JPH07139397A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To timely discharge NOX, absorbed by an NOx absorbent, from the NOX absorbent CONSTITUTION:An NOX absorbent 18 to absorb NOx when an air-fuel ratio of inflow exhaust gas is lean and discharge absorbed NOX when the air-fuel ratio of the inflow exhaust gas is a theoretical air-fuel ratio or rich is arranged in an engine exhaust passage. An engine running region where lean fuel-air mixture is burnt and a running region where fuel air mixture having a theoretical air-fuel ratio is burnt are predetermined. When an NOX absorbing amount of the NOX absorbent 18 exceeds a specified amount, an engine running region wherein fuel-air mixture having a theoretical air-fuel ratio is burnt is increased.

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]

2. Description of the Related Art When the air-fuel ratio of the exhaust gas that flows in is lean,
No when_xAnd the air-fuel ratio of the inflowing exhaust gas is
The absorbed NO when the air-fuel ratio or rich_xEmit
NO _xThe absorbent is placed in the engine exhaust passage and NO_xabsorption
Agent to NO_xNO when should be released_xInflow into absorbent
The air-fuel ratio of the exhaust gas to be changed from lean to stoichiometric air-fuel ratio or
Switch for a predetermined time, then NO_xabsorption
To return the air-fuel ratio of the exhaust gas flowing into the agent to lean again
The internal combustion engine that has been modified has already been proposed by the applicant.
(See International Publication WO93 / 07363).

[0003]

In this internal combustion engine,
NO_xThe release action of NO is basically NO_xTo some extent on the absorbent
NO_xNO is absorbed_xAbsorbed by the absorbent
NO_xBased on the idea of letting all
Therefore, in this internal combustion engine, NO _xabsorption
Agent to NO_xNO when should be released_xAbsorbed in absorbent
All NO being done _xN for the time required to release
O_xThe air-fuel ratio of the exhaust gas flowing into the absorbent is controlled from lean.
The theoretical air-fuel ratio or rich is selected.

However, NO_xAbsorbent to NO_xLet go
NO when it should be issued_xAbsorbent to all NO_xDon't let go
NO if partially released_xThe absorbent is good again
NO _xWill be absorbed, and NO_xAbsorbent is
NO because it also functions as a three-way catalyst_xAbsorbent to total N
O_xThe air-fuel ratio is maintained at the stoichiometric air-fuel ratio continuously after
NO even if_xIs NO_xPurify with absorbent
To be Moreover, if the air-fuel mixture is made rich to some extent,
Almost no from the institution_xIs not emitted, so NO_xabsorption
All NO from agent_xKeeps the air-fuel ratio rich after
Almost NO_xIs not released into the atmosphere
Yes. Thinking this way is NO_xNO_xRelease from absorbent
NO when it should_xAbsorbent to all NO_xTo release
The air-fuel ratio to the stoichiometric air-fuel ratio or rich only for the time required for
Is not always necessary and therefore NO
From scavenger to NO_xAnother perspective on how to release
You will be able to think of another method.

An object of the present invention is to provide an exhaust emission control device which releases NO _x from a NO _x absorbent based on a concept which is basically different from the method adopted in the above-mentioned internal combustion engine.

[0006]

That is, according to the present invention, NO _x is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and it is absorbed when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio or rich. the _the NO _x absorbent to release the NO _x arranged in the engine exhaust passage, the engine the NO _x discharged from the engine burned lean mixture when the operating condition is a lean combustion region set in advance _the NO _x absorbent When the engine operating condition is in a predetermined stoichiometric air-fuel ratio or rich combustion region, the air-fuel mixture or the rich air-fuel mixture having the stoichiometric air-fuel ratio is burned and absorbed in the NO _x absorbent.
In an internal combustion engine which is adapted to release _x, and means for estimating the amount of NO _x is absorbed in the NO _x absorbent, NO
_{When the} NO _x amount estimated to be absorbed by the _x absorbent exceeds a predetermined amount, the lean combustion region is narrowed and the stoichiometric air-fuel ratio or the rich combustion region is widened.

Further, according to the present invention, the above-mentioned lean combustion region is set to a lower load side than the stoichiometric air-fuel ratio or rich combustion region, and the NO _x amount estimated to be absorbed by the NO _x absorbent is When the amount exceeds a predetermined amount, the lean combustion region is narrowed to the low load side and the stoichiometric air-fuel ratio or the rich combustion region is widened to the low load side by the region changing means.

