JP2000080913A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent any nitrogen oxide once emitted out of a nitrogen-oxide absorbent from being absorbed by this absorbent again. SOLUTION: A catalytic converter is equipped with a plurality of cells demarcated by a cellular wall 14 almost parallelly extending to an exhaust passage axis, and it is so constituted that an upstream end open cell 16u where an exhaust upstream end 15u is opened and an exhaust downstream end 15d is closed, and a downstream end open cell 16d where the exhaust upstream end 15u is closed and the exhaust downstream end 15d is opened both are alternately and repeatedly placed side by side. When an air-fuel ratio of incoming exhaust is in a lean side, a nitrogen-oxide absorbent 10 which absorbs nitrogen oxide when an air-fuel ratio of incoming exhaust is in a lean side, and emits the nitrogen oxide being once absorbed when oxygen content in the incoming exhaust is lowered, is set up in each inner wall surface of the upstream end open cell 16u and the downstream end open cell 16d. When the nitrogen oxide should be emitted out of the NOX absorbent 10, the air-fuel ratio to be taken into the NOX absorbent 10 is temporarily set to a rich side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine.

[0002]

2. Description of the Related Art The ratio of the total amount of air to the total amount of fuel and the total amount of reducing agent supplied into an exhaust passage, a combustion chamber, and an intake passage upstream of a certain position in an exhaust passage flows through the position. when referred to as air-fuel ratio of the exhaust gas, in an internal combustion engine which is adapted allowed to combust a lean air-fuel mixture, air-fuel ratio of the exhaust flowing absorbs NO _X when the lean, the oxygen concentration in the inflowing exhaust gas is lowered and the absorption place _the NO _X absorbent in the engine exhaust passage, and temporarily rich air-fuel ratio of the exhaust gas flowing to the NO _X absorbent N that release to that NO _X
O exhaust purification system of an internal combustion engine so as to reduce with the _X absorbent to release NO _X that is absorbed is known (see Japanese Patent Laid-Open No. 6-66129). Furthermore, in this exhaust gas purifying apparatus, the release of SO _X, which is absorbed from _the NO _X absorbent to temporarily rich air-fuel ratio of the exhaust gas flowing to the NO _X absorbent while temporarily heated _the NO _X absorbent Like that.

[0003]

However, NO_Xabsorption
Is generally provided in a monolithic catalytic converter
It is. This monolithic catalytic converter has an exhaust passage axis
Cells extending almost in parallel to each other, both ends open
Cells on the inner wall surface of these cells. _Xabsorption
The agent is placed. In this case, NO on the exhaust upstream side_XAbsorbent
NO released from the part_XOr SO_XCirculates in the cell
NO on the exhaust downstream side_XCan reach the absorbent part
There is a potential. However, NO_XExhaust flowing into the absorbent
Even if it is said that the air-fuel ratio of air is rich, for example, NO_Xabsorption
Immediately after the air-fuel ratio of the exhaust gas flowing into the
Outflow side NO_XAlmost no oxygen concentration around the absorbent
And therefore this NO_XNO in absorbent part_X
Or SO_XIs reached and this NO_XOr SO_XIs NO_X
The problem is that it will be absorbed again in the absorbent.
You.

[0004]

According to a first aspect of the present invention, there is provided an exhaust gas purifying apparatus for an internal combustion engine having a catalytic converter in an exhaust passage. A plurality of cells defined by porous cell walls extending substantially parallel to each other, the cells being open at the upstream end and closed at the downstream end; A downstream end open cell whose downstream end is open is formed by alternately and repeatedly arranging the upstream end open cell and the downstream end open cell. When the air-fuel ratio of the inflowing exhaust gas is lean, NO _X absorb, the oxygen concentration in the inflowing exhaust gas is disposed on the inner wall surface of the inner wall and on the downstream end open cells of the upstream end opening cells to _the NO _X absorbent to release the NO _X that is absorbed decreases, NO NO _X absorbed from _X absorbent
So that it allowed to temporarily reduce the oxygen concentration in the exhaust gas flowing to the NO _X absorbent when it should emit.
That is, in the first aspect, the exhaust gas flowing into the catalytic converter first flows into the upstream open cell, then passes through the cell wall, flows into the downstream open cell, and thus flows out of the catalytic converter. As a result, the exhaust gas flows into the downstream end open cell from the surrounding cell wall, so that the NO _X flowing into the downstream end open cell is prevented from reaching the NO _X absorbent again, and therefore the NO _X absorbent is prevented. N released from
O _X is prevented from being absorbed again by the NO _X absorbent.
Further, since _the NO _X absorbent to the inner wall surface on the inner wall and on the downstream end open cells of the upstream end opening cells are arranged together with the NO _X is reliably absorbed in the NO _X absorbent, pass through _the NO _X absorbent The exhaust gas flow rate at the time of performing is reduced. As a result, the contact time between the exhaust and _the NO _X absorbent increases,
Oxygen concentration becomes possible to effectively utilize the exhaust gas is caused to decrease for of the NO _X emission.

According to the second invention, in the first invention, when the SO _X absorbed from the NO _X absorbent is to be released, the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is temporarily enriched. I try to. That is, in the second invention, SO _X released from the NO _X absorbent is
There are prevented from being absorbed again in the NO _X absorbent. According to a third aspect, in the first aspect, the internal combustion engine includes a plurality of cylinders, and the cylinders are divided into a plurality of cylinder groups, and each of the cylinder groups is shared via a branch exhaust passage. of which is connected to the junction exhaust passage, the catalytic converter disposed in the branch passages, arranged selectively reducible selective reduction catalyst to the confluence exhaust passage a NO _X in an oxidizing atmosphere, all of the catalytic converter The air-fuel ratio of the exhaust gas flowing into the fuel cell is prohibited from becoming rich at the same time, so that the selective reduction catalyst is maintained in an oxidizing atmosphere. That is, in the third invention, a large amount of HC exhausted from _the NO _X absorbent, CO is reliably oxidized in the selective reduction catalyst.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a case where the present invention is applied to a spark ignition type engine. However, the present invention can be applied to a diesel engine. Referring to FIG. 1, the engine body 1 includes, for example, four cylinders. Each cylinder is connected to a surge tank 3 via a corresponding branch pipe 2, and the surge tank 3 is connected to an air cleaner 5 via an intake duct 4. An intake control valve 6 is arranged in the intake duct 4. Each cylinder is provided with a fuel injection valve 7 for directly injecting fuel into the cylinder. On the other hand, each cylinder through an exhaust manifold 8 and an exhaust pipe 9 NO _X absorbent 10
The catalytic converter 11 is connected to a wall-flow type catalytic converter 11 provided with an exhaust pipe 12.
The catalytic converter 11 is provided with an electric heater 17. The electric heater 17 is connected to a power supply 19 via a switch 18 that is turned on and off based on an output signal from an electronic control unit 20.

