JP2000274230A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent SOx poisoning of a NOx catalyst in an exhaust emission control device provided with an occlusion reduced form NOx catalyst in an exhaust passage. SOLUTION: Downstream of a SOx absorbent 17, a NOx absorbent 20 and a bypass pipe 26 detouring the NOx absorbent 20, and at the initial section of the bypass pipe 26, a switch valve 28 is provided to switch the pass of exhaust gas. When treating the SOx exhaust by the SOx absorbent 17, at the same time of the start of stoichiometric or rich air fuel ration control, the switch valve 28 is held at a bypass opening position to guide exhaust gas to the bypass pipe 26 to prevent the gas from flowing to the NOx catalyst 20. At the same time as finishing the treatment of SOx emission, stoichiometric or rich air fuel ration control is switched to lean air/fuel ratio control. After the lapse of prescribed delay time from the switching of the control of the air/fuel ratio, the switching valve 28 is held at a bypass closing position to guide exhaust gas to the NOx catalyst 20 so as to prevent exhaust gas from flowing into the bypass pipe 26.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing nitrogen oxides (NO
The present invention relates to an exhaust gas purification device capable of purifying x).

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for purifying NOx from exhaust gas discharged from an internal combustion engine capable of lean combustion, there is a NOx absorbent represented by an NOx storage reduction catalyst. N
The Ox absorbent has a lean (ie, air-fuel) ratio of the incoming exhaust gas.
NOx is absorbed when the oxygen concentration of the inflowing exhaust gas is reduced, and the absorbed NOx is released when the oxygen concentration of the inflowing exhaust gas is reduced. air-fuel ratio of the exhaust gas is lean (i.e., under oxygen-rich atmosphere) to absorb NOx when the oxygen concentration of the inflowing exhaust gas is the catalyst for reducing the absorbed NOx release and N ₂ when dropped.

When this storage-reduction type NOx catalyst (hereinafter sometimes simply referred to as a catalyst or NOx catalyst) is disposed in an exhaust passage of an internal combustion engine capable of lean combustion, when exhaust gas having a lean air-fuel ratio flows, the exhaust gas contains NOx is absorbed by the catalyst, and NO is absorbed by the catalyst when exhaust gas having a stoichiometric (stoichiometric air-fuel ratio) or rich air-fuel ratio flows.
x is released as NO ₂ , and HC and C
It is reduced to N ₂ by a reducing component such as O, that is, NOx is purified.

In general, the fuel of an internal combustion engine contains sulfur, and when the fuel is burned in the internal combustion engine, the sulfur in the fuel burns and sulfur oxides such as SO ₂ and SO ₃ (SOx ) Occurs. The storage reduction type NOx catalyst includes N 2
Since SOx in the exhaust gas is absorbed by the same mechanism as that for absorbing Ox, when this NOx catalyst is arranged in the exhaust passage of the internal combustion engine, not only NOx but also SOx is absorbed by the NOx catalyst.

However, the sulfur absorbed by the NOx catalyst
Ox forms stable sulfate over time,
Under the same conditions as those for releasing and reducing NOx from the NOx catalyst, there is a tendency that the NOx catalyst is hardly decomposed and released and is easily accumulated in the catalyst. When the accumulated amount of SOx in the NOx catalyst increases,
The NOx absorption capacity of the catalyst decreases, so that NOx in the exhaust gas cannot be sufficiently removed, and the NOx purification efficiency decreases. This is so-called SOx poisoning.

In order to maintain the NOx purification performance of the NOx storage reduction catalyst high for a long period of time, an SOx absorbent that exclusively absorbs SOx in exhaust gas is disposed upstream of the NOx catalyst. An exhaust gas purifying apparatus has been developed which prevents SOx poisoning by preventing SOx from flowing into the exhaust gas.

[0007] The SOx absorbent, the air-fuel ratio of the inflow gas absorbs SOx when the lean, but the SOx absorbed when the oxygen concentration in the inlet gas is low is to release as SO _2, the SOx absorption Since the SOx absorbing capacity of the agent is limited, it is necessary to execute a process of actively releasing SOx from the SOx absorbent before the SOx absorbent is saturated with SOx, that is, a regeneration process.

[0008] Regarding the regeneration treatment technology of the SOx absorbent,
For example, it is disclosed in Japanese Patent Publication No. 2605580. According to this publication, in order to release the SOx absorbed by the SOx absorbent, it is necessary to lower the oxygen concentration by increasing the air-fuel ratio of the inflowing exhaust gas to stoichiometric or rich, and to increase the temperature of the SOx absorbent. It is said that SOx is more easily released.

Further, in the regeneration treatment technology disclosed in this publication, when the SOx is released from the SOx absorbent, the released SOx is prevented from being absorbed by the NOx catalyst disposed downstream. An exhaust path switching valve that branches off from an exhaust pipe connecting the SOx absorbent and the NOx catalyst and bypasses the NOx catalyst, and selectively switches exhaust gas to either the NOx catalyst or the bypass path. When the regeneration process of the SOx absorbent is being performed, the exhaust gas is caused to flow to the bypass passage by controlling the exhaust path switching valve so as not to flow to the NOx catalyst, and when the regeneration process is not being performed, the exhaust path switching valve is provided. By controlling the exhaust gas, the exhaust gas flows to the NOx catalyst and does not flow to the bypass passage. With this configuration, during the execution of the regeneration process, the SOx released from the SOx absorbent does not flow into the NOx catalyst, so that it is possible to prevent the NOx catalyst from poisoning the SOx.

[0010]

By the way, in the conventional technique, when performing the regeneration process of the SOx absorbent, the air-fuel ratio of the exhaust gas is made stoichiometric or rich to lower the oxygen concentration, and at the same time, the exhaust path switching valve is used. To control the exhaust gas to flow through the bypass passage, maintain this state during the regeneration process, and when the regeneration process is completed, make the air-fuel ratio of the exhaust gas lean and at the same time control the exhaust path switching valve to control the exhaust gas. The exhaust gas path is switched so that the NO.

However, if the exhaust path switching valve is controlled to be switched at the above timing, the regeneration process is completed and the air-fuel ratio of the exhaust gas is made lean. The released SOx is absorbed by the NOx catalyst, and the NOx catalyst
There was a problem of poisoning.

The cause is presumed as follows. During the regeneration process, exhaust gas having a stoichiometric or rich air-fuel ratio flows through the SOx absorbent. At that time, the SOx absorbent not only releases SOx but also adsorbs HC in the exhaust gas. Therefore, at the end of the regeneration process, a large amount of HC is contained in the SOx absorbent.
Is adsorbed. On the other hand, even at the end of the regeneration process, not all of the SOx absorbed by the SOx absorbent is released, and the absorbed SOx remains in the SOx absorbent.

At the end of the regeneration process in such a state, SOx
When exhaust gas having a lean air-fuel ratio is flown into the absorbent, a large amount of oxygen contained in the exhaust gas is consumed in large amounts to oxidize HC adsorbed on the SOx absorbent, and as a result, S
Inside the Ox absorbent, the air-fuel ratio of the exhaust gas becomes a stoichiometric atmosphere, and immediately after the end of the regeneration process, the SOx absorbent is still at a high temperature. Released from the drug. This SO
The desorption of SOx from the x-absorbent reduces the amount of HC adsorbed on the SOx-absorbent, reduces the oxygen consumption for oxidizing HC, and increases the air-fuel ratio inside the SOx-absorbent. Continue until.

At the end of the regeneration process, the exhaust path switching valve is controlled so that the air-fuel ratio of the exhaust gas becomes lean and at the same time the exhaust gas flows to the NOx catalyst.
SOx released from the x absorbent is combined with exhaust gas to NOx
It seems to flow to the catalyst and poison the NOx catalyst.

