JP3142679B2 - 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 air-fuel ratio control device for an engine.

[0002]

2. Description of the Related Art In general, in a typical spark ignition type engine for automobiles, a mixture of fuel (for example, gasoline) and air is compressed by a piston in a combustion chamber and then ignited and burned by a spark plug. However, the air-fuel ratio (air / fuel ratio A / F) of the air-fuel mixture can be basically set arbitrarily within the flammable range.

[0003] The leaner the air-fuel mixture, the lower the engine output, but the higher the fuel efficiency and emission performance. Note that the NOx generation rate peaks in the air-fuel ratio range (around 16 in A / F) slightly leaner than the stoichiometric air-fuel ratio. Therefore, in recent years, in a low / medium load region where a very high engine output is not required, the mixture is made as lean as possible as long as the ignitability is not impaired (for example, A / A
F = 19 to 24), and lean-burn engines designed to improve fuel efficiency and emission performance are frequently used.

In such a lean burn engine,
Generally, in the low / medium load region, a lean target air-fuel ratio is set according to the operation state, and the fuel injection amount is set so that the actual air-fuel ratio follows this target air-fuel ratio. In particular,
For example, a linear O ₂ sensor (linear air-fuel ratio sensor) is provided in the exhaust passage, the air-fuel ratio is grasped from the O ₂ concentration in the exhaust gas detected by the linear O ₂ sensor, and the air-fuel ratio follows the target air-fuel ratio. Feedback control is performed such that the fuel injection amount is set so as to perform the control. In the following, for convenience, the air-fuel ratio determined from the O ₂ concentration detected by the linear O ₂ sensor is referred to as “the air-fuel ratio detected by the linear O ₂ sensor”.

However, since the linear O ₂ sensor is provided in the exhaust passage downstream of the control target (intake system or combustion chamber), the air-fuel ratio detected by the linear O ₂ sensor is the actual air-fuel ratio of the control target. Will have a response delay. Here, the response delay time is the time required for the change in the fuel injection amount (air-fuel ratio) to reach the linear O ₂ sensor, that is, the dead time, and the change in the fuel injection amount (air-fuel ratio) reaches the linear O ₂ sensor. It consists of a time required for the output of the subsequent linear O ₂ sensor to change to a predetermined degree, that is, a temporary delay time. For this reason, if the fuel injection amount is simply set according to the deviation of the air-fuel ratio from the target air-fuel ratio detected by the linear O ₂ sensor, the control delay is disturbed by the response delay, and cycling or hunting occurs.

Therefore, an approximate target air-fuel ratio in which the target air-fuel ratio is corrected according to the dead time and the temporary delay time of the linear O ₂ sensor is set, and the fuel injection amount is determined according to the deviation of the air-fuel ratio from the approximate target air-fuel ratio. There has been proposed an air-fuel ratio control device that is set, that is, an air-fuel ratio control device that basically sets a target air-fuel ratio preceding by a time corresponding to a response delay to an approximate target air-fuel ratio (for example, Japanese Unexamined Patent Publication No. -60746).

[0007]

However, in the conventional air-fuel ratio control device disclosed in, for example, Japanese Patent Application Laid-Open No. Sho 60-60746, the dead time or the temporary delay time of the linear O ₂ sensor is taken into consideration. However, the air-fuel ratio control essentially sets the fuel injection amount according to the deviation of the air-fuel ratio from the target value detected by the linear O ₂ sensor, that is, the result does not take into account the dynamic characteristics of the controlled object. Since the air-fuel ratio control based on the so-called classical control in which the fuel injection amount is set based only on the deviation is performed, the control accuracy is limited.

Therefore, in recent years, the dynamic characteristics related to the air-fuel ratio to be controlled, that is, the temporal relationship between the fuel injection amount and the output of the linear O ₂ sensor have been elucidated, and a model expression (state Equation), and an air-fuel ratio control device using a so-called modern control theory, such as setting the fuel injection amount so as to minimize the deviation of the actual air-fuel ratio from the target air-fuel ratio based on the model formula, has been proposed. I have.

However, in the conventional air-fuel ratio control device using the modern control theory, since the dynamic characteristics of the controlled object are not sufficiently clarified, there is a discrepancy between the dynamic characteristics of the controlled object and the model formula. Also, despite the fact that the control target is a non-linear system, the model formula is linearized to simplify the arithmetic processing, so the error in the model formula increases and the control accuracy of the air-fuel ratio control does not increase much. is there. SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned conventional problems, and has as its object to provide an air-fuel ratio control device capable of causing an air-fuel ratio to accurately follow a target air-fuel ratio.

