JPH10184424A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately calculate a storage oxygen desorption amount of a three-dimensional catalyst. SOLUTION: An upstream air-fuel ratio sensor 23 is arranged in an exhaust passage situated upper stream from a three-dimensional catalyst 18 and a downstream air-fuel ration sensor 24 is arranged in an exhaust passage situated downstream from the three-dimensional catalyst 18. From an air-fuel ratio and an intake air amount detected by the upstream air-fuel ratio sensor 23 and a speed ratio between a desorption speed and an adsorption speed of oxygen at the three-dimensional catalyst 18, a storage oxygen desorption amount is calculated. A speed ratio is corrected such that a storage oxygen desorption amount by calculation coincides with an actual storage oxygen desorption amount.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an air-fuel ratio control device for an internal combustion engine.

[0002]

2. Description of the Related Art A three-way catalyst has an air-fuel ratio almost stoichiometric.
Function to oxidize HC and CO and reduce NOx
Therefore, if the air-fuel ratio can be maintained almost at the stoichiometric air-fuel ratio, ternary
Simultaneous purification of HC, CO and NOx using a catalyst
Can be. However, maintain the air-fuel ratio almost at the stoichiometric air-fuel ratio
Is difficult, and the air-fuel ratio is not
I will be. However, the air-fuel ratio deviates from the theoretical air-fuel ratio
Even if the three-way catalyst O _TwoSlage function, ie oxygen storage
HC, CO, NOx can be purified by the function
You.

That is, the three-way catalyst has a function of taking in and storing excess oxygen in exhaust gas when the air-fuel ratio is lean,
With this function, NOx is reduced. On the other hand, when the air-fuel ratio becomes rich, the unburned HC and CO in the exhaust gas rob oxygen stored in the three-way catalyst, thereby oxidizing the unburned HC and CO. Therefore, in order to reduce NOx when the air-fuel ratio deviates from the stoichiometric air-fuel ratio, the three-way catalyst must be in a state capable of storing oxygen, that is, the oxygen storage amount of the three-way catalyst is larger than the maximum oxygen storage amount. There must be room, while the three-way catalyst must store some oxygen to oxidize unburned HC and CO. That is, NOx can be reduced when the air-fuel ratio deviates from the stoichiometric air-fuel ratio to the lean side, and when the air-fuel ratio deviates to the rich side with respect to the stoichiometric air-fuel ratio, the unburned HC, C
In order to be able to oxidize O, the oxygen storage amount of the three-way catalyst must be maintained at about half of the maximum oxygen storage amount.

The amount of oxygen adsorbed on the three-way catalyst and the amount of oxygen desorbed from the three-way catalyst can be calculated from the amount of intake air and the amount of deviation of the air-fuel ratio from the stoichiometric air-fuel ratio. The stored oxygen amount of the three-way catalyst can be calculated from the deviation amount of the air-fuel ratio. Therefore, a target storage oxygen amount to be stored in the three-way catalyst is predetermined, and the fuel injection amount is controlled so that the calculated storage oxygen amount of the three-way catalyst becomes the target storage oxygen amount. The engine is known (see JP-A-6-249028).

[0005]

However, in order to control the amount of oxygen to be stored in the three-way catalyst on the basis of the calculated amount of stored oxygen, the amount of oxygen stored in the three-way catalyst must be accurately calculated. Must. However, while calculating the stored oxygen amount from the difference between the intake air amount and the air-fuel ratio as described above, a deviation occurs from the actual stored oxygen amount, and therefore, the stored oxygen amount is accurately calculated. In order to do so, it is necessary to correct this deviation amount. However, in the above-mentioned internal combustion engine, the deviation is not corrected, and there is a problem that the calculated stored oxygen amount does not accurately represent the actual stored oxygen amount.

[0006]

According to a first aspect of the present invention, an upstream air-fuel ratio sensor is disposed in an engine exhaust passage upstream of a three-way catalyst and an engine downstream of the three-way catalyst is provided. In an internal combustion engine having a downstream air-fuel ratio sensor disposed in the exhaust passage, the three-way catalyst uses the air-fuel ratio detected by the upstream air-fuel ratio sensor and the speed ratio of the oxygen desorption speed and adsorption speed in the three-way catalyst. Calculating means for calculating a stored oxygen desorption amount indicating the desorption amount of oxygen stored in the source catalyst; andcontrol means for controlling an air-fuel ratio of the engine such that the stored oxygen desorption amount becomes the target desorption amount. When the air-fuel ratio detected by the downstream air-fuel ratio sensor is lean even though the calculated stored oxygen desorption amount has not reached zero, the desorption speed is reduced relatively to the adsorption speed. Speed ratio corrected and calculated If the air-fuel ratio detected by the downstream air-fuel ratio sensor is not lean even though the stored oxygen desorption amount has reached zero, the speed that corrects the speed ratio to increase the desorption speed relative to the adsorption speed Ratio correcting means.

That is, when the air-fuel ratio detected by the downstream air-fuel ratio sensor is lean even though the calculated stored oxygen desorption amount has not reached zero, that is, the calculated storage oxygen desorption amount is calculated. It is considered that the calculated desorption rate of oxygen is too fast than the actual rate when does not reach zero but actually reaches zero. Therefore, at this time, the desorption speed is reduced relatively to the adsorption speed. On the other hand, when the calculated stored oxygen desorption amount is zero, but the air-fuel ratio detected by the downstream air-fuel ratio sensor is not lean, that is, the calculated stored oxygen desorption amount is zero. However, when it is not actually zero, the calculated desorption rate of oxygen is considered to be too slow. Therefore, at this time, the desorption speed is increased relatively to the adsorption speed.

[0008] In the second invention, in the first invention,
If the calculated amount of stored oxygen desorbed does not reach the maximum desorbed amount and the air-fuel ratio detected by the downstream air-fuel ratio sensor is rich, the maximum desorbed amount is reduced and corrected. The maximum desorption amount correcting means for increasing and correcting the maximum desorption amount when the air-fuel ratio detected by the downstream air-fuel ratio sensor is not rich even though the stored oxygen desorption amount has reached the maximum desorption amount. I have it.

That is, when the air-fuel ratio detected by the downstream air-fuel ratio sensor is rich even though the calculated stored oxygen desorption amount has not reached the maximum desorption amount, that is, when the stored oxygen desorption amount is calculated, When the desorption amount reaches the maximum desorption amount but does not actually reach the maximum desorption amount, it is considered that the calculated maximum desorption amount is too large. Therefore, at this time, the maximum desorption amount is reduced. On the other hand, when the calculated stored oxygen desorption amount has reached the maximum desorption amount, but the air-fuel ratio detected by the downstream air-fuel ratio sensor is not rich, that is, the calculated stored oxygen desorption amount is not calculated. When the amount has reached the maximum desorption amount but has not actually reached the maximum desorption amount, the calculated maximum desorption amount is considered to be too small. Therefore, at this time, the maximum desorption amount is increased.

According to a third aspect of the present invention, there is provided an internal combustion engine having an upstream air-fuel ratio sensor disposed in an engine exhaust passage upstream of a three-way catalyst and a downstream air-fuel ratio sensor disposed in an engine exhaust passage downstream of the three-way catalyst. Storage oxygen desorption indicating the amount of desorbed oxygen stored in the three-way catalyst based on the intake air amount, the air-fuel ratio detected by the upstream air-fuel ratio sensor, and the speed ratio of the desorption speed and the adsorption speed of the three-way catalyst. Calculating means for calculating the desorbed amount; control means for controlling the air-fuel ratio of the engine such that the stored desorbed oxygen amount becomes the target desorbed amount; and the calculated stored oxygen desorbed amount reaches the maximum desorbed amount. When the air-fuel ratio detected by the downstream air-fuel ratio sensor is rich despite the fact that it is not, the speed ratio is corrected to increase the desorption speed relative to the adsorption speed, and the calculated stored oxygen The separation amount has reached the maximum desorption amount Air-fuel ratio detected by the downstream side air-fuel ratio sensor regardless is when not rich has and a speed ratio correction means for correcting the speed ratio so as to relatively decrease the desorption rate relative adsorption rates.