The present invention has been made in view of such problems of the prior art, and an object to be solved by the present invention is to reduce the air-fuel ratio of the exhaust gas flowing into the SOx absorbent from stoichiometric or rich. S immediately after switching to lean
An object of the present invention is to prevent SOx poisoning of the NOx absorbent more reliably by preventing SOx released from the Ox absorbent from flowing into the NOx absorbent.

[0016]

The present invention has the following features to attain the object mentioned above. (1) The exhaust gas purifying apparatus for an internal combustion engine according to the first invention of the present application is (A) arranged in an exhaust passage of an internal combustion engine capable of lean combustion, and when the air-fuel ratio of the inflowing exhaust gas is lean, S
An SOx absorbent that absorbs Ox and releases the absorbed SOx when the oxygen concentration of the exhaust gas flowing in is low;
NOx is disposed in the exhaust passage downstream of the Ox absorbent and absorbs NOx when the inflowing exhaust gas has a lean air-fuel ratio and absorbs when the inflowing exhaust gas has a low oxygen concentration.
a NOx absorbent that releases x, (c) a bypass passage that branches downstream of the SOx absorbent and bypasses the NOx absorbent and flows exhaust gas, and (d) an exhaust gas that flows out of the SOx absorbent. Exhaust path switching means for selectively switching to which of the NOx absorbent and the bypass path,
(E) the exhaust passage so that the exhaust gas flowing out of the SOx absorbent is guided to the bypass passage during the SOx release process of releasing the SOx absorbed by the SOx absorbent by setting the air-fuel ratio of the exhaust gas to stoichiometric or rich. The switching means is controlled, and after a predetermined time has passed since the air-fuel ratio of the exhaust gas was switched to lean upon completion of the SOx release process, the SOx
Exhaust path switching control means for controlling the exhaust path switching means so that exhaust gas flowing out of the absorbent is guided to the NOx absorbent.

In the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect of the present invention, the exhaust gas having the stoichiometric or rich air-fuel ratio flows into the SOx absorbent during the SOx release process, whereby the SOx absorbed by the SOx absorbent is removed. Released and released. Since the released SOx flows into the bypass passage, it does not flow into the NOx absorbent. Therefore, SOx
During the release process, the NOx absorbent does not poison SOx.
Incidentally, SOx released from the SOx absorbent is caused to reduced by the unburned HC, CO in the exhaust gas, it is discharged as SO _2.

After the end of the SOx release process, the air-fuel ratio of the exhaust gas is switched to lean. However, even if the air-fuel ratio is switched to the lean air-fuel ratio, the exhaust gas is guided to the bypass passage until a predetermined time elapses, and flows into the NOx absorbent. Absent. Therefore, after the end of the SOx release process, SOx released from the SOx absorbent immediately after the switching of the lean air-fuel ratio of the exhaust gas also flows through the bypass passage, and does not flow into the NOx absorbent.
Therefore, even during this time, the NOx absorbent does not poison SOx.

The SOx that has been released from the SOx absorbent immediately after switching to the lean air-fuel ratio also becomes Sx by the time the predetermined time elapses after the switching to the lean air-fuel ratio.
It is no longer released from the Ox absorbent. Therefore, when the exhaust gas starts flowing to the NOx absorbent after the elapse of the predetermined time, since the SOx is not released from the SOx absorbent, the NOx absorbent does not poison the SOx.

(2) The exhaust gas purifying apparatus for an internal combustion engine according to the second invention of the present application is (a) disposed in an exhaust passage of an internal combustion engine capable of lean combustion, when the air-fuel ratio of the inflowing exhaust gas is lean. An SOx absorbent that absorbs SOx and releases the absorbed SOx when the oxygen concentration of the exhaust gas flowing in is low;
(B) disposed in the exhaust passage downstream of the SOx absorbent, and when the air-fuel ratio of the inflowing exhaust gas is lean, N
A NOx absorbent that absorbs Ox and releases the absorbed NOx when the oxygen concentration of the exhaust gas flowing in is low;
(D) whether the exhaust gas flowing out of the SOx absorbent is led to the NOx absorbent or the bypass passage, which branches off downstream of the Ox absorbent and flows the exhaust gas around the NOx absorbent. Exhaust path switching means for selectively switching, (e) oxygen concentration detecting means provided between the SOx absorbent and the NOx absorbent, and (f) the SOx by making the air-fuel ratio of exhaust gas stoichiometric or rich. At the time of SOx release processing for releasing SOx absorbed by the absorbent, SOx
Controlling the exhaust path switching means so that the exhaust gas flowing out of the absorbent is guided to the bypass passage;
When it is determined that the air-fuel ratio of the exhaust gas downstream of the SOx absorbent is lean based on the oxygen concentration detected by the oxygen concentration detecting means after the end of the release process, the exhaust gas flowing out of the SOx absorbent becomes the NOx absorbent. Exhaust path switching control means for controlling the exhaust path switching means so as to be guided to

In the exhaust gas purifying apparatus for an internal combustion engine according to the second aspect of the present invention, the exhaust gas having the stoichiometric or rich air-fuel ratio flows into the SOx absorbent during the SOx release process, whereby the SOx absorbed by the SOx absorbent is removed. Released and released. Since the released SOx flows into the bypass passage, it does not flow into the NOx absorbent. Therefore, SOx
During the release process, the NOx absorbent does not poison SOx.
Incidentally, SOx released from the SOx absorbent is caused to reduced by the unburned HC, CO in the exhaust gas, it is discharged as SO _2.

The air-fuel ratio of the exhaust gas is switched to lean by the end of the SOx release process. However, even if the air-fuel ratio is switched to the lean air-fuel ratio, the exhaust gas downstream of the SOx absorbent is determined based on the oxygen concentration detected by the oxygen concentration detecting means. Until the air-fuel ratio of the gas is determined to be lean, the exhaust gas is guided to the bypass passage and does not flow into the NOx absorbent. Therefore, after the end of the SOx release processing, SOx released from the SOx absorbent immediately after switching to the lean air-fuel ratio also flows through the bypass passage, and does not flow into the NOx absorbent. Therefore, even during this time, the NOx absorbent does not poison SOx.

On the other hand, if the air-fuel ratio of the exhaust gas downstream of the SOx absorbent is lean, SOx should not have been released from the SOx absorbent at that time. Therefore,
When it is determined that the air-fuel ratio of the exhaust gas downstream of the SOx absorbent is lean and the exhaust gas flows to the NOx absorbent, SOx is not released from the SOx absorbent. Will not poison.

In the exhaust gas purifying apparatus for an internal combustion engine according to the first or second aspect of the present invention, examples of the internal combustion engine capable of lean burn include a direct-injection lean-burn gasoline engine and a diesel engine. it can. In the case of a lean burn gasoline engine, the air-fuel ratio control of the exhaust gas can be realized by the air-fuel ratio control of the air-fuel mixture supplied to the combustion chamber. In the case of a diesel engine, the air-fuel ratio of exhaust gas is controlled by performing so-called sub-injection in which fuel is injected during an intake stroke, an expansion stroke, or an exhaust stroke, or by supplying a reducing agent into an exhaust passage upstream of the SOx absorbent. It can be realized by doing. Here, the air-fuel ratio of the exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the upstream NOx absorbent.

In the exhaust gas purifying apparatus for an internal combustion engine according to the first invention or the second invention, the NOx absorbent may be an occlusion reduction type NOx catalyst. NOx storage reduction catalyst, the air-fuel ratio of the inflowing exhaust gas absorbs NOx when the lean, the oxygen concentration in the inflowing exhaust gas to release NOx absorbed to decrease is the catalyst for reducing the N ₂ . This storage-reduction NOx catalyst uses, for example, alumina as a carrier, and on the carrier, for example, alkali metals such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earths such as barium Ba, calcium Ca, and lanthanum La. And at least one selected from rare earth elements such as yttrium Y and a noble metal such as platinum Pt.

