JP7204426B2 - Fuel injection control device for internal combustion engine - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel injection control device for an internal combustion engine, and more particularly to technology for controlling an air-fuel ratio after a fuel cut is completed.

The exhaust gas purification control device for an internal combustion engine disclosed in Patent Document 1 includes first sensors for detecting the air-fuel ratio or rich/lean of the exhaust gas on the upstream and downstream sides of the upstream side catalyst and on the downstream side of the downstream side catalyst, respectively. ~ In an internal combustion engine equipped with a third sensor, rich control is performed after the fuel cut is completed, and the target air-fuel ratio on the upstream side of the upstream catalyst ( The degree of richness of the air-fuel ratio is changed by changing the target output of the first sensor), and then when the output of the third sensor (the air-fuel ratio on the downstream side of the downstream side catalyst) becomes richer than the judgment value, the rich The control is ended and the normal air-fuel ratio F/B control is resumed.

Japanese Patent Application Laid-Open No. 2002-276433

By the way, when the fuel injection control device determines the release timing of the rich control after fuel cut based on the output of the post-catalyst sensor that detects the exhaust air-fuel ratio downstream of the catalyst, the response speed of the post-catalyst sensor changes due to deterioration etc. Then (in other words, when the output of the post-catalyst sensor changes over time), there is a possibility that the period of the rich control deviates from the optimum value and the exhaust performance deteriorates.

The present invention has been made in view of the conventional circumstances, and its object is to provide a fuel injection control device that determines the release timing of rich control after a fuel cut based on the output of a post-catalyst sensor. To suppress deterioration of exhaust performance when speed changes.

According to one aspect of the present invention, the fuel injection control device for an internal combustion engine reduces the air-fuel ratio of the internal combustion engine during a rich control period from the end of the fuel cut until the output of the post-catalyst sensor reaches the release determination value. a rich control unit that controls the air-fuel ratio to a rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio; and a fuel ratio changing unit.

According to the present invention, the rich air-fuel ratio is changed in accordance with the change in the rich control period caused by the change in the response speed of the post-catalyst sensor, so deterioration of exhaust performance can be suppressed.

1 is a system configuration diagram of a vehicle internal combustion engine; FIG. 4 is a time chart showing rich control after fuel cut; 5 is a time chart showing changes in the rich control period due to deterioration of the oxygen sensor; 4 is a flowchart showing a procedure for changing the rich target air-fuel ratio according to the degree of deterioration DE; 4 is a time chart showing an output waveform of an oxygen sensor at the start of fuel cut; 4 is a time chart showing changes in output change speed due to deterioration of an oxygen sensor; FIG. 4 is a graph showing the correlation among maximum value ΔMAX, reference value, and intake air flow rate QA; FIG. 4 is a graph showing the correlation among maximum value ΔMAX, reference value, and intake air flow rate QA; 4 is a time chart showing extension of the threshold reaching time due to a decrease in the response speed of the oxygen sensor; 4 is a time chart showing extension of threshold reaching time due to extension of dead time of the oxygen sensor; FIG. 4 is a graph showing the correlation between the degree of deterioration DE and the rich target air-fuel ratio; FIG. 4 is a time chart showing the timing of calculating the degree of deterioration DE and the timing of changing the rich target air-fuel ratio based on the degree of deterioration DE; 4 is a flowchart showing a procedure of rich control after fuel cut; 4 is a time chart showing timing of rich control after fuel cut and change of rich target air-fuel ratio; 4 is a time chart showing the timing of calculating the degree of deterioration DE and the timing of changing the rich target air-fuel ratio based on the degree of deterioration DE; 4 is a flowchart showing a procedure for changing the rich target air-fuel ratio according to the degree of deterioration DE; 4 is a time chart showing an output waveform of an oxygen sensor when fuel injection is restarted;

An embodiment of a fuel injection 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 to which a fuel injection control device according to the present invention is applied. The internal combustion engine 11 is a gasoline engine for vehicles.
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 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 combusted by the spark generated by the spark plug 23, and the exhaust gas generated in the combustion chamber 17 by the combustion is discharged to the exhaust pipe 26 provided in each cylinder through the exhaust valve 25. Ejected.
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) contained in exhaust gas by oxidation and reduction, and has oxygen storage capacity.

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 (exhaust 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 detects the exhaust gas of the first catalyst device 31. It is a post-catalyst sensor that detects the air-fuel ratio of the exhaust gas flowing out from the purification catalyst (exhaust air-fuel ratio).

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.
Note that the control device 51 includes a non-volatile memory such as an EEPROM in which data can be erased and rewritten.

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 is a fuel injection control device that generates an air-fuel ratio control signal, outputs it to the fuel injection valve 21 (fuel injection device), and controls fuel injection by the fuel injection valve 21 .

In controlling the fuel injection amount, the control device 51 performs air-fuel ratio feedback control for controlling 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. do.
Further, the control device 51 stops the fuel injection by the fuel injection valve 21, for example, when the accelerator opening ACC of the internal combustion engine 11 is fully closed and the engine speed NE is higher than the cut speed. When the engine rotation speed NE falls below the recovery rotation speed or the accelerator pedal is stepped on, fuel cut (deceleration fuel cut) is performed to resume (reset) the fuel injection by the fuel injection valve 21 .

The exhaust gas purification catalyst (three-way catalyst) of the first catalyst device 31 can efficiently purify both rich components (HC, CO) and lean components (NOx) when the oxygen storage amount is near the stoichiometric air-fuel ratio.
However, during the fuel cut, the oxygen sucked into the cylinder is discharged as it is to the exhaust system, so the oxygen storage amount of the first catalyst device 31 excessively increases during the fuel cut and becomes saturated, and after the fuel cut ends reduction performance of

Therefore, the control device 51 temporarily controls the air-fuel ratio to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio when ending the fuel cut and resuming the fuel injection, and the oxygen storage of the first catalyst device 31. Implement rich control to quickly reduce the amount.
That is, the control device 51 changes the air-fuel ratio of the internal combustion engine 11 to a rich air-fuel ratio richer than the stoichiometric air-fuel ratio during a period from the end of the fuel cut until the output of the oxygen sensor 35 (post-catalyst sensor) reaches the release determination value. It has a software function as a rich control unit that controls

FIG. 2 is a time chart showing control characteristics of the rich control section.
In FIG. 2, when the fuel cut condition is satisfied at time t1, the control device 51 starts the fuel cut to stop the fuel injection by the fuel injection valve 21, and the condition (recover condition) to restart the fuel injection at time t2. is established, the fuel injection by the fuel injection valve 21 is restarted.