The electronic control unit 20 is composed of a digital computer, and a ROM (read only memory) 22, a RAM (random access memory) 23, a CPU (microprocessor) 24, and a power supply which are always connected to each other by a bidirectional bus 21. It has a supplied B-RAM (backup RAM) 25, an input port 26 and an output port 27. A pressure sensor 2 that generates an output voltage proportional to the pressure in the surge tank 3
The exhaust pipe 12 is provided with a temperature sensor 29 for generating an output voltage proportional to the temperature of exhaust gas flowing through the exhaust pipe 12. These sensors 28 and 29
Are input to the input port 26 via the corresponding AD converters 30, respectively. The CPU 24 calculates the intake air amount Q from the output voltage of the pressure sensor 28. The input port 26 is connected to a rotation speed sensor 31 that generates an output pulse representing the engine rotation speed N. On the other hand, the output port 27 is connected to the fuel injection valve 7 and the switch 18 via the corresponding drive circuits 32.

FIG. 2 shows the catalytic converter 11 in detail. Referring to FIG. 2, the catalytic converter 11 of the wall flow type comprises a plurality of cells defined by porous cell walls 14 extending substantially parallel to the axis of the exhaust passage, these cells being open at the exhaust upstream end 15u and An upstream-end open cell 16u in which the exhaust downstream end 15d is closed and a downstream-end open cell 16d in which the exhaust upstream end 15u is closed and the exhaust downstream end 15d is open are provided. The cells 16d are alternately and repeatedly arranged. The number of cells per 1 in ^{2 of the} cross section is, for example, 300, and the thickness of the cell wall 14 is, for example, 0.1.
3 mm. _The NO _X absorbent 10 is disposed on the inner wall surface of the upstream end opening cells 16u, _the NO _X absorbent 10 is also arranged on the inner wall surface of the downstream end open cells 16d. Accordingly, as shown by the arrow EG in FIG.
flows into the u, then the upstream end open cells 16u side NO _X
Absorbent 10, cell wall 14, downstream end open cell 16d side N
O _X absorbent 10 sequentially passes through the flow into the downstream end open cells in 16d, and flows out from the catalytic converter 11 and thus.
Incidentally, _the NO _X absorbent 10 is also on the inner wall surface of the pores of the cell walls 14 in this embodiment made of a ceramic for example is disposed, thus exhaust _the NO _X absorbent even when passing through the cell walls 14 10 Contact with.

[0009] _the NO _X absorbent 10 is potassium, for example, on a carrier K, sodium Na, alkaline earth such as lithium Li, cesium Cs, barium Ba, calcium Ca, lanthanum La, such as yttrium Y At least one selected from rare earths and platinum P
Noble metals such as t, palladium Pd, rhodium Rh and iridium Ir are supported. This NO _X absorbent 1
0 do absorbing and releasing action of the NO _X when the air-fuel ratio of the exhaust gas flowing into absorbs NO _X when the lean, the oxygen concentration in the inflowing exhaust gas to release NO _X absorbed and reduced. In addition,
_The NO _X absorbent 10 air-fuel ratio of the exhaust gas when the fuel or air is not supplied to the exhaust passage flowing into _the NO _X absorbent 10 of the upstream of the air amount to the total amount of fuel supplied to the combustion chamber of each cylinder Match the ratio of the sum.

If the above-mentioned NO _X absorbent 10 is disposed in the engine exhaust passage, the NO _X absorbent 10 actually performs the absorption and release of NO _X , but the detailed mechanism of the absorption and release is not clear. There is also. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIGS. 3 (A) and 3 (B). Next, regarding this mechanism, platinum Pt and barium Ba are deposited on the carrier.
Will be described as an example in the case of carrying other precious metals,
The same mechanism is obtained by using an alkali metal, an alkaline earth, or a rare earth.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), these oxygens O ₂ adhere to the surface of platinum Pt in the form of O ₂ ⁻ or O ²⁻ . On the other hand, NO in the exhaust gas that flows in reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Then part of the produced NO ₂ while bonding with the barium oxide BaO is absorbed into the absorbent while being further oxidized on the platinum Pt, 3 nitric acid as shown in (A) ions NO ₃ ^- in It diffuses into the absorbent in form. Thus N
O _X is absorbed in the NO _X absorbent 10.

[0012] NO ₂ is produced on the surface of the oxygen concentration is as high as platinum Pt in the inflowing exhaust gas, as long as NO ₂ to NO _X absorbing capacity of the absorbent is not saturated is absorbed in the absorbent and nitrate ions NO ₃ ^- Is generated. On the other hand, when the oxygen concentration in the exhaust gas that flows in decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus the nitrate ions NO ₃ ⁻ There are released from the absorbent in the form of NO _2. That is, the oxygen concentration in the inflowing exhaust gas are released NO _X from _the NO _X absorbent 10 when lowered. The oxygen concentration in the exhaust gas lean degree of the exhaust gas flowing to flow the lower the lowered, thus from _the NO _X absorbent 10 when lowering the lean degree of the exhaust gas flowing N
O _X is to be released.

On the other hand, if the air-fuel ratio of the inflowing exhaust gas is made rich at this time, a large amount of unburned HC and CO is discharged from the engine, and the unburned HC and CO are converted to oxygen O ₂ ^- or O ^2- on platinum Pt. And oxidize. Further, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas extremely decreases, so that NO ₂ is released from the absorbent, and this NO ₂ is unreacted as shown in FIG. H
It is reduced by reacting with C and CO. In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is released from the absorbent one after another. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, NO _X
NO _X will be released from the absorbent 10.

In the internal combustion engine shown in FIG. 1, the air-fuel ratio of the air-fuel mixture burned in the combustion chamber is normally maintained at a lean level.
O _X fuel ratio of the exhaust gas flowing into the absorbent 10 is usually lean, thus NO _X in the exhaust gas at this time is absorbed in the NO _X absorbent 10. However, of the NO _X absorbent 10 N
Since the O _x absorption capacity is limited, the N _x
Before the O _X absorption capacity is saturated, the NO _X absorbent 10
_X must be released. Therefore, in the internal combustion engine shown in FIG. 1, temporarily rich air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 when the NO _X absorption of the NO _X absorbent 10 becomes more than the set predetermined amount of so that the reduction with the release of NO _X from _the NO _X absorbent 10 Te.

By the way, the NO _X from the NO _X absorbent 10
Considering the release mechanism and the reducing action of the released NO _x , if oxygen is present in the NO _x absorbent 10, NO
It can also be considered that the _X absorbent 10 cannot purify NO _X satisfactorily. However, according to the present inventors, it has been confirmed to be able to satisfactorily purify NO _X in _the NO _X absorbent 10 when the amount of oxygen in some degree to _the NO _X absorbent 10 is present.