That is, when the air-fuel ratio detected by the downstream air-fuel ratio sensor is rich even though the calculated stored oxygen desorption amount has not reached the maximum desorption amount, that is, when the stored oxygen desorption amount is calculated, When the desorption amount does not reach the maximum desorption amount but actually reaches the maximum desorption amount, the calculated desorption rate of oxygen is considered to be too slow.
Therefore, at this time, the desorption speed is increased relatively to the adsorption speed. On the other hand, when the calculated stored oxygen desorption amount has reached the maximum desorption amount, but the air-fuel ratio detected by the downstream air-fuel ratio sensor is not rich, that is, the calculated stored oxygen desorption amount is not calculated. When the amount has reached the maximum desorption amount but has not actually reached the maximum desorption amount, the calculated oxygen desorption rate is considered to be too high.
Therefore, at this time, the desorption speed is reduced relatively to the adsorption speed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is a spark plug, 5 is an intake valve, 6 is an intake port, 7 is an exhaust valve, and 8 is exhaust gas. Each port is shown. The intake ports 6 are connected to the surge tank 10 via the corresponding branch pipes 9, and each branch pipe 9 has an intake port 6 respectively.
A fuel injection valve 11 for injecting fuel toward the inside is mounted. The surge tank 10 is connected to an air cleaner 14 via an intake duct 12 and an air flow meter 13, and a throttle valve 15 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected to the exhaust manifold 16 and the exhaust pipe 1.
7 is connected to a casing 19 containing a three-way catalyst 18.

The electronic control unit 30 is composed of a digital computer, and is connected to a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and a power source connected to each other by a bidirectional bus 31. Backup RAM connected
35, an input port 36 and an output port 37. The air flow meter 13 generates an output voltage proportional to the amount of intake air, and the output voltage
Is input to the input port 36 via the. Throttle valve 1
An idle switch 21 for generating an output signal indicating that the throttle valve 15 is at the idling position is attached to 5, and an output signal of the idle switch 21 is input to an input port 36. Further, a water temperature sensor 22 that generates an output voltage proportional to the engine cooling water temperature is attached to the engine body 1, and the output voltage of the water temperature sensor 22 corresponds to A
The data is input to the input port 36 via the D converter 38.

On the other hand, an air-fuel ratio sensor 23 (hereinafter referred to as an upstream air-fuel ratio sensor) is arranged in the exhaust pipe 17 upstream of the three-way catalyst 18, and the air-fuel ratio sensor is also provided in the exhaust pipe 20 downstream of the three-way catalyst 18. A sensor 24 (hereinafter, referred to as a downstream air-fuel ratio sensor) is provided. The output signals of the air-fuel ratio sensors 23 and 24 are input to the input port 3 via the corresponding AD converter 38.
6 is input. In the three-way catalyst 18, the three-way catalyst 1 is provided.
Temperature sensor 25 that generates an output voltage proportional to the temperature of 8
And the output voltage of the temperature sensor 25 is input to the input port 36 via the corresponding AD converter 38.
The input port 36 is connected to a rotation speed sensor 26 that generates an output pulse indicating the engine rotation speed. On the other hand, the output port 37 is connected to the ignition plug 4 and the fuel injection valve 11 via the corresponding drive circuit 39, respectively.

The upstream air-fuel ratio sensor 23 generates a current I according to the air-fuel ratio A / F as shown in FIG.
This current I is converted into a voltage and input to the input port 36 via the AD converter 38. Accordingly, based on the output signal of the upstream air-fuel ratio sensor 23, the air-fuel ratio A upstream of the three-way catalyst 18 is determined.
/ F. On the other hand, the downstream air-fuel ratio sensor 24 generates an output voltage V that changes abruptly at the stoichiometric air-fuel ratio as shown in FIG. That is, when the air-fuel ratio is lean, the downstream air-fuel ratio sensor 24 outputs 0.1 (V).
Output voltage V of about 0.9 (V) when the air-fuel ratio is rich. In the embodiment according to the present invention, the output voltage V of the downstream air-fuel ratio sensor 24 is
Is lower than the set value V _L , for example, 0.2 (V), it is determined that the air-fuel ratio on the downstream side of the three-way catalyst 18 is lean, and the output voltage V of the downstream air-fuel ratio sensor 24 becomes the set value V
_R , for example, when it is higher than 0.7 (V),
It is determined that the air-fuel ratio on the downstream side is rich.

The three-way catalyst 18 has a function of oxidizing HC and CO and reducing NOx when the air-fuel ratio is substantially at the stoichiometric air-fuel ratio, that is, a function of simultaneously purifying HC, CO and NOx. However the three-way catalyst 18 as mentioned in the introduction the O ₂ storage function, i.e. a function of storing oxygen therein, the O ₂ to ternary though the air-fuel ratio deviates from the stoichiometric air-fuel ratio by the storage function Catalyst 1
8 can purify HC, CO and NOx. This oxygen storage action is performed by cerium Ce contained in the three-way catalyst 18.

That is, cerium Ce is unstable when it is a simple metal, and becomes stable as ceria CeO ₂ when oxygen is bonded thereto. Therefore, if oxygen exists around cerium Ce, that is, if the air-fuel ratio is lean, oxygen is immediately deprived and becomes ceria CeO ₂ . On the other hand, when the air-fuel ratio becomes rich, that is, when a large amount of unburned HC and CO is present in the exhaust gas, the unburned HC and CO deprive ceria CeO ₂ of oxygen, so that ceria CeO ₂ becomes unstable cerium Ce again. Becomes In this case, the time required for cerium Ce to deprive oxygen from the surroundings is extremely short, that is, the adsorption speed of oxygen is extremely high, while the time required for unburned HC and CO to deprive ceria CeO ₂ of oxygen. Is slightly longer, that is, the desorption rate of oxygen is slower than the adsorption rate.

As described above, when the air-fuel ratio becomes lean, oxygen is depleted from the exhaust gas, so that the NO contained in the exhaust gas is reduced.
When x is reduced and the air-fuel ratio becomes rich, the unburned HC and CO of the exhaust gas deprive ceria CeO ₂ of oxygen, so that the unburned HC and CO are oxidized. Therefore, even if the air-fuel ratio deviates from the stoichiometric air-fuel ratio, HC, CO and NOx can be purified. However, in this case, the three-way catalyst 18 must be in a state capable of storing oxygen when the air-fuel ratio becomes lean, so that HC, CO and NOx can be purified, and the air-fuel ratio becomes rich. Sometimes the three-way catalyst 18 must have stored some oxygen.

The amount of oxygen that can be stored by the three-way catalyst 18 is limited, and the three-way catalyst 18 cannot store more oxygen than the three-way catalyst 18 can store. On the other hand, when the air-fuel ratio deviates from the stoichiometric air-fuel ratio, it is not known whether the air-fuel ratio deviates toward the lean side or the rich side. It is necessary to equalize the amount of oxygen that can be obtained and the amount of oxygen that can be desorbed therefrom. That is, considering that there is a limit to the amount of oxygen that the three-way catalyst 18 can store, the amount of oxygen stored in the three-way catalyst 18 is maintained at half of the maximum amount of oxygen that the three-way catalyst 18 can store. There is a need.

Incidentally, the amount of oxygen stored in the three-way catalyst 18 cannot be directly measured. Therefore, this amount of oxygen is usually obtained by calculation. In this case, the oxygen storage amount is usually calculated based on the case where the oxygen storage amount is zero. However, in order to reliably create such a state in which the oxygen storage amount is zero, the air-fuel ratio must be rich. There is a problem that a state must be actively created. Furthermore, the desorption speed of oxygen from the three-way catalyst 18 is relatively slow, and the desorption speed is greatly affected by the ambient temperature. Therefore, even if the air-fuel ratio becomes temporarily rich, the amount of stored oxygen is reduced. There is a disadvantage that it cannot be said that it is always zero. In other words, there is a danger that the reference value will be out of order if the storage amount of oxygen is zero.

