JPH1047133A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve air-fuel ratio feed back control performance of an internal combustion engine. SOLUTION: An engine is judged to have been activated when the output of an air-fuel ratio sensor attains target output SL1H on a rich side, and begins air-fuel ratio feed back control (S1-S4), and a rich continuation time TTSL is integrated during rich control (S7-S42), and after reversing toward a lean side, the rich continuation time TTSL is compared with a standard time TTS, and when the former is larger, the response of the air-fuel, ratio sensor is judged to be later for decreasing the variation width (velocity) DSL of the target output SL, and when the former is smaller, the response is judged to be fast for increasing DSL (S21S24), and the target output SL is decreased DSL by DSL and varied up to the target output SLN equivalent to a stoichiometric air-fuel ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for feedback-controlling the air-fuel ratio of an internal combustion engine, and more particularly to a technique for controlling the output of an air-fuel ratio sensor provided in an exhaust passage to a predetermined target value. The present invention relates to a technique for changing a target output.

[0002]

2. Description of the Related Art Conventionally, in an internal combustion engine for a vehicle, feedback control of the air-fuel ratio is performed based on the output of an air-fuel ratio sensor. The target output of the air-fuel ratio sensor corresponding to the above is changed.

For example, after the engine is started, it is determined whether or not the air-fuel ratio sensor has been activated by determining whether the output of the air-fuel ratio sensor is equal to a first predetermined value corresponding to a target air-fuel ratio at the time of air-fuel ratio feedback after engine warm-up. May be determined based on whether a different second predetermined value has been reached. In such a case, it is confirmed that the air-fuel ratio sensor has been activated, and the air-fuel ratio feedback control is started. The target output is changed from the second predetermined value to the first predetermined value.

[0004] In order to follow the change of the target output, the air-fuel ratio feedback amount increases in the direction of change of the target output as compared with the case where the target output is not changed.
For example, when the target output is changed to the lean side, feedback is performed so that the actual air-fuel ratio is also corrected to the lean side, so that the amount of the feedback amount in the lean direction is increased.

In a situation where the response of the air-fuel ratio sensor is low, such as at low temperatures, feedback overcorrection is likely to occur.
When the target output is changed as described above, the overcorrection increases as the feedback correction amount increases.
Various devices have been proposed to prevent over-compensation for a decrease in the response of the air-fuel ratio sensor.

One of such ideas is, for example,
There is a technique disclosed in JP-A-61-232348. In this example, if the air-fuel ratio feedback cycle becomes longer due to a decrease in the air-fuel ratio sensor response or other reasons, the feedback gain is reduced to reduce the feedback overcorrection.

[0007]

However, in these conventional methods, the response of the air-fuel ratio sensor is detected by some method, and the feedback gain is reduced in accordance with the reduction in the response. When the target output of the fuel ratio sensor changes, the required margin for reducing the feedback gain varies depending on the variation width of the target output. As described above, since the response of the air-fuel ratio sensor has to be detected by some method as described above, at the time when the target output changes, the required margin for reducing the feedback gain cannot be calculated. I will.

Therefore, in order to prevent the air-fuel ratio feedback overcorrection, the feedback gain must be reduced with a margin, and consequently, the correction speed for the air-fuel ratio turbulence is reduced, and the air-fuel ratio is not reduced. Control accuracy is reduced. Alternatively, if an attempt is made to reduce the feedback gain without making it too small, the rate of change of the target output must be suppressed.
The period in which the air-fuel ratio is rich or lean away from the target is lengthened. In such a case, the air-fuel ratio deviates from an appropriate value due to a decrease in the correction speed of the air-fuel ratio or a deviation of the target air-fuel ratio, which may be undesirable for engine operability or exhaust emission. .

The present invention has been made in view of such a conventional problem, and suppresses a large deviation in the air-fuel ratio by changing the target output at an appropriate speed according to the response of the air-fuel ratio sensor. It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine in which favorable feedback control of the air-fuel ratio is performed.

[0010]

Therefore, according to the present invention, as shown in FIG. 1, an air-fuel ratio for detecting an air-fuel ratio of an air-fuel mixture supplied to an engine in response to a specific component in exhaust gas. A sensor, air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio so that the output of the air-fuel ratio sensor becomes a target output, and target output changing means for changing the target output of the air-fuel ratio sensor. In an air-fuel ratio control device for an internal combustion engine, a response measuring means for measuring a response of the air-fuel ratio sensor, and a target for setting a change speed of a target output of the air-fuel ratio sensor in accordance with the measured response of the air-fuel ratio sensor Output change speed setting means,
When the target output is changed, the target output is changed at the set change speed.

