JPH06229230A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To inhibit emission of NOX discharged from NOX absorbent, into the atmosphere while preventing the NOX absorbent from being poisoned by sulfur. CONSTITUTION:An NOX absorbent 19 for absorbing NOX in a lean condition and for discharging absorbed NOX in a rich condition is located in an engine exhaust passage, and an SOX absorbent 18 carrying metal having an oxidization- reduction function, for absorbing SOX in a lean condition and for discharging absorbed SOX in a rich condition is located in the engine exhaust gas passage, upstream of the NOX absorbent 19. Means for obtaining a degree of richness adapted to be decreased by the reaction of oxidization-reduction in the SOX absorbent 18 when the air-fuel ratio is set in a rich condition is provided. The degree of rich when the SOX being discharged from the SOX absorbent 18 and NOX 19 being to be discharged from the NOX absorbent 19 is increased by a degree of richness decreased by the reaction of oxidization-reduction.

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]

In the internal combustion engine which is adapted allowed to combust a lean air-fuel mixture, the air-fuel ratio of the inflowing exhaust gas is absorbed NO _x when the lean, the NO _x concentration of oxygen absorbed to decrease in the inflowing exhaust gas emission the _the NO _x absorbent arranged in the engine exhaust passage, the NO _x generated when burned lean mixture is absorbed by _the NO _x absorbent before the absorption of NO _x capacity of the NO _x absorbent is saturated to temporarily already proposed by the internal combustion engine is the applicant which is adapted to reduce the released NO _x with the release of NO _x in the rich from _the NO _x absorbent the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent Has been done.

However, in the fuel and the lubricating oil of the engine,
SO is included in the exhaust gas because it contains OH._xIncluded
Therefore, in this internal combustion engine, this SO_xAlso NO_xWhen
Both NO_xIt is absorbed by the absorbent. However, this SO
_xIs NO_xRich air-fuel ratio of exhaust gas flowing into the absorbent
Even if it is NO_xNot released from the absorbent and therefore NO_xabsorption
SO in the agent_xWill gradually increase. By the way
Is NO_xSO in absorbent_xNO when the amount of_xabsorption
NO that the agent can absorb_xGradually decreases, and finally N
O_xAbsorbent is NO_xCan hardly absorb
U Therefore NO _xSulfur in the engine exhaust passage upstream of the absorbent
Equipped with a trap, this sulfur trap allows exhaust gas
SO contained in_xThe internal combustion engine that captures
It has already been proposed by the applicant (Japanese Patent Application No. 4-2080).
90). In this internal combustion engine, the S discharged from the engine
O_xIs captured by the sulfur capture device, so NO_xabsorption
NO for the agent_xOnly will be absorbed.

[0004]

However, in this internal combustion engine, the SO _x trapped by the sulfur trapping device is not released from the sulfur trapping device even if the exhaust gas flowing into the sulfur trapping device has a rich air-fuel ratio. Continues to be captured in the capture device. Therefore S by sulfur capture device
The trapped amount of O _x gradually increases, and when the SO _x trapping capacity of the sulfur trap is saturated, SO _x passes through the sulfur trap, so SO _x is absorbed by the NO _x absorbent and NO is absorbed.
_x The problem of progressive accumulation in the absorbent arises.

[0005]

According to the present invention, in order to solve the above problems, when the air-fuel ratio of the inflowing exhaust gas is lean, NO _x is absorbed and the oxygen concentration in the inflowing exhaust gas is increased. NO releases released NO _x
with placing the _x absorbent engine exhaust passage, the air-fuel ratio of the exhaust gas air-fuel ratio of the exhaust gas flowing though carrying a metal having a redox function to absorb SO _x when a lean, flows There is disposed a SO _x absorbent to release the SO _x absorbed and becomes rich in _the NO _x absorbent in the engine exhaust passage upstream of, further air-fuel ratio of the exhaust gas flowing into the SO _x absorbent and _the NO _x absorbent is rich When the lean mixture is burned, the SO _x absorbent is provided with means for determining the degree of richness that decreases due to the redox reaction, and when the lean mixture is burned, the SO _x in the exhaust gas discharged into the engine exhaust passage is provided. the absorbing the NO _x in the exhaust gas in _the NO _x absorbent with absorbed into SO _x absorbent, SO from SO _x absorbent
rich degree redox above at this time along with the time to release the NO _x from the NO _x absorbent when the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent and _the NO _x absorbent rich with releasing _x The amount of rich that decreases due to the reaction is increased.

[0006]

When the lean air-fuel mixture is burned, SO _x in the exhaust gas is absorbed by the SO _x absorbent, so SO _x is absorbed.
Only NO _x is absorbed by the NO _x absorbent arranged downstream of the absorbent. On the other hand, when the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent and the NO _x absorbent is made rich, SO _x
SO _x is released from the absorbent, NO _x from _the NO _x absorbent
Is released. However a large amount of unburned H in SO _x absorbent when SO _x absorbent has a redox function at this time
C, unburned HC for reducing the NO _x in _the NO _x absorbent for CO is oxidized, CO is insufficient, no longer E reduced the total NO _x released from _the NO _x absorbent and thus I will end up. However non-sufficient amount of the degree of rich as described above to reduce the total NO _x in decreases when caused to increase by the degree of rich _the NO _x absorbent released from _the NO _x absorbent by oxidation-reduction reaction Fuel HC,
CO flows, the total NO _x is to be satisfactorily reduced released from _the NO _x absorbent and thus.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, 1 is an engine body, 2 is a piston
Ton, 3 is combustion chamber, 4 is spark plug, 5 is intake valve, 6 is intake
A port, 7 is an exhaust valve, and 8 is an exhaust port. Intake
Port 6 is connected to surge tank 10 via corresponding branch pipe 9.
Each of the branch pipes 9 is connected to the inside of the intake port 6 and burns.
A fuel injection valve 11 for injecting fuel is attached. Surger
Ink 10 includes intake duct 12 and air flow meter 13
Is connected to the air cleaner 14 through the intake duct 12
A throttle valve 15 is arranged inside. On the other hand, the exhaust port
8 through the exhaust manifold 16 and the exhaust pipe 17
O_xAbsorbent 18 and NO_xCase with built-in absorbent 19
Connected to the sing 20. SO_xAbsorbent 18 is NO_xSucking
It is located upstream of the sorbent 19, and in the embodiment shown in FIG.
Is SO _xAbsorbent 18 and NO_xIf the absorbent 19 is
Using a monolithic carrier made of Lumina as a unit
Has been formed.