Furthermore when the air-fuel ratio is lean of the exhaust gas flowing in accordance with the present invention absorbs the NO _x, when the air-fuel ratio of the inflowing exhaust gas of the stoichiometric air-fuel ratio or rich to release the absorbed NO _x NO _x The absorbent is placed in the engine exhaust passage, and when the engine operating state is in the predetermined lean combustion region, the lean air-fuel mixture is burned and the NO _x discharged from the engine is absorbed by the NO _x absorbent, so that the engine operating state. in There internal combustion engine so as to release the NO _x absorbed in _the NO _x absorbent by burned air-fuel mixture or a rich mixture of the theoretical air-fuel ratio to the stoichiometric air-fuel ratio or rich combustion region predetermined, NO lean combustion region when it exceeds a means for estimating the amount of NO _x absorbed in the _x absorbent, the amount of _the amount of NO _x is a predetermined estimated to be absorbed in _the NO _x absorbent And a region changing means for widening the stoichiometric air-fuel ratio or the rich combustion region to make the entire combustion region the stoichiometric air-fuel ratio or the rich combustion region.

[0009]

In the first aspect of the invention, when the NO _x amount estimated to be absorbed by the NO _x absorbent exceeds a predetermined amount, the lean combustion region is narrowed and the stoichiometric air-fuel ratio or the rich combustion region is reduced. Since it is spread, the chances of releasing NO _x from the NO _x absorbent are increased and NO
_The chances of NO _x being absorbed by the _x absorbent are reduced.

In the second invention, the lean combustion region is theoretical
Defined on the low load side of the air-fuel ratio or rich combustion range
And NO_xNO estimated to be absorbed by the absorbent
_xWhen the amount exceeds a predetermined amount, the lean burn region
The region is narrowed to the lower load side and the theoretical air-fuel ratio or
The combustion area is expanded to the low load side. Theoretical air-fuel ratio or
NO when the rich combustion region is expanded to the low load side_xAbsorbent
To NO_xThe opportunity to be released is increased and
NO emitted from the engine as the duty decreases _xAbsolute of
Since the amount is small, the lean combustion region is narrowed to the lower load side.
NO when you get it _xNO for absorbent_xLess chance of being absorbed
NO and less_xNO for absorbent_xAs is absorbed
However, the amount is extremely small.

In the third aspect of the invention, when the NO _x amount estimated to be absorbed by the NO _x absorbent exceeds a predetermined amount, the lean combustion region is eliminated and the stoichiometric air-fuel ratio or rich combustion region is widened. is _the NO _x absorbent from the NO _x in the entire range of operation since the entire combustion zone is the stoichiometric air-fuel ratio or rich combustion region is released.

[0012]

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 comprises a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 which are mutually connected by a bidirectional bus 31. And an output port 36. 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 AD converter 37. . A throttle switch 20 that is turned on when the throttle opening reaches an idling opening is attached to the throttle valve 14, and an output signal of the throttle switch 20 is input to an input port 35. An air-fuel ratio sensor 21 is arranged in the exhaust manifold 15, and the output voltage of the air-fuel ratio sensor 21 is input to the input port 35 via the AD converter 38. Further, the input port 35 is connected to the rotation speed sensor 22 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 36 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 = f · TP · K · FAF where f is a constant, TP is the basic fuel injection time, K is a correction coefficient, and FAF is a feedback correction coefficient. 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 obtained in advance by experiments and is stored in advance in the ROM 32 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. 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, K <
When 1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and when K> 1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes empty. The fuel ratio becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The feedback correction coefficient FAF is K = 1.
When 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, a coefficient for accurately matching the air-fuel ratio with the stoichiometric air-fuel ratio based on the output signal of the air-fuel ratio sensor 21. Is. This feedback correction coefficient FAF
Moves up and down about 1.0, and this FAF
Decreases when the mixture becomes rich, and increases when the mixture becomes lean. When K <1.0 or K> 1.0, FAF is fixed at 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 according to the operating state of the engine. In the embodiment of 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, ce
Alkali metals such as sium Cs, barium Ba, cal
Alkaline earths such as 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 air and fuel (hydrocarbons) fed is 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 exhaust passage of the engine, the NO _x absorbent 18 actually acts to absorb and release NO _x , but the detailed mechanism of this absorbing and releasing effect is not clear. There is also. However, it is considered that this absorbing / releasing action is performed by the mechanism 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.

