JPH0437260B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an air-fuel ratio control method that detects an air-fuel ratio based on engine exhaust gas components and performs feedback control on the air-fuel ratio of an air-fuel mixture supplied to the engine.

Conventional air-fuel ratio control means simply performs integral control based on the output of an air-fuel ratio sensor. Therefore, during transient operation of the engine, if the basic air-fuel ratio changes faster than the correction speed of the integral control, the correction control cannot follow it. Further, when the air-fuel ratio sensor is inactive, feedback control of the air-fuel ratio cannot be performed, so sufficient air-fuel ratio control cannot be performed and exhaust gas cannot be reliably purified.

This invention was made in view of the above points, and in addition to integral processing control based on the output of an air-fuel ratio sensor, a value corresponding to this integral information is stored as non-volatile memory correction information for each state of the engine, Among the stored correction information, the air-fuel ratio is feedback-controlled using correction information corresponding to the engine state at that time and integral information at that time.

In other words, the device according to the present invention uses the integral information of the air-fuel ratio sensor only when the combustion state of the engine is relatively stable, even if the engine is in feedback control based on integral information according to the output of the air-fuel ratio sensor. By storing a value corresponding to the information as correction information and controlling the air-fuel ratio using this correction information, it is possible to quickly control the air-fuel ratio to a predetermined level without delay in response even during engine transients, and it also prevents exhaust gas and drivability from deteriorating. The purpose of this invention is to provide an air-fuel ratio control method that does not cause problems.

That is, in the present invention, when at least one of the following operating conditions occurs, storage of the value corresponding to the integral information as correction information is stopped.

The first case is when the width of the injection pulse applied to the electromagnetic fuel injection valve for forming an air-fuel mixture is very small. FIG. 7 shows the relationship between the injection pulse width T applied to the electromagnetic fuel injection valve and the injection amount q.
In general, the injection pulse width T and the injection amount q are expressed as a linear function, but when the pulse width T becomes extremely small, the relationship between the pulse width T and the injection amount q is no longer linear. Therefore, in an operating state using such a small pulse width, storage as correction information is stopped.

Second, in a deceleration state when the engine does not stop fuel supply, the air-fuel ratio is likely to be disturbed, so as shown in Figure 8, the operating range used during deceleration operation, that is, the predetermined engine speed N ₁
Larger and predetermined injection pulse width
In an operating range smaller than _T1 , storage as correction information is stopped. Note that the deceleration state may be set to a state where the intake air amount Q ₁ is smaller than the predetermined intake air amount and larger than the predetermined engine rotation speed N ₁ as shown in FIG.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the engine section. The engine 11 is a known four-stroke pyrotechnic ignition type engine installed in a car. Combustion air is taken in sequentially through an air cleaner 12, an intake pipe 13, and a throttle valve 14. do. Further, fuel is supplied from a fuel system (not shown) through electromagnetic fuel injection valves 15a, 15b, . . . provided corresponding to each cylinder of the engine 11. The exhaust gas after combustion is transferred to exhaust manifold 1.
6, it is released into the atmosphere through the exhaust pipe 17, three-way catalytic converter 18, etc.

The intake pipe 13 includes a potentiometer-type intake air amount sensor 19 that detects the amount of intake air taken into the engine 11 and outputs an analog voltage according to the amount of intake air, and detects the temperature of the air taken into the engine 11. A thermistor-type intake temperature sensor 20 that outputs an analog voltage (analog detection signal) according to the intake temperature
is installed.

Further, a thermistor-type water temperature sensor 21 is installed in the engine 11 to detect the coolant temperature and output an analog voltage (analog detection signal) according to the coolant temperature, and the exhaust manifold 16 is further equipped with an oxygen concentration sensor 21 in the exhaust gas. Detects the air-fuel ratio and outputs a voltage of approximately 1 volt (high level) when the air-fuel ratio is lower than the stoichiometric air-fuel ratio (rich), and approximately 0.1 volt (low level) when it is higher than the stoichiometric air-fuel ratio (lean). An air-fuel ratio sensor 22 is installed. The rotational speed of the crankshaft of the engine 11 is detected by a rotational speed (number) sensor 23, and a frequency pulse signal corresponding to the rotational speed is output. As this rotational speed (number) sensor 23, for example, an ignition coil of an ignition device may be used, and an ignition pulse signal from the primary terminal of the ignition coil may be used as the rotational speed signal. The detection signals of the sensors 19 to 23 are supplied to the control circuit 24, which calculates the fuel injection amount based on the detection signals and controls the opening time of the electromagnetic fuel injection valves 15a, 15b, etc. In this way, the amount of fuel injection is adjusted.