During the fuel cut from time t1 to time t2, the oxygen sucked into the cylinder is discharged to the exhaust system as it is, so the oxygen storage amount of the first catalytic device 31 increases, and the output of the oxygen sensor 35 is turned lean. do.
When resuming fuel injection from time t2, the control device 51 controls the air-fuel ratio to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio in order to quickly reduce the oxygen storage amount of the first catalyst device 31. Enforce controls.

The rich control is divided into a first stage of enriching the air-fuel ratio by feedforward control and a second stage of enriching the air-fuel ratio by feedback control.
In the first stage, the control device 51 switches the injection rate (target equivalence ratio) TFBYA, which is a correction term for the basic fuel injection pulse width TP, from the default value of 1.0 to a predetermined value higher than 1.0, and changes the injection rate TFBYA (TFBYA >1.0) is continued for a predetermined period (from time t2 to time t3 in FIG. 2).

The control device 51 calculates the basic fuel injection pulse width TP as an injection pulse width corresponding to the amount of fuel required to generate a mixture having a stoichiometric air-fuel ratio, and uses this basic fuel injection pulse width TP as an injection rate TFBYA (TFBYA>1.0). By performing a rich spike that corrects an increase with , the air-fuel ratio is feedforward controlled to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio.
When the first stage rich control ends at time t3, that is, after the injection rate TFBYA is made higher than 1.0 for a predetermined period from the end of the fuel cut and the injection amount is corrected to increase, the control device 51 performs the second Go to stage rich control.

In the second-stage rich control, the control device 51 returns the injection rate TFBYA to 1.0, and then sets the target air-fuel ratio in the air-fuel ratio feedback control to the rich target richer than the stoichiometric air-fuel ratio (excess air ratio λ=1). Air-fuel ratio feedback control is performed to correct the fuel injection pulse width TI so that the air-fuel ratio detected by the air-fuel ratio sensor 34 approaches the rich target air-fuel ratio.
At time t4 during the air-fuel ratio feedback control based on the rich target air-fuel ratio, when the output of the oxygen sensor 35 reaches the cancellation determination value defined as the condition for canceling the rich control, the control device 51 performs the rich control (second stage), and shift to air-fuel ratio feedback control in which the stoichiometric air-fuel ratio is set as the target air-fuel ratio.

Here, if the oxygen sensor 35 deteriorates and the response speed becomes slow, the output of the oxygen sensor 35 reaches the release determination value during the rich control later than in the initial state, and the rich control is carried out for an excessively long time. It will be.
Note that deterioration in the present application means a state in which the response speed (output change, transient characteristic) of the oxygen sensor 35 changes over time from the initial state, and includes changes over time within the normal range (within the allowable range).
FIG. 3 shows the release timing of the rich control when the oxygen sensor 35 is in the initial state (non-deteriorated state) and the release timing of the rich control when the response speed of the oxygen sensor 35 becomes slow due to aging (deterioration). and

When the oxygen sensor 35 deteriorates, the change in the output of the oxygen sensor 35 when the exhaust air-fuel ratio downstream of the first catalyst device 31 reverses from rich to lean with the start of fuel cut is delayed, and the rich control causes the first catalyst device to change. 31, the output change of the oxygen sensor 35 is delayed when the exhaust air-fuel ratio is reversed from lean to rich.
Therefore, when the oxygen sensor 35 deteriorates, the output of the oxygen sensor 35 reaches the release determination value during the rich control later than in the initial state (non-deteriorated state), and the period during which the rich control (second stage) is performed is reduced. becomes excessively long.

If the rich control period becomes excessively long, the amount of oxygen storage in the first catalyst device 31 is excessively reduced. emissions will increase.
Therefore, when the oxygen sensor 35 deteriorates and the response speed of the output change to the change in the exhaust air-fuel ratio becomes slow, the control device 51 performs the rich control after the fuel cut so that the oxygen storage amount of the first catalyst device 31 becomes excessive. , the rich target air-fuel ratio of the rich control (second stage) is changed so as to approach the stoichiometric air-fuel ratio in accordance with the decrease in the response speed.
That is, the control device 51 has a software function as a rich air-fuel ratio changing unit that changes the rich air-fuel ratio (rich target air-fuel ratio) in the rich control after fuel cut based on the response speed of the oxygen sensor 35 (post-catalyst sensor). Prepare as

Note that the control device 51 can perform the rich control after the end of the fuel cut by dividing it into a first stage by feedforward control and a second stage by feedback control. It is also possible to perform only one of feedback control.
Also when the rich control is performed by feedforward control, the control device 51 changes the rich target air-fuel ratio in the feedforward control so as to approach the stoichiometric air-fuel ratio in accordance with the decrease in the response speed of the oxygen sensor 35. can be done.

FIG. 4 is a flow chart showing the procedure of correction processing of the rich target air-fuel ratio in the rich control after the end of the fuel cut.
When the internal combustion engine 11 starts, the control device 51 determines in step S101 whether the oxygen sensor 35 is activated based on whether the output of the oxygen sensor 35 exceeds the rich determination threshold value, for example.

Then, until the oxygen sensor 35 is activated, the control device 51 proceeds to step S108 and sets a parameter (a parameter correlated to the response speed) as an index of the degree of decrease in the response speed due to deterioration (change over time) of the oxygen sensor 35. ), the deterioration degree DE obtained during the previous operation of the internal combustion engine 11 is held.
As will be described later, when the internal combustion engine 11 is stopped, the control device 51 writes and stores the degree of deterioration DE obtained in the operating state before the stop in a non-volatile memory. It is read out as the degree of deterioration DE obtained during the previous operation.