On the other hand, a state in which the amount of oxygen stored in the three-way catalyst 18 is maximized can be easily and reliably created. That is, since the three-way catalyst 18 is exposed to the atmosphere during the stop of the engine, the stored amount of oxygen in the three-way catalyst 18 is the maximum, which is the normal state of the three-way catalyst 18. Therefore, it is extremely natural to refer to the state where the oxygen storage amount is the maximum. It is common to stop the fuel supply during the deceleration operation.
Is exposed to the atmosphere. Since a large amount of oxygen is present in the atmosphere and the oxygen adsorption speed of the three-way catalyst 18 is high, the amount of oxygen stored in the three-way catalyst 18 can be reduced even when the fuel supply stop time is extremely short. It will definitely be maximum. Therefore, in the embodiment according to the present invention, the oxygen storage amount is calculated based on the maximum oxygen storage amount.

The fact that the oxygen storage amount is the maximum means that the oxygen stored in the three-way catalyst has not been desorbed at all. Therefore, in the embodiment according to the present invention, the desorbed amount of oxygen is determined based on such a state, that is, the state where the desorbed amount of stored oxygen is zero. This stored oxygen desorption amount is represented by the symbol OSC below. Therefore, the fact that the stored oxygen desorption amount OSC is zero indicates that the stored oxygen amount is the maximum, and when the stored oxygen desorption amount OSC is the maximum, the desorbable oxygen is completely desorbed. Is shown. The time when the stored oxygen desorption amount OSC is maximum is represented by the symbol OSCmax below.

Next, a method of calculating the amount of adsorption and desorption of oxygen will be described. In the embodiment according to the present invention, the amount of adsorbed oxygen and the amount of desorbed oxygen per time Δt are calculated using the following equations. Adsorption amount = KO ₂ · {Ga · (ΔA / F) / (A / F)}
· Delta] t desorption amount _{= KO 2 · {Ga · (} ΔA / F) / (A / F) /
k｝ · Δt Here, KO ₂ represents the oxygen concentration in the air, Ga represents the intake air amount (g / s), and (A / F) represents the upstream air-fuel ratio sensor 23.
(ΔA / F) represents the deviation (A / F-theoretical air-fuel ratio) between this air-fuel ratio (A / F) and the stoichiometric air-fuel ratio.

When the air-fuel ratio is lean, the three-way catalyst 18
Since oxygen is adsorbed, the above-mentioned amount of adsorption is determined at this time.
Is used. In this equation, KO_Two・ Ga is an institution
The amount of oxygen supplied in the cylinder per unit time (g /
s), and (ΔA / F) / (A / F) burns
Shows the ratio of surplus oxygen at the time. Therefore KO
_Two・ {Ga ・ (ΔA / F) / (A / F)} ・ Δt is time
It indicates the surplus oxygen amount (g) per Δt. Where ΔA
/ F is positive. The presence of such excess oxygen
Excess oxygen is considered to be immediately absorbed by the three-way catalyst 18.
The amount of adsorption per time Δt is the same as the amount of excess oxygen
Therefore, the adsorption amount per time Δt is expressed as
Will be done.

As described above, when the air-fuel ratio is lean, the above-mentioned surplus oxygen amount is stored per time Δt. Therefore, the above-mentioned stored oxygen desorption amount OSC is reduced by the above-mentioned surplus oxygen amount per time Δt. Will do. Therefore, the stored oxygen desorption amount OSC is represented by the following equation. OSC = OSC-KO ₂ · ｛Ga · (ΔA / F) / (A
/ F)｝ · Δt On the other hand, when the air-fuel ratio is rich, oxygen is desorbed from the three-way catalyst 18. At this time, the above equation for determining the desorption amount is used. Also in this equation, KO ₂ · Ga indicates the amount of oxygen (g / s) supplied into the engine cylinder per unit time. On the other hand, in this equation, (ΔA / F) /
(A / F) indicates the proportion of oxygen deficient during combustion, and therefore, KO ₂ · ｛Ga · (ΔA / F) / (A /
F)｝ · Δt represents the amount of oxygen deficiency (g) per time Δt. Here, ΔA / F is negative.

If there is a shortage of oxygen during combustion, unburned HC and CO are generated by the amount of the insufficient oxygen, and the unburned HC and C are generated.
The amount of oxygen proportional to the amount of generated O, that is, the amount of oxygen proportional to the amount of oxygen deficiency is desorbed from the three-way catalyst 18. However, as described above, the rate of desorption of oxygen from the three-way catalyst 18 is lower than the rate of adsorption of oxygen to the three-way catalyst 18, and the amount of oxygen desorbed from the three-way catalyst 18 at this time is smaller than the rate of adsorption. As a result, the desorption rate is lower and the amount becomes smaller than the oxygen deficiency. In other words, the amount of oxygen desorbed from the three-way catalyst 18 is the amount of oxygen deficient (desorption rate / adsorption rate).
Double. Accordingly, if the speed ratio between the desorption speed and the adsorption speed is 1 / k (= desorption speed / adsorption speed), the desorption amount per time Δt is the amount obtained by multiplying the above-described oxygen deficient amount by the speed ratio 1 / k. Therefore, the desorption amount per time Δt is expressed by the above equation.

As described above, when the air-fuel ratio is rich, the oxygen-deficient amount / speed ratio 1 / k per time Δt is desorbed. Therefore, the above-mentioned stored oxygen desorbed amount OSC is the oxygen-deficient amount / speed per time Δt. It will increase by the ratio 1 / k. Therefore, considering that ΔA / F <0, the stored oxygen desorption amount OSC is expressed by the following equation. OSC = OSC-KO ₂ · ｛Ga · (ΔA / F) / (A
/ F) / k｝ · Δt Note that when the air-fuel ratio is maintained at the stoichiometric air-fuel ratio, it is considered that the oxygen adsorbing action and the desorbing action are not performed, and at this time, the stored oxygen desorbed amount OSC does not change. .

As described above, the stored oxygen desorption amount OSC is determined by the intake air amount, the air-fuel ratio detected by the upstream air-fuel ratio sensor 23, and the speed ratio of the desorption speed and the adsorption speed of oxygen in the three-way catalyst 18 to 1 / k. On the other hand, the maximum desorption amount OSCmax of the stored oxygen is basically the product of the maximum desorption amount G (Tc) when the three-way catalyst 18 is new and the degradation coefficient DK of the three-way catalyst 18 (= G ( Tc) / DK)
Represented by The maximum desorption amount G (Tc) when the three-way catalyst 18 is new is a function of the temperature Tc of the three-way catalyst 18 as shown in FIG.
c) increases as the temperature Tc of the three-way catalyst 18 increases.

On the other hand, the deterioration coefficient DK of the three-way catalyst 18 is 1.0 when the three-way catalyst 18 is new. However, as the service period of the three-way catalyst 18 becomes longer, the three-way catalyst 1
8 gradually deteriorates, and the O ₂ storage function gradually weakens. In the embodiment according to the present invention, the cumulative operation time TD is used as a representative value of the use period of the three-way catalyst 18, and in this case, the deterioration coefficient DK of the three-way catalyst 18 is calculated as shown in FIG. It becomes smaller gradually as the TD becomes longer.

G (Tc) shown in FIGS. 3A and 3B
And DK were determined experimentally and therefore these G (T
c) The maximum desorption amount OSCmax obtained from the product of DK
Represents the actual maximum desorption amount. However, depending on how the three-way catalyst 18 is used, G (Tc) and DK
There is a risk that the maximum desorption amount OSCmax determined from the product of the products may not accurately represent the actual maximum desorption amount. Therefore, in the first embodiment according to the present invention, the maximum desorption amount O
The maximum desorption amount OSCmax is corrected by the correction coefficient KOSC so that SCmax accurately represents the actual maximum desorption amount. On the other hand, in the second embodiment, as will be described later, the maximum desorption amount OSCmax obtained from the product of G (Tc) and DK is used without modification.