(Operation / Effect) The air-fuel ratio feedback control is performed so that the output of the air-fuel ratio sensor becomes the target output. When the target output is changed, the response of the air-fuel ratio sensor is measured, and according to the response, Changes the target output at the set change speed. By this, when the response of the air-fuel ratio sensor is slow, the correction speed of the target output is slowed accordingly to prevent overcorrection of the air-fuel ratio feedback control, and when the response of the air-fuel ratio sensor is fast, By accelerating the change speed of the target output, it is possible to quickly converge to the final target output, so that appropriate air-fuel ratio feedback control can always be performed, and exhaust purification performance can be improved.

In the invention according to a second aspect, the target output changing means determines the target output from an activation determination target output for determining whether the air-fuel ratio sensor has been activated after the engine is started. After activation, the air-fuel ratio is changed to a target air-fuel ratio determination target output for feedback control of the air-fuel ratio to the target air-fuel ratio.

(Operation / Effect) After the engine is started, the air-fuel ratio sensor is activated and the maximum output increases as the warm-up proceeds. Therefore, the activation of the air-fuel ratio sensor is determined by comparing the output of the air-fuel ratio sensor with the target output for activation determination. After it is determined that the air-fuel ratio sensor has been activated, the output is changed to the target output for determining the target air-fuel ratio, and the feedback control for bringing the air-fuel ratio close to the target air-fuel ratio is started.

According to a third aspect of the present invention, the target air-fuel ratio is a stoichiometric air-fuel ratio, and the activation determination target output is set to a value corresponding to an air-fuel ratio richer than the stoichiometric air-fuel ratio. Features. (Operation / Effect) By performing the air-fuel ratio feedback control using the target air-fuel ratio as the stoichiometric air-fuel ratio, it is possible to purify all of CO, HC, and NOx in a well-balanced manner by a purifying means such as a three-way catalyst. Further, in order to determine the activation of the air-fuel ratio sensor, it is necessary to determine whether the maximum output of the air-fuel ratio sensor is sufficiently high. Therefore, the value is set to a value corresponding to an air-fuel ratio richer than the stoichiometric air-fuel ratio. Thus, activation can be determined.

Further, in the invention according to claim 4, the target output changing means sets a final target output, and the target output changed at the set change speed reaches the final target output. The change of the target output is terminated at a point in time. (Operation / Effect) In the case of changing from the target output for the activation determination to the target output for the target air-fuel ratio determination, a final target output for the target air-fuel ratio determination is set, and according to the response of the air-fuel ratio sensor. When the target output changed at the set change speed reaches the final target output, further change is prohibited. This makes it possible to easily and reliably change the target output.

Further, according to a fifth aspect of the present invention, the response measuring means includes: after the start of the air-fuel ratio feedback control,
The response is measured by the time from when the output of the air-fuel ratio sensor crosses the previous target output to when it crosses the current target output. (Operation / Effect) When the response of the air-fuel ratio sensor is relatively fast, after the output of the air-fuel ratio sensor once crosses the target output, the air-fuel ratio is controlled to be lean or rich so that the air-fuel ratio approaches the target output, As a result, if the time required to cross the target output again is short, and conversely, if the response of the air-fuel ratio sensor is slow, the time from once crossing the target output to re-crossing becomes long, so this time is measured. This makes it possible to measure the response of the air-fuel ratio sensor.

According to a sixth aspect of the present invention, the response measuring means sets a time required for changing from an output value of the air-fuel ratio sensor at the start of the air-fuel ratio feedback control to a set value shifted by a predetermined amount from the output value. Is used to measure the response. (Operation / Effect) The time required for the output value of the air-fuel ratio sensor to change from the output value at the start of the air-fuel ratio feedback control to a set value shifted by a predetermined amount from the output value is determined when the response of the air-fuel ratio sensor is fast. When the response is short, the response becomes long when the response is slow. Therefore, by measuring this time, the response of the air-fuel ratio sensor can be measured.

Further, since the response can be measured within a short time, the target output can be changed at a change speed according to the response at each time. The invention according to claim 7 is characterized in that the air-fuel ratio sensor detects stepwise whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio.