The electronic control unit 30 is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, and a constant power source by a bidirectional bus 31. Connected backup RAM
33 a, an input port 35 and an output port 36. The air flow meter 13 generates an output voltage proportional to the intake air amount, and this output voltage is input to the input port 35 via the AD converter 37. A temperature sensor 21 that generates an output voltage proportional to the exhaust gas temperature is disposed in the exhaust pipe 17 upstream of the SO _x absorbent 18, and the output voltage of the temperature sensor 21 is input to the input port 35 via the AD converter 38. Is entered. 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 = TP · K Here, TP represents the basic fuel injection time, and K represents the 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 by an experiment in advance, and the engine load Q / N
As a function of (intake air amount Q / engine speed N) and engine speed N, the ROM 3 is previously stored in the form of a map as shown in FIG.
It is stored in 2. 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 theoretical air-fuel ratio, that is, becomes lean, and K> 1.0.
If so, 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 correction coefficient K is controlled according to the operating state of the engine, and FIG. 3 shows an embodiment of the control of the correction coefficient K. In the embodiment shown in FIG. 3, the correction coefficient K is gradually decreased as the engine cooling water temperature increases during warm-up operation, and when warm-up is completed, the correction coefficient K becomes a constant value smaller than 1.0, that is, the engine. The air-fuel ratio of the air-fuel mixture supplied into the cylinder is maintained lean. Next, when the acceleration operation is performed, the correction coefficient K is set to 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio,
If the full load operation is performed, the correction coefficient K is made larger than 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made rich. As can be seen from FIG. 3, in the embodiment shown in FIG. 3, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is maintained at a constant lean air-fuel ratio except during warm-up operation, acceleration operation and full load operation. Therefore, the lean air-fuel mixture is burned in most engine operating regions.

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 _x contained in the casing 20
The absorbent 19 uses, for example, alumina as a carrier, and alkali metals such as potassium K, sodium Na, and cesium Cs, alkaline earths such as barium Ba and calcium Ca, and rare earths such as lanthanum La and yttrium Y are used on the carrier. At least one selected from the above and a noble metal such as platinum Pt are supported. In addition, it is preferable to add lithium Li to the NO _x absorbent 19. NO of Toko to refer to the ratio of the engine intake passage and _the NO _x absorbent 19 the exhaust passage upstream of the supply air and fuel into the (hydrocarbon) and the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent 19
_x absorbent 19 absorbs the NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, perform absorption and release action of the NO _x concentration of oxygen in the inflowing exhaust gas to release NO _x absorbed to decrease. When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NO _x absorbent 19, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3, Thus absorbs NO _x when the air-fuel ratio of the mixture is _the NO _x absorbent 19 to be supplied into the combustion chamber 3 in this case is lean, lowering the oxygen concentration in the mixture fed into the combustion chamber 3 Then, the absorbed NO _x is released.

If the above-mentioned NO _x absorbent 19 is arranged in the exhaust passage of the engine, the NO _x absorbent 19 actually performs the NO _x absorption and release action, but the detailed mechanism of this absorption and release action 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 attaches 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 → NO 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 becomes NO _x
It is absorbed in the absorbent 19.

Platinum as long as the oxygen concentration in the inflowing exhaust gas is high
NO on the surface of Pt₂Is 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 19 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 19
To NO _xWill be released.

On the other hand, at this time, if 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, 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 is extremely lowered, so NO ₂ is released from the absorbent, and this NO ₂ is unrecovered as shown in FIG. 5 (B). It is reduced by reacting with fuel HC and CO.
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. Thus the air-fuel ratio of the inflowing exhaust gas becomes the NO _x is released from the NO _x absorbent 19 in a short time when the rich.

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 immediately and is oxidized, then white
O on gold Pt₂ ^-Or O^2-Is consumed, but still unburned H
If C and CO remain, the unburned HC and CO absorb them.
NO released from sorbent_xAnd N emitted from the engine
O_xIs reduced. Therefore, the air-fuel ratio of the inflowing exhaust gas
Rich in NO in a short time_xSoak up in absorbent 19
NO is stored_xWas released, and this was released
NO_xIs reduced to NO in the atmosphere_xIs discharged
Will be able to be blocked. Also, NO_xabsorption
Agent 19 has the function of a reducing catalyst, so inflowing exhaust gas
NO even if the air-fuel ratio of_xRelease from absorbent 19
NO issued_xIs reduced. However the inflow
NO when the air-fuel ratio of the exhaust gas is made the stoichiometric air-fuel ratio_xSucking
NO from collecting agent 19_xIs released only gradually, so N
O _xAll NO absorbed in the absorbent 19_xLet out
Takes a little longer.