The oxygen concentration in the inflowing exhaust gas is generated NO ₂ on the surface of as high as platinum Pt, as long as NO ₂ of absorption of NO _x capacity of the absorbent is not saturated is absorbed in the absorbent and nitrate ions NO ₃ ^- Is generated. In contrast the reaction with the amount of NO ₂ oxygen concentration is lowered in the inflowing exhaust gas is lowered backward (NO ₃ ^- → NO ₂₎ proceeds to, thus nitrate ions to the absorber NO ₃ ^- Are released from the absorbent in the form of NO ₂ . Namely, when the oxygen concentration in the inflowing exhaust gas is released NO _x from the NO _x absorbent 18 when lowered. As shown in FIG. 4, when the lean degree of the inflowing exhaust gas is low, the oxygen concentration in the inflowing exhaust gas is low. Therefore, even if the leaning degree of the inflow exhaust gas is low, the air-fuel ratio of the inflowing exhaust gas is lean. Even NO _x absorbent 18
Will release NO _x .

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG. 4, a large amount of unburned H
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 this released NO
_xNO is reduced to the atmosphere_xIs discharged
You will be able to prevent it. Also, NO_xAbsorbent
Since 18 has the function of a reduction catalyst,
NO even if the air-fuel ratio is the theoretical air-fuel ratio_xRelease from absorbent 18
NO done_xIs reduced. However, inflow and outflow
NO when the air-fuel ratio of air gas is set to the theoretical air-fuel ratio_xabsorption
Agent 18 to NO_xNO is released only gradually
_xAll NO absorbed in the absorbent 18_xTo release
Takes a little longer.

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 absorption of NO _x capacity of the NO _x absorbent 18, _the NO _x absorbent 18 when saturation absorption of NO _x capacity of the NO _x absorbent 18 is not E longer absorb NO _x.
Therefore NO absorption _of NO _x capacity of the _x absorbent 18 must be released the NO _x from the NO _x absorbent 18 before the saturation, how much to _the NO _x absorbent 18 to the NO _x
It is necessary to estimate whether is absorbed. Then this N
A method of estimating the O _x absorption amount will be described.

[0025] is _the amount of NO _x absorbed per 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 absorbent per unit time for increasing _x amount 1
The NO _x absorbed in 8 increases. Thus _the amount of NO _x absorbed per unit time per _the NO _x absorbent 18 becomes 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, N per unit time
The NO _x amount NOXA absorbed by the O _x absorbent 18 is previously obtained as a function of the absolute pressure PM and the engine speed N, and this NO _x amount NOXA is a function of PM and N. The map shown in FIG. Is stored in the ROM 32 in advance.

On the other hand, 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 _the NO _x releasing 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 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
_The amount of _x increases. In this case, the amount of exhaust gas, that is, the amount of intake air can be represented by the product of the engine speed N and the absolute pressure PM in the surge tank 10, and therefore per unit time as shown in FIG. 6 (B). _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. 6 (C). To do. The NO _x amount NOXD released from the NO _x absorbent 18 per unit time is stored in advance in the ROM 3 in the form of a map shown in FIG. 7A as a function of N · PM and K.
It is stored in 2.

Further, _the NO _x absorbent 18 nitrate ions NO ₃ and the absorbent temperature is high in ^- that _the NO _x releasing rate from _the NO _x absorbent 18 is increased so easily decomposed. In this case, since the temperature of the NO _x absorbent 18 is almost proportional to the exhaust gas, the NO _x release rate Kf as shown in FIG.
Becomes larger as the exhaust gas temperature T becomes higher. Therefore NO
_{x When the} emission rate Kf is taken into consideration, NO per unit time
_The amount of NO _x released from the _x absorbent 18 is represented by the product of NO XD and NO _x release rate Kf shown in FIG. 7 (A). In the embodiment according to the present invention, the exhaust gas temperature T is previously RO in the form of a map shown in FIG. 7C as a function of the absolute pressure PM in the surge tank 10 and the engine speed N.
It is stored in M32.

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 Kf NOXD_xSoak up in absorbent 18
NO estimated to have been collected_xThe amount ΣNOX is represented by the following formula
Will be forgotten.

