JP2002364425A - Air-fuel ratio controller for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain high exhaust emission control efficiency by calculating an oxygen adsorption amount of an exhaust emission control catalyst and controlling the oxygen adsorption amount to an optimum amount. SOLUTION: In this controller, an oxygen amount calculation means calculates an oxygen amount unused in the exhaust emission control catalyst on the basis of an intake air flow rate and an output value of a first oxygen concentration detection means provided on the upstream side of the catalyst. A catalyst model identification means sequentially identifies a catalyst model set such that the subsequently adsorbed oxygen amount is reduced when the oxygen amount already adsorbed in the catalyst is increased. An oxygen adsorption amount calculation means calculates the oxygen adsorption amount of the catalyst by use of a parameter of the identified catalyst model, and a target air-fuel ratio setting means compares the calculated oxygen adsorption amount with an optimum oxygen adsorption amount, and sets and outputs a target air-fuel ratio.

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 apparatus for an internal combustion engine, and more particularly to a technique for controlling an amount of oxygen adsorbed on an exhaust purification catalyst to an optimum amount.

[0002]

2. Description of the Related Art Conventionally, an exhaust purification catalyst has been disposed in an exhaust passage of an internal combustion engine, and the oxidation and reduction of the exhaust purification catalyst has been balanced to achieve a high exhaust purification rate. 2. Description of the Related Art An exhaust gas purification system for an internal combustion engine that performs feedback control so as to achieve a target air-fuel ratio is known.

In this type of exhaust gas purification system, the amount of oxygen adsorbed in the catalyst greatly affects the exhaust gas purification rate. For example, if the amount of oxygen adsorbed in the catalyst exceeds the optimum value, C
When the oxidation reaction of O and HC is promoted, the reduction reaction of NOx is slowed down. Conversely, when the oxygen adsorption amount is smaller than the optimum value, the reduction reaction of NOx is promoted, while the oxidation reaction of CO and HC is slowed down. Will be.

Therefore, it is desirable to directly control the exhaust air-fuel ratio by the amount of oxygen adsorbed so that the amount of adsorbed oxygen in the catalyst is within the optimum range. Japanese Patent Application Laid-open No. Hei 9 (1997) discloses a method for controlling the exhaust air-fuel ratio by the amount of adsorbed substances in the catalyst.
Japanese Unexamined Patent Application Publication No. -72235 discloses an example. This method estimates the amount of adsorbed material using a catalyst model that takes into account the adsorption reaction of the gas component flowing into the catalyst, the oxidation-reduction reaction between the flowing gas component and the adsorbed material in the catalyst, and the desorption reaction of the adsorbed material in the catalyst. However, the exhaust air-fuel ratio is controlled so that the estimated amount of adsorbed substance falls within a predetermined range, whereby it is possible to control the amount of adsorbed oxygen in the catalyst within an optimum range.

[0005]

However, in the above method, the amount of the adsorbed substance is calculated based on the chemical reaction in the catalyst, so that there is a problem that the calculation becomes very complicated. In addition, a catalyst that has already adsorbed a large amount and a catalyst that has not adsorbed it at all have different adsorption states (that is, ease of adsorption to the catalyst), but this is not taken into account, and high accuracy is not considered. However, there is a problem that the amount of adsorption cannot be estimated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problem, and by setting a simple catalyst model, the oxygen adsorption amount of the catalyst can be accurately estimated while keeping the estimated oxygen adsorption amount within an optimum range. It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine that controls an exhaust air-fuel ratio such that

[0007]

Therefore, according to the first aspect of the present invention, as shown in FIG. 1, the oxygen concentration in the exhaust gas is measured on the upstream side of the exhaust purification catalyst provided in the exhaust passage of the internal combustion engine. First oxygen concentration detecting means for detecting, and second oxygen concentration detecting means for detecting the oxygen concentration in the exhaust gas passing through the exhaust purification catalyst downstream of the exhaust purification catalyst;
Oxygen amount calculating means for calculating the amount of oxygen not used for oxidation / reduction by the exhaust gas purification catalyst based on the output value of the first oxygen concentration detecting means and the amount of intake air to the engine, and the oxygen amount calculating means Using the detected oxygen amount as input and the detection value of the second oxygen concentration detecting means as output, the larger the amount of oxygen already adsorbed to the exhaust gas purification catalyst, the smaller the amount of oxygen subsequently adsorbed to the exhaust gas purification catalyst. Catalyst model identification means for sequentially identifying the catalyst model, and oxygen adsorption amount calculation means for calculating the oxygen adsorption amount of the exhaust purification catalyst using the parameters of the catalyst model identified by the catalyst model identification means. And target air-fuel ratio setting means for setting a target exhaust air-fuel ratio on the upstream side of the exhaust purification catalyst based on the oxygen adsorption amount calculated by the oxygen adsorption amount calculating means. That.

