JPH0552141A - Air fuel ratio control device for engine - Google Patents

Abstract

PURPOSE:To attain accurate air-fuel ratio control even in the case of releasing HC, adsorbed by an HC adsorbing catalyst, by increasing a temperature, relating to an air-fuel ratio control device for an engine wherein the HC adsorbing catalyst, a three way catalyst and an air-fuel ratio sensor are provided in order from upstream in an exhaust system of the engine. CONSTITUTION:A control means 53 for controlling at least either of fuel or intake air, so that air-fuel ratio of a supply mixture to an engine combustion chamber is vibratively given having a required period in rich and lean sides from the theoretical air-fuel ratio, is provided. A correcting means 60 for correcting a means value of air-fuel ratio of the supply mixture, set vibratively by the control means 63, based on an output of an air-fuel ratio sensor 17, so that a means value of air-fuel ratio of exhaust passing through a three way catalyst 9 may come into the vicinity of a stoichiometric value, is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an engine exhaust system,
The present invention relates to an air-fuel ratio control device for an engine, which includes an HC adsorption catalyst, a three-way catalyst, and an air-fuel ratio sensor in this order from the upstream side.

[0002]

2. Description of the Related Art Conventionally, for purification of exhaust gas, there is an engine having a three-way catalyst in the exhaust system of the engine. In this case, in order to ensure the purification efficiency of the three-way catalyst, the exhaust gas flowing into the three-way catalyst. It is necessary to change the air-fuel ratio of in the vicinity of the stoichiometric air-fuel ratio (Stoichio) in a relatively short cycle.
An oxygen concentration sensor (O ₂ sensor) is provided upstream of the three-way catalyst to perform air-fuel ratio feedback control with a small response delay.

On the other hand, recently, it has been particularly required to reduce HC discharged from the engine at a low temperature, and as one of the measures against this, a Y-type zeolite, mordenite or the like is provided on the upstream side of the three-way catalyst. It has been proposed to provide an HC adsorbent or an adsorbent catalyst (Japanese Patent Application Laid-Open No. 2-7532).
No. 7).

[0004]

However, even with such an HC adsorption catalyst, other harmful components (for example, CO, NOx) and HC after warm-up have been purified at least to the conventional level. Efficiency is of course required,
Therefore, even in a system having this HC adsorption catalyst, air-fuel ratio feedback control or some purification system replacing it is required.

Considering the technique for performing the air-fuel ratio feedback control as described above in the system having the HC adsorption catalyst as described above, first, in order to secure the feedback response, It is premised that an O ₂ sensor is provided on the upstream side. However, in a system having such an HC adsorption catalyst, when the engine warms up and the temperature of the HC adsorption catalyst exceeds a predetermined temperature, the engine HC adsorbed on the HC adsorption catalyst at low temperature
Is released and is led to the three-way catalyst. Therefore, during the period when HC is released from the HC adsorption catalyst, the air-fuel ratio of the exhaust flowing into the three-way catalyst can be grasped by the O ₂ sensor. However, there is a problem that accurate feedback control of the air-fuel ratio becomes impossible.

The present invention was devised in view of the above problems, and the HC adsorbed by the HC adsorbing catalyst as the temperature rises.
It is an object of the present invention to provide an engine air-fuel ratio control device that enables accurate air-fuel ratio control by making it possible to grasp the air-fuel ratio of exhaust gas that flows into a three-way catalyst even when is released.

[0007]

Therefore, according to the air-fuel ratio control device for an engine of the present invention as set forth in claim 1, an HC exhaust catalyst, a three-way catalyst and an air-fuel ratio sensor are arranged in the engine exhaust system in order from the upstream side. Of the fuel or intake air so that the air-fuel ratio of the air-fuel mixture supplied to the engine combustion chamber is given oscillatingly with a required period on the rich side and the lean side of the stoichiometric air-fuel ratio. The supply air-fuel mixture is provided with a control means for controlling at least one of them, and the supply air-fuel mixture is oscillatingly set by the control means so that the average value of the air-fuel ratio of the exhaust gas passing through the three-way catalyst is approximately near stoichiometry. It is characterized in that it is provided with a correction means for correcting the average value of the air-fuel ratio of the above-mentioned item based on the output of the air-fuel ratio sensor.