By the way, as described above, the inflow of exhaust gas is
If the leanness of the fuel ratio is reduced,
NO even if the air-fuel ratio is lean_xAbsorbent 19 to NO_x
Is released. Therefore NO_xAbsorbent 19 to NO_xLet go
To release it, reduce the oxygen concentration in the exhaust gas
It will be good. However, NO_xAbsorbent 19 to NO _x
If the air-fuel ratio of the inflowing exhaust gas is lean even if
NO_xNO in absorbent 19_xIs not reduced, so
NO in this case_xNO downstream of the absorbent 19_xReduce
A catalytic converter or NO_xDownstream of absorbent 19
It is necessary to supply a reducing agent. Of course NO like this_x
NO downstream of the absorbent 19_xCan be reduced
But rather NO_xN in the absorbent 19
O_xIs preferably reduced. Implementation according to the invention
NO in the example_xAbsorbent 19 to NO _xWhen to release
Is the stoichiometric air-fuel ratio or rich
And thereby NO_xNO released from absorbent 19
_xNO_xTry to reduce in the absorbent 19
It

An embodiment according to the present invention as shown in FIG.
In the warm-up operation and full load operation,
The supplied air-fuel mixture is rich and mixed during acceleration operation.
Aiki is assumed to be the theoretical air-fuel ratio, but most other operating areas
In the region, the lean mixture is burned in the combustion chamber 3.
Be done. In this case, it is burned in the combustion chamber 3.
The air-fuel ratio of the air-fuel mixture is almost 18.0 or higher, which is shown in Fig. 1.
In a preferred embodiment, a lean mixture with an air-fuel ratio of 20 to 24 is used.
Qi is burned. When the air-fuel ratio becomes 18.0 or more
Three-way catalysts have reducing properties even under lean air-fuel ratios.
Even if NO _xCan not be fully returned, obey
NO under lean air-fuel ratio like this_xTo reduce
It is not possible to use a three-way catalyst. Also, the air-fuel ratio is 1
NO even if it is 8.0 or more_xAs a catalyst capable of reducing C
There is a u-zeolite catalyst, but this Cu-zeolite catalyst
Uses this Cu-zeolite catalyst because it lacks heat resistance
It is not preferable to have it. So after all,
NO when the air-fuel ratio is 18.0 or higher_xBook to purify
NO used in the invention_xUse absorbent 19
There is no other way.

In the embodiment according to the present invention, as described above, the air-fuel mixture supplied into the combustion chamber 3 is rich during full-load operation, and the air-fuel mixture has a stoichiometric air-fuel ratio during acceleration operation. NO _x during acceleration and acceleration
NO _x will be released from the absorbent 19. However, if the frequency of the full load operation or the acceleration operation is low, the lean air-fuel mixture is burned even when the NO _x absorbent 19 releases NO _x during the full load operation and the acceleration operation. Between NO _x absorbent 19
The NO _x absorption capacity due to is saturated, and thus N
The O _x absorbent 19 cannot absorb NO _x . Therefore, when the lean air-fuel mixture is being continued, the air-fuel ratio of the inflowing exhaust gas is periodically made rich, or the air-fuel ratio of the inflowing exhaust gas is periodically changed to the stoichiometric air-fuel ratio and the NO _x absorbent 19 is periodically supplied. It is necessary to release NO _x into the.

By the way, the exhaust gas contains SO _x , and the NO _x absorbent 19 contains not only NO _x but also SO _x.
Is also absorbed. It is considered that the SO _x absorption mechanism into the NO _x absorbent 19 is the same as the NO _x absorption mechanism. That is, similar to the case of explaining the NO _x absorption mechanism, the case of supporting platinum Pt and barium Ba on the carrier will be described as an example. As described above, when the air-fuel ratio of the inflowing exhaust gas is lean, the oxygen O ₂ Is O ₂
^- or O ^{2 -} is attached in the form of the surface of the platinum Pt, SO ₂ in the inflowing exhaust gas on the surface of the platinum Pt O ₂ ^- or O
It reacts with ^2- and becomes SO ₃ . SO generated next
A part of ₃ is further oxidized on platinum Pt, absorbed in the absorbent and bound to barium oxide BaO, and diffused in the absorbent in the form of sulfate ion SO ₄ ²⁻ to form a stable sulfate B.
Generate aSO ₄ .

However, this sulfate BaSO ₄ is stable and difficult to decompose, and even if the air-fuel ratio of the inflowing exhaust gas is made rich, the sulfate BaSO ₄ remains without being decomposed. Thus will be sulfates BaSO ₄ increases as NO _x time to absorbent 19. elapses, that _the amount of NO _{x the} NO _x absorbent 19 can absorb as thus to time has elapsed is reduced Become.

[0023] Therefore, in this embodiment of the present invention absorbs SO _x when the air-fuel ratio of the exhaust gas flowing to not flow into the SO _x in _the NO _x absorbent 19 is lean, the air-fuel ratio of the exhaust gas flowing The SO _x absorbent 18 that releases the absorbed SO _x when it becomes rich is arranged upstream of the NO _x absorbent 19. When the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent 18 is lean, it also absorbs NO _x together with SO _x , but when the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent 18 is made rich, only the absorbed NO _x is obtained. S absorbed without
It also releases O _x .

As described above, when the NO _x absorbent 19 is SO
_{When x} is absorbed, stable sulfate BaSO ₄ is formed. As a result, SO _x is not released from the NO _x absorbent 19 even if the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 19 is made rich. Therefore, when the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent 18 is made rich, the SO _x absorbent 18
In order to release SO _x from the absorbed S
As present in the absorbent in the state where no stable sulfates BaSO ₄ as it is generated either, or sulfate BaSO ₄ to be present in the absorbent in the form of ^- O _x is sulfate ion SO ₄ ² It is necessary to As the SO _x absorbent 18 that enables this, an absorbent in which at least one selected from transition metals such as iron Fe, manganese Mn, nickel Ni, and titanium Ti and lithium Li is supported on a carrier made of alumina. Can be used. in this case,
It has been found that an absorbent in which at least lithium Li is supported on a carrier made of alumina is most preferable.

In this SO _x absorbent 18, SO _x absorbent 1
When the air-fuel ratio of the exhaust gas flowing into 8 is lean, SO ₂ contained in the exhaust gas is absorbed in the absorbent in the form of sulfate ion SO ₄ ²⁻ while being oxidized on the surface of the absorbent, and then the absorbent. Diffused in. In this case, when platinum Pt is supported on the carrier of the SO _x absorbent 18, SO ₂ easily sticks to the platinum Pt in the form of SO ₃ ^2- , and thus SO ₂
Is easily absorbed in the absorbent in the form of sulfate ion SO ₄ ²⁻ . Therefore, in order to promote the absorption of SO ₂ , it is preferable to support platinum Pt on the carrier of the SO _x absorbent 18. As described above, when the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent 18 becomes lean, the SO _x becomes SO _x absorbent 18
Therefore, the NO _x absorbent 19 provided downstream of the SO _x absorbent 18 absorbs only NO _x .