According to a second aspect of the present invention, the catalyst model is characterized in that the exhaust gas purifying catalyst has two characteristics of adhesion to a noble metal and metal oxidation.
It is characterized in that the transfer function calculated for each of the two oxygen adsorption elements is synthesized. The invention according to claim 3 is characterized in that the first transfer function and the second transfer function
The transfer function is characterized by being represented by a first-order lag type.

According to a fourth aspect of the present invention, the first oxygen concentration detecting means is a wide-range oxygen concentration sensor having a characteristic that an output value changes linearly with a change in an exhaust air-fuel ratio. I do. The invention according to claim 5 is characterized in that the second oxygen concentration detecting means is a stoichiometric oxygen concentration sensor having a characteristic that an output value changes rapidly near a stoichiometric air-fuel ratio of the exhaust air-fuel ratio.

According to a sixth aspect of the present invention, the target air-fuel ratio setting means sets the optimum oxygen adsorption of the exhaust purification catalyst set according to the oxygen adsorption amount calculated by the oxygen adsorption amount calculating means and the operating state of the engine. The target exhaust air-fuel ratio on the upstream side of the exhaust purification catalyst is set so that the difference from the amount becomes small.

The invention according to claim 7 is characterized in that the catalyst model identification means identifies a catalyst model using a sequential least squares method.

[0012]

According to the first aspect of the present invention, although the actual air-fuel ratio detected by the first oxygen concentration detecting means deviates from the stoichiometric air-fuel ratio and the amount of intake air, the air-fuel ratio is introduced into the catalyst. And the amount of oxygen that is not used for reduction is calculated, the amount of oxygen is used as an input, and the detection value of the second oxygen concentration detecting means (ie,
A catalyst model is set with the output (the amount of oxygen discharged from the catalyst) as an output.

Here, the catalyst model is designed so that the larger the amount of oxygen already adsorbed, the smaller the amount of oxygen adsorbed thereafter, that is, the “easiness of adsorption” changes depending on the oxygen adsorption state of the catalyst. It is set in consideration of, and by sequentially identifying this, it is possible not only to cope with changes in the operating state and deterioration of the catalyst, etc., but also to cope with the ease of oxygen adsorption of the catalyst. .

Then, using the parameters of the identified catalyst model, the amount of change in the amount of oxygen adsorbed on the catalyst and the amount of oxygen adsorption calculated by integrating the amount of change are calculated. Accurate calculation can be performed in response to changes in the characteristics of the catalyst including ease of adsorption. By setting the target exhaust air-fuel ratio based on the calculated amount of oxygen adsorption, the exhaust gas purification efficiency can be maintained at a high level. can do.

According to the second aspect of the invention, the oxygen purification of the exhaust purification catalyst includes two oxygen adsorption elements, one due to the adhesion to the noble metal and the other to oxidize the metal. By setting the catalyst model based on the final transfer function of the exhaust purification catalyst calculated by transfer function synthesis of the exhaust purification catalyst, a catalyst model that better expresses the oxygen adsorption state of the exhaust purification catalyst can be set.

According to the third aspect of the invention, a relatively simple second order (= first order + 1) is used without using a complicated reaction equation.
The oxygen adsorption amount can be accurately calculated using the catalyst model of the following (2). According to the fourth aspect of the present invention, the first oxygen concentration detecting means is a wide-range oxygen concentration sensor having a characteristic that an output value changes linearly with a change in exhaust air-fuel ratio, so that exhaust gas purification is performed. The amount of oxygen not used for oxidation / reduction by the catalyst can be accurately calculated.

According to the fifth aspect of the present invention, the second oxygen concentration detecting means is a stoichiometric oxygen concentration sensor having a characteristic that the output value changes rapidly near the stoichiometric air-fuel ratio of the exhaust air-fuel ratio. The oxygen concentration in the exhaust gas can be detected while suppressing an increase in cost. According to the sixth aspect of the invention, the target air-fuel ratio is set so that the amount of oxygen adsorbed by the exhaust gas purification catalyst becomes an optimum amount, so that the exhaust gas purification efficiency can be kept high.

