JPH08232644A - Emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE: To contrive accurate detection a degree of deterioration of an absorbent by setting a decision level capable of estimating a NOx absorbent to be a maximum absorbing amount, switching air-fuel ratio of inflow exhaust gas from lean ratio to rich ratio when an estimated NOx amount exceeds the decision level, and judging a degree of deteriorating the absorbent from a difference in a change process between sensor output levels. CONSTITUTION: A NOx absorbent 18 is arranged in an engine exhaust passage, to arranged an O2 sensor 22 of generating a current in proportion to air-fuel ratio in the engine exhaust exhaust passage in the downstream of the NOx absorbent 18. A lean mixture is burned till absorbing the most NOx amount by the NOx absorbent 18, to make it absorb NOx . In order to release NOx from the NOx absorbent 18, a time form setting air-fuel ratio rich of a mixture to completing release operation of NOx is measured from a generated current valve of the O2 sensor 22, to obtain a degree of deteriorating the NOx absorbent 18 from this measured time. In accordance with a degree of deteriorating the absorbent 18, its maximum absorbing amount is lessened. consequently to update a decision level, so as to enable accuracy of detecting deterioration to enhance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine.

[0002]

Absorbs NO _X when the air-fuel ratio is lean of the Related Art inflowing exhaust gas air-fuel ratio of the inflowing exhaust gas to release NO _X absorbed and becomes rich _the NO _X absorbent to the engine exhaust passage There is known an internal combustion engine which is arranged in a NOx adsorbent and which normally burns a lean air-fuel mixture and absorbs NO _X generated at this time in a NO _X absorbent. NO _X absorption to _the NO _X absorbent is in the air-fuel ratio temporarily rich exhaust gas flowing into the NO _X absorbent in order to release the NO _X from _the NO _X absorbent exceeds a certain amount in the internal combustion engine It N
Air-fuel ratio of the exhaust gas flowing into the O _X absorbent NO _X release action from when it is made rich _the NO _X absorbent is started.

However, such NO_XAbsorbent is used
It gradually deteriorates over time, and the more it deteriorates, the more NO_XSucking
Finally, NO_XCan no longer absorb
I will end up. Therefore, such NO_XWhen using an absorbent
Is NO_XIt is possible to detect how deteriorated the absorbent is.
And are required. By the way NO_XExhaust flowing into the absorbent
When the air-fuel ratio of gas is made rich, NO _XFrom absorbent
NO_XNO during release action_XFlow from absorbent
The air-fuel ratio of the exhaust gas emitted is slightly lean
Yes, NO_XNO from absorbent_XThe release action of
And NO_XThe air-fuel ratio of the exhaust gas flowing out of the absorbent is rich
Has been found to be. In this case, NO_XFor absorbent
NO absorbed_XNO decreases as the amount decreases_XAbsorbent
NO after the air-fuel ratio of the exhaust gas flowing into the_X
The exhaust gas flowing out of the absorbent becomes rich in air-fuel ratio.
It will be no longer from this time._Xabsorption
The degree of deterioration of the agent can be detected.

[0004] Therefore NO _X fuel ratio of the absorbent downstream of the engine exhaust passage in the exhaust gas is arranged an air-fuel ratio sensor capable of detecting whether is rich or lean, N from _the NO _X absorbent
After switching the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent to release O _X from lean to rich, measure the time until the air-fuel ratio of the exhaust gas flowing out of the NO _X absorbent becomes rich. An internal combustion engine is known in which the degree of deterioration of the NO _X absorbent is detected from this time (see PCT International Publication WO94 / 17291).

[0005]

The degree of deterioration of the invention It is an object of the way _the NO _X absorbent, i.e. _the NO _X absorbent of the NO _X absorbent capacity in order to detect the NO _X when switching to rich the air-fuel ratio of the exhaust gas from lean The maximum amount of NO that the absorbent can absorb
Must absorb _X. But whether the amount of absorption of _the NO _X absorbent in the above-mentioned internal combustion engine exceeds the maximum NO _X absorption not is determined, a state where _the amount of NO _X is absorbed in the NO _X absorbent is not known how much Thus, the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from lean to rich. However, in this way NO
NO to the state where NO _X absorption amount of _X absorbent do not know
_{Even if} the air-fuel ratio of the exhaust gas flowing into the _X absorbent is switched from rich to lean and the air-fuel ratio of the exhaust gas flowing out of the NO _X absorbent becomes rich after this switching, this time is not always NO _X. It does not indicate the degree of deterioration of the absorbent. Therefore, the known internal combustion engine described above has a problem that it is difficult to accurately detect the degree of deterioration of the NO _X absorbent.

[0006]

According to the first invention,
In order to solve the above problems, the air-fuel ratio of the inflowing exhaust gas
NO when lean_XOf exhaust gas flowing in
NO absorbed when the air-fuel ratio becomes rich_XReleases NO
_XThe absorbent is placed in the engine exhaust passage to ensure that the exhaust gas air-fuel ratio
The air-fuel ratio sensor that produces an output at a level proportional to
_XThe internal combustion engine installed in the exhaust passage downstream of the absorbent
And NO_XEstimated N estimated to be absorbed by the absorbent
O_XNO to obtain quantity_XQuantity estimation means and NO_XAbsorbent N
O_XMaximum absorption NO_XLet's presume that it is absorbed
Estimated judgment level NO_XDetermine if the amount has been exceeded
Discriminating means and estimated NO_XWhen the amount exceeds the judgment level
NO _XAbsorbent to NO_XNO to release_XFlowing into the absorbent
Switch the air-fuel ratio of incoming exhaust gas from lean to rich
Equipped with air-fuel ratio switching means, the air-fuel ratio of exhaust gas is lean
The output level of the air-fuel ratio sensor when
Bell is NO_XNO absorbed in the absorbent_XAccording to quantity
Supports lean air-fuel ratio through different output level changes
Output level corresponding to rich air-fuel ratio
Based on the difference in the output level change process
NO_XDeterioration judging means for judging the degree of deterioration of the absorbent
And NO_XChange the judgment level according to the degree of deterioration of the absorbent.
A new judgment level updating means is provided.

In the second invention, N in the first invention
The greater the degree of deterioration of the O _X absorbent, the shorter the time required to change from the output level corresponding to the lean air-fuel ratio to the output level corresponding to the rich air-fuel ratio.
_X A predetermined period of time elapses from the time when the air-fuel ratio of the exhaust gas flowing into the absorbent is switched from lean to rich until the output level of the air-fuel ratio sensor reaches the output level corresponding to the rich air-fuel ratio. It is judged that the shorter the time, the greater the degree of deterioration of the NO _X absorbent.

In the third invention, in the second invention,
And the period until the output level of the air-fuel ratio sensor is substantially equal to the output level corresponding to the rich air-fuel ratio from the lean air-fuel ratio of the exhaust gas predetermined period flows into _the NO _X absorbent from switching to the rich To be done. 2 in the 4th invention
In the second aspect of the invention, the output level of the air-fuel ratio sensor corresponds to the rich air-fuel ratio after a predetermined period has elapsed after the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent has been switched from lean to rich for a predetermined period. It is the period until it almost matches the output level.

In the fifth invention, in the second invention,
Air-fuel ratio of the exhaust gas flowing into the NO _X absorbent output level immediately before the air-fuel ratio is changed from lean to rich of the exhaust gas flowing out from _the NO _X absorbent changes rapidly in after being switched from lean to rich The above-mentioned predetermined period is a period from a lapse of a certain period after the switching of the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent from lean to rich until a sudden change in the output level.

In the sixth invention, in the first invention,
NO_XThe air-fuel ratio of the exhaust gas flowing into the absorbent is lean
The change rate of the output level when switching to rich is NO
_XThe greater the degree of deterioration of the absorbent, the greater the
As the rate of change in output level increases,
_XIt is judged that the degree of deterioration of the absorbent is large. 7th departure
In the first invention, Ming says NO_XFlowing into the absorbent
Exhaust gas air-fuel ratio after switching from lean to rich
The output level of the sensor corresponds to the rich air-fuel ratio.
The time-integrated value of the output level during the period until
NO _XThe greater the degree of deterioration of the absorbent, the smaller
Therefore, the deterioration judgment means can reduce the time integrated value of the output level.
The more NO_XIt is judged that the degree of deterioration of the absorbent is large.

In the eighth invention, in the first invention,
NO by deterioration judgment means_XThe deterioration of the absorbent was judged.
After that, the deterioration determination means returns NO._XDeterioration judgment of absorbent
NO until it is done_XNO from absorbent_Xof
The release action takes place and this NO _XWhen the release action of
Deterioration judgment means compared with the degree of rich air-fuel ratio
NO_XThe air-fuel ratio limit when the deterioration of the absorbent is judged.
I try to reduce the degree of chi.

In the ninth invention, in the first invention,
Another air-fuel ratio sensor is arranged in the engine exhaust passage upstream of the NO _x absorbent so that the degree of richness of the air-fuel ratio becomes a predetermined degree when the deterioration determination means determines the deterioration of the NO _x absorbent. In addition, the air-fuel ratio is feedback-controlled based on the output signal of the other air-fuel ratio sensor. In the tenth invention, in the first invention, NO _X by the deterioration determining means is used.
After judging the deterioration of the absorbent is performed, it is performed releasing action of the NO _X from _the NO _X absorbent during the period until the deterioration determination of the NO _X absorbent is carried out by the deterioration determination means again, this N
The cycle in which the O _X releasing action is performed is set shorter as the degree of deterioration of the NO _X absorbent increases.

In the eleventh aspect of the invention, in the first aspect of the invention, the deterioration determining means determines the deterioration of the NO _x absorbent when the engine is operating in a predetermined condition suitable for determining the deterioration. In the twelfth invention, in the first invention, the representative value representing the difference in the changing process of the output level of the air-fuel ratio sensor is N regardless of the operating state of the engine.
And comprising a correction means for correcting the representative value of the engine operating state so as to be substantially the same for the same NO _X absorption amount of O _X absorbent.