[0016] air-fuel ratio of the exhaust gas flowing into _the NO _X absorbent 10 is oxygen present in _the NO _X absorbent 10 when the rich been clarified why NO _X is satisfactorily purified Not. However, the following reasons are considered. That is, even though the air-fuel ratio of the air-fuel mixture burned in each cylinder during normal operation is lean, the exhaust gas discharged from each cylinder contains HC. Some of the HC is oxidized in _the NO _X absorbent 10 and the rest of the HC catalyst particles such as platinum Pt without being oxidized
Adheres on the surface of Further, NO from _the NO _X absorbent 10
_The NO _X absorbent as described above in when releasing the _X 1
Since the air-fuel ratio of the exhaust gas flowing into the exhaust gas 0 is made rich, NO _X
A large amount of HC flows into the absorbent 10, and a part of the HC adheres to the platinum Pt surface. However, this HC is platinum P
When the t surface is covered, when the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 10 is lean, oxygen O ₂ is deposited on the platinum Pt surface by O ₂ ⁻
Or to not be attached at O ^2- in the form NO _X is NO
_It becomes difficult to be absorbed by the _X absorbent 10, and thus a large amount of NO _X is discharged from the NO _X absorbent 10. On the other hand, NO _X
NO air-fuel ratio of the exhaust gas flowing into the absorbent 10 is at a rich released from _the NO _X absorbent 10 on the platinum Pt surface
_X becomes less likely to react with HC and CO in the exhaust gas, so that also in this case, a large amount of NO _X is discharged from the NO _X absorbent 10.

Meanwhile, NO from _X absorbent 10 when the air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 to be released NO _X is rich, platinum when oxygen is present in the NO _X absorbent in 10 Pt The surroundings can locally become an oxidizing atmosphere. At this time, at elevated temperature of the NO _X absorbent 10 is in the temperature of the exhaust gas flowing to the NO _X absorbent 10 is increased as compared with normal operation, HC on a result platinum Pt surface is oxidized by oxygen. Therefore, from the surface of platinum Pt, HC
There are removed, good of the NO _X absorbent 10 and thus NO _X
Purification action is ensured. Alternatively, when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 is made rich, first, the NO _X
HC and CO in the exhaust gas flowing into the absorbent 10 react with, for example, oxygen on the platinum Pt surface. As a result, the area around the platinum Pt is locally heated, whereby the reaction between HC and oxygen adhering to the surface of the platinum Pt is promoted,
Thus, HC is removed from the surface of the platinum Pt.

However, NO_XOxygen concentration in absorbent 10
Is excessively high when oxygen and HC on the platinum Pt surface or
Excessive reaction with HC in the incoming exhaust gas occurs, resulting in catalyst
When the temperature of the converter 11 becomes excessively high, the catalytic converter
May be melted. Therefore, NO_Xabsorption
NO in agent 10_XNO for good purification of NO _X
The amount of oxygen in the absorbent 10 is set within a predetermined set range.
That is, NO_XPlatinum P without causing erosion of the absorbent 10
Maintain within a range where HC on the t surface can be removed satisfactorily.
Is required.

Therefore, in this embodiment, NO_XAbsorbent 10
From NO_XTo release NO _XFlow into absorbent 10
N so that the oxygen concentration in the exhaust gas falls within this set range.
O_XThe air-fuel ratio of the exhaust gas flowing into the absorbent 10 is controlled.
I have to. Note that spark ignition as in the present embodiment is performed.
In a gasoline engine, the setting range is, for example, from 0.3% to 1.
It is about 0%. In contrast, for diesel engines
The range is, for example, about 1.0% to 2.0%. Dee
The diesel engine has a higher setting range than the gasoline engine.
Because the exhaust temperature of diesel engines is lower than that of gasoline engines
This is because erosion of the catalytic converter 11 hardly occurs.
Light oil, fuel for diesel engines, is more active than gasoline
Low oxygen demands a relatively large amount of oxygen.
You.

[0020] However, the oxygen around the platinum Pt is to be supplied by the inclusion of oxygen in the exhaust gas flowing into _the NO _X absorbent 10, the oxygen in the exhaust gas is not always platinum P
does not reach around t, so oxygen cannot be used effectively for HC removal on the platinum Pt surface. On the other hand, most of the oxygen be supplied oxygen from _the NO _X absorbent 10 around platinum Pt is possible to reach the platinum Pt.

[0021] In this embodiment therefore, stored oxygen when the oxygen concentration in the inflowing exhaust gas is high, NO _X of the oxygen storage agent around platinum Pt to the oxygen concentration in the exhaust gas releases oxygen that accumulated a lower flowing Provided in the absorbent 10, NO _X
Air-fuel ratio of the exhaust gas flowing into the absorbent 10 stored oxygen is an oxygen storage agent when the lean air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 so as to release the NO _X from _the NO _X absorbent is rich Then, oxygen is supplied from the oxygen storage agent around the platinum Pt.

On the other hand, N from the platinum Pt oxidation and removal action of the HC on the platinum Pt surface when the temperature is high around is promoted or _the NO _X absorbent 10, as described above
O _X release effect and the reduction reaction of the released NO _X is promoted. When oxygen reacts with a reducing agent such as HC on the surface of platinum Pt, the temperature around platinum Pt rises. Therefore, if a reducing agent is supplied around platinum Pt, the temperature around platinum Pt can be increased. On the other hand, as described above, when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 becomes rich, oxygen is supplied from the oxygen storage agent around the platinum Pt. Therefore, in this embodiment, the air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 is adapted to supply HC to the platinum Pt around when the rich.

The exhaust gas flowing into the NO _x absorbent 10 contains HC
Can be effectively utilized HC to those who supplied the HC from _the NO _X absorbent 10 around platinum Pt than to include increases the temperature around the platinum Pt. Therefore, in the present embodiment, the HC adsorbent that adsorbs HC when the temperature is low and desorbs the adsorbed HC when the temperature is high becomes N
Is provided on O _X absorbent 10, the air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 is low the temperature of the exhaust gas when the lean of the exhaust gas flowing to the NO _X absorbent 10 air-fuel ratio is rich Sometimes the temperature of this exhaust is raised.

That is, when the oxygen storage agent is represented by OC and the HC adsorbent is represented by AD, as shown in FIG.
Air-fuel ratio of the exhaust gas flowing into _the NO _X absorbent 10 is absorbed NO _X in the exhaust gas flowing to the NO _X absorbent 10 when the lean, the oxygen O in the exhaust gas flowing into the oxygen absorbent OC
₂ is stored and the temperature of the HC adsorbent is lowered,
HC in the exhaust gas flowing into the adsorbent AD is adsorbed. In contrast, NO _X from _the NO _X absorbent 10 as the air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 is shown in FIG. 4 (B) becomes rich is released, oxygen from an oxygen storage agent OC O ₂ Is released and the temperature of the HC adsorbent is raised, so that H
HC desorbs from the C adsorbent. The oxygen O2 released from the oxygen storage agent OC and the HC desorbed from the HC adsorbent then move on the surface of the platinum Pt and react therewith.
The temperature around t is increased. Incidentally, NO part of the HC desorbed from the HC absorbent is released from _the NO _X absorbent 10
₂ could be reduced.

As the oxygen storage agent, for example, ceria CeO ₂
Can be used. As the HC adsorbent, zeolite or mordenite can be used, and zeolite or mordenite can be used as a carrier. Therefore, in this embodiment, the the _the NO _X absorbent 10, for example zeolite or mordenite as a support, such as the carrier on, for example, potassium K, sodium Na, lithium Li, cesium Cs, barium Ba, calcium Ca At least one selected from alkaline earths, rare earths such as lanthanum La and yttrium Y, platinum Pt, palladium Pd, rhodium Rh and iridium I
It is formed by supporting a noble metal such as r and ceria CeO ₂ .

