JP3513880B2 - Engine air-fuel ratio control device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine air-fuel ratio control device for controlling an air-fuel ratio of an air-fuel mixture supplied to an engine to a lean side from a stoichiometric air-fuel ratio under predetermined operating conditions.

[0002]

2. Description of the Related Art Conventionally, for the purpose of improving fuel efficiency, the air-fuel ratio of an air-fuel mixture supplied to an engine is leaner than the stoichiometric air-fuel ratio (Stoichi; A / F = 14.7) under predetermined operating conditions. / F = around 22), and
In some cases, various conditions are set when switching to the lean side air-fuel ratio.

For example, Japanese Patent Laid-Open No. 63-12851 prohibits switching to the lean side air-fuel ratio during a gear change and a predetermined time thereafter, and in Japanese Patent Laid-Open No. 63-12846, a lean control range is set for each gear position. In addition, in Japanese Patent Laid-Open No. 61-187552, a delay time is provided for switching to the lean side air-fuel ratio.

[0004]

However, such a conventional air-fuel ratio control device for an engine is designed to switch the air-fuel ratio immediately or simply with a certain delay time in response to changes in operating conditions. Since the O ₂ storage effect of ceria contained in is not maximized, NOx cannot be converted under the condition that the air-fuel ratio is changed from stoichiometric to lean, in other words, when the air-fuel ratio is changed from stoichiometric to lean, it will not be There has been a problem that the NOx emission amount sharply increases due to a gradual change in the air-fuel ratio in the original catalyst.

In view of such conventional problems, the present invention can prevent a rapid increase in NOx emission amount due to a gradual change of the air-fuel ratio in the three-way catalyst when the air-fuel ratio is changed from stoichiometric to lean. The purpose is to do so.

[0006]

Therefore, the present invention provides an air-fuel ratio control device having the following configuration (1) or (2) in an engine having a three-way catalyst containing ceria as a cocatalyst in the exhaust passage. provide. (1) As shown in (1) in FIG. 1, the engine speed, based on
Based on the present fuel injection amount and the cooling water temperature, and the lean condition determining means A determines a predetermined operating condition for controlling the air-fuel ratio leaner than the stoichiometric air-fuel ratio of a mixture supplied to the engine operating conditions Is changed and the predetermined operating condition is determined, the switching enrichment means B for enriching the air-fuel ratio from the stoichiometric air-fuel ratio for a predetermined period prior to switching to the lean side air-fuel ratio; An air-fuel ratio lean controller for switching the air-fuel ratio to the lean-side air-fuel ratio after the lapse of the period is provided, and an air-fuel ratio control device for the engine is configured. (2) As shown in (2) of FIG. 1, lean condition determination means D for determining a predetermined operating condition for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine to an air-fuel ratio leaner than the stoichiometric air-fuel ratio.
And an in-catalyst rich state determination means E for determining whether or not the inside of the three-way catalyst is in a rich state based on a signal from an O ₂ sensor arranged downstream of the three-way catalyst, and the lean condition determination means. The air-fuel ratio that switches the air-fuel ratio to the lean side air-fuel ratio only under the condition that the inside-catalyst rich state determination means E determines that the inside of the three-way catalyst is rich when the predetermined operating condition is determined by D. Lean switching means F
And are provided to constitute an engine air-fuel ratio control device.

[0007]

In the configuration of (1) above, after the lean condition is satisfied and before the air-fuel ratio is changed from stoichiometric to lean, the air-fuel ratio is shifted to the rich side to generate a reducing atmosphere in the three-way catalyst, and 2CeO ₂ → As Ce ₂ O ₃ +1/2 O ₂ , maximize the O ₂ storage capacity to Ce, then stoichiometric →
Switch to lean. Therefore, when switching from stoichiometric to lean, the O ₂ storage effect (Ce ₂ O ₃ +1/2 O ₂ → 2
When the air-fuel ratio in the three-way catalyst changes to lean due to CeO ₂ ), the time for which the intermediate A / F of A / F = 14.7 to 22 becomes short, and the NOx emission amount can be reduced.