On the other hand, as described above, SO_xIn absorbent 18
SO absorbed_xIs sulfate ion SO _Four ^2-Inside the absorbent in the form of
Sulfate BaS that has diffused into or is unstable
O_FourHas become. Therefore SO_xFlowing into the absorbent 18
When the air-fuel ratio of exhaust gas becomes rich, SO_xIn absorbent 18
SO absorbed_xIs SO_xReleased from absorbent 18
Will be. NO at this time_xAbsorbent 19 to N
O_xIs released.

By the way, as described above, the NO _x absorbent 19
In reaction with NO ₂ on the platinum Pt surface is not present immediately ^- advances in the direction of _{(NO 3 → NO 2),} NO x is immediately released from the absorbent. When the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 19 is made rich, NO ₂ on the platinum Pt surface is immediately reduced by unburned HC and CO, so NO ₂ on the platinum Pt surface immediately disappears. As shown in FIG. 6, NO _x is released from the NO _x absorbent 19 within a short time. That is, the NO _x release rate of the NO _x absorbent 19 is considerably high.

[0028] In contrast SO _x SO _x absorbed in the absorbent 18 is absorbed in _the NO _x absorbent 19 NO
_It is more stable than _x and is difficult to decompose, so this SO
decomposition of _x does not occur unless the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent 18 rich. That is, when the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent 18 is made rich, SO
SO _x in the _x absorbent 18 is decomposed and released from the absorbent. This decomposition rate is considerably slow, and therefore, as shown in FIG. 6, even if the air-fuel ratio of the exhaust gas flowing into the SO _x absorbent 18 is made rich, it takes a long time compared to NO _x until the release of SO _x is completed. Will be required. That is, the SO _x release rate is considerably slower than the NO _x release rate.

In the embodiment according to the invention, NO _x and SO
_The air-fuel mixture supplied to the combustion chamber 3 to release _x is periodically made rich, and FIG. 7 shows the timing at which the air-fuel mixture is made rich in this way. Incidentally, in FIG. 7 P shows the timing of releasing the NO _x from the NO _x absorbent 19, Q indicates the timing for releasing the SO _x from the SO _x absorbent 18. As can be seen from FIG. 7, the period for enriching the air-fuel mixture to release NO _x from the NO _x absorbent 19 is quite short, and the air-fuel mixture is enriched once every few minutes. On the other hand, since the amount of SO _x contained in the exhaust gas is much smaller than the amount of NO _x , it takes a considerable time for the SO _x absorbent 18 to be saturated with SO _x . Therefore, the period for enriching the air-fuel mixture in order to release the SO _x from the SO _x absorbent 18 is considerably long, and the air-fuel mixture is enriched, for example, once every several hours.

By the way, as shown in FIG. 6, inside the combustion chamber 3.
NO when the air-fuel ratio of the air-fuel mixture supplied to
_xIs NO in a short time_xReleased from absorbent 19,
SO _xSorbent 18 to SO_xBy the time is released
Takes more time. Therefore SO_xMixed to release
No time to keep feeling rich_xTo emit
Much longer than the time the mixture remains rich. An example
For example NO_xThe mixture is rich for several seconds when releasing
Whereas SO_xWhen releasing the
It will be rich for a few minutes. SO like this_xEmit
Sometimes the mixture becomes rich over a long period of time
SO like_xThe mixture is rich due to the release of
The fuel consumption increases drastically due to the long cycle
There is nothing to do.

The ideal rich control solid line in (P in Figure 7) the air-fuel mixture when from _the NO _x absorbent 19 to release the NO _x in the case of not provided SO _x absorbent 18 FIG 8 (A) Shows. Note that Kt is the basic fuel injection time TP
The correction coefficient for is shown. As shown by the solid line in FIG. 8 (A), when NO _x should be released from the NO _x absorbent 19, the correction coefficient Kt is increased to KK (> 1.0) to supply the mixture into the combustion chamber 3. The air-fuel ratio of the air is made rich, and then this rich state is maintained for C ₁ hour. Next, the correction coefficient Kt is gradually decreased, and then the correction coefficient Kt is maintained at 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is maintained at the stoichiometric air-fuel ratio. Next, when C ₂ hours have elapsed after the rich control was started, the correction coefficient Kt is made smaller than 1.0 again, and the combustion of the lean mixture is started again.

The air-fuel ratio of the mixture supplied to the combustion chamber 3 becomes rich (Kt = KK), a large part of the NO _x absorbed in _the NO _x absorbent 19 is abruptly released. The value of the correction coefficient KK and the time C ₁ at this time are O on platinum Pt.
It is specified that just the amount of unburned HC, CO that is necessary for consuming ₂ ⁻ or O ^{2 −} and reducing all NO _x is generated. In other words, if the rich degree of the air-fuel mixture supplied into the combustion chamber 3 at this time is made too high, unburned HC,
Excessive amount of CO causes excessive surplus of unburned HC and CO to be released into the atmosphere. At this time, if the degree of richness of the air-fuel mixture is too low, unburned HC and CO cannot be reduced due to too little NO. _x is released into the atmosphere and therefore the correction factor KK
The value of and the time C ₁ at this time are unburned HC, CO and N
Any of the O _x is determined so as not to be released into the atmosphere.

On the other hand, when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich (Kt = KK) as described above, N
O Although _x absorbent 19 most of the NO _x absorbed in is suddenly released thereafter NO _x is not released little by from _the NO _x absorbent 19 even leave the air-fuel ratio rich. Therefore, if the air-fuel ratio is kept rich, unburned HC, C
O will be released to the atmosphere. Therefore, as shown by the solid line in FIG. 8A, the air-fuel ratio is made rich (Kt = K
K), the degree of richness is gradually reduced, and then the air-fuel ratio is maintained at the stoichiometric air-fuel ratio (Kt = 1.0) and NO
_The NO _x released little by little from the _x absorbent 18 is gradually reduced. Then, after the action of releasing NO _x from the NO _x absorbent 18 is completed, the air-fuel ratio is returned to lean again.