In the embodiment according to the present invention, even if the air-fuel ratio deviates to the lean side or the rich side with respect to the stoichiometric air-fuel ratio, half of the maximum desorption amount OSCmax is reduced so that harmful components in the exhaust gas can be purified. The target injection amount OSCref is set, and the fuel injection amount is controlled such that the storage oxygen release amount OSC obtained by the calculation becomes the target release amount OSCref.

That is, in the embodiment according to the present invention, the basic fuel injection time TAUB necessary for bringing the air-fuel ratio to the stoichiometric air-fuel ratio is obtained in advance by experiments, and this basic fuel injection time TAUB is as shown in FIG. Are stored in advance in the ROM 32 as a function of the engine load (intake air amount Q / engine speed N) and the engine speed N. When the fuel injection time is maintained at the basic fuel injection time TAUB, the air-fuel ratio is maintained at the stoichiometric air-fuel ratio. Therefore, when the stored oxygen desorption amount OSC is maintained at the target desorption amount OSCref, the fuel injection time is set at the basic value. The fuel injection time is maintained at TAUB. In contrast, the stored oxygen desorption amount OSC is equal to the target desorption amount OSCre.
f, the stored oxygen desorption amount OSC becomes equal to the target desorption amount OS
The fuel injection time is increased or decreased to return to Cref. Next, this will be described with reference to FIG.

As shown in FIG. 5, the stored oxygen desorption amount OS
When C is maintained at the target desorption amount OSCref, the fuel injection time TAU is maintained at the basic fuel injection time TAUB. Next, when the amount of oxygen desorbed increases and the stored oxygen desorbed amount OSC becomes larger than the target desorbed amount OSCref, the fuel injection time TAU is reduced with respect to the basic fuel injection time TAUB to make the engine air-fuel ratio lean. . When the engine air-fuel ratio becomes lean, oxygen is adsorbed on the three-way catalyst 18,
Thus, the stored oxygen desorption amount OSC again becomes the target desorption amount OSC.
Return to ref.

Next, when the amount of adsorbed oxygen increases and the amount of stored oxygen desorbed OSC becomes smaller than the target desorbed amount OSCref, the fuel injection time TAU increases with respect to the basic fuel injection time TAUB to enrich the engine air-fuel ratio. I'm sick. When the engine air-fuel ratio becomes rich, oxygen is desorbed from the three-way catalyst 18, and thus the stored oxygen desorption amount OSC returns to the target desorption amount OSCref again.

As described above, by controlling the fuel injection time TAU in accordance with the stored oxygen desorption amount OSC, the stored oxygen desorption amount OSC can be maintained at the target desorption amount OSCref. In this case, when the engine air-fuel ratio becomes lean, excess oxygen is robbed by the three-way catalyst 18 so that NOx in the exhaust gas is reduced. When the engine air-fuel ratio becomes rich, oxygen is robbed from the three-way catalyst 18 to thereby reduce the exhaust gas. Unburned HC and CO in
Is oxidized, so that the exhaust gas flowing out of the three-way catalyst 18 contains almost no unburned HC, CO and NOx. At this time, as shown in FIG. 5, the output voltage V of the downstream air-fuel ratio sensor 24 is maintained near 0.45 (V) indicating that the air-fuel ratio is the stoichiometric air-fuel ratio.

In the embodiment according to the present invention, when the gas flowing into the three-way catalyst 18 is excessive in air, for example, when the supply of fuel is stopped during deceleration operation, that is, when the stored oxygen desorption amount OSC is zero. The calculation of the stored oxygen desorption amount OSC is started based on. At this time, the calculated stored oxygen desorption amount OSC and the calculated maximum desorption amount OSCm
If ax deviates from the actual stored oxygen desorption amount or the actual maximum desorption amount, the stored oxygen desorption amount OSC cannot be maintained at the target desorption amount which is half the actual maximum desorption amount.
Therefore, in the first embodiment of the present invention, the desorption rate is set so that the calculated stored oxygen desorption amount OSC matches the actual stored oxygen desorption amount and the maximum desorption amount OSCmax matches the actual maximum desorption amount. And the speed ratio 1 / k of the adsorption speed and the maximum desorption amount OSCmax are corrected by the correction coefficient KOSC. Next, FIG. 6 to FIG.
This will be described with reference to FIG.

In the first embodiment, the maximum desorption amount OS
The maximum desorption amount OSCmax is corrected by multiplying Cmax by the correction coefficient KOSC. That is, the following equation is calculated. OSCmax = OSCmax · KOSC Therefore, when the correction coefficient KOSC increases, the maximum desorption amount OSC
When max increases and the correction coefficient KOSC decreases, the maximum desorption amount OSCmax decreases.

FIG. 6 shows that the stored oxygen desorption amount OSC and the maximum desorption amount OSCmax completely correspond to the actual stored oxygen desorption amount and the actual maximum desorption amount, respectively. This shows a case where the engine air-fuel ratio temporarily becomes significantly large. Assuming that the engine air-fuel ratio becomes significantly lean, oxygen in the exhaust gas is rapidly adsorbed by the three-way catalyst 18, so that the stored oxygen desorption amount OSC sharply decreases and reaches zero. When the stored oxygen desorbed amount OSC reaches zero, the three-way catalyst 18 can no longer adsorb oxygen, so that the air-fuel ratio downstream of the three-way catalyst 18 also becomes lean, and thus, as shown in FIG. The output voltage V of the side air-fuel ratio sensor 24 becomes lower than 0.2 (V). That is, the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is lean. At this time, as shown in FIG.
SC does not change, and the value of k of the speed ratio 1 / k does not change.

FIG. 7 shows the air detected by the downstream air-fuel ratio sensor 24 before the calculated stored oxygen desorption amount OSC reaches zero when the air-fuel ratio temporarily becomes largely lean for some reason. This shows a case where the fuel ratio becomes lean.
At this time, it is considered that the desorption rate of oxygen was set too high. At this time, the fact that the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 becomes lean before the stored oxygen desorption amount OSC reaches zero indicates that the stored oxygen desorption amount OSC does not reach zero. This means that the stored oxygen desorption amount is zero. Thus, the difference between the calculated stored oxygen desorption amount OSC and the actual stored oxygen desorption amount OSC is caused by setting the oxygen desorption rate too high, and as a result, the calculated stored oxygen desorption amount This is because the amount OSC becomes larger than the actual stored oxygen desorption amount. Therefore, in this case, in order to reduce the desorption speed of oxygen, that is, the speed ratio 1 / k
As shown in FIG. 7, the value of k of the speed ratio 1 / k is increased in order to reduce the speed. At this time, since the actual stored oxygen desorption amount is zero, the calculated stored oxygen desorption amount OSC is set to zero.

FIG. 8 shows the downstream air-fuel ratio sensor 24 when the calculated stored oxygen desorption amount OSC reaches zero when the air-fuel ratio temporarily becomes significantly large for some reason. Shows a case in which the air-fuel ratio detected by is not lean. At this time, it is considered that the desorption rate of oxygen was set too low. At this time, the fact that the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is not lean even though the stored oxygen desorption amount OSC has reached zero means that the storage oxygen desorption amount OSC has reached zero. Means that the actual amount of stored oxygen desorbed has not reached zero. Thus, the difference between the calculated stored oxygen desorption amount OSC and the actual stored oxygen desorption amount OSC is caused by setting the oxygen desorption rate too low, and as a result, the calculated storage oxygen desorption amount This is because the amount OSC becomes smaller than the actual amount of stored oxygen desorbed. Therefore, in this case, the value of k of the speed ratio 1 / k is reduced as shown in FIG. 8 in order to increase the desorption speed of oxygen, that is, to increase the speed ratio 1 / k.