[0014]

In the first invention, NO_XAbsorbent NO_Xabsorption
The judgment level that can be estimated that the amount is the maximum absorption amount
Set, NO_XPresumed to be absorbed by the absorbent
Constant NO_XNO when the amount exceeds this judgment level_Xabsorption
The air-fuel ratio of the exhaust gas flowing into the agent is switched from lean to rich
available. At this time, if the output level of the air-fuel ratio sensor changes
NO from the difference_XThe degree of deterioration of the absorbent is judged.
NO_XThe greater the degree of deterioration of the absorbent, the more NO_Xabsorption
The maximum absorption of the agent is reduced and therefore NO _XInferior absorbent
The determination level is updated according to the degree of conversion.

In the second aspect of the invention, the period from when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from lean to rich until the output level of the air-fuel ratio sensor reaches the output level corresponding to the rich air-fuel ratio. A certain period of time is set in advance, and NO _X increases as the elapsed time of this period becomes shorter.
It is judged that the degree of deterioration of the absorbent is large. In the third invention, the switching from the lean air-fuel ratio of the exhaust gas flowing into the NO _X absorbent to a rich output level of the air-fuel ratio sensor is time to substantially match the output level corresponding to the rich air-fuel ratio It is judged that the shorter the elapsed time, the greater the degree of deterioration of the NO _X absorbent.

According to the fourth aspect of the invention, the output level of the air-fuel ratio sensor corresponds to the rich air-fuel ratio after a certain period of time has passed after the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent was switched from lean to rich. It is determined that the degree of deterioration of the NO _X absorbent increases as the elapsed time until the period of time substantially coincides with. 5 th In the invention, NO elapsed time is short indeed NO in the period from the lean air-fuel ratio of the exhaust gas flowing into the _X absorbent until the output level changes rapidly from after a lapse after switching a period of time to the rich It is judged that the degree of deterioration of the _X absorbent is large.

In the sixth aspect, the degree of deterioration of the NO _X absorbent increases as the rate of change of the output level when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from lean to rich. To be judged. In the seventh invention, the output level after switching the air-fuel ratio sensor from a lean air-fuel ratio of the exhaust gas flowing into the NO _X absorbent to rich of the output level in the period until the output level corresponding to the rich air-fuel ratio It is judged that the degree of deterioration of the NO _X absorbent increases as the time integral value decreases.

[0018] In the eighth invention, after the decision of deterioration of _the NO _X absorbent is performed, the releasing action of the NO _X from _the NO _X absorbent even until deterioration determination of the NO _X absorbent is performed again Done. The degree of rich air-fuel ratio when the releasing action of the NO _X deterioration determination of the NO _X absorbent as compared with the degree of rich air-fuel ratio when takes place takes place is reduced.

In the ninth aspect of the invention, the air-fuel ratio is feedback controlled so that the degree of richness of the air-fuel ratio becomes a predetermined degree when the deterioration determination of the NO _X absorbent is made.
The 10 th invention, after the decision of deterioration of _the NO _X absorbent is performed, releasing action of the NO _X from _the NO _X absorbent is carried out even in until deterioration determination of the NO _X absorbent is performed again. This cycle is carried out the releasing action of the NO _X is hidden short as the degree of deterioration of the NO _X absorbent increases.

In the eleventh aspect of the invention, the judgment of deterioration of the NO _X absorbent is made when the engine is in a predetermined operating condition suitable for judgment of deterioration. In the twelfth aspect of the invention, the representative value representing the difference in the change process of the output level of the air-fuel ratio sensor is set to be almost the same for the same NO _x absorption amount of the NO _x absorbent regardless of the operating state of the engine. The representative value is corrected according to the engine operating condition.

[0021]

DETAILED 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 an exhaust port. Show. The intake port 6 is connected to the surge tank 10 via a corresponding branch pipe 9, and each branch pipe 9 is provided with a fuel injection valve 11 for injecting fuel into the intake port 6. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 17 containing a NO _X absorbent 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, a constant power source by a bidirectional bus 31. Connected backup RAM
35, an input port 36 and an output port 37. A pressure sensor 19 that generates an output voltage proportional to the absolute pressure in the surge tank 10 is arranged in the surge tank 10, and the output voltage of the pressure sensor 19 is input to the input port 35 via the corresponding AD converter 38. To be done. An air-fuel ratio sensor (hereinafter referred to as an O ₂ sensor) 20 is arranged in the exhaust manifold 15, and the O ₂ sensor 20 is input to the input port 36 via the corresponding AD converter 38. Another air-fuel ratio sensor (hereinafter referred to as an O ₂ sensor) 22 is arranged in the exhaust pipe 21 downstream of the NO _x absorbent 18, and the O ₂ sensor 22 is connected to the input port 36 via the corresponding AD converter 38. Connected to. Also, input port 3
A rotation speed sensor 23 for generating an output pulse indicating the engine speed and a vehicle speed sensor 24 for generating an output pulse indicating the vehicle speed are connected to the valve 6. On the other hand, the output port 37 is connected to the spark plug 4 and the fuel injection valve 11 via the corresponding drive circuit 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on the following equation, for example. TAU = TP · K · FAF where TP is the basic fuel injection time, K is the correction coefficient, FAF
Indicates the feedback correction coefficient, respectively. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is previously obtained by an experiment, and is previously RO in the form of a map as shown in FIG. 2 as a function of the absolute pressure PM in the surge tank 10 and the engine speed N.
It is stored in M32. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder, and if K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, when K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and K>
When it becomes 1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The feedback correction coefficient FAF is basically the output signal of the O ₂ sensor 20 when K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder should be the stoichiometric air-fuel ratio. This is a coefficient for accurately matching the air-fuel ratio with the theoretical air-fuel ratio. The feedback correction coefficient FAF moves up and down about 1.0, and the FAF decreases when the air-fuel mixture becomes rich and increases when the air-fuel mixture becomes lean. Note that K <1.0 or K> 1.
When it is 0, the FAF is normally fixed to 1.0.

The target air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder, that is, the value of the correction coefficient K is changed in accordance with the operating state of the engine. In the embodiment according to the present invention, it is basically shown in FIG. Thus, it is predetermined as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. That is, as shown in FIG. 3, K <1.0, that is, the air-fuel mixture is lean in the low load operation region on the load lower side than the solid line R, and K in the high load operation region between the solid line R and the solid line S. = 1.
0, that is, the air-fuel ratio of the air-fuel mixture is the stoichiometric air-fuel ratio, and K> 1.0, that is, the air-fuel mixture is rich in the full-load operation region on the higher load side than the solid line S.

FIG. 4 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 4, the concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer, and is discharged from the combustion chamber 3. The concentration of oxygen O _{2 in} the generated exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NO stored in the casing 17_X
The absorbent 18 uses, for example, alumina as a carrier, and
For example, potassium K, sodium Na, lithium Li,
Alkali metals such as sium Cs, barium Ba, potassium
Alkaline earths such as calcium Ca, lanthanum La,
At least one selected from rare earths such as thorium Y
And a noble metal such as platinum Pt. organ
Intake passage and NO_XProvided in the exhaust passage upstream of the absorbent 18.
The ratio of supplied air and fuel (hydrocarbon) is set to NO _Xabsorption
When the air-fuel ratio of the exhaust gas flowing into the agent 18 is called NO_X
The absorbent 18 is used when the air-fuel ratio of the inflowing exhaust gas is lean.
NO_XIs absorbed, and the oxygen concentration in the exhaust gas is reduced.
NO absorbed by_XReleases NO_XTo absorb and release
U Note that NO_XFuel in the exhaust passage upstream of the absorbent 18
(Hydrocarbon) or inflow / outflow when air is not supplied
The air-fuel ratio of the air-gas is the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3.
The ratio and therefore NO in this case_XAbsorbent 18 burns
When the air-fuel ratio of the air-fuel mixture supplied to the firing chamber 3 is lean
Is NO_XOf the air-fuel mixture in the combustion chamber 3
NO absorbed when oxygen concentration decreases_XTo release
Become.

If the above-mentioned NO _X absorbent 18 is arranged in the engine exhaust passage, the NO _X absorbent 18 actually performs the NO _X absorption and release operation, but the detailed mechanism of this NO _X absorption operation is not clear. There is also. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking the case where platinum Pt and barium Ba are supported on the carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), the oxygen O ₂ is O ₂ ⁻ or O _2.
It adheres to the surface of platinum Pt in the form of ^2- . On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ^{2 −} on the surface of platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the generated NO ₂ is absorbed on the platinum Pt while being absorbed in the absorbent and combined with barium oxide BaO to be absorbed in the form of nitrate ion NO ₃ ⁻ as shown in FIG. 5 (A). Diffuses in the agent. In this way, NO _X is absorbed in the NO _X absorbent 18.

Platinum as long as the oxygen concentration in the inflowing exhaust gas is high
NO on Pt surface₂Is generated and the absorbent NO_XAbsorption capacity
NO unless power is saturated₂Is absorbed in the absorbent and nitric acid
Ionic NO₃ ^-Is generated. On the other hand, the inflow exhaust gas
NO in the oxygen₂When the production of
Reaction is in the opposite direction (NO₃ ^-→ NO₂), And thus suck
Nitrate ion NO in the sorbent₃ ^-Is NO₂From the absorbent in the form of
Is released. That is, the oxygen concentration in the inflowing exhaust gas decreases
And NO_XAbsorbent 18 to NO_XWill be released
It As shown in Fig. 4, the degree of leanness of the inflowing exhaust gas
The lower the value, the lower the oxygen concentration in the inflowing exhaust gas.
Even if the lean degree of the inflowing exhaust gas is lowered by
NO even if the air-fuel ratio of the exhaust gas is lean_XAbsorbent 18
To NO _XWill be released.

On the other hand, at this time, if the air-fuel mixture supplied to the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, a large amount of unburned H is emitted from the engine as shown in FIG.
C and CO are discharged, and these unburned HC and CO are platinum Pt.
It is oxidized by reacting with the oxygen O ₂ ⁻ or O ^{2 −} above.
Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas drops extremely, so
O ₂ is released, and this NO ₂ is reduced by reacting with unburned HC and CO as shown in FIG. 5 (B). When NO ₂ is no longer present on the surface of platinum Pt in this way, NO ₂ is released one after another from the absorbent. Therefore NO _X from _the NO _X absorbent 18 in a short time when the air-fuel ratio of the inflowing exhaust gas is made rich, that is released.