(Operation / Effect) The present invention can be applied to air-fuel ratio feedback control using an air-fuel ratio sensor for detecting stepwise whether the air-fuel ratio is richer or leaner than a stoichiometric air-fuel ratio of an oxygen sensor or the like.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 2 showing a configuration of one embodiment, an intake passage 12 of an engine 11 is provided with an air flow meter 13 for detecting an intake air flow rate Q and a throttle valve 14 for controlling the intake air flow rate Q in conjunction with an accelerator pedal. An electromagnetic fuel injection valve 15 as fuel supply means is provided for each cylinder in the downstream manifold portion.

The fuel injection valve 15 is driven to open by an injection pulse signal from a control unit 16 containing a microcomputer, and is injected from a fuel pump (not shown) to inject and supply fuel controlled to a predetermined pressure by a pressure regulator. . Further, a water temperature sensor 17 for detecting a cooling water temperature Tw in a cooling jacket of the engine 11 is provided, and an air-fuel ratio sensor 19 for detecting an air-fuel ratio of an intake air-fuel mixture by detecting an oxygen concentration in exhaust gas in an exhaust passage 18.
And a three-way catalyst 20 for oxidizing CO and HC in the exhaust gas on the downstream side and reducing NO _X to purify the exhaust gas.

The distributor (not shown) has a built-in crank angle sensor 21 which counts a crank unit angle signal output from the crank angle sensor 21 in synchronization with the engine rotation for a certain period of time, or The engine rotation speed N is detected by measuring the period of the reference angle signal.
The calculation of the fuel injection amount from the fuel injection valve 15 may be controlled so as to be other than the stoichiometric air-fuel ratio depending on conditions such as the cooling water temperature. Based on the product of the correction coefficient for calculating the fuel injection amount and the air-fuel ratio feedback correction coefficient α shown below, the final fuel injection amount is calculated as in the following equation.

Ti = Tp × COEF × α where Ti: final fuel injection amount Tp: basic fuel injection amount COEF: correction coefficient α: air-fuel ratio feedback correction coefficient Further, the air-fuel ratio feedback correction coefficient α is added to the above equation. The learned air-fuel ratio learned value may be added.

When the stoichiometric air-fuel ratio is to be controlled, the air-fuel ratio is feedback-corrected based on the output of the air-fuel ratio sensor 19 in order to more accurately control the stoichiometric air-fuel ratio. The air-fuel ratio sensor 19, for example, what is called an oxygen sensor is used. As shown in FIG. 3, the oxygen sensor has a rich output (about 1 V) and a lean output (about 0) near the stoichiometric air-fuel ratio. Has the characteristic of rapidly reversing.

The air-fuel ratio is compared with the output of the air-fuel ratio sensor 19 (hereinafter referred to as VO2) and the target output SL (= SLN) corresponding to the stoichiometric air-fuel ratio. If VO2> SL, it is determined that the air-fuel ratio is rich. If VO2 <SL, it is determined to be lean. When the air-fuel ratio sensor detects that the air-fuel ratio is rich, the fuel is reduced and the air-fuel ratio is corrected to be lean. When the air-fuel ratio is detected as lean, the fuel is increased and the air-fuel ratio is corrected to be rich. In this way, the air-fuel ratio is corrected based on the output value of the air-fuel ratio sensor 19 on the upstream side of the catalyst 20.

After the engine is started, the output V of the air-fuel ratio sensor is first determined to determine whether the air-fuel ratio sensor 19 has been activated.
O2 is compared with a target output SL (= SL1H) for activation determination which is set to a richer side than a value corresponding to the stoichiometric air-fuel ratio. When the output of the air-fuel ratio sensor 19 reaches the target output SLH for activation determination, it is determined that the air-fuel ratio sensor 19 has been activated. Thereafter, the target output is changed from SLH to the target output for stoichiometric air-fuel ratio determination. Change to SL (= SLN)
Air-fuel ratio feedback control for bringing the air-fuel ratio close to the stoichiometric air-fuel ratio is started.

When the target output of the air-fuel ratio sensor is changed in this way, the air-fuel ratio correction is extraly performed by the amount that the air-fuel ratio output follows the change. If the detection of the air-fuel ratio by the air-fuel ratio sensor is not delayed or the delay is small, this chasing does not matter much, but if the response of the air-fuel ratio sensor is slow, this chasing time is prolonged, and The air-fuel ratio correction is largely overcorrected.

The output VO2 of the air-fuel ratio sensor is the target output SL
So that it does n’t take much time to catch up
Although it is conceivable to reduce the amount of movement of the SL, if the SL is moved too slowly, it takes a lot of time for the SL to reach the final target output, and it is difficult to control the air-fuel ratio appropriately. This affects the operating performance and exhaust purification performance during that time.