In this embodiment, in order to enrich the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10, the fuel injection valve 7 separates from the fuel injection in the engine intake stroke or the compression stroke for engine output. A second fuel injection, that is, a secondary fuel injection is performed in the expansion stroke or the exhaust stroke. The fuel from the secondary fuel injection hardly contributes to the engine output. The secondary fuel injection time TAUS when NO _X is to be released from the NO _X absorbent 10 is set to TN.
This TN is a secondary fuel injection time required to maintain the oxygen concentration in the exhaust gas in the NO _X absorbent 10 within the above-mentioned set range, and is equal to the engine load Q / N (intake air amount Q / engine speed). N) and a function of the engine speed N are obtained in advance by experiments. This TN is stored in the ROM 22 in advance in the form of a map shown in FIG.

[0027] when the temperature of _the NO _X absorbent 10 is high H
Reaction between C and oxygen can be effectively removed the HC on the platinum Pt surface when supplying a large amount of oxygen to _the NO _X absorbent 10 when the temperature of the NO _X absorbent 10 is high since active. On the other hand, when the temperature of the NO _X absorbent 10 is low N
O _X absorbent it can supply a large amount of oxygen to 10 this oxygen H
It cannot be used effectively for C removal. Rather, to reduce the temperature of the NO _X absorbent 10 or _the NO _X absorbent 10,
Inhibiting the reducing action of the NO _X release action or NO _X from. On the other hand, the temperature TEX of the exhaust gas discharged from the NO _X absorbent 10 detected by the temperature sensor 29 represents the temperature of the NO _X absorbent 10. Therefore, in this embodiment, FIG.
As shown in (A), the higher the exhaust temperature TEX, the more N
O _X secondary fuel injection time TN as the oxygen concentration COX increases in the exhaust gas of the absorbent 10 correction coefficient KRN (> 0)
I am trying to correct it.

Further, if the amount of HC desorbed from the HC adsorbent when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 becomes rich, the amount of oxygen necessary for the HC becomes necessary. Therefore, in the present embodiment obtains the HC amount SHC adsorbed by the HC adsorbent, the oxygen concentration in the exhaust in _the NO _X absorbent 10 as when adsorbed HC amount SHC often as shown in FIG. 6 (B) The secondary fuel injection time TN is corrected by the correction coefficient KRN so that COX becomes higher. The correction coefficient KRN is stored in the ROM 22 in advance in the form of a map shown in FIG. 6C as a function of the exhaust gas temperature TEX and the adsorbed HC amount SHC.

On the other hand, the secondary fuel injection timing FIT when NO _X is to be released from the NO _X absorbent 10 is set to ADV. This ADV is, for example, after compression top dead center (ATDC) 9
It is determined from 0 ° crank angle (CA) to about 120 ° CA. As described above, HC is supplied to _the NO _X absorbent 10 from the HC absorbent when the _the NO _X absorbent 10 to be released the NO _X. However, since the amount of HC discharged from the engine during normal operation is small, a sufficient amount of HC cannot be adsorbed on the HC adsorbent during normal operation. Therefore, in this embodiment, secondary fuel injection is performed during normal operation,
Thus, HC is supplied to and adsorbed to the HC adsorbent.

However, during normal operation, that is, when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 is lean, secondary fuel injection is performed and if the oxygen concentration in the exhaust gas flowing into the NO _X absorbent 10 decreases, NO NO _X is released from the _X absorbent 10. In addition, HC due to secondary fuel injection is NO _X
If the HC adsorbent is oxidized in the absorbent 10, the temperature of the HC adsorbent increases and HC is desorbed from the HC adsorbent. Then, H
Secondary fuel injection time T when HC should be supplied to C adsorbent
AUS is changed to a secondary fuel injection time T at which NO _X is not released from the NO _X absorbent 10 and HC is not desorbed from the HC adsorbent.
A. This TA is obtained by an experiment in advance as a function of the engine load Q / N and the engine speed N, and is stored in the ROM 22 in advance in the form of a map shown in FIG.

On the other hand, the secondary fuel injection timing FIT for supplying HC to the HC adsorbent is set to RTD.
Is retarded from ADV, for example, ATDC 150 ° CA
From about 180 ° CA. When the secondary fuel injection timing is delayed in this way, the ratio of HC that is burned in the combustion chamber or in the exhaust passage of the HC due to the secondary fuel injection decreases, so that the temperature of the exhaust gas flowing into the NO _X absorbent 10 is kept low. Is done. At this time, since the HC supplied to the HC adsorbent is heavy HC (polymer HC), NO
It is hardly oxidized in the _X absorbent 10. Therefore, the temperature rise of the HC adsorbent during the normal operation can be suppressed, and thus the desorption of HC from the HC adsorbent can be suppressed.

Conversely, if the secondary fuel injection timing is advanced, such as when NO _X should be released from the NO _X absorbent 10, the proportion of HC burned in the combustion chamber or in the exhaust passage increases, so that NO _X absorption is performed. The temperature of the exhaust gas flowing into the agent 10 is increased, and therefore, the desorption of HC from the HC adsorbent is promoted. Further, at this time, the HC supplied to the NO _X absorbent 10 is light HC (low molecular HC), so that the HC easily reacts in the NO _X absorbent 10. Therefore, NO _X absorbent 1
The NO _X released from 0 can be easily reduced.

As described above, in the internal combustion engine shown in FIG.
The O _X can be satisfactorily purified. However, since the fuel and the lubricating oil of the engine contain sulfur, the exhaust gas that flows in contains sulfur, for example, SO _X ,
The NO _X absorbent 10 absorbs not only NO _X but also SO _X. It is considered that the mechanism of absorbing SO _X into the NO _X absorbent 10 is the same as the mechanism of absorbing NO _X.

That is, in the same manner as when the NO _X absorption mechanism was described, platinum Pt and barium Ba were deposited on the carrier.
When the air-fuel ratio of the inflowing exhaust gas is lean as described above, the oxygen O ₂
Is attached to the surface of platinum Pt in the form of O ₂ ⁻ or O ²⁻ , and SO _X in the flowing exhaust gas, for example, SO ₂ reacts with O ₂ ⁻ or O ^{2− on} the surface of platinum Pt to form SO ₃ Becomes Next, the generated SO ₃ is further oxidized on the platinum Pt, is absorbed in the absorbent, and is bonded to barium oxide BaO, and diffuses into the absorbent in the form of sulfate ions SO ₄ ^2- . Next, this sulfate ion SO ₄ ^2- is combined with barium ion Ba ²⁺ to form sulfate BaSO ₄ .

However, this sulfate BaSO_FourDecomposes
It is difficult to simply enrich the air-fuel ratio of the inflowing exhaust
Sulfate BaSO_FourRemains undisassembled. But
NO_XAs time passes, sulfuric acid
Acid salt BaSO_FourIncrease, and thus the time
NO as time goes by_XNO that can be absorbed by the absorbent 10 _X
The amount will be reduced.