[0010]

In order to achieve the above-mentioned object, as shown in FIG. 1, the first aspect of the present invention is a linear air-fuel ratio detecting apparatus for detecting an air-fuel ratio by detecting an oxygen concentration in exhaust gas. A fuel ratio sensor a, a fuel injection amount setting means b for setting the fuel injection amount based on the air-fuel ratio grasped by the linear air-fuel ratio sensor a so that the actual air-fuel ratio follows a predetermined target air-fuel ratio, In an air-fuel ratio control device for an engine provided with a fuel injection amount correcting means c for correcting the fuel injection amount set value according to the primary delay time of the air-fuel ratio sensor a, the fuel injection amount correcting means c comprises a linear air-fuel ratio The air-fuel ratio grasped by the sensor a, the temporary delay time of the linear air-fuel ratio sensor, and the previous air-fuel ratio are predicted based on the past fuel injection amount results, and the deviation of the predicted air-fuel ratio from the target air-fuel ratio is calculated. To minimize In the direction in which the change rate of the air-fuel ratio prediction value becomes slower when the intake air amount is small than when it is large,
There is provided an air-fuel ratio control device for an engine, comprising a fuel injection amount correction characteristic changing means d for changing a correction characteristic of a fuel injection amount correction means c with respect to a temporary delay time.

According to a second aspect of the present invention, in the air-fuel ratio control device for an engine according to the first aspect, the fuel injection amount setting means b and the fuel injection amount correction means c correspond to the primary delay time of the linear air-fuel ratio sensor a. Using a predetermined model formula that includes a parameter for performing the relationship between the fuel injection amount and the air-fuel ratio,
Predict the previous air-fuel ratio based on the air-fuel ratio grasped by the linear air-fuel ratio sensor a, the temporary delay time of the linear air-fuel ratio sensor, and the past fuel injection amount results, and calculate the target air-fuel ratio of the air-fuel ratio prediction value. It is a modern control means e that sets and corrects the fuel injection amount so as to minimize the deviation with respect to the fuel injection amount correction characteristic changing means d, by changing the model formula according to the intake air amount, An air-fuel ratio control device for an engine, wherein a correction characteristic relating to a temporary delay time is changed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below.
As shown in FIGS. 2A and 2B, in each cylinder (only one is shown) of the gasoline engine 1, when the intake valve 2 is opened, the combustion from the independent intake passage 4 via the intake port 3 is performed. The air-fuel mixture is sucked into the chamber 5, the air-fuel mixture is compressed by the piston 6, ignited and burned by the ignition plug 7, and when the exhaust valve 8 is opened, the combustion gas is discharged through the exhaust port 9. The process of discharging to 10 is repeated. The engine 1 has an air-fuel ratio A during normal operation.
/ F is leaner (A / F = 19 to 24) than the stoichiometric air-fuel ratio (A / F = 14.7, ie, excess air ratio λ = 1).

In the exhaust passage 10, a linear O ₂ sensor 11 (linear air-fuel ratio sensor) for detecting the O ₂ concentration in the exhaust gas is provided.
The air-fuel ratio is determined from the O ₂ concentration. In the following, this air-fuel ratio is referred to as “the air-fuel ratio detected by the linear O ₂ sensor 11” for convenience. On the downstream side of the linear O ₂ sensor 11,
A catalytic converter 1 for purifying exhaust gas is provided in an exhaust passage 10.
2 are interposed.

In order to supply air to the combustion chamber 5 of each cylinder, a single common intake passage 14 whose upstream end is open to the atmosphere is provided.
The common intake passage 14 includes an air cleaner (not shown), an air flow sensor (not shown) for detecting an intake air amount, and an accelerator pedal (not shown) in order from the upstream side when viewed in the direction of air flow. ) And a throttle valve 15 which is opened and closed according to the amount of depression. The downstream end of the common intake passage 14 is connected to a surge tank 16 for stabilizing the flow of intake air, and the upstream end of the independent intake passage 4 of each cylinder is connected to the surge tank 16.

A bypass intake passage 17 for bypassing the throttle valve 15 and passing air to the common intake passage 14
The bypass intake passage 17 is provided with an ISC valve 18 for controlling the idle speed by adjusting the flow rate of the air flowing through the bypass intake passage 17.