According to the invention of claim 7, the catalyst model can be easily and well identified by using the sequential least squares method (RLS method).

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a system diagram of an internal combustion engine showing one embodiment of the present invention. In FIG. 2, an air flow meter 3 for detecting an intake air flow rate Qa is provided in an intake passage 2 of an engine 1, and an intake air amount Qa is controlled by a throttle valve 4.

Each cylinder of the engine 1 has a fuel injection valve (injector) 6 for injecting fuel into the combustion chamber 5, a combustion chamber 5
A spark plug 7 for performing spark ignition is provided therein, and a fuel mixture is formed by injecting fuel from the fuel injection valve 6 with respect to air sucked in through an intake valve 8, and the air-fuel mixture is formed. The fuel is compressed in the combustion chamber 5 and ignited by spark ignition by the spark plug 7.

The exhaust passage 10 is provided with an exhaust purification catalyst (hereinafter, simply referred to as an exhaust purification catalyst).
The catalyst 12 is interposed, and is provided upstream of the catalyst 12.
Makes the air-fuel ratio linear according to the oxygen concentration in the exhaust.
Wide-range oxygen concentration sensor (A / F sensor) 11 to detect
However, downstream of the catalyst 12, near the stoichiometric air-fuel ratio of the exhaust air-fuel ratio
A stoichiometric oxygen concentration sensor (O
_TwoSensor 13 is provided.

The exhaust gas of the engine 1 is discharged from the combustion chamber 5 to an exhaust passage 10 through an exhaust valve 9,
And released into the atmosphere via mufflers. C / U20
The, A / F sensor 11, O ₂ sensor 13, (not shown) the crank angle sensor, a water temperature sensor (not shown) signals from the air flow meter 3 and the like are input.

The C / U 20 sets the target exhaust air-fuel ratio (at the inlet side of the catalyst 12) by processing the various input signals in accordance with the control block diagram shown in FIG. The injection fuel amount and the like are controlled so as to be as follows. Next, the setting of the target air-fuel ratio on the catalyst 12 inlet side according to the present embodiment will be described with reference to the block diagram shown in FIG.

The oxygen amount calculator 21 calculates the amount of oxygen sucked into the catalyst 12 and not used for oxidation and reduction (ie, the amount of oxygen that affects the amount of adsorbed oxygen). In particular,
By multiplying the difference between the air-fuel ratio (actual λ) detected by the A / F sensor 11 and the stoichiometric air-fuel ratio (λ = 1) by the intake air amount Qa (Equation (1)), the oxygen amount of the catalyst 12 is adsorbed. Calculate the amount of oxygen inhaled that affects the amount.

U (t) = (actual λ−1) × Qa (1) The catalyst model identification unit 22 inputs the oxygen intake amount u (t) calculated by the oxygen amount calculation unit 21, O ₂ downstream
A catalyst model (identification model) having a sensor detection value (discharged oxygen amount) y (t) as an output is identified using a sequential least squares method (RLS method). Here, the setting of the catalyst model will be described.

In the present embodiment, a catalyst model is set by converting the catalyst 12 into a transfer function in consideration of the following points. Catalyst 12
There is a limit to the amount of oxygen that can be adsorbed, and the ease with which oxygen is adsorbed changes depending on the amount of oxygen already adsorbed (ie, can be expressed by a first-order lag). I do.

Further, there are two factors that cause the catalyst 12 to adsorb oxygen, namely, adhesion of oxygen to the noble metal and oxidation of the metal (for example, cerium is oxidized to cerium oxide).
A transfer function is formed for each of them, and a composite of these is defined as a final catalyst transfer function. Assuming that the amount of change in adsorption (the amount of adsorbed oxygen) decreases as the amount of oxygen adsorbed by the catalyst 12 increases, it can be expressed as in equation (2), where V is the amount of adsorption and k is the adsorption coefficient.

D (V) / dt = −kV (2) When this is solved, V = e− ^kT (3). When this is Laplace transformed, V (s) = 1 / (k + s) (4) ).

The Laplace transform of the oxygen inflow variation Δp from the equilibrium state where the oxygen adsorption amount ν of the catalyst 12 is equal to the oxygen emission amount q gives the following equation (5). Δp (s) = sP (5) Here, assuming that the oxygen adsorption amount ν of the catalyst 12 is a function of the oxygen inflow change amount Δp and the adsorbed amount V, the equations (4), (5) ) Is given by equation (6).