Therefore, in the present invention, while measuring the response of the air-fuel ratio sensor, the SL is changed at a change speed set in accordance with the response result, so that the air-fuel ratio feedback when the response of the air-fuel ratio sensor is slow is obtained. And when the response of the air-fuel ratio sensor is fast, the convergence to the final target value is accelerated so that appropriate air-fuel ratio feedback control is performed.

The air-fuel ratio control routine including the air-fuel ratio feedback control performed by the control unit 16 while changing the target output of the air-fuel ratio sensor will be described with reference to the flowcharts of FIGS. In step (denoted as S in the figure) 1, it is determined whether or not the condition is such that air-fuel ratio feedback control is possible. Conditions under which the air-fuel ratio feedback control is not possible include when the air-fuel ratio sensor 19 is not activated, during so-called fuel cut control, when the operability of the engine is unstable, and the like. Is possible.

If it is determined in step 1 that the conditions are not such that the air-fuel ratio feedback control is possible, step 13
Then, the value of the flag FLGFB is reset to 0, and the process further proceeds to step 100, where the air-fuel ratio feedback correction coefficient α is clamped at 100% (= 1), the air-fuel ratio feedback control is stopped, and the air-fuel ratio is fed. Control forward.

On the other hand, if it is determined in step 1 that the conditions permit the air-fuel ratio feedback control, the process proceeds to step 2, in which the air-fuel ratio sensor 19 has reached a state where it can generate a sufficient output, that is, whether the air-fuel ratio sensor 19 has been activated. The flag FLG
The determination is made by reading the value of FB. Step 2
If FLGFB = 0, the air-fuel ratio sensor
19 was not in a state capable of generating sufficient output,
That is, it is determined that it is in the inactive state, and the process proceeds to step 10. In steps 10 and 11, it is determined whether or not the output of the air-fuel ratio sensor 19 has become a sufficient value in this determination.

That is, in step 10, the target output SL of the air-fuel ratio sensor 19 is set to the target output SL1H for activation determination, and in step 11, the air-fuel ratio sensor sets the target output SL1H to SL1H.
It is determined whether the sufficient output VO2 has been generated.
Then, the output VO2 of the air-fuel ratio sensor 19 is sufficiently output.
If it is determined that (VO2 ≧ SL1H), step 12
Then, the process proceeds to step 13 to set the value of the flag FLGFB to 1. If it is determined that the output VO2 is not sufficiently output (VO2 <SL1H), the process proceeds to step 13 to reset the value of the flag FLGFB to 0. At this stage, since the air-fuel ratio feedback control has not been started yet,
Proceeding to 100, α = 100%, and feed-forward control of the air-fuel ratio is continued.

As described above, immediately after the air-fuel ratio feedback control condition is satisfied and before the activation determination of the air-fuel ratio sensor 19 is performed, the process proceeds from step 2 to step 10, and the activation determination is performed once after the activation determination is performed at least once. The fuel ratio feedback control is started. In this way, when the air-fuel ratio sensor 19 is activated and it is determined that a sufficient output VO2 can be generated, the flag FL is determined in step 2 in the next routine.
It is determined that the value of GFB is 1, and the process proceeds to step 3 and thereafter to start the air-fuel ratio feedback control.

In step 3, the output V of the air-fuel ratio sensor 19
O2 is read, and if rich, proceed to step 4; if lean, proceed to step 5. In step 4, the previous determination result is referred to. If the previous time is lean (that is, the current state has changed from lean to rich), the process proceeds to step 6. If the previous time is also rich, the process proceeds to step 7.

The output of the air-fuel ratio sensor 19 is equal to the target value SL.
Immediately after it is determined that activation has occurred beyond 1H, this state is maintained and the air-fuel ratio rich determination state is continued.
Proceeding to step 7, subtract the integral IR in the lean correction direction from the air-fuel ratio feedback correction coefficient α, and proceed to step 42 to integrate TTSL as the duration of the rich state.

The accumulated time TTSL is a continuation time of the rich state when the flow of this figure is executed in time synchronization, and the continuation time of the rich state is executed when the flow is executed in synchronization with the rotation of the engine. Number of rotations. Also, for example, this flow is executed in synchronization with the rotation of the engine, but when it is desired to measure the time for the rich continuation,
The ΔT to be integrated in step 7 refers to the elapsed time measured by a timing means different from the present flow, and
May be used as the difference between the time when the command is executed and the current time.