FIG. 2 explains the control circuit 24,
100 is a microprocessor (CPU) that calculates the fuel injection amount. Reference numeral 101 denotes a rotational speed counter that counts the engine rotational speed based on a signal from the rotational speed (number) sensor 23, and this rotational speed counter 101 sends an interrupt command signal to the interrupt control unit 102 in synchronization with the engine rotation. . When interrupt control section 102 receives this signal, it outputs an interrupt signal to microprocessor 100 through common path CB. A digital input port 103 transmits to the microprocessor 100 a signal from the air-fuel ratio sensor 22 and a digital signal of a starter signal from a starter switch 25 that turns on and off the operation of a starter (not shown). Reference numeral 104 is an analog input port consisting of an analog multiplexer and an A/D converter, and has the function of A/D converting each signal from the intake air amount sensor 19, absorption temperature sensor 20, and cooling water temperature 21 and sequentially reading it into the microprocessor 100. have 105 is a power supply circuit and a RAM which will be described later.
107 is directly supplied with power from the battery 26. This battery 26 circuit is provided with a key switch 27, but the power supply circuit 105 directly connects the battery 26 without passing through the key switch 27.
6, and RAM107 is connected to key switch 27.
Power is always applied regardless of the The battery 26 is connected to the other power supply circuit 1 via the key switch 27.
This power supply circuit 106 supplies power to parts other than the RAM 107, which will be described later. This RAM 107 is a temporary storage unit that is used temporarily during program operation, but as mentioned above, power is always applied regardless of the key switch 27, so even if the key switch 27 is turned off and engine operation is stopped, the stored contents are constitutes a non-volatile memory that does not disappear. A second correction amount k3, which will be described later, is also stored in this RAM 107. A read-only memory (ROM) 108 stores programs, various constants, and the like. Reference numeral 109 denotes a fuel injection time control counter including a register, which is composed of a down counter and receives a digital signal representing the opening time of the electromagnetic fuel injection valves 15a, 15b, calculated by the microprocessor 100, that is, the fuel injection amount. It is converted into a pulse signal with a pulse time width that gives the actual opening time of the electromagnetic fuel injection valves 15a, 15b, . 110 is an electromagnetic fuel injection valve 15a, 15b
111 is a timer that measures the elapsed time and drives the microprocessor 10.
0.

That is, the rotational speed counter 101 measures the engine rotational speed, for example, once per engine rotation based on the output of the rotational speed sensor 23, and supplies an interrupt command signal to the interrupt control section 102 when the measurement is completed. The interrupt control unit 102 generates an interrupt signal based on the interrupt command, and causes the microprocessor 100 to execute an interrupt processing routine for calculating the fuel injection amount. FIG. 3 shows a schematic flowchart of the microprocessor 100. Based on this flowchart, the functions of the microprocessor 100 will be explained.
The operation of the entire arrangement is also explained. That is, when the key switch 27 and starter switch 25 are turned on to start the engine, the first step 120 is started.
A start-up command is issued, and the main routine arithmetic processing is started. First, initialization is executed in step 121, and digital values corresponding to the cooling water temperature and intake air temperature from the analog input port 104 are read in step 122. In step 123, based on the results, a correction amount k1 according to the cooling water temperature and intake air temperature is calculated.
(It goes without saying that this correction amount k1 is different from the integral information of the present invention), and this correction amount k1 is stored in the RAM 107. Step 12
4, the air-fuel ratio sensor 22 is connected to the digital input port.
The correction amount k2, which will be described later, is increased or decreased as a function of the elapsed time by the timer 111, and this correction amount k2, that is, the integral processing information is stored in the RAM 107.
Store in.

Figure 4 shows the correction amount k2 as this integral processing information.
increase or decrease. In other words, the processing step 124 for integrating
A detailed flowchart is shown.