Then, in step S108, the control device 51 holds the degree of deterioration DE obtained during the previous operation, and uses it to set the rich target air-fuel ratio in the rich control.
Since the control device 51 cannot obtain the response speed correctly and cannot update the deterioration degree DE when the oxygen sensor 35 is not activated, the deterioration degree DE obtained during the previous operation is used until the deterioration degree DE is newly obtained. By setting the rich target air-fuel ratio in the rich control, it is possible to set the rich target air-fuel ratio substantially corresponding to the actual deterioration state (response speed).

When the control device 51 determines in step S101 that the oxygen sensor 35 has been activated, the process proceeds to step S102 to determine whether or not the degree of deterioration DE has already been calculated during the current operation of the internal combustion engine 11 .
Here, if the degree of deterioration DE has already been calculated, the control device 51 proceeds to step S109 (rich air-fuel ratio changing section), and after starting the internal combustion engine 11 (after activating the oxygen sensor 35), A rich target air-fuel ratio in rich control is set based on the degree DE.

On the other hand, if the degree of deterioration DE has not been calculated from after the start of the internal combustion engine 11 (after the oxygen sensor 35 is activated) to the present time, the control device 51 proceeds to step S103 to check whether fuel cut has started. to decide.
If the fuel cut has not started, the control device 51 proceeds to step S108 and holds the degree of deterioration DE obtained during the previous operation.

As will be described later, the control device 51 calculates the degree of deterioration DE based on the change in the output of the oxygen sensor 35 after the fuel cut is started, so the deterioration degree DE cannot be updated until the fuel cut is started. Therefore, the control device 51 holds the degree of deterioration DE obtained during the previous operation until fuel cut is started.
Then, when the fuel cut is started, the control device 51 proceeds to step S104, the duration of the fuel cut exceeds a predetermined time, and the output of the oxygen sensor 35 straddles the threshold THA after the fuel cut is started and is leaned. , in other words, whether the condition for calculating the degree of deterioration DE is satisfied.

At least one of the first condition that the duration of the fuel cut exceeds a predetermined time and the second condition that the output of the oxygen sensor 35 changes in the lean direction across the threshold value THA after the start of the fuel cut is met. If not, the control device 51 proceeds to step S107 and determines whether or not the fuel cut has ended.
If the fuel cut continues, the controller 51 returns to step S104.
On the other hand, if the fuel cut ends without calculating (updating) the degree of deterioration DE of the oxygen sensor 35, the controller 51 proceeds to step S108 and holds the degree of deterioration DE obtained during the previous operation.

Further, when the control device 51 determines in step S104 that the duration of the fuel cut has exceeded a predetermined time and that the output of the oxygen sensor 35 has changed in the lean direction across the threshold value THA after the start of the fuel cut, the deterioration degree Assuming that the DE calculation condition is satisfied, the process proceeds to step S105 (response detection unit).
In step S105, the control device 51 controls the oxygen sensor 35 based on the change in the output of the oxygen sensor 35 in the monitoring interval from the time when the current fuel cut is started until the output of the oxygen sensor 35 changes in the lean direction across the threshold value THA. The degree of deterioration DE of 35 is calculated.

Below, the calculation processing of the degree of deterioration DE in step S105 will be described in detail.
FIG. 5 is a diagram showing the output waveform of the oxygen sensor 35 at the start of fuel cut, showing the output waveform in the initial state of the oxygen sensor 35 (new state, non-deteriorated state) and the output waveform in the degraded state of the oxygen sensor 35. shows the output waveform of

When the fuel cut starts, the air flows into the first catalyst device 31 as it is. During this period, the output of the oxygen sensor 35 downstream of the first catalyst device 31 is substantially maintained at the level before the start of the fuel cut.
Then, when the oxygen storage amount of the first catalyst device 31 reaches saturation, oxygen begins to flow downstream of the first catalyst device 31, and the output of the oxygen sensor 35 starts to change in the lean direction.

Here, when the oxygen sensor 35 is in the initial state, the output of the oxygen sensor 35 suddenly changes in the lean direction after the oxygen storage amount of the first catalyst device 31 reaches saturation.
On the other hand, when the oxygen sensor 35 deteriorates, the output change in the lean direction after the oxygen storage amount of the first catalyst device 31 reaches saturation becomes slower than in the initial state.
Therefore, in step S105, the control device 51 calculates the degree of deterioration DE of the oxygen sensor 35 based on the waveform when the output of the oxygen sensor 35 changes in the lean direction with the start of fuel cut.

FIG. 6 is a time chart showing one aspect of the processing for calculating the degree of deterioration DE.
The control device 51 samples the detection signal VO2R of the oxygen sensor 35 at regular intervals after the start of fuel cut, and for each sampling, the deviation ΔVO2R between the previous value and the current value of the detection signal VO2R of the oxygen sensor 35, that is, the unit time Calculate the amount of voltage change per unit.

Furthermore, every time the control device 51 calculates the deviation ΔVO2R, the absolute value of the deviation ΔVO2R is compared with the maximum value ΔMAX so far, and the maximum value ΔMAX is updated based on the larger value. . Thereby, the control device 51 obtains the maximum value ΔMAX of the absolute values of the deviation ΔVO2R in the monitoring interval from the start of the fuel cut until the output of the oxygen sensor 35 reaches the threshold value THA.
The maximum value ΔMAX corresponds to the maximum slope of the detection signal VO2R or the maximum rate of change of the detection signal VO2R in the monitoring interval from the start of fuel cut until the output of the oxygen sensor 35 reaches the threshold value THA.

Here, when the oxygen sensor 35 deteriorates, as shown in FIG. 6, the change in the detection signal VO2R of the oxygen sensor 35 after the start of fuel cut becomes moderate, so the maximum value ΔMAX becomes smaller as the deterioration progresses. be a value.
The control device 51 stores the maximum value ΔMAX in the initial state of the oxygen sensor 35 as a reference value ΔMAXST (response determination threshold value) in a non-volatile memory, and stores the absolute value of the deviation between the obtained maximum value ΔMAX and the reference value ΔMAXST as deterioration. Set the degree to DE.