FIG. 9 shows that the stored oxygen desorption amount OSC and the maximum desorption amount OSCmax completely correspond to the actual stored oxygen desorption amount and the actual maximum desorption amount, respectively. This shows a case where the engine air-fuel ratio becomes temporarily rich. If the engine air-fuel ratio becomes significantly rich, oxygen in the exhaust gas is rapidly desorbed from the three-way catalyst 18, so that the stored oxygen desorption amount OSC rapidly increases and reaches the maximum desorption amount OSCmax. When the stored oxygen desorption amount OSC reaches the maximum desorption amount OSCmax, there is no longer any oxygen to be desorbed in the three-way catalyst 18, so that the air-fuel ratio downstream of the three-way catalyst 18 also becomes rich,
Thus, as shown in FIG.
Is higher than 0.7 (V). That is, the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 becomes rich. At this time, as shown in FIG.
OSC does not change, nor does the value of k for the speed ratio 1 / k.

FIG. 10 shows that when the calculated stored oxygen desorption amount OSC reaches the maximum desorption amount OSCmax when the air-fuel ratio temporarily becomes considerably rich for some reason, the downstream air-fuel ratio sensor 24 detects the stored oxygen desorption amount OSC. This shows a case where the detected air-fuel ratio becomes rich. That is, a case where the calculated stored oxygen desorption amount OSC has not reached the maximum desorption amount OSCmax but the actual stored oxygen desorption amount OSC has reached the maximum desorption amount OSCmax is shown.

Thus, the calculated maximum desorption amount OSCma
The difference between x and the actual maximum desorption amount occurs because the desorption rate of oxygen is set too low or the calculated maximum desorption amount OSCmax is incorrect. In this case, since the desorption rate of oxygen is corrected by the method shown in FIGS. 7 and 8, the desorption rate of oxygen is considered to be correct.
Thus, it is considered that the calculated maximum desorption amount OSCmax is incorrect. Therefore, in this case, as shown in FIG. 10, the correction coefficient KOSC is decreased in order to decrease the maximum desorption amount OSCmax. Further, at this time, the stored oxygen desorption amount OSC is set to the maximum desorption amount OSCmax.

FIG. 11 shows that the calculated stored oxygen desorption amount OSC reaches the maximum desorption amount OSCmax when the air-fuel ratio temporarily becomes significantly rich for some reason. This shows a case where the air-fuel ratio detected by the air-fuel ratio sensor 24 is not rich. That is,
The calculated stored oxygen desorption amount OSC is the maximum desorption amount OSCma
This shows a case where the stored oxygen desorption amount OSC has not reached the maximum desorption amount OSCmax even though it has reached x.

As described above, the calculated maximum desorption amount OSCma
The difference between x and the actual maximum desorption amount occurs because the desorption speed of oxygen is set too high or the calculated maximum desorption amount OSCmax is incorrect. In this case, as described above, the desorption rate of oxygen
The desorption rate of oxygen is considered to be correct because it has been corrected by the method shown in FIG.
It is considered that x is wrong. Therefore, in this case, as shown in FIG. 11, the correction coefficient KOSC is increased in order to increase the maximum desorption amount OSCmax.

Thus, in the first embodiment, the calculated stored oxygen desorption amount OSC and the calculated maximum desorption amount OSCmax
Are exactly matched to the actual stored oxygen desorption and the actual maximum desorption, respectively. Therefore, even if the air-fuel ratio deviates from the stoichiometric air-fuel ratio to either the lean side or the rich side, the harmful components in the exhaust gas are reliably purified because the actual amount of stored oxygen desorbed is maintained at half of the actual maximum desorbed amount. You can do it.

Next, a routine for calculating the stored oxygen desorption amount OSC used in the first embodiment will be described with reference to FIG. This routine is executed by interruption every predetermined time. Referring to FIG. 12, first, at step 100, the count value TD for obtaining the cumulative operation time is incremented by one. Next, at step 101, it is determined whether or not a fixed time has elapsed since the engine was started. In step 113, the stored oxygen desorption amount OSC is made zero, and then the processing cycle is completed. On the other hand, when it is determined in step 101 that the predetermined time has elapsed after the engine is started, the routine proceeds to step 102, where the temperature T of the three-way catalyst 18 detected by the temperature sensor 25 is detected.
It is determined whether or not c has become higher than the fixed value Tco. When Tc ≦ Tco, the routine proceeds to step 113, where Tc
When c> Tco, the routine proceeds to step 103.

In step 103, it is determined whether or not the fuel injection has been stopped during the deceleration operation. If the fuel injection has been stopped, the routine proceeds to step 113. On the other hand, when the fuel injection has not been stopped, the routine proceeds to step 104, and from step 104 to step 112, the stored oxygen desorption amount OSC is calculated. Before the calculation of the stored oxygen desorption amount OSC is started, the stored oxygen desorption amount OSC is set to zero in step 113, and therefore, the calculation may be started from a state in which the stored oxygen desorption amount OSC is zero. Recognize.

In step 104, the upstream air-fuel ratio sensor 2
The air-fuel ratio A / F calculated based on the relationship shown in FIG. Next, at step 105, it is determined whether or not the air-fuel ratio A / F is the stoichiometric air-fuel ratio. When the air-fuel ratio A / F is the stoichiometric air-fuel ratio, the routine proceeds to a correction routine shown in FIG. On the other hand, if the air-fuel ratio A / F is not the stoichiometric air-fuel ratio, the routine proceeds to step 106, where the air-fuel ratio A
It is determined whether / F is lean. Air / fuel ratio A / F
Is lean, the routine proceeds to step 107, where the deviation ΔA / F of the air-fuel ratio calculated based on the intake air amount Ga obtained by the air flow meter 13 and the output of the upstream air-fuel ratio sensor 23 (= air-fuel ratio A / F− Using the theoretical air-fuel ratio) and the interruption time interval Δt, the stored oxygen desorption amount OS
C is calculated.

OSC ← OSC-KO ₂ · Ga · (ΔA /
F) / (A / F) · Δt Next, at step 108, it is determined whether or not the stored oxygen desorption amount OSC is larger than zero. When OSC ≧ 0, the process proceeds to the correction routine shown in FIG. 13. When OSC <0, the process proceeds to step 109, where OSC is set to zero, and then the process proceeds to the correction routine shown in FIG.

On the other hand, in step 106, the air-fuel ratio A /
When it is determined that F is not lean, that is, the air-fuel ratio A / F
Is rich, the routine proceeds to step 110, where the deviation ΔA / F of the air-fuel ratio calculated based on the intake air amount Ga obtained by the air flow meter 13 and the output of the upstream air-fuel ratio sensor 23 (= air-fuel ratio A / F− Using the theoretical air-fuel ratio) and the interruption time interval Δt, the stored oxygen desorption amount OS
C is calculated.

OSC ← OSC-KO ₂ · Ga · (ΔA /
F) / (A / F) / k · Δt Next, at step 111, it is determined whether or not the stored oxygen desorption amount OSC is larger than the maximum desorption amount OSCmax.
When OSC ≦ OSCmax, the process proceeds to the correction routine shown in FIG. 13. When OSC> OSCmax, the process proceeds to step 112, where OSC is set to OSCmax, and then the process proceeds to the correction routine shown in FIG.

In the correction routine shown in FIG. 13, first, at step 114, it is determined whether or not the stored oxygen desorption amount OSC is between zero and the maximum desorption amount OSCmax. If OSCmax>OSC> 0, the routine proceeds to step 115, where it is determined whether or not the output voltage V of the downstream air-fuel ratio sensor 24 is between 0.2 (V) and 0.7 (V), that is, the downstream air-fuel ratio. It is determined whether the air-fuel ratio detected by the sensor 24 is the stoichiometric air-fuel ratio. When 0.2 <V <0.7, the processing cycle is completed. 0.2
If not <V <0.7, the routine proceeds to step 116.

In step 116, it is determined whether or not V ≧ 0.7. When V ≧ 0.7, that is, when the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is rich, the routine proceeds to step 117, where the correction coefficient KOSC is updated based on the following equation. KOSC = KOSC · (1−β1) Here, β1 is a positive constant smaller than 1. Therefore, at this time, the correction coefficient KOSC is reduced as shown in FIG. Next, at step 118, the OSC is set to the maximum desorption amount OSCmax.