First, in step 400, it is determined whether the air-fuel ratio detector is in an active state or whether feedback control of the air-fuel ratio can be performed based on the cooling water temperature, etc., and if feedback control is not possible, that is, when the engine is in a predetermined first engine state. If there is no state and the loop is open, the process proceeds to step 406, where the correction amount k2 is set to k2=1, and the process proceeds to step 405. If feedback control is possible in the first engine state, the process advances to step 401.
In step 401, it is determined whether the elapsed time has passed the unit time Δt1 or not, and if it has not passed, the processing step 124 is terminated without correcting k2. When the time Δt1 has elapsed, the process proceeds to step 402, and if the air-fuel ratio is rich and the output of the air-fuel ratio sensor 22 is a rich high level signal, the process proceeds to step 4.
Proceed to step 08 and convert k2 obtained in the previous cycle to Δk
The correction amount k2 is decreased by 2, and the process proceeds to step 405, where this new correction amount k2 is stored in the RAM 107. If the air-fuel ratio is lean in step 402 and the output of the air-fuel ratio sensor 22 is a low level signal indicating lean, the process proceeds to step 404, where k2 is changed to Δk2.
The process proceeds to step 405. In this way, the correction amount k2 is increased or decreased. At step 125 in FIG. 3, the correction amount k3 engine condition correction information is increased or decreased, and the result is stored in the RAM 107.

Next, in FIG. 5, the above correction amount k3 is processed and stored. That is, this is a detailed flowchart of step 125 for memory processing. Prior to explaining this flowchart, we will explain the judgment conditions for demarcating the region (second engine state) in which the combustion state of the engine is relatively stable when performing the above-mentioned memory processing. do. By the way, the purpose of the calculation process of the correction amount k3 is to make the basic (base) fuel amount based on the basic calculation match the fuel amount currently required by the engine as much as possible without performing feedback correction of the air-fuel ratio. By making adjustments over time, we can improve responsiveness during engine transients where air-fuel ratio feedback control does not function adequately, better compensate for changes in parts and characteristics over time, and improve performance using atmospheric pressure sensors. It is possible to compensate for atmospheric pressure changes at high altitudes, or to match the basic air-fuel ratio (basic fuel amount) to the target air-fuel ratio (required fuel amount) as much as possible even when air-fuel ratio feedback is stopped (during open loop control). It is to make it possible to do so.

However, even in normal operating ranges, the engine condition changes widely, and the combustion condition of the engine is extremely unstable, especially during transient periods or in the output increase range, so it is difficult to detect the engine condition at that point. It is not preferable to perform calculation processing for the correction amount k3. In particular, in this embodiment, as shown in FIG. 6, a map of the correction amount k m _o in which the engine speed N and the intake air Q are divided and allocated as the correction amount ^k3 is used, and each k ^m _o is rewritten. Yes, for example, once the value in an unstable combustion state is memorized, it can be used in other operating states (e.g. steady operating state).
In some cases, the stored value k ^m _o may be used, but in that case, the air-fuel ratio may fluctuate or the drivability may be significantly impaired. Therefore, it is extremely important to calculate the correction amount k3 in a state where the combustion state of the engine is relatively stable.

In this embodiment, the conditions shown in FIGS. 7 to 10 are taken into consideration as the determination conditions.

First, there is a case where the injection pulse width and fuel injection amount applied to the electromagnetic fuel injection valve for forming an air-fuel mixture are very small. Figure 7 shows the injection pulse width T and injection amount q applied to the electromagnetic fuel injection valve.
The relationship between is shown. In general, the injection pulse width T and the injection amount q are expressed as a linear function, but if the pulse width T is very small (for example, smaller than the pulse width To (approximately 1.5 ms)), the structure and accuracy of the injection valve itself may be affected. Due to this, the relationship between pulse width T and injection amount q is no longer linear. This value To is set to be sufficiently smaller than the injection pulse width T _D during idling operation. Therefore, in an operating state using such a small pulse width (T<To), it is not preferable to store it as correction information. Of course, it goes without saying that this lower limit value To may have different values depending on the injection valve and its fuel supply system. This boundary condition corresponds to boundary line A in FIG. Note that point A in FIG. 10 corresponds to the idle operation, and has an idle rotational speed N _D and an injection pulse width T _D.

Second, in a deceleration state when the engine does not stop fuel supply, the air-fuel ratio is likely to be disturbed, so as shown in Figure 8 (or Figure 9), the operating range used during deceleration operation, i.e. It is not preferable to store an operating region higher than a predetermined engine speed N ₁ and smaller than a predetermined injection pulse width T ₁ (or intake air Q ₁ ) as correction information.