In the exhaust gas purifying apparatus for an internal combustion engine according to the first or second invention, the SOx absorbent may be a three-way catalyst, a storage reduction type NOx catalyst, or a selective reduction type NOx.
A catalyst and the like can be exemplified. Here, the selective reduction type N
Ox catalyst refers to a catalyst that reduces or decomposes NOx in the presence of hydrocarbons in an oxygen-excess atmosphere, and a catalyst in which a transition metal such as Cu is ion-exchanged on zeolite and a noble metal is supported on zeolite or alumina. Catalysts and the like.

In the exhaust gas purifying apparatus for an internal combustion engine according to the first or second aspect of the present invention, the exhaust path switching means may be constituted by a single switching valve provided at a branch portion of a bypass passage. Alternatively, a first opening / closing valve is provided in an exhaust passage located closer to the NOx absorbent downstream of the branch portion, and a second opening / closing valve is provided in a bypass passage. When one opening / closing valve is opened, the other opening / closing valve is opened. It can also be configured to be controlled to close.

In the exhaust gas purifying apparatus for an internal combustion engine according to the first invention or the second invention, "SOx release processing"
This means, of course, that the air-fuel ratio of the exhaust gas is controlled to be stoichiometric or rich in order to positively release the SOx absorbed by the SOx absorbent. As a result of controlling the air-fuel ratio of the combustion gas to stoichiometric or rich for the combustion in the fuel cell, the air-fuel ratio of the exhaust gas becomes stoichiometric or rich and SOx is released from the SOx absorbent. Here, the “request from the operating state of the internal combustion engine” is, for example,
At the time of high load operation of the internal combustion engine, at the time of full load operation, at the time of warm-up operation at the time of starting, and at the time of acceleration in the case of an internal combustion engine for driving a vehicle,
For example, during high-speed constant-speed operation.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to FIGS.

[First Embodiment] FIG. 1 is a diagram showing a schematic configuration in a case where the present invention is applied to a gasoline engine for a vehicle capable of lean combustion. In this figure, reference numeral 1 denotes an engine body, reference numeral 2 denotes a piston, reference numeral 3 denotes a combustion chamber, reference numeral 4 denotes a spark plug, reference numeral 5 denotes an intake valve, reference numeral 6 denotes an intake port, reference numeral 7 denotes an exhaust valve, reference numeral 8 denotes an exhaust port. Are shown respectively.

The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and each branch pipe 9 is provided with a fuel injection valve 11 for injecting fuel into the intake port 6. The surge tank 10 is connected to an air cleaner 14 via an intake duct 12 and an air flow meter 13, and a throttle valve 15 is arranged in the intake duct 12.

On the other hand, the exhaust port 8 is connected to the exhaust manifold 16.
Is connected to a casing 18 containing an SOx absorbent 17 via an exhaust pipe 19, and an outlet of the casing 18 is connected to a casing 21 containing a storage-reduction type NOx catalyst (NOx absorbent) 20 via an exhaust pipe 19, Is the exhaust pipe 2
2, and is connected to a muffler (not shown). still,
In the following description, the NOx storage reduction catalyst 20 is referred to as a NOx catalyst 20. The SOx absorbent 17 and the NOx catalyst 20 will be described later in detail.

The inlet 21a of the casing 21 and the exhaust pipe 2
2 is connected by a bypass pipe (bypass passage) 26 that bypasses the NOx catalyst 20, and an exhaust switching in which a valve element is actuated by an actuator 27 is provided at an inlet 21 a of a casing 21 which is a branch of the bypass pipe 26. A valve (exhaust path switching means) 28 is provided. This exhaust switching valve 28 (hereinafter, abbreviated as switching valve 28) is closed by an actuator 27 so as to close the inlet of the bypass pipe 26 and fully open the inlet to the NOx catalyst 20 as shown by the solid line in FIG. One of a closed position and a bypass open position where the inlet to the NOx catalyst 20 is closed and the inlet of the bypass pipe 26 is fully opened as shown by a broken line in FIG. .

An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, and a CPU (Central Processor) by a bidirectional bus 31. Unit) 3
4, an input port 35 and an output port 36 are provided. The air flow meter 13 generates an output voltage proportional to the amount of intake air, and this output voltage is input to an input port 35 via an AD converter 37.

On the other hand, the exhaust pipe 1 downstream of the SOx absorbent 17
9 is provided with a temperature sensor 23 for generating an output voltage proportional to the temperature of the exhaust gas discharged from the SOx absorbent 17, and the output voltage of the temperature sensor 23 is supplied to an input port 35 via an AD converter 38. Is entered. The input port 35 is connected to a rotation speed sensor 41 that generates an output pulse indicating the engine rotation speed. The output port 36 is connected to the ignition plug 4, the fuel injection valve 11, and the actuator 27 via a corresponding drive circuit 39, respectively.

In this gasoline engine, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = TP · K Here, TP indicates a basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by an experiment, 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.
2 is stored. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, if K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and K> 1.
When it becomes 0, the air-fuel ratio of the mixture supplied to the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

In the gasoline engine of this embodiment, the lean air-fuel ratio control is performed with the value of the correction coefficient K being smaller than 1.0 in the low engine load operation region of the engine, and the engine high load operation region is performed. During a warm-up operation, an acceleration, and a constant speed operation at 120 km / h or more when the engine is started, the value of the correction coefficient K is set to 1.0, and the stoichiometric control is performed. Is set to a value larger than 1.0 and the rich air-fuel ratio control is performed.

In an internal combustion engine, low-medium load operation is usually performed most frequently, and therefore, during most of the operation period, the value of the correction coefficient K is made smaller than 1.0, and the lean mixture is burned. become.

FIG. 3 schematically shows the concentration of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from this figure, 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. The concentration of oxygen O _{2 in} the discharged exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NOx contained in the casing 21
The catalyst 20 has, for example, alumina as a carrier, on which potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, barium Ba, alkaline earth such as calcium Ca, lanthanum La, yttrium Y At least one selected from such rare earths and a noble metal such as platinum Pt are supported.

When the NOx catalyst 20 is disposed in the exhaust passage of the engine, the NOx catalyst 20 is arranged such that when the air-fuel ratio of the inflowing exhaust gas (hereinafter referred to as exhaust air-fuel ratio) is lean, the NOx catalyst 20
And absorbs and releases NOx when the oxygen concentration in the inflowing exhaust gas decreases. Here, the exhaust air-fuel ratio means the engine intake passage and the NOx catalyst 2
It means the ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of zero.

When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx catalyst 20, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. Therefore, in this case, the NOx catalyst 2
0 means that when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, NOx is absorbed, and when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases, the absorbed NOx is released. .

The detailed mechanism of the NOx absorption / release operation of the NOx catalyst 20 is not clear in some parts. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking platinum Pt and barium Ba supported on a carrier as an example.
The same mechanism is obtained by using an alkali metal, an alkaline earth, or a rare earth.

That is, when the air-fuel ratio of the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases,
As shown in FIG. 4A, oxygen O ₂ adheres to the surface of platinum Pt in the form of O ₂ ⁻ or O ²⁻ . On the other hand, NO contained in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ).

Next, a part of the generated NO ₂ is absorbed in the NOx catalyst 20 while being oxidized on the platinum Pt and combined with the barium oxide BaO, and as shown in FIG. NO ₃ ^- diffuses into the NOx catalyst 20 in the form of. In this way, NOx is absorbed in the NOx catalyst 20.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt,
As long as the absorption capacity is not saturated, NO ₂ is absorbed in the NOx catalyst 20 and nitrate ions NO ₃ ⁻ are generated.