[0036] However, _the NO _X absorbent sulfate BaSO ₄ produced in the 10 to the air-fuel ratio rich or stoichiometric air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 when the temperature of the NO _X absorbent 10 is high Then, it is decomposed and sulfate ion SO ₄ ^2-
Is released from the absorber in the form of SO ₃ . Therefore, in the present embodiment, when the SO _X absorption amount of the NO _X absorbent 10 becomes larger than a predetermined amount, the NO _X absorbent 10 flows into the NO _X absorbent 10 while heating the NO _X absorbent 10 by the electric heater 17. temporarily rich air-fuel ratio of the exhaust are from _the NO _X absorbent 10 so as to release the sO _X. That is, when SO _X is to be released from the NO _X absorbent 10, the secondary fuel injection is performed while the switch 18 is turned on. The SO ₃ released at this time is H
It is immediately reduced to SO2 by C and CO.
It is also possible to from _the NO _X absorbent 10 if the air-fuel ratio of the exhaust gas flowing into _the NO _X absorbent 10 to the stoichiometric air-fuel ratio so as to release the SO _X.

The secondary fuel injection time TAUS when SO _X is to be released from the NO _X absorbent 10 is set to TS. The TS is the same as when releasing the NO _X from _the NO _X absorbent 10, the oxygen concentration in the exhaust gas in _the NO _X absorbent 10 has been determined to be within the set range mentioned above, the engine load Q
As a function of / N and the engine speed N, it is stored in the ROM 22 in advance in the form of a map shown in FIG. In addition, as the oxygen concentration in the exhaust gas in _the NO _X absorbent 10 as at high exhaust temperature TEX increases, and in _the NO _X absorbent 10 as when HC amount SHC adsorbed by the HC adsorbent is often The secondary fuel injection time TS is corrected by the correction coefficient KRS so that the oxygen concentration in the exhaust gas becomes higher. This correction coefficient KRS is stored in advance in the ROM 22 in the form of a map shown in FIG. 9 as a function of the exhaust gas temperature TEX and the adsorbed HC amount SHC. Further, the secondary fuel injection timing FIT at this time is set to ADV. Therefore, NO _X absorbent 1
From 0, SO _X can be released well.

[0038] As described above _the NO _X absorbent 1
0 The NO _X NO _X and SO _X released from the absorbent 10 is absorbed in the NO _X absorbent 10 again so is provided in the catalytic converter 11 of the wall-flow is prevented. That is, as described with reference to FIG. 2, since exhaust gas flows into the downstream end open cell 16d from the surrounding cell wall, the NO _X flowing into the downstream end open cell 16d
And SO _X are NO _X on the inner wall surface of the downstream open cell 16d.
It is prevented from reaching the absorbent 10. Therefore, N
NO _X and SO _X can be reliably and promptly released from the O _X absorbent 10.

Further, all of the exhaust gas flowing into the catalytic converter 11 is _the NO _X absorbent, HC adsorbent, and so flows through the oxygen storage agent, _the NO _X absorbent during the normal operation 1
0 can be reliably absorbed NO _X, HC can be effectively adsorbed to the HC adsorbent can be stored efficiently oxygen to an oxygen storage agent. Furthermore, NO _X absorbent 10 is disposed on the inner wall surface of the inner wall and on the downstream end open cells 16d of the upstream end open cells 16u, it is further also arranged on the pore inner wall of the cell walls 14. Therefore, NO
_X is more reliably absorbed by the NO _x absorbent 10.

[0040] Moreover, the exhaust flow rate or contact time of the space velocity of the exhaust gas in _the NO _X absorbent 10 is made to decrease as _the NO _X absorbent 10 and the exhaust, is increased as it passes through _the NO _X absorbent 10. As a result, it is possible to effectively utilize the exhaust gas oxygen concentration was allowed to decrease to of the NO _X release action or SO _X release action. FIG. 10 shows a NO _X release control routine in this embodiment. This routine is executed by interruption every predetermined set time.

Referring to FIG. 10, first, at step 40, it is determined whether or not the NO _X flag which is set when NO _X is to be released from the NO _X absorbent 10 and which is reset otherwise is set. . When the NO _X flag is reset, the routine proceeds to step 41, where the NO _X amount SN absorbed in the NO _X absorbent 10 is calculated based on, for example, the engine operating state. For example, N
Since the NO _X amount flowing into the O _X absorbent 10 increases as the engine load Q / N increases, and increases as the engine speed N increases, the product Q / N of the engine load Q / N and the engine speed N increases. It is possible to estimate the absorbed NO _X amount SN based on the integrated value of N · N. In the following step 42, it is determined whether or not the absorbed NO _X amount SN is larger than a certain value SN1.
This constant value SN1 is about 30% of the maximum NO _X amount that the NO _X absorbent 10 can absorb. When SN ≦ SN1, the processing cycle ends. On the other hand, when SN> SN1, the routine proceeds to step 43, where the NO _X flag is set.

NO_XWhen the flag is set,
From step 40 to step 44, NO_XFlag is set
Whether a certain period of time has passed since the
O_XNO of absorbent 10_XThe release action takes place over a certain period of time
Is determined. NO _XWhether the flag is set
If the specified time has not elapsed, the processing cycle
finish. In contrast, NO_XWhether the flag is set
If a certain period of time has elapsed after that, then go to step 45
Proceed, NO_XThe flag is reset.

FIG. 11 shows an SO _X release control routine in this embodiment. This routine is executed by interruption every predetermined set time. Referring to FIG. 11, first, at step 50, it is determined whether or not the SO _X flag that is set when SO _X is to be released from the NO _X absorbent 10 is reset is set otherwise. When the SO _X flag has been reset, the routine proceeds to step 51, where the SO _X amount SS absorbed in the NO _X absorbent 10 is calculated based on, for example, the engine operating state. For example, the SO _X amount flowing into the NO _X absorbent 10 increases as the integrated fuel injection amount increases, so that the absorbed SO _X amount SS can be estimated based on the integrated fuel injection amount. In the following step 52, it is determined whether or not the absorbed NO _X amount SS is larger than a fixed value SS1.
This constant value SS1 is about 30% of the maximum SO _X amount that the NO _X absorbent 10 can absorb. When SS ≦ SS1, the processing cycle ends. On the other hand, when SS> SS1, the routine proceeds to step 53, where the SO _X flag is set. In the following step 54, the switch 18 is turned on and the electric heater 17 is turned on.

SO_XWhen the flag is set,
From step 50 to step 55,_XFlag is set
Whether a certain period of time has passed since the
O_XSO of absorbent 10_XThe release action takes place over a certain period of time
Is determined. SO _XWhether the flag is set
If the specified time has not elapsed, the processing cycle
finish. In contrast, SO_XWhether the flag is set
If a certain period of time has passed since,
Go, SO_XThe flag is reset. Next step 5
At 7, the switch 18 is turned off and the electric heater 17 is turned off.
To In the following step 58, NO_XThe flag is reset
Reset or maintained in a reset state.