Ν (s) = sP · 1 / (k + s) (6) Further, the amount of oxygen p flowing into the catalyst 12 can be considered as the sum of the amount of oxygen adsorbed ν and the amount of oxygen exhausted q. Equation (7)
Can be represented by P (s) = ν (s) + Q (s) = sP · 1 / (k + s) + Q (s) (7) Accordingly, the transfer function Q (s) / P (s) is expressed by the following equation (8). Become.

Q (s) / P (s) = k / (k + s) (8) As described above, the elements of the catalyst 12 for adsorbing oxygen include adhesion to a noble metal and oxidation of the metal. Therefore, each of them is represented by a transfer function, and a composite of these is defined as a final transfer function of the catalyst. First, a transfer function for adhesion to a noble metal is represented by equation (9) as G1.

G1 = k1 / (k1 + s) (9) Similarly, the transfer function G2 for the oxidation of metal is expressed by the following equation (1).
0). G2 = k2 / (k2 + s) (10) From equations (9) and (10), the transfer function Gs of the catalyst 12 is obtained.
(Z), since given by Gs = G1 × G2 = k1 · k2 / ^{[s 2 + (k1 + k2)} s + k1 · k2 ] ... (11), represented as follows.

Gs (z) = b1 / [z ² + a1z + a2] (12) where a1, a2, and b1 are parameters. And ARX
The catalyst model is represented using the model as shown in Expression (13), and y (k) + a1y (k-1) + a2y (k-2) = b1u (k-1) + e (k) (13) The parameter vector θ and the data vector ψ are expressed by the formula (1)
If defined as 4) and (15), y (k) is given by equation (16)
Can be expressed as

Θ = [a1, a2, b1] ^T (14) ψ = [-y (k-1),-y (k-2), u (k-1)] ^T (15) y ( k) = θ ^T ψ (k) + e (k) (16) Next, the sequential identification (parameter estimation) of the catalyst model will be described. Since the characteristics of the catalyst 12 change depending on the operating state and the degree of deterioration of the catalyst 12, the parameters (a1, a2, b1) of the catalyst model shown in Expression (13) are sequentially estimated. The least squares method is used for the estimation, and the parameter that minimizes the square of the error between the actual value and the estimated value is calculated.

The arithmetic expression is the same as that of the general weighted successive least squares method, and is composed of the parameter estimation expressions of Expressions (17) to (18).

[0036]

(Equation 1) Here, the derivation of the parameter estimation formula will be described. (A) Application of least squares method The following equation is set as an evaluation criterion for estimating parameters.

[0037]

(Equation 2) Note that l (k, θ, ε (k, θ)) is an arbitrarily positive scalar function that measures the magnitude of the prediction error. In the least squares method, l (k,
The parameters are estimated such that θ, ε (k, θ)) = ε ² (k, θ) to minimize the evaluation criterion J _N (θ).

Here, in the ARX model, since the one-step ahead predicted value of the output is linear with respect to θ, that is, θ ^T ψ (k), the prediction error ε (k, θ) is given as follows. ε (k, θ) = y (k) −θ ^T ψ (k) (19) Then, the evaluation criterion J _N (θ) when the least squares method is applied
Is as shown in Expression (20).

[0039]

(Equation 3) Calculating this further,

[0040]

(Equation 4) Becomes However,

[0041]

(Equation 5) And m is the dimension of θ. To find the minimum value of J _N (θ), differentiating equation (21) with respect to θ and setting it to 0,

[0042]

(Equation 6) Is obtained. Next, the above formulas (17) and (19) are derived using formulas (22) to (26). (B) Derivation of Constitutive Equation (19) Equation (26) is derived from Equations (22) and (23).

[0043]

(Equation 7) Becomes The first term on the right side of Expression (27) is defined as follows,

[0044]

(Equation 8) Taking the inverse matrix of both sides of equation (28) and decomposing one element of the product sum,

[0045]

(Equation 9) Taking the inverse matrix of equation (29),

[0046]

(Equation 10) Becomes Here, applying the inverse matrix lemma to equation (30),

[0047]

[Equation 11] Becomes Note that the inverse matrix lemma means that the following equation holds for a certain regular matrix A. (A + BC) ^-1 = A ^-1 -A ^-1 B (I + CA ^-1 B) ^-1 CA ^-1 (3) Since the derived equation (23) of the constitutive equation (17) is an m-dimensional vector, Is replaced by