On the other hand, the oxygen concentration in the inflowing exhaust gas
Decreases and NO_TwoThe reaction goes in the reverse direction when the amount of
(NO_Three ^-→ NO_Two), And nitrate ion in the NOx catalyst 20
NO _Three ^-Is NO_TwoAlternatively, the NOx catalyst 20 is released in the form of NO.
Will be issued. That is, the oxygen concentration in the inflow exhaust gas decreases.
Then, NOx is released from the NOx catalyst 20.
As shown in FIG. 3, the degree of lean of the incoming exhaust gas
Lower the oxygen concentration in the incoming exhaust gas
Therefore, if the degree of leanness of the inflow exhaust gas is reduced, NO
NOx is released from the x catalyst 20.

On the other hand, at this time, when the mixture supplied to the combustion chamber 3 becomes stoichiometric or rich, a large amount of unburned HC and CO is discharged from the engine as shown in FIG.
These unburned HC and CO are converted to oxygen O ₂ ^- or O ₂ on platinum Pt.
Reacts with ^2- and is oxidized.

When the exhaust air-fuel ratio becomes stoichiometric or rich, the concentration of oxygen in the inflowing exhaust gas is extremely reduced, so that NO ₂ or NO is released from the NOx catalyst 20, and this NO ₂ or NO is shown in FIG. As shown in B), it reacts with unburned HC and CO and is reduced to N ₂ .

[0050] That is, HC in the inflowing exhaust gas, CO, first oxygen O ₂ on the platinum Pt ^- or O ^2- immediately be reacted with oxidized, then the platinum Pt on the oxygen O ₂ ^- or O ^2- If HC and CO still remain after consumption, this H
C, NOx discharged from the released NOx, the engine from the NOx catalyst 20 by the CO is made to reduction to N _2.

In this way, NO _{2 is} deposited on the surface of platinum Pt.
Alternatively, when NO is no longer present, NO ₂ or NO is released from the NOx catalyst 20 one after another, and is further reduced to N ₂ . Therefore, if the exhaust air-fuel ratio is made stoichiometric or rich, the NOx catalyst 20
Ox will be released.

[0052] Thus, NOx when the exhaust air-fuel ratio becomes lean is absorbed in the NOx catalyst 20, NOx when the exhaust air-fuel ratio to the stoichiometric or rich is released in a short time from the NOx catalyst 20, reduced to N ₂ Is done. Therefore, emission of NOx into the atmosphere can be prevented.

During the full load operation, the mixture supplied to the combustion chamber 3 is made rich. At the time of high load operation, at the time of warm-up operation at the time of engine start, at the time of acceleration, and at 120 km / min.
When the air-fuel mixture is set to the stoichiometric air-fuel ratio at the time of constant speed operation at h or more, and when the air-fuel mixture is lean at the time of low-medium load operation, NOx in the exhaust gas is absorbed by the NOx catalyst 20 at the time of low-medium load operation. At full load operation and at high load operation
NOx is released from the Ox catalyst 20 and reduced. However, if the frequency of full-load operation or high-load operation is low, and the frequency of low-medium load operation is high and the operation time is long, the release and reduction of NOx cannot be made in time, and N
The NOx absorption capacity (NOx absorption capacity) of the Ox catalyst 20 is saturated, so that the NOx cannot be absorbed.

Therefore, in such a case, when the lean air-fuel mixture is being burned, that is, when the medium-low load operation is being performed, the stoichiometric or short-time stoichiometric or stoichiometric operation is performed in a relatively short cycle. In some cases, a method of controlling the air-fuel ratio of the air-fuel mixture so that the rich air-fuel mixture is burned and releasing and reducing NOx in a short cycle may be employed. Thus N
Due to the absorption and release of Ox, the exhaust air-fuel ratio (in this embodiment, the air-fuel ratio of the air-fuel mixture) is “lean” and “spike-like stoichiometric or rich” (hereinafter referred to as “rich spike”) in a relatively short cycle. Is alternately repeated is referred to as lean-rich spike control, and this embodiment also employs lean-rich spike control. In this application, the lean-rich spike control is included in the lean air-fuel ratio control.

On the other hand, the fuel contains sulfur (S), and when the sulfur in the fuel burns, sulfur oxides (SOx) such as SO ₂ and SO ₃ are generated, and the NOx catalyst 20 These SOx also absorb. It is considered that the SOx absorption mechanism of the NOx catalyst 20 is the same as the NOx absorption mechanism. That is, assuming that platinum Pt and barium Ba are supported on the carrier in the same manner as when the NOx absorption mechanism is described, as described above, when the exhaust air-fuel ratio is lean, oxygen O ₂ is reduced. O ₂ ^- or O ^2-
The SOx (for example, SO ₂ ) in the inflowing exhaust gas becomes platinum Pt in the form of
Is oxidized to SO ₃ on the surface.

Thereafter, the generated SO ₃ is further oxidized on the surface of the platinum Pt, absorbed in the NOx catalyst 20 and combined with the barium oxide BaO, and then enters the NOx catalyst 20 in the form of sulfate ion SO ₄ ^2−. Diffuses to produce sulfate BaSO ₄ . This sulfate BaSO ₄ is stable and hard to decompose, and remains in the NOx catalyst 20 without being decomposed even if the air-fuel ratio of the inflowing exhaust gas is stoichiometric or rich for a short time by the lean-rich spike control described above. I will. Therefore, the BaSO ₄ in the NOx catalyst 20 with the passage of time
When the generation amount of NOx increases, the amount of BaO that can participate in the absorption of the NOx catalyst 20 decreases, and the NOx absorption ability decreases. This is SOx poisoning.

Therefore, in this embodiment, SOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean and is absorbed when the oxygen concentration of the inflowing exhaust gas is low so that SOx does not flow into the NOx absorbent 20. S that releases oxidized SOx
The Ox absorbent 17 is disposed upstream of the NOx absorbent 20. When the air-fuel ratio of the exhaust gas flowing into the SOx absorbent 17 is lean, the SOx absorbent 17
NOx is absorbed together with Ox, but when the air-fuel ratio of the inflowing exhaust gas is made stoichiometric or rich and the oxygen concentration becomes low, not only the absorbed SOx but also NOx is released.

As described above, in the NOx catalyst 20, SOx
Is absorbed to produce stable sulfate BaSO ₄ ,
As a result, even if the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 is stoichiometric or rich,
No longer released from Therefore, the SOx absorbent 17
In order to release SOx from the SOx absorbent 17 when the air-fuel ratio of the exhaust gas flowing into the exhaust gas is made stoichiometric or rich, the absorbed SOx is converted into sulfate ions SO ₄ ^{2− in} the form of SOx absorbent 17. Or if the sulfate BaSO ₄ is formed, the sulfate B
It is necessary that aSO ₄ be present in the SOx absorbent 17 in an unstable state. SOx that makes this possible
As the absorbent 17, copper C on a carrier made of alumina is used.
u, iron Fe, manganese Mn, transition metals such as nickel Ni, sodium Na, titanium Ti and lithium Li
SOx absorbent 1 carrying at least one selected from the group consisting of:
7 can be used.

In this SOx absorbent 17, the SOx absorbent 1
Air-fuel ratio of the exhaust gas flowing into the 7 is absorbed while SO ₂ in the exhaust gas is oxidized on the surface of the SOx absorbent 17 in the sulfate ions SO ₄ ^2-form in the SOx absorbent 17 when the lean,
Next, it is diffused into the SOx absorbent 17. In this case, S
Platinum Pt, palladium Pd on the carrier of the Ox absorbent 17,
If any of rhodium Rh is supported, SO
₂ platinum Pt in SO ₃ ^2-form, palladium Pd, easily adsorbed on the rhodium Rh, thus SO ₂ is likely to be absorbed in the SOx absorbent 17 in the sulfate ions SO ₄ ^2-form.
Therefore, in order to promote the absorption of SO ₂ , it is preferable that any one of platinum Pt, palladium Pd and rhodium Rh be supported on the carrier of the SOx absorbent 17.