On the other hand, at step 116, V <0.7
Is determined, that is, the downstream air-fuel ratio sensor 24
When the air-fuel ratio detected by the above is lean, the routine proceeds to step 119, where the value of k of the speed ratio 1 / k is updated based on the following equation. k = k · (1 + γ1) Here, γ1 is a positive constant smaller than 1. Therefore, at this time, the value of k is increased as shown in FIG. Next, at step 120, the OSC is set to zero.

On the other hand, in step 114, OSCma
When it is determined that x>OSC> 0 is not satisfied, step 1 is executed.
21 and the stored oxygen desorption amount OSC becomes the maximum desorption amount OSC
It is determined whether it is max. OSC = OSCma
If x, the routine proceeds to step 122, where it is determined whether or not V ≧ 0.7. When V <0.7, that is, when the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is not rich, the routine proceeds to step 123, where the correction coefficient KOSC is updated based on the following equation.

KOSC = KOSC · (1 + β2) where β2 is a positive constant smaller than 1. Therefore, at this time, the correction coefficient KOSC is increased as shown in FIG. On the other hand, when it is determined in step 121 that OSC is not OSCmax, the routine proceeds to step 124, where it is determined whether or not the stored oxygen desorption amount OSC is zero. When OSC = 0, the routine proceeds to step 125, where it is determined whether or not V ≦ 0.2.
If V> 0.2, that is, if the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is not lean, step 12
Proceeding to 6, the value of k of the speed ratio 1 / k is updated based on the following equation.

K = k · (1−γ2) Here, γ2 is a positive constant smaller than 1. Therefore, at this time, the value of k is reduced as shown in FIG. Next, a calculation routine of the fuel injection time TAU used in the first embodiment will be described with reference to FIG. This routine is repeatedly executed.

Referring to FIG. 14, first, step 2
At 00, the maximum desorption amount OSCmax which is the product of the maximum desorption amount G (Tc) obtained from the relationship shown in FIG. 3A and the deterioration coefficient DK obtained from the relationship shown in FIG. 3B.
(= G (Tc) · DK) is calculated. Next, at step 201, the maximum desorption amount OSCmax is multiplied by the correction coefficient KOSC as shown in the following equation to obtain the maximum desorption amount
SCmax is updated.

OSCmax = OSCmax.KOSC Next, in step 202, half of the maximum desorption amount OSCmax is set as the target desorption amount OSCref. Next, at step 203, the basic fuel injection time TA is obtained from the map shown in FIG.
UB is calculated. Next, at step 204, it is determined whether or not the stored oxygen desorption amount OSC is equal to the target desorption amount OSCref. When OSC = OSCref, the routine proceeds to step 205, where the fuel injection time TAU is set to the basic fuel injection time TAUB. OSC = OSCre
If not f, the routine proceeds to step 206, where it is determined whether or not the stored oxygen desorption amount OSC is smaller than the target desorption amount OSCref. When OSC <OSCref, the routine proceeds to step 207, where the fuel injection time TAU (= TAUB · KR) is calculated by multiplying the basic fuel injection time TAUB by the rich correction coefficient KR (KR> 1.0).
On the other hand, when OSC> OSCref, the routine proceeds to step 208, where the basic fuel injection time TAUB is multiplied by a lean correction coefficient KL (0 <KL <1.0) to calculate the fuel injection time TAU (= TAUB · KL). Is done.

The value of the correction coefficient KOSC and the value of k of the speed ratio 1 / k are stored in the backup RAM 35. 15 to 23 show a second embodiment. In the second embodiment, the maximum desorption amount G when the three-way catalyst 18 is new is
The case where the maximum desorption amount OSCmax obtained from the product of (Tc) and the deterioration coefficient DK matches the actual maximum desorption amount. Therefore, in the second embodiment, the maximum desorption amount OSCm
ax is not required to be corrected by the correction coefficient KOSC, so the correction coefficient KOSC is not used, and the calculated stored oxygen desorption amount OSC matches the actual stored oxygen desorption amount, and the maximum desorption amount OSCmax is the actual storage oxygen desorption amount. Only the speed ratio 1 / k between the desorption speed and the adsorption speed is modified to match the maximum desorption amount. Next, this will be described with reference to FIGS.

FIG. 15 shows that the stored oxygen desorption amount OSC completely matches the actual stored oxygen desorption amount, and in such a state, the engine air-fuel ratio temporarily became considerably lean for some reason. Shows the case. If the engine air-fuel ratio becomes significantly lean, the oxygen in the exhaust gas rapidly drops
8, the stored oxygen desorbed amount OSC rapidly decreases and reaches zero. When the stored oxygen desorbed amount OSC reaches zero, the three-way catalyst 18 can no longer adsorb oxygen, so that the air-fuel ratio downstream of the three-way catalyst 18 becomes lean. The output voltage V of the side air-fuel ratio sensor 24 becomes lower than 0.2 (V). That is, the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is lean. At this time, as shown in FIG.
The value of k of k does not change.

FIG. 16 shows that the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 before the calculated stored oxygen desorption amount OSC reaches zero when the air-fuel ratio temporarily becomes largely lean for some reason. This shows a case where the fuel ratio becomes lean. At this time, it is considered that the desorption rate of oxygen was set too high. At this time, the fact that the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 becomes lean before the stored oxygen desorption amount OSC reaches zero indicates that the stored oxygen desorption amount OSC is low.
This means that the actual amount of stored oxygen released is zero even if C has not reached zero. Thus, the difference between the calculated stored oxygen desorption amount OSC and the actual stored oxygen desorption amount OSC occurs because the oxygen desorption speed is set too high,
As a result, the calculated stored oxygen desorption amount OSC becomes larger than the actual stored oxygen desorption amount. Therefore, in this case, in order to reduce the desorption speed of oxygen, that is, the speed ratio l
To reduce the speed ratio l as shown in FIG.
The value of k in / k is increased. At this time, since the actual stored oxygen desorption amount is zero, the calculated stored oxygen desorption amount OSC is set to zero.

FIG. 17 shows the downstream air-fuel ratio sensor 24 when the calculated stored oxygen desorption amount OSC has reached zero when the air-fuel ratio temporarily becomes significantly large for some reason. Shows a case in which the air-fuel ratio detected by is not lean. At this time, it is considered that the desorption rate of oxygen was set too low. At this time, even though the stored oxygen desorption amount OSC has reached zero, the fact that the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is not lean means that the storage oxygen desorption amount OSC becomes zero. Despite this, the actual amount of stored oxygen desorbed is not zero. Thus, the calculated stored oxygen desorption amount OSC and the actual stored oxygen desorption amount OSC
The difference is caused by setting the oxygen desorption rate too low, and as a result, the calculated stored oxygen desorption amount OSC becomes smaller than the actual stored oxygen desorption amount. Therefore, in this case, the value of k of the speed ratio 1 / k is reduced as shown in FIG. 17 in order to increase the desorption speed of oxygen, that is, to increase the speed ratio 1 / k.

FIG. 18 shows that the stored oxygen desorption amount OSC completely coincides with the actual stored oxygen desorption amount, and in such a state, the engine air-fuel ratio temporarily became considerably rich for some reason. Shows the case. Assuming that the engine air-fuel ratio becomes significantly rich, the oxygen in the exhaust gas suddenly increases.
8, the stored oxygen desorption amount OSC rapidly increases and reaches the maximum desorption amount OSCmax. When the stored oxygen desorption amount OSC reaches the maximum desorption amount OSCmax, there is no oxygen to be desorbed in the three-way catalyst 18, so that the air-fuel ratio downstream of the three-way catalyst 18 also becomes rich. As shown in (2), the output voltage V of the downstream side air-fuel ratio sensor 24 becomes higher than 0.7 (V). That is, the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 becomes rich. At this time, as shown in FIG.
Does not change.