That is, the air-fuel ratio of the inflowing exhaust gas is made rich.
Then, first, unburned HC and CO are O on platinum Pt.₂ ^-or
Is O^2-Reacts instantly with and is oxidized, then platinum
O on Pt₂ ^-Or O^2-Is consumed but unburned HC,
If CO remains, the unburned HC and CO absorb the absorbent.
NO released from_XAnd NO emitted from the engine _X
Is reduced. Therefore, the air-fuel ratio of the inflowing exhaust gas is
No in a short time if you switch_XAbsorbed by absorbent 18
NO_XIs released, and the released NO
_XNO is reduced to the atmosphere_XIs discharged
You will be able to prevent it.

When the lean air-fuel mixture is burned as described above, NO _X is absorbed by the NO _X absorbent 18. However there is a limit to the NO _X absorbing capacity of the NO _X absorbent 18, _the NO _X absorbent 18 when saturation NO _X absorbing capacity of the NO _X absorbent 18 is not E longer absorb NO _X.
Therefore NO NO _X absorbing capacity of the _X absorbent 18 must emit NO _X from _the NO _X absorbent 18 before the saturation degree of the NO _X in _the NO _X absorbent 18 in order that
It is necessary to estimate whether is absorbed. Then this N
A method for estimating the O _X absorption amount will be described.

[0034] is _the amount of NO _X absorbed in unit time per _the NO _X absorbent 18 to the amount of NO _X discharged from the higher unit time per engine becomes higher the engine load increases when the lean air-fuel mixture is burned As the engine speed increases and the engine speed increases, NO emitted from the engine per unit time
NO _X is increased _{to X} amount is absorbed per unit time _the NO _X absorbent 18 to increase. Therefore, NO per unit time
_The amount of NO _X absorbed by the _X absorbent 18 is a function of the engine load and the engine speed. In this case, the engine load is absolute pressure PM and the engine speed N of the absolute amount of NO _X that have at pressure representative absorbed in unit time per _the NO _X absorbent 18 since it is the surge tank 10 in the surge tank 10 Is a function of. Therefore, in the embodiment according to the present invention, NO _X per unit time
Absolute pressure PM of NO _X amount NOXA absorbed by the absorbent 18
And as a function of the engine speed N, previously obtained by experiments,
The NO _X absorption NOXA is stored in advance in the ROM32 in the form of a map shown in FIG. 6 (A) as a function of PM and N.

Meanwhile, although the air-fuel ratio of the mixture fed into the engine cylinder NO _X is released from _the NO _X absorbent 18 becomes the stoichiometric air-fuel ratio or rich NO _X emission amount at this time primarily exhaust gas It is affected by quantity and air-fuel ratio. That is, NO of _the amount of NO _X amount of exhaust gas is discharged from the higher per unit time _the NO _X absorbent 18 increases increases, the air-fuel ratio is discharged from the higher per unit time _the NO _X absorbent 18 becomes rich
_X amount increases. In this case, it can be represented by the product of the exhaust gas amount, that is, the intake air amount, the engine speed N, and the absolute pressure PM in the surge tank 10. Therefore, as shown in FIG. _the amount of NO _X NOXD released from _the NO _X absorbent 18 increases as N · PM increases. Further, since the air-fuel ratio corresponds to the value of the correction coefficient K, the NO _X amount NOXD released from the NO _X absorbent 18 per unit time increases as the value of K increases as shown in FIG. 7B. To do. The NO _X amount NOXD released from the NO _X absorbent 18 per unit time is previously stored in the ROM 3 in the form of a map shown in FIG. 6B as a function of N · PM and K.
It is stored in 2.

As described above, the lean air-fuel mixture is burned.
NO per unit time when given _XAbsorption amount is NOXA
The air-fuel mixture with the stoichiometric air-fuel ratio or the rich air-fuel mixture is burned.
NO per unit time when burnt_XThe amount released
NO as it is represented by NOXD_XAbsorbed by absorbent 18
Estimated to be NO_XThe quantity ΣNOX is expressed by the following equation
Will be.

ΣNOX = ΣNOX + NOXA-NOXD Therefore, in the embodiment according to the present invention, as shown in FIG.
O_XNO estimated to be absorbed by the absorbent 18_Xamount
When ΣNOX reaches the maximum allowable value MAX,
Temporarily enriches the air-fuel ratio, which results in NO_Xabsorption
Agent 18 to NO _XIs to be released.

However, the exhaust gas contains SO _X , and the NO _X absorbent 18 contains not only NO _X but also SO _X.
Is also absorbed. It is considered that the absorption mechanism of SO _x into the NO _x absorbent 18 is the same as the absorption mechanism of NO _x . That is, assuming that platinum Pt and barium Ba are supported on the carrier in the same manner as when the NO _X absorption mechanism is described, as described above, when the air-fuel ratio of the inflowing exhaust gas is lean, oxygen O ₂ Is O ₂
^- or O is deposited on the surface of the platinum Pt in ^2-form, SO ₂ in the inflowing exhaust gas on the surface of the platinum Pt O ₂ ^- or O ^2-
And SO ₃ . Next, a part of the generated SO ₃ is further oxidized on the platinum Pt, is absorbed in the absorbent and combines with the barium oxide BaO, and the sulfate ion SO 3
₄ Diffusion into the absorbent in the form of ^2- , stable sulfate BaSO
Generate ₄ .

However, the sulfate BaSO ₄ is stable and difficult to decompose, and almost all the sulfate BaSO ₄ is not decomposed even if the air-fuel ratio of the air-fuel mixture is made rich for a short time as shown in FIG. It remains as it is. Therefore NO
_{In the X} absorbent 18, sulfate Ba as time passes
SO ₄ will increase, and thus the maximum NO _x absorption amount that can be absorbed by the NO _x absorbent 18 will gradually decrease over time. In other words, in other words, the NO _X absorbent 18 gradually deteriorates as time passes. Maximum NO that NO _x absorbent 18 can absorb
Maximum _X absorption amount must to release NO _X from _the NO _X absorbent 18 while less NO _X absorption of the _the NO _X absorbent 18 when lowered, which can be initially absorbed _the NO _X absorbent 18 in order that It is necessary to accurately detect the NO _X absorption amount, that is, the degree of deterioration of the NO _X absorbent 18.

NO in the present invention_XAbsorbent 18 can absorb
Maximum NO_XAbsorption amount, ie NO_XDegree of deterioration of the absorbent 18
O₂Detect from the air-fuel ratio detected by the sensor 22
This will be explained below. Immediately
If the air-fuel mixture supplied to the combustion chamber 3 becomes rich,
Oxygen O from the combustion chamber 3 as shown in FIG.₂And unburned
Exhaust gas containing HC and CO is discharged, but this oxygen O
₂Hardly reacts with unburned HC and CO.
Oxygen O₂Is NO_XNO past the absorbent 18_Xabsorption
It will be discharged from the agent 18. On the other hand, in the combustion chamber 3
NO when the supplied air-fuel mixture becomes rich_XAbsorbent 18?
Et NO _XIs released. At this time, it is included in the exhaust gas
Unburned HC and CO released NO_XUsed to return
NO because it is used_XAbsorbent 18 to NO_XIs released
No while_XExhaust of unburned HC and CO from the absorbent 18
Will not be served. Therefore NO_XAbsorbent 18 to N
O_XIs continuously released, NO_XExhausted from absorbent 18
Oxygen O is contained in the exhaust gas emitted.₂But unburned
HC and CO are not contained at all, so during this period NO
_XThe air-fuel ratio of the exhaust gas discharged from the absorbent 18 is small
It is just lean. Then NO_XIn absorbent 18
All NO absorbed_XWhen is released, it is included in the exhaust gas.
Unburned HC and CO that are rare are NO_XNO in the absorbent 18
_XNO as it is without being used for the reduction of_XSucking
It is discharged from the collecting agent 18. Therefore, at this time NO_XAbsorbent
The air-fuel ratio of the exhaust gas discharged from 18 becomes rich.
That is, NO_XAll NO absorbed in the absorbent 18_XLet go
NO when issued_XThe exhaust gas discharged from the absorbent 18
It will change from lean to rich. In this case, N
O_XNO absorbed by the absorbent 18_XThe amount is NO_Xabsorption
The air-fuel ratio of the exhaust gas flowing into the agent 18 is from lean to rich
NO after switching to_XExhaust gas discharged from the absorbent 18
Approximately the elapsed time until the air-fuel ratio of the gas becomes rich
Proportional, so from this elapsed time NO_XAbsorbent 1
NO absorbed in 8_XYou will know the amount. next
I will explain this in a little more detail.

The O ₂ sensor 22 shown in FIG. 1 comprises a cup-shaped cylindrical body made of zirconia arranged in the exhaust passage, and an anode made of a platinum thin film is provided on the inner side surface of the cylindrical body. Cathodes made of platinum thin films are respectively formed on the outer surfaces of the strips. The cathode is covered with a porous layer, and a constant voltage is applied between the cathode and the anode. This O ₂
In the sensor 22, as shown in FIG. 9, a current I (mA) proportional to the air-fuel ratio A / F flows between the cathode and the anode. In FIG. 9, I _O is the air-fuel ratio A / F and is the theoretical air-fuel ratio (= 14.
The current value at the time of 6) is shown. As can be seen from FIG. 9, when the air-fuel ratio A / F is lean, the current value I is I> I _O
In this range, the larger the air-fuel ratio A / F, the larger it becomes, and the current value I becomes zero when the air-fuel ratio A / F becomes rich at approximately 13.0 or less.