ΣNOX = ΣNOX + NOXA−Kf · NOXD As described above, in the embodiment according to the present invention, basically, FIG.
At A, the air-fuel ratio is controlled according to the value of the correction coefficient K distinguished by the solid lines R and S. Therefore, in the region on the lower load side than the solid line R in FIG. 3, the lean air-fuel mixture (K <1.0) is burned, so NO _x is absorbed by the NO _x absorbent 18, and the load higher than the solid line R in FIG. In the region on the side of, the stoichiometric air-fuel ratio mixture (K = 1.0) or rich mixture (K> 1.
0) is burned, so NO _x from NO absorbent 18
Will be released. Thus by the solid line R in FIG. 3 in Sakai low-load operation and high-load operation are alternately repeated when the NO _x
Absorption of NO _x capacity of the absorbent 18 without saturation, when the lean air-fuel mixture has been burned and thus NO _x
The NO _x is well absorbed in the absorbent 18.

[0030] However for example the operating state continues as indicated by the point Y in FIG. 3, therefore absorption of NO _x capacity of the lean mixture combustion is continued _the NO _x absorbent 18 is saturated. Therefore, in the embodiment according to the present invention, the lean air-fuel mixture combustion is continuously performed and the N adsorbed in the NO _x absorbent 18 is absorbed.
When the amount of O _x ΣNOX exceeds a predetermined amount, a region (K = 1.
The region (0) is expanded to the low load side up to the region indicated by the broken line T, and the region where the lean air-fuel mixture is burned (K <1.0 region) is narrowed to the region lower than the broken line T.
Therefore, when the operating state indicated by the point Y in FIG. 3 continues, the lean air-fuel mixture is switched to the stoichiometric air-fuel ratio when the NO _x amount ΣNOX exceeds a predetermined amount, and therefore NO
_The action of releasing NO _x from the _x absorbent 18 is started.

On the other hand, even if the operating region (the region of K = 1.0) where the air-fuel ratio is the stoichiometric air-fuel ratio is widened by the region shown by the broken line T, the region where the lean air-fuel mixture should be burned (K <
1.0) remains, so if the lean air-fuel mixture is burned in this region (K <1.0), NO _x absorbent 1
The absorption of NO _{x on} 8 is performed. However the NO _x absorbing capability of the NO so _x amount is very small even when the operating region of the low load side is performed than the broken line T of Figure 3 NO _x absorbent 18 to be discharged from the engine is saturated when the low-load operation without first, nO _x amount of absorption of nO _x capacity of the nO _x absorbent 18 is even released into the atmosphere even if saturation does not occur particularly large problem because very small. In this case, NO _x
In order to completely prevent the NO _x absorption capacity of the absorbent 18 from being saturated, when the NO _x amount ΣNOx exceeds a predetermined amount, all the regions on the low load side from the solid line S of FIG. It may be set as the combustion region (K = 1.0) of the air-fuel mixture.

The map is used to switch the air-fuel ratio between the solid line R and the broken line T shown in FIG. That is, the value of the correction coefficient K divided by the solid line S and the solid line R in FIG. 3 is a function of the absolute pressure PM in the surge tank 10 and the engine speed N in the form of the map I shown in FIG.
The value of the correction coefficient K stored in FIG. 3 and divided by the solid line S and the broken line T in FIG. 3 is a map II shown in FIG. 8B as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. Is stored in the ROM 32 in advance.
Therefore, normally, the correction coefficient K is determined based on the map I shown in FIG. 8A, and when the NO _x amount ΣNOX exceeds a predetermined amount, the correction coefficient K is determined based on the map II shown in FIG. 8B. Is determined.

Next, the air-fuel ratio control will be described in more detail with reference to FIG. For example _the amount of NO _x ΣNO operating conditions have continued combustion of the lean air-fuel mixture to be absorbed in _the NO _x absorbent 18 when the being performed indicated at a point Y in FIG. 3
X gradually increases. Next, at t ₁ , the amount of NO _x ΣN
When OX reaches a predetermined first upper limit value MAX1, the map defining the correction coefficient K is switched from the map I in FIG. 8A to the map II in FIG. 8B. As a result, the air-fuel ratio changes from lean to the theoretical air-fuel ratio, and thus the NO _x amount Σ
NOX gradually decreases. Next, when the NO _x amount ΣNOX becomes smaller than a predetermined second upper limit value MAX2 (<MAX1) and the throttle valve is closed to the idling opening degree (t _{2 in} FIG. 9), the correction coefficient The map defining K is switched from Map II to Map I. This is repeated below.