In the configuration of (2) above, the lean condition
After the establishment of the₂Rich sensor output
Then, the inside of the three-way catalyst is in a reducing atmosphere, in other words, Ce
To O ₂With the maximum storage capacity, stoichi →
Switch to the next. Therefore, when switching from stoichiometric to lean
Is O₂Storage effect (Ce₂O₃+1/2 O₂→ 2C
eO₂) Changes the air-fuel ratio in the three-way catalyst to lean
When A / F = 14.7 to 22
It shortens and can reduce NOx emissions.

[0009]

EXAMPLES Examples of the present invention will be described below. FIG. 2 shows the system configuration of the first embodiment. Air is drawn into the combustion chamber of each cylinder of the engine 1 from the air cleaner 2 through the throttle valve 3 and the intake manifold 4. An electromagnetic fuel injection valve 5 is provided in each branch portion of the intake manifold 4, and each fuel injection valve 5
Air-fuel mixture is generated by the fuel injected from. Then, the air-fuel mixture is ignited by the spark plug 6 and burned in the combustion chamber.

The fuel injection valve 5 is opened by being energized by a drive pulse signal output at a predetermined timing in synchronization with engine rotation from a control unit 12 which will be described later.
The fuel adjusted to a predetermined pressure is injected. Therefore, the fuel injection amount is controlled by the pulse width of the drive pulse signal. Exhaust gas from the engine 1 reaches an exhaust pipe 8 through an exhaust manifold 7.

A three-way catalyst 10 is interposed in the exhaust pipe 8. This three-way catalyst 10 has an H value near the stoichiometric air-fuel ratio.
It has a three-way catalytic function that oxidizes C and CO and reduces NOx, and a lean NOx reducing catalytic function that can reduce NOx under lean conditions. The three-way catalyst 10 contains ceria as a cocatalyst. Then, the exhaust gas passes through the three-way catalyst 10 and is then discharged through the muffler 11.

A controller for controlling the operation of the fuel injection valve 5.
The unit 12 contains a microcomputer
Thus, signals are input from various sensors. The various
As the sensor of the engine, the engine is provided on the upstream side of the throttle valve 3.
1. An air flow meter 13, which detects the intake air flow rate Q of 1,
Reference crank angle signal and unit from engine 1 camshaft rotation
Outputs a crank angle signal to indirectly detect engine speed N
Crank angle sensor 14 that can be output, engine 1 water
A water temperature sensor 15 for detecting the cooling water temperature Tw in the jacket,
It is attached to the exhaust manifold 7 and sucked into the engine 1.
Corresponding to the oxygen concentration in the exhaust gas related to the air-fuel ratio of the air-fuel mixture
Output voltage signal ₂Sensor 16 etc. are provided
It

Here, the control unit 12
Controls the operation of the fuel injection valve 5 by performing arithmetic processing as described below based on signals from the various sensors. Next, the contents of arithmetic processing of the control unit 12 will be described with reference to the flowcharts of FIGS. It should be noted that this flow is executed, for example, every 50 ms. Step 1 (S in the figure
It is written as 1. The same shall apply hereinafter)
The intake air flow rate Q is detected based on the signal from the.

In step 2, the engine speed N is detected based on the signal from the crank angle sensor 14. In step 3, from the intake air flow rate Q and the engine speed N,
Basic fuel injection amount Tp = K × Q / N (K
Is a constant). In step 4, the cooling water temperature Tw is detected based on the signal from the water temperature sensor 15.

In step 5, the engine speed N is 800
It is determined whether or not it is within the range of rpm to 4000 rpm, and if it is out of the range, it proceeds to step 8 (FIG. 4) as a stoichiometric condition, and if it is within the range, it proceeds to step 6. In step 6, it is determined whether the basic fuel injection amount Tp is within the range of 0.5 ms to 5 ms,
If it is out of range, step 8 is set as stoichiometric condition (Fig. 4)
If it is within the range, proceed to step 7.

In step 7, the cooling water temperature Tw is 70.degree.
It is determined whether the temperature is within the range of 100 ° C. If the temperature is out of the range, the stoichiometric condition is set and the process proceeds to step 8 (FIG. 4). That is, in this embodiment, 800 rpm <N <4000 rpm, 0.5 ms <T
The lean condition is p <5 ms, and 70 ° C. <Tw <100 ° C., and the stoichiometric condition is when even one of them is not satisfied.