According to a second aspect of the present invention, there is provided an air-fuel ratio control system for an engine according to the present invention, when the engine is started in a low temperature state, when a set time has elapsed since the engine was started, the temperature of the engine or the HC. A limiting means is provided for temporarily limiting the correction of the average value of the air-fuel ratio of the supplied mixture to the lean side from the time when the temperature of the adsorption catalyst exceeds the set temperature. There is.

[0009]

In the above-described air-fuel ratio control system for an engine of the present invention, the control means causes the air-fuel ratio of the air-fuel mixture supplied to the engine combustion chamber to have a required period on the rich side and the lean side with respect to the theoretical air-fuel ratio. At least one of the fuel and the intake air is controlled so as to be provided in an oscillatory manner. At this time, the average value of the air-fuel ratio of the exhaust gas that has passed through the three-way catalyst becomes approximately stoichiometric by the correction means. As described above, the average value of the air-fuel ratio of the supply air-fuel mixture that is set oscillatingly by the control means is corrected based on the output of the air-fuel ratio sensor (Claim 1).

Further, when the engine is started in a low temperature state, the limiting means temporarily corrects when a set time has elapsed since the engine was started or when the temperature of the engine or the temperature of the HC adsorption catalyst exceeds the set temperature. The correction of the mean value of the air-fuel ratio of the supply air-fuel mixture by the means to the lean side is limited (claim 2).

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An engine air-fuel ratio control system as an embodiment of the present invention will be described below with reference to the drawings.
2 is a block diagram showing the overall configuration of this device, FIG. 2 is an overall configuration diagram of an engine system having this device, FIG. 3 is a control hardware block diagram of this device, and FIGS. 6 is a waveform diagram for explaining the operation of the present apparatus.

An engine system having this device is as shown in FIGS. 1 and 2. As shown in these figures, an engine (internal combustion engine) EG has an intake passage 2 communicating with a combustion chamber 1 and an exhaust gas. It has a passage 3 and an intake passage 2
The combustion chamber 1 and the combustion chamber 1 are controlled to communicate with each other by the intake valve 4, and the exhaust passage 3 and the combustion chamber 1 are controlled to communicate with each other by the exhaust valve 5.

In the intake passage 2, from the upstream side,
An air cleaner 6, a throttle valve 7 and an electromagnetic fuel injection valve (injector) 8 are provided. In the exhaust passage 3, an HC adsorbing catalyst 9A and an exhaust gas purifying catalyst converter (three-way exhaust valve) are arranged in this order from the upstream side. A catalyst) 9 and a muffler (silencer) not shown are provided. The intake passage 2
Is provided with a surge tank 2a.

Here, the HC adsorption catalyst 9A is composed of Y-type zeolite, mordenite, etc., and adsorbs HC at low temperatures, while releasing the adsorbed HC when the temperature rises.

Further, the injectors 8 are provided in the intake manifold portion by the number of cylinders. Now, assuming that the engine EG of this embodiment is an in-line four-cylinder engine, four injectors 8 are provided. That is, it can be said that the engine is a so-called multi-point fuel injection (MPI) type multi-cylinder engine.

Further, the throttle valve 7 is connected to an accelerator pedal via a wire cable, so that the opening can be changed according to the amount of depression of the accelerator pedal. The opening / closing drive is also performed by an ISC motor, which allows the opening degree of the throttle valve 7 to be changed without pressing the accelerator pedal during idling.

With such a configuration, the air sucked through the air cleaner 6 according to the opening degree of the throttle valve 7 is mixed with the fuel from the injector 8 in the intake manifold portion so as to have an appropriate air-fuel ratio, and the inside of the combustion chamber 1 is mixed. After the ignition plug 35 is ignited at an appropriate timing to be burned to generate an engine torque, the air-fuel mixture is discharged to the exhaust passage 3 as exhaust gas, and the HC adsorption catalyst 9A and the catalytic converter 9 exhaust the exhaust gas. CO, HC, N inside
After the harmful components of Ox are purified, they are silenced by the muffler and released to the atmosphere side.

Further, various sensors are provided to control the engine EG. First, on the intake passage 2 side, an air flow sensor 11 for detecting the intake air amount from the Karman vortex information, an intake temperature sensor 12 for detecting the intake air temperature, and an atmospheric pressure sensor 13 for detecting the atmospheric pressure are provided in the portion where the air cleaner is provided. The throttle valve is provided with a potentiometer-type throttle sensor 14 for detecting the opening of the throttle valve 7, an idle switch 15 for detecting an idling state, and the like.