[0034] After the amount of NO _x NOX when the operating state at the point Y in FIG. 3 continues maps is larger than the first upper limit value MAX1 is switched from the map I on the map II, NO _x the amount NOX is will first immediately _the amount of NO _x NOX again map switch the map II on the map I when it is smaller than the upper limit value MAX1 exceeds the first upper limit value MAX1, lean and theory was thus The air-fuel ratio will be repeatedly switched in a short cycle. Thus, in order to prevent the lean and stoichiometric air-fuel ratios from being repeatedly switched in a short cycle, in the embodiment according to the present invention, after the map is switched from the map I to the map II, the NO _x amount ΣNOX.
The map is not switched from the map II to the map I unless the value becomes less than the second upper limit MAX2.

FIG. 10 and FIG. 11 show a routine for controlling the air-fuel ratio, and this routine is executed by interruption at regular time intervals. Referring to FIG. 10 and FIG. 11, first, at step 50, the NO _x amount ΣNOx absorbed in the NO _x absorbent 18 is the upper limit value MAX.
Is determined to be greater than or equal to. This upper limit value MAX is the first value as an initial value during initialization processing at the time of engine start.
Is set to the maximum value MAX1. When ΣNOX ≤ MAX, the routine proceeds to step 51, where it is judged if the flag F indicating that the map II (Fig. 8 (B)) should be used is set or not. If the flag F is not set, the process jumps to step 54 and the map I shown in FIG.
The correction coefficient K is calculated from Next, at step 55, the maximum value MAX is made the first maximum value MAX1, and then the routine proceeds to step 56.

At step 56, the basic fuel injection time TP is calculated from the map shown in FIG. Then in Step 57.
Then, it is determined whether or not the correction coefficient K is smaller than 1.0. When K <1.0, that is, in the operating state in which the lean air-fuel mixture should be burned, the routine proceeds to step 58, where the NO _x absorption amount NOXA per unit time is calculated from the map shown in FIG. 6 (A). Next, at step 59, the NO _x release amount NO
XD is made zero, and then in step 60, the feedback correction coefficient FAF is fixed to 1.0. Next, at step 61, the fuel injection time TAU is calculated based on the following equation.

TAU = f · TP · K · FAF On the other hand, when it is judged at step 57 that K ≧ 1.0, the routine proceeds to step 62, where NO _x released per unit time is calculated from the map shown in FIG. 7 (A). The quantity NOXD is calculated. Next, at step 63, the NO _x release rate Kf is calculated from the relationship shown in FIG. 7 (B) and the map shown in FIG. 7 (C), and then at step 64, the NO _x absorption amount NOXA per unit time is made zero. Next, at step 65, it is judged if the correction coefficient K is larger than 1.0. K>
When it is 1.0, that is, in the operating state in which the rich air-fuel mixture is to be burned, the routine proceeds to step 61 through step 60.

On the other hand, when K = 1.0, that is, when the stoichiometric air-fuel ratio air-fuel mixture is to be burned, the routine proceeds to step 66, where the feedback correction coefficient FAF is calculated based on the output signal of the air-fuel ratio sensor 21, and then, Go to step 61. In step 66, FAF is decreased when the air-fuel ratio sensor 21 detects that the air-fuel ratio has become rich, and FA is detected when it is detected that the air-fuel ratio has become lean.
Since AF is increased, the air-fuel ratio is maintained at the stoichiometric air-fuel ratio.

In step 67 following step 61, the NO _x amount Σ absorbed in the NO _x absorbent 18 is calculated based on the following equation.
NOX is calculated. ΣNOX = ΣNOX + NOXA−Kf · NOXD Next, at step 68, it is judged if the NO _x amount ΣNOX becomes negative. If ΣNOX <0, the routine proceeds to step 69, where ΣNOX is made zero.

On the other hand, in step 50, ΣNOX> M
If it is determined that AX (= MAX1), step 70
8 and the flag F is set, then the routine proceeds to step 71, where the correction coefficient K is calculated from the map II shown in FIG. 8 (B). That is, the map for determining the correction coefficient K is switched from the map I to the map II. Then step 72
Then, MAX is set to the second maximum value MAX2, and step 5
Go to 6. When MAX = MAX2, ΣNOX ≦ MA
Steps 70, 71, 72 until X (= MAX2)
And proceeds to step 56, where ΣNO ≦ MAX (= MAX
When it becomes 2), the process proceeds to step 51. At this time, since the flag F is set, the routine proceeds to step 52, where it is judged based on the output signal of the throttle switch 20 whether or not the throttle valve 14 is closed to the idling opening degree, that is, whether or not the deceleration operation is performed. To be done. When the throttle valve 14 continues to open, the routine proceeds to step 71.