The stoichiometric condition and the lean condition will be described below separately. [Stoichiometric condition] In step 8 (FIG. 4), the lean flag Flean is set to zero. In step 9, the output voltage V of the O ₂ sensor 16 is read. The output voltage read this time is V _n
And the previously read output voltage is V _n-1 .

In step 10, the O ₂ sensor read this time is read.
The 16 output voltages V _n are compared with a predetermined slice level voltage SL (for example, 400 mV) to determine whether V _n > SL (rich). If V _n > SL (rich), step 11
To the output voltage V _{n-1 of the} O ₂ sensor 16 read last time.
Is compared with a predetermined slice level voltage SL, and V _n-1 > S
It is determined whether L (previous rich).

If V _n-1 > SL (previous rich) in the determination in step 11, the rich state is continuing, so the routine proceeds to step 12 and the air-fuel ratio feedback correction coefficient α is set to a predetermined value with respect to the previous value. Decrease the integrated amount ΔI. If V _n-1 ≦ SL (lean last time) in the determination at step 11, it means that the lean-to-rich change has occurred, so the routine proceeds to step 13, where the air-fuel ratio feedback correction coefficient α is set to a predetermined value with respect to the previous value. Proportional decrease ΔP (>> ΔI).

When V _n ≤ SL (lean), the step
Proceed to 14 and output voltage V of the O ₂ sensor 16 read last time
_n-1 is compared with a predetermined slice level voltage SL, and V _n-1
> SL (previous rich) is determined. If V _n-1 ≦ SL (previously lean) in the determination in step 14, the lean state is continuing, so the routine proceeds to step 15, where the air-fuel ratio feedback correction coefficient α is a predetermined integral Δ with respect to the previous value.
I increase.

If V _n-1 > SL (previously rich) in the determination in step 14, it means that there has been a change from rich to lean, so the routine proceeds to step 16, and the air-fuel ratio feedback correction coefficient α is set to the previous value. On the other hand, a predetermined proportional amount ΔP (>> ΔI) is increased. After setting the increase / decrease of the air-fuel ratio feedback correction coefficient α, the routine proceeds to step 17. In step 17, the basic fuel injection amount Tp and various correction coefficients COE including the water temperature correction coefficient etc.
From F, the air-fuel ratio feedback correction coefficient α, and the voltage correction amount Ts based on the battery voltage, the fuel injection amount Ti is calculated according to the following equation.

Ti = Tp × COEF × α + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a drive pulse signal having a pulse width of this Ti is injected at a predetermined timing synchronized with the engine rotation. It is output to the valve 5 and fuel injection is performed. At this time, the air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio (Stoichi; A / F = 14.7).

[Lean Condition] In step 18 (FIG. 5),
It is determined whether the lean flag Flean is 1. When the stoichiometric condition is changed to the lean condition, the lean flag Flean is still 0, so in step 19, the lean flag Flean.
After an is set to 1 and the value of the timer TM for measuring the time from the satisfaction of the lean condition is set to 0 in step 20, the process proceeds to step 22. On the other hand, when the lean condition continues, the lean flag Flean = 1 has already been set, and therefore the value of the timer TM is the execution time interval of this flow in order to measure the time from the satisfaction of the lean condition in step 21. After increasing by 50 ms, proceed to step 22.

In step 22, it is judged whether or not the value of the timer TM indicating the time from the satisfaction of the lean condition exceeds 5 seconds. If TM ≦ 5 sec, it is within 5 sec after the lean condition is reached, so steps 23 to 31 are executed to shift the air-fuel ratio feedback control to the rich side and make the inside of the three-way catalyst 10 a reducing atmosphere. .

In step 23, the output voltage V of the O ₂ sensor 16 is read. The output voltage read this time is V _n, and the output voltage read last time is V _n-1 . In step 24,
The output voltage V _n of the O ₂ sensor 16 read this time is compared with a predetermined slice level voltage SL (for example, 400 mV), and V _n >
It is determined whether or not it is SL (rich).

When V _n > SL (rich), step
Go to 25 and output voltage V of the O ₂ sensor 16 read last time
_n-1 is compared with a predetermined slice level voltage SL, and V _n-1
> SL (previous rich) is determined. If V _n-1 > SL (previous rich) in the determination in step 25, the rich state is continuing, so the routine proceeds to step 26, where the air-fuel ratio feedback correction coefficient α is a predetermined integral Δ with respect to the previous value.
I decrease.