Here, _the engine speed N ₁ is determined by taking into account the variation in the idle speed _N The idle speed is slightly higher than the idle speed N _D (for example, if the idle speed is 700 rpm, the speed is 1000~
1200rpm). Also, the judgment pulse width T ₁ (or intake air amount Q ₁ ) during deceleration operation
is a value that is larger than the injection pulse width TD at idle and can be arbitrarily set in consideration of the degree of deceleration operation, and it goes without saying that it can have a different value depending on each engine. This boundary condition corresponds to boundary line B in FIG.

Thirdly, when the engine is operated at high speed and high load, when the engine is in a so-called output increase range, and when the air-fuel ratio is enriched in order to lower the combustion temperature and exhaust temperature. Therefore, it is not recommended to store it as correction information when the intake air amount becomes larger than the preset intake air amount Q ₂ , or when the engine speed becomes higher than N ₂ , or when the injection pulse width becomes larger than T ₂ . .

It should be noted that each of the above values T ₂ , Q ₂ , and N ₂ can be arbitrarily set in consideration of the above points, and it goes without saying that different values are obtained depending on various engines.

These boundary conditions correspond to boundary lines C, D, and E in FIG. 10 in order from the former. The shaded area surrounded by the boundary lines A, B, C, D, and E shown so far is the so-called ``second state in which the combustion state of the engine is relatively stable'' in this engine. This corresponds to the engine condition area.Of course, even within this area, when the engine is warming up from being cold, or immediately after the engine status has entered this area from outside this area, or when the engine status is within this area, The combustion state may become unstable when fuel is being increased or when the throttle switch is turned on, so storing the correction amount k3 is prohibited as necessary. .of course,
This is a condition that can be changed in consideration of the required control accuracy of the air-fuel ratio, the frequency of occurrence of storage processing operations, etc.

Next, the operation of storing the correction amount k3 will be explained according to the flowchart shown in FIG. Here, block 496 is a block for determining whether or not the combustion state of the engine is in a second engine state where the combustion state is relatively stable.

First, in step 497, in the first detection step,
It is determined whether the injection amount in the fuel injection valves 15a, 15b... or the pulse width T applied to the fuel injection valves 15a, 15b... is within a set value 'To≦T≦ _T2 ), and if it is outside the set value. If the value is within the set value, step 125 is completed, and if the value is within the set value, the process proceeds to step 498. In step 498, the air amount Q detected by the intake air amount sensor 19 is within the set value (Q≦Q ₂ ).
If it is other than the set value, proceed to step 1.
25, and if it is within the set value, proceed to step 49.
Proceed to step 9. In this step 499, it is determined whether the engine rotation speed N detected by the rotation speed sensor 23 is within a set value (N≦N ₂ ), and if it is outside the set value, step 125 is terminated and the engine speed is within the set value. If so, proceed to step 500. This step 5
00 (second detection step), it is determined whether the engine speed N is within a set value (N ₁ ≦N),
If it is outside the set value, the process advances to step 502, and if it is within the set value, the process advances to step 501. In this step 501 (second detection step), it is determined whether or not the injection pulse width T is within a set value (T ₁ ≦T). If the pulse width T is smaller than T ₁ , step 125 is ended, and If it is ₁ or more, step 50
Proceed to step 2 and determine the value of K2.

If k2=1, nothing is done and this processing step 125 is ended. Note that the correction amount k3 forms a map as shown in FIG. 6 based on the intake air amount Q and the engine rotational speed N. The correction amount K3 on the map corresponding to the m-th position with respect to the intake air amount Q and the N-th position with respect to the engine speed N is expressed as ^km _o .
In this embodiment, the map in the RAM 107 divides the engine speed N into every 200 rpm, and the intake air amount Q from idle to full throttle into 32 parts. If k2<1 in step 502, the process proceeds to step 503, where k ^m _o is decreased by Δk3, and the result is stored in step 505.
Store in RAM107. In step 502, “k
2>1'', the process proceeds to step 504, where the correction amount k ^m _o obtained in the previous cycle is increased by Δk3, and the process proceeds to step 505, where this processing step 125 is ended. Here, block diagram 496 and step 502 constitute the determination step of the present invention. Next, when step 125 in the main routine is completed, the process returns to step 122.