Similarly, if the process proceeds from step 5 to step 9, the air-fuel ratio continues to be lean.
Measure the lean duration with. When the process proceeds to step 6, the air-fuel ratio has changed from lean to rich.
(PL at lean time) is added, and at step 21, the accumulated result of the TTSL, that is, the final lean duration time is referred to, and this time is compared with the first reference time TTS.

When it is determined that the lean continuation time TTSL is longer than the reference time TTS, if the change width of the SL per unit time, that is, the change speed is increased, the overshoot is concerned as described above. twenty four
Then, the SL change width DSL is reduced by DDSL. Conversely, when it is determined that the lean continuation time TTSL is shorter than the reference time TTS, it is determined that the response of the air-fuel ratio sensor 19 has become faster, and the routine proceeds to step 23 where the change width DSL is set to DD.
Increase by SL.

None of the above, the lean duration time TT
When it is determined that SL is substantially equal to the reference time TTS,
The value of the change width DSL is maintained as it is. Using the change width DSL appropriately corrected as described above, in step 25, S
L is changed by DSL, and the result is compared with the lower limit SLN in step 26. When SL becomes lower than the lower limit SLN, in step 27, the SL is reduced to the lower limit SLN.
Fixed to

In step 41, DDSL = 0 is set to prepare for the subsequent measurement of the rich time. Similarly, if it is determined in step 5 that the air-fuel ratio has reversed from rich to lean, the process proceeds to step 8 and thereafter, where the rich continuation time T
While changing the change width DSL according to the TSL,
After changing with L, TTSL = 0 is set to prepare for the subsequent measurement of the lean time.

As described above, in the present embodiment, the SL is changed at a change speed (change width) corresponding to the response of the air-fuel ratio sensor 19, so that when the response is slow, the change speed is reduced and the overcorrection is performed. In addition, when the response is fast, the change speed is increased and the SL is quickly changed to the stoichiometric air-fuel ratio, thereby obtaining good air-fuel ratio control accuracy. In the present embodiment, an example in which the target output is changed to the final target output has been described.
The configuration may be such that the change is made until the integrated value reaches a predetermined value.

In the present embodiment, the response of the air-fuel ratio sensor 19 is measured based on the time required from when the output of the air-fuel ratio sensor 19 crosses the previous target output to when it crosses the current target output. However, in addition to this, the air-fuel ratio sensor 19
May be measured based on the time required for the output of the first to change from the first set value to the second set value. The operation of another embodiment for measuring the response of the air-fuel ratio sensor 19 in this manner is shown in the flowcharts of FIGS.

4 and 5 will be described. First, the output VO2 of the air-fuel ratio sensor 19 is set to the first value.
In order to measure the time required to change from the set value VO2H to the second set value VO2L, if the output VO2 is rich, that is, go from step 4 to step 7, the output VO2 is integrated from the air-fuel ratio feedback correction coefficient α. When the minute IR is reduced, a change in the output VO2 of the air-fuel ratio sensor 19 is observed, and the time during which the output VO2 is equal to or less than the first set value VO2H and equal to or more than the second set value VO2L is measured.

That is, when it is determined in step 51 that VO2> VO2H, the routine proceeds to step 41, in which TTSL is reset to 0, and by reducing α, the air-fuel ratio becomes lean and the output VO2 decreases. When it is equal to or less than VO2H, the process proceeds from step 51 to step 52 to step 42, and T
The TSL is counted up by ΔT. Further, the output VO
When the value of TTSL is lower than VO2L, the process proceeds from step 52 to step 53 to determine whether TTSL is 0 or not. Here, if VO2H> VO2L is set, since VO2 passes through step 42 once, VO2 becomes VO2L.
When TTSL (≧ ΔT) is not 0 at the time when step 53 is passed for the first time after it becomes less than
Proceed to the following, determine the response of the air-fuel ratio sensor 19 while comparing the TTSL with the reference time TTS as in the above-described embodiment,
The SL is changed according to the response.
Reset L to 0. However, the reference time TTS is set to a different value from the above embodiment.

Thereafter, when the process proceeds from step 52 to step 53, TTSL = 0, so that step
SL does not change without going to 31. That is, the SL is changed once each time the rich becomes one time. Of course, it is also possible to change the SL in the same manner at the time of the lean operation. However, in the present embodiment, an example in which the SL is changed only at the time of the rich operation is shown. In step 44, TTSL is reset to 0 without changing SL.