The maximum value ΔMAX is a parameter that correlates with the response speed of the oxygen sensor 35, and the deviation between the maximum value ΔMAX and the reference value ΔMAXST is a parameter that indicates the degree of decrease in the response speed from the initial state (degree of change over time). , the degree of deterioration DE indicates that the deterioration of the oxygen sensor 35 progresses as the value increases.
Note that the control device 51 sets the deviation between the maximum value ΔMAX and the reference value ΔMAXST as the degree of deterioration DE, but can set the ratio between the maximum value ΔMAX and the reference value ΔMAXST as the degree of deterioration DE.

Further, the control device 51 can set the reference value ΔMAXST as a one-point constant, but it can be set variably according to the operating conditions of the internal combustion engine 11 that affect the response speed of the oxygen sensor 35 .
When the output of the oxygen sensor 35 changes in the lean direction due to a fuel cut, the greater the intake air flow rate QA of the internal combustion engine 11, the greater the amount of oxygen flowing into the first catalyst device 31 per unit time. The change speed of the output of 35 in the lean direction becomes faster.
Therefore, the control device 51 stores the maximum value ΔMAX for each condition of the intake air flow rate QA in the initial state of the oxygen sensor 35 as the reference value ΔMAXST in the memory, and based on the intake air flow rate QA when the maximum value ΔMAX is obtained. A reference value ΔMAXST is determined, and the absolute value of the deviation between the reference value ΔMAXST and the maximum value ΔMAX can be used as the degree of deterioration DE.

FIG. 7 illustrates the correlation between the intake air flow rate QA and the reference value ΔMAXST.
When the oxygen sensor 35 is in the initial state, the greater the intake air flow rate QA, the faster the change in the lean direction of the output of the oxygen sensor 35 accompanying the fuel cut.

Therefore, in order to change the reference value ΔMAXST of the maximum value ΔMAX more as the intake air flow rate QA increases, a conversion table for converting the intake air flow rate QA to the reference value ΔMAXST, or a reference value ΔMAXST using the intake air flow rate QA as a variable. is set, and the conversion table or function is stored in the memory of the control device 51 .
Then, the control device 51 uses a conversion table or a function to determine the reference value ΔMAXST based on the intake air flow rate QA when the maximum value ΔMAX was obtained, and calculates the absolute value of the deviation between the reference value ΔMAXST and the maximum value ΔMAX. Deterioration degree DE.

With such a configuration, the degree of deterioration DE can be obtained with high accuracy even if the conditions of the intake air flow rate QA are different.
On the other hand, when the reference value ΔMAXST is a one-point constant, as shown in FIG. It is defined as ΔMAXST.

For example, the minimum intake air flow rate, which is the intake air flow rate QA when the internal combustion engine 11 is completely warmed up and under no-load conditions, can be used as the reference intake air flow rate QA.
If the reference value .DELTA.MAXST is a one-point constant, the calculation load of the controller 51 can be reduced, and a simple air-fuel ratio control system with reduced memory capacity can be obtained.

Note that the processing for calculating the degree of deterioration DE is not limited to calculation based on the maximum value ΔMAX indicating the maximum rate of change in the output of the oxygen sensor 35 .
For example, the control device 51 can set the degree of deterioration DE based on the time from the start of fuel cut until the output of the oxygen sensor 35 reaches the threshold value THA.

When the oxygen sensor 35 deteriorates and its response speed drops, the time TL from the start of fuel cut until the output of the oxygen sensor 35 reaches the determination threshold value THA (set value) becomes longer.
Therefore, the control device 51 uses the time TL in the initial state as the reference time TLST, and can estimate that the oxygen sensor 35 is more deteriorated as the actually obtained time TL becomes longer than the reference time TLST. .

Therefore, the control device 51 can set the absolute value or the ratio of the deviation between the reference time TLST and the measured time TL as the degree of deterioration DE.
Note that the control device 51 can use a one-point constant or a value corresponding to the intake air flow rate QA as the reference time TLST. The reference time TLST is shortened as the flow rate QA increases.

FIG. 9 is a time chart showing that the time TL is lengthened due to deterioration of the oxygen sensor 35. As shown in FIG.
When the oxygen sensor 35 is degraded, the rate of change in the output is slow. Therefore, the time TL from the start of fuel cut until the output of the oxygen sensor 35 reaches the threshold THA is as compared to the initial state in which the rate of change in the output is fast. become longer.

FIG. 10 shows the output waveform of the oxygen sensor 35 when the dead time from the start of the fuel cut until the output of the oxygen sensor 35 starts to change in the lean direction is lengthened due to deterioration of the oxygen sensor 35 .
Since the time TL is extended even when the dead time is lengthened due to deterioration, the calculation processing of the degree of deterioration DE based on the time TL is based on a deterioration pattern in which the output change speed (slope) is small and a deterioration pattern in which the dead time is long. In both cases, it is possible to set the degree of deterioration DE that matches the actual situation.

On the other hand, when the degree of deterioration DE is calculated based on the maximum value ΔMAX, if the rate of change in the output in the lean direction after the dead time has passed is the same as in the initial state, the deterioration state is such that the dead time is extended. , the maximum value ΔMAX does not change from the initial state, and an error occurs in the degree of deterioration DE.
Therefore, calculation of the degree of deterioration DE based on the maximum value ΔMAX is not suitable for a deterioration pattern in which dead time is long but the maximum value ΔMAX does not change significantly.

Further, the control device 51 can set the degree of deterioration DE from the maximum value ΔMAX and the time TL.
For example, the control device 51 calculates the "maximum value ΔMAX/time TL" as a parameter that correlates with the response speed, and calculates the calculated "maximum value ΔMAX/time TL" and the "maximum value ΔMAX/time TL" in the initial state. The absolute value or ratio of the deviation from the reference value corresponding to the value of can be used as the degree of deterioration DE.

In the initial state of the oxygen sensor 35, the maximum value ΔMAX is larger and the time TL is shorter than in the deteriorated state. becomes longer.
Therefore, the "maximum value ΔMAX/time TL" becomes smaller as the deterioration of the oxygen sensor 35 progresses, and the controller 51 compares the "maximum value ΔMAX/time TL" in the initial state with the actually measured "maximum value ΔMAX/time TL". The absolute value or ratio of the deviation from the time TL" can be set as the degree of deterioration DE.