In the independent intake passage 4, in order from the upstream side, a throttle valve 19 for narrowing the passage cross section in order to strengthen the swirl in a predetermined low load region, and a fuel in the air in the independent intake passage 4.
Fuel injection valve 20 for injecting (gasoline) to form an air-fuel mixture
Are provided. The fuel injection amount of the fuel injection valve 20 is:
As will be described later, the air-fuel ratio A / F (excess air ratio λ) of the air-fuel mixture is controlled by the control unit 25 so as to follow a target air-fuel ratio set according to the operating state. Also, the combustion temperature of the air-fuel mixture rises excessively.
In order to prevent (increase in the amount of generated NOx), the exhaust passage 10
Independent intake passage 4 uses a part of exhaust gas in the interior as EGR gas.
An EGR passage 21 for recirculating the EGR gas and an EGR valve 22 for adjusting the EGR gas amount are provided.

In order to control the air-fuel ratio of the engine 1, a control unit 2 composed of a microcomputer or the like is used.
5 are provided. This control unit 25
Is a "fuel injection amount setting means" described in the claims
And a "fuel injection amount correction means" and a "fuel injection amount correction characteristic changing means", and are comprehensive modern control means for performing air-fuel ratio control based on modern control theory. The predetermined air-fuel ratio control is performed using the amount, the engine speed detected by the speed sensor, the air-fuel ratio detected by the linear O ₂ sensor 11 and the like as control information.

The control rules of the air-fuel ratio control by the control unit 25 based on modern control theory will be described below. In the following, lowercase letters with an underline (for example, x ) represent a vector, and uppercase letters with an underline (for example, A ) represent a matrix and are not underlined. Things shall represent scalars. Also, ( x ) ^T
Represents the transposed vector of the vector x , and ( A ) ^T represents the matrix A
And the transposed matrix of

In the air-fuel ratio control in the present embodiment, basically, a dynamic model of a controlled object (control system) is clarified to create a model that accurately represents the dynamic characteristic (step 1), and the model is expressed by a mathematical formula. To form a state equation (model equation) (Step 2), set a predetermined evaluation function (Step 3), and determine an optimal fuel injection amount (optimum input) that minimizes the evaluation function based on the state equation (Step 3). The calculation is performed (step 4), and the fuel injection is performed with the optimum fuel injection amount. The specific contents of these steps will be described below.

[0020] (1) Step 1 (model decision) control object (control system), the fuel injection quantity u as input information, the exhaust air-fuel ratio (excess air ratio) lambda i.e. the linear O ₂ sensor output or the linear O ₂ sensor This is a fuel system of the engine 1 which uses the air-fuel ratio at the arrangement position as output information.
It is represented as

Λ [k + 1] = α · λ [k] + (1−α) · u [k−L] Equation 1 where k: time λ [k + 1]: Exhaust air-fuel ratio at time k + 1 (excess air ratio) λ [k]: Exhaust air-fuel ratio at time k u [k−L]: Fuel injection amount at time k−L α: Temporary delay characteristic value (0 <α <1) L: Dead time (for example, 3 to 6)

In equation (1), k represents the current time, and k +
Reference numeral 1 denotes a time earlier than the current time by a time necessary to execute the control routine once. The dead time L is the time required for this effect to reach the linear O ₂ sensor 11 when the fuel injection amount changes, and the temporary delay characteristic value α is a value representing the temporary delay characteristic of the linear O ₂ sensor 11. Here, α is calculated from the time constant T. That is, as shown in FIG. 6, line fuel injection amount at time t ₁
> If changes stepwise as H _1, exhaust air-fuel ratio
(The output value of the linear O ₂ sensor) changes like the curve H ₂ .
Here, the exhaust air-fuel ratio is initially changed at time t _2, the the proportion to the total amount of change in time t ₃ reaches a predetermined value (e.g. 63%), a t ₂ -t ₁ dead time L, t ₃ −t ₂ is the time constant T.

(2) Step 2 (model equation of state) Assuming that the current time is k, the elements that affect the exhaust air-fuel ratio (output value of the linear O ₂ sensor) after time k + 1 are λ [k], u [k
−1], u [k−2],..., U [k−L]. Therefore, the state x [k] of the control target at the current time k is expressed by the following Expression 2.

X [k] = ([lambda] [k], u [k-1], u [k-2], ..., u [k-L]) ^T ........................... , The state x [k + 1] of the control target at time k + 1 is expressed by Expression 3.