Note that the initialization process in step 121 also executes the following. That is, the battery may be removed when inspecting or repairing a vehicle. For this reason
The correction amount k3 stored in the RAM 107 may be corrupted and become a meaningless value. Normally RAM1 is used to detect whether the battery has come off or not.
A constant with a predetermined pattern is placed in a specific address of 07. When the program starts, it is determined whether the value of this constant is corrupted, that is, it is an incorrect value, and if it is an incorrect value, it is assumed that the battery has been disconnected, and all values of the correction amount k3 are is initialized to "1" and the constant of the determined pattern is reset. If the pattern constant is not corrupted at the next startup, k3 will not be initialized.

Normally, the main routine processing of steps 122 to 125 is repeatedly executed according to the control program. In FIG. 2, when an interrupt signal for fuel injection amount calculation is input from the interrupt control unit 102, the microprocessor 100 immediately interrupts the main routine processing even if it is processing the main routine, and starts the interrupt processing at step 130. Move on to the routine. In step 131, a signal representing the engine speed N from the rotation speed counter 101 is taken in. Next, in step 132, a signal representing the intake air amount (intake amount) Q is taken in from the analog input port 104. Next, in step 133, a signal representing the engine speed N is taken in from the analog input port 104. Number N and intake air amount Q
is the correction amount k3 in the calculation process of the main routine.
is stored in the RAM 107 for use as a parameter for storage processing. Next step 13
4, the basic fuel injection amount determined from the engine speed N and intake air Q (that is, the electromagnetic fuel injection valve 1
The injection time width t) of 5a, 15b... is calculated. The calculation formula is "t=F×Q/N" (F: constant). Next, in step 135, various correction amounts for fuel injection determined in the main routine are read out from the RAM 107, and correction calculations are made for the injection amount (injection time width) for determining the air-fuel ratio. The formula for calculating the injection time width T is “T=t×
K1×k2×k3”. Next, in step 136, the data of the fuel injection amount corrected and calculated is sent to the counter 10.
Set to 9. Next, the process advances to step 137 and returns to the main routine. When returning to the main routine, the process returns to the processing step at which it was interrupted due to interrupt processing.

The general functions of the microprocessor 100 are as described above.

As described above, a large number of second correction amounts k3 (=k ^m _o ) are prepared according to the intake air amount and engine speed, so the appropriate correction amount corresponding to the engine operating condition is immediately used. be able to. Control with quick response is possible under all operating conditions, including transient conditions. Furthermore, since the second correction amount k3 is corrected in accordance with the operating state, it can be automatically corrected for changes over time and deterioration of the engine and sensors.

In the above embodiment, if the engine continues to be operated under certain conditions, the correction amount k3 will be corrected by only the same k ^m _o out of the whole, and k ^m +1 o +1 or k _m ⁺¹ _{o +1} or
Since the difference between values near k ^m _o such as k ^m-1 _o-1 may be too large, it is also possible to learn and correct the surroundings of k ^m _o at the same time. In this case, step 1 of the calculation process of the correction amount k3 in the main routine of the above embodiment is performed.
25, the correction amount k2 as integral processing information
When “k2>1”, step 5 in Fig. 5
04 is km n=km n+3Δkn km±1 n±1=km±1 n±1+2Δkn km±2 n±2=km±2 n±1+Δkn km±1 n±2=km±1 n±1+Δkn km±2 n The program is programmed to execute the process of ±2=km±2 n±2+Δkn. In other words, if the correction amount of the central k ^m _o is "3",
If there is only one, it is modified by "2", and if there are two, it is modified by "1" in the same direction. When "k2<1", subtraction processing is performed in the same manner as described above in step 503, and each is stored in the RAM 107.

In addition, in the above embodiment, the correction amount "k3=k ^m _o "
is a predetermined correction amount Δk3 (or
3Δkn, 2Δkn, Δkn), but it is also possible to obtain k3 by multiplying the correction amount k2 by a constant a or an that changes depending on the engine condition.

In addition, in the above embodiment, the correction amount k3 is stored in the RAM.
The intake air amount and engine speed are used as parameters to be divided into 107 parts and stored, and the map is created by dividing the map at predetermined intervals as shown in Fig. 6. Since the number of parameters increases and there is a concern that the cost will increase and the reliability decreases, the parameter may be set to the intake air amount Q only, and the correction amount k3 may be set to k ¹ , k ² , k ³ , . . . ^km .

In addition, in each of the above embodiments, the correction amount k3 is
Although the intake air amount is used as the engine parameter for dividing the engine into 7 parts and storing the engine parameters, it goes without saying that the opening degree of the intake negative pressure throttle valve may also be used.