FIG. 19 shows that the downstream air-fuel ratio sensor 24 detects the calculated stored oxygen desorption amount OSC before reaching the maximum desorption amount OSCmax when the air-fuel ratio becomes temporarily rich for some reason. This shows a case where the detected air-fuel ratio becomes rich. That is, a case where the calculated stored oxygen desorption amount OSC has not reached the maximum desorption amount OSCmax but the actual stored oxygen desorption amount OSC has reached the maximum desorption amount OSCmax is shown.

As described above, the calculated maximum desorption amount OSCma
The difference between x and the actual maximum desorption amount occurs because the desorption rate of oxygen is set too low. Therefore, in this case, the value of k of the speed ratio 1 / k is reduced as shown in FIG. 19 in order to increase the desorption speed of oxygen, that is, to increase the speed ratio 1 / k. Further, at this time, the stored oxygen desorption amount OSC is the maximum desorption amount OSCmax.
It is said.

FIG. 20 shows that when the air-fuel ratio becomes temporarily rich for some reason, the calculated stored oxygen desorbed amount OSC reaches the maximum desorbed amount OSCmax even though the calculated stored oxygen desorbed amount OSC has reached the maximum desorbed amount OSCmax. This shows a case where the air-fuel ratio detected by the air-fuel ratio sensor 24 is not rich. That is,
The calculated stored oxygen desorption amount OSC is the maximum desorption amount OSCma
This shows a case where the stored oxygen desorption amount OSC has not reached the maximum desorption amount OSCmax even though it has reached x.

As described above, the calculated maximum desorption amount OSCma
The difference between x and the actual maximum desorption amount occurs because the desorption rate of oxygen is set too high. Therefore, in this case, the value of k of the speed ratio l / k is increased as shown in FIG. 20 in order to reduce the desorption speed of oxygen, that is, to reduce the speed ratio l / k. As described above, in the second embodiment, even when the air-fuel ratio becomes lean or rich, when it is determined that the desorption speed of oxygen is low, the desorption speed of oxygen is increased, and the air-fuel ratio becomes lean. When it is determined that the desorption speed of oxygen is high even when the air becomes rich or when the air becomes rich, the desorption speed of oxygen is reduced. That is, in the second embodiment, the opportunity to update the oxygen desorption speed is increased, so that the oxygen desorption speed can be made to match the actual desorption speed at an early stage.

As described above, also in the second embodiment, the calculated stored oxygen desorption amount OSC can be made to exactly match the actual stored oxygen desorption amount, so that the actual storage oxygen desorption amount is equal to the actual maximum desorption amount. Therefore, even if the air-fuel ratio deviates from the stoichiometric air-fuel ratio to either the lean side or the rich side, harmful components in the exhaust gas can be reliably purified. Next, a routine for calculating the stored oxygen desorption amount OSC used in the second embodiment will be described with reference to FIG. This routine is executed by interruption every predetermined time.

Referring to FIG. 21, first, in step 3
At 00, a count value T for obtaining the cumulative operation time
D is incremented by one. Then step 30
At 1, it is determined whether or not a fixed time has elapsed since the engine was started.
If the fixed time has not elapsed since the engine was started, step 3
Proceed to 13. In step 313, the stored oxygen desorption amount OSC
Is set to zero, and then the processing cycle is completed. On the other hand, when it is determined in step 301 that a predetermined time has elapsed after the engine is started, the routine proceeds to step 302, where the temperature sensor 2
The temperature Tc of the three-way catalyst 18 detected by the
It is determined whether it has become higher than co. Tc ≦ Tc
When o, the process proceeds to step 313, and when Tc> Tco, the process proceeds to step 303.

At step 303, it is determined whether or not the fuel injection has been stopped during the deceleration operation. If the fuel injection has been stopped, the routine proceeds to step 313. On the other hand, when the fuel injection has not been stopped, the routine proceeds to step 304, and in steps 304 to 312, the stored oxygen desorption amount OSC is calculated. Also in the second embodiment, before the calculation of the stored oxygen desorption amount OSC is started, the stored oxygen desorption amount OSC is set to zero in step 313, and therefore, the stored oxygen desorption amount OSC is calculated from the zero state. Is started.

In step 304, the upstream air-fuel ratio sensor 2
The air-fuel ratio A / F calculated based on the relationship shown in FIG. Next, at step 305, it is determined whether or not the air-fuel ratio A / F is the stoichiometric air-fuel ratio. When the air-fuel ratio A / F is the stoichiometric air-fuel ratio, the routine proceeds to a correction routine shown in FIG. On the other hand, if the air-fuel ratio A / F is not the stoichiometric air-fuel ratio, the routine proceeds to step 306, where the air-fuel ratio A
It is determined whether / F is lean. Air / fuel ratio A / F
Is lean, the routine proceeds to step 307, where the air-fuel ratio deviation ΔA / F (= air-fuel ratio A / F− Using the theoretical air-fuel ratio) and the interruption time interval Δt, the stored oxygen desorption amount OS
C is calculated.

OSC ← OSC-KO ₂ · Ga · (ΔA /
F) / (A / F) · Δt Next, at step 308, it is determined whether or not the stored oxygen desorption amount OSC is larger than zero. When OSC ≧ 0, the process proceeds to the correction routine shown in FIG. 22, and when OSC <0, the process proceeds to step 309 to set OSC to zero and then proceeds to the correction routine shown in FIG.

On the other hand, at step 306, the air-fuel ratio A /
When it is determined that F is not lean, that is, the air-fuel ratio A / F
Is rich, the process proceeds to step 310, where the air-fuel ratio deviation ΔA / F (= air-fuel ratio A / F−) calculated based on the intake air amount Ga obtained by the air flow meter 13 and the output of the upstream air-fuel ratio sensor 23. Using the theoretical air-fuel ratio) and the interruption time interval Δt, the stored oxygen desorption amount OS
C is calculated.

OSC ← OSC-KO ₂ · Ga · (ΔA /
F) / (A / F) / k · Δt Next, at step 311, it is determined whether or not the stored oxygen desorption amount OSC is larger than the maximum desorption amount OSCmax.
When OSC ≦ OSCmax, the process proceeds to the correction routine shown in FIG. 22, and when OSC> OSCmax, the process proceeds to step 312, where OSC is set to OSCmax, and then the process proceeds to the correction routine shown in FIG.

In the correction routine shown in FIG. 22, first, at step 314, it is determined whether or not the stored oxygen desorption amount OSC is between zero and the maximum desorption amount OSCmax. If OSCmax>OSC> 0, the routine proceeds to step 315, where it is determined whether or not the output voltage V of the downstream air-fuel ratio sensor 24 is between 0.2 (V) and 0.7 (V), that is, the downstream air-fuel ratio. It is determined whether the air-fuel ratio detected by the sensor 24 is the stoichiometric air-fuel ratio. When 0.2 <V <0.7, the processing cycle is completed. 0.2
If not <V <0.7, the routine proceeds to step 316.

At step 316, it is determined whether or not V ≧ 0.7. When V ≧ 0.7, that is, when the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is rich, the process proceeds to step 317, and the value of k of the speed ratio 1 / k is updated based on the following equation. . k = k · (1−δ1) Here, δ1 is a positive constant smaller than 1. Therefore, at this time, the value of k is reduced as shown in FIG. Next, at step 318, the OSC is set to the maximum desorption amount OSCmax.

On the other hand, at step 316, V <0.7
Is determined, that is, the downstream air-fuel ratio sensor 24
When the air-fuel ratio detected by the above is lean, the process proceeds to step 319, and the value of k of the speed ratio 1 / k is updated based on the following equation. k = k · (1 + δ2) Here, δ2 is a positive constant smaller than 1. Therefore, at this time, the value of k is increased as shown in FIG. Next, at step 320, the OSC is set to zero.

On the other hand, in step 314, OSCma
If it is determined that x>OSC> 0 is not satisfied, step 3
21 and the stored oxygen desorption amount OSC becomes the maximum desorption amount OSC
It is determined whether it is max. OSC = OSCma
If x, the routine proceeds to step 322, where it is determined whether or not V ≧ 0.7. When V <0.7, that is, when the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is not rich, the routine proceeds to step 323, where the speed ratio l is calculated based on the following equation.
The value of k in / k is updated.