X [k + 1] = ([lambda] [k + 1], u [k], u [k-1], ..., u [k-L + 1]) ^T ... Equation 3

Therefore, when the dead time L is, for example, 3, the state equation of the model is expressed by the following equation (4).

[Equation 4] x [k + 1] = A · x [k] + bu · u [k] ············································ 4 where x [k + 1] = (λ [k + 1], u [ k], u [k-1], u [k-2]) T x [k] = (λ [k], u [k-1], u [k-2], u [k-3]) ^T A = ( a ₁ , a ₂ , a ₃ , a ₄ ) ^T a ₁ = (α, 0,0,1-α) a ₂ = (0,0,0,0) a ₃ = (0, 1, 0, 0) a ₄ = (0, 0, 1, 0) b = (0, 1, 0, 0) ^T

(3) Step 3 (setting of evaluation function) The evaluation function J is set as in the following Expression 5.

Equation 5] J = Σ [0 → ∞] {(x [k] - m) T · R · (x [k] - m) + u [k] · Q · u [k]} ...... Formula 5 Formula In 5, Σ [0 → ∞] represents a sigma (integration) in which the initial value of the counter is 0 and the final value is ∞. And, m is a vector representing the target value of the control object. R is a constant matrix representing the weight of the first term on the right side, Q is a constant representing the weight of the second term on the right side,
R and Q are set, for example, as follows. _{R = (r 1, r 2} , r 3, r 4) T r 1 = (1,0,0,0) r 2 = (0,1,0,0) r 3 = (0,0,1, 0) r ₄ = (0,0,0,1) Q = 1

In Equation 5, the first term on the right-hand side is a term representing the accumulation of the deviation from the target value. The smaller the accumulation of the deviation, the more the actual air-fuel ratio follows the target air-fuel ratio. Therefore, from the viewpoint of making the air-fuel ratio follow the target value, the first term on the right side is sufficient. However, no matter how small the accumulation of the deviation is,
If the fuel injection amount u [k] is excessively large, the fuel efficiency deteriorates. Therefore, the second term on the right side is designed to stop the fuel injection amount u [k] from becoming excessive. And R ,
By changing the value of Q, it is possible to adjust whether emphasis is placed on followability to a target value (target air-fuel ratio) or on fuel efficiency.

FIG. 7 (a) shows that when the weight of R is set to be large while the weight of Q is set to be small, when the fuel injection amount is changed stepwise as indicated by a polygonal line G ₁ ′,
The change over time in the exhaust air-fuel ratio (curve G ₁ ) and the change over time in the feedback coefficient, that is, the fuel injection amount (curve G ₂ ) are shown.
In this case, as is clear from FIG. 7A, the response of the exhaust air-fuel ratio is good, but the fuel consumption increases. Also,
FIG. 7B shows that when the weight of R is set to be small while the weight of Q is set to be large, the fuel injection amount is changed by a polygonal line G ₃ ′.
(The same as G ₁ ′ in FIG. 7 (a)), the change over time of the exhaust air-fuel ratio (curve G ₃ ) and the change over time of the feedback coefficient, that is, the fuel injection amount (curve G ₄ ). In this case, as is clear from FIG. 7B, the response of the exhaust air-fuel ratio is deteriorated, but the fuel consumption is reduced.

(4) Step 4 (Calculation of Optimal Fuel Injection Amount) In the control target (control system) whose state equation is expressed by the equation 4, the fuel injection that minimizes the value of the evaluation function J expressed by the equation 5 Amount, optimal fuel injection amount (optimal input)
Compute u '[k]. This operation is an optimal regulator problem of a linear system, and is performed by numerically solving the Riccati difference equation by a commonly known ordinary method, and an optimal fuel injection amount u ′ as shown in the following equation (6). [k] is obtained.

[6] u '[k] = c · x [k] ..................................................................... formula 6 In the equation 6, c is optimally Feedback gain.

In such a control law, it is necessary that the state equation (Equation 4) accurately represents the actual dynamic characteristic of the controlled object (control system) as described above. And
Since the controlled object is strictly a non-linear system, it is necessary to make the state equation nonlinear in order to accurately express the dynamic characteristics of the controlled object. However, if the state equation is made nonlinear, the calculation becomes very complicated, and the control unit 25 becomes large.