FIG. 10 shows NO_XExhaust flowing into the absorbent 18
Changes in the gas air-fuel ratio (A / F) in and O₂Sensor 22
Of the current I flowing between the cathode and the anode of the_XAbsorbent
Of the air-fuel ratio (A / F) out of the exhaust gas flowing out from 18
And change. NO as shown in FIG._XSucking
The air-fuel ratio (A / F) in of the exhaust gas flowing into the sorbent 18 is
NO switched from lean to rich_XFrom absorbent 18
NO_XNO when the release action starts_XFrom absorbent 18
The air-fuel ratio (A / F) out of the exhaust gas that flows out is the theoretical air-fuel
It rapidly decreases to near the ratio, so the current value I becomes I_ONear
Decrease rapidly. Then NO_XFrom absorbent 18
NO_XNO during release action _XAbsorbent 18?
The air-fuel ratio (A / F) out of the exhaust gas flowing out from the
However, the current value I is kept at the lean state._O
Is held at a value slightly larger than. Then NO
_XAll NO absorbed in the absorbent 18_XIs emitted
NO_XAir-fuel ratio of the exhaust gas flowing out from the absorbent 18 (A /
F) out rapidly becomes smaller and becomes rich, so
The current value I rapidly drops to zero.

FIG. 11 shows changes in the current value I when the NO _X amount absorbed by the NO _X absorbent 18 is different. Incidentally, the numerical values in FIG. 11 shows the amount of NO _X is absorbed in the NO _X absorbent 18. As shown in FIG. 11, when the NO _X amount absorbed by the NO _X absorbent 18 is different, the change process of the current value I is different accordingly,
Will therefore be seen that _the amount of NO _X is absorbed from the difference of the change process in _the NO _X absorbent 18. As one of the representative values representing this difference in the changing process, the current value I changes after the air-fuel ratio (A / F) in of the exhaust gas discharged from the NO _x absorbent 18 is switched from lean to rich. substantially has elapsed time t and until zero, as the amount of NO _X is reduced, which is absorbed in the NO _X absorbent 18 as can be seen from Figure 11 the elapsed time t becomes shorter. Thus, it is possible to know the amount of NO _X is absorbed from the elapsed time t in the NO _X absorbent 18.

By the way up to NO _X absorption of _the NO _X absorbent 18 can absorb, i.e. NO _X absorption maximum NO _X absorption of the NO _X absorbent 18 in order to detect the degree of deterioration of the NO _X absorbent 18 It is necessary to switch the air-fuel ratio (A / F) in of the exhaust gas from lean to rich when the amount is reached and to determine the elapsed time t at this time. In FIG. 8, SAT
Shows the determination level NO _X absorption of the NO _X absorbent 18 is estimated that the maximum NO _X absorption, estimated that in the embodiment according to the present invention are absorbed in the NO _X absorbent 18 The determined NO _X amount ΣNOX is the judgment level SA
Air-fuel ratio in order to determine the deterioration of the NO _X absorbent 18 when exceeding the T is temporarily rich, the maximum NO to _the NO _X absorbent 18 from the elapsed time t of the current value I in this case can absorb _The amount of _X absorption, that is, the degree of deterioration of the NO _X absorbent 18 is determined.

As shown in FIG. 8, the NO _X amount ΣN
The maximum allowable value MAX for OX is set to a value smaller than the determination level SAT, and ΣNO _x is the maximum allowable value M.
Judging the deterioration of the NO _X absorbent 18 when it reaches the AX only NO _X release action from _the NO _X absorbent 18 without takes place. Frequently only NO _X release action from _the NO _X absorbent 18 is performed is higher than the frequency of the deterioration determination of the NO _X absorbent 18 is made, thus after the decision of deterioration of _the NO _X absorbent 18 is performed, the following By the time the deterioration of the NO _X absorbent 18 is judged, the NO _X releasing action is performed a plurality of times.

The current I flowing between the cathode and the anode of the O ₂ sensor 22 is converted into a voltage and input into the input port 36,
In the electronic control unit 30, this voltage is again converted into the corresponding current value I, and the air-fuel ratio is controlled based on this current value I. 12 and 13 show the current value I shown in FIG.
The air-fuel ratio control routine for judging the degree of deterioration of the NO _X absorbent 18 from the elapsed time t of is shown. This routine is executed by interruption every fixed time.

Referring to FIGS. 12 and 13, first, at step 100, the basic fuel injection time TP is calculated from the relationship shown in FIG. Next, at step 101, it is judged if the deterioration judgment flag indicating that the degree of deterioration of the NO _X absorbent 18 should be judged is set or not. When the deterioration determination flag is not set, the routine proceeds to step 102, where the NO _X absorbent 18 is changed to NO _X.
It is determined whether or not the NO _X release flag indicating that should be released is set. When the NO _X release flag is not set, the routine proceeds to step 103.

In step 103, the correction coefficient K is calculated based on FIG. Next, at step 104, the correction coefficient K
Is determined to be 1.0 or not. When K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture should be the stoichiometric air-fuel ratio, the routine proceeds to step 125, where feedback control I of the air-fuel ratio is performed.
Is done. This feedback control I is shown in FIG. On the other hand, if K = 1.0, step 1
In step 05, it is determined whether the correction coefficient K is smaller than 1.0. When K <1.0, that is, when the air-fuel ratio of the lean mixture should be made lean, the routine proceeds to step 126, where the air-fuel ratio feedback control II is performed. This feedback control II is shown in FIG. On the other hand, K <
If it is not 1.0, the routine proceeds to step 106, FAF is fixed at 1.0, and then the routine proceeds to step 107. In step 107, the fuel injection time TAU is calculated based on the following equation.

TAU = TP.K.FAF Next, at step 108, it is judged if the correction coefficient K is smaller than 1.0. When K <1.0, that is, when the lean air-fuel mixture should be burned, the routine proceeds to step 109, where the NO _X absorption amount NOXA is calculated from FIG. 6 (A). Next, at step 110, the NO _X release amount NOXD is made zero, and then the routine proceeds to step 113. On the other hand, when it is determined in step 108 that K ≧ 1.0,
That is, when the stoichiometric air-fuel ratio mixture or rich mixture should be burned, the routine proceeds to step 111, where NO _{x is obtained} from FIG. 6 (B).
The release amount NOXD is calculated. Next, at step 112, the NO _X absorption amount NOXA is made zero, and then at step 1
Proceed to 13. In step 113, the NO _X is gradually increased.
NO _X amount ΣN estimated to be absorbed by the absorbent 18
OX is calculated.

ΣNOX = ΣNOX + NOXA-NOXD Next, at step 114, it is judged if ΣNOX becomes negative. If ΣNOX <0, then step 1
Proceeding to 15, ΣNOX is made zero. Then step 1
At 16, the current vehicle speed SP is added to ΣSP. This Σ
SP indicates the cumulative traveling distance of the vehicle. Next, at step 117, it is judged if the cumulative traveling distance ΣSP is larger than the set value SP _O. When ΣSP ≦ SP _O, the routine proceeds to step 118, where ΣNOX is the maximum allowable value MAX.
(Fig. 8) is determined. ΣNOX> MA
When it becomes X, the routine proceeds to step 123, where the NO _X release flag is set.

Meanwhile, when it is judged that .SIGMA.SP> SP _O at step 117 proceeds to step 120 NO
_The amount of NO _X estimated to be absorbed by the _X absorbent 18
It is determined whether or not NOX becomes larger than the determination level SAT (FIG. 8). When ΣNOX> SAT, the routine proceeds to step 121, where the deterioration determination flag is set, and then at step 122, ΣSP is made zero.

When the deterioration determination flag is set, step
From 101 to step 123, the deterioration determination is performed.
It This deterioration determination is shown in FIG. On the other hand, NO
_XWhen the release flag is set, the step 102 is started.
Go to step 124 and NO_XRelease processing is performed. This N
O_XThe release process is shown in FIG. Next, the screen of FIG.
Feedback control I performed at step 125,
That is O ₂Theoretical air-fuel ratio based on the output signal of the sensor 20
Diagram of feedback control for maintaining air-fuel ratio
This will be described with reference to FIGS. 14 and 15.

As shown in FIG. 15, the O ₂ sensor 20 generates an output voltage V of about 0.9 (V) when the air-fuel mixture is rich, and about 0.1 (V) when the air-fuel mixture is lean. Generate an output voltage V. The feedback control I shown in FIG. 14 is performed based on the output signal of the O ₂ sensor 20. Referring to FIG. 14, first, at step 130, it is judged if the output voltage V of the O ₂ sensor 20 is smaller than the reference voltage Vr of about 0.45 (V). V
When ≦ Vr, that is, when the air-fuel ratio is lean, the routine proceeds to step 131, where the delay count value CDL is decremented by 1. Next, at step 132, it is judged if the delay count value CDL has become smaller than the minimum value TDR. If CDL <TDR, the routine proceeds to step 133, where the CDL is set to TDR, and then step 1
Proceed to 37. Therefore, as shown in FIG. 15, when V ≦ Vr, the delay count value CDL is gradually decreased, and then the CDL is maintained at the minimum value TDR.

On the other hand, when it is judged at step 130 that V> Vr, that is, when the air-fuel ratio is rich, the routine proceeds to step 134, where the delay count value CDL is 1
Is incremented only. Next, at step 135, it is judged if the delay count value CDL has become larger than the maximum value TDL. If CDL> TDL, the routine proceeds to step 136, where the CDL is set to TDL, and then the routine proceeds to step 137. Therefore, as shown in FIG.
When> Vr, the delay count value CDL is gradually increased, and then the CDL is maintained at the maximum value TDL.

In step 137, the delay count value CDL is changed from the previous processing cycle to the current processing cycle.
It is determined whether the sign of is inverted from positive to negative or from negative to positive. When the sign of the delay count value CDL is inverted, the routine proceeds to step 138, where it is determined whether or not the inversion from positive to negative, that is, whether or not the inversion from rich to lean. At the time of inversion from rich to lean, the routine proceeds to step 139, where the rich skip value RSR is added to the feedback correction coefficient FAF, and thus the FAF is rapidly increased by the rich skip value RSR as shown in FIG. On the other hand, when the lean to rich inversion is performed, the routine proceeds to step 140, where the lean skip value RSL is subtracted from the FAF, and thus the FAF is rapidly decreased by the lean skip value RSL as shown in FIG.

On the other hand, when it is determined in step 137 that the sign of the delay count value CDL is not inverted, the routine proceeds to step 141, where the delay count value CDL
It is determined whether L is negative. When CDL ≦ 0, the routine proceeds to step 142, where the feedback correction coefficient FA
The rich integrated value KIR (KIR <RSR) is added to F, and thus FAF is gradually increased as shown in FIG. On the other hand, when CDL> 0, the routine proceeds to step 143, where the lean integral value KIL is subtracted from FAF, and therefore FAF is gradually decreased as shown in FIG. In this way, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio.