Further, on the exhaust passage 3 side, the oxygen concentration (O
An oxygen concentration sensor 17 (hereinafter, simply referred to as an O ₂ sensor 17) is provided as an air-fuel ratio sensor for detecting ( ₂ concentration).

Further, as other sensors, a water temperature sensor 19 for detecting an engine cooling water temperature, and a crank angle sensor 2 for detecting a crank angle as shown in FIGS. 1 and 3.
1 (this crank angle sensor 21 also serves as a rotation speed sensor for detecting the engine rotation speed) and a TDC sensor (cylinder discrimination sensor) 22 for detecting the top dead center of the first cylinder (reference cylinder) are provided in the distributor. Has been.

The detection signals from these sensors are input to the electronic control unit (ECU) 23.

A voltage signal from the battery sensor 25 that detects the voltage of the battery and a signal from the cranking switch 20 or the ignition switch (key switch) that detects the start time are input to the ECU 23. There is.

By the way, the hardware configuration of the ECU 23 is as shown in FIG. 3, and the ECU 23 has a CPU 27 as its main part.
Detection signals from the intake air temperature sensor 12, the atmospheric pressure sensor 13, the throttle sensor 14, the O ₂ sensor 17, the water temperature sensor 19 and the battery sensor 25 are input interfaces 28.
And the air flow sensor 11, the crank angle sensor 21, T
Detection signals from the DC sensor 22, the idle switch 15, the cranking switch 20, the ignition switch and the like are input via the input interface 29.

Further, the CPU 27 is a ROM for storing program data and fixed value data via a bus line.
31. Data is exchanged with a battery backup RAM (not shown) that is backed up by holding the stored contents of the RAM 32 that is updated and sequentially rewritten and the battery is connected. Is becoming

The data in the RAM 32 is erased and reset when the ignition switch is turned off.

Further, the fuel injection control signal based on the calculation result in the CPU 27 is sent through the four injection drivers (injector drive circuits) 34 to the solenoid (injector solenoid) 8a of the injector 8 (more precisely, for the injector solenoid 8a). Transistor).

Focusing now on fuel injection control (air-fuel ratio control), the CPU 27 outputs a fuel injection control signal calculated by a method described later through the driver 34 to sequentially drive, for example, four injectors 8. It is designed to continue.

For such fuel injection control (injector drive time control), the ECU 23 first
Basic drive time for injector 8 (basic pulse width)
It has a basic pulse width setting means 51 as a basic drive time determining means for determining TB, and this basic pulse width setting means 51 is an A / N calculating means 52, which is the intake air amount A information from the air flow sensor 11. And the engine 1 obtained from the engine speed N information from the crank angle sensor 21.
The basic drive time TB is determined based on the intake air amount A / N information per rotation.

Further, the forced modulation means 53 and the base air-fuel ratio setting means 54 are provided together, and the outputs from these forced modulation means 53 and the base air-fuel ratio setting means 54 are selectively output by the switching means 55. It is like this.

Here, the forced modulation means 53 provides the air-fuel ratio of the air-fuel mixture supplied to the engine combustion chamber 1 oscillatingly with a required period on the rich side and the lean side of the stoichiometric air-fuel ratio. As described above, the air-fuel ratio correction coefficient KAF (KAF = KAV + K
V1; KAV sets the average value of the air-fuel ratio of the supply air-fuel mixture, and KV1 sets the air-fuel ratio fluctuation amount having a required period).

The base air-fuel ratio setting means 54 has an air-fuel ratio correction coefficient K corresponding to the engine speed N and the engine load A / N.
AF (= Kafo) is set.

When the engine cooling water temperature Tw is equal to or higher than a predetermined value Tw1 (this Tw1 is referred to as a feedback start water temperature) and is within a predetermined engine operating range (feedback zone; engine low / medium load zone), the switching means 55 Switching to select the output from the forced modulation means 53, when the engine cooling water temperature Tw is lower than the predetermined value Tw1 and is not in the predetermined engine operating range (feedback zone), the output from the base air-fuel ratio setting means 54 is selected. It switches to do so.

Further, the average value of the air-fuel ratio of the exhaust gas that has passed through the catalytic converter 9 is set to be near stoichiometry,
Base A / F correction means 60 is provided for correcting the average value of the air-fuel ratio of the supply air-fuel mixture that is set oscillatingly by the forced modulation means 53 based on the output of the O ₂ sensor 17.