On the other hand, when the throttle valve 14 is closed to the idling opening degree, the routine proceeds to step 53, where the flag F is reset, then the routine proceeds to step 54, where the correction coefficient is calculated from the map I shown in FIG. 8 (A). K is calculated. That is, the map for determining the correction coefficient K is switched from the map II to the map I. Then step 55
Then, MAX is set to the first maximum value MAX1 again.

[0042]

[Effect of the Invention] first of the NO _x absorbing capability of the NO _x absorbent so opportunities absorption of the NO _x is made to decrease with opportunities NO _x emissions can be made to increase the present invention that can be inhibited from saturated it can. Amount of NO _x released into the atmosphere even if the absorption of NO _x capacity of the NO _x absorbent is saturated with absorption of NO _x capacity of the NO _x absorbent in the second invention it is possible to further suppress the saturation Can be extremely small.

The absorption of NO _x capacity of the NO _x absorbent in the third invention can be completely prevented from saturation.

[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 showing a correction coefficient K.

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

FIG. 5 is a diagram for explaining the action of absorbing and releasing NO _x .

FIG. 6 is a diagram showing a NO _x absorption amount NOXA and the like.

FIG. 7 is a diagram showing a NO _x release amount NOXD and the like.

FIG. 8 is a diagram showing a map of a correction coefficient K.

FIG. 9 is a time chart of air-fuel ratio control.

FIG. 10 is a flowchart showing air-fuel ratio control.

FIG. 11 is a flowchart showing air-fuel ratio control.

[Explanation of symbols]

16 ... Exhaust pipe 18 ... NO _x absorbent

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F01N 3/28 ZAB 301 C F02D 41/04 301 C 8011-3G

Claims (3)

[Claims] 1. When the air-fuel ratio of the inflowing exhaust gas is lean
No when_xAnd the air-fuel ratio of the inflowing exhaust gas is
The absorbed NO when the air-fuel ratio or rich _xEmit
NO_xPlace the absorbent in the engine exhaust passage to
If the state is in the lean combustion region that is set in advance, lean mixing is performed.
NO emitted from the engine after burning Aiki_xNO_x
Absorb with an absorbent to keep the engine operating condition
Theoretical air-fuel ratio or rich combustion region
NO by burning Aiki or rich mixture_xAbsorb with absorbent
NO is stored_xTo the internal combustion engine that is designed to release
By the way, NO_xNO absorbed in the absorbent_xEstimate quantity
Means to do, NO_xPresumed to be absorbed by the absorbent
NO_xWhen the amount exceeds the predetermined amount, the above
The stoichiometric air-fuel ratio or rich
Of an internal combustion engine equipped with a region changing means for expanding the combustion region
Exhaust purification device. 2. The lean combustion region is defined on the lower load side than the stoichiometric air-fuel ratio or the rich combustion region, and NO _x
When the NO _x amount estimated to be absorbed by the absorbent exceeds a predetermined amount, the region changing means narrows the lean combustion region to a lower load side and lowers the theoretical air-fuel ratio or the rich combustion region. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust emission control device is expanded to the load side. 3. When the air-fuel ratio of the inflowing exhaust gas is lean
No when_xAnd the air-fuel ratio of the inflowing exhaust gas is
The absorbed NO when the air-fuel ratio or rich _xEmit
NO_xPlace the absorbent in the engine exhaust passage to
If the state is in the lean combustion region that is set in advance, lean mixing is performed.
NO emitted from the engine after burning Aiki_xNO_x
Absorb with an absorbent to keep the engine operating condition
Theoretical air-fuel ratio or rich combustion region
NO by burning Aiki or rich mixture_xAbsorb with absorbent
NO is stored_xTo the internal combustion engine that is designed to release
By the way, NO_xNO absorbed in the absorbent_xEstimate quantity
Means to do, NO_xPresumed to be absorbed by the absorbent
NO_xWhen the amount exceeds the predetermined amount, the above
The stoichiometric air-fuel ratio or rich
Expand the combustion range to cover the entire combustion range with stoichiometric air-fuel ratio or rich combustion.
Exhaust of an internal combustion engine provided with a region changing means for setting a combustion region
Purification device.