Next, at step 126 in FIG.
Feedback control II, that is, O ₂Current of sensor 22
A target lean corresponding to the correction coefficient K of the air-fuel ratio based on the value I
Diagram of feedback control for maintaining air-fuel ratio
This will be described with reference to 16. First referring to FIG.
In step 150, the target
Target current value I corresponding to the air-fuel ratio_OIs calculated. Next
In step 151, O₂The current value I of the sensor 22 is
Standard current value I_OIs determined to be greater than or equal to. I> I
_OIf it is, go to step 152 and perform feedback compensation.
A constant value ΔF is added to the positive coefficient FAF, and I ≦ I_OWhen
In step 153, the feedback correction coefficient F
A constant value ΔF is subtracted from AF. Air fuel in this way
The ratio is maintained at the target lean air / fuel ratio.

Next, the NO _X release control performed in step 124 of FIG. 12 will be described with reference to FIG. Referring to FIG. 17, first, at step 160, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Next, at step 161, the feedback correction coefficient FAF is fixed at 1.0. Therefore, when the NO _X releasing process of FIG. 17 is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 162, it is judged if the current value I of the O ₂ sensor 22 is lower than a predetermined constant value α (FIG. 11). When I <α, the routine proceeds to step 163, where the NO _x release flag is reset, and thus the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio determined by the operating state at that time, normally to lean. Next, at step 164, ΣNOX is made zero.

Next, the deterioration determination performed in step 123 of FIG. 12 will be described with reference to FIG. Referring to FIG. 18, first, at step 170, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Next, at step 171, the feedback correction coefficient FAF
Is fixed at 1.0. Therefore, when the deterioration determination of FIG. 18 is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 172, the elapsed time t is incremented by 1. Next, at step 175, it is judged if the current value I of the O ₂ sensor 22 is lower than a predetermined constant value α (FIG. 11). When I <α, the routine proceeds to step 174, where the degree of deterioration of the NO _X absorbent 18 is calculated based on the elapsed time t from the relationship shown in FIG. FIG.
As shown in (A), the shorter the elapsed time t, the NO _X
The degree of deterioration of the absorbent 18 increases. This NO _x absorbent 1
When the deterioration degree of 8 exceeds a predetermined value, for example, a warning light is turned on.

[0060] Then the maximum NO _X absorption W _max of the NO _X absorbent 18 on the basis of the elapsed time t from the relationship shown in step 175 in FIG. 19 (B) is calculated. As shown in FIG. 19B, the maximum NO _X absorption amount W _max becomes larger as the elapsed time t becomes longer. Next, at step 176, the determination level SAT (= 1.1 · W _max ) is calculated by multiplying the maximum NO _X absorption amount W _max by a constant value, for example 1.1. That is, the determination level SAT is updated according to the degree of deterioration of the NO _X absorbent 18. NO _X absorbent 18
Suppose that the deterioration of
When the O _X amount ΣNOX exceeds the determination level SAT, N
NO _X absorption of O _X absorbent 18 is always a maximum NO _X absorption, therefore the decision level SAT is the NO _X absorption of the NO _X absorbent 18 is the largest NO _X absorption amount estimation This represents the possible NO _X amount ΣNOX.

Needless to say, the determination level SAT is required.
No. other than 1.1 is the maximum_XAbsorption W_maxMultiply by
May be any number up to 1.0_Xabsorption
Quantity W _maxThe decision level SAT is obtained by multiplying
Can be However, maximum NO_XAbsorption W_maxTo
NO if the value to be multiplied is too large_XAbsorbent 18 N
O_XMaximum absorption NO_XAfter reaching the absorption amount, NO_XRelease of
Since it takes a long time for the action to take place,
NO_XEmissions will increase. Therefore maximum NO_XSucking
Yield W_maxIt is better to multiply the value by
This is not desirable, and this value is preferably about 1.3 or less.

At step 176, the decision level SAT
Is calculated, the routine proceeds to step 177, where the NO _X absorption amount W
_{The maximum} allowable value MAX is calculated by multiplying _max by a positive value less than or equal to 1.0, for example, 0.7. That is, the maximum allowable value MAX is also updated according to the degree of deterioration of the NO _X absorbent 18. Next, at step 178, the deterioration determination flag is reset. When the deterioration determination flag is reset, the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio according to the operating state at that time, normally to lean.
Next, at step 179, t and ΣNOX are made zero.

A second embodiment is shown in FIGS. 20 to 22. Figure
NO as shown in 20_XExhaust flowing into the absorbent 18
Gas air-fuel ratio (A / F) in switched from lean to rich
If you get O₂The current value I of the sensor 22 is I_OSuddenly near
The current value I is I _ONecessary to lower to near
Air-fuel ratio (A / F) before switching to rich
The greater the degree of lean in, the longer it becomes. in this case,
Current value I is I_OThe time required to drop to near is NO
_XNO absorbed in the absorbent 18_XDirectly related to quantity
Absent. Therefore NO_XMaximum NO of absorbent 18_XAbsorption amount is positive
For accurate detection, the current value I is I_ODrop to near
It is preferable not to include the time required for
Yes. Therefore, in this second embodiment, the current value I is I_OThan
Elapsed time from when a high β was reached to when I = α
Time t is calculated and NO is calculated from this elapsed time t._XOf absorbent 18
The degree of deterioration is calculated.

Also in the second embodiment, the routines shown in FIGS. 12 and 13 are used for the air-fuel ratio control, but the routine shown in FIG. 21 is used for the deterioration process performed in step 123 of FIG. Referring to FIG. 21, first, at step 200, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Next, at step 201, the feedback correction coefficient FAF is 1.
It is fixed at 0. Therefore, when the deterioration determination is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 202, it is judged if the current value I of the O ₂ sensor 22 is lower than the set value β (FIG. 20). When I <β, the routine proceeds to step 203, where the elapsed time t is incremented by 1. Next, at step 204, it is judged if the current value I of the O ₂ sensor 22 is lower than the set value α (FIG. 20), and if I <α, the routine proceeds to step 205.

At step 205, from the elapsed time t, as shown in FIG.
The deterioration degree of the NO _X absorbent 18 is calculated based on (A). Next, at step 206, from the elapsed time t, as shown in FIG.
Maximum NO _X absorption amount W of the NO _X absorbent 18 on the basis of the (B)
_max is calculated. Next, at step 207, the maximum NO
_The determination level SAT is calculated by multiplying the _X absorption amount W _max by a constant value, for example, 1.1. Next, at step 208, the maximum allowable value MAX is calculated by multiplying the maximum NO _X absorption amount W _max by a constant value, for example 0.7. Next, at step 209, the deterioration determination flag is reset. When the deterioration determination flag is reset, the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio that is determined by the operating state at that time, and is normally switched to lean. Next, at step 210, t and ΣNOX are made zero.

The third embodiment is shown in FIGS. 23 to 26. Figure
NO as shown in 23_XExhaust flowing into the absorbent 18
The air-fuel ratio of the gas is switched from lean to rich, then
NO _XAll NO absorbed in the absorbent 18_XRelease action of
Is completed, the air-fuel ratio (A / F) out changes from lean to
The current value I suddenly changes immediately before
The sudden change point P of the flow value appears. This current value sudden change point P is NO_X
The air-fuel ratio (A / F) because it represents the completion point of the release action
After in has been switched from lean to rich, the current value I
The elapsed time t until reaching the current value sudden change point P is NO_XSucking
Maximum NO of collecting agent 18_XIt represents the amount of absorption
It Also in this third embodiment, NO _XAbsorbent 18
Maximum NO_XThe air-fuel ratio (A
/ F) in after the change from lean to rich
Time t₁Is not included in the elapsed time t.

Further, as shown in FIG. 23, the air-fuel ratio (A / F) out is not yet rich at the current value sudden change point P, so that when the current value I reaches the current value sudden change point P. If the air-fuel ratio of the air-fuel mixture is changed from rich to lean, the air-fuel ratio of the exhaust gas flowing out from the NO _X absorbent 18 (A
/ F) the out completely it can release all NO _X that is absorbed in the NO _X absorbent 18 without being rich. That is, if the air-fuel ratio of the air-fuel mixture is switched from rich to lean when the current value I reaches the current value sudden change point P, the release action of all the absorbed NO _X from the NO _X absorbent 18 can be completed, Moreover, since the air-fuel ratio (A / F) out of the exhaust gas flowing out from the NO _X absorbent 18 does not become rich, it is possible to prevent a large amount of unburned HC and CO from being released into the atmosphere. Therefore, in this third embodiment, the current value I
When the current value sudden change point P is reached, the air-fuel ratio of the air-fuel mixture is switched from rich to lean.

Also in this third embodiment, the routine shown in FIGS. 12 and 13 is used for the air-fuel ratio control, but the routine shown in FIG. 24 is used for the NO _X releasing process performed in step 124 of FIG. Figure 1
The routine shown in FIG. 25 is used for the deterioration process performed in step 123 of No. 2. Referring to FIG. 24 showing the NO _X releasing process, first, at step 300, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Next, at step 301, the feedback correction coefficient FAF is fixed at 1.0. Therefore, when the NO _X releasing process is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 302, it is judged if a fixed time t ₁ (FIG. 23) has elapsed since the air-fuel ratio of the air-fuel mixture was made rich. When the fixed time t ₁ has elapsed, the routine proceeds to step 303, where the change rate ΔI (= I ₁ −I) of the current value I is obtained by subtracting the current value I ₁ at the previous interrupt from the current value I at the current interrupt. It is calculated. Next, at step 304, it is judged if the rate of change ΔI of the current value I exceeds a preset set value X. When ΔI> X, that is, when the current value I reaches the current value sudden change point P, the routine proceeds to step 305, where the NO _X release flag is reset, and thus the air-fuel ratio of the air-fuel mixture is determined from the rich state to the operating state at that time. The air-fuel ratio is usually switched to lean. Next, at step 306, ΣNOX is made zero.