Further, when the engine is started in the low temperature state, it is temporarily stopped from the time when the set time has elapsed from the engine start or the temperature of the engine (engine cooling water temperature) or the temperature of the HC adsorption catalyst 9A exceeds the set temperature. The clip means 61 is provided as a restriction means for restricting the correction of the average value of the air-fuel ratio of the supplied mixture to the lean side by the base A / F correction means 60.

Further, a warming-up increasing means 56 is provided as a water temperature correcting means for setting a warming-up coefficient KWT (= Kwt) as a water temperature correcting coefficient according to the engine cooling water temperature Tw detected by the water temperature sensor 19. Intake temperature sensor 1
The correction means 57 is provided to set the correction coefficient K according to the intake air temperature detected in 2, the atmospheric pressure detected by the atmospheric pressure sensor 13 and the like, and further for correcting the driving time according to the battery voltage. A dead time correction means 58 for setting the dead time (ineffective time) TD is also provided.

Then, the basic pulse width setting means 51, the forced modulation means 53 or the base air-fuel ratio setting means 54, the warm-up increasing means 56, the correcting means 57, the dead time correcting means 58.
Driving time Tin of the injector 8 by adding the outputs from
A pulse width setting means 59 for setting j (= TB × KAF × KWT × K + TD) is provided.

Next, the control procedure of this apparatus will be described with reference to the flow charts of FIGS.

First, in step A1 of FIG. 4, the engine cooling water temperature Tw is Tw1 (feedback start water temperature).
If it is lower, if it is lower, then in step A2, the air-fuel ratio correction coefficient Ka is calculated from the air-fuel ratio map according to A / N, N (A is the intake air amount, N is the engine speed).
Read fo and set it to KAF. Such setting of the air-fuel ratio correction coefficient Kafo is performed by the base air-fuel ratio correction means 54.

If the engine cooling water temperature Tw is equal to or higher than Tw1 (feedback start water temperature) in step A1, it is determined in step A3 whether or not the engine is in the low / medium load engine operating range. A4
Then, it is determined whether the O ₂ sensor output V is higher than the determination voltage Vs. The above V may use the average value of the O ₂ sensor output in the required sampling period.

If V> Vs, that is, in the rich state,
In step A5, the rich flag FR is set to 1 and V ≦ V
If s, that is, the lean state, the rich flag FR is set to 0 in step A6. Then, after that, step A7
Then, the air-fuel ratio correction coefficient KAF is set to KAV + KV1. Here, KAV is an average value of the air-fuel ratio of the supply air-fuel mixture, and KV1 is an air-fuel ratio fluctuation amount having a required period.

Then, step A2 or step A7
After that, in step A8, the water temperature correction coefficient Kwt is set according to the engine cooling water temperature Tw, and this is set in KWT. The setting of the water temperature correction coefficient Kwt is performed by the warm-up increasing means 56.

After that, in step A9, another correction coefficient K and dead time TD are set, and then step A10.
Then, after calculating the basic drive time TB, step A11
And the fuel injection time Tinj is TB × KAF × KWT × K
After setting with + TD, in step A12, this Ti
The injector 8 is driven according to nj.

By the way, the average value KAV of the air-fuel ratio and the variation KV1 used in the above step A7 are updated according to the flow shown in FIG.

That is, in step B1 of FIG. 5, it is first determined whether the count-up flag CUP is 1, and if so, the count value C is incremented by 1 in step B2. After incrementing the count value C in this way, in step B3, the count value C is the maximum value C.
It is determined whether or not MAX has been reached, and if not, the flag TO for checking the timing of updating the average value KAV is set to 0.
Then (step B4), the variation KV1 is updated (step B5). In this case, the variation KV1 is P + (Cβ /
CMAX). Here, P and β are constants.

When the count value C reaches the maximum value CMAX, TO = 1 is set and then CUP is set to 1 (steps B6 and B7).

On the other hand, if the count-up flag CUP is not 1 in step B1, the count value C is decremented by 1 in step B8. After decrementing the count value C in this way, in step B9, it is determined whether or not the count value C becomes the minimum value −CMAX,
Otherwise, the flag TO is set to 0 (step B1
0), and the variation KV1 is updated (step B11). In this case, the variation KV1 is calculated by (Cβ / CMAX) -P.

Even when the count value C reaches the minimum value −CMAX, TO = 1 is set and then CUP is set to 1 (steps B12 and B13).