However, in a narrow operating range, the controlled object may be considered to be a linear system. Therefore, in this embodiment, the operating region is divided into a plurality (two) of small operating regions, a linear state equation is created for each of the small operating regions, and the state equation is selectively used according to the operating state (the amount of intake air). Like that. In other words, while the calculation (control unit 25) is prevented from being complicated by taking the state equation as linear, the state equation is switched according to the intake air amount, that is, the exhaust gas volume, and the state equation and the dynamic characteristic of the controlled object are switched. There is no discrepancy (error). Specifically, in Equation 4, an optimal temporary delay characteristic value α is set for each small operation region, that is, an optimal matrix A is set for each small operation region, and these state equations are properly used, so that the entire operation region is used. Is a state equation that accurately represents the dynamic characteristic of the controlled object.

Hereinafter, a control method of the switching control for switching the state equation according to the intake air amount will be described with reference to a flowchart shown in FIG. First, in step # 1, various control information such as the intake air amount and the engine speed are read. Subsequently, in step # 2, the charging efficiency Ce, that is, the amount of air charged into the combustion chamber 5 in one combustion stroke, is calculated based on the intake air amount and the engine speed. The amount of intake air, that is, the amount of air per unit time, is basically represented by the product (CeNe) of the charging efficiency Ce and the engine speed Ne, and the exhaust gas volume is proportional to this amount of intake air.

Next, in step # 3 and step # 4,
Whether or not the charging efficiency Ce is smaller than a predetermined boundary filling efficiency Tce and whether or not the engine speed Ne is smaller than a predetermined boundary engine speed Tne, respectively, are compared and determined. Here, when it is determined that Ce <Tce and Ne <Tne (steps # 3 and # 4 are both YES), that is, when the operating state of the engine 1 is equal to the intake air amount indicated by the area A in FIG. If it is determined that the vehicle is in the small area, the air-fuel ratio control is performed in step # 5 according to the control law based on the model 1. Here, in the control law based on the model 1, Expression 4
The temporary delay characteristic value α included in the matrix A in the state equation shown in (1) is set to a predetermined relatively large value.

As described above, when α is large, as is apparent from Equation 1, the current (current time) exhaust air-fuel ratio λ [k] is strongly reflected in the next exhaust air-fuel ratio λ [k + 1]. , The effect of the fuel injection amount u [k-L], that is, the effect of the fuel injection amount change. That is, the changing speed of the exhaust air-fuel ratio λ [k + 1] decreases (dulls).

As described above, when the intake air amount (exhaust gas volume) is small, α is increased to decrease the changing speed of the exhaust air-fuel ratio because the exhaust gas in the exhaust passage 10 is reduced when the intake air amount is small. This is to compensate for the temporary delay because the temporary delay becomes large due to the non-uniformity of the flow. Thereafter, the process returns to step # 1.

On the other hand, when it is determined in step # 3 that Ce ≧ Tce (NO), or in step # 4, Ne ≧ Tne
Is determined (NO), that is, if it is determined that the operating state of the engine 1 is in the region where the amount of intake air is large as shown by the region B in FIG. The air-fuel ratio control is performed according to a control law based on the air-fuel ratio. Here, in the control law based on the model 2, the temporary delay characteristic value α in the state equation shown in Expression 4 is set to a relatively small value.

When α is small, the fuel injection amount u [k−L], that is, a change in the fuel injection amount is strongly reflected in the next exhaust air-fuel ratio λ [k + 1], contrary to the case of the model 1. And the speed of change of the exhaust air-fuel ratio λ [k + 1] increases. That is, when the intake air amount (exhaust gas volume) is large, the temporary delay becomes small, and α is set small to cope with this. Therefore, the temporary delay is accurately compensated according to the intake air amount (exhaust gas volume), the control accuracy of the air-fuel ratio control is increased, and the air-fuel ratio can accurately follow the target air-fuel ratio. Thereafter, the process returns to step # 1.

As shown in FIG. 5, the operating range is divided depending on whether or not the engine speed Ne is equal to or more than a predetermined value Tne '.
In a region A 'where Ne <Tne' (a region where the amount of intake air is small), Model 1 in which the temporary delay characteristic value α is set large is used, and a region B 'where Ne≥Tne' (a region where the amount of intake air is large).
Then, the model 2 in which α is set small may be used. In this case, there is no need to compare the charging efficiencies Ce, so the control logic is simplified.

