exhaust gas flowing out from the exhaust gas purifying catalyst;
An air-fuel ratio control device applied to the internal combustion engine comprising
The target air-fuel ratio is set to a lean target air-fuel ratio that is leaner than the stoichiometric air-fuel ratio when the output of the post-catalyst sensor is reversed to rich, and the target air-fuel ratio is set to the stoichiometric when the output of the post-catalyst sensor is reversed to lean. a target air-fuel ratio setting unit that sets a rich target air-fuel ratio that is richer than the air-fuel ratio;
An oxygen storage amount estimating unit for estimating an oxygen storage amount in the exhaust gas purification catalyst,
resetting the estimated amount of the oxygen storage amount to a predetermined minimum amount when the output of the post-catalyst sensor is inverted to rich, and resetting the estimated amount of the oxygen storage amount when the output of the post-catalyst sensor is inverted to lean; resetting to a predetermined maximum amount, setting the minimum amount or the maximum amount as an initial value , and updating the estimated amount of the oxygen storage amount based on the output of the pre-catalyst sensor ;
the oxygen storage amount estimation unit;
The rich target air-fuel ratio is corrected based on the deviation between the estimated amount and the target oxygen storage amount when the output of the post-catalyst sensor is reversed to rich, and when the output of the post-catalyst sensor is reversed to lean. a target air-fuel ratio correction unit that corrects the lean target air-fuel ratio based on the deviation between the estimated amount and the target amount of
an air-fuel ratio control unit that generates an air-fuel ratio control signal based on the air-fuel ratio detected by the pre-catalyst sensor and the target air-fuel ratio;
An air-fuel ratio control device for an internal combustion engine. The target air-fuel ratio correction unit
reducing the rich level of the rich target air-fuel ratio when the estimated amount when the output of the post-catalyst sensor is reversed to rich is greater than the target amount;
reducing the lean level of the lean target air-fuel ratio when the estimated amount when the output of the post-catalyst sensor is reversed to lean is smaller than the target amount;
2. An air-fuel ratio control device for an internal combustion engine according to claim 1. The target air-fuel ratio setting unit
shifting the target air-fuel ratio to be leaner than the stoichiometric air-fuel ratio when the output of the post-catalyst sensor is reversed to rich, and then gradually bringing the target air-fuel ratio closer to the stoichiometric air-fuel ratio;
shifting the target air-fuel ratio to be richer than the stoichiometric air-fuel ratio when the output of the post-catalyst sensor is turned lean, and then gradually bring the target air-fuel ratio closer to the stoichiometric air-fuel ratio;
The target air-fuel ratio correction unit
When the estimated amount when the output of the post-catalyst sensor is reversed to rich is greater than the target amount, the rich shift amount of the target air-fuel ratio is decreased and the target air-fuel ratio is brought closer to the stoichiometric air-fuel ratio after the rich shift. slow down,
When the estimated amount when the output of the post-catalyst sensor is reversed to lean is smaller than the target amount, the lean shift amount of the target air-fuel ratio is decreased and the target air-fuel ratio is brought closer to the stoichiometric air-fuel ratio after the lean shift. slow down,
3. An air-fuel ratio control device for an internal combustion engine according to claim 2. A target amount changing unit that decreases the target amount when the degree of correction of the lean target air-fuel ratio is greater than a first threshold and the degree of correction of the rich target air-fuel ratio is greater than a second threshold. An air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 3. A difference between the estimated amount and the target amount when the output of the post-catalyst sensor is reversed to lean is larger than a first abnormality determination value, and the estimated amount when the output of the post-catalyst sensor is reversed to rich. and the target amount, further comprising a catalyst diagnostic unit that determines occurrence of a failure of the exhaust gas purification catalyst when a deviation between the target amount and the second abnormality determination value is greater than a second abnormality determination value. air-fuel ratio control device for an internal combustion engine. When one of the degree of correction of the rich target air-fuel ratio and the degree of correction of the lean target air-fuel ratio exceeds a failure determination threshold and the other falls below a normality determination threshold that is smaller than the failure determination threshold, the pre-catalyst sensor is 6. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, further comprising a pre-catalyst sensor diagnostic section for determining occurrence of failure. a return-time rich control unit that controls the air-fuel ratio of the internal combustion engine after the fuel cut is completed to a return-time rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio;
a return-time rich target air-fuel ratio changing unit that changes the return-time rich target air-fuel ratio based on one of a correction degree of the lean target air-fuel ratio and a change degree of the target amount;
5. The air-fuel ratio control system for an internal combustion engine according to claim 4 , further comprising:

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