When the control device 51 calculates the degree of deterioration DE based on the "maximum value ΔMAX/time TL", the deterioration corresponding to the actual condition is obtained for both the deterioration pattern in which the output change speed is slow and the deterioration pattern in which the dead time is long. Degree DE can be set.
Furthermore, when both the output change speed and dead time change due to deterioration, the degree of deterioration can be calculated with higher sensitivity than when calculating the degree of deterioration DE from only the output change speed or from only the dead time.

Note that the control device 51 can use a one-point constant or a value corresponding to the intake air flow rate QA as the reference value for the "maximum value ΔMAX/time TL". When changing the reference value of ΔMAX/time TL″, the larger the intake air flow rate QA, the greater the change in the reference value.

After calculating the degree of deterioration DE of the oxygen sensor 35 as described above in step S105, the control device 51 proceeds to step S106 to calculate the degree of deterioration DE used for setting the rich target air-fuel ratio in the rich control after the fuel cut. The value calculated in the previous operating state of the internal combustion engine 11 is updated to the value calculated in step S105 in the current operating state of the internal combustion engine 11 .
After step S106 or step S108, the control device 51 proceeds to step S109, and performs processing for setting the rich target air-fuel ratio in the rich control after the fuel cut according to the degree of deterioration DE.

In step S109, as shown in FIG. 11, the control device 51 adjusts the rich target air-fuel ratio based on the rich target air-fuel ratio as the degree of deterioration DE increases, in other words, as the response speed of the oxygen sensor 35 decreases. Closer to the stoichiometric air-fuel ratio than the fuel ratio.
Therefore, the control device 51 holds the degree of deterioration DE obtained in the previous operating state from the start of the internal combustion engine 11 until the degree of deterioration DE is newly calculated in step S105, and performs the rich control after the fuel cut. , the rich target air-fuel ratio is changed based on the degree of deterioration DE obtained in the previous operating state.

Then, in the rich control after the degree of deterioration DE is newly obtained, the control device 51 sets the rich target air-fuel ratio based on the degree of deterioration DE obtained in the current operation.
Next, in step S110, the control device 51 determines whether or not a command to stop the operation of the internal combustion engine 11 has been issued, based on the signal of the operation/stop switch of the internal combustion engine 11 such as the ignition switch, the engine switch, and the key switch. do.

If the signal from the run/stop switch is on and the internal combustion engine 11 continues to run, the controller 51 returns to step S102.
On the other hand, when the operation of the internal combustion engine 11 is to be stopped because the signal of the operation/stop switch is off, the control device 51 proceeds to step S111, and stores the degree of deterioration DE obtained in step S105 in the current operation in the non-volatile memory. , to set the rich target air-fuel ratio in the first rich control in the next operation.

The control device 51 can update the degree of deterioration DE each time the fuel cut is executed. In the embodiment, the update calculation of the degree of deterioration DE is performed only once during operation of the internal combustion engine 11 .

FIG. 12 is a time chart for explaining the timing of calculating the degree of deterioration DE and the timing of setting the rich target air-fuel ratio based on the degree of deterioration DE.
When the internal combustion engine 11 is started for the first time after going offline, the degree of deterioration DE obtained during the previous operation does not exist in the memory.

Then, in the subsequent rich control after the fuel cut, the control device 51 sets the rich target air-fuel ratio according to the degree of deterioration DE stored in the memory.
When the internal combustion engine 11 is stopped after obtaining the degree of deterioration DE, the control device 51 stores and retains the data of the degree of deterioration DE during the stop period. A rich target air-fuel ratio is set based on the degree of deterioration DE obtained during operation.

Further, the control device 51 calculates the degree of deterioration DE at the first (first) fuel cut, and stores the newly calculated degree of deterioration DE instead of the degree of deterioration DE obtained during the previous operation.
Then, in the second and subsequent rich control, the control device 51 sets the rich target air-fuel ratio based on the degree of deterioration DE obtained in the first fuel cut.

Further, the control device 51 stores the deterioration degree DE newly obtained in the current operation while the internal combustion engine 11 is stopped so that it can be used in the first rich control in the next operation.
Note that the control device 51 performs the setting of the rich target air-fuel ratio based on the degree of deterioration DE obtained during the previous operation, limited to the initial rich control after the restart. As a result, even if there is an error in the degree of deterioration DE obtained during the previous operation, the degree of deterioration DE is continuously used to set the rich target air-fuel ratio, thereby suppressing deterioration of the exhaust gas performance.

FIG. 13 is a flow chart showing the procedure of rich control after fuel cut.
FIG. 14 is a time chart showing changes in the output of the oxygen sensor 35 due to fuel cut and rich control, and changes in the rich target air-fuel ratio due to deterioration in the response of the oxygen sensor 35 .
Note that FIG. 14 omits the description of the rich spike by the feedforward control as the first stage of the rich control.

The procedure of rich control by the control device 51 will be described below with reference to FIG. 14 .
In step S201, the control device 51 determines whether or not the fuel cut has started, and when the fuel cut has started, the process proceeds to step S202.
In step S202, the control device 51 determines whether or not the fuel cut has ended, and proceeds to step S203 when the fuel cut ends and the fuel injection is restarted and the rich control is started.

In step S203, the control device 51 sets the injection rate (target equivalence ratio) TFBYA in the control of the fuel injection amount to a predetermined value higher than 1.0 to 1.0, that is, the target air-fuel ratio in the feedforward control, as the first stage of the rich control. By switching to a value that makes the air-fuel ratio richer than the stoichiometric air-fuel ratio, a rich spike that instantaneously supplies excessive fuel is performed.
Then, in the next step S204, the control device 51 determines whether or not the duration of the first stage has reached a predetermined period, in other words, determines the end timing of the first stage, and performs the first stage rich control. If it continues for a predetermined period, the process proceeds to step S205.