Further, in the above embodiment, in step 125 where the correction amount k3 is calculated and stored, k3 is calculated and rewritten (stored) every unit time _Δt2 , but the unit rotation of the engine ΔN Of course, it is also possible to rewrite the calculation of k3 every time, and in this case, the unit rotation ΔN is about 30 rotations when the engine is steady, and about 20 rotations during transients such as acceleration and deceleration, which is the control response. Good in terms of control accuracy.

As described above, the present invention is a method that includes an air-fuel ratio sensor that detects an air-fuel ratio based on exhaust gas components of an engine, and controls the air-fuel ratio based on a signal from the air-fuel ratio sensor. an integral processing step for integrating the output signal of the air-fuel ratio sensor in the engine state; and a value corresponding to the integral information obtained in this integral processing step for a predetermined second engine state that is narrower than the range of the first engine state. includes a storage processing step for storing engine state correction amount information in a read/write non-volatile memory in accordance with the engine state at the time of processing only in a stable combustion state; The engine air-fuel ratio is controlled by the correction information corresponding to the engine state at that time among the engine state correction information stored in the nonvolatile memory, and the correction information stored in the nonvolatile memory Based on this, the air-fuel ratio can be controlled accurately over all engine conditions, even during engine transients and when the air-fuel ratio sensor is inactive, and also prevents changes over time in the engine, deterioration of the air-fuel ratio sensor, and even production variations. This has the excellent effect of being able to compensate and control the air-fuel ratio with high precision.

In particular, in the present invention, when the fuel injection amount is small and the fuel injection valve does not operate normally, or during deceleration when fuel injection is not stopped, the storage of engine condition correction information according to the integral information is stopped, so the engine idle It is possible to update and store the engine state correction amount at the time of engine state correction, and there is an effect that there is no need to update and store inaccurate engine state correction information at the time of unstable combustion of the engine.

[Brief explanation of drawings]

FIG. 1 is an overall configuration diagram showing one embodiment of the present invention, FIG. 2 is a block diagram of the control circuit shown in FIG. 1, FIG. 3 is a schematic flowchart of the microprocessor shown in FIG. 2, and FIG. 4 is a detailed flowchart of the steps for obtaining the correction amount k2 shown in FIG. 3, FIG. 5 is a detailed flowchart of the steps for obtaining the correction amount k3 shown in FIG. 3, and FIG. 6 is the operation of this embodiment. 7 to 10, which are used to explain the correction amount k3, are diagrams illustrating how a specific driving region is bounded.

Claims (1)

[Scope of Claims] 1. An air-fuel ratio sensor 22 that detects an air-fuel ratio based on exhaust gas components of the engine 11, wherein the air-fuel ratio sensor 22 detects an air-fuel ratio when the engine 11 is in a predetermined first engine state in which feedback control of the air-fuel ratio is possible. Step 124 of integrating the output of 22
and a storage processing step 503 in which a value corresponding to the integral information k2 obtained in the integral processing step 124 is updated and stored in the read/write nonvolatile memory 107 as engine state correction information k3 in correspondence with the engine state at the time of the processing. ,504,50
5, the engine 1 is adjusted according to the integral information k2 obtained in the integral processing step 124 and the correction information corresponding to the engine condition at that time among the engine condition correction information k3 stored in the nonvolatile memory 107.
1, the air-fuel ratio control method corrects the air-fuel ratio of the air-fuel mixture given to the engine 11, wherein the fuel injection amount T is the fuel injection amount when the engine 11 is idling.
a first detection step 497 for detecting that the fuel injection amount _T by _the fuel injection valves 15a, 15b, . When the intake air amount Q is less than or equal to the second set value T ₁ that is set larger than the fuel injection amount T _D or when the intake air amount Q is less than or equal to the predetermined value Q ₁ that is set larger than the intake air amount Q _D at the time of idling, the engine rotation speed a second detection step 500, 501 that detects that N is a predetermined rotation speed _N1 or more set slightly higher than the idle rotation speed N _D ; and the first and second detection steps 497, 50.
When the state is detected at 0,501, execution of the memory processing steps 503, 504, and 505 is prohibited, and the first engine state in which the integral processing of the output signal of the air-fuel ratio sensor 22 is being performed is performed. An air-fuel ratio control method characterized by comprising determining steps 496 and 502 in which the storage processing steps 503, 504, and 505 are executed only in a second engine state in which the combustion state of the engine 11 is stable.