Next, the deterioration determination routine in the third embodiment will be described with reference to FIG. Referring to FIG. 25, first, at step 310, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Next, at step 311, the feedback correction coefficient FAF is fixed at 1.0. Therefore, when the deterioration determination is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 312, the elapsed time t is incremented by 1. Then step 3
At 13, it is judged if the elapsed time t has become longer than the fixed time t ₁ (FIG. 23). When t ≦ t _1, the routine proceeds to step 322, where t is made zero. On the other hand, t
When> t ₁ , the process proceeds to step 314 and the current value I _{1 at the} time of this interrupt is subtracted from the current value I ₁ at the time of the previous interrupt to change ΔI (= I) of the current value I.
_1- I) is calculated. Next, at step 315, it is judged if the rate of change ΔI of the current value I exceeds a preset value X. When ΔI> X, that is, when the current value I reaches the current value sudden change point P, the process proceeds to step 316.

At step 316, from the elapsed time t, as shown in FIG.
The deterioration degree of the NO _X absorbent 18 is calculated based on (A). Next, at step 317, from the elapsed time t, as shown in FIG.
Maximum NO _X absorption amount W of the NO _X absorbent 18 on the basis of the (B)
_max is calculated. Next, at step 318, the maximum NO
_The determination level SAT is calculated by multiplying the _X absorption amount W _max by a constant value, for example, 1.1. Next, at step 319, the maximum allowable value MAX is calculated by multiplying the maximum NO _X absorption amount W _max by a constant value, for example 0.7. Next, at step 320, the deterioration determination flag is reset. When the deterioration determination flag is reset, the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio that is determined by the operating state at that time, and is normally switched to lean. Next, at step 321, t and ΣNOX are made zero.

27 to 29 show the fourth embodiment. As described above, the current value I has the current value sudden change point P as shown in FIG. 27, and the current value I drops to zero after reaching the current value sudden change point P. Depends change process on the type of metal contained in the NO _X absorbent 18 of the current value I when the current value I is lowered from the current value sudden change points P, as shown in FIG. 27 by _the NO _X absorbent 18 The rate of change of the current value I when the current value I falls from the current value sudden change point P is NO
_The larger the _X absorption amount, the smaller it becomes. In such a case, the maximum NO _x absorption amount can be obtained from the rate of change of the current value I when the current value I falls from the current value sudden change point P. Therefore, in the fourth embodiment, the maximum NO _x absorption amount is obtained from the elapsed time t until the current value I changes from I ₁ to I ₂ .

Also in the fourth embodiment, the routine shown in FIGS. 12 and 13 is used for the air-fuel ratio control, but the routine shown in FIG. 28 is used for the deterioration processing performed in step 123 of FIG. Referring to FIG. 28, first, in step 400, the correction coefficient K is set to a constant value KK of about 1.3, for example. Next, at step 401, the feedback correction coefficient FAF is 1.
It is fixed at 0. Therefore, when the deterioration determination is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 402, it is judged if the current value I of the O ₂ sensor 22 is lower than the set value I ₁ (FIG. 27). When I <I ₁ , the process proceeds to step 403 and the elapsed time t is incremented by 1. Next, at step 404, it is judged if the current value I of the O ₂ sensor 22 becomes equal to the set value I ₂ (FIG. 27), and if I = I ₂ , the routine proceeds to step 405.

At step 405, from the elapsed time t, as shown in FIG.
The deterioration degree of the NO _X absorbent 18 is calculated based on (A). Next, at step 406, from the elapsed time t, as shown in FIG.
Maximum NO _X absorption amount W of the NO _X absorbent 18 on the basis of the (B)
_max is calculated. Next, at step 407, the maximum NO
_The determination level SAT is calculated by multiplying the _X absorption amount W _max by a constant value, for example, 1.1. Next, at step 408, the maximum allowable value MAX is calculated by multiplying the maximum NO _X absorption amount W _max by a constant value, for example 0.7. Next, at step 409, the deterioration determination flag is reset. When the deterioration determination flag is reset, the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio that is determined by the operating state at that time, and is normally switched to lean. Next, at step 410, t and ΣNOX are made zero.

A fifth embodiment is shown in FIGS. 30 to 32. N
Considering that the current value I of the O ₂ sensor 22 is maintained at a certain value while the NO _X is released from the O _X absorbent 18, the maximum NO _X absorption amount is the hatched area in FIG. It is considered to be proportional to. Even if the current value I changes in a spike shape due to disturbance or the like in the course of the change of the current value I, this area is hardly affected by the spike shape change, so that the maximum NO _x absorption amount can be accurately detected. Therefore, in this fifth embodiment, the current value I is integrated over time to obtain the hatched area in FIG. 30, and the maximum NO _X absorption amount is obtained from this area, that is, the integrated value of the current value I.

Also in the fifth embodiment, the routine shown in FIGS. 12 and 13 is used for the air-fuel ratio control, but the routine shown in FIG. 31 is used for the deterioration process performed in step 123 of FIG. Referring to FIG. 31, first, at step 500, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Next, at step 501, the feedback correction coefficient FAF is 1.
It is fixed at 0. Therefore, when the deterioration determination is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 502, it is judged if the current value I of the O ₂ sensor 22 is lower than the set value I _S (FIG. 30). When I <I _S , the routine proceeds to step 503, where the multiplication result I · Δt of the current value I and the time interrupt interval Δt is added to the integral value S. Next, at step 504, it is judged if the current value I of the O ₂ sensor 22 has become zero, and if I = 0, the routine proceeds to step 505.

In step 505, the integrated value S is calculated as shown in FIG.
The deterioration degree of the NO _X absorbent 18 is calculated based on (A). Next, at step 506, the integrated value S is calculated as shown in FIG.
Maximum NO _X absorption amount W of the NO _X absorbent 18 on the basis of the (B)
_max is calculated. Next, at step 507, the maximum NO
_The determination level SAT is calculated by multiplying the _X absorption amount W _max by a constant value, for example, 1.1. Next, at step 508, the maximum allowable value MAX is calculated by multiplying the maximum NO _X absorption amount W _max by a constant value, for example 0.7. Next, at step 509, the deterioration determination flag is reset. When the deterioration determination flag is reset, the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio that is determined by the operating state at that time, and is normally switched to lean. Next, at step 510, S and ΣNOX are made zero.

A sixth embodiment is shown in FIGS. 33 to 36. N
The NO _X amount released from the O _X absorbent 18 per unit time is the unburned HC in the exhaust gas flowing into the NO _X absorbent 18,
It is proportional to the inflow of CO. In this case, as can be seen from FIG. 4, the amount of unburned HC and CO in the exhaust gas increases as the air-fuel ratio of the air-fuel mixture becomes richer, and therefore the NO _x absorbent 1
The amount of NO _X released per unit time from 8 increases as the air-fuel ratio of the air-fuel mixture becomes richer. Therefore, when the rich degree of the air-fuel ratio of the air-fuel mixture is increased, a large amount of NO _X is released from the NO _X absorbent 18 in a short time, so that FIG.
As shown in (A), when the current value I of the O ₂ sensor 22 rapidly decreases to zero and the air-fuel ratio of the air-fuel mixture has a low degree of richness, a large amount of NO _X is gradually discharged from the NO _X absorbent 18. 33B, the current value I of the O ₂ sensor 22 decreases to zero relatively slowly as shown in FIG.

By the way, when the air-fuel ratio of the air-fuel mixture is made rich for the purpose of releasing NO _X from the NO _X absorbent 18, as can be seen by comparing FIGS. 33 (A) and 33 (B), the degree of richness can be understood. The fuel consumption rate can be reduced by making the fuel consumption as high as possible and making the rich time as short as possible. Therefore, in the embodiment according to the present invention, NO _x absorbent 1
When NO _X is released from No. 8, as shown in FIG. 33 (A), the degree of richness of the air-fuel ratio of the air-fuel mixture is increased, and the rich time is shortened. However NO _X as it is shown in FIG. 33 (A) in this case
Even if the absorption amount is different, only a small difference occurs in the changing process of the current value I. Therefore, in such a state, if the maximum NO _x absorption amount is obtained from the difference in the changing process of the current value I, a large error will occur.

On the other hand, if the rich degree is lowered,
NO as shown in 33 (B)_XIf the absorption amount is different,
There is a large difference in the changing process of the flow value I, and therefore the maximum NO
_XThe amount of absorption can be accurately detected. Therefore this first
NO in 6 examples_XWhen judging the deterioration of the absorbent 18
As shown in FIG. 33 (B), the degree of richness is reduced.
I am trying to do it. In addition, the degree of rich like this
When it is made smaller, the degree of richness is the changing process of the current value I.
Has a large effect on the
Must be maintained to a predetermined degree. So the first
O in 6 examples ₂Rich using the output signal of the sensor 20
The air-fuel ratio is adjusted so that the degree of
It is designed to control the feedback. Then this feed
The back control will be described.

FIG. 34A shows a case where the air-fuel ratio is maintained at the stoichiometric air-fuel ratio by the feedback control I routine shown in FIG. At this time, the actual air-fuel ratio fluctuates up and down around the stoichiometric air-fuel ratio 14.6. Therefore, at this time, the average value of the actual air-fuel ratio is the theoretical air-fuel ratio 14.6.
Becomes On the other hand, FIG. 34B shows the rich integrated value KI.
The case where R'is made larger than the lean integral value KIL 'is shown. In this case, the actual air-fuel ratio fluctuates while deviating to the rich side as a whole, and the rich time and the rich degree during this time are larger than the lean time and the lean degree during this time. Therefore, at this time, the average value of the air-fuel ratio is slightly richer than the theoretical air-fuel ratio.

Therefore, in the sixth embodiment, the rich integrated value KI
By making R ′ larger than the lean integrated value KIL ′, the average value of the air-fuel ratio is slightly shifted to the rich side with respect to the theoretical air-fuel ratio. Note that the rich skip value RSR may be made larger than the lean skip value RSL (FIG. 15) in order to make the average value of the air-fuel ratio a little richer than the theoretical air-fuel ratio, and the absolute value of the minimum value TDR is the maximum. It may be larger than the value TDL (FIG. 15).