When the SOx absorbent 17 is arranged upstream of the NOx catalyst 20, when the air-fuel ratio of the exhaust gas flowing into the SOx absorbent 17 becomes lean, SOx in the exhaust gas is absorbed by the SOx absorbent 17, and Downstream NOx catalyst 2
At 0, SOx does not flow, and the NOx catalyst 20 absorbs only NOx in the exhaust gas.

[0061] On the other hand, the SOx absorbed by the SOx absorbent 17 as described above sulfate ions SO ₄ ^{2 -} a form on whether you are diffused in the SOx absorbent 17, or sulfate BaSO ₄ in an unstable state ing. Therefore, SOx absorbent 1
When the air-fuel ratio of the exhaust gas flowing into the exhaust gas 7 becomes stoichiometric or rich and the oxygen concentration decreases, the SOx absorbed in the SOx absorbent 17 is easily released from the SOx absorbent 17.

By the way, according to the research of the present applicant, SOx
The following was found regarding the absorption / release action of the absorbent 17.
When the amount of SOx absorbed by the SOx absorbent 17 is small, the SOx absorbent 17 has a strong SOx adsorbing power,
If exhaust gas having a stoichiometric or rich air-fuel ratio is allowed to flow in the x absorbent 17 for a short time (for example, 5 seconds or less), SOx is not released from the SOx absorbent 17. Regarding this, the applicant has determined that the lean / rich spike control for releasing NOx from the NOx catalyst 20 when the amount of SOx absorbed in the SOx absorbent 17 is small has a stoichiometric or rich air-fuel ratio. It is confirmed that SOx is not released from the SOx absorbent 17 during the continuous time. Where S
Even when the amount of SOx absorbed in the Ox absorbent 17 is small, if the stoichiometric or rich air-fuel ratio exhaust gas is allowed to flow through the SOx absorbent 17 for a long time, SOx is released from the SOx absorbent 17. You.

However, when the amount of SOx absorbed by the SOx absorbent 17 increases, the SOx adsorbing power of the SOx absorbent 17 becomes weak, and thus the stoichiometric or rich air-fuel ratio exhaust gas is supplied to the SOx absorbent 17 for a short time. Even when flowing, SOx leaks out of the SOx absorbent 17 and the downstream NOx
There is a possibility that the catalyst 20 may be poisoned.

Therefore, in this embodiment, the SOx absorbed by the SOx absorbent 17 is determined from the history of the operating state of the engine.
The amount is estimated, the time when the estimated SOx absorption amount reaches a predetermined amount is determined to be the time to release SOx from the SOx absorbent 17, and the process of actively releasing SOx from the SOx absorbent 17 (hereinafter, referred to as “ This processing is referred to as reproduction processing). In performing the regeneration process of the SOx absorbent 17, E
The CU 30 determines the engine operating state at that time from the engine speed N and the engine load Q / N, and substitutes the temperature of the exhaust gas detected by the temperature sensor 29 as the temperature of the SOx absorbent 17 to obtain the engine operating state. Based on the temperature of the SOx absorbent 17 and the stoichiometric or rich condition that can release SOx most efficiently with little deterioration in fuel efficiency, the engine is operated at the selected air-fuel ratio to reduce the exhaust gas discharged for a long time.
It is performed by flowing the x absorbent 17.

When the temperature of the SOx absorbent 17 is raised to a predetermined temperature (for example, 550 ° C.) or higher, it is easy for SOx to be released from the SOx absorbent 17, in other words, it is possible to accelerate the release of SOx. know. Therefore, in this embodiment, during execution of the regeneration process of the SOx absorbent 17, the ECU 30 controls the temperature of the exhaust gas by appropriate means, and adjusts the temperature of the SOx absorbent 17 to the predetermined temperature (hereinafter referred to as SOx absorbent). Release temperature).

When the SOx absorbent 17 is regenerated, the exhaust gas flowing out of the SOx absorbent 17 (hereinafter referred to as “SO
The exhaust gas discharged from the x-absorbent 17 is referred to as regeneration exhaust, and may be distinguished from the exhaust gas discharged from the sox-absorbent 17 during non-regeneration processing. Will be included,
When this regenerated exhaust gas flows into the NOx catalyst 20, SOx in the regenerated exhaust gas is absorbed by the NOx catalyst 20, and the NOx catalyst 20 is poisoned by SOx, so that the provision of the SOx absorbent 17 is lost. Therefore, in this embodiment, SO
In order to prevent the SOx released from the SOx absorbent 17 from being absorbed by the NOx catalyst 20 during the regeneration processing of the x absorbent 17, the SOx absorbent 1
The regeneration exhaust flowing out of the pipe 7 is guided into the bypass pipe 26.

In this embodiment, the air-fuel ratio is stoichiometrically controlled at the time of high-load operation of the engine, at the time of warm-up operation at the time of engine start, at the time of acceleration, and at the time of constant speed operation of 120 km / h or more. Sometimes, the air-fuel ratio is controlled to be rich. Therefore, in these operating states, the air-fuel ratio of the exhaust gas becomes stoichiometric or rich, and the exhaust gas having the stoichiometric or rich air-fuel ratio becomes SOx
It will flow into the absorbent 17.

As described above, even if the stoichiometric or rich air-fuel ratio exhaust gas flows into the SOx absorbent 17, the SOx is not released from the SOx absorbent 17 if it is instantaneous, so that no problem occurs. However, if the exhaust gas flows into the NOx catalyst 20 downstream, the SOx absorbent 17 may release SOx. If the exhaust gas flows into the downstream NOx catalyst 20, the NOx catalyst 20 may be poisoned with SOx. In particular,
When the exhaust gas temperature becomes equal to or higher than the SOx release temperature, the release of SOx from the SOx absorbent 17 is promoted, and the risk of SOx poisoning of the NOx catalyst 20 increases.

For example, the warm-up operation at the time of starting the engine is continued until the engine body 1 is warmed up, so that it may take a long time, and the acceleration may be continued for a certain period of time. The constant speed operation of / h or more may be continued for a long time on the highway, and in these cases, SOx may be released from the SOx absorbent 17.

Therefore, in this embodiment, during high-load operation, during warm-up operation when starting the engine, during acceleration,
The stoichiometric or rich control of the air-fuel ratio was performed according to a request from the operating state of the engine, such as at a constant speed operation of km / h or more, or at full load operation (hereinafter, this is referred to as stoichiometric or rich air-fuel ratio control depending on the engine operating state). As a result, when the air-fuel ratio of the exhaust gas becomes stoichiometric or rich,
The exhaust gas flowing out of the SOx absorbent 17 is passed through the bypass pipe 2
6 and is prevented from flowing into the NOx catalyst 20.

That is, in this embodiment, regardless of whether the regeneration process of the SOx absorbent 17 is being executed or not.
When the stoichiometric control or the rich control of the air-fuel ratio is performed, the exhaust gas flowing out of the SOx absorbent 17 is guided into the bypass pipe 26 to prevent the exhaust gas from flowing into the NOx catalyst 20.

Hereinafter, the case where SOx is released from the SOx absorbent 17 by executing the regeneration processing of the SOx absorbent 17 will be described.
As described above, the stoichiometric or rich air-fuel ratio control is performed in response to a request from the engine operating state.
Is generally referred to as SOx of the SOx absorbent 17
Called release processing.

Next, switching of the exhaust gas path will be described in detail. When the lean / rich spike control of the air-fuel ratio is being executed in order to absorb and release NOx in the exhaust gas by the NOx catalyst 20 for reduction purification, the switching valve 28 is maintained at the bypass closed position as shown by a solid line in FIG. Is done. Therefore, at this time, the exhaust gas flowing out of the SOx absorbent 17 flows into the NOx catalyst 20 and does not flow into the bypass pipe 26. The SOx in the exhaust gas is the SOx absorbent 1
The SOx does not flow into the NOx catalyst 20 and SOx poisoning of the NOx catalyst 20 is prevented. Then, NOx in the exhaust gas is absorbed and released by the NOx catalyst 20,
It is reduced and purified.