[0048]

(Equation 12) Here, from equations (27) and (28)

[0049]

(Equation 13) Where the second term on the right side is

[0050]

[Equation 14] Substituting this gives

[0051]

(Equation 15) Becomes Here, equation (27) is

[0052]

(Equation 16) Can be transformed into

[0053]

[Equation 17] Get. Here, by substituting equation (29) into equation (33),

[0054]

(Equation 18) And expand this,

[0055]

[Equation 19] Becomes By multiplying equation (31) by ψ (N),

[0056]

(Equation 20) And substituting equation (35) into equation (34),

[0057]

(Equation 21) Becomes Since the brackets on the right side correspond to the prediction error,

[0058]

(Equation 22) I can go. Therefore, equation (36) becomes

[0059]

(Equation 23)Becomes Equations (31), (37), and (38) above are usually
Of the (unweighted) recursive least squares method
This is an estimation formula.
Exponentially forgetting past values gives a good estimate of convergence
The evaluation standard J _N(θ) is set as follows.

[0060]

(Equation 24) Here, λ is a forgetting factor and is a positive number equal to or less than 1 (for example, 0.98). Then, the above parameter estimation formula (3
1), (37), and (38) are transformed with weights, and the parameter estimation expressions of Expressions (17) to (19) are obtained.

The above parameter estimation equations (17) to (17)
The parameters a1, a2, and b1 are sequentially estimated using the sequential least squares method constituted by (19). Here, the system identification may be started with the parameter initial value set to 0 as in the case of normal system identification, but a certain amount of time is required until convergence, so a value determined in advance offline (offline estimation parameter value) May be used as an initial value. For example, data of an oxygen intake amount (input u (t)) and an O ₂ sensor detection value (output y (t)) at the time of idling operation when a catalyst whose characteristics have a substantially central value are sampled, The (off-line) parameter estimation value is calculated using a general prediction error method (for example, the maximum likelihood estimation method or the least squares method).

Here, from the above equations (11) and (12), a1 = k1 + k2 (39) a2 = b1 = k1 · k2 (40) Therefore, k1 and k2 are successively estimated. It can be calculated as follows using the parameters (a1, a2, b1).

K1 = a2 / k2 (41) k2 = [(a1^Two/ 4) -a2]^1/2+ a1 / 2 (41) The oxygen adsorption amount calculation unit 23 calculates the oxygen adsorption amount. Above
As described above, the oxygen adsorption amount ν of the catalyst 12 depends on the oxygen inflow
Since it was given as a function of the amount of change Δp and the amount of adsorption V,
The amount of the catalyst 12 adhering to the noble metal is Δp × e^{- k1t}, Metal
The amount to be oxidized is Δp × e^-k2tAnd the amount of change in oxygen adsorption
Calculate (However, k1 = a2 / k2, k2 = [(a1^Two/ 4) -a2]^1/2+
a1 / 2). Then, integration processing is further performed to
Is calculated.

The target air-fuel ratio setting unit 24 calculates the oxygen adsorption amount of the catalyst 12 calculated by the oxygen adsorption amount calculation unit 23 and the operating state of the engine (for example, engine load Tp, rotation speed Ne, etc.).
Is compared with the optimal oxygen adsorption amount set based on the above, the difference is calculated, and the difference is converted into a target air-fuel ratio and output. Note that the optimal oxygen adsorption amount is an oxygen adsorption amount in a range where the exhaust gas purification efficiency of the catalyst 12 is maximum, and the target air-fuel ratio is an A / F sensor 11 on the upstream side of the catalyst 12.
Is the target value of the exhaust air-fuel ratio detected at.

Air-fuel ratio feedback (F / B) controller 2
Reference numeral 5 sets the amount of fuel to be injected and the like based on the target air-fuel ratio (target λ) set by the target air-fuel ratio setting unit 24 and the actual air-fuel ratio (actual λ) detected by the A / F sensor 11. As described above, the catalyst model (identification model) is set in which the catalyst 12 is represented by a transfer function in consideration of the easiness of adsorbing oxygen, and the A / F sensor 11 on the upstream side of the catalyst 12 and the O ₂ on the downstream side are set. Since the catalyst model is sequentially identified based on the detection value of the sensor 13, the oxygen adsorption behavior can be accurately expressed using a relatively simple model.