K = k · (1 + δ3) where δ3 is a positive constant smaller than 1. Therefore, at this time, the value of k is increased as shown in FIG. On the other hand, in step 321, OSC =
If it is determined that it is not OSCmax, step 3
Proceeding to 24, it is determined whether the stored oxygen desorption amount OSC is zero. When OSC = 0, the routine proceeds to step 325, where it is determined whether or not V ≦ 0.2. V> 0.2
In other words, when the air-fuel ratio detected by the downstream air-fuel ratio sensor 24 is not lean, the routine proceeds to step 326, where the value of k of the speed ratio 1 / k is updated based on the following equation.

K = k · (1−δ4) where δ4 is a positive constant smaller than 1. Therefore, at this time, the value of k is reduced as shown in FIG. Next, a routine for calculating the fuel injection time TAU used in the second embodiment will be described with reference to FIG. This routine is repeatedly executed.

Referring to FIG. 23, first, in step 4
At 00, the maximum desorption amount OSCmax which is the product of the maximum desorption amount G (Tc) obtained from the relationship shown in FIG. 3A and the deterioration coefficient DK obtained from the relationship shown in FIG. 3B.
(= G (Tc) · DK) is calculated. Next, at step 401, half of the maximum desorption amount OSCmax is set as the target desorption amount OSCref. Next, at step 402, FIG.
The basic fuel injection time TAUB is calculated from the map shown in FIG.

Next, at step 403, it is determined whether or not the stored oxygen desorption amount OSC is equal to the target desorption amount OSCref. Step 40 when OSC = OSCref
4 and the fuel injection time TAU is changed to the basic fuel injection time TA.
UB. On the other hand, if OSC is not OSCref, the routine proceeds to step 405, where the stored oxygen desorption amount OS
It is determined whether C is smaller than the target desorption amount OSCref. Step 40 when OSC <OSCref
Proceeding to 6, the fuel injection time TAU (= TAUB · KR) is calculated by multiplying the basic fuel injection time TAUB by the rich correction coefficient KR (KR> 1.0). On the other hand, when OSC> OSCref, step 407
To the basic fuel injection time TAUB and the lean correction coefficient K
The fuel injection time TAU (= TAUB · KL) is calculated by multiplying L (0 <KL <1.0).

[0085]

According to the present invention, the actual amount of stored oxygen desorbed can be accurately calculated.

[Brief description of the drawings]

FIG. 1 is an overall view of an internal combustion engine.

FIG. 2 is a diagram showing an output of an air-fuel ratio sensor.

FIG. 3 shows the maximum desorption amount G (Tc) when the three-way catalyst is new.
FIG. 4 is a diagram showing a deterioration coefficient DK.

FIG. 4 is a diagram showing a map of a basic fuel injection time.

FIG. 5 is a time chart for explaining a method of controlling a stored oxygen desorption amount.

FIG. 6 is a time chart for explaining a method of updating the correction coefficient KOSC and the value of k of the speed ratio 1 / k in the first embodiment.

FIG. 7 is a time chart for explaining a method of updating the correction coefficient KOSC and the value of k of the speed ratio 1 / k in the first embodiment.

FIG. 8 is a time chart for explaining a method of updating the correction coefficient KOSC and the value of k of the speed ratio 1 / k in the first embodiment.

FIG. 9 is a time chart for explaining a method of updating the correction coefficient KOSC and the value of k of the speed ratio 1 / k in the first embodiment.

FIG. 10 is a time chart for explaining a method of updating the correction coefficient KOSC and the value of k of the speed ratio 1 / k in the first embodiment.

FIG. 11 is a time chart for explaining a method of updating the correction coefficient KOSC and the value of k of the speed ratio 1 / k in the first embodiment.

FIG. 12 is a flowchart for calculating a stored oxygen desorption amount OSC.

FIG. 13 shows a maximum desorption amount OSCmax and a speed ratio 1 / k.
9 is a flowchart for correcting the value of k of FIG.

FIG. 14 is a flowchart for calculating a fuel injection time TAU.

FIG. 15 is a time chart for explaining a method of updating the value of k of the speed ratio 1 / k in the second embodiment.

FIG. 16 is a time chart for explaining a method of updating the value of k of the speed ratio 1 / k in the second embodiment.

FIG. 17 is a time chart for explaining a method of updating the value of k of the speed ratio 1 / k in the second embodiment.

FIG. 18 is a time chart for explaining a method of updating the value of k of the speed ratio 1 / k in the second embodiment.

FIG. 19 is a time chart for explaining a method of updating the value of k of the speed ratio 1 / k in the second embodiment.

FIG. 20 is a time chart for explaining a method of updating the value of k of the speed ratio 1 / k in the second embodiment.

FIG. 21 is a flowchart for calculating a stored oxygen desorption amount OSC.

FIG. 22 is a flowchart for correcting the value of k of the speed ratio 1 / k.

FIG. 23 is a flowchart for calculating a fuel injection time TAU.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Fuel injection valve 13 ... Air flow meter 16 ... Exhaust manifold 18 ... Three-way catalyst 23 ... Upstream air-fuel ratio sensor 24 ... Downstream air-fuel ratio sensor

Claims (3)

[Claims] 1. An internal combustion engine having an upstream air-fuel ratio sensor disposed in an engine exhaust passage upstream of a three-way catalyst and a downstream air-fuel ratio sensor disposed in an engine exhaust passage downstream of the three-way catalyst. From the air-fuel ratio detected by the upstream air-fuel ratio sensor and the speed ratio between the desorption speed and the adsorption speed of the three-way catalyst, the stored oxygen desorption amount indicating the amount of desorption of oxygen stored in the three-way catalyst is calculated. Calculating means for controlling the air-fuel ratio of the engine so that the stored oxygen desorbed amount becomes the target desorbed amount, and the calculated stored oxygen desorbed amount has not reached zero. When the air-fuel ratio detected by the downstream air-fuel ratio sensor is lean, the speed ratio is corrected so as to decrease the desorption speed relatively to the adsorption speed, and the calculated stored oxygen desorption amount reaches zero. Despite the downstream air-fuel ratio sensor An air-fuel ratio control device for an internal combustion engine, comprising: speed ratio correction means for correcting the speed ratio so as to increase the desorption speed relative to the adsorption speed when the air-fuel ratio detected is not lean. 2. The maximum desorption amount is reduced when the air-fuel ratio detected by the downstream air-fuel ratio sensor is rich even though the calculated stored oxygen desorption amount has not reached the maximum desorption amount. If the air-fuel ratio detected by the downstream air-fuel ratio sensor is not rich even though the calculated stored oxygen desorption amount has reached the maximum desorption amount, the maximum desorption amount is increased and corrected. 2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, further comprising a separation amount correcting unit. 3. An internal combustion engine in which an upstream air-fuel ratio sensor is disposed in an engine exhaust passage upstream of a three-way catalyst and a downstream air-fuel ratio sensor is disposed in an engine exhaust passage downstream of the three-way catalyst. From the air-fuel ratio detected by the upstream air-fuel ratio sensor and the speed ratio between the desorption speed and the adsorption speed of the three-way catalyst, the stored oxygen desorption amount indicating the amount of desorption of oxygen stored in the three-way catalyst is calculated. Calculating means for controlling the air-fuel ratio of the engine such that the stored oxygen desorbed amount becomes the target desorbed amount, and a control means for controlling the air-fuel ratio of the engine so that the calculated stored oxygen desorbed amount has not reached the maximum desorbed amount. Regardless, when the air-fuel ratio detected by the downstream air-fuel ratio sensor is rich, the speed ratio is corrected so as to increase the desorption speed relative to the adsorption speed,
If the air-fuel ratio detected by the downstream air-fuel ratio sensor is not rich even though the calculated stored oxygen desorption amount has reached the maximum desorption amount, the desorption speed is reduced relative to the adsorption speed. An air-fuel ratio control device for an internal combustion engine, comprising: a speed ratio correcting means for correcting the speed ratio.