Next, when shifting from the above state to the SOx release process, the control of the air-fuel ratio is switched from the lean / rich spike control to the stoichiometric or rich control, and at the same time, the switching valve 28 is moved from the bypass closed position to the dashed line in FIG. The position is switched to the bypass open position shown in FIG.

If the air-fuel ratio is controlled to stoichiometric or rich during the SOx release process, the air-fuel ratio of the exhaust gas becomes stoichiometric or rich. This exhaust gas is the SOx absorbent 17
, SOx is released from the SOx absorbent 17.
At this time, since the switching valve 28 is in the bypass open position, the exhaust gas containing a large amount of SOx flowing out of the SOx absorbent 17 does not flow into the NOx catalyst 20 and the bypass pipe 2
Flow through 6. Therefore, it is possible to prevent the NOx catalyst 20 from being poisoned by the SOx released from the SOx absorbent 17. Incidentally, SOx in the exhaust gas is unburned HC in the exhaust gas, is made to reduction by CO, it is released as SO _2.

Next, when switching from the stoichiometric or rich air-fuel ratio control to the lean / rich spike control due to the end of the SOx release processing, the switching valve 28 is moved from the bypass open position after a predetermined delay time has elapsed from the switching of the air-fuel ratio control. Switch to the bypass closed position. The reason why the delay time is provided between the switching of the air-fuel ratio control and the switching of the exhaust gas path by the switching valve 28 is as follows.

During the SOx release process, exhaust gas having a stoichiometric or rich air-fuel ratio flows through the SOx absorbent 17, but at that time, the SOx absorbent 17 not only releases SOx but also adsorbs HC in the exhaust gas. Therefore, at the end of the SOx release process, a large amount of HC is adsorbed on the SOx absorbent 17. On the other hand, even at the end of the SOx release process, not all of the SOx absorbed by the SOx absorbent 17 is released, and the SOx absorbent 17
Remains. This is no exception immediately after the regeneration process of the SOx absorbent 17.

When exhaust gas having a lean air-fuel ratio flows into the SOx absorbent 17 at the end of the SOx release process in such a state, oxygen contained in the exhaust gas oxidizes HC adsorbed on the SOx absorbent 17. Consumed in large quantities for
As a result, the exhaust gas having the lean air-fuel ratio is removed from the SOx absorbent 17
Even when the SOx absorbent 17 flows into the SOx absorbent 17, a stoichiometric atmosphere is formed inside the SOx absorbent 17, and immediately after the end of the SOx release processing, the SOx absorbent 17 is still at a high temperature and is in an atmosphere where SOx can be easily desorbed. SOx remaining in the agent 17 is desorbed and released from the SOx absorbent 17.
The desorption of SOx from the SOx absorbent 17 reduces the amount of HC adsorbed on the SOx absorbent 17 and reduces the amount of oxygen consumed for oxidizing HC.
Until the air-fuel ratio inside becomes lean.

Therefore, when the switching valve 28 is switched from the bypass open position to the bypass closed position at the same time as the switching from the stoichiometric or rich air-fuel ratio control to the lean / rich spike control so that the exhaust gas flows to the NOx catalyst 20, the SOx release processing Immediately after the termination, the SOx released from the SOx absorbent 17 flows into the NOx catalyst 20, and the NOx catalyst 2
0 may be poisoned by SOx.

Therefore, in this embodiment, the SOx releasing process is completed, and the exhaust gas having the lean air-fuel ratio is supplied to the SOx absorbent 1
7, the SOx absorbent 17
Until a predetermined time elapses until no more is released
The switching valve 28 is held at the bypass open position to allow the exhaust gas to flow to the bypass pipe 26 and not to flow into the NOx catalyst 20.

Thus, even if SOx is desorbed from the SOx absorbent 17 after the end of the SOx release process, the desorbed SOx does not flow into the NOx catalyst 20.
The SOx poisoning of the NOx catalyst 20 can be prevented.

It should be noted that the switching of the air-fuel ratio control from the switching valve 28
The length of the delay time until the exhaust gas path switching by the engine is determined based on the operating state of the engine at the time of switching from stoichiometric or rich air-fuel ratio control to lean / rich spike control.
Is selected with reference to the delay time map stored in the.

Here, the delay time map is obtained by conducting an experiment on this engine in advance and determining the operating state of the engine such as the exhaust gas temperature, the vehicle speed and the engine speed, and the SOx desorption of the SOx absorbent 17 immediately after the switching of the air-fuel ratio control. The relationship with the time until the separation ends is determined, and based on the results of this experiment, an optimal delay time is set according to the operating state of the engine, and is created by mapping. Incidentally, the relationship between the delay time and the exhaust gas temperature is such that the higher the exhaust gas temperature, the shorter the delay time, the higher the vehicle speed, the shorter the delay time, and the higher the engine speed, the shorter the delay time. There is a tendency.

Then, after the elapse of the delay time,
The switching valve 28 is switched from the bypass open position to the bypass closed position, and the exhaust gas flowing out of the SOx absorbent 17 is subjected to NOx
It flows to the catalyst 20 and does not flow to the bypass pipe 26. After the delay time has elapsed, the exhaust gas with lean air-fuel ratio becomes S
SOx from the SOx absorbent 17 even when flowing into the Ox catalyst 17
Since there is no desorption, and SOx in the exhaust gas is absorbed by the SOx absorbent 17, SOx does not flow into the NOx catalyst 20, and SOx poisoning of the NOx catalyst 20 is prevented. Then, NOx in the exhaust gas is absorbed and released by the NOx catalyst 20,
It is reduced and purified.

FIG. 5 shows an example of the air-fuel ratio control in this embodiment. In this embodiment, in the lean-rich spike control, for example, 60 km
This is alternately repeated with the lean operation continuation time of about 40 seconds and the stoichiometric operation continuation time of about 2 seconds in the constant speed running at / h. On the other hand, during the regeneration process of the SOx absorbent 17, the air-fuel ratio is set to the stoichiometric control, and the duration thereof is set to a time sufficiently longer than the stoichiometric operation duration in the lean / rich spike control, for example, about 1 hour.

Next, an exhaust path switching processing execution routine according to this embodiment will be described with reference to FIG. A flowchart including the steps constituting this routine is stored in the ROM 32 of the ECU 30, and the processing in each step of the flowchart is entirely performed by the CP of the ECU 30.
Performed by U34.

<Step 101> First, the ECU 30
In step 101, it is determined whether the current air-fuel ratio control is stoichiometric control or rich control. The air-fuel ratio is controlled by the stoichiometric control during regeneration of the SOx absorbent 17, during high-load operation of the engine, during warm-up operation when starting the engine, during acceleration, and when operating at a constant speed of 120 km / h or more. In such cases, the ECU 30 makes an affirmative determination in step 101 and proceeds to step 102. On the other hand, the lean / rich spike control of the air-fuel ratio is performed during low-medium load operation of the engine. In this case, the ECU 30 makes a negative determination in step 101 and repeats step 101.

<Step 102> When the ECU 30 makes an affirmative decision in step 101 and proceeds to step 102, the ECU 30
It is determined whether the temperature of the x absorbent 17 is equal to or higher than the SOx release temperature. In this embodiment, the temperature of the exhaust gas at the outlet of the SOx absorbent 17 detected by the temperature sensor 23 is used as the temperature of the SOx absorbent 17.

<Step 103> If an affirmative determination is made in step 102, the SOx is easily released from the SOx absorbent 17, so the ECU 30 proceeds to step 103 and sets the switching valve 28 to the bypass open position indicated by a broken line in FIG. And the exhaust gas flowing out of the SOx absorbent 17 is guided into the bypass pipe 26 so as not to flow into the NOx catalyst 20. As a result, the exhaust gas flowing out of the SOx absorbent 17 is discharged to the atmosphere through the bypass pipe 26. Therefore, even if SOx is released from the SOx absorbent 17, the SOx is not absorbed by the NOx catalyst 20, and the NOx catalyst 20 can be prevented from being poisoned by SOx.