In step S205, the control device 51 reads the rich target air-fuel ratio set according to the degree of deterioration DE of the oxygen sensor 35 (calculated result in step S109).
Then, in the next step S206, the control device 51 calculates the fuel injection pulse width TI (fuel injection amount) based on the deviation between the rich target air-fuel ratio read in step S205 and the actual air-fuel ratio obtained from the output of the air-fuel ratio sensor 34. air-fuel ratio feedback control for correcting is performed as the second stage rich control.

In the next step S207, the control device 51 compares the output of the oxygen sensor 35 with the rich control cancellation determination value. to continue the second stage of the rich control, that is, the feedback control based on the rich target air-fuel ratio.
On the other hand, when the desorption of oxygen from the first catalyst device 31 progresses with the rich control, and the output of the oxygen sensor 35 changes from the lean output to the rich direction and reaches the release determination value, the control device 51 performs step S208. proceed to

Then, in step S208, the control device 51 cancels the rich control (second stage) after the fuel cut, and normal air-fuel ratio feedback control, for example, the stoichiometric air-fuel ratio or leaner than the stoichiometric air-fuel ratio, is set as the target air-fuel ratio. air-fuel ratio feedback control.
As described above, if the rich target air-fuel ratio in the rich control after fuel cut is changed according to the degree of deterioration DE of the oxygen sensor 35, the rich control (second stage) end determination ( release determination) becomes slower than the initial state due to a decrease in response speed due to deterioration of the oxygen sensor 35, it is possible to prevent excessive rich control that excessively reduces the oxygen storage amount of the first catalyst device 31, Exhaust gas performance after fuel cut can be maintained.
In other words, the control device 51 brings the rich target air-fuel ratio in the rich control closer to the stoichiometric air-fuel ratio by the extent that the rich control is performed excessively long due to the deterioration of the response of the oxygen sensor 35. 31's oxygen storage is not depleted excessively.

By the way, the control device 51 does not store the degree of deterioration DE obtained during the previous operation during the stop period of the internal combustion engine 11, and sets the rich target air-fuel ratio to the reference rich target air-fuel ratio in the first rich control. The degree of deterioration DE can be calculated at the time of the first fuel cut, and the rich target air-fuel ratio can be changed based on the degree of deterioration DE obtained at the first time in the rich control after the second and subsequent fuel cuts.
FIG. 15 shows the deterioration degree DE when the control device 51 sets the rich target air-fuel ratio to the reference rich target air-fuel ratio in the first rich control without carrying over the deterioration degree DE obtained during the previous operation to the next operation. and the timing of setting the rich target air-fuel ratio based on the degree of deterioration DE (the timing of reflecting the degree of deterioration DE).

At the first fuel cut after the start of the internal combustion engine 11, there is no calculation history of the degree of deterioration DE of the oxygen sensor 35. Therefore, the control device 51 sets the reference rich target air-fuel ratio as the rich target air-fuel ratio in the rich control after the fuel cut. set.
The reference rich target air-fuel ratio is, for example, a rich target air-fuel ratio that is suitable when the oxygen sensor 35 is in the initial state.
Then, the control device 51 calculates the degree of deterioration DE from the change in the output of the oxygen sensor 35 during the first fuel cut, and sets the rich target air-fuel ratio set based on the calculated degree of deterioration DE to rich control after the second and subsequent fuel cuts. apply to

Further, the control device 51 can calculate the degree of deterioration DE of the oxygen sensor 35 based on the change in the output of the oxygen sensor 35 from lean to rich when the fuel injection is restarted after the fuel cut.
Furthermore, when the control device 51 calculates the degree of deterioration DE based on the change in the output of the oxygen sensor 35 when fuel injection is restarted after the fuel cut, the control device 51 cancels the fuel cut (deceleration fuel cut) by depressing the accelerator pedal. When the resuming state of fuel injection after fuel cut is the acceleration state of the internal combustion engine 11, the calculation of the degree of deterioration DE can be canceled and the degree of deterioration DE obtained during the previous operation can be held.

This is because the acceleration operation of the internal combustion engine 11 may change the air-fuel ratio, the amount of intake air, etc., affecting the output change of the oxygen sensor 35 and lowering the accuracy of calculating the degree of deterioration DE.
Therefore, the control device 51 restarts the fuel injection based on the decrease in the engine rotation speed in the fuel cut state, and determines the degree of deterioration based on the change in the output of the oxygen sensor 35 when the internal combustion engine 11 shifts to the idling state after the fuel cut. Calculate the DE.

FIG. 16 calculates the deterioration degree DE of the oxygen sensor 35 based on the change in the output of the oxygen sensor 35 when fuel injection is restarted after the fuel cut is finished, and when the fuel cut is canceled by turning on the accelerator, the deterioration degree DE of the previous operation is calculated. 10 is a flow chart showing a procedure for correcting the rich target air-fuel ratio so as to hold .
When the internal combustion engine 11 starts, the control device 51 determines in step S301 whether the oxygen sensor 35 is activated.

Until the oxygen sensor 35 is activated, the control device 51 proceeds to step S307 and holds data of the degree of deterioration DE obtained during the previous operation of the internal combustion engine 11 .
When determining that the oxygen sensor 35 is activated in step S301, the control device 51 proceeds to step S302 to determine whether or not the degree of deterioration DE has been calculated during the current operation of the internal combustion engine 11, in other words, whether or not the degree of deterioration DE has been calculated during the previous operation. It is determined whether or not the degree of deterioration DE obtained at the time has been updated to the degree of deterioration DE newly obtained this time.

Here, if the degree of deterioration DE has already been calculated, the control device 51 proceeds to step S310 and sets the rich target air-fuel ratio in the rich control based on the degree of deterioration DE newly obtained this time.
That is, once the control device 51 calculates the degree of deterioration DE after the internal combustion engine 11 is started, it does not repeat the calculation of the degree of deterioration DE thereafter. Rich control is executed each time fuel cut is performed according to the rich target air-fuel ratio based on the degree of deterioration DE.

On the other hand, if the degree of deterioration DE has not been calculated from the start of the internal combustion engine 11 to the present time, the control device 51 proceeds to step S303 to determine whether fuel cut has started.
If the fuel cut has not started, the control device 51 proceeds to step S307 and holds the degree of deterioration DE obtained during the previous operation.