If V _n-1 ≤SL (previously lean) is judged in step 25, it means that the lean-to-rich change has occurred. Therefore, the routine proceeds to step 27, where the air-fuel ratio feedback correction coefficient α is set to the previous value. On the other hand, it decreases by a predetermined proportional amount ΔP (>> ΔI). If V _n ≦ SL (lean), step 28
To the output voltage V _{n-1 of the} O ₂ sensor 16 read last time.
Is compared with a predetermined slice level voltage SL, and V _n-1 > S
It is determined whether L (previous rich).

If V _n-1 ≤SL (previously lean) as determined in step 28, the lean state is continuing, so the routine proceeds to step 29, where the air-fuel ratio feedback correction coefficient α is set to a predetermined value with respect to the previous value. Increase ΔI by the integral. If V _n-1 > SL (previously rich) is determined in step 28, it means that the state has changed from rich to lean, so the routine proceeds to step 30, and the air-fuel ratio feedback correction coefficient α is set to a predetermined value with respect to the previous value. It is increased twice the proportional amount ΔP (>> ΔI). Therefore,
In this case, the proportion of the rich side when rich → lean is
Lean → Double the proportion to the lean side when rich (2 × Δ
P), and the air-fuel ratio during the air-fuel ratio feedback control is shifted to the rich side.

After setting the increase / decrease of the air-fuel ratio feedback correction coefficient α, the routine proceeds to step 31. In step 31, the basic fuel injection amount Tp and various correction coefficients CO including the water temperature correction coefficient
From the EF, the air-fuel ratio feedback correction coefficient α, and the voltage correction amount Ts based on the battery voltage, the fuel injection amount Ti is calculated according to the following equation. Ti = Tp × COEF × α + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a drive pulse signal having a pulse width of Ti is supplied to the fuel injection valve 5 at a predetermined timing in synchronization with the engine rotation. It is output and fuel injection is performed. At this time, the air-fuel ratio is shifted to the rich side by increasing the proportional amount to the rich side during rich → lean as described above.

As described above, the first five immediately after the lean condition is satisfied.
During sec, the air-fuel ratio is not immediately made lean and the air-fuel ratio
In the three-way catalyst 10, shift the feedback control to the rich side.
To reduce the 2C inside the three-way catalyst 10
eO₂→ Ce₂O₃+1/2 O ₂Let the reaction of. this
O to Ce₂Maximize storage capacity. Re
If 5 seconds have passed after the condition is met, go to step 22.
If it is judged that TM> 5 sec, the process proceeds to step 32.

In step 32, the basic fuel injection amount Tp is corrected to a lean side air-fuel ratio (A / F = 22), and then various correction coefficients COEF including a water temperature correction coefficient and a voltage correction amount based on the battery voltage are calculated. The fuel injection amount Ti is calculated from Ts according to the following equation. Ti = Tp × (14.7 / 22) × COEF + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and the drive pulse signal with the pulse width of Ti is set at a predetermined timing synchronized with the engine rotation. The fuel is injected by being output to the injection valve 5. At this time, the air-fuel ratio is controlled to the lean side air-fuel ratio (A / F = 22).

FIG. 6 shows the effect of this embodiment in comparison with the conventional example. In the conventional example, when the lean flag Flean = 1 due to satisfaction of the lean condition, the air-fuel ratio is immediately switched from stoichiometric to lean. At this time, since the air-fuel ratio in the three-way catalyst 10 gradually changes, the A / F change time in the catalyst is long, and during this period, the NOx emission amount increases as shown by S ₁ in the figure.

On the other hand, in the present embodiment, after the lean condition is satisfied, the air-fuel ratio is shifted to the rich side for a predetermined period (5 seconds) to generate a reducing atmosphere in the three-way catalyst, and 2CeO ₂
→ As Ce ₂ O ₃ +1/2 O ₂ , maximize the O ₂ storage capacity to Ce, and then switch from stoichiometric to lean. Therefore, when switching from stoichiometric to lean, the O ₂ storage effect (Ce ₂ O ₃ +1/2 O ₂ ) during the period indicated by A in the figure is
→ 2CeO ₂ ) shortens the A / F change time in the catalyst when the air-fuel ratio in the three-way catalyst changes to lean, in other words, shortens the time to reach the intermediate A / F of A / F = 14.7 to 22. Therefore, the NO emission amount becomes as shown in S _{2 in} the figure, and the NOx emission amount decreases as compared with the conventional example.