Also in the sixth embodiment, the routines shown in FIGS. 12 and 13 are used for the air-fuel ratio control, but the routines shown in FIGS. 35 and 36 are used for the deterioration processing performed in step 123 of FIG. . 35 and 36, first, at step 600, the output voltage V of the O ₂ sensor 20 is 0.4.
It is determined whether or not it is lower than the reference voltage Vr of about 5 (V). When V ≦ Vr, that is, when the air-fuel ratio is lean, the routine proceeds to step 601, where the delay count value CDL is decremented by 1. Next, at step 602, it is judged if the delay count value CDL has become smaller than the minimum value TDR. If CDL <TDR, the routine proceeds to step 603, where the CDL is set to TDR, and then the routine proceeds to step 607. Therefore, when V ≦ Vr, the delay count value CDL is gradually decreased, and then C
DL is maintained at the minimum value TDR. On the other hand, step 60
When it is determined that V> Vr at 0, that is, when the air-fuel ratio is rich, the routine proceeds to step 604, where the delay count value CDL is incremented by 1. Next, at step 605, it is judged if the delay count value CDL has become larger than the maximum value TDL, and the CDL
> When TDL is reached, the process proceeds to step 606 and the CD
After setting L to TDL, the process proceeds to step 607. Therefore V>
When it reaches Vr, the delay count value CDL is gradually increased, and then the CDL is maintained at the maximum value TDL.

In step 607, the delay count value CDL from the previous processing cycle to the current processing cycle is
It is determined whether the sign of is inverted from positive to negative or from negative to positive. When the sign of the delay count value CDL is inverted, the routine proceeds to step 608, where it is determined whether or not the inversion from positive to negative, that is, whether or not the inversion from rich to lean. When reversing from rich to lean, the routine proceeds to step 609, where the rich skip value RSR is added to the feedback correction coefficient FAF, and thus the FAF is rapidly increased by the rich skip value RSR. On the other hand, when reversing from lean to rich, step 6
Proceeding to 10, the lean skip value RSL is subtracted from the FAF, and thus the FAF is rapidly reduced by the lean skip value RSL.

On the other hand, when it is judged in step 607 that the sign of the delay count value CDL is not inverted, the routine proceeds to step 611, where the delay count value CDL
It is determined whether L is negative. When CDL ≦ 0, the routine proceeds to step 612, where the feedback correction coefficient FA
The rich integral value KIR '(KIR'> KIL ') is added to F, and as a result, as shown in FIG.
Is increased relatively quickly. On the other hand, when CDL> 0, the routine proceeds to step 613, where the lean integral value KIL 'is subtracted from FAF, and thus FAF is decreased relatively slowly as shown in FIG. 34 (B).
Next, the routine proceeds to step 614, where the elapsed time t is incremented by 1. Next, at step 615, it is judged if the current value I of the O ₂ sensor 22 is lower than the set value α (FIG. 33), and if I <α, the routine proceeds to step 616.

In step 616, the time from the elapsed time t is changed to that shown in FIG.
The deterioration degree of the NO _X absorbent 18 is calculated based on (A). Next, at step 617, from the elapsed time t, as shown in FIG.
Maximum NO _X absorption amount W of the NO _X absorbent 18 on the basis of the (B)
_max is calculated. Next, at step 618, the maximum NO
_The determination level SAT is calculated by multiplying the _X absorption amount W _max by a constant value, for example, 1.1. Next, at step 619, the maximum allowable amount MAX is calculated by multiplying the maximum NO _X absorption amount W _max by a constant value, for example 0.7. Next, at step 620, the deterioration determination flag is reset. When the deterioration determination flag is reset, the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio that is determined by the operating state at that time, and is normally switched to lean. Next, at step 621, t and ΣNOX are made zero.

37 to 40 show the seventh embodiment. N
The NO _X release rate from the O _X absorbent 18 changes according to the operating state of the engine, and changes according to the temperature of the NO _X absorbent 18. That is, even if the air-fuel ratio of the air-fuel mixture has the same degree of richness, the larger the intake air amount, that is, the larger the exhaust gas amount, the unburned HC, C flowing into the NO _x absorbent 18 per unit time.
NO _X from _the NO _X absorbent 18 in the amount of O is increased
Release rate is faster. Therefore, at this time, for example, when the elapsed time t as shown in FIG. 11 of the first embodiment is detected, this elapsed time t becomes shorter as the intake air amount increases. Further, NO nitrate ions NO enough in the absorbent temperature is high in the _X absorbent 18 ₃ ^- NO from about NO _X absorbent 18 temperature so is liable to decompose NO ₂ NO _X absorbent 18 is higher _X emission speed becomes faster. Therefore, also in this case, the elapsed time t becomes shorter as the temperature of the NO _X absorbent 18 becomes higher.

[0087] Incidentally, it is preferable to accurately detect the maximum NO _X absorption of the NO _X absorbent 18 to ensure that the elapsed time t by the temperature of the intake air amount and _the NO _X absorbent 18 is not affected. Therefore, in the seventh embodiment, the correction factors KQ and KT are added to the elapsed time t so that the elapsed time t does not change even if the intake air amount or the temperature of the NO _x absorbent 18 changes.
I am trying to multiply. Here, the correction coefficient KQ is a coefficient relating to the intake air amount, and this correction coefficient KQ is shown in FIG.
As shown in (A), the absolute pressure P in the surge tank 10
It increases as M increases, and increases as engine speed N increases. This correction coefficient KQ is the surge tank 10
37 as a function of the absolute pressure PM and the engine speed N in FIG.
It is stored in advance in the ROM 32 in the form of the map shown in FIG.

On the other hand, the correction coefficient QT is a coefficient relating to the temperature of the NO _X absorbent 18, and this correction coefficient KT is shown in FIG.
As shown in (A), it increases as the temperature T of the NO _x absorbent 18 increases. Here, since the temperature T of the NO _x absorbent 18 is determined by the operating state of the engine, the NO _x absorbent 18 that changes according to the operating state of the engine in the seventh embodiment.
Temperature T obtained in advance in the ROM 32 in the form of a map shown in FIG. 38B as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. Remembered in.

Also in the seventh embodiment, the routine shown in FIGS. 12 and 13 is used for the air-fuel ratio control, but the routine shown in FIG. 39 is used for the deterioration process performed in step 123 of FIG. Referring to FIG. 39, first, in step 700, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Next, at step 701, the feedback correction coefficient FAF is 1.
It is fixed at 0. Therefore, when the deterioration determination is started, the air-fuel ratio of the air-fuel mixture is made rich. Next, at step 702, the elapsed time t is incremented by 1. Next, at step 703, the current value I of the O ₂ sensor 22 is set to the set value α.
It is determined whether or not it is lower than (FIG. 11), and if I <α, the process proceeds to step 704. In step 704, FIG.
3B using the correction coefficient KQ calculated from the map of FIG. 7B and the temperature T obtained from the map of FIG.
By multiplying the elapsed time t by the correction coefficient KT calculated from the map of 8 (A), the final elapsed time t (= K
Q · KT · t) is calculated.

Next, at step 705, the final elapsed time
From time t based on FIG. 40 (A) NO _XDeterioration of absorbent 18
The degree is calculated. Then, in step 706, the final process
From the excess time t, based on FIG. 40 (B), NO_XOf absorbent 18
Maximum NO_XAbsorption W_maxIs calculated. Then step
Maximum NO at 707_XAbsorption W_maxA constant value, for example
The decision level SAT is calculated by multiplying by 1.1
Is done. Next, at step 708, the maximum NO_XAbsorption W
_maxIs multiplied by a constant value, for example 0.7
The maximum value MAX is calculated. Then in step 709
Indicates that the deterioration determination flag is reset. The deterioration determination flag is
When reset, the air-fuel ratio of the air-fuel mixture changes from rich to
When the air-fuel ratio is determined by the operating condition of
Be done. Next, at step 710, t and ΣNOX are zero.
It is said that

FIG. 41 shows the eighth embodiment. NO _X release rate from _the NO _X absorbent 18 and is large intake air quantity as described above is faster, NO _X release rate from _the NO _X absorbent 18 when the temperature increases of the NO _X absorbent 18 is increased.
NO _X of the NO _X release rate becomes faster when _the NO _X absorbent 18
Even if the absorption amount is different, a large difference does not occur in the changing process of the current value I. In order to accurately detect the maximum NO _x absorption amount, it is preferable to make a large difference in the changing process of the current value I. For this reason, when the intake air amount is small and the temperature of the NO _x absorbent 18 is low, the maximum NO _x absorption amount is reduced. It is preferable to detect the _X absorption amount. Therefore, in this eighth embodiment, FIG.
Using the map shown in (B) and the map shown in FIG. 38 (B), the maximum NO _x of the NO _x absorbent 18 is small when the intake air amount is small and the temperature of the NO _x absorbent 18 is low.
The amount of absorption is detected.

Also in the eighth embodiment, the routine shown in FIGS. 12 and 13 is used for the air-fuel ratio control, but the routine shown in FIG. 41 is used for the deterioration process performed in step 123 of FIG. Referring to FIG. 41, first, in step 800, as shown in FIG.
Whether correction coefficient KQ calculated from the map shown in (B) is smaller than the set value KQ _O predetermined are determined. That is, at step 800, it is judged if the operating state of the engine is the low speed low load operation with a small intake air amount. When KQ ≧ KQ _O , step 10 in FIG.
Go to 3. On the other hand, when KQ <KQ _O, the routine proceeds to step 801, where it is determined whether or not the temperature T of the NO _X absorbent 18 calculated from the map shown in FIG. 38 (B) is lower than a preset temperature T _O. Is determined. When T ≧ T _O, the routine proceeds to step 103 in FIG. On the other hand, T <
When it is T _O, the routine proceeds to step 802.

In step 802, the correction coefficient K is set to a constant value KK of, for example, about 1.3. Then step 80
In 3, the feedback correction coefficient FAF is fixed to 1.0. Therefore, at this time, the air-fuel ratio of the air-fuel mixture is made rich. Next, the routine proceeds to step 804, where the elapsed time t is incremented by 1. Then, in step 805, O ₂
It is determined whether or not the current value I of the sensor 22 is lower than the set value α (FIG. 11), and if I <α, step 806
Proceed to.