The SOx released from the SOx absorbent 17
Is reduced by unburned HC and CO in the exhaust gas, and is released as SO ₂ . When the air-fuel ratio is under stoichiometric or rich control, unburned HC, CO and NOx are discharged from the engine body 1. However, since the SOx absorbent 17 has a three-way catalytic function, the unburned HC, CO and NOx are discharged. C
O and NOx are purified by the SOx absorbent 17,
There is no risk of being released into the atmosphere.

On the other hand, if a negative determination is made in step 102, SOx is hardly released from the SOx absorbent 17, so that
The ECU 30 returns to step 101.

<Step 104> Next, the ECU 30
Proceeding from step 103 to step 104, it is determined whether the current air-fuel ratio control is lean control (to be precise, lean
Is determined to be rich spike control). Step 10
If a negative determination is made in step 4, it means that the stoichiometric or rich air-fuel ratio control is still ongoing, and in that case, the ECU 30 repeats step 104.

<Step 105> If an affirmative determination is made in step 104, the ECU 30 proceeds to step 105 and refers to the delay time map to select a delay time corresponding to the engine operating state at that time.

<Step 106> Next, the ECU 30
Proceeding to step 106, the elapsed time since the affirmative determination in step 104 is counted.

<Step 107> Next, the ECU 30
Proceeding to step 107, it is determined whether the elapsed time counted in step 106 has reached the delay time selected in step 105. If a negative determination is made in step 107, the ECU 30 returns to step 106 and continues counting the elapsed time. While the counting of the elapsed time is continued, the switching valve 28 is held at the bypass open position, so that the exhaust gas flowing out of the SOx absorbent 17 flows to the bypass pipe 26 and does not flow to the NOx catalyst 20. Therefore, lean / rich air-fuel ratio control
Immediately after switching to rich spike control, SOx absorbent 1
Even if SOx is desorbed from 7, the desorbed SOx does not flow into the NOx catalyst 20, and the SOx poisoning of the NOx catalyst 20 is prevented.

<Step 108> If an affirmative determination is made in step 107, the ECU 30 proceeds to step 108, holds the switching valve 28 at the bypass closed position indicated by the solid line in FIG. The gas is introduced into the NOx catalyst 20 so as not to flow into the bypass pipe 26. After the delay time has elapsed, even if exhaust gas having a lean air-fuel ratio flows into the SOx catalyst 17, SOx does not desorb from the SOx absorbent 17, and SOx in the exhaust gas is
The NOx catalyst 20 absorbs SOx
The x does not flow, and the SOx poisoning of the NOx catalyst 20 is prevented. Then, NOx in the exhaust gas is converted to NOx catalyst 20
Is absorbed and released, and reduced and purified.

As described above, according to this embodiment, S
When SOx is likely to be released from the Ox absorbent 17, the exhaust gas flowing out of the SOx absorbent 17 flows into the bypass pipe 26 and does not flow into the NOx catalyst 20, so that the NOx catalyst 20 is poisoned with SOx. Can be reliably prevented. As a result, the NOx purification rate of the NOx catalyst 20 can always be kept high.

In this embodiment, a series of signal processing (steps 101 to 108) by the ECU 30 can be regarded as an exhaust path switching control means for controlling a switching valve (exhaust path switching means).

In this control routine, when the air-fuel ratio is under stoichiometric or rich control and the temperature of the SOx absorbent 17 is equal to or higher than the SOx release temperature, the exhaust gas is
6, the exhaust gas can be introduced into the bypass pipe 26 when the air-fuel ratio becomes stoichiometric or rich control regardless of the temperature of the SOx absorbent 17. In such a case, step 102 is deleted, and if a positive determination is made in step 101, the process proceeds to step 103.

[Second Embodiment] Next, a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described with reference to FIGS. This second embodiment is the first embodiment.
The point different from the embodiment is as follows. In the second embodiment, as shown in FIG. 7, an O ₂ for generating an output voltage proportional to the oxygen concentration of the exhaust gas exiting the SOx catalyst 17 is supplied to an exhaust pipe 19 disposed downstream of the SOx catalyst 17. A sensor (oxygen concentration detecting means) 24 is attached,
The output voltage of the O ₂ sensor 24 becomes E via the AD converter 40.
It is input to the input port 35 of the CU 30.

In the first embodiment described above, the timing for switching the switching valve 28 from the bypass open position to the bypass closed position after the end of the SOx release process is switched from stoichiometric or rich air-fuel ratio control to lean / rich spike control. The elapsed time has reached a preset delay time. If the delay time has elapsed, SOx is desorbed from the SOx absorbent 17 even if exhaust gas having a lean air-fuel ratio flows into the SOx absorbent 17. It is based on the idea that there is not.

On the other hand, in the second embodiment, S
If the air-fuel ratio of the exhaust gas flowing out of the SOx absorbent 17 is stoichiometric or rich after the air-fuel ratio is switched from the stoichiometric or rich control to the lean-rich spike control by the end of the Ox release processing, the SOx absorbent 17 It can be estimated that there is a possibility that SOx may be desorbed from the fuel gas. If the air-fuel ratio of the exhaust gas flowing out of the SOx absorbent 17 is lean, the air-fuel ratio inside the SOx absorbent 17 is also lean. From SOx absorbent 17 to S
Since it can be estimated that Ox does not desorb, SOx absorbent 1
The timing of switching the path of the exhaust gas after the SOx release process is determined based on the air-fuel ratio of the exhaust gas flowing out of the exhaust gas. Then, in order to detect the air-fuel ratio of the exhaust gas flowing out of the SOx absorbent 17, O ₂
The sensor 24 is provided.

Next, an exhaust path switching process execution routine according to the second embodiment will be described with reference to FIG. A flowchart including steps constituting this routine is stored in the ROM 32 of the ECU 30.
This is executed by the PU.

<Steps 201 to 204> Steps 201 to 20 in the flowchart shown in FIG.
Step 4 is exactly the same as steps 101 to 104 of the flowchart in the first embodiment shown in FIG.

If the affirmative determination is made in step 204, the ECU 30 proceeds to step 205, where the O ₂ sensor 24
Thus, it is determined whether the air-fuel ratio of the exhaust gas flowing out of the SOx absorbent 17 is lean.

If a negative determination is made in step 205,
The ECU 30 repeats Step 205. Step 2
Since the switching valve 28 is held at the bypass open position while the step 05 is repeated, the exhaust gas flowing out of the SOx absorbent 17 flows to the bypass pipe 26 and does not flow to the NOx catalyst 20. This means that the air-fuel ratio of the exhaust gas downstream of the SOx absorbent 17 is not lean, which means that even if the exhaust gas having the lean air-fuel ratio flows into the SOx absorbent 17, the SOx absorbent 1
7, the air-fuel ratio becomes stoichiometric, and the SOx absorbent 1
This is because there is a possibility that SOx may be desorbed from the exhaust gas 7, and the exhaust gas possibly containing such SOx may be decomposed into N 2.
This is so as not to flow into the Ox catalyst 20. This prevents SOx poisoning of the NOx catalyst 20.

If the determination in step 205 is affirmative,
Since it can be estimated that SOx has not been desorbed from the SOx absorbent 17, the ECU 30 proceeds to step 206, holds the switching valve 28 at the bypass closed position shown by the solid line in FIG. Into the NOx catalyst 20 so as not to flow into the bypass pipe 26. As a result, the exhaust gas having a lean air-fuel ratio is
7 to the NOx catalyst 20. Then, the SOx in the exhaust gas is absorbed by the SOx absorbent 17 and the NOx
SOx does not flow into the catalyst 20 and the NOx catalyst 20
SOx poisoning is prevented. And NOx in exhaust gas
Is absorbed and released by the NOx catalyst 20, and is reduced and purified.