However, iron F is provided upstream of the NO _x absorbent 19.
When the SO _x absorbent 18 containing a transition metal such as e, manganese Mn, nickel Ni, and titanium Ti is arranged, unburned HC and CO in the SO _x absorbent 18 are present because these transition metals have a redox function. Will be oxidized. That is, the unburned HC and CO deprive the transition metal bonded to oxygen of oxygen and oxidize it. Therefore, when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is switched from lean to rich (A / F) _R as shown by the broken line in FIG. 9, unburned HC and CO generated at this time absorb SO _x. Since the agent 18 is oxidized, the air-fuel ratio of the exhaust gas flowing out of the SO _x absorbent 18 does not immediately become rich (A / F) _R as shown by the solid line in FIG. 9 (A). That is, immediately after the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is switched from lean to rich (A / F) _R , most of unburned HC and CO that are generated in large amounts are SO _x absorbent 18
The air-fuel ratio of the exhaust gas flowing out from the SO _x absorbent 18 does not become rich because it is oxidized in the above, and the degree of richness gradually increases as the amount of oxygen bound to the transition metal decreases thereafter. Therefore, as shown by the solid line in FIG. 8A, even if the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is made rich, the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 19 is not so large as shown by the chain line. It does not become rich, and therefore the amount of unburned HC and CO is not sufficient, so N
O _x will be released into the atmosphere.

At this time, in order to prevent NO _x from being released to the atmosphere, N as shown by a chain line in FIG.
It suffices that the rich degree of the air-fuel ratio of the exhaust gas flowing into the O _x absorbent 19 reaches the rich degree corresponding to the correction coefficient KK, for that purpose, as shown by the solid line in FIG. Needs to be increased by ΔK ₁ . Therefore, in the embodiment according to the present invention, when NO _x should be released from the NO _x absorbent 19, a value obtained by adding ΔK ₁ to the correction coefficient KK is set as Kt.

By the way, the redox reaction in the SO _x absorbent 18 has a temperature dependency, and therefore, when the air-fuel mixture supplied into the combustion chamber 3 is switched from lean to rich, S
The degree of richness reduction in the O _x absorbent 18 is SO _x
It changes according to the temperature of the absorbent 18. Figure 9 (B) shows SO
and the degree of rich ΔD to reduce the SO _x absorbent 18 when using iron Fe as the transition metal contained in the _x absorbent 18, the relationship between the exhaust gas temperature T representative of the temperature of the SO _x absorbent 18 Is shown. The redox reaction becomes stronger as the temperature of the SO _x absorbent 18 becomes higher, so that FIG.
As shown in FIG. ₅ , in the range of T <T ₀ , the higher the exhaust gas temperature T, the larger the decrease amount ΔD of the rich degree. On the other hand, when T> T ₀ , the Fe ₂ Fe ₃ supported on the carrier changes to FeO, which has a weak redox power, and the rich degree decreases when the exhaust gas T exceeds T _0. Amount Δ
D becomes smaller.

Therefore, when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is switched from lean to rich, the correction coefficient KK
The value of ΔK ₁ to be added to is changed with the same pattern as the decrease amount ΔD of the rich degree with respect to the exhaust gas temperature T, as shown by the solid line in FIG. 9 (C). Figure correction coefficient KK, as previously described in the embodiment according to the present invention is shown in FIG. 8 (A) to when releasing the NO _x from the NO _x absorbent 19 as shown by the solid line in FIG. 8 (B) 9
The value obtained by adding ΔK ₁ shown in (C) is taken as Kt. Note that ΔK ₁ shown in FIG. 10A (ΔK ₁ shown in FIG. 9C)
(Same as ₁ ) and the relationship between exhaust gas temperature T and ROM3
It is stored in 2. Further, from the NO _x absorbent 19 to N
When the air-fuel mixture in order to release the O _x is rich in high exhaust gas temperature T, thus the amount of the NO _x which the temperature of _the NO _x absorbent 19 is released from the more _the NO _x absorbent 19 becomes high is increased. Therefore, as shown in FIG. 10B, the value of the correction coefficient KK is increased as the exhaust gas temperature T increases.
As shown in FIG. 10C, the time C ₁ is the exhaust gas temperature T
The higher the height, the shorter the length. Note that the relationship between the correction coefficient KK and the exhaust gas temperature T shown in FIG.
The relationship between the time C ₁ shown in (C) and the exhaust gas temperature T is R in advance.
It is stored in the OM32.

Next, when the time C ₁ has elapsed, the value of Kt is gradually decreased, and the rich degree is maintained constant at ΔK ₂ . As shown in FIG. 9A, after the lean-to-rich switching, the rich degree gradually increases, so that ΔK ₂ becomes a value smaller than ΔK ₁ . Further, the [Delta] K ₂ This [Delta] K ₂ because changes in the same pattern as [Delta] K ₁ with respect to the exhaust gas temperature T is caused to change as shown by the broken line in FIG. 9 (C). Therefore, the correction coefficient K ₀ (FIG. 8 (B)) corresponding to ΔK ₂ is also changed in the same pattern as ΔK ₁ with respect to the exhaust gas temperature T as shown in FIG. 10 (D). ) Correction coefficient K shown in
The relationship between ₀ and the exhaust gas temperature T is also stored in the ROM 32 in advance.

When the air-fuel ratio is made rich, NO _x
The amount of the NO _x amount of the NO _x released from the absorbent 19 is released from the more then _the NO _x absorbent 19 often less, therefore _the NO _x absorbent 19 is shorter the time until you release the NO _x I will get sick. As described above, the higher the exhaust gas temperature T, the larger the amount of NO _x released from the NO _x absorbent 19 when the air-fuel ratio is made richer, and therefore the amount shown in FIG.
As shown in 1 (A), the time C ₂ from when the air-fuel ratio is made rich to when it is returned to lean again is made shorter as the exhaust gas temperature T becomes higher. Note that time C shown in FIG.
The relationship between ₂ and the exhaust gas temperature T is stored in the ROM 32 in advance.