In the present embodiment, steps 5-7 are performed.
Corresponds to the lean condition determining means, and steps 23 to 31 are performed.
The portion of corresponds to the switching enrichment means, and the portion of step 32 corresponds to the lean switching means. Further, in this embodiment,
Although the rich shift during the air-fuel ratio feedback control is performed in proportion to the rich side, it may be performed by changing the slice level voltage.

FIG. 7 shows the system configuration of the second embodiment. The difference from FIG. 2 is that the exhaust pipe 8 is located downstream of the three-way catalyst 10.
A rear O ₂ sensor 17 is provided in the control unit 12 and its signal is input to the control unit 12. Next, the contents of the arithmetic processing of the control unit 12 will be described with reference to the flowchart of FIG. It should be noted that this flow is executed, for example, every 50 ms.

In step 51, the intake air flow rate Q is detected based on the signal from the air flow meter 13. Step
At 52, the engine speed N is detected based on the signal from the crank angle sensor 14. In step 53, the basic fuel injection amount Tp = K × Q / N (K is a constant) corresponding to the theoretical air-fuel ratio is calculated from the intake air flow rate Q and the engine speed N.

In step 54, the cooling water temperature Tw is detected based on the signal from the water temperature sensor 15. In step 55, it is determined whether the engine speed N is in the range of 800 rpm to 4000 rpm. If it is out of the range, the stoichiometric condition is set and the process proceeds to step 61. If it is in the range, the process proceeds to step 56.

In step 56, the basic fuel injection amount Tp is
It is determined whether it is within the range of 0.5 ms to 5 ms. If it is out of the range, the stoichiometric condition is set and the process proceeds to step 61. In step 57, the cooling water temperature Tw is
Whether it is within the range of 70 ° C. to 100 ° C. is judged, and if it is out of the range, it goes to step 61 as a stoichiometric condition, and if it is out of the range, it goes to step 58 as a lean condition.

That is, in this embodiment, 800 rpm <N <40
00rpm, 0.5ms <Tp <5ms, and 70 ℃ <Tw <100 ℃
Is the lean condition, and the stoichiometric condition is when even one of them is not satisfied. In the case of stoichiometric conditions, after the lean flag Flean is set to 0 in step 61, in step 62, the basic fuel injection amount Tp, various correction coefficients COEF including the water temperature correction coefficient and the voltage correction amount Ts based on the battery voltage The fuel injection amount Ti is calculated according to the formula.

Ti = Tp × COEF + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a drive pulse signal having a pulse width of Ti is set at a predetermined timing synchronized with the engine rotation. Is output to fuel injection. The description of the air-fuel ratio feedback control is omitted.

In the case of the lean condition, the routine further proceeds to step 58, and it is determined whether or not the lean flag Flean is 1. When the stoichiometric condition is changed to the lean condition, the lean flag Flean = 0 is still set, and thus the process proceeds to step 59. In step 59, the output voltage V _R of the rear O ₂ sensor 17 is read.

Then, in step 60, the rear O ₂ sensor
The output voltage V _R of 17 predetermined slice level voltage (e.g.
400 mV) to determine whether V _R > SL (rich). When V _R ≦ SL (lean), the inside of the three-way catalyst 10 is not in the reducing atmosphere, so the stoichiometric → lean switching is not performed, and the process proceeds to steps 61 and 62 to perform stoichiometric control.

When V _R > SL (rich), a three-way catalyst
Since the inside of 10 is in a reducing atmosphere, the routine proceeds to step 63, where the lean flag Flean is set to 1, and then the routine proceeds to step 64. In step 64, the basic fuel injection amount Tp is set to the lean side air-fuel ratio (A
/ F = 22), the fuel injection amount Ti is calculated according to the following equation from various correction coefficients COEF including a water temperature correction coefficient and the like and the voltage correction amount Ts based on the battery voltage.