When the fuel cut is started, the control device 51 proceeds to step S304 and determines whether or not the fuel cut has been terminated and the fuel injection has been restarted.
Here, if the fuel cut is continuing, the control device 51 proceeds to step S307 and holds the degree of deterioration DE obtained during the previous operation.

On the other hand, when the fuel injection is restarted, the control device 51 proceeds to step S305, and the time after the fuel injection is restarted exceeds a predetermined time, and the output of the oxygen sensor 35 crosses the threshold THB after the fuel injection is restarted. , it is determined whether or not there is a change in the rich direction.
That is, in step S305, the control device 51 determines whether or not the change in the output of the oxygen sensor 35 necessary for obtaining the degree of deterioration DE has been monitored.

In step S305, the control device 51 satisfies the conditions that the time from the resumption of fuel injection exceeds a predetermined time, and that the output of the oxygen sensor 35 crosses the threshold value THB and changes in the rich direction after the resumption of fuel injection. If it is determined that it is not, the process proceeds to step S306.
In step S306, the control device 51 determines whether the accelerator pedal has been depressed, in other words, whether the accelerator opening (throttle opening) has increased or changed, or whether the accelerator is in the accelerator ON state or in the accelerator OFF state. determine if there is

If the accelerator pedal is not depressed and the accelerator is off, the control device 51 returns to step S305 to determine whether or not the conditions for calculating the degree of deterioration DE are satisfied.
Further, when the accelerator pedal is depressed and the accelerator is on, it is impossible to correctly calculate the degree of deterioration DE from the output change of the oxygen sensor 35 due to fluctuations in the air-fuel ratio and the intake air flow rate QA accompanying the acceleration operation of the internal combustion engine 11. Since it is difficult, the controller 51 proceeds to step S307 and holds the degree of deterioration DE obtained during the previous operation.

In other words, when the fuel cut is canceled by stepping on the accelerator pedal (the accelerator is turned on), the control device 51 postpones the calculation of the degree of deterioration DE and postpones the calculation of the degree of deterioration DE to the next and subsequent fuel cuts.
Then, the control device 51 cancels the fuel cut based on the decrease in the engine speed and calculates the degree of deterioration DE based on the change in the output of the oxygen sensor 35 when the internal combustion engine 11 shifts to idle operation. To calculate DE with high accuracy.

On the other hand, in step S305, the control device 51 sets the condition that the time from restarting the fuel injection exceeds a predetermined time and that the output of the oxygen sensor 35 crosses the threshold THB after the restart of the fuel injection and changes in the rich direction. If it is determined that (the condition for calculating the degree of deterioration DE) is satisfied, the process proceeds to step S308.
Then, the control device 51 monitors the change in the output of the oxygen sensor 35 after the fuel injection is restarted, and calculates the degree of deterioration DE based on the monitored output change.
As fuel injection is restarted, the output of the oxygen sensor 35 changes from lean output to rich output, but there is a delay before reaching the rich output due to a decrease in response speed due to deterioration.

FIG. 17 is a diagram showing the output waveform of the oxygen sensor 35 when fuel injection is restarted after the end of the fuel cut, showing the output waveform of the oxygen sensor 35 in the initial state (new state, non-deteriorated state) and the oxygen sensor. 35 shows the output waveform in response deterioration state.
When the fuel cut ends and fuel injection resumes, rich exhaust gas flows into the first catalyst device 31 and oxygen is desorbed from the first catalyst device 31. However, the oxygen storage of the first catalyst device 31 Since the amount is saturated, there is a dead time from the start of fuel injection until the downstream of the first catalyst device 31 is reversed to rich, and after the dead time has passed, the output of the oxygen sensor 35 starts to change in the rich direction.

Here, if the oxygen sensor 35 is in the initial state, the output of the oxygen sensor 35 suddenly changes in the rich direction after the dead time has passed. The change in the output in the rich direction is slower than at a certain time.
Therefore, in the same manner as in the case where the degree of deterioration DE is obtained from the change in the output at the start of fuel cut, the control device 51 determines the unit time between monitoring from the resumption of fuel injection until the output of the oxygen sensor 35 reaches the rich side threshold. The maximum value ΔMAX (maximum slope or maximum change speed) of the amount of change in output per hit is obtained, and the absolute value or ratio of the deviation between the maximum value ΔMAX and the reference value is set as the degree of deterioration DE.

In addition, the control device 51 sets the absolute value or the ratio of the deviation between the reference and the time TR until the output of the oxygen sensor 35 reaches the lean-to-rich reversal threshold from the resumption of fuel injection to the degree of deterioration DE.
Furthermore, the control device 51 sets the degree of deterioration DE from the maximum value ΔMAX, which is the maximum amount of change in output per unit time after fuel injection is restarted, and the time TR from lean to rich reversal after fuel injection is restarted. can do.

For example, the control device 51 calculates the “maximum value ΔMAX/time TR” as a parameter that correlates with the response speed of the oxygen sensor 35, and the absolute value or the deviation between the calculated “maximum value ΔMAX/time TR” and the reference value The ratio can be used as the degree of deterioration DE.
The reference value for the maximum value ΔMAX, time TR or "maximum value ΔMAX/time TR" is a value corresponding to a one-point constant or intake air flow rate QA.

After calculating the degree of deterioration DE in step S308, the control device 51 proceeds to step S309, and uses the degree of deterioration DE calculated in step S308 as the degree of deterioration DE used for setting the rich target air-fuel ratio in the rich control after the fuel cut. Update.
After holding the degree of deterioration DE obtained during the previous operation in step S307, or after updating to the degree of deterioration DE newly obtained in step S309, the control device 51 proceeds to step S310 and cuts the fuel based on the degree of deterioration DE. A rich target air-fuel ratio is set for subsequent rich control.

Here, as shown in FIG. 11, the control device 51 adjusts the rich target air-fuel ratio based on the rich target air-fuel ratio as the degree of deterioration DE increases, in other words, as the response speed of the oxygen sensor 35 decreases. closer to the stoichiometric air-fuel ratio.
The control device 51 then proceeds to step S311 to determine whether or not a command to stop the internal combustion engine 11 has been issued, based on the signal of the operation/stop switch of the internal combustion engine 11 such as the ignition switch, the engine switch, and the key switch. .