[0045] that is, NO when the _the NO _X absorbent 10 to be released the SO _X is an air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 is absorbed from the time _the NO _X absorbent 10 because they are rich _X Is also released. _The NO _X absorbent 10 NO _X release action time required to complete the considerably shorter than the time required to complete the SO _X release action of _the NO _X absorbent 10, therefore SO of the NO _X absorbent 10 _{When the X} release operation is completed, the NO _X release operation is also completed. Therefore, when a certain period of time has elapsed since the SO _X flag was set, not only the SO _X flag is reset, but also the NO _X flag is reset or maintained in a reset state.

FIG. 12 shows a routine for controlling the secondary fuel injection. This routine is executed by interruption every predetermined set crank angle. FIG.
First, at step 60, it is determined whether or not the SO _X flag is set. When the SO _X flag is reset, that is, when it is not time to release SO _X from the NO _X absorbent 10, the routine proceeds to step 61, where it is determined whether the NO _X flag is set. When NO _X flag is reset, i.e. NO from _the NO _X absorbent 10 _X and SO _X
If it is not time to release, then step 6
Proceeding to 2, the HC amount SHC adsorbed on the HC adsorbent of the NO _X absorbent 10 is calculated. For example, the adsorbed HC amount S
Since HC increases as the integrated fuel injection amount increases, the adsorbed HC amount SHC can be estimated based on the integrated fuel injection amount. In the following step 63, the secondary fuel injection time T for supplying HC to the HC adsorbent from the map of FIG.
A is calculated. In the subsequent step 64, the secondary fuel injection time TAUS is set to this TA. In the following step 65, the secondary fuel injection timing FIT is set to RTD.

On the other hand, when the NO _X flag is set, the routine proceeds from step 61 to step 66, where the secondary fuel injection time TN for releasing NO _X from the NO _X absorbent 10 is calculated from the map of FIG. Is done. In the following step 67, the correction coefficient KRN is calculated from the map of FIG. In the following step 68, the secondary fuel injection time TAUS is calculated in the form of the product of TN and KRN. In the following step 69, the secondary fuel injection timing FIT is set to ADV. On the other hand, when the SO _X flag is set, the process proceeds from step 60 to step 70, and the secondary fuel injection time TS for releasing SO _X from the NO _X absorbent 10 is calculated from the map of FIG. In the following step 71, the correction coefficient KRS is calculated from the map of FIG. In the following step 72, the secondary fuel injection time TAUS is calculated in the form of the product of TS and KRS. In the following step 69, the secondary fuel injection timing FIT is set to ADV.

FIG. 13 shows another embodiment. Referring to FIG. 13, the wall flow type catalyst converter 11 provided with _the NO _X absorbent 10 in the branch pipe 8a of the exhaust manifold 8
Are respectively arranged. That is, refers generally a plurality of cylinders of the internal combustion engine is divided into a plurality of cylinder groups, it comes to have arranged _the NO _X absorbent 10 in the exhaust passage connected to each cylinder group. In this embodiment, each cylinder group is formed from one cylinder, but each cylinder group may be formed from a plurality of cylinders.

On the other hand, the selective reduction catalyst 80 is located downstream of the exhaust pipe 9.
Is disposed. This selective reduction catalyst 80 is composed of platinum Pt, palladium Pd, rhodium Rh, on a porous support such as zeolite and mordenite.
Noble metals such as iridium Ir, or copper Cu, iron F
e, a transition metal such as cobalt Co and nickel Ni is supported and formed. The selective reduction catalyst 80 selectively reacts NO _X with HC and CO in an oxygen atmosphere containing a reducing agent such as hydrocarbon HC and carbon monoxide CO, thereby reducing NO _X to nitrogen N _2. Can be. That is, when the inflowing exhaust gas contains a reducing agent, the selective reduction catalyst 80 reduces NO _X in the inflowing exhaust gas even in an oxygen atmosphere. Although not shown in FIG. 13, each catalytic converter 11 is provided with an electric heater similarly to the embodiment of FIG. 1, and these electric heaters are connected to a power supply via corresponding switches.

During normal operation, the air-fuel ratio of the air-fuel mixture burned in each cylinder is lean, so that NO _X discharged from each cylinder at this time is absorbed in the corresponding NO _X absorbent 10. NO _X are also maintained in the selective reduction catalyst 80 at this time is an oxidizing atmosphere as discharged from _the NO _X absorbent 10 without being absorbed in the NO _X absorbent 10, HC in the exhaust flowing into the selective reduction catalyst 80 the NO _X because CO is included is reduced in the selective reduction catalyst 80. Moreover, H in HC adsorbent in this case _the NO _X absorbent 10
Secondary fuel injection is performed in each cylinder in order to supply C. Of the HC, H which is not adsorbed by the HC adsorbent is used.
C also reduce NO _X in the selective reduction catalyst 80, or is oxidized.

In this embodiment, the amount of absorbed NO _{X and the} amount of absorbed SO _X are obtained for each NO _X absorbent 10. When the absorbed NO _X amount of the NO _X absorbent 10 in the i-th cylinder becomes larger than the set amount, the secondary fuel injection is performed in the i-th cylinder, and the air-fuel ratio of the exhaust gas flowing into the corresponding NO _X absorbent 10 is reduced. It is rich, thereby NO _X is released and reduced. The secondary fuel injection time in this case is the same as that of the above-described embodiment, and the description is omitted.

On the other hand, when the absorbed SO _X amount of the NO _X absorbent 10 in the i-th cylinder becomes larger than the set amount, the secondary fuel injection is performed in the i-th cylinder and the corresponding NO _X absorbent 1
Air-fuel ratio of the exhaust gas flowing into the 0 is made rich, whereby SO _X is released. In this case, the richer the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10, the richer the air-fuel ratio.
There is also an idea that SO _X can be satisfactorily released from the NO _X absorbent 10. Therefore, in this embodiment, NO
_When SO _X is to be released from the _X absorbent 10, the secondary fuel injection time at this time is determined so that the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 becomes as rich as possible.

[0053] However, when the air-fuel ratio of the exhaust gas flowing from _the NO _X absorbent 10 in the NO _X absorbent 10 so as to release the NO _X or SO _X is made rich, _the NO _X absorbent 10
A large amount of HC may be discharged from the fuel cell. In this case, if the selective reduction catalyst 80 is in an oxidizing atmosphere, the selective reduction catalyst 80
Oxidizing the HC in, can be purified, the selective reduction catalyst 80 to the selective reduction catalyst 80 becomes a reducing atmosphere when the example is an air-fuel ratio of the exhaust gas flowing in all of the NO _X absorbent 10 is made rich at the same time In this case, HC cannot be oxidized well.

Therefore, in the present embodiment, the air-fuel ratio of the exhaust gas flowing into all the NO _X absorbents 10 is simultaneously inhibited from becoming rich, and the selective reduction catalyst 80 is maintained in a reducing atmosphere. That is, for example, if the absorbed SO _X amount of all the NO _X absorbents 10 becomes larger than the set amount, at this time, for example, the secondary fuel injection is performed in the first cylinder # 1, and the corresponding NO _X absorbent 10 SO _X is released, while the remaining cylinder # 2, # 3
3, and # 4 air-fuel ratio of the exhaust gas cylinder # 4, the secondary fuel injection flows into _the NO _X absorbent 10 corresponding prohibited is maintained lean. As a result, it is possible to satisfactorily oxidized in the selective reduction catalyst 80 of HC discharged from _the NO _X absorbent 10 of the first cylinder.