In this way, the variation KV1 is updated. An example of the variation waveform of the variation KV1 is shown in FIG.
It becomes like (c).

When the variation KV1 is updated in this way, it is then determined in step B14 whether the flag TO has become 1. That is, the count value C
When becomes the maximum value or the minimum value, TO = 1, so
At this time, the average value KAV is updated as follows.

First, in step B15, the rich flag F
It is determined whether R is 1, and if FR = 1, that is, when the O ₂ sensor output V is larger than Vs (in the rich state)
Sets KAV = KAV-α, reduces the average value KAV, and shifts this KAV to the lean side (step B16).

After that, in step B17, it is determined whether or not this KAV is smaller than a certain set value −KAVM1. If so, in step B18,
After setting KAV = -KAVM1, and otherwise, the process of step B18 jumps to determine whether the HC adsorbing catalyst 9A is in an engine operating state (specific operating state) in which HC can be released (step). B19).

Here, in the following cases, it is determined that the above-mentioned specific operation state is established. (1) For example, engine cooling water temperature T as engine temperature
w is between Tw2 (first engine temperature) and Tw3 (second engine temperature) higher than Tw2 (Tw1 <Tw2
≦ Tw ≦ Tw3 <Tw4; Tw4 is the temperature after warm-up). (2) When the temperature Tc of the HC adsorption catalyst 9A is between Tc1 (first adsorption catalyst temperature) and Tc2 (second adsorption catalyst temperature) higher than Tc1 (Tc1 ≦ Tc ≦ Tc2). (3) From when the engine cooling water temperature Tw exceeds Tw2 (set engine temperature) until the set time elapses. (4) From when the temperature Tc of the HC adsorption catalyst 9A exceeds Tc1 (set adsorption catalyst temperature) until the set time elapses. (5) During the second set time t2 after exceeding the first set time t1 set according to the ambient temperature of the engine (at least one of the intake air temperature and the engine cooling water temperature).

If it is determined in step B19 that the engine is in the specific operating state (the operating state in which HC can be released by the HC adsorbing catalyst 9A), KA is determined in step B20.
V is another set value-KAVM2 (where 0 <KAVM2 <
It is determined whether it is smaller than KVM1). If so, in step B21, KAV = -KAVM2
far. As a result, the average value KA of the air-fuel ratio of the supply air-fuel mixture
V is clipped to limit the correction of the average value KAV to the lean side.

In step B15, the rich flag F
When R is not 1, that is, when the O ₂ sensor output V is Vs or less (in the lean state), KAV = KAV + α is set, the average value KAV is increased, and this KAV is shifted to the rich side (step B22).

Then, in step B23, it is determined whether this KAV is larger than the set value KVM1. If not, it returns, but if so, it sets KAV = KAVM1 in step B24, and then returns.

In this way, the average value KAV is updated. An example of the fluctuation waveform of this average value KAV is shown in FIG.
It becomes like (b). An example of O ₂ sensor output is shown in FIG.
It becomes like (a).

Thus, when the rich atmosphere in the three-way catalyst 9 due to the HC released from the HC adsorbing catalyst 9A is detected by the O ₂ sensor 17, the base A / F correction means 60 causes the median value of the forced modulation. When the exhaust gas modulated in the vicinity of stoichio is always flown into the three-way catalyst 9 due to the gradual shift to the lean side, the temperature rises and the HC adsorbed catalyst releases the adsorbed HC. However, good purification efficiency can be achieved.

Further, since the HC releasing operation region is detected by the water temperature or the like and the clip value is set to the lean shift with respect to the center value of the forced modulation, the exhaust gas flowing into the three-way catalyst 9 due to the HC releasing is emptied. It is possible to prevent the air-fuel ratio in the combustion chamber from becoming excessively lean in order to maintain the fuel ratio, and thereby to prevent deterioration of drivability.

If the main purpose is to improve the exhaust gas purification efficiency of the catalyst, steps B19 to B21 in FIG.
Can be omitted.

It is also possible to use a three-way catalyst 9 with a heater. In this case, when the engine is started, the heater is energized, and when the three-way catalyst 9 is warmed by exhaust heat, the heater is cut off. Is done.