When the operation/stop switch is turned on and the operation of the internal combustion engine 11 is to be continued, the control device 51 returns to step S302. .
In step S312, the controller 51 stores the degree of deterioration DE obtained in step S308 in the current operation in the non-volatile memory, and uses it to set the rich target air-fuel ratio in the first rich control in the next operation.

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.

For example, the control device 51 monitors one of the output waveform of the oxygen sensor 35 after the start of the fuel cut and the output waveform of the oxygen sensor 35 after the end of the fuel cut (after the resumption of fuel injection), and the unit time in the monitored output waveform Calculate the degree of deterioration DE based on the amount of voltage change per unit and/or the time from the start of monitoring until the sensor output reaches the threshold, and store the calculated degree of deterioration DE for use in the initial rich control during the next operation, or In the initial rich control, the rich air-fuel ratio is set as the default, and from the second time onwards, the rich air-fuel ratio can be changed based on the degree of deterioration DE obtained in the current operation.

Further, the control device 51 can determine that a deterioration failure of the oxygen sensor 35 has occurred when the degree of deterioration DE exceeds the failure determination threshold value.
Then, when it is determined that the deterioration failure of the oxygen sensor 35 has occurred, the control device 51 saves the diagnosis of the deterioration failure of the oxygen sensor 35 as a diagnosis history in the memory, or cancels the rich control. It is possible to implement a fail-safe process to stop the control used, or operate a warning device to warn the driver of the vehicle of an abnormality in the oxygen sensor 35 (internal combustion engine 11).

In addition, the control device 51 monitors both the change in the output of the oxygen sensor 35 after the start of the fuel cut and the change in the output of the oxygen sensor 35 after the end of the fuel cut (after the resumption of fuel injection). Based on this, the degree of deterioration DE can be calculated.
In addition, the control device 51 can enrich the air-fuel ratio by the feedforward control over the entire period of the rich control, and the rich target air-fuel ratio (rich air-fuel ratio) in the feedforward control is set to the degree of deterioration DE (of the oxygen sensor 35). response speed).

In addition, the control device 51 may reduce the rich control amount during the entire rich control period in anticipation that the rich control period will be extended due to the delayed release of the rich control due to the decrease in the response speed of the oxygen sensor 35 .
Therefore, the change of the rich target air-fuel ratio based on the degree of deterioration DE is not limited to a uniform shift of the rich target air-fuel ratio during rich control. The timing of starting to gradually approach the . It can be made faster as it increases.

In addition, the control device 51 repeats the calculation of the degree of deterioration DE based on the change in the output of the oxygen sensor 35 at the time of starting and/or canceling the fuel cut a plurality of times. It can be used for setting the target air-fuel ratio.
In addition, the control device 51 obtains the output change amount per unit time after the fuel cut is started and/or after the fuel injection is restarted for each sampling of the sensor output. It is possible to obtain the degree of deterioration DE.
In this case, the threshold is adjusted so that the number of samples of the amount of change in output that is equal to or greater than the threshold decreases due to deterioration of the oxygen sensor 35, and the controller 51 initially sets the number of samples of the amount of change in output that is equal to or greater than the threshold. The degree of deterioration DE is increased in accordance with the increase in the number decreased from the state.

Further, the control device 51 monitors the output waveform of the oxygen sensor 35 after the start of fuel cut, calculates the degree of deterioration DE (a parameter that correlates with the response speed) based on the monitored output waveform, and determines the degree of deterioration based on the calculated degree of deterioration. It is possible to change the rich air-fuel ratio in rich control after the end of fuel cut.
Further, the post-catalyst sensor is not limited to an oxygen sensor that outputs a detection signal (voltage signal) 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. It may be an air-fuel ratio sensor that outputs a detection signal corresponding to the fuel ratio.

DESCRIPTION OF SYMBOLS 11... Internal combustion engine, 21... Fuel injection valve, 31... First catalyst device (exhaust gas purification catalyst), 34... Air-fuel ratio sensor, 35... Oxygen sensor (post-catalyst sensor), 51... Control device (fuel injection control device)

Claims (6)

A fuel injection control device applied to an internal combustion engine provided with a post-catalyst sensor that detects an exhaust air-fuel ratio downstream of an exhaust gas purification catalyst,
a rich control unit that controls the air-fuel ratio of the internal combustion engine to a rich target air-fuel ratio that is richer than the stoichiometric air-fuel ratio during a rich control period from the end of the fuel cut until the output of the post-catalyst sensor reaches a release determination value;
a rich air-fuel ratio changing unit that brings the rich target air-fuel ratio closer to the stoichiometric air-fuel ratio in the rich control period in accordance with a decrease in the response speed of the post-catalyst sensor;
A fuel injection control device for an internal combustion engine. The rich air-fuel ratio change unit includes a response detection unit that detects a response speed based on a change in the output of the post-catalyst sensor after the fuel cut is started or after the fuel cut is finished.
2. A fuel injection control device for an internal combustion engine according to claim 1. The response detection unit detects the amount of change per unit time in the output of the post-catalyst sensor or the time until the output of the post-catalyst sensor reaches a set value as a parameter that correlates with the response speed.
3. A fuel injection control device for an internal combustion engine according to claim 2. The rich air-fuel ratio changing unit changes the rich target air-fuel ratio based on a comparison between a parameter correlated with the response speed and a response determination threshold value based on the intake air amount of the internal combustion engine when the parameter is detected.
4. A fuel injection control device for an internal combustion engine according to claim 3. The response detection unit detects the response speed in the first fuel cut after activation of the post-catalyst sensor,
The rich air-fuel ratio changing unit changes the rich target air-fuel ratio in second and subsequent fuel cuts based on the response speed detected in the first fuel cut.
3. A fuel injection control device for an internal combustion engine according to claim 2. The rich air-fuel ratio changing unit sets the rich target air-fuel ratio in the first fuel cut after the post-catalyst sensor is activated based on the response speed detected and stored in memory by the response detecting unit during the previous operation of the internal combustion engine. change,
6. A fuel injection control device for an internal combustion engine according to claim 5.

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