At step 806, the process shown in FIG.
The deterioration degree of the NO _X absorbent 18 is calculated based on (A). Next, at step 807, from the elapsed time t, as shown in FIG.
Maximum NO _X absorption amount W of the NO _X absorbent 18 on the basis of the (B)
_max is calculated. Next, at step 808, the maximum NO
_The determination level SAT is calculated by multiplying the _X absorption amount W _max by a constant value, for example, 1.1. Next, at step 809, the maximum allowable amount MAX is calculated by multiplying the maximum NO _X absorption amount W _max by a constant value, for example, 0.7. Next, at step 810, the deterioration determination flag is reset. When the deterioration determination flag is reset, the air-fuel ratio of the air-fuel mixture is switched from rich to an air-fuel ratio that is determined by the operating state at that time, and is normally switched to lean. Next, at step 811, t and ΣNOX are made zero.

[0095]

The degree of deterioration of the NO _X absorbent can be accurately detected.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a map of a basic fuel injection time.

FIG. 3 is a diagram illustrating a correction coefficient K;

FIG. 4 Unburned HC and C in exhaust gas discharged from the engine
It is a diagram which shows the concentration of O and oxygen roughly.

FIG. 5 is a diagram for explaining the action of NO _X absorption and release.

FIG. 6 NO _X absorption amount NOXA and NO _X release amount NO
It is a figure which shows XD.

FIG. 7 is a diagram showing a NO _X release amount NOXD.

FIG. 8 is a time chart of air-fuel ratio control.

FIG. 9 is a diagram showing a current value flowing between an anode and a cathode of an O ₂ sensor.

FIG. 10 is a time chart showing changes in the current value flowing between the anode and the cathode of the O ₂ sensor.

FIG. 11 is a time chart showing changes in the current value flowing between the anode and the cathode of the O ₂ sensor.

FIG. 12 is a flowchart for controlling the air-fuel ratio.

FIG. 13 is a flowchart for controlling the air-fuel ratio.

FIG. 14 is a flowchart for performing feedback control I.

FIG. 15 is a time chart showing changes in the feedback correction coefficient FAF.

FIG. 16 is a flowchart for performing feedback control II.

FIG. 17 is a flow chart for performing NO _X release processing.

FIG. 18 is a flowchart for performing deterioration determination.

19 is a diagram showing a deterioration degree and the maximum NO _X absorption of the NO _X absorbent.

FIG. 20 is a time chart showing changes in the current value I for explaining the deterioration detecting method in the second embodiment.

FIG. 21 is a flow chart for performing deterioration determination in the second embodiment.

22 is a diagram showing a deterioration degree and the maximum NO _X absorption of the NO _X absorbent.

FIG. 23 is a time chart showing changes in the current value I for explaining the deterioration detecting method in the third embodiment.

FIG. 24 is a flow chart for performing a NO _X releasing process in the third embodiment.

FIG. 25 is a flowchart for performing deterioration determination in the third embodiment.

26 is a diagram showing a deterioration degree and the maximum NO _X absorption of the NO _X absorbent.

FIG. 27 is a time chart showing changes in the current value I for explaining the deterioration detecting method in the fourth embodiment.

FIG. 28 is a flow chart for performing deterioration determination in the fourth embodiment.

29 is a diagram showing a deterioration degree and the maximum NO _X absorption of the NO _X absorbent.

FIG. 30 is a time chart showing changes in the current value I for explaining the deterioration detecting method in the fifth embodiment.

FIG. 31 is a flow chart for performing deterioration determination in the fifth embodiment.

32 is a diagram showing a deterioration degree and the maximum NO _X absorption of the NO _X absorbent.

FIG. 33 is a time chart showing changes in the current value I for explaining the deterioration detection method in the sixth embodiment.

FIG. 34 is a time chart showing changes in the feedback correction coefficient.

FIG. 35 is a flow chart for performing deterioration determination in the sixth embodiment.

FIG. 36 is a flowchart for making deterioration determination in the sixth embodiment.

FIG. 37 is a diagram showing a correction coefficient KQ.

FIG. 38 is a diagram showing a correction coefficient KT.

FIG. 39 is a flowchart for performing deterioration determination in the seventh embodiment.

40 is a diagram showing a deterioration degree and the maximum NO _X absorption of the NO _X absorbent.

FIG. 41 is a flowchart for making a deterioration determination in the eighth embodiment.

[Explanation of symbols]

15 ... Exhaust manifold 18 ... NO _X absorbent 20, 22 ... O ₂ sensor

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location F01N 3/24 ZAB F01N 3/24 ZABR F02D 41/04 305 F02D 41/04 305Z (72) Inventor Tetsu Aichi 1 Toyota Town, Toyota City, Japan Toyota Motor Co., Ltd.

Claims (12)

[Claims] 1. A fuel ratio of the inflowing exhaust gas is absorbed NO _X when the lean air-fuel ratio of the inflowing exhaust gas to release NO _X absorbed and becomes rich _the NO _X absorbent to the engine exhaust passage arranged in an internal combustion engine air-fuel ratio sensor arranged in _the NO _X absorbent in the exhaust passage downstream of which generates an output level proportional to the air-fuel ratio of the exhaust gas, it is estimated to be absorbed in _the NO _X absorbent and _the amount of NO _X estimating means for obtaining an that the estimated amount of NO _X, whether NO _X absorption amount of the NO _X absorbent exceeds the above estimated amount of NO _X a determination level can be estimated to have become the maximum NO _X absorption discriminating means for discriminating, the estimated amount of NO _X is switched from lean to rich the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent in order to release the NO _X from _the NO _X absorbent when exceeds the determination level Equipped with air-fuel ratio switching means, exhaust gas When the air-fuel ratio of the gas is switched from lean to rich, the output level of the air-fuel ratio sensor corresponds to the lean air-fuel ratio through a change process of different output levels according to the amount of NO _X absorbed in the NO _X absorbent. Deterioration judging means for judging the degree of deterioration of the NO _x absorbent based on the difference in the output level changing process from the output level to the output level corresponding to the rich air-fuel ratio, and the deterioration of the NO _x absorbent. An exhaust gas purification apparatus for an internal combustion engine, comprising: a judgment level updating means for updating the judgment level according to the degree. 2. The greater the degree of deterioration of the NO _X absorbent, the shorter the time required to change from the output level corresponding to the lean air-fuel ratio to the output level corresponding to the rich air-fuel ratio, and the deterioration determining means is NO _X absorption. The elapsed time of a predetermined period of time from the time when the air-fuel ratio of the exhaust gas flowing into the agent is changed from lean to rich until the output level of the air-fuel ratio sensor becomes the output level corresponding to the rich air-fuel ratio. The exhaust emission control device for an internal combustion engine according to claim 1, wherein it is determined that the shorter the degree, the greater the degree of deterioration of the NO _x absorbent. 3. The output level of the air-fuel ratio sensor is substantially equal to the output level corresponding to the rich air-fuel ratio from the time when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from lean to rich for the predetermined period. The exhaust emission control device for an internal combustion engine according to claim 2, which is a period until the coincidence. 4. The output level of the air-fuel ratio sensor corresponds to the rich air-fuel ratio after a lapse of a certain period after the predetermined period has been changed from lean to rich in the exhaust gas flowing into the NO _x absorbent. The exhaust emission control system for an internal combustion engine according to claim 2, wherein the exhaust purification device is for a period until the output level substantially matches the output level. NO _X after the 5. A fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from lean to rich
Immediately before the air-fuel ratio of the exhaust gas flowing out of the absorbent changes from lean to rich, the output level suddenly changes, and the predetermined period is from the lean air-fuel ratio of the exhaust gas flowing into the NO _x absorbent. The exhaust emission control device for an internal combustion engine according to claim 2, which is a period from a lapse of a certain period after switching to the rich state to a rapid change in the output level. 6. The change rate of the output level when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from lean to rich, the greater the degree of deterioration of the NO _X absorbent becomes, the greater the degree of deterioration becomes. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the determination means determines that the degree of deterioration of the NO _X absorbent increases as the rate of change of the output level increases. 7. The above-mentioned output level during the period until the output level of the air-fuel ratio sensor after switching the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent from lean to rich reaches the output level corresponding to the rich air-fuel ratio. becomes smaller as the time integral value increases the degree of deterioration of the NO _X absorbent, in claim 1, the above deterioration determining means determines that the degree of deterioration of the NO _X absorbent as the time integral value of the output level is reduced is large An exhaust gas purification device for an internal combustion engine as described. 8. After the deterioration judgment means judges that the NO _x absorbent has deteriorated, the deterioration judgment means again makes N.
NO in until deterioration determination of O _X absorbent it is performed
The release action of NO _X from the _X absorbent is performed, and this NO _X is released.
Compared to the degree of the rich air-fuel ratio when the releasing action is performed to claim 1 which is adapted to reduce the degree of rich air-fuel ratio when the deterioration determination of the NO _X absorbent by said deterioration determination means is made of An exhaust gas purification device for an internal combustion engine as described. 9. Place another air-fuel ratio sensor in _the NO _X absorbent in the engine exhaust passage upstream of, NO by the deterioration determination means
_The air-fuel ratio is feedback-controlled based on the output signal of the other air-fuel ratio sensor so that the degree of richness of the air-fuel ratio becomes a predetermined degree when the deterioration determination of the _X absorbent is performed. Exhaust gas purification device for internal combustion engine. After 10. A deterioration determination of the NO _X absorbent by said deterioration determining means is carried out, during the period until again deterioration determination of the NO _X absorbent by said deterioration determining means is carried out N
The release of NO _X from the O _X absorbent is performed, and this NO
_2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the cycle in which the release action of _X is performed is made shorter as the degree of deterioration of the NO _X absorbent increases. 11. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the deterioration determining means determines whether the NO _X absorbent is deteriorated when the engine is operating in a predetermined state suitable for determining the deterioration. 12. The representative value representing the difference in the changing process of the output level of the air-fuel ratio sensor is substantially the same for the same NO _x absorption amount of the NO _x absorbent regardless of the operating state of the engine. The exhaust emission control device for an internal combustion engine according to claim 1, further comprising a correction unit that corrects the representative value according to an operating state of the engine.