According to the second embodiment, when there is a possibility that SOx is released from the SOx absorbent 17,
The exhaust gas flowing out of the SOx absorbent 17 is
6 and no longer flows into the NOx catalyst 20,
The SOx poisoning of the Ox catalyst 20 can be reliably prevented. As a result, the NOx purification rate of the NOx catalyst 20 can always be kept high.

In this embodiment, the series of signal processing (steps 201 to 06) by the ECU 30 can be regarded as an exhaust path switching control means for controlling the switching valve (exhaust path switching means).

[Other Embodiments] In the above-described embodiment, the present invention is applied to a gasoline engine. However, it goes without saying that the present invention can be applied to a diesel engine. In the case of a diesel engine, the combustion in the combustion chamber is performed in a much leaner region than the stoichiometric region, so that the air-fuel ratio of the exhaust gas flowing into the SOx absorbent 17 and the NOx catalyst 20 is very lean under a normal engine operating condition. Although SOx and NOx are absorbed, SOx and NOx are hardly released.

In the case of a gasoline engine, as described above, the air-fuel mixture supplied to the combustion chamber 3 is made stoichiometric or rich so that the SOx absorbent 17 and the NOx
Although the air-fuel ratio of the exhaust gas flowing into the catalyst 20 can be made stoichiometric or rich, SOx and NOx absorbed by the SOx absorbent 17 and the NOx catalyst 20 can be released.
In the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is made stoichiometric or rich, there is a problem that soot is generated at the time of combustion.

Therefore, when the present invention is applied to a diesel engine, in order to make the air-fuel ratio of the inflowing exhaust gas stoichiometric or rich, it is necessary to separate the reducing agent ( For example, light oil as a fuel) must be supplied to the exhaust gas. The supply of the reducing agent to the exhaust gas can be performed by sub-injecting the fuel into the cylinder during the intake stroke, the expansion stroke, or the exhaust stroke, or the reducing agent can be supplied into the exhaust passage upstream of the SOx catalyst 17. It is also possible by supplying.

When a diesel engine is provided with an exhaust gas recirculation device (a so-called EGR device),
By introducing a large amount of exhaust gas recirculation gas into the combustion chamber, the air-fuel ratio of the exhaust gas can be made stoichiometric or rich.

[0114]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, when there is a possibility that SOx is released from the SOx absorbent, the exhaust gas flowing out of the SOx absorbent is prevented from flowing into the NOx absorbent. NO
SOx poisoning of the x absorbent can be reliably prevented. As a result, an excellent effect that the NOx purification rate of the NOx absorbent can be kept high over a long period of time is exhibited.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
It is a schematic structure figure of an embodiment.

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

FIG. 3 Unburned HC in exhaust gas discharged from the engine,
FIG. 3 is a diagram schematically showing the concentrations of CO and oxygen.

FIG. 4 is a diagram for explaining the NOx absorbing / releasing action of a NOx storage reduction catalyst.

FIG. 5 is a diagram showing an example of air-fuel ratio control in the first embodiment.

FIG. 6 is a diagram illustrating an example of an exhaust path switching processing execution routine according to the first embodiment.

FIG. 7 shows a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
It is a schematic structure figure of an embodiment.

FIG. 8 is a diagram illustrating an example of an exhaust path switching process execution routine according to the second embodiment.

[Explanation of symbols]

1 engine body (internal combustion engine) 3 combustion chamber 4 a spark plug 11 the fuel injection valves 16, 19 and 22 an exhaust pipe (exhaust passage) 17 SOx absorbent 20 NOx catalyst (NOx absorber) 23 Temperature sensor 24 0 ₂ sensor (oxygen concentration Detecting means) 26 bypass pipe (bypass passage) 28 switching valve (exhaust path switching means) 30 ECU (exhaust path switching control means)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F01N 3/28 301C F02D 41/04 305 F02D 41/04 305Z 41/14 310 41/14 310P 43/00 301 43/00 301E 301T F-term (reference) 3G084 AA01 AA03 AA04 BA13 BA17 BA20 BA24 CA09 DA10 DA19 DA27 EA11 EB11 FA05 FA07 FA26 FA27 FA29 FA33 FA35 FA38 3G091 AA02 AA11 AA12 AA13 AA18 AB23 AB23 BA14 BA15 BA19 BA33 CA12 CA13 CB02 CB05 DA03 DA07 DB06 DB10 DB13 DC01 EA01 EA03 EA05 EA17 EA31 EA34 EA39 FA01 FA04 FA09 FA12 FA13 FA14 FA17 FA18 FB10 FB11 HA12 GB02 GB01 HAB HB03 HB05 3G301 HA01 HA02 HA13 HA15 HA18 JA15 JA21 JA25 JA26 JA33 JB09 LB02 MA0 1 MA11 NA06 NA08 NA09 ND01 NE01 NE06 NE13 NE14 NE15 NE21 PA01A PA01B PD02A PD02B PD11A PD11B PE01A PE01B PF01A PF01B

Claims (2)

[Claims] (1) The exhaust gas is disposed in an exhaust passage of an internal combustion engine capable of lean combustion and absorbs SOx when the air-fuel ratio of the inflowing exhaust gas is lean and absorbs it when the oxygen concentration of the inflowing exhaust gas is low. An SOx absorbent that releases SOx, and (b)
NOx is disposed in the exhaust passage downstream of the SOx absorbent when the air-fuel ratio of the inflowing exhaust gas is lean.
A NOx absorbent for absorbing NOx and releasing the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low;
A bypass passage for branching downstream of the absorbent and flowing exhaust gas bypassing the NOx absorbent, and (d) selecting which of the NOx absorbent and the bypass passage the exhaust gas flowing out of the SOx absorbent is guided to. (E) exhaust gas flowing out of the SOx absorbent during the SOx release process of releasing the SOx absorbed by the SOx absorbent by setting the air-fuel ratio of the exhaust gas to stoichiometric or rich. The exhaust path switching means is controlled so as to be guided to the passage, and the exhaust gas flowing out of the SOx absorbent after a predetermined time has elapsed after the air-fuel ratio of the exhaust gas has been switched to lean after the end of the SOx release processing is reduced by the NOx absorbent. Exhaust path switching control means for controlling the exhaust path switching means so as to be guided to the exhaust gas. 2. (a) Arranged in an exhaust passage of an internal combustion engine capable of lean burn, absorbs SOx when the air-fuel ratio of the inflowing exhaust gas is lean, and absorbs it when the oxygen concentration of the inflowing exhaust gas is low. An SOx absorbent that releases SOx, and (b)
NOx is disposed in the exhaust passage downstream of the SOx absorbent when the air-fuel ratio of the inflowing exhaust gas is lean.
A NOx absorbent for absorbing NOx and releasing the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low;
A bypass passage for branching downstream of the absorbent and flowing exhaust gas bypassing the NOx absorbent, and (d) selecting which of the NOx absorbent and the bypass passage the exhaust gas flowing out of the SOx absorbent is guided to. Exhaust gas switching means, (e) oxygen concentration detecting means provided between the SOx absorbent and the NOx absorbent, and (f) the SOx absorption by making the air-fuel ratio of exhaust gas stoichiometric or rich. And controlling the exhaust path switching means so that exhaust gas flowing out of the SOx absorbent is guided to the bypass passage at the time of SOx releasing processing for releasing SOx absorbed by the agent, and the oxygen concentration detecting means after the end of the SOx releasing processing. If it is determined that the air-fuel ratio of the exhaust gas downstream of the SOx absorbent is lean based on the oxygen concentration detected in the above step, the exhaust gas flowing out of the SOx absorbent absorbs the NOx Exhaust gas switching control means for controlling the exhaust path switching means so as to be guided by the agent.