[0037]

According to the first invention, the exhaust air-fuel ratio is predicted, and the fuel injection amount is set (corrected) so that the deviation between the predicted value of the air-fuel ratio and the target air-fuel ratio is minimized. Therefore, the responsiveness (responsiveness) of the air-fuel ratio to the target air-fuel ratio is improved, and the control accuracy of the air-fuel ratio control is improved. Furthermore, when the intake air amount is small, the correction characteristic is changed so that the change speed of the air-fuel ratio predicted value becomes slower than when the intake air amount is large, so that the temporary delay is accurately compensated according to the intake air amount (exhaust gas volume). Accordingly, the ability of the air-fuel ratio to follow the target air-fuel ratio is further enhanced.

According to the second aspect, basically the same operation and effect as those of the first aspect can be obtained. Further, the air-fuel ratio control is performed based on a model equation (state equation) representing the dynamic characteristic of the controlled object, and the model equation is preferably changed according to the intake air amount, that is, the exhaust gas volume. Is obtained, and the control accuracy of the air-fuel ratio control is further improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration according to first and second aspects of the present invention.

FIG. 2A is a system configuration diagram of an engine provided with an air-fuel ratio control device according to the present invention, and FIG. 2B shows input / output to / from a control target (control system) of the engine shown in FIG. FIG.

FIG. 3 is a flowchart illustrating a control method of model switching control in air-fuel ratio control.

FIG. 4 is a map showing an operation region using a model 1 and an operation region using a model 2 in model switching control.

FIG. 5 is a modified example of a map showing an operation region using a model 1 and an operation region using a model 2 in model switching control.

FIG. 6 shows a case where the fuel injection amount changes stepwise.
FIG. 4 is a diagram illustrating characteristics of an exhaust air-fuel ratio with respect to time.

7A is a diagram showing characteristics of the exhaust air-fuel ratio and the feedback coefficient with respect to time when the weight of the deviation accumulation term of the evaluation function is large, and FIG. 7B is a diagram showing the case where the weight of the deviation accumulation term is small; It is a figure similar to (a) in FIG.

[Explanation of symbols]

1 ... engine 5 ... combustion chamber 11 ... linear O ₂ sensor 20 ... Fuel injection valve 25 ... Control Unit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-60746 (JP, A) JP-A-3-233154 (JP, A) JP-A-3-233155 (JP, A) JP-A-2- 27135 (JP, A) JP-A-1-125541 (JP, A) JP-A-61-244850 (JP, A) JP-A-3-175125 (JP, A) JP-A-3-202650 (JP, A) JP-A-1-147135 (JP, A) JP-A-3-210036 (JP, A) JP-A-1-280648 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) F02D 41/14 310 F02D 41/04 305 F02D 45/00 370

Claims (2)

(57) [Claims] 1. A linear air-fuel ratio sensor for detecting an air-fuel ratio by detecting an oxygen concentration in exhaust gas, and an actual air-fuel ratio is determined based on an air-fuel ratio detected by the linear air-fuel ratio sensor to a predetermined target air-fuel ratio. An engine provided with fuel injection amount setting means for setting the fuel injection amount so as to follow the fuel ratio, and fuel injection amount correction means for correcting the fuel injection amount setting value in accordance with the primary delay time of the linear air-fuel ratio sensor In the air-fuel ratio control device of the above, the fuel injection amount correction means uses the air-fuel ratio detected by the linear air-fuel ratio sensor, the temporary delay time of the linear air-fuel ratio sensor, and the previous air-fuel ratio based on the past fuel injection amount results. And the fuel injection amount set value is corrected so as to minimize the deviation of the predicted value of the air-fuel ratio from the target air-fuel ratio. An air-fuel ratio of an engine, wherein a fuel injection amount correction characteristic changing means for changing a correction characteristic of the fuel injection amount correction means with respect to a temporary delay time is provided in a direction in which a change speed of the air-fuel ratio prediction value becomes slower. Control device. 2. The air-fuel ratio control device for an engine according to claim 1, wherein the fuel injection amount setting means and the fuel injection amount correction means include a parameter corresponding to a first-order lag time of the linear air-fuel ratio sensor. Using a predetermined model formula representing the relationship between the fuel injection amount and the air-fuel ratio, the air-fuel ratio grasped by the linear air-fuel ratio sensor, the temporary delay time of the linear air-fuel ratio sensor,
It is a modern control means for predicting the air-fuel ratio based on the past fuel injection amount results and setting and correcting the fuel injection amount so as to minimize the deviation of the air-fuel ratio predicted value from the target air-fuel ratio. An air-fuel ratio control for the engine, wherein the fuel injection amount correction characteristic changing means changes the correction characteristic relating to the temporary delay time by changing the model formula according to the intake air amount. apparatus.