Since the amount of adsorbed oxygen is calculated using the parameters (estimated parameters) of the sequentially identified catalyst model, the catalyst 1 corresponding to the fluctuation of the oxygen adsorption characteristics of the catalyst 12 can be used.
2 can accurately calculate the oxygen adsorption amount. Then, the calculated oxygen adsorption amount and the optimum oxygen adsorption amount are compared, and the difference is converted and output as the target air-fuel ratio.
The amount of oxygen adsorbed is always the optimal amount of oxygen adsorbed, and the exhaust gas purification efficiency can be kept high.

[Brief description of the drawings]

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

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

[Explanation of symbols]

1 engine 2 intake passage 3 air flow meter 4 throttle valve 6 fuel injection valve 7 spark plug 8 intake valves 9 an exhaust valve 10 exhaust passage 11 A / F sensor 12 exhaust purification catalyst 13 O ₂ sensor 20 control unit (C / U) 21 Oxygen Amount calculation unit 22 Catalyst model identification unit 23 Oxygen adsorption amount calculation unit 24 Target air-fuel ratio setting unit 25 Air-fuel ratio F / B control unit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) F02D 45/00 314 F02D 45/00 314Z F term (reference) 3G084 AA04 BA09 BA11 DA05 DA07 DA10 EA03 EB11 FA07 FA20 FA26 FA30 FA38 3G091 AA02 AA12 AA17 AA23 AB06 BA01 BA07 BA11 BA13 BA27 BA33 CB01 DC01 DC07 EA01 EA05 EA16 EA34 EA36 FB10 FC01 HA36 HA37 HA42 3G301 HA01 HA15 JA25 JA26 JA33 LB02 LC01 MA01 MA11 NA03 ND01 PE03 PD03

Claims (7)

[Claims] A first oxygen concentration detecting means for detecting an oxygen concentration in exhaust gas on an upstream side of an exhaust gas purification catalyst provided in an exhaust passage of an internal combustion engine; A second oxygen concentration detecting means for detecting an oxygen concentration in the exhaust gas having passed through the exhaust gas purification catalyst, and the second oxygen concentration detecting means based on an output value of the first oxygen concentration detecting means and an intake air amount to an engine. An oxygen amount calculation means for calculating an oxygen amount not used for oxidation / reduction by the exhaust gas purification catalyst; an oxygen amount calculated by the oxygen amount calculation means being input; and a detection value of the second oxygen concentration detection means being an output, the exhaust gas Catalyst model identification means for setting a catalyst model such that the greater the amount of oxygen already adsorbed on the purification catalyst, the smaller the amount of oxygen subsequently adsorbed on the exhaust purification catalyst, and sequentially identifying the catalyst model; Tactile means identified Oxygen adsorption amount calculating means for calculating an oxygen adsorption amount of the exhaust purification catalyst using parameters of a medium model; and a target exhaust air-fuel ratio upstream of the exhaust purification catalyst based on the oxygen adsorption amount calculated by the oxygen adsorption amount calculating means. An air-fuel ratio control device for an internal combustion engine, comprising: target air-fuel ratio setting means for setting 2. The exhaust purification catalyst according to claim 1, wherein the catalyst model is obtained by combining a first transfer function calculated in consideration of an amount attached to a noble metal and a second transfer function calculated in consideration of an amount of metal oxidized. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio control device is set based on a final transfer function. 3. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the first transfer function and the second transfer function are each represented by a first-order lag type. 4. The oxygen concentration sensor according to claim 1, wherein said first oxygen concentration detection means is a wide-range oxygen concentration sensor having a characteristic that an output value changes linearly with a change in exhaust air-fuel ratio. Item 4. An air-fuel ratio control device for an internal combustion engine according to any one of items 3. 5. The stoichiometric oxygen concentration sensor according to claim 1, wherein said second oxygen concentration detecting means has a characteristic that an output value changes abruptly near a stoichiometric air-fuel ratio of an exhaust air-fuel ratio. 5. The air-fuel ratio control device for an internal combustion engine according to any one of 4. 6. The target air-fuel ratio setting means reduces a difference between an oxygen adsorption amount calculated by the oxygen adsorption amount calculating means and an optimum oxygen adsorption amount of the exhaust purification catalyst set according to an operating state of an engine. The air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 5, wherein a target exhaust air-fuel ratio on the upstream side of the exhaust purification catalyst is set so as to be as follows. 7. The air-fuel ratio control for an internal combustion engine according to claim 1, wherein said catalyst model identification means identifies the catalyst model using a sequential least squares method. apparatus.