As long as the selective reduction catalyst 80 is maintained in the oxidizing atmosphere, the air-fuel ratio of the exhaust gas flowing into the plurality of NO _X absorbents 10 may be made rich at the same time. In this embodiment, however, it limits the _the NO _X absorbent 10 should NO _X or SO _X is released to one, the remaining three of the NO _X absorbent 1
The air-fuel ratio of the exhaust gas flowing to zero is maintained lean. This is done for the following reasons. That is, when SO _X is to be released from the NO _X absorbent 10 as described above, it is preferable to make the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 as rich as possible. On the other hand, if the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 is made considerably rich, a large amount of HC is discharged from the NO _X absorbent 10, and a large amount of oxygen is required to oxidize the large amount of HC. Therefore, in the present embodiment, the air-fuel ratio of the exhaust gas discharged from the three cylinders is maintained lean, whereby a large amount of oxygen flows into the selective reduction catalyst 80.

In other words, SO_XTo release N
O_XThe amount of HC discharged from the absorbent 10 is determined by the selective reduction catalyst.
Until the oxygen flowing into the 80 reaches the maximum amount that can be oxidized.
NO _XIncrease the air-fuel ratio of the exhaust gas flowing into the absorbent 10 to the rich side
can do. Therefore, in this embodiment, the selective reduction
SO 2 based on the amount of oxygen flowing into the medium 80_XPerform release action
NO to do_XThe air-fuel ratio of the exhaust gas flowing into the absorbent 10,
So as to determine the secondary fuel injection time in the corresponding cylinder
are doing.

That is, the amount of oxygen flowing into the selective reduction catalyst 80 increases as the engine operating state, for example, the engine load Q / N increases, and increases as the engine speed N increases. Therefore, the secondary fuel injection time TSS when the SO _X releasing action is to be performed is set to be longer as the engine load Q / N is higher, and to be longer as the engine speed N is higher. This secondary fuel injection time TSS is shown in FIG.
4 is stored in the ROM 22 in advance in the form of a map shown in FIG.

FIG. 15 shows a NO _X release control routine in this embodiment. This routine is executed by interruption every predetermined set time. Referring to FIG. 15, first, in step 100, 1 is sequentially assigned to a variable i.
2, 3, and 4 are repeatedly substituted. Subsequent step 101
Is set when NO _X is to be released from the NO _X absorbent 10 of the i-th cylinder, and reset otherwise.
It is determined whether the O _X (i) flag is set. When the NO _X (i) flag is reset, the routine proceeds to step 102, where the NO _X amount SN (i) absorbed in the NO _X absorbent 10 of the i-th cylinder is calculated based on, for example, the engine operating state. . In the following step 103, it is determined whether or not the absorbed NO _X amount SN (i) is larger than a certain value SN1. When SN (i) ≦ SN1, the processing cycle ends. On the other hand, SN (i)> SN
When the value is 1, the routine then proceeds to step 104, where it is determined whether the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 other than the i-th cylinder is made rich, that is, the NO _X release of the NO _X absorbent 10 other than the i-th cylinder. It is determined whether the action or the SO _X releasing action is being performed. the processing cycle is ended when the air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 other than the i-th cylinder is made rich. NO other than the i-th cylinder
_If the air-fuel ratio of the exhaust gas flowing into the _X absorbent 10 is not made rich, that is, if the air-fuel ratio of the exhaust gas flowing into all the NO _X absorbents 10 is lean, then step 1
Proceeding to 05, the NO _X (i) flag is set.

When the NO _X (i) flag is set, the process proceeds from step 101 to step 106, where NO
_X (i) flag whether a predetermined time has elapsed or after being set, i.e. the i-th cylinder NO _X absorbent 10 NO
_It is determined whether or not the _X release action has been performed for a predetermined time or more. If the predetermined time has not elapsed since the NO _X (i) flag was set, the processing cycle ends. On the other hand, when a predetermined time or more has elapsed since the NO _X (i) flag was set, the routine proceeds to step 107, where the NO _X (i) flag is reset.

That is, when, for example, i = 1 is set in step 100, the absorbed NO _X amount SN (1) of the NO _X absorbent 10 of the first cylinder is calculated in step 102, and in step 103, SN (1) is changed to SN1. It is determined whether or not it is greater than Proceeds to step 104 when the SN (1)> SN1, 2 cylinder, # 3 cylinder, the air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 of the fourth cylinder is equal to or rich is determined. If the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 of the second cylinder, the third cylinder, and the fourth cylinder is not rich, NO in step 105
_X (1) The flag is set. When the NO _X (1) flag is set, the process proceeds from step 101 to step 106. When a predetermined time has elapsed since the NO _X (1) flag was set, the NO _X (1) flag is reset in step 107.

FIG. 16 shows the SO _X release control routine in this embodiment. This routine is executed by interruption every predetermined set time. Referring to FIG. 16, first, at step 110, 1 is sequentially assigned to a variable i.
2, 3, and 4 are repeatedly substituted. Next step 111
Is set when SO _X is to be released from the NO _X absorbent 10 of the i-th cylinder, and reset otherwise.
It is determined whether the O _X (i) flag is set. When the SO _X (i) flag has been reset, the process then proceeds to step 112, where the SO _X amount SS (i) absorbed by the NO _X absorbent 10 of the i-th cylinder is calculated based on, for example, the engine operating state. . In the following step 113, it is determined whether or not the absorbed NO _X amount SS (i) is larger than a fixed value SS1. When SS (i) ≦ SS1, the processing cycle ends. On the other hand, SS (i)> SS
If it is 1, then the routine proceeds to step 114, where it is determined whether the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 10 other than the i-th cylinder is made rich, that is, the NO _X release of the NO _X absorbent 10 other than the i-th cylinder. It is determined whether the action or the SO _X releasing action is being performed. the processing cycle is ended when the air-fuel ratio of the exhaust gas flowing to the NO _X absorbent 10 other than the i-th cylinder is made rich. NO other than the i-th cylinder
_If the air-fuel ratio of the exhaust gas flowing into the _X absorbent 10 is not made rich, that is, if the air-fuel ratio of the exhaust gas flowing into all the NO _X absorbents 10 is lean, then step 1
Proceeding to 15, the SO _X (i) flag is set. In the following step 116, the electric heater 17 of the i-th cylinder is turned on.

When the SO _X (i) flag is set, the process proceeds from step 111 to step 117, where the SO _X (i) flag is set.
_X (i) Whether a predetermined time or more has elapsed since the flag was set, that is, the SO _X of the SO _X absorbent 10 of the i-th cylinder
_It is determined whether or not the _X release action has been performed for a predetermined time or more. If the predetermined time has not elapsed since the SO _X (i) flag was set, the processing cycle ends. On the other hand, when a certain time or more has elapsed since the SO _X (i) flag was set, the routine proceeds to step 118, where the SO _X (i) flag is reset. In the following step 119, the electric heater 17 of the i-th cylinder is turned off. In the following step 120, the NO _X (i) flag is reset or maintained in a reset state.