As described above, ΔK ₁ , KK, C ₁ , K ₀ and C ₂ are controlled according to the exhaust gas temperature T, and when T <T ₀ , the correction coefficient Kt is increased as the exhaust gas temperature T is higher. It That is, the degree of richness is increased. In this case, almost no SO _x is released from the SO _x absorbent 18 because the time during which the air-fuel mixture is rich is short,
Substantially only the action of releasing NO _x from the NO _x absorbent 19 is performed.

FIG. 12 shows the case of releasing SO _x (see FIG. 7).
Q) shows rich control of air-fuel mixture. As shown in FIG. 12, even when SO _x should be released from the SO _x absorbent 18, the correction coefficient Kt is increased to (KK + ΔK ₁ ) to make the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 rich. Then, the rich state is maintained for C ₁ hour. Next, the correction coefficient Kt is gradually decreased, and then the correction coefficient Kt is kept at K ₀ (> 1.0), that is, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is kept rich. That is, when releasing the SO _x is to first, first rich degree (Kt = KK + ΔK ₁₎ mixed gas until greatly rich, then than the first rich degree (Kt = KK + ΔK ₁₎ A small second rich degree (Kt = K ₀ ) is maintained. Next, when C ₂ hours have elapsed since the rich control was started, the correction coefficient Kt becomes 1.
It is made smaller than 0 and combustion of the lean mixture is started again. The time C ₂ for releasing SO _x is as shown in FIG.
It is considerably longer than the time C ₂ at the time of releasing NO _x shown in (B), for example, about 3 minutes to 10 minutes.

As described above, S from the SO _x absorbent 18
The O _x release rate is considerably slow, and even if the air-fuel ratio of the air-fuel mixture is made extremely rich, the SO _x release rate does not increase in proportion thereto. That is, making the air-fuel ratio of the air-fuel mixture extremely rich only unnecessarily increases the fuel consumption amount. Therefore, when SO _x should be released, if the correction coefficient Kt is maintained at a relatively small value of K ₀ , SO _x becomes S
Good release from the O _x absorbent 18. Nevertheless, when SO _x is to be released, the air-fuel mixture is first made extremely rich (Kt = KK + ΔK ₁ ). Next, the reason will be described.

When the correction coefficient Kt is maintained at K ₀ when SO _x should be released, SO ₂ is gradually released from the SO _x absorbent 18. At this time, at the same time, the NO _x absorbent 19
₂ is released, but since the degree of richness is small, NO ₂ is also gradually released from the NO _x absorbent 19. However, when SO ₂ released from SO _x absorbent 18 flows into _the NO _x absorbent 19 when the NO ₂ is gradually released in this way from _the NO _x absorbent 19, the NO ₂ and SO ₂ is It reacts (SO ₂ + NO ₂ → SO ₃ + NO), and the SO ₃ thus generated is absorbed by the NO _x absorbent 19 in the form of SO ₄ ⁻ . Such a reaction does not occur unless NO ₂ is present, and thus S in the NO _x absorbent 19
SO _x absorbent 1 to prevent absorption of O ₂
NO _x absorbent 1 when SO ₂ is released from 8
It is necessary to prevent NO _{2 from} being released from 9. Therefore, as shown in FIG. 12, when SO _x is to be released, first, the air-fuel ratio of the air-fuel mixture is extremely rich (Kt
= KK + ΔK ₁ ).

That is, the air-fuel mixture is extremely rich (Kt = KK +
ΔK ₁ ) makes most of NO ₂ from the NO _x absorbent 19
Are released all at once, and thereafter, almost no NO ₂ is released from the NO _x absorbent 19. Therefore, after that, the correction coefficient Kt
SO ₂ released from the SO _x absorbent 18 does not react with NO ₂ when is maintained at K ₀ , thus eliminating the risk of SO ₂ being absorbed by the NO _x absorbent 19 .

The air-fuel ratio of the air-fuel mixture is rich (Kt =
The larger the amount of SO _x released from the SO _x absorbent 18 when it is maintained at K ₀ ), the more the SO _x absorbent 18 becomes SO _x.
It takes less time to finish releasing. In this case, as the exhaust gas temperature T becomes higher, the SO _x decomposition rate becomes faster and the SO _x release rate becomes faster. Therefore, as shown in FIG. 11B, the air-fuel ratio is made rich and then returned to lean again. The time C _{2 up to} is shortened as the exhaust gas temperature T increases. The relationship between the time C ₂ and the exhaust gas temperature T shown in FIG. 11B is stored in the ROM 32 in advance.

As shown in FIGS. 10 and 11, each value ΔK ₁ , KK, C ₁ , K ₀ , C ₂ is a function of the exhaust gas temperature T, and in the embodiment according to the present invention, this exhaust gas temperature T is It is detected by the temperature sensor 21. As described above, the exhaust gas temperature T can be directly detected, but can also be estimated from the intake air amount Q and the engine speed N. In this case, the relationship between the exhaust gas temperature T, the intake air amount Q, and the engine speed N is previously obtained by an experiment, and this relationship is stored in advance in the ROM 32 in the form of a map as shown in FIG. Then, the exhaust gas temperature T may be calculated from this map.

Next, referring to FIGS. 14 to 17, one embodiment of the control of NO _x and SO _x absorption and release according to the present invention will be described. 14 to 16 show a routine for calculating the correction coefficient KK at the time of rich control, and this routine is executed by interruption at regular time intervals. 14 to 1
Referring to FIG. 6, first, at step 50, it is judged if the correction coefficient K is smaller than 1.0, that is, if the lean air-fuel mixture is burned. K <1.0
When, that is, when the lean air-fuel mixture is being combusted, the routine proceeds to step 51, where the NO _x amount Wn absorbed in the NO _x absorbent 19 is calculated. That, NO _x amount exhausted from the combustion chamber 3 is increased The more the intake air quantity Q, so increases as the engine load Q / N becomes higher NO _x
The amount of NO _x Wn absorbed by the absorbent 19 is Wn and k ₁
· Q · Q / N (k 1 is a constant) will be represented by the sum of the.

Next, at step 52, the SO _x absorbent 18
Amount of SO _x Ws being absorbed is calculated. That is, the SO _x amount discharged from the combustion chamber 3 increases as the intake air amount Q increases, so the SO _x amount Wn absorbed by the SO _x absorbent 18 is Wn and K ₂ · Q (K ₂ is a constant). Will be represented by the sum of and. This SO _x amount Ws is stored in the backup RAM 33a. Then release SO _x flag indicating that should be released in step 53 SO _x is whether it is set or not. Release of SO _x flag whether _the NO _x releasing flag showing that should be released NO _x proceeds to step 54 is set when not been set or not. When the NO _x release flag is not set, the routine proceeds to step 55.

At step 55, SO_xAbsorbed in absorbent 18
Is SO_xThe amount W is a predetermined set amount Ws₀Than
Is also large or not. This set amount Ws₀Is for example
SO _xMaximum SO that the absorbent 18 can absorb_xQuantity of 30 par
It is about a cent. Ws ≦ Ws₀If so, go to step 63
move on. NO in step 63_xAbsorbed by absorbent 19
NO_xThe amount Wn is a predetermined set amount Wn₀Than
It is determined whether or not it occurs. This set amount Wn₀Is for example N
O_xMaximum NO that the absorbent 19 can absorb_xAmount of 30%
It is about Wn ≦ Wn₀When, the processing cycle is completed
Finish.

On the other hand, in step 55, Ws> Ws₀so
When it is determined that there is an SO, the routine proceeds to step 56, where SO_x
The release flag is set. Then in step 57
The correction coefficient KK is calculated from the relationship shown in FIG.
In step 58, ΔK is calculated from the relationship shown in FIG.
₁Is calculated. Next, at step 59, KK is changed to ΔK. ₁
Is added to KK, and then in step 60
From the relationship shown in FIG.₁Is calculated. Next
Then, in step 61, correction is made from the relationship shown in FIG.
Coefficient K₀Is calculated, and then in step 62, as shown in FIG.
From the relationship shown in (B), time C₂Is calculated. Then
Complete the processing cycle. In addition, SO_xThe emission flag is set
Each value ΔK₁, KK, C₁, K₀, C₂Is calculated
Then, the air-fuel mixture becomes rich as will be described later.

On the other hand, in step 63, Wn> Wn₀so
If it is determined that there is, go to step 64 and NO._x
The release flag is set. Then in step 65
The correction coefficient KK is calculated from the relationship shown in FIG.
In step 66, ΔK is calculated from the relationship shown in FIG.
₁Is calculated. Next, at step 67, KK is changed to ΔK. ₁
KK is obtained by adding. Then in step 68
Is the time C from the relationship shown in FIG.₁Is calculated.
Next, at step 69, the relationship shown in FIG.
Positive coefficient K₀Is calculated, and then in step 70, as shown in FIG.
From the relationship shown in (A), time C₂Is calculated. Then
Complete the processing cycle. Note that NO_xThe emission flag is set
Each value ΔK₁, KK, C₁, K₀, C₂Is calculated
Then, the air-fuel mixture becomes rich as will be described later.

When the SO _x release flag or NO _x release flag is set, the routine proceeds from step 53 or step 54 to step 71, where the count value C is incremented by one. Next, at step 72, the count value C is the time C.
_It is determined whether it is smaller than ₁ . When C <C ₁ , the processing cycle is completed, so that the correction coefficient is kept at KK (= KK + ΔK ₁ ) during the time C ₁ . Then C
When ≧ C ₁ , the routine proceeds to step 73, where it is judged if the count value C is smaller than the time C ₂ . When C <C _2, the routine proceeds to step 74, where the correction coefficient KK is set to a constant value α
Is subtracted. Therefore, the value of the correction coefficient KK gradually decreases.

Next, at step 75, the correction coefficient KK is K
_It is determined whether or not it has become smaller than ₀ . KK> K ₀
In the case of, the processing cycle is completed, and when KK ≦ K ₀ , the routine proceeds to step 76, where KK is made K ₀ . Therefore KK =
After reaching K ₀ , the correction coefficient is maintained at K ₀ (> 1.0). Next, when it is judged at step 73 that C ≧ C ₂ , the routine proceeds to step 77, where it is judged if the SO _x release flag is set or not. When the SO _x release flag is set, the routine proceeds to step 78, where the SO _x release flag is reset. When the SO _x release flag is reset, combustion of the lean mixture is started again as described later. Then amount of SO _x Ws absorbed in the SO _x absorbent 18 in step 79 is made zero, then _the amount of NO _x Wn that is absorbed in the NO _x absorbent 19 in step 81 is made zero. Then step 82
At, the count value C is set to zero.

On the other hand, if it is determined at step 77 that the SO _x release flag is not set, then the routine proceeds to step 80, at which the NO _x release flag is reset. N
When the O _x release flag is reset, combustion of the lean mixture is started again as described later. Then step 81
Amount of NO _x W that is absorbed in the NO _x absorbent 19 in
n is set to zero. Next, at step 82, the count value C is made zero.

On the other hand, when it is judged in step 50 that K ≧ 1.0, that is, when the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is the stoichiometric air-fuel ratio or rich, the routine proceeds to step 83, where K ≧ 1. It is determined whether or not the state of 0 has continued for a certain time t ₁ , for example, 10 seconds. K ≧ 1.0
When the state of _No. has not continued for the fixed time t ₁ , the processing cycle is completed, and when the state of K ≧ 1.0 has continued for the constant time t ₁ , the routine proceeds to step 84, where Wn is made zero. That is, if the time period during which the air-fuel mixture supplied to the engine cylinder is at the stoichiometric air-fuel ratio or rich continues for about 10 seconds, NO
Most of the NO _x absorbed in the _x absorbent 19 is considered to have been released, and therefore Wn is made zero in step 84 in this case.

Next, at step 85, it is judged if the state of K> 1.0 has continued for a fixed time t ₂ (t ₂ > t ₁ ), for example, 10 minutes. The condition of K> 1.0 is a constant time t
_{2 If} not continued, complete the processing cycle and K>
When the state of 1.0 continues for a certain period of time t ₂ , the routine proceeds to step 56, where Ws is made zero. That is, it is considered that most of the SO _x absorbed in the SO _x absorbent 18 is released if the time period during which the air-fuel mixture supplied to the engine cylinder is rich is continued for about 10 minutes. In this case, Ws is made zero in step 86.

FIG. 17 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed. Referring to FIG. 17, first, in step 90, the correction coefficient K is calculated. The correction coefficient K is set to, for example, 0.6 in the operating state where the lean air-fuel mixture should be burned.
Further, the correction coefficient K is a function of the engine cooling water temperature during the engine warm-up operation, and is made smaller as the engine cooling water temperature becomes higher in the range of K ≧ 1.0. Further, the correction coefficient K is set to 1.0 during acceleration operation, and the correction coefficient K is set to a value larger than 1.0 during full load operation.

Next, at step 91, the correction coefficient K is 1.
It is determined whether it is smaller than 0. When K ≧ 1.0, the routine proceeds to step 95, where K is set to Kt. On the other hand, when K <1.0, the routine proceeds to step 92, where it is judged if the SO _x release flag is set. SO
Step 9 when the _x release flag is not set
Then, the routine proceeds to step 3, where it is judged if the NO _x releasing flag is set. When the NO _x release flag is not set, the routine proceeds to step 95. Next, at step 96, the basic fuel injection time TP is calculated from the map shown in FIG. 2, and then at step 97, the fuel injection time TAU (= T
P · Kt) is calculated. Therefore, when K ≧ 1.0, or even when K> 1.0, the SO _x release flag and NO _x are released.
When neither the release flag is set, the air-fuel ratio of the air-fuel mixture is set to the air-fuel ratio according to the correction coefficient K.

On the other hand, when the SO _x release flag or the NO _x release flag is set, the routine proceeds from step 92 or step 93 to step 94, where Kt is KK calculated by the routine shown in FIGS. 14 to 16 (= KK + ΔK ₁ ). It is said that Next, through step 96, the fuel injection time TAU is calculated at step 97. Therefore, at this time, the air-fuel ratio of the air-fuel mixture is forcibly made rich.

FIG. 18 shows another embodiment. In this embodiment, the same components as those of the embodiment shown in FIG. 1 are designated by the same reference numerals. As shown in FIG. 18, in this embodiment, the exhaust manifold 16 is connected to the inlet of the casing 41 containing the SO _x absorbent 40, and the outlet of the casing 40 contains the NO _x absorbent 43 via the exhaust pipe 42. Casing 4
4 is connected to the inlet part. NO with SO _x is absorbed in the SO _x absorbent 40 when the lean air-fuel mixture is burned in the combustion chamber 3 in this embodiment
NO _x is absorbed by the _x absorbent 43. On the other hand, when the air-fuel mixture supplied into the combustion chamber 3 is made rich, the SO _x absorbent 4
SO _x is released from 0, and NO _{x is} absorbed from the NO _x absorbent 43.
Is released.

[0061]

High long period of time over _the NO _x absorbent, according to the present invention NO _x
It is possible to prevent NO _x from being released into the atmosphere when releasing NO _x from the NO _x absorbent while maintaining the absorption rate.

[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 changes in 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 absorbing and releasing NO _x .

FIG. 6 is a diagram showing SO _x and NO _x release rates.

FIG. 7 is a diagram showing SO _x and NO _x release timings.

FIG. 8 is a diagram showing rich control at the time of releasing NO _x .

FIG. 9 is a diagram for explaining the degree of richness that decreases in the SO _x absorbent and the like.

FIG. 10 is a diagram showing a relationship between various parameters and exhaust gas temperature.

FIG. 11 is a diagram showing a relationship between time C ₂ and exhaust gas temperature.

FIG. 12 is a diagram showing rich control at the time of SO _x release.

FIG. 13 is a diagram showing a map of exhaust gas temperature.

FIG. 14 is a flowchart for calculating a correction coefficient KK.

FIG. 15 is a flowchart for calculating a correction coefficient KK.

FIG. 16 is a flowchart for calculating a correction coefficient KK.

FIG. 17 is a flowchart for calculating a fuel injection time TAU.

FIG. 18 is an overall view showing another embodiment of the internal combustion engine.

[Explanation of symbols]

16 ... Exhaust manifold 18,40 ... SO _x absorbent 19,43 ... NO _x absorbent

Claims (1)

[Claims] 1. The air-fuel ratio of the inflowing exhaust gas is lean.
NO when_xAnd the oxygen concentration in the inflowing exhaust gas
NO absorbed when the degree is lowered_xReleases NO_xabsorption
The agent is placed in the engine exhaust passage, and the redox function is
The air-fuel ratio of the inflowing exhaust gas is
SO when lean_xExhaust gas that absorbs and flows in
Absorbed when the air-fuel ratio becomes rich_xReleasing S
O_xLocated in the engine exhaust passage upstream of the absorbent, and
_xAbsorbent and NO_xAir-fuel of exhaust gas flowing into the absorbent
SO when the ratio becomes rich_xRedox in absorbent
Equipped with means to determine the degree of richness that decreases due to the original reaction
However, when the lean air-fuel mixture is being burned, the engine
SO in the exhaust gas discharged into the exhaust passage_xSO_xSucking
NO in exhaust gas as well as absorbed in sorbent_xNO_xabsorption
Absorbed by the agent, SO _xAbsorbent from SO_xWith releasing
NO_xAbsorbent to NO_xSO should be released_xSucking
Agent and NO_xThe air-fuel ratio of the exhaust gas flowing into the absorbent
At the same time as making it rich, the rich degree at this time
Increase by the degree of richness reduced by redox reaction
Exhaust gas purification device for internal combustion engine.