Ti = Tp × (14.7 / 22) × COEF + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a drive pulse having a pulse width of this Ti is set at a predetermined timing in synchronization with engine rotation. A signal is output to the fuel injection valve 5 and fuel injection is performed. At this time, the air-fuel ratio is controlled to the lean side air-fuel ratio (A / F = 22).

FIG. 8 shows the effect of this embodiment in comparison with the conventional example. The output of the rear O ₂ sensor 17 indicates the average value of the air-fuel ratio in the three-way catalyst 10. When this output is 400 mV or less, it is leaner than stoichiometric and when it is 400 mV or more, it is richer than stoichiometric.
Therefore, above 400 mV, 2CeO ₂ → Ce ₂ O ₃ + 1 /
The reaction of 2 O ₂ is performed, and the O ₂ storage capacity of Ce is large.

Accordingly, when the rear O ₂ sensor output is leaner when the rear O ₂ sensor output is 400 mV or more as in the present embodiment, as compared with the case where the rear O ₂ sensor output is lean as in the conventional example, the air-fuel ratio is switched. Inside the three-way catalyst 10 A / F = 14.7〜
Since the time to reach the intermediate A / F of 22 (the A / F change time in the catalyst) is short, the NOx generation amount decreases from S ₁ to S ₂ in the figure. This is the O ₂ storage effect (Ce
_{This is because 2} O ₃ +1/2 O ₂ → 2CeO ₂ ) is exhibited.

In this embodiment, steps 55 to 57 are used.
Part corresponds to the lean condition judging means, and steps 59, 60
Part corresponds to the rich condition determination means in the catalyst,
The portion 64 corresponds to lean switching means.

[0048]

As described above, according to the present invention,
After the condition is met, empty before switching from stoichiki to lean.
By shifting the fuel ratio to the rich side, the ceria O₂Stray
After maximizing the ability, or rear O₂Sensor output
Ceria based on O ₂Large storage capacity
Switch from stoichiki to lean when
The effect of reducing NOx emissions at the time of replacement can be obtained.
It

[Brief description of drawings]

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

FIG. 2 is a system diagram showing a first embodiment.

FIG. 3 is a flowchart showing the contents of arithmetic processing of the first embodiment.

FIG. 4 is a flowchart following FIG.

FIG. 5 is a flowchart following FIG.

FIG. 6 is a diagram showing an effect of the first embodiment.

FIG. 7 is a system diagram showing a second embodiment.

FIG. 8 is a flowchart showing arithmetic processing contents of the second embodiment.

FIG. 9 is a diagram showing the effect of the second embodiment.

[Explanation of symbols]

1 engine 5 fuel injection valve 10 three-way catalyst 12 control unit 13 air flow meter 14 crank angle sensor 15 water temperature sensor 16 O ₂ sensor 17 rear O ₂ sensor

Claims (2)

(57) [Claims] 1. An engine having a three-way catalyst containing ceria as a co-catalyst in an exhaust passage , based on engine speed, basic fuel injection amount and cooling water temperature.
And a lean condition determining means for determining a predetermined operating condition for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, and the operating condition changes to determine the predetermined operating condition. When the air-fuel ratio of the lean side is changed to a lean side air-fuel ratio, the switching-time enrichment means for making the air-fuel ratio richer than the stoichiometric air-fuel ratio for a predetermined period prior to switching to the lean-side air-fuel ratio. An air-fuel ratio control device for an engine, comprising: an air-fuel ratio lean switching means for switching to. 2. An engine having a three-way catalyst containing ceria as a co-catalyst in an exhaust passage, a predetermined operating condition for controlling an air-fuel ratio of an air-fuel mixture supplied to the engine to an air-fuel ratio leaner than a stoichiometric air-fuel ratio. Lean condition determining means for determining, and O arranged downstream of the three-way catalyst.
_{2 In-} catalyst rich state determination means for determining whether the inside of the three-way catalyst is in a rich state based on a signal from a sensor, and when the lean operating condition is determined to be the predetermined operating condition An air-fuel ratio control device for an engine, comprising: an air-fuel ratio lean switching means for switching the air-fuel ratio to a lean-side air-fuel ratio only under a condition where the inside-catalyst rich state determination means determines that the inside of the three-way catalyst is rich.