As described above, in this embodiment, the air-fuel ratio sensor
The response of the 19 output is measured with the time from the first set value to the second set value. As shown in FIG. 8, the response measurement result is compared with the first embodiment by SL. Since the lag until the change is reflected is shortened, when the response characteristic of the air-fuel ratio sensor 19 changes suddenly, the SL can be changed in a form more suitable for the characteristic at that time.
In the first embodiment, the reason why the rich continuation period indicated by the dotted arrow in FIG. 8 is not measured is that when the rich continuation period is measured, the SL is changed based on the measurement result. In this case, the output of the air-fuel ratio sensor crosses the SL again, so that a plurality of SL changes are performed, and the SL change width must be corrected according to the number of changes, which complicates the algorithm. That's why.

The case where the response characteristic of the air-fuel ratio sensor changes abruptly means, for example, that the sensor is in the process of being activated at a low temperature and a low load immediately after the start of the engine, and that the gas exchange rate near the sensor surface is high. From a state in which the sensor response decreases and the sensor response decreases, the engine shifts to an acceleration state, the exhaust gas temperature increases, and the gas exchange rate also recovers, so that the sensor response may suddenly change.

As described above, according to the present invention, when the target output is changed from the target output for determining the activation of the air-fuel ratio sensor to the target output for the air-fuel ratio feedback control,
If the response of the air-fuel ratio sensor is decreasing, reduce the target output change speed to reduce overcorrection.If the response is good, increase the change speed and quickly approach the final target output. This makes it possible to appropriately correct the air-fuel ratio, thereby improving the exhaust gas purification performance and obtaining good driving performance.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration and functions of the present invention.

FIG. 2 is a diagram showing a system configuration according to an embodiment of the present invention.

FIG. 3 is a diagram showing output characteristics of an air-fuel ratio sensor used in the embodiment.

FIG. 4 is a flowchart showing a first stage of an air-fuel ratio control routine according to the first embodiment.

FIG. 5 is a flowchart showing a latter part of the flowchart of the above.

FIG. 6 is a flowchart showing a first stage of an air-fuel ratio control routine according to the second embodiment.

FIG. 7 is a flowchart showing a latter part of the flowchart of the above.

FIG. 8 is a time chart showing the state of each state under the control of the first embodiment and the second embodiment.

[Description of Signs] 11 Engine 15 Fuel injection valve 16 Control unit 19 Air-fuel ratio sensor

Claims (7)

[Claims] An air-fuel ratio sensor for detecting an air-fuel ratio of an air-fuel mixture supplied to an engine in response to a specific component in exhaust gas, and feedback of an air-fuel ratio so that an output of the air-fuel ratio sensor becomes a target output. An air-fuel ratio control device for an internal combustion engine, comprising: an air-fuel ratio feedback control unit for controlling; and a target output changing unit for changing a target output of the air-fuel ratio sensor. A response measuring unit for measuring a response of the air-fuel ratio sensor And target output change speed setting means for setting a change speed of a target output of the air-fuel ratio sensor in accordance with the measured response of the air-fuel ratio sensor. An air-fuel ratio control device for an internal combustion engine, wherein a target output is changed at the set change speed. 2. The method according to claim 1, wherein the target output changing means sets the target output to a target air-fuel ratio after the activation based on an activation determination target output for determining whether the air-fuel ratio sensor is activated after the engine is started. 2. The method according to claim 1, wherein the output is changed to a target output for determining a target air-fuel ratio for feedback control to a fuel ratio.
3. The air-fuel ratio control device for an internal combustion engine according to claim 1. 3. The target air-fuel ratio is a stoichiometric air-fuel ratio, and the activation determination target output is set to a value corresponding to an air-fuel ratio richer than the stoichiometric air-fuel ratio. Air-fuel ratio control device for an internal combustion engine. 4. The target output changing means sets a final target output, and ends the change of the target output when the target output changed at the set change speed reaches the final target output. The air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 3, wherein: 5. The response measuring means measures a response based on a time period from when the output of the air-fuel ratio sensor crosses a previous target output to when the output of the air-fuel ratio sensor crosses a current target output after the start of the air-fuel ratio feedback control. The air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 4, wherein: 6. The response measuring means measures a response based on a time required until the output value of the air-fuel ratio sensor at the start of the air-fuel ratio feedback control changes to a set value shifted by a predetermined amount from the output value. The air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 4, characterized in that: 7. The internal combustion engine according to claim 1, wherein the air-fuel ratio sensor detects stepwise whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio. Air-fuel ratio control device.