[0061]

As described above in detail, according to the air-fuel ratio control system for an engine of the present invention (claim 1), the HC adsorbing catalyst, the three-way catalyst and the exhaust gas are sequentially arranged in the exhaust system of the engine from the upstream side. In the one provided with a fuel ratio sensor, the air-fuel ratio of the air-fuel mixture supplied to the engine combustion chamber is supplied with the fuel or The supply is provided with a control means for controlling at least one of the intake air, and the supply is oscillatingly set by the control means so that the average value of the air-fuel ratio of the exhaust gas passing through the three-way catalyst is approximately near stoichiometry. Since the air-fuel ratio average value of the air-fuel mixture is modified based on the output of the air-fuel ratio sensor, the rich atmosphere in the three-way catalyst due to the HC released from the HC adsorption catalyst is SE When detected by the sensor, the median value of the forced modulation is gradually shifted to the lean side, whereby the three-way catalyst always receives the modulated exhaust gas near Stoichio, resulting in Even when the temperature rises and the HC adsorbing catalyst releases the adsorbed HC, there is an advantage that good purification efficiency can be achieved.

Further, in the air-fuel ratio control system for an engine of the present invention (claim 2), when the engine is started in a low temperature state, when the set time has elapsed from the engine start, or the temperature of the engine or the HC adsorption. Since the limiting means is provided for temporarily limiting the correction of the average value of the air-fuel ratio of the supply air-fuel mixture to the lean side by the correcting means from the time when the temperature of the catalyst exceeds the set temperature, the HC is controlled. The discharge operation range is detected by the water temperature, etc., and the
A clip value can be set for the engine shift, which prevents the air-fuel ratio in the combustion chamber from becoming excessively lean in order to maintain the air-fuel ratio of the exhaust gas flowing into the three-way catalyst due to HC release. The advantage is that the deterioration of drivability can be prevented.

[Brief description of drawings]

FIG. 1 is a block diagram showing the overall configuration of an apparatus according to an embodiment of the present invention.

FIG. 2 is an overall configuration diagram of an engine system having an apparatus according to an embodiment of the present invention.

FIG. 3 is a control hardware block diagram according to an embodiment of the present invention.

FIG. 4 is a flowchart illustrating the operation of the embodiment of the present invention.

FIG. 5 is a flowchart illustrating the operation of the embodiment of the present invention.

FIG. 6 is a waveform diagram for explaining the operation of one embodiment of the present invention.

[Explanation of symbols]

1 combustion chamber 2 intake passage 2a surge tank 3 exhaust passage 4 intake valve 5 exhaust valve 6 air cleaner 7 throttle valve 8 injector 8a injector solenoid 9 catalytic converter (three-way catalyst) 9A HC adsorption catalyst 11 air flow sensor 12 intake temperature sensor 13 atmospheric pressure Sensor 14 Throttle sensor 15 Idle switch 17 O ₂ sensor (air-fuel ratio sensor) 19 Water temperature sensor 20 Cranking switch 21 Crank angle sensor 22 Cylinder discrimination sensor 23 Electronic control unit (ECU) 25 Battery sensor 27 CPU 28, 29 Input interface 30 A / D converter 31 ROM 32 RAM 34 Injection driver 35 Spark plug 51 Basic pulse width setting means 52 A / N calculation means 53 Forced modulation means 54 Base air-fuel ratio setting means 55 Switching means 56 warm-up increasing means 57 correcting means 58 dead time correcting means 59 pulse width setting means 60 base A / F correcting means (correcting means) 61 clip means (limiting means)

Claims (2)

[Claims] 1. An exhaust system of an engine, in order from an upstream side,
In an engine equipped with an HC adsorption catalyst, a three-way catalyst, and an air-fuel ratio sensor, the air-fuel ratio of the air-fuel mixture supplied to the engine combustion chamber has a required cycle between the rich side and the lean side of the theoretical air-fuel ratio. Control means is provided for controlling at least one of fuel and intake air so as to be provided in an oscillating manner, and the control means is provided so that the average value of the air-fuel ratio of the exhaust gas that has passed through the three-way catalyst is approximately in the vicinity of stoichiometry. An air-fuel ratio control device for an engine, comprising an amending means for amending an average value of the air-fuel ratio of the supply air-fuel mixture which is set oscillatingly by means of the output of the air-fuel ratio sensor. 2. When the engine is started at a low temperature, the set time is passed from the start of the engine, or the temperature of the engine or the temperature of the HC adsorbing catalyst exceeds the set temperature. 2. The air-fuel ratio control device for an engine according to claim 1, further comprising a limiting means for limiting the correction of the average value of the air-fuel ratio of the supplied mixture to the lean side by the correcting means.