FIG. 17 shows a routine for controlling the secondary fuel injection. This routine is executed by interruption every predetermined set crank angle. FIG.
First, at step 130, 1, 2, 3, and 4 are repeatedly assigned to a variable i sequentially. Next step 1
At 31, it is determined whether the SO _X (i) flag is set. When SO _X (i) flag is reset, that is, from _the NO _X absorbent 10 of the i-th cylinder SO
_If it is not time to release _X , then the routine proceeds to step 132, where it is determined whether the NO _X (i) flag is set. When the NO _X (i) flag is reset, that is, when it is not necessary to release NO _X and SO _X from the NO _X absorbent 10 of the i-th cylinder, the routine proceeds to step 133, where the NO _X absorbent of the i-th cylinder is HC amount adsorbed on the 10 HC adsorbents SHC
(I) is calculated. In the following step 134, the secondary fuel injection time TA for supplying HC to the HC adsorbent is calculated from the map of FIG. In the following step 135, i
The secondary fuel injection time TAUS (i) of the cylinder No. is defined as TA. In the following step 136, the secondary fuel injection timing FIT (i) of the i-th cylinder is set to RTD.

On the other hand, when the NO _X (i) flag is set, steps 132 to 137 are performed.
The process proceeds, NO _X absorbent 10 of the i-th cylinder from the map of FIG. 5
, A secondary fuel injection time TN for releasing NO _X is calculated. In the following step 138, the correction coefficient KRN is calculated from the map of FIG. Subsequent step 13
In step 9, the secondary fuel injection time TAUS (i) of the i-th cylinder is calculated in the form of the product of TN and KRN. Next step 140
Then, the secondary fuel injection timing FIT (i) of the i-th cylinder is ADV
It is said. On the other hand, when the SO _X (i) flag is set, the process proceeds from step 131 to step 141,
The secondary fuel injection time TSS for releasing SO _X from the NO _X absorbent 10 is calculated from the map of FIG. In the following step 142, the secondary fuel injection time TA of the i-th cylinder
US (i) is the TSS. Next step 140
Then, the secondary fuel injection timing FIT (i) of the i-th cylinder is ADV
It is said. Note that other configurations and operations of the exhaust gas purification device are the same as those of the above-described embodiment, and thus description thereof will be omitted.

[0065]

According to the present invention, it is possible to prevent NO _X released from the NO _X absorbent from being absorbed again by the NO _X absorbent.

[Brief description of the drawings]

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

FIG. 2 is a partially enlarged sectional view of a catalytic converter.

FIG. 3 is a diagram for explaining the NO _X absorbing / releasing action of a NO _X absorbent.

FIG. 4 is a diagram for explaining the NO _X absorbing / releasing action of a NO _X absorbent, the oxygen absorbing / releasing action of an oxygen storage agent, and the HC adsorption / desorption action of an HC adsorbent.

FIG. 5 is a diagram showing a map of a secondary fuel injection time TN.

FIG. 6 is a diagram showing a map of a correction coefficient KRN.

FIG. 7 is a diagram showing a map of a secondary fuel injection time TA.

FIG. 8 is a diagram showing a map of a secondary fuel injection time TS.

FIG. 9 is a diagram showing a map of a correction coefficient KRS.

FIG. 10 is a flowchart for executing NO _X release control.

FIG. 11 is a flowchart for executing SO _X release control.

FIG. 12 is a flowchart for controlling secondary fuel injection.

FIG. 13 is an overall view of an internal combustion engine showing another embodiment.

FIG. 14 is a diagram showing a map of a secondary fuel injection time TSS.

FIG. 15 is a flowchart for executing NO _X release control in the embodiment of FIG.

FIG. 16 is a flowchart for executing SO _X release control in the embodiment of FIG.

FIG. 17 is a flowchart for controlling secondary fuel injection in the embodiment of FIG.

[Explanation of symbols]

1 ... engine body 7 ... fuel injector 8 ... exhaust manifold 10 ... NO _X absorbent 11 ... catalytic converter 14 ... cell walls 15u ... exhaust upstream end 15d ... exhaust downstream end 16u ... upstream end open cells 16d ... downstream end open cells

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) F01N 3/24 F01N 3/24 R 3/28 301 3/28 301M 301C 301H F02D 41/04 305 F02D 41 / 04 305A 355 355 41/14 310 41/14 310B 41/36 41/36 B 41/40 41/40 NF term (reference) 3G091 AA12 AA13 AA17 AA18 AA24 AA28 AA29 AB05 AB06 BA01 BA11 BA14 CA04 CA18 CB02 CB03 CB06 DA03 DA04 DA08 DB06 DB10 DB13 EA01 EA03 EA06 EA17 EA30 EA31 FA07 FB10 FB11 FB12 GA06 GA17 GA18 GA23 GB01W GB02W GB03W GB04W GB05W GB06W GB07W GB09X GB17X HA08 HA11 HA12 HA36 HA45 HB02 3G301 MA01 HA01 HA04 HA06 NE15 PA07A PD11A PE01A PE03A PE04A PE05A

Claims (3)

[Claims] An exhaust purification device for an internal combustion engine having a catalytic converter in an exhaust passage, the catalytic converter comprising a plurality of cells defined by porous cell walls extending substantially parallel to an exhaust passage axis. These cells include an upstream end open cell in which the exhaust upstream end is opened and the exhaust downstream end is closed, and a downstream end open cell in which the exhaust upstream end is closed and the exhaust downstream end is open. And the downstream end open cell are alternately and repeatedly arranged. When the air-fuel ratio of the inflowing exhaust gas is lean, NO
Absorbs _X, the oxygen concentration in the inflowing exhaust gas is disposed on the inner wall surface of the inner wall and on the downstream end open cells of the upstream end opening cells to _the NO _X absorbent to release the NO _X that is absorbed decreases, an exhaust purification system of an internal combustion engine which is adapted allowed to temporarily reduce the oxygen concentration in the exhaust gas flowing to the NO _X absorbent when it should emit NO _X that is absorbed from _the NO _X absorbent. 2. SO _X absorbed from a NO _X absorbent
An exhaust purification system of an internal combustion engine according to claim 1 which is adapted to temporarily rich air-fuel ratio of the exhaust gas flowing to the NO _X absorbent when it should emit. 3. The internal combustion engine includes a plurality of cylinders, the cylinders are divided into a plurality of cylinder groups, and each of the cylinder groups is connected to a common combined exhaust passage via a branch exhaust passage. placing the catalytic converter in the branch passages, arranged NO _X selectively reducible selective reduction catalyst to the confluence exhaust passage in an oxidizing atmosphere, all of the air-fuel ratio of the exhaust gas flowing into the catalytic converter rich simultaneously 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the selective reduction catalyst is maintained in an oxidizing atmosphere by preventing the selective reduction